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Comparative EmLryolo^y
of the
Vertebrates
Comparative
EmDryolo^y
or tne
Vertebrates
by
Olin E. Nelsen, M.A., Ph.D.
Department of Zoology
University of Pennsylvania
With 2057 Drawings and Photographs
Grouped as 380 Illustrations
New York Toronto London
McGRAW-HILL BOOK COMPANY, INC.
W43'
Pref
ace
A study of the comparative embryology of a group of animals such as the
vertebrates when followed to its logical conclusion leads to a consideration
of the comparative anatomy of the group. Students claim, and justly so, that
they learn best through the association of events, things, and concepts. As
applied to the study of vertebrate embryology and anatomy, the principle
of learning by association means this: observations upon the adult anatomy
of the various organ-systems of a particular vertebrate species when corre-
lated with the earlier stages of embryonic development of these systems lead
to a more ready perception and understanding of structural principles and
relationships involved. Furthermore, when the developmental anatomy and
the adult anatomy of any one species is associated with similar phenomena
in other species of the vertebrate group it naturally produces a clearer under-
standing of the development and morphology of the group as a whole. This
broad, comprehensive approach is a fundamental one and it is a requirement
for the furtherance of research in vertebrate biology, whether it be on the
level of cellular chemistry or the physiology of organ-systems.
An endeavor to satisfy a demand for a comprehensive approach to verte-
brate development by an extension of the descriptions of the earlier phases
of the embryology of several representative vertebrate species into their final
stages of development, and hence into the realm of comparative anatomy, is
the main purpose of this book. This goal is the greatest defense which the
author can give for his effort to assemble the material and data contained
herein.
On the other hand, though the book correlates comparative vertebrate em-
bryology with comparative vertebrate anatomy, its arrangement is such that
the fundamental features of comparative vertebrate embryology readily can
be divorced from the intricate phases of comparative anatomy. For example,
Chaps. 1-11, 20, 21, and 22 are devoted to a consideration of basic embryo-
logical principles whereas Chaps. 12-20 treat particularly the relationships of
comparative embryology and comparative anatomy. A proper selection of
descriptive material in Chaps. 12-20 (which may be done readily by a survey
of the outline heading each chapter) added to the basic embryological data
affords a basis for a thorough course in comparative vertebrate embryology.
The selection of material suggested in the previous paragraph brings forth
another motive for writing this text. It has been the author's habit — one com-
VIII PREFACE
mon to many other teachers — never to give a course in exactly the same way
two years in succession. This procedure enlivens a course and keeps suc-
cessive groups of students out of the rut of looking forward to the same identical
lectures and laboratory approach year after year. As a result, in reality this
book is a compilation of the different aspects of embryology presented by the
author over a period of years to classes in comparative vertebrate embryology.
Consequently, by the use of certain chapters and the outlines at the headings
of each chapter, various facets of embryology may be presented one year
while other aspects are selected the following year, and so on. Moreover, a
selective procedure allows the book to be used readily for short courses in
embryology as well as longer courses. For example. Chaps. 3, 5-1 1 , and 20-22
may serve as the basis for a short course in vertebrate embryology.
Another feature of the text is the presentation of many illustrations well
prepared. Illustrations are an important adjunct to the teaching of embryology.
This is true especially where the teacher is burdened with the teaching of other
courses and thus is handicapped by lack of time to make adequate blackboard
drawings and illustrations of laboratory and lecture material. In Chaps. 3,
5-1 1, and 20-22, one finds illustrative material adequate to enable the student
to gain an appreciation of the fundamental features of vertebrate development.
Thus, this part of the book may be used extensively as a laboratory guide to
the fundamental principles involved in vertebrate development.
A final aspect of the text may be mentioned, namely, the references given
at the close of the chapters. References to literature are important especially
in courses of embryology where small groups of students are assembled.
Under these conditions the teacher often prefers to give the course on a
seminar basis. With this approach, references are most valuable in the assign-
ment of special reports and student lectures which the student later gives to
the class as a whole.
Acknowledgments
The author expresses his great obUgation to and appreciation for the supe-
rior artistic abilities, continual patience, and conscientious effort of Elisabeth
R. Swain who executed the difficult task of preparing — with certain exceptions
— the illustrations for this text. He also wishes to express his sincere thanks
to Edna R. White and Julia A. Lloyd who contributed illustrations. These
three artists were most exact in carrying out the author's instructions for illus-
trations, and also in transforming his preliminary sketches into finished draw-
ings.
The author is indebted greatly to Wistar Institute of Anatomy and Biology,
Philadelphia, for permission to redraw various illustrations from the journals
published by the Wistar Institute. Appreciation similarly goes to the Carnegie
Institution of Washington; The Marine Biological Laboratory, Woods Hole,
Mass.; Williams and Wilkins Co., Baltimore; University of Chicago Press,
Chicago; Yale University Press, New Haven; Academic Press, Inc., New
York; Museum of Comparative Zoology at Harvard College; Oxford Uni-
versity Press, Inc., New York; Ginn and Co., Boston; W. B. Saunders Co.,
Philadelphia; McGraw-Hill Book Co., Inc., New York; Henry Holt and Co.,
Inc., New York; W. W. Norton and Co., Inc., New York; John Wiley and
Sons, Inc., New York; J. B. Lippincott Co., Philadelphia; The Macmillan Co.,
New York and London; Knopf, Inc., New York; Appleton-Century Co.,
Inc., New York; Sidgewick and Jackson, Ltd., London; Cambridge Univer-
sity Press, England; and Columbia University Press, New York.
To his colleagues in the Department of Zoology of the University of Penn-
sylvania the author owes a debt of appreciation for encouragement during the
writing of the manuscript, especially to Dr. J. Percy Moore, Dr. D. H.
Wenrich, and Dr. L. V. Heilbrunn. Acknowledgments and appreciation go
to Mrs. Anna R. Whiting, also of the Department of Zoology, and to Dr.
Miles D. McCarthy of the Harrison Department of Surgical Research of the
University of Pennsylvania Medical School and the Department of Zoology,
Pomona College, Claremont, California, who read much of the manuscript and
offered valuable suggestions. Frances R. Houston, Librarian of the University
of Pennsylvania Medical School, and Elizabeth D. Thorp, Librarian of the
Botany-Zoological Library of the University of Pennsylvania, deserve sincere
thanks for cooperative understanding and help in securing and placing many
periodicals at the author's disposal. Various students contributed clerical efforts
toward the completion of this work, especially Barbara Neely Gilford, Carolyn
Kerr, and Louise Mertz. Their endeavors are appreciated greatly.
X ACKNOWLEDGMENTS
Any attempt of the author to acknowledge obUgations would be incomplete,
indeed, without mention of the extreme readiness to serve and cooperate on
the part of Dr. James B. Lackey, then Science Editor of The Blakiston Co.
(presently Research Professor, School of Engineering, University of Florida),
and also to Irene Claire Moore, then Assistant Manuscript Editor (presently
Book Editor, United Lutheran Publication House, Philadelphia), and to W. T.
Shoener, Production Manager.
The Author
Autumn 1952
Philadelphia, Pa.
Contents
Preface vii
Acknowledgments ix
Orientation xiii
Part I
The Period of Preparation
1 . The Testis and Its Relation to Reproduction 3
2. The Vertebrate Ovary and Its Relation to Reproduction 52
3. The Development of the Gametes or Sex Cells 112
Part II
The Period of Fertilization
4. Transportation of the Gametes (Sperm and Egg) from the Germ
Glands to the Site where Fertilization Normally Occurs 177
5. Fertilization 210
Part III
The Development of Primitive Embryonic Form
6. Cleavage (Segmentation) and Blastulation 279
7. The Chordate Blastula and Its Significance 340
8. The Late Blastula in Relation to Certain Innate Physiological
Conditions: Twinning 373
9. Gastrulation 388
10. Tubulation and Extension of the Major Organ-forming Areas:
Development of Primitive Body Form 454
11. Basic Features of Vertebrate Morphogenesis 516
Part IV
Histogenesis and Morphogenesis of the Organ Systems
12. Structure and Development of the Integumentary System 555
13. Structure and Development of the Digestive System 596
14. Development of the Respiratory-buoyancy System 634
15. The Skeletal System 653
xi
Xil CONTENTS
1 6. The Muscular System 699
1 7. The Circulatory System 724
18. The Excretory and Reproductive System 768
19. The Nervous System 805
20. The Development of Coelomic Cavities 857
21. The Developing Endocrine Glands and Their Possible Relation to
Definitive Body Formation and the Differentiation of Sex 874
Part V
The Care of the Developing Embryo
22. Care and Nourishment of the Developing Young 899
Index 933
Orientation
I. Some Definitions Relative to Embryology
The word embryo has various shades of meaning. In general, it is applied
to the rudimentary or initial state of anything while it remains in an unde-
veloped or primitive condition. As used in zoology, it designates in one sense
the earlier stages of the development of an animal before the definitive or
adult form of the species is assumed; or, in a second sense, it signifies the
entire period of prenatal existence.
The word development not only is used to denote the various changes evi-
dent in prenatal emergence, but also it applies to postnatal changes as well.
Moreover, in the development of a particular animal it may be extended
beyond the period of structural and physiological maturity to the changes
involved in eventual senescence.
The developing young of viviparous animals while undergoing the later
stages of development within the uterus is spoken of as a fetus. This term is
used also, on occasion, to designate the later stages of development of oviparous
species. The phrase mammary fetus is applied to the young of marsupial mam-
mals such as the opossum while it remains attached to the nipple within the
marsupial pouch of the mother.
The term descriptive embryology is applied to the method of embryological
study concerned with the direct observation and description of embryological
development. Up to the latter part of the last century embryology was con-
cerned mainly with the direct observation of the changes going on in the
intact embryo. However, beginning in the 1880's Wilhelm Roux and others
initiated the expeirmental approach in embryological study and the school of
experimental or causal embryology was formed. In experimental embryology
various parts of the developing embryo are removed, transplanted, parts are
exchanged, or the environmental conditions are altered. The end sought by
this method is an analysis of the respective roles played during development
by different parts of the developing organism and by different environmental
factors, in an endeavor to give a mechanical and functional explanation of
development. One of the outstanding results of the experimental method
applied to embryological study is the great body of evidence which points
to the fact that in the vertebrate group one of the main processes in devel-
opment (morphogenesis) is the induction of organs and organ-systems by
so-called organizer cellular areas present in certain parts of the developing
embryo. Organization of the developing body, in other words, is dependent
ORIENTATION
upon a series of changes mediated by cellular groups known as organizers
which appear at the correct time and locus in development.
It soon became apparent, however, that the terminology employed in ex-
perimental embryology was vague because it substituted indefinite terms such
as "inductors" or "organizers" as an explanation of developmental events.
The use of the word organizer means little unless one is able to describe the
manner of operation of the physical and chemical substances which effect
the results produced by the organizer. Consequently, embryologists with physi-
ological and biochemical training are concerned now with the effort of deter-
mining the specific chemical factors concerned with the various processes and
steps involved in development. This type of embryological study is called
biochemical or chemical embryology. Chemical embryology is divisible into
two main lines of attack, namely, an investigation of the chemistry of cells
and cellular parts or cytochemistry, and a study of the chemistry of groups
of cells or histochemistry.
11. Free-living Versus Sheltered Embryological Forms; Periods
of Development
The independence of a free-Uving existence on the part of developing young
is assumed at different stages of development depending upon the species
involved. For example, in the case of the frog, the developing embryo becomes
free-living at an early stage and it experiences a free-living larval existence
for an extended period before its metamorphosis into the adult or definitive
form of the frog. In the chick, the young undertakes a kind of free-living
existence at the time of hatching or about a week after it has assumed the
definitive body form. The human young, on the other hand, experiences an
extensive period of fetal development for about five months in utero after
it has achieved definitive body form. Moreover, it is most helpless and de-
pendent even after birth.
Regardless of the time during its development when an animal species
assumes a free-living, independent existence, it is apparent that the develop-
ment of the individual as a whole may be divided into two general periods,
viz., embryonic and post-embryonic periods. The embryonic period of devel-
opment begins at fertilization of the egg and continues for a time after definitive
body form is achieved. The end of the embryonic period may be regarded as
the time of birth in viviparous forms, hatching in oviparous species, and the
end of metamorphosis in free-living larval species. This is an arbitrary and,
for some forms, quhte comprehensive definition. Nevertheless, for comparative
purposes this definition is suitable. The post-embryonic period begins at the
termination of the embryonic phase of development and continues through
sexual maturity into later life.
ORIENTATION
The embryonic period of development in all vertebrate species may be
resolved into three distinct phases:
a. An early embryonic period which begins at the time when the egg starts
to develop and which reaches its culmination when the embryo has attained
the state of primitive, generalized body form (see Chaps. 10 and 11, and
fig. 255).
b. A period of transition then follows during which the structural condi-
tions prevalent in primitive body form are transformed into the morphology
present in definitive body form. Definitive body form is reached when the
embryo assumes a general resemblance to the adult form of the species. The
changes described in Chaps. 12-20 are concerned to a considerable extent
with this phase of development.
c. The late embryonic period. This phase of development comprises the
changes which the embryo experiences for a time after it has achieved definitive
body form. In the human embryo, it includes several months of fetal growth
in the uterus, and in the chick it is of about a week's duration continuing
from day 14 of incubation to the time of hatching around day 20. In the frog
it is a brief period during the close, and possibly shortly after, metamorphosis.
The period of transition may be regarded as the larval period of develop-
ment. If so conceived, two types of larval forms exist, namely ( 1 ) free-living
larval forms such as the frog tadpole in which the body structures are adapted
to a free-living existence outside of protective embryonic structures, and (2)
non-free-living larval forms in which the larval or transitional period is passed
within the confines of covering egg membranes or within the protective tissues
of the female or male parent. Free-hving larval forms include Amphioxus,
most fishes, and amphibia, while some fishes and all reptiles, birds, and mam-
mals may be regarded as having a protected larval existence.
III. Summary of Developmental Phenomena Associated with the Life
of an Individual Vertebrate Animal
A. Period of Preparation
During this period the parents are prepared for reproduction and the repro-
ductive cells or gametes are elaborated.
B. Embryonic Development
1. Early embryonic period
This period begins with fertilization of the egg and ends with the develop-
ment of primitive embryonic body form with its basic conditions of the various
systems. The basic or group condition of a particular vertebrate organ-system
is that stage of development of the system when it possesses structural features
common to all embryos of the vertebrate group. When the common or primi-
XVI ORIENTATION
tive embryonic conditions of the various systems are present, a common,
basic, primitive embryonic body form also is present. Hence, all vertebrate
embryos tend to pass through a stage of development in which the shape
and form of the developing body resembles that of all other vertebrate species
at this stage of development. This stage of body formation is known as the
primitive, embryonic body- form stage.
2. The larval period or period of transition
During this phase of development, the basic conditions of the organ-systems,
which are present at the end of primitive body formation, are transformed
into the structural conditions present in definitive body form. At the end of
this period of development the general form of the organ-systems, and of the
embryo as a whole, resembles the adult morphology of the species. Hence the
term: "definitive body form."
3. The late embryonic period
This part of development intervenes between the time when definitive body
form is established and the episode of hatching or birth. In free-living larval
species it comprises a brief period at the end of metamorphosis.
C. Post-embryonic Development
Post-embryonic development may be divided into the following periods:
1 . Prepuberal period
During this time the organ-systems grow and enlarge, and the reproductive
mechanisms mature.
2. Puberal period and the adult
The organism now is capable of reproduction, and in size, activity, and
appearance is recognized as an adult.
3. Period of senescence and decline
The sexual activities lessen and the organ-systems of the body may very
slowly undergo regressive changes.
IV. A Classification of the Vertebrates and Related Species
A. Characteristics of the Phylum Chordata
The vertebrates belong to the phylum Chordata. This phylum is character-
ized by three main features which appear in the early embryo, viz., ( 1 ) a
dor sally situated nerve cord which in most instances is hollow or tube-like;
(2) a dorsally placed notochordal or median skeletal axis located always
immediately ventral to the nerve cord, and (3) a complicated anterior portion
ORIENTATION Xvii
of the digestive tract known as the pharynx. The pharyngeal area of the di-
gestive tract is composed of a series of paired skeletogenous arches known
as the visceral or branchial arches, between which are found the branchial
pouches and branchial furrows or grooves.
B. Major Divisions of the Phylum
The entire phylum Chordata may be divided into the lower chordates and
the higher chordates.
Lower Chordata (Acraniata)
Subphylum: Hemichordata
These are small, soft-bodied animals living along the shores of the sea, and
in some instances to considerable depths into the sea. Dorsal and ventral
nerve cords are present in the class Enteropneusta or the "tongue worms."
The notochord is a short structure confined to the anterior end. Gill slits are
present.
Subphylum: Urochordata (Tunicata)
These forms inhabit the sea from the polar regions to the equator, and
from the shores outward to considerable depths. It is in the larval form that
this group lays most of its claim to a right to be placed among the Chordata,
for the young hatches as a larva which resembles the amphibian tadpole super-
ficially. In this tadpole a dorsal nerve cord is present, and in the tail region
a well-formed notochord as well. Gill slits also are found. Later in life the
larva settles down to a sessile existence and the tail with its notochord is lost.
Examples: Styela partita; Molgula manhattensis; Ciona intestinalis.
Subphylum: Cephalochordata (Lancets)
To this group belong the familiar forms known as Amphioxus. Of all the
lower chordates, the lancets possess characteristics closely resembling the
higher chordate group. A dorsal tubular nerve cord is present, below which
is an elongated notochord, and an extensive pharyngeal area is developed.
The basic plan of the circulatory system resembles that of the vertebrate
group, although many pulsating "hearts" are to be found, one in each of
the numerous blood vessels coursing through the pharyngeal area.
Examples: Branchiostoma virginiae; B. calif orniense; Asymmetron
macricaudatum.
Higher Chordata (Craniata)
Subphylum: Vertebrata
Group I: Agnathostomata
To this group belong the cyclostomes or the vertebrates without jaws. The
cyclostomes include the lampreys (Hyperoartia) and the hagfishes (Hyper-
ORIENTATION
otreta). They are parasitic on other fishes in the adult. The notochord and
its surrounding sheaths serve as the main skeletal axis. True vertebral elements
do not reinforce the notochord, although certain vertebral elements are present
in some species.
Examples: The California hagfish, PoUstotrema (Bdellostoma) stoiiti, and
the common sea lampreys, Petromyzon marinus, Okkelbergia lamotteni, Lam-
pet ra ayresii.
The California hagfish has 12 pairs of gill slits whereas the sea lamprey,
Petromyzon marinus, has 7 pairs.
Group II: Gnathostomata
The Gnathostomata are vertebrates which possess jaws. In a sense, they
are the only true vertebrates in the chordate phylum, for the notochordal axis
always is supplemented or displaced by vertebral elements.
1. Class: Pisces
Division 1 : Chondrichthyes
To this group belong the selachian or elasmobranch fishes. The word chon-
drichthyes means cartilaginous fishes, i.e. the fishes with endoskeletons of
cartilage. The adjective selachian has a similar meaning, whereas the term
elasmobranch means plate-like gill.
The sharks, skates, rays, and chimaeras comprise the numerous species of
cartilaginous fishes. The skin is covered with small placoid scales; median and
paired fins are present; the sexes are separate, and elaborate reproductive ducts
are developed. The heart, exclusive of the sinus venosus, is two chambered.
Examples: Sqiialus acanthias, the dog fish; Rhineodon typus, the whale
shark; Manta birostus, the "great devil ray."
Division 2: Dipnoi
The dipnoan or lungfishes effect external respiration by means of gills and
well-formed lungs. The heart, in harmony with its respiratory mechanisms,
is practically three-chambered. Paired fins have a segmented, cartilaginous,
central axis.
Examples: The African lungfish, Protopterus annectens; the South Ameri-
can lungfish, Lepidosiren paradoxa; and the Australian lungfish, Neoceratodus
forsteri.
Division 3: Teleostomi
In this group, the skeleton, in most species, is bony. A single opening for
the gill-chamber is present -on each side of the pharynx, the gills being cov-
ORIENTATION
ered by an operculum. An air bladder is found in most species. Paired fins
are not supported by a median axis.
Series 1. The Ganoidei. The ganoid fishes, possessing ganoid or cycloid
scales. An air bladder is to be found with an open duct united to the post-
pharyngeal area. A spiral valve is developed in the intestine. There are two
groups of ganoid fishes, viz. the Chondrostei, which possess a cartilaginous
skeleton and dermal bony plates, and the Holostei which have a bony skeleton.
Examples of Chondrostei are Acipenser fulvescens, Scaphirhynchus pla-
torhynchus, and Parascaphirhynchus albiis. Lepisosteus osseus and Amia calva
are representatives of the Holostei.
Series 2. Teleostei. In the bony fishes an air bladder is present but usually
the pneumatic duct connecting the air bladder with the esophagus is rudi-
mentary or absent. A spiral valve is absent in the intestine. The scales are
cycloid or ctenoid, and in some instances are absent altogether.
Examples: Oncorhynchus tschawytscha, the chinook or king salmon, the
most important source of food fish in the country; Salmo salar, the Atlantic
salmon; Trutta irideus, the rainbow trout; Salvelinus fontinalis, the speckled
brook trout, and a host of other genera and species.
2. Class: Amphibia
The amphibians are cold-blooded vertebrates adapted to an existence in a
watery or moist medium. Some species such as Necturus maculosus and the
axolotl, Ambystoma mexicanum, spend their entire life within water, while
others such as the frogs and salamanders are in and out of the water. The
toads, on the other hand, are able to get along under fairly dry conditions.
The skin is soft, moist, and glandular, and, with the exception of the Gymno-
phiona, it is devoid of scales. External respiration is carried on by means of
gills in the larva, but in the adult the lungs and skin are the principal areas
concerned with respiration. However, in those adults which live exclusively
in the water, gills may be retained. Some species do not possess lungs and in
these the skin and lining surfaces of the pharynx accommodate respiratory
functions. In forms such as Necturus and the Axolotl, external gills function
as the principal mechanism of external respiration in the adult. Excluding
the sinus venosus, a three-chambered heart is typical of the group.
Order 1 : Caudata (Urodela)
The salamanders and newts form a large number of amphibian species.
They have an elongate body with a conspicuous tail and the body muscles
tend to retain a segmental condition. Many vertebrae are present.
Examples: Cryptobranchus alleganiemis, Triturus viridescens, Ambystoma
maculatum, Desmognathus fuscus, Plethodon cinereus, Amphiuma means,
Necturus maculatus, Siren lacertina, Triton cristatus, etc.
XX ORIENTATION
Order 2: Anura (Salienta)
The frogs and toads. Short compact body; tail absent in adult; only nine
vertebrae present; ribs ankylosed to vertebrae as short processes; hind legs
long and muscular.
Examples: Ascaphus truei, Scaphiopus holbrookii, Bufo americanus, Rana
pipiens, R. sylvatica, R. catesbiana, Hyla crucijer, Discoglossus pic t us, Xe no-
pus laevis, Pipa pipa, Nectophrynoides vivipara.
Order 3; Gymnophiona
The caecilians are long-bodied, limbless amphibians resembling earthworms.
They are inhabitants of the tropics with the exception of Madagascar. Scales
are present in the dermal layer of the skin.
Examples: Hypogeophis alternans, Scoleconiorphus uluguruensis, Caecilia
tentaculata.
3. Class: Re pt ilia
Scale-covered, cold-blooded, claw-digited vertebrates with a three- or four-
chambered heart, and generally inhabitants of dry land or streams. External
respiration carried on exclusively by means of lungs.
Order 1 : Crocodila
The crocodilians include the alligators and crocodiles. These are large
greatly elongated reptiles covered with scales and bony plates. The eye has
an upper and lower lid and a nictitating membrane. Teeth are thecodont. All
species are oviparous. The anus is a longitudinal opening.
Examples: Alligator mississippiensis and Crocodylus acutus.
Order 2: Lacertilia
The lizards are elongated reptiles of diverse sizes. Teeth are pleurodont or
acrodont. The eye has an upper and lower eyelid and a nictitating membrane.
The tympanum is not at the surface, and the ear opening may be covered by
scales. A vestigial pineal or median eye is often present, and the tongue is
well developed and protusile. Most species are oviparous, a few are ovovi-
parous, and some may be classed as viviparous. The anus is a transverse slit.
Examples: Anolis carolinensis, the chameleon; Sphaerodactylus notatus,
the reef gecko; Phyrynosoma cornutum, the horned toad; Heloderma sus-
pectum, the Gila Monster; the Tuatera of New Zealand, and the dragon lizard
of the Dutch East Indies.
Order 3: Serpentes
Snakes are crawling reptiles who have lost their legs. They form a large
number of reptilian species. Acrodont teeth always are present. Functional
ORIENTATION
eyelids are absent and they lack a tympanum or external ear opening. Some
species are oviparous and others are ovoviviparous.
Examples: Natrix sipedon, the common water snake; Thamnophis radix,
the common garter snake; Crotalus horridus, the common rattler.
Order 4: Testudinata
Turtles possess short, compact bodies encased more or less completely in
a box constructed of bony plates integrated to form a dorsal covering, the
carapace, and a ventral shield, the plastron. The jaws are toothless and cov-
ered by a horny cutting edge. The tympanum is at the surface of the body
and eyelids and nictitating membrane are present. All species are oviparous.
Examples: Sternotherus odoratus, the musk turtle; Chelydra serpentina,
the snapping turtle; Clemmys guttata, the spotted turtle; and Terrapene Caro-
lina, the common box turtle.
4. Class: Aves
Birds are warm-blooded, lung-breathing vertebrates with feathers, without
teeth, and with a horny beak. The body is built for flight and most species fly.
All species are oviparous. Other than the extinct birds or Archaeornithes, all
modern birds may be grouped together under the heading Neornithes. The
Neornithes may be divided into two main groups:
Series 1: Ratitae (running birds)
The flightless running birds such as the recently extinct moas, and present
living forms such as the kiwi, Apteryx; the cassowary, Casuarius sp., and the
ostrich, Struthio sp., belong in this group.
Series 2: Carinatae (flying birds)
This group contains many orders. The following orders are intimately as-
sociated with man:
Anserijormes: Geese, ducks, swans
Gallijormes: The common fowl, turkey, pheasants, guinea hen, etc.
Columbijormes: Doves, pigeons
Passeriformes: Canary and other common song birds
5. Class: Mammalia
The mammals are warm-blooded, lung-breathing vertebrates with a coating
of hair. They produce a nutritive substance for the young which is elaborated
in glandular areas known as the mammae or breasts.
Division 1 : Prototheria
These are highly specialized egg-laying mammals found only in Australia,
Tasmania, and New Guinea. The spiny anteater, Echidna aculeata, is found
ORIENTATION
in all of these localities and the Platypus or Ornithorynchus paradoxus, is
an inhabitant of Australia. The urogenital ducts and intestine open posteriorly
into a common chamber, the cloaca.
Division 2: Theria or true mammals
The Theria bring forth their young alive, possess true mammary glands with
nipples, and all produce a small egg with little stored food material. They also
possess separate openings to the exterior for the urogenital ducts and the in-
testine, a cloaca being absent in the adult condition.
Series 1 : Metatheria. These are the marsupial or pouched mammals such
as the Virginia opossum, Didelphys virginiana.
Series 2: Eutheria. The following orders are given:
Subseries 1. Unguiculata or mammals with claws
Order 1. Insectivora or insect-eating mammals
Examples: Moles and shrews
Order 2. Chiroptera or flying mammals
Example: The bats
Order 3. Carnivora or flesh-eating mammals
Examples: Wolves, dogs, foxes, raccoons, otters, skunks, weasels,
mink, hyenas, cats, lions, tigers
Order 4: Rodentia or gnawing mammals
Examples: Rats, mice, rabbits, hares, guinea pigs, squirrels, bea-
vers, gophers (ground squirrels), prairie dogs
Order 5. Edentata or mammals without teeth or with reduced con-
dition of the teeth
Examples: Armadillos, three-toed sloths, anteaters
Order 6. Pinnipedia or mammals with bilateral appendages adapted
for swimming
Examples: Seals, sea-lions, walruses
Subseries 2. Ungulata or mammals with hoofs
Order 7. Artiodactyla or even-toed mammals
Examples: Hippopotami, peccaries, swine, deer, moose, elk,
pronghorn antelope, cows, sheep, goats, camels, giraffe,
llamas, antelopes, gazelles
Order 8. Perissodactyla or odd-toed mammals
Examples: Horses, zebras, asses, tapirs, rhinoceroses
Order 9. Sirenia or mammals with hind limbs absent and adapted
to living in the water
Example: The manatees or sea cows
Order 10. Proboscidea
Examples: The elephants
ORIENTATION XXIU
Subseries 3. Cetacea or marine mammals
Order 1 1 . Odontoceti or toothed whales
Examples: Porpoises, sperm whales, killer whales, narwhals
Order 12. Mystacoceti or whalebone whales
Examples: Sulphur-bottom whales, right whales, finback whales
Subseries 4. Primates or mammals with flattened, distal modifications
of the digits known as nails
Order 13. Primates
Examples: Man, monkeys, lemurs, apes
PART I
Tne Period or Preparation
The events which precede the initiation of the new individual's development are:
(1) The preparation of the male and female parents and their reproductive structures
for the act of reproduction (Chaps. 1 and 2).
(2) The preparation of the gametes (Chap. 3).
The anterior lobe of the pituitary gland, because of its secretion of the gonadotrophic
(gonad-stimulating) hormones, is the pivotal structure in the reproductive mechanism.
The gonadotrophic hormones are:
(1) Follicle-stimulating hormone, FSH;
(2) Luteinizing hormone, LH (ICSH), and
(3) Luteotrophin, LTH.
Tne Testis and Its Relation to Reprod-uction
A. Introduction
1. General description of the male reproductive system
2. Importance of the testis
B. Anatomical features of the male reproductive system.
1. Anatomical location of the testis
2. Possible factors involved in testis descent
3. General structure of the scrotum and the testis in mammals
a. Structure of the scrotum
b. General structure of the testis
4. Specific structures of the mammalian testis which produce the reproductive cells
and the male sex hormone
a. Seminiferous tubules
b. Interstitial tissue
5. The testis of vertebrates in general
6. Accessory reproductive structures of the male
a. The reproductive duct in forms utilizing external fertilization
b. The reproductive duct in species practicing internal fertilization
C. Specific activities of the various parts of the male reproductive system
1. Introduction
a. Three general functions of the male reproductive system
b. Some definitions
2. Activities of the testis
a. Seasonal and non-seasonal types of testicular activity
b. Testicular tissue concerned with male sex-hormone production
c. Testicular control of body structure and function by the male sex hormone
1 ) Sources of the male sex hormone
2) Biological effects of the male sex hormone
a) Effects upon the accessory reproductive structures
b) Effects upon secondary sex characteristics and behavior of the individual
c) Effects upon the seminiferous tubules
d. Seminiferous-tubule activity and formation of sperm
e. The seminiferous tubule as a sperm-storing structure
3. Role of the reproductive duct in sperm formation
a. Vertebrates without a highly tortuous epididymal portion of the reproductive
duct
b. The epididymis as a sperm-ripening structure
c. The epididymis and vas deferens as sperm-storage organs
d. Two types of vertebrate testes relative to sperm formation
4 THE TESTIS AND ITS RELATION TO REPRODUCTION
4. Function of the seminal vesicles (vesicular glands)
5. Function of the prostate gland
6. Bulbourethral (Cowper's) glands
7. Functions of seminal fluid
a. Amount of seminal fluid discharged and its general functions
b. Coagulation of the semen
c. Hyaluronidase
d. Accessory sperm
e. Fructose
f. Enzyme-protecting substances
D. Internal and external factors influencing activities of the testis
1. Internal factors
a. Temperature and anatomical position of the testis
b. Body nourishment in relation to testicular function
c. The hypophysis and its relation to testicular function
2. External environmental factors and testis function
a. Light as a factor
b. Temperature influences
E. Internal factors which may control seasonal and continuous types of testicular
function
F. Characteristics of the male reproductive cycle and its relation to reproductive con-
ditions in the female
A. Introduction
1. General Description of the Male Reproductive System
The male reproductive system of most vertebrate animals consists of two
testis with a sperm-conveying duct and attendant auxiliary glands associated
with each testis. In some species, such as the frog and many teleost fishes, the
sperm-conveying duct is a simple structure, but in most vertebrate forms there
is a tendency for the duct to be complicated. The cyclostomatous fishes do not
possess sperm-conveying ducts from the testis to the outside.
In reptiles, some birds and all mammals, in gymnophionan amphibia and
in the "tailed" frog, Ascaphus, in sharks and certain teleost fishes, an intro-
mittent organ is added to the sperm-conveying structures for the purpose of
internal fertilization. But an intromittent organ is not present in all species
which practice internal fertilization. In many salamanders, internal fertiliza-
tion is effected by the spawning of a spermatophore filled with sperm; the
latter is picked up by the cloaca of the female. The sperm in these salamanders
are stored in special pockets or tubules within the dorsal wall of the cloaca.
These storage tubules form the spermatheca (fig. 10). Direct transfer of sperm
to the female by cloacal contact may occur in some species.
2. Importance of the Testis
The word testis or testicle was formerly applied to the ovary of the female,
as well as to the male sperm-producing organ, and the term "female testicle"
ANATOMICAL FEATURES
was used in reference to the female organ. The use of the word "ovary" was
introduced by Steno in 1667, and also by de Graaf (fig. 1) in 1672 in his
work on the female generative organs. To quote from de Graaf: "Thus, the
general function of the female testicles is to generate the ova, to nourish
them, and to bring them to maturity, so that they serve the same purpose
in women as the ovaries of birds. Hence, they should rather be called ovaries
than testes because they show no similarity, either in form or contents, with
the male testes properly so called." (See Corner, '43.) From the time of
de Graaf the word "testis" has been restricted to designate the male organ
essential to reproduction.
The phrase "essential to reproduction" does not describe fully the impor-
tance of testicular function. As we shall see later on, the testis not only assumes
the major role in the male's activities during the period of reproduction, but
also, in the interim between specific reproductive periods, it governs in many
instances male behavior leading to protection and preservation of the species.
Thus, the testis is the organ responsible for maleness in its broader, more
vigorous sense.
B. Anatomical Features of the Male Reproductive System
Before endeavoring to understand the general functions of the testis in re-
lation to reproduction, it is best to review some of the structural relationships
of the testis in the vertebrate group.
Fig. 1. Reinier de Graaf. Born in Holland, 1641; died in Delft, Holland. 1673. Author
of important works on the generative organs of the female. Described the Graafian fol-
licle in the ovary of mammals but erroneously believed it to be the mammalian egg. (From
Corner, '43.)
6 THE TESTIS AND ITS RELATION TO REPRODUCTION
1. Anatomical Location of the Testis
In most vertebrates other than mammals, the testes are suspended well
forward within the peritoneal cavity. In the Mammalia, however, the con-
dition is variable. In the monotrematous mammals, Echidna and Ornithoryn-
chus, the testes are located within the peritoneal cavity near the kidneys. In
the elephant the testes also are located in this area. Schulte ('37) describes
the position of the testes in an Indian elephant (Elephas indicus), 20 years
old, as being "retroperitoneal lying on each side medial to the lower pole of
the kidney." (The kidneys were found to lie retroperitoneally on either side of
the lower thoracic and lumbar vertebrae, and each measured about 275 mm.
in length.) However, in the majority of mammals the testes descend pos-
teriad from the original embryonic site, the extent varying with the species.
In some there is a slight posterior migration, and the testes of the adult are
situated well forward in the pelvic region. Examples of this condition are
found in conies, whales, sea cows, African jumping shrews, and in arma-
dillos. In sloths and American anteaters, the testes may descend into the
pelvic cavity and lie in the area between the urinary bladder and the posterior
body wall. However, in most of the eutherian and marsupial mammals, a
dual outpushing of the postero-ventral body wall occurs into which the testes
come to lie either permanently, or, in some forms, temporarily during the
breeding season. This outward extension of the body-wall tissues is known
as the scrotum; it involves not only the skin, muscle and connective tissues
of the body wall but the peritoneal lining as well (fig. 2). (The interested
student may consult Weber ('28) and Wislocki ('33) for data concerning
the extent of testis descent in mammals.)
The peritoneal evaginations into the scrotal sac are two in number, one
for each testis; each evagination is known as a processus vaginalis (figs. 3E,
F; 4A, B). In many mammals this evagination becomes separated entirely
from the peritoneal cavity, and the testis, together with a portion of the
sperm-conveying duct, lies suspended permanently in a small antechamber
known as the inguinal bursa or serous cavity of the scrotum (fig. 4B). (See
Mitchell, '39.) This condition is found in the horse, man, opossum, bull,
ram, dog, cat, etc. In certain other mammals, such as the rat, guinea pig, and
ground hog, the inguinal bursa does not become separated from the main
peritoneal cavity, and a persistent inguinal canal remains to connect the in-
guinal bursa with the peritoneal cavity (fig. AC). In some rodents the testes
pass through this persisting inguinal canal into the scrotum as the breeding
season approaches, to be withdrawn again after the breeding period is termi-
nated. The ground squirrel, Citellus tridecemlineatus (Wells, '35) and the
ground hog, Marmota monax (Rasmussen, '17) are examples of mammals
which experience a seasonal descent of the testis.
In the majority of those mammals possessing a scrotum, it is a permanent
structure. In a few, however, it is a temporary affair associated with the
ANATOMICAL FEATURES
RECTUM
URINARY BLADDER
AMPULLARY PORTION
OF VAS DEFERENS
SEMINAL VESICLE
EJACULATORY DUCT
PUBIC BONE
PROS TATE GLAND
VAS DEFERENS
SPERMATIC BLOOD
AND LYMPH VESSELS
EPIDIDYMIS
TESTIS
INGUINAL BURSA
SCROTUM
Fig. 2. Sketch of male reproductive system in man.
breeding season, as in the bat, Myotis, where the testes pass into a temporary
perineal pouch or outpushing of the posterior abdominal wall during the
reproductive season, to be withdrawn again together with the scrotal wall
when the breeding period is past (fig. 4D). A similar periodic behavior is
true of many insectivores, such as the common shrews, the moles, and the
European hedgehog (Marshall, '11).
The permanent scrotum is a pendent structure, in some species more so
than others. In the bull and ram, it extends from the body for a considerable
distance, whereas in the cat, hippopotamus, tapir, guinea pig, etc., it is closely
applied to the integumentary wall. In primates, including man, in most carni-
vores, and many marsupials, the pendency of the scrotum is intermediate
between the extremes mentioned above.
An exceptional anatomical position of the testes in the lower vertebrates
is found in the flatfishes, such as the sole and flounder, where they lie in a
caudal outpouching of the peritoneal cavity (fig. 5). The testis on either side
may even lie within a special compartment in the tail. (The ovaries assume
the latter position in the female.)
ANATOMICAL FEATURES
SUPRARENAL
Fig. 3. Diagrammatic representations of the urogenital structures in the developing
male pig, with special emphasis upon testicular descent. (A) Early relationship of the
genital fold (genital ridge), mesonephric kidney and its duct, together with the meta-
nephric kidney and the ureter in 20-mm. pig embryo. The relationship of the mesonephric
and metanephric ducts to the urogenital sinus is shown. The MiJllerian duct is omitted.
(B) Male pig embryo about 45-mm., crown-rump length, showing relationship of gonad
and metanephric kidney. The metanephric kidney is shown below (dorsal to) the meso-
nephric kidney. The gonad (testis) is now a well-defined unit. The portion of the genital
fold tissue anterior to the testis becomes the anterior suspensory ligament of the testis,
while the genital fold tissue caudal to the testis continues back to join the inguinal liga-
ment of the mesonephros (the future gubernaculum). (C) About 80-mm., crown-rump,
pig embryo. Observe that the metanephros is now the dominant urinary organ and has
grown cephalad, displacing the mesonephric kidney which is regressing and moving
caudally with the testis. The remains of the mesonephric kidney at this time are gradually
being transformed into epididymal structures. (D) About 130-mm.. crown-rump, pig
embryo. Observe that the test-is is approaching the internal opening of the inguinal canal.
The anterior suspensory ligament is now an elongated structure extending over the latero-
ventral aspect of the metanephric kidney; the gubernacular tissue is shown extending
downward into the inguinal canal. (E) Later stage in testicular descent. The anterior
suspensory ligament of the testis is a prominent structure, while the gubernaculum is
compact and shortened. (F) The condition found in the full-term, fetal pig. The testis
is situated in the scrotal swelling; the gubernaculum is much shortened, while the anterior
suspensory ligament remains as a prominent structure, extending cephalad to the caudal
portions of the metanephric kidney.
2. Possible Factors Involved in Testis Descent
The descent of the testis within the peritoneal cavity and into the scrotum
poses an interesting problem. In embryonic development extensive migration
of cell substance, or of cells, tissues, and organ structures is one of many
processes by which the embryonic body is formed. That is to say, the dynamic
movement or displacement of developing body structures from their original
position is a part of the pattern of development itself. The casual factors in-
PROCESSUS
VAGINALIS
PERITONEAL
CAVITr
f^ OBLITERATED
INGUINAL
CANAL
TESTIS
GUBERNACULUM
SEROUS CAVITY
. (INGUINAL BURSA)
WALL TISSUE
(EE FIG S
VAS DEFERENS
EPI 01 DYMIS
TESTIS
TESTICULAR
LIGAMENT
PATENT INGUINAL CANAL
SCROTAL BULGE
Fig. 4. Diagrammatic drawings portraying the relationship of the testis to the processus
vaginalis (peritoneal evagination) and the scrotum. The testis is at all times retroperi-
toneal, i.e., outside the peritoneal cavity and membrane. (A) Earlier stage of testicular
descent at the time the testis is moving downward into the scrotum. (B) Position of
the testis at the end of its scrotal journey in a form possessing permanent descent of the
testis, e.g., man, dog, etc. (C) Testis-peritoneal relationship in a form which does not
have a permanent descent of the testis — the testis is withdrawn into the peritoneal cavity
at the termination of each breeding season. Shortly before the onset of the breeding
period or "rut," the testis once again descends into the scrotum, e.g., ground hog. (D)
Position of testis in relation to body wall and peritoneum in the mole, shrew, and hedge-
hog in which there is no true scrotum. The testis bulges outward, pushing the body wall
before it during the breeding season. As the testis shrinks following the season of rut,
the bulge in the body wall recedes. True also of bat, Myotis.
STOMA CH
PECTORAL FIN
MEMAL PUBIC PROCESS
TAIL MUSCLE
LATERAL LINE
ANUS
UROGENITAL OPENING / / \ "TAIL COELOM
ABDOMINAL COELOM / TESTIS
DUCTUS DEFERENS
CONTAINING TESTIS
Fig. 5. Opened peritoneal cavity of a common flounder, Limanda ferruginea, showing
the position occupied by the testes. Each testis is situated partly in a separate compart-
ment on either side of the hemal processes of the tail vertebrae.
10
ANATOMICAL FEATURES 11
volved in such movements are still unknown, and the study of such behavior
forms one of the many interesting aspects of embryological investigation
awaiting solution.
Various theoretical explanations have been proposed, however, to explain
the movement of the testis posteriad from its original embryonic site. Classical
theory mentions the mechanical pull or tightening stress of the gubernaculum,
a structure which develops in relation to the primitive genital fold or genital
ridge (figs. 3B, C; 351C-7).
The genital ridge extends along the mesial aspect of the early developing
mesonephric kidney from a point just caudal to the heart to the posterior
extremity of the mesonephric kidney near the developing cloacal structures
(Hill, '07). Anteriorly, the genital ridge (fold) merges with the diaphragmatic
ligament of the mesonephros (fig. 3A). The gonad (testis or ovary) develops
in a specialized region of the more cephalic portion of the genital ridge
(Allen, '04). (See fig. 3A.) The caudal end of the mesonephric kidney even-
tually becomes attached to the posterior ventral body wall by means of a
secondary formation of another cord of tissue, the inguinal fold (fig. 3A).
The latter is attached to the posterior ventral body wall near the area where
the scrotal outpushing (evagination) later occurs. This inguinal fold later
becomes continuous with the genital fold (fig. 3B). The inguinal fold thus
becomes converted into a ligament, the inguinal ligament of the mesonephros,
uniting the caudal portion of the mesonephric kidney and adjacent genital
fold tissue with the area of scrotal evagination (fig. 3B). The gubernaculum
represents a later musculo-connective tissue development of the inguinal liga-
ment and the adjacent genital fold tissue. It contains smooth muscle fibers
as well as connective tissue. As the scrotal evagination forms at the point
where the gubernaculum attaches to the body wall, the gubernaculum from
the beginning of its formation is connected with the developing scrotal sac.
As the testis migrates posteriad, the anterior suspensory ligament of the
testis elongates and the gubernaculum shortens (fig. 3A-F). This decrease
in length of the gubernaculum is both real and relative. It is real in that an
actual shortening occurs; it is relative because the rapid enlargement of the
developing pelvic cavity and its contained organs makes the length of the
gubernaculum appear less extensive. This enlargement of the pelvic space
and increase in size of its contained structures and a corresponding failure
of the gubernaculum to elongate, certainly are factors in bringing about the
intra-abdominal descent of the testis; that is, testis descent within the peritoneal
cavity itself (Felix, '12).
Developmental preparations precede the extra-abdominal descent of the
testes, for the scrotal chambers must be prepared in advance of the arrival
of the testes. These developmental events are:
( I ) two outpocketings of the abdominal wall which come to lie side by
side below the skin to form the walls of the scrotal chamber, and
12 THE TESTIS AND ITS RELATION TO REPRODUCTION
(2) an evagination of the peritoneum into each of the abdominal out-
pocketings which act as peritoneal linings for each pocket.
It is worthy of mention that the above outpushings of the abdominal wall
and of the peritoneum precede the movement of the testes into the scrotum.
They serve to illustrate the theory that a shortening of the gubernaculum is
not sufficient to explain testis descent. Rather, that in this descent a whole
series of developmental transformations are involved; the shortening of the
gubernaculum and scrotal development merely represent isolated phases of
the general pattern of movement and growth associated with this descent.
More recent research emphasizes the importance of certain physiological
factors relative to the descent problem. It has been determined, for example,
that administration of the gonadotrophic hormone of pregnancy urine (cho-
rionic gonadotrophin) or of the male sex hormone, testosterone, aid the
process of extra-abdominal descent (i.e., descent from the inguinal ring area
downward into the scrotum). Hormone therapy, using chorionic gonadotrophin
together with surgery, is used most often in human cryptorchid conditions.
The androgen, testosterone, aids testicular descent mainly by stimulating the
growth of the scrotal tissues and the vas deferens; however, it is not too
successful in effecting the actual descent of the testis (Robson, '40; Wells,
'43; Pincus and Thimann, '50).
The phenomenon of testicular migration thus is an unsolved problem.
Many activities and factors probably play a part in ushering the testis along
the pathway to its scrotal residence.
3. General Structure of the Scrotum and the Testis in Mammals
a. Structure of the Scrotum
The scrotal modification of the body wall generally occurs in the postero-
ventral area between the anus and the penial organ. However, in marsupials
it is found some distance anterior to the latter.
Each scrotal evagination consists of three general parts: the skin with
certain attendant muscles, the structures of the body wall below the skin,
and the peritoneal evagination. The skin, with its underlying tunica dartos
muscle tissue and superficial perineal fascia, forms the outer wall of the
scrotum (fig. 6). Within this outer cutaneous covering lie the two body-wall
and two peritoneal evaginations. The body-wall evaginations involve con-
nective and muscle tissues of the external oblique, internal oblique, and
transversus muscles. The caudal part of each peritoneal outpocketing forms
the serous cavity or inguinal bursa in which the testis is suspended after its
descent, and its more anterior portion forms the inguinal canal (figs. 2,
4B, 6). The oblique and transversus layers of tissues thus are molded into
a musculo-connective tissue compartment around each serous cavity. The
median septum of tfie scrotum represents the area of partial fusion between
ANATOMICAL FEATURES 13
the two musculo-connective tissue compartments, whereas the median raphe
of the scrotum denotes the area of fusion of the two cutaneous coverings of
the body-wall outpushings (fig. 6).
Consequently, passing inward from the superficial perineal fascia of the
skin or outer wall, one finds the following tissue layers surrounding the testis:
( 1 ) The external spermatic fascia represents the modified fascia of the ex-
ternal oblique muscle layer of the embryo.
(2) The middle spermatic fascia is a modification of the internal oblique
muscular layer, whose tissue forms the cremaster muscle loops within
the scrotum (fig. 6). (Some of the cremasteric musculature may be
derived from the transversus layer.)
( 3 ) The internal spermatic fascia or tunica vaginalis communis is derived
from the transverse muscle layer of the embryo.
(4) Along the inner surface of the tunica vaginalis communis is the peri-
toneal membrane. The latter is reflected back over the surface of the
suspended testis, and thus forms the visceral peritoneal covering of
the testis. This lining tissue of the common vaginal tunic and the
peritoneal membrane which covers the testis are derived from the
original peritoneal evagination into the scrotal pocket; as such it
forms the tunica vaginalis propria.
b. General Structure of the Testis
The testis is composed of the following structural parts:
( 1 ) The inner layer of the tunica vaginalis propria, the tunica vaginalis
internus, envelops the testis. The cavity between the outer and inner
layers of the tunica vaginalis propria is the inguinal bursa. Oblitera-
tion by injury or infection of this inguinal bursa may cause degen-
erative changes in the testis. In other words, the testis normally must
be free to move within its serous (peritoneal) cavity.
(2) Within the tunica vaginalis internus of the testis is a thick fibrous
layer of connective tissue, the tunica albuginea (fig. 7). From this
tunic, connective tissue partitions, the septula of the testis, extend
inward and converge toward that testicular zone where supplying
blood vessels enter and leave, including the lymphatics. The latter
zone is known as the mediastinum testis and it represents a regional
thickening of the tunica albuginea. Here the connective tissue fibers
form a latticework which acts as a framework for the larger blood
and lymph vessels and efferent ducts of the testis. The testis is attached
to the scrotal wall in the mediastinal area.
(3) The spaces between the various septula partitions form the septula
compartments. In the human testis there are about 250 septula com-
partments, each containing a lobule of the testis. The lobuli testis
SPERMATIC CORD
DUCTUS DEFERENS
NICA VAGINALIS PROPRIA
CAVITY OF TUNICA VAGINALIS
( SEROU S BURSA )
TUNICA VAGINALIS PROPRIA
TUNICA VAGINALIS COMMUNIS
MIDDLE SPERMATIC FASCIA ,
(CREMASTER MUSCLE)
EXTERNAL SPERMATIC FASCIA
PERFICIAL PERINEAL fascia]
ICA DARTOS
PERM
DERI
ONEAL
ATIVES
USCLE LAYER
DERIVATIVE S
DERIVATIVES
Fig. 6. Schematic drawing of the testis and its relationship within the scrotum. On the
right side of the drawing the muscle and connective-tissue layers surrounding the inguinal
bursa and testis are shown; on the left side may be seen the loops of the cremaster muscle
surrounding the tunica vaginalis communis.
CAPUT EPIDIDYMIDIS
LOBULI TES
CONTAINING
CONVOLUTED
PORTIONS OF
SEMINIFEROUS
TUBULES
TUNICA
SEPT
Fig. 7. Diagrammatic representation of the general structural relationship of the parts of
the human testis. (Modified from Corner, 1943. after Spalteholz and Huber.)
14
ANATOMICAL FEATURES 15
contain the convoluted portions of the seminiferous tubules. From
one to three seminiferous tubules are found in each lobule; they
may anastomose at their distal ends. The combined length of all the
seminiferous tubules approaches 250 meters in the human. The con-
voluted portions of the seminiferous tubules empty into the straight
tubules (tubuli recti) and these in turn unite with the rete tubules
located within the substance of the mediastinum. Connecting with the
rete tubules of the testis, there are, in man, from 12 to 14 ductuli
efferentes (efferent ductules of the epididymis) of about 4 to 6 cm.
in length which emerge from the mediastinum and pass outward to
unite with the duct of the epididymis. The epididymal duct represents
the proximal portion of the reproductive duct which conveys the male
gametes to the exterior.
4. Specific Structures of the Mammalian Testis Which Produce
THE Reproductive Cells and the Male Sex Hormone
Two very essential processes involved in reproduction are the formation
of the sex cells or gametes and the elaboration of certain humoral substances,
known as sex hormones. Therefore, consideration will be given next to those
portions of the testis which produce the sperm cells and the male sex hormone,
namely, the seminiferous tubules and the interstitial tissue.
a. Seminiferous Tubules
The seminiferous tubules lie in the septula compartments (fig. 7). The
word seminiferous is derived from two Latin words: semen, denoting seed,
and ferre, which means to bear or to carry. The seminiferous tubule, therefore,
is a male "seed-bearing" structure. Within this tubule the male gametes or
sperm are formed, at least morphologically. However, the word semen has
a broader implication in that it is used generally to denote the entire repro-
ductive fluid or seminal fluid. The seminal fluid is a composite of substances
contributed by the seminiferous tubules and various parts of the accessory
reproductive tract.
The exact form and relationship of the various seminiferous tubules (tubuli
seminiferi) which occupy each testicular compartment have been the object
of much study. It is a generally accepted belief at present that the tubules
within each testicular lobule are attached at their distal ends; that is, that
they anastomose (fig. 7). Some investigators also believe that there may be
other anastomoses along the lengths of these very much contorted and twisted
structures. Moreover, it appears that the septula or testicular compartmental
partitions are not always complete; the seminiferous tubules of one lobule
thus have the opportunity to communicate with those of adjacent lobules.
The seminiferous tubules of any one lobule join at their proximal ends and
empty into a single straight seminiferous tubule. The straight tubules or
16
THE TESTIS AND ITS RELATION TO REPRODUCTION
tubuli recti pass into the mediastinum and join the anastomosing rete tubules
of the rete testis.
The convoluted portions of the seminiferous tubules produce the sperm
(spermia; spermatozoa). In the human testis, the length of one of these
tubules is about 30 to 70 cm. and approximately 150 /i, to 250 fJ^ in diameter.
Each tubule is circumscribed by a basement membrane of connective tissue
and contains two cell types:
( 1 ) supporting or Sertoli cells, and
(2) spermatogenic cells or spermatogonia (see fig. 8 and Chap. 3).
The cells of Sertoli are relatively long, slender elements placed perpen-
dicularly to the basement membrane to which they firmly adhere. These
cells may undergo considerable change in shape, and some observers believe
that they may form a syncytium, known as the "Sertolian syncytium." Others
believe them to be distinct elements. It is said that Sertoli cells may round
up and form phagocytes which become free from the basement membrane
and move, ameba-like, in the lumen of the seminiferous tubule, phagocytizing
degenerating sperm cells. However, their main function appears to be asso-
ciated with the development of sperm during the period when the latter
undergo their transformation from the spermatid condition into the adult
SERTOLI CELL
SEMINIFEROUS TUBULE
CELLS OF L E YD IG
BASEMENT MEMBRANE
CAPILLARY
NTERSTITIAL TISSUE
SPERMATOGONIUM
Fig. 8. Semidiagrammatic representation of section of cat testis, showing seminiferous
tubules and interstitial tissue, particularly the cells of Leydig.
ANATOMICAL FEATURES 17
sperm form. The Sertolian cells thus may act as nursing elements during
sperm metamorphosis.
The spermatogenic cells or spermatogonia (germinal epithelium of the
tubule) lie toward the outer portion of the seminiferous tubule between the
various Sertoli elements. As a rule spermatogonia lie apposed against the base-
ment membrane of the tubule (see fig. 8 and Chap. 3).
b. Interstitial Tissue
The interstitial tissue of the testis is situated between the seminiferous
tubules (fig. 8). It consists of a layer of connective tissue applied to the
basement membrane of the seminiferous tubule and of many other structures,
such as small blood and lymph vessels, connective tissue fibers, connective
tissue cells, mast cells, fixed macrophages, etc. The conspicuous elements of
this tissue are the so-called interstitial cells or cells of Leydig (fig. 8). In
man, cat, dog, etc., the cells of Leydig are relatively large, polyhedral ele-
ments, possessing a granular cytoplasm and a large nucleus.
5. The Testis of Vertebrates in General
In the vertebrate group, the testis shows marked variations in shape and
size. In many fishes, the testes are irregular, lobular structures, but in other
fishes, amphibia, reptiles, birds, and mammals, they assume an ovoid shape.
The size of the testis is extremely variable, even in the same species. The
testis of the human adult approximates 4 to 5 cm. in length by 3 cm. wide
and weighs about 14 to 19 Gm. The testis of the horse averages 11 cm.
long by 7 cm. wide with a weight of 30 to 35 Gm., while that of the cat is
1.6 cm. long and 1.1 cm. wide with a weight of 1.5 Gm. In the mud puppy,
Necturus, the testis is approximately 3.5 cm. long and 0.8 cm. wide with a
weight of 0.3 Gm. The testis of the large bullfrog is 1.2 cm. by 0.5 cm. with
a weight of 0.8 Gm. In comparison to the foregoing, Schulte ('37) gives the
weight of each testis of an Indian elephant as two kilograms!
Regardless of size or shape, the presence of seminiferous tubules and inter-
stitial tissue may be observed in all vertebrate testes. In some species the
seminiferous tubule is long; in others it is a short, blunt affair. The interstitial
cells may be similar to those described above, or they may be small, incon-
spicuous oval elements.
6. Accessory Reproductive Structures of the Male
a. The Reproductive Duct in Forms Utilizing External Fertilization
The accessory reproductive organs of the vertebrate male are extremely
variable in the group as a whole. A relatively simple reproductive duct (or
in some no duct at all) is the rule for those forms where fertilization is
effected in the external medium. In cyclostome fishes, for example, the repro-
ductive cells are shed into the peritoneal cavity and pass posteriad to emerge
18 THE TESTIS AND ITS RELATION TO REPRODUCTION
externally by means of two abdominal pores. Each pore empties into the
urogenital sinus. In teleost fishes (perch, flounder, etc.) the conveying repro-
ductive duct is a short, simple tube continuous with the testis at its caudal
end and passing posteriorly to the urogenital sinus (fig. 9A). In frogs and
toads, as well as in certain other fishes, such as Amia and Polypterus, the
male reproductive duct is a simple, elongated tube associated with the testis
by means of the eff'erent ductules of the latter, coursing posteriad to open
into the cloaca (frogs and toads) or to the urogenital sinus (Amia; Polypterus)
(fig. 9B, C). Simplicity of sperm duct development and external union of
the gametes are associated reproductive phenomena in the vertebrate group.
b. The Reproductive Duct in Species Practicing Internal Fertilization
An entirely different, more complex male reproductive duct is found (with
some exceptions) in those vertebrates where gametic union occurs within
the protective structures of the maternal body. Under these circumstances
there may be a tendency for one male to serve several females. Enlargement
of the duct with the elaboration of glandular appendages, and structures or
areas for sperm storage is the rule under these conditions (fig. 9D-F). This
form of the male genital tract is found not only in those species where an
intromittent organ deposits the sperm within the female tract, but also where
the sperm are deposited externally in the form of spermatophores (fig. 10).
In many species, the reproductive duct is greatly lengthened and becomes
a tortuous affair, especially at its anterior or testicular end. In fact, the cephalic
end of the duct may be twisted and increased to a length many times longer
than the male body itself. This coiled, cephalic portion is called the duct of
the epididymis (epididymides, plural). (See figs. 7, 9E.) The word epididymis
is derived from two Greek words: epi = upon, and didymis = testicle. The
epididymis, therefore, is the body composed of the tortuous epididymal duct
and the efferent ducts of the testis which lie upon or are closely associated
with the testis. The complex type of reproductive duct is composed thus of
two main portions, an anterior, contorted or twisted portion, the epididymal
duct, and a less contorted posterior part, the vas deferens or sperm duct
proper (fig. 9D, E).
In some vertebrates, in addition to the above complications, the caudal
end of the reproductive duct has a pronounced swelling or diverticulum, the
seminal vesicle (e.g., certain sharks and certain birds). The latter structures
are true seminal vesicles in that they store sperm during the reproductive
period.
The epididymal duct in man is a complex, coiled canal composed of a head
(caput), a body (corpus), and a tail (cauda). (See fig. 7.) It is C-shaped
with its concavity fitting around the dorsal border of the testis, the head
portion being located at the anterior end of the latter. The total length of
the epididymal duct in man is said to be about 4 to 7 m. In other mammals
ANATOMICAL FEATURES
19
EPIDIDYMIS
VAS DEFERENS
PI D I 0 Y M I S
-^ K 1 ON E
Y
U RETE n
J-DEFERENT
DUCT
Fig. 9. Various vertebrate testes and reproductive ducts, emphasizing the relative sim-
plicity of the duct where external fertilization is the rule while complexity of the duct is
present when internal fertilization is utilized. There are exceptions to this rule, however.
(A) Flounder (Limanda ferruginea). (B) Frog (Rana catesbiana). (C) Urodele
(Cryptobranchiis alleganiensis). (D) Dog shark (Squalus acanthias). (E) Urodele
(Nee turns maculosus). (F) Rooster (Gall us doinesticus).
the epididymal duct may be much longer. For example, in the ram, from 40
to 60 m.; in the boar, 62 to 64 m.; in the stallion, 72 to 86 m. (Asdell, '46).
At its caudal end it becomes much less tortuous and gradually passes into
the vas deferens (ductus deferens).
The ductus deferens has a length of about 30 to 35 cm. in man. Leaving
the scrotum it passes anteriad together with accompanying nerves and blood
vessels in the subcutaneous tissue over the front of the pelvic bone into the
peritoneal cavity through the inguinal ring (fig. 2). Here it separates from
the other constituents of the spermatic cord (i.e., it separates from the nerves
20 THE TESTIS AND ITS RELATION TO REPRODUCTION
and blood vessels) and passes close to the dorsal aspect of the bladder and
dorsally to the ureter. It then turns posteriad along the dorsal aspect of the
neck of the bladder and the medial region of the ureter, and accompanied
by its fellow duct from the other side, it travels toward the prostate gland
and the urethra. Just before it enters into the substance of the prostate, it
receives the duct of the seminal vesicle. The segment of the vas deferens
from the ureter to the seminal vesicle is considerably enlarged and is called
the ampulla. After receiving the duct of the seminal vesicle, the vas deferens
becomes straightened and highly muscularized — as such it is known as the
ejaculatory duct. The latter pierces the prostate gland located at the caudal
end of the bladder and enters the prostatic portion of the urethra; from this
point the urethra conveys the genital products.
The auxiliary glands associated with the genital ducts of the human male
consist of the seminal vesicles, the prostate gland, Cowper's glands, and the
glands of Littre.
The seminal vesicles are hollow, somewhat tortuous bodies (fig. 2). Each
vesicle arises in the embryo as an outpushing (evagination) of the vas deferens.
The prostate gland has numerous excretory ducts which empty into the urethra.
It represents a modification of the lining tissue of the urethra near the urinary
bladder together with surrounding muscle and connective tissues. Cowper's
(bulbourethral) glands are small pea-shaped structures placed at the base of
the penial organ; their ducts empty into the urethra. The glands of Littre are
small, glandular outgrowths along the urethra and are closely associated with it.
To summarize the matter relative to the structural conditions of the repro-
ductive duct in the male of those species which practice internal fertilization:
( 1 ) A lengthening and twisting of the duct occurs.
(2) A sperm-storage structure is present, either as a specialized portion
of the duct or as a sac-like extension.
(3) Certain auxiliary glands may be present. These glands are sometimes
large and vesicular structures, such as the seminal vesicles of the
human duct, or they may be small glands distributed along the wall
of the duct, such as the glands of Littre.
C. Specific Activities of the Various Parts of the Male Reproductive System
1. Introduction
a. Three General Functions of the Male Reproductive System
The activities of the testes and the accessory parts of the male reproductive
system result in the performance of three general functions as follows:
(1 ) formation of the semen,
(2) delivery of the semen to the proper place where the sperm may be
utilized in the process of fertilization, and
(3) elaboration of the male sex hormone.
ACTIVITIES OF THE MALE REPRODUCTIVE SYSTEM
21
b. Some Definitions
Semen or seminal fluid is the all-important substance which the male con-
tributes during the reproductive event. It is the product of the entire repro-
ductive system, including special glands of the accessory reproductive struc-
tures. The semen is composed of two parts:
(1) The sperm (spermatozoa, spermia) are the formed elements which
take part in the actual process of fertilization.
The seminal plasma, a fluid part, is a lymph-like substance contain-
ing various substances dissolved or mixed in it. These contained sub-
stances are important as a protection for the sperm and as an aid to
the process of fertilization.
With regard to the second function of the male genital system, namely,
the delivery of sperm to the site of fertilization, it should be observed that
(2)
Fig. 10. Spermatophores of common urodeles. (Redrawn from Noble: Biology of the
Amphibia, New York, McGraw-Hill.) (A) Triturus viridescens. (After Smith.) (B)
Desmognathiis fuse us. (After Noble and Weber.) (C) Eurycea bislineata.
in some vertebrates this is a more simple problem than in others. In those
forms which practice external fertilization, the male system simply discharges
the seminal fluid into the surrounding external medium. However, in those
vertebrates where internal fertilization is the rule, the female system assumes
some of the burden in the transport of the semen to the region where fertili-
zation is consummated, thus complicating the procedure. In these instances,
the male genital tract is called upon to produce added substances to the seminal
fluid which aid in protecting the sperm en route to the fertilization site.
The elaboration of the androgenic or male sex hormone is a most impor-
tant function. Androgenic or male sex hormone substances are those organic
compounds which induce maleness, for they aid the development of the male
secondary sex characteristics, enhance the growth and functional development
of the male accessory reproductive structures, and stimulate certain aspects
of spermatogenesis." Like the estrogens, androgens are not confined to a
particular sex; they have been extracted from the urine of women and other
female animals. The androgens derived from urinary concentrates are andros-
22
THE TESTIS AND ITS RELATION TO REPRODUCTION
terone and dehydroisoandrosterone. These two androgens are not as powerful
as that prepared from testicular tissue. Testicular androgen was first isolated
from testicular tissue in 1935 and was given the name testosterone. It also
has been synthesized from cholesterol. It is the most powerful of the androgens
and probably similar, if not identical, with the substance produced in the
testis (Koch, '42).
2. Activities of the Testis
a. Seasonal and Non-seasonal Types of Testicular Activity
The testis has two main functions: the production of sperm and formation
of the male sex hormone. In many vertebrates these two activities represent
a continuous procedure during the reproductive life of the male animal. This
.'-.
,'0/ \ 1 *
// ^i \ '
e. >1
AUG
1 1
SEPT
OCT
I
NOV
■ ■
1.-1 1 ■ -K N\\\\\NK\\\\\\\\N 1
JULY
DEC.
JAN FEB MAR APRIL MAY JUNE
Fig. II. Seasonal spermatogenesis and accessory gland development in the ground
squirrel, Citellus tridecemlineatus. Stippling below base line shows period of hibernation,
whereas crosshatching reveals the reproductive period. (From Turner: General Endo-
crinology, Philadelphia, Saunders, after L. J. Wells.)
condition is found in certain tropical fish, in the common fowl and various
wild tropical birds, and in many mammals, such as man, the dog, bull, stallion,
cat, etc. On the other hand, in the majority of vertebrates these activities of
the testis are a seasonal affair. This condition is found in most fish, practically
all amphibia, all temperate-zone-inhabiting reptiles, most birds, and many
mammals. Among the latter, for example, are the ferret, deer, elk, fox, wolf,
and many rodents, such as the midwestern ground squirrel. Seasonal periodicity
is true also of the common goose and turkey.
Sperm-producing periodicity is not correlated with any particular season,
nor is spermatogenesis always synchronized with the mating urge, which in
turn is dependent upon the male sex hormone. In some forms, these two
testicular functions may actually occur at different seasons of the year, as
for example, in the three-spined stickleback, Gasterosteus aculeatus (fig. 15).
(See Craig-Bennett, '31.) In general, it may be stated that sperm are produced
ACTIVITIES OF THE MALE REPRODUCTIVE SYSTEM 23
during the weeks or months which precede the development of the mating
instinct. Many species follow this rule. For example, in the bat of the genus
Myotis, sperm are produced during the late spring and summer months, while
mating or copulation takes place during the fall or possibly early the next
spring (Guthrie, '33). In the common newt, Triturus viridescens, spermato-
genesis comes to pass during the warm months of the summer, and sperm
are discharged from the testis into the reproductive ducts during the late fall
and early spring, while copulation is accomplished in the early spring. The
testes in this species are quiescent during the cool winter months. In the
midwestern ground squirrel, Citelliis tridecemlineatus, spermatogenesis begins
in November and is marked during February and March (fig. 11). TTie animal
hibernates away the winter months and emerges the first part of April in a
breeding condition. Mating occurs in the early spring (Wells, '35). In the
garter snake, Thamnophis radix, sperm are produced in the testes in the sum-
mer months, stored in the epididymides during the hibernation period in the
fall and winter, and used for copulation purposes in the spring (Cieslak,
'45). Again, in the Virginia deer, Odocoileus virginianus borealis, studied by
Wislocki ('43), active spermatogenesis is realized during the summer and
early autumn months, while the mating season or "rut" which results from
the driving power of the male sex hormone, is at its peak in October and
November (fig. 12). In the fox, Bishop ('42) observed spermatogenesis to
begin in the late fall months, while mating is an event of the late winter and
early spring. In April and May the seminiferous tubules again assume an
inactive state (fig. 13). In the common frog, Rana pipiens, spermatogenesis
is present in the summer months and morphogenesis of spermatids into sperm
happens in large numbers during September, October, and November. Sperm
are stored in the testis over the winter, and the mating instinct is awakened
in the early spring (Glass and Rugh, '44). Following the mating season in
spring and early summer the testis of the teleost, Fimdulus heteroclitiis, is
depleted of sperm until the next winter and spring (Matthews, '38).
As the seasonal type of testicular activity is present in a large number of
vertebrate species, it seems probable that it represents the more primitive or
fundamental type of testicular functioning.
b. Testicular Tissue Concerned with Male Sex-hormone Production
While one cannot rule out the indirect effects which activities of the semi-
niferous tubules may have upon the functioning of the testis as a whole, in-
cluding the interstitial tissue, direct experimental evidence and other obser-
vations suggest that the interstitial tissue holds the main responsibility for
the secretion of the male sex hormone, testosterone, or a substance very
closely allied to it. For example, if a testis from an animal possessing a per-
manent scrotum is removed from the inguinal bursa and placed within the
peritoneal cavity, the seminiferous tubules tend to degenerate, but the inter-
24
THE TESTIS AND ITS RELATION TO REPRODUCTION
cV
^
s
i^-^
■^'.^
Ui0
5-> si ^ -^ a
^ \»<J
^^1^
»..^ ^^^%
■':^^n ^^^ mr,
Fig. 12. Sections of the testis of the deer, Odocoileus virginianus borealis. (After
Wislocki.) (A) Seminiferous tubules of deer in June. Observe repressed state of tubules
and absence of sperm. (B) Epididymal duct of same deer. Observe absence of sperm
and smaller diameter of duct compared with (D). (C) Seminiferous tubules of October
deer; spermatogenic activity is marked. (D) Epididymal duct, showing well-developed
epididymal tube and presence of many sperm.
stitial tissue remains. The sex hormone, under these circumstances, continues
to be produced. Again, males having cryptorchid testis (i.e., testes which have
failed in their passage to the scrotum) possess the secondary sex character-
istics of normal males but fail to produce sperm cells. Also, it has been demon-
strated that the mammalian fetal testis contains the male sex hormone. How-
ever, in this fetal condition, the seminiferous tubules are present only in an
undeveloped state, whereas interstitial tissue is well differentiated. It is probable
in this case that the interstitial tissue of the fetal testis responds to the
luteinizing hormone in the maternal blood.
In hypophysectomized male rats injected with dosages of pure follicle-
stimulating hormone (FSH) or with small doses of pure luteinizing hormone
ACTIVITIES OF THE MALE REPRODUCTIVE SYSTEM
25
(LH; ICSH), the seminiferous tubules of the testis respond and spermato-
genesis occurs. However, the interstitial tissue remains relatively unstimu-
lated and the accessory structures continue in the atrophic state. If larger
doses of the luteinizing factor are given, the interstitial tissue responds and
the secondary sexual characters are developed, showing a relationship between
interstitial activity and sex-hormone production. (Consult Evans and Simpson
in Pincus and Thimann, '50, pp. 355, 356.)
From certain species whose reproductive activities are confined to a par-
ticular season of the year, there also comes evidence that the interstitial tissue
.**?<
4) "
%. i
a?) e *■ • «4,^» , „, B
Fig. 13. Sections of seminiferous tubules of silver fox. (After Bishop.) (A) Re-
gressed state of tubules following breeding season. (B) Tubule from fox during the
breeding season, characterized by active spermatogenesis.
is the site of sex-hormone production. In the behavior of testicular tissue in
the stickleback, Gasterosteus, as shown by van Oordt ('23) and Craig-Bennett
('31) sperm are produced actively in the seminiferous tubules during one
period of the year when the interstitial tissue is in an undeveloped condition.
The secondary sex characters also are in abeyance at this season of the year.
However, during the months immediately following sperm production, sperm
are stored within the seminiferous tubules and active spermatogenesis is absent.
When the seminiferous tubules thus have completed their spermatogenic ac-
tivity, the interstitial tissue begins to increase, followed by a development of
secondary sex characteristics (figs. 14, 15). A similar difference in the rhythm
of development of these two testicular tissues can be shown for many other
vertebrates. All of these suggestive facts thus serve to place the responsibility
for male sex-hormone production upon the interstitial tissue, probably the
cells of Leydig.
26
THE TESTIS AND ITS RELATION TO REPRODUCTION
Fig. 14. Sections of the testis of the stickleback (Gasterosteus pungitius). (Modified
from Moore, '39, after Van Oordt.) Cf. fig. 13. (A) Spermatogenic activity with many
formed sperm in seminiferous tubules before the mating season, interstitial tissue in
abeyance. (B) At mating period. Interstitial tissue well developed, spermatozoa stored
in the tubules with spermatogenic activity absent.
-^:^\^^^^^f^^^^v^'^^^.^^k^^v^^^^^^^^j
APRIL MAY JUNE JULY AUG
SEPT. OCT.
Fig. 15. Seasonal reproductive cycle in the stickleback (Gasterosteus aculeatus). Cf.
fig. 14. Breeding season is indicated by crosshatching below base line. Observe that
spermatogenic activity follows rise of temperature, whereas interstitial-tissue and sex-
character development occur during ascending period of light. (Redrawn from Turner;
General Endocrinology, Philadelphia, Saunders, modified from Craig-Bennett, 1931.)
c. Testicular Control of Body Structure and Function by the Male
Sex Hormone
1) Sources of the Male Sex Hormone. Testosterone is prepared from tes-
ticular extracts. It is the most potent of the androgens and is believed to be
the hormone produced by the testis. The chemical formula of testosterone is:
OH
CHa
^\
ACTIVITIES OF THE MALE REPRODUCTIVE SYSTEM 27
The testis, however, is not the only site of androgen formation. As men-
tioned above, androgens are found in the urine of female animals, castrates,
etc. It seems probable that the suprarenal (adrenal) cortex may secrete a
certain androgenic substance, possibly adrenosterone, a weak androgen. Many
androgens have been synthesized also in the laboratory (Schwenk, '44).
2) Biological Effects of the Male Sex Hormone. The presence of the male
sex hormone in the male arouses the functional development of the accessory
reproductive structures, the secondary sexual characters, and also stimulates
the development of the seminiferous tubules.
a) Effects upon the Accessory Reproductive Structures. Castra-
tion or removal of the testes from an animal possessing a continuous type
of testicular activity produces shrinkage, and a general tendency toward atro-
phy, of the entire accessory reproductive structures. Injection of testosterone
or other androgens under such conditions occasions a resurgence of func-
tional development and enlargement of the accessory structures (fig. 16).
Moreover, continued injections of the androgen will maintain the accessories
in this functional state (Moore, '42; Dorfman in Pincus and Thimann, '50).
Similarly, under normal conditions in those vertebrates which possess the
seasonal type of testicular function, the accessory reproductive organs shrink
in size with a loss of functional activity when the testis undergoes regression
during the period immediately following the active season. An enlargement
and acquisition of a normal functional condition of the accessories follows
testicular development as the breeding season again approaches (Bishop, '42;
Wislocki, '43; Matthews, '38; Turner, C. L., '19). (Compare figs. 12A-D.)
b) Effects upon Secondary Sex Characteristics and Behavior of
THE Individual. In addition to the primary effects upon the reproductive
system itself, the androgens induce many other secondary structures and altera-
tions of the physiology and behavior of the individual. The influence of the
testicular hormone has been demonstrated in all of the vertebrate groups
from fishes to mammals (Dorfman in Pincus and Thimann, '50). Examples
of testosterone stimulation are: the singing and plumage of the male bird;
hair development of certain mammals; the crowing and fighting, together
with spur, comb, and wattle growth in the rooster. The disagreeable bellig-
erency and positive energy drive of the bull, stallion, or human male may
be attributed, largely, to the action of testicular hormone. However, lest we
disparage this aggressive demeanor unduly, it should be recognized that upon
such explosive force rests the preservation of species and races in some in-
stances. As an example, witness that hairy dynamo of the barren northern
tundras, the bull muskox, whose fiery pugnaciousness when the need arises
undoubtedly has been a strong factor in the preservation of this species.
An excellent example of the effect of testosterone is shown in the develop-
ment of antlers and change in behavior of the Virginia deer, Odocoileus vir-
ginianus borealis (Wislocki, '43). In the northern climate, the testes and male
28 THE TESTIS AND ITS RELATION TO REPRODUCTION
accessory organs reach a profound condition of regression in April and May.
Growtii of the new antlers starts at this time, and during the late summer the
antlers grow rapidly and begin to calcify. During the summer, also, the testes
develop rapidly, and spermatogenesis results. Loss of the "velvet" covering of
the antlers is experienced during September, and mating is the rule in October
and November. The antlers are shed in midwinter. If the testes are removed
after the naked antler condition is reached, the antlers are shed rapidly. Testos-
terone administered to does or to young males which have been castrated
induces the development of antlers. The general scheme of antler development
suggests, possibly, that the testicular hormone acts upon an anterior pituitary
factor, and this activated factor in turn initiates antler growth. Hardening of
the antlers and loss of velvet results from testosterone stimulation. Loss of
the antler is synchronized with a decrease in the amount of testosterone in
the blood stream, accompanied by the acquisition of a docile, non-belligerent,
more timid behavior.
c) Effects upon the Seminiferous Tubules. Testosterone has a stim-
ulating effect upon the seminiferous tubule and sperm formation. This matter
is discussed in Chap. 3.
d. Seminiferous-tubule Activity and Formation of Sperm
See Chap. 3.
e. The Seminiferous Tubule as a Sperm-storing Structure
See p. 31.
3. Role of the Reproductive Duct in Sperm Formation
a. Vertebrates Without a Highly Tortuous Epididymal Portion of the
Reproductive Duct
In a large number of vertebrates, morphologically developed sperm pass
from the testis through the efferent ductules of the epididymis (vasa efferentia)
to the epididymal duct where they remain for varying periods. However, in
many vertebrates the anterior (proximal) portion of the sperm duct does not
form a tortuous epididymal structure similar to that found in other verte-
brates. This condition is present in the common frog, Rana; in the hellbender,
Cryptobranchus; in the bowfin, Amia; etc. Because of this fact, the sperm
pass directly into the vas deferens or sperm duct (Wolffian duct) without
undergoing a sojourn through a convoluted epididymal portion of the duct.
Correlated with the type of testis and sperm-duct relationship in the frog,
is the fact that one may obtain viable, fertilizing sperm directly from the
testis. For example, if one removes the testis from a living frog and macerates
it in pond water or in an appropriate saline solution, active sperm are obtained
which are capable of fertilizing eggs in a normal manner. That is, the frog
testis matures sperm morphologically and physiologically. This type of tes-
ACTIVITIES OF THE MALE REPRODUCTIVE SYSTEM
29
MORPHOLOGICAL
MATURATION
THE TESTES
Fig. 16
MORPHOLOGICAL
AND PHYSIOLOGICAL
MATURATION IN THE
TESTES - NO
CONVOLUTED EPIOIDYMAL
DUCTS .
Fig. 17
PHYSIOLOGICAL
MATURATION AND
SPERM STORAGE
IN HIGHLY DEVELOPED
EPIDIDYMAL DUCTS
Fig. 16. Effects of the male sex hormone upon the functional development of the
accessory reproductive structures of the male rat. (After Turner: General Endocrinology,
Philadelphia, Saunders, p. 324.) (A) Normal male rat condition produced by injection
of crystalline male sex hormone for 20 days into castrate before autopsy. (B) Castrated
male litter mate of (A) receiving no replacement therapy.
Fig. 17. Diagrammatic drawings of the two types of testicular-reproductive relation-
ships occurring in the vertebrate group. (A) Simplified type of reproductive duct con-
nected with the testis by means of efferent ductules. The duct-testis relationship of many
teleost fishes is similar to this but does not possess the efferent ductules, the sinus-like
reproductive duct being attached directly to the testis. Sperm cells (spermatozoa) are
matured and stored within the testis. This type of relationship generally is found where
fertilization is external or where sperm are discharged all at once during a short repro-
ductive period. (B) More complicated variety of reproductive duct, connected with the
testis by means of efferent ducts, but possessing an anterior twisted portion, the epididymal
duct in which the sperm are stored and physiologically matured. This type of duct
generally is found in those vertebrates which utilize internal fertilization and where
sperm are discharged over a short or extended reproductive period.
ticular maturation is characteristic of many of the lower vertebrates possessing
simple reproductive ducts.
b. The Epididymis as a Sperm-ripening Structure
On the other hand, in those forms which possess an anterior convoluted
epididymal portion of the reproductive duct, the journey of the sperm through
this portion of the duct appears to be necessary in order that fertilizable sperm
may be produced. In mammals it has been shown that the epididymal journey
somehow conditions the physiological ripening of the sperm. Sperm taken
30 THE TESTIS AND ITS RELATION TO REPRODUCTION
from the mammalian testis will not fertilize; those from the caudal portion
of the epididymis will, provided they have been in the epididymis long enough.
Under normal conditions sperm pass through the epididymis slowly, and retain
their viability after many days' residence in this structure. Sperm prove to be
fertile in the rabbit epididymis up to about the thirty-eighth day; if kept
somewhat longer than this, they become senile and lose the ability to fertilize,
although morphologically they may seem to be normal (Hammond and Asdell,
'26). In the rat, they may live up to 20 to 30 days in the epididymis and still
be capable of fertilization (Moore, '28). It has been estimated that the epi-
didymal journey in the guinea pig consumes about two weeks, although they
may live and retain their fertilizing power as long as 30 days in epididymides
which have been isolated by constriction (Moore and McGee, '28; Young,
'31; Young, '31b). It is said that in the bull, sperm within the epididymis
may live and be motile for two months. As a result of these facts, it may be
concluded that the epididymal journey normally is a slow process, and that
it is beneficial for the development of sperm "ripeness" or ability to fertilize.
c. The Epididymis and Vas Deferens as Sperm-storage Organs
Along with the maturing faculty, the epididymal duct and vas deferens also
act as sperm-storage organs. As observed on p. 23, in the bat, Myotis, sperm
are formed in great numbers in the seminiferous tubules and pass to the
epididymal duct where they are stored during the fall, winter, and early spring
months; the epididymal journey thus is greatly prolonged in this species. In
the ovoviviparous garter snake, Thamnophis radix, sperm are produced during
the summer months; they pass into the epididymides during early autumn and
are stored there during the fall and winter. In the mammal, sperm are stored
in the epididymal duct.
Aside from its main purpose of transporting sperm to the exterior (see
sperm transport, p. 177), the caudal portion of the sperm duct or vas deferens
also is capable of storing sperm for considerable periods of time. In the
common perch, Perca flavescens, sperm are developed in the testes in the
autumn, pass gradually into the accessory reproductive ducts, and are stored
there for five or six months until the breeding season the following spring
(Turner, C. L., '19). Again, in mammals, the ampullary region of the vas
deferens appears to be a site for sperm storage. For example, the ampulla of
the bull sometimes is massaged through the rectal wall to obtain sperm for
artificial insemination. In this form sperm may be stored in the ampulla and
still be viable, for as long as three days. Similarly, in lower vertebrates large
numbers of sperm may be found in the posterior extremities of the vas deferens
during the breeding season. Thus, the reproductive duct (and its epididymal
portion when present) is instrumental in many vertebrate species as a tem-
porary storage place for the sperm.
ACTIVITIES OF THE MALE REPRODUCTIVE SYSTEM 31
d. Two Types of Vertebrate Testes Relative to Sperm Formation
The importance of the epididymal duct in many vertebrates and its relative
absence in others, focuses attention upon the fact that in many vertebrate
species sperm are produced, stored, and physiologically matured entirely
within the confines of the testis (frog, bowfin, stickleback, etc.). The repro-
ductive duct under these circumstances is used mainly for sperm transport.
In many other vertebrate species sperm are morphologically formed in
the testis and then are passed on into the accessory structures for storage and
physiological maturation. Functionally, therefore, two types of testes and two
types of accessory reproductive ducts are found among the vertebrate group
of animals (fig. 17). It naturally follows that the testis which produces, stores,
and physiologically matures sperm is best adapted for seasonal activity, par-
ticularly where one female is served during the reproductive activities. That
is, it functions as an "all at one time" spawning mechanism. On the other hand,
that testis which produces sperm morphologically and passes them on to a
tortuous epididymal duct for storage and physiological maturing is best adapted
for the continuous type of sperm production or for the service of several
females during a single seasonal period. The sperm, under these conditions,
pass slowly through the epididymal duct, and, therefore, may be discharged
intermittently.
4. Function of the Seminal Vesicles (Vesicular Glands)
The seminal vesicles show much diversity in their distribution among vari-
ous mammals. Forms like the cat, dog, opossum, rabbit, sloth, armadillo, whale,
do not possess them, while in man, rat, elephant, mouse, they are well-
developed structures. It was formerly thought that the seminal vesicles in
mammals acted as a storehouse for the sperm, hence the name. In reality
they are glandular structures which add their contents to the seminal fluid
during the sexual act.
5. Function of the Prostate Gland
The prostate gland also is a variable structure and is found entirely in the
marsupial and eutherian mammals. In marsupials it is confined to the pros-
tatic portion of the urethral wall; in man it is a rounded, bulbous structure
which surrounds the urethra close to the urinary bladder. In many other
mammals it is a much smaller and less conspicuous structure. It discharges
its contents into the seminal fluid during the orgasm. It is probable that the
prostatic and vesicular fluids form the so-called "vaginal plug" in the vagina
of the rat, mouse, etc.
6. Bulbourethral (Cowper's) Glands
The bulbourethral glands are absent in the dog but present in most other
mammals. In marsupials and monotremes these structures are exceptionally
32 THE TESTIS AND ITS RELATION TO REPRODUCTION
well formed. In the opossum there are three pairs of bulbourethral glands.
The mucous contents of these and other small urethral glands are discharged
at the beginning of the sexual climax and, as such, become part of the semi-
nal fluid.
7. Functions of Seminal Fluid
a. Amount of Seminal Fluid Discharged and Its General Functions
As stated previously, the semen or seminal fluid is composed of two parts,
the sperm cells (spermia; spermatozoa) and the seminal plasma. The presence
of the sperm cells represents the most constant feature, although they may
vary considerably from species to species in size, shape, structure, and number
present. The seminal plasma varies greatly as to composition and amount
discharged.
The quantity of seminal fluid discharged per ejaculate and the relative
numbers of sperm present in man and a few other vertebrate species associ-
ated with him are as follows:*
Voliane of Sini>le Ejaculate, Sperni Density in Semen,
Species Most Common Value, in CC. Average Value, per CC.
Boar 250 cc. 100,000,000 per cc.
Bull 4-5 cc. 1,000,000,000 per cc.
Cock 0.8 cc. 3,500,000,000 per cc.
Dog 6 cc. 200,000,000 per cc.
Man 3.5 cc. 100,000,000 per cc.
Rabbit 1 cc. 700,000,000 per cc.
Ram 1 cc. 3,000,000.000 per cc.
Stallion 70 cc. 120,000,000 per cc.
Turkey 0.3 cc. 7,000,000,000 per cc.
* Modified from Mann ('50).
Two important branches of study involving the semen pertain to:
(1) the chemical and physiological nature and numerical presence of the
sperm, and
(2) the physiology and biochemistry of the seminal plasma.
(See Mann, '50, for discussion and bibliography.) As a result of the studies
thus far, a considerable body of information has been accumulated.
The main function of the semen, including the plasma and accessory sperm,
appears to be to assist the sperm cell whose chance fortune it is to make con-
tact with the egg. Once this association is accomplished, the egg seemingly
takes over the problem of fertilization. The seminal plasma and the accessory
numbers of sperm appear to act as an important protective bodyguard and
ACTIVITIES OF THE MALE REPRODUCTIVE SYSTEM 33
also as an aid for this event. Modern research emphasizes, therefore, that the
work of the male reproductive system is not complete until this contact is
made.
b. Coagulation of the Semen
In many mammalian species, the semen tends to coagulate after its dis-
charge from the male system. In the mouse, rat, guinea pig, opossum, rhesus
monkey, etc., the semen coagulates into a solid mass, the vaginal plug, once
it reaches the vagina of the female. The probable function of the vaginal
plug is to prevent the semen from seeping out of the vagina. The formation
of this plug may be due to a protein present in the contents of the seminal
vesicle which comes in contact with the enzyme, vesiculase. In the rat and
guinea pig this catalyst probably is produced by the ''coagulating gland," a
specialized structure associated with the seminal vesicles in these forms. Some
of it also may come from the prostate.
Coagulation of the seminal fluid also occurs in man, stallion, and boar but
it is entirely absent in the dog, bull, and many other animals. Human semen
coagulates immediately after discharge but liquefies a short time afterward.
This liquefaction may be due to the presence of two enzymes, fibrinogenase
and fibrinolysin, found in human semen and both derived from the prostate.
These enzymes are found also in dog semen. In the latter their property of
inhibiting blood coagulation may be of use where considerable amounts of
blood may be present in the female genital tract at the onset of full estrous
conditions. Another important contribution of the prostate gland is citric acid.
Its role is not clear but it may enter into the above coagulation-liquefaction
process (Mann, '50, p. 348).
c. Hyahironidase
Various enzymes have been demonstrated to be present in the semen of
certain invertebrates and vertebrates. One such enzyme is hyaluronidase which
appears to be produced in the testes of the rat, rabbit, boar, bull, and man.
It is not found in the testes of vertebrates below the mammals. Its specific
function is associated with the dispersal of the follicle cells surrounding the
egg; in so doing it may aid the process of fertilization in mammals.
d. Accessory Sperm
One sperm normally effects a union with the egg in fertilization. Accessory
sperm may enter large-yolked eggs, but only one is intimately involved in
the union with the egg pronucleus. However, what is meant by accessory
sperm here is the large number of sperm which normally clusters around
the egg during the fertilization process in many animal species. A suggestion
of a function for these accessory sperm follows from the fact that hyaluroni-
dase may be extracted from the semen, presumably from the sperm them-
34 THE TESTIS AND ITS RELATION TO REPRODUCTION
selves. Rowlands ('44) and also Leonard and Kurzrok ('46) have shown
that a seminal fluid deficient in sperm numbers may fertilize if hyaluronidase
extracted from sperm (?) is added to such a weakened sperm suspension.
The implication is that the accessory sperm thus may act as "cupbearers"
for the one successful sperm in that they carry hyaluronidase which aids in
liquefying the follicle cells and other gelatinous coating material around the egg.
e. Fructose
An older concept in embryology maintained that sperm were unable to
obtain or utilize nourishment after they departed from the testis. More recent
investigation has shown, however, that sperm do utilize certain sugar materials,
and that their survival depends upon the presence of a simple sugar in the
medium in which they are kept. (See Mann, '50.)
The sugar that is found normally in semen is fructose. It varies in quantity
from species to species, being small in amount in the semen of the boar or
stallion but considerably larger in quantity in the seminal fluid of the bull,
man, and rabbit. The seat of origin of this sugar appears to be the seminal
vesicle, at least in man, although the prostate may also be involved, particu-
larly in the rabbit and also in the dog. The dog, however, has but a small
amount of fructose in the seminal discharge. The real function of seminal
fructose "might be as a readily utilizable store of energy for the survival of
motile spermatozoa" (Mann, '50, p. 360).
/. Enzyme-protecting Substances
Runnstrom (personal communication) and his co-workers have demon-
strated that the fertilizing life of sea-urchin sperm is increased by certain sub-
stances found in the jelly coat of the sea-urchin egg. Presumably these
substances are protein in nature, and, according to Runnstrom, they may
act to preserve the enzyme system of the sperm. Similarly, the seminal fluid
may act to preserve the enzyme system of the sperm, while en route to the
egg, especially within the female genital tract.
D. Internal and External Factors Influencing Activities of the Testis
Conditions which influence testicular activity are many. Many of the fac-
tors are unknown. Nevertheless, a few conditions which govern testis function
have been determined, especially in certain mammalian species. The general
results of experimental determination of some of the agents which affect
testicular function are briefly outlined below.
1. Internal Factors
a. Temperature and Anatomical Position of the Testis
It is well known that in those mammals which have a permanent scrotal
residence of the testes failure of the testis or testes to descend properly into
FACTORS INFLUENCING ACTIVITIES OF THE TESTIS
35
the scrotum results in a corresponding failure of the seminiferous tubules
to produce sperm. In these instances the testis may appear shriveled and
shrunken (fig. 18). However, such cryptorchid (ectopic) conditions in most
cases retain the ability to produce the sex hormone at least to some degree.
A question therefore arises relative to the factors which inhibit seminiferous
tubule activity within the cryptorchid testis.
The failure of cryptorchid testes to produce viable sperm has been of
interest for a long time. Observations have demonstrated that the more hidden
Fig. 18. Experimental unilateral cryptorchidism in adult rat. The animal's left testis
was confined within the abdominal cavity for six months, whereas the right testis was
permitted to reside in the normal scrotal position. Observe the shrunken condition of the
cryptorchid member. (After Turner: General Endocrinology, Philadelphia, Saunders.)
the testis (i.e., the nearer the peritoneal cavity) the less likely are mature
sperm to be formed. A testis, in the lower inguinal canal or upper scrotal
area is more normal in sperm production than one located in the upper
inguinal canal or inside the inguinal ring. Studies made upon peritoneal and
scrotal temperatures of rats, rabbits, guinea pigs, etc., demonstrate a tem-
perature in the scrotum several degrees lower than that which obtains in
the abdomen. These observations suggest that the higher temperature of the
non-scrotal areas is a definite factor in bringing about seminiferous tubule
injury and failure to produce sperm.
With this temperature factor in mind. Dr. Carl R. Moore (in Allen,
Danforth, and Doisy, '39) and others performed experiments designed to
test its validity as a controlling influence. They found that confinement alone
of an adult guinea pig testicle in the abdomen led to marked disorganization
of all seminiferous tubules in seven days. After several months of such con-
36
THE TESTIS AND ITS RELATION TO REPRODUCTION
finement the seminiferous tubules experience marked degenerative changes
and only Sertoli cells remain (fig. 19 A, B). The interstitial tissue, however,
is not greatly impaired. If such a testis is kept not too long within the abnormal
position and once again is returned to the scrotum, spermatogenesis is reju-
venated (fig. 20A, B). In a second experiment, the scrotum of a ram was
encased loosely with insulating material; a rapid degeneration of the seminif-
erous tubules followed. Young ('27, '29) in a third type of experiment found
that water 6 to 7° warmer than the body temperature applied to the external
aspect of the guinea-pig testis for a 15-minute period evoked degenerative
Fig. 19. Sections of experimental, cryptorchid, guinea-pig, seminiferous tubules and
interstitial tissue. (Modified from C. R. Moore in Sex & Internal Secretions, Williams &
Wilkins, Baltimore, 1939.) (A) Testis confined to abdomen for three months. (B)
Testis confined to abdomen for six months. Observe degenerate state of seminiferous
tubule after six months' confinement. Interstitial tissue not greatly affected by confinement.
changes with temporary sterility (fig. 21). Recovery, however, is the rule in
the latter instance. Summarizing the effects of such experiments involving
temperature, Moore (in Allen, Danforth, and Doisy, '39, p. 371) concludes:
"The injury developing from applied heat, although more rapidly effective,
is entirely similar to that induced by the normal body temperature when the
testicle is removed from the scrotum to the abdomen."
The position of the scrotum and its anatomical structure is such as to
enhance its purpose as a regulator of testicular temperature (figs. 2, 6). When
the surrounding temperature is cold, the contraction of the dartos muscle
tissue of the scrotal skin contracts the scrotum as a whole, while the con-
traction of the cremaster muscle loops pulls the testes and the scrotum closer
to the body, thus conserving the contained heat. When the surrounding tem-
perature is warm, these muscles relax, producing a more pendulous condition
to permit heat loss from the scrotal wall.
FACTORS INFLUENCING ACTIVITIES OF THE TESTIS
37
In accordance with the foregoing description of the scrotum as a necessary
thermoregulator for the testis, it has been further shown for those mammals
which possess a scrotum that testis grafts fare much better when transplanted
to the scrotal wall or into the anterior chamber of the eye (Turner, C. D.,
Fig. 20. Sections of testis during and after abdominal confinement. (Modified from
C. R. Moore in Sex & Internal Secretions, Williams & Wilkins, Baltimore, 1939.) (A)
Section of left testis to show degenerate state of seminiferous tubules after 24 days of
abdominal confinement. (B) Section of right testis 74 days after replacement in scrotum.
Observe spermatogenic activity in tubules.
0^'^
Fig. 21. Effect of higher temperature applied to external surface of guinea-pig testis.
Water, 47°, was applied to surface of scrotum for period of 10 minutes. Testis was
removed from animal 12 days after treatment. Seminiferous tubules are degenerate.
(Modified from Moore, '39; see also Young, '27, J. Exp. Zool., 49.)
38 THE TESTIS AND ITS RELATION TO REPRODUCTION
'48). The anterior chamber of the eyeball possesses a temperature much
cooler than the internal parts of the body.
Two types of seminiferous tubules are thus found in mammals. In a few
mammalian species (see p. 6) the temperature of the peritoneal cavity is
favorable to the well-being of the seminiferous tubule; in most mammalian
species, however, a lower temperature is required. On the other hand, the
activities of the interstitial tissue of the testis appear to be much less sensitive
to the surrounding temperature conditions, and the male sex hormone may
be produced when the testes are removed from the scrotum and placed within
the peritoneal cavity.
With regard to the functioning of the testis within the peritoneal cavity
of birds it has been suggested that the air sacs may function to lower the
temperature around the testis (Cowles and Nordstrom, '46). In the sparrow,
Riley ('37) found that mitotic activity in the testis is greatest during the
early morning hours when the bird is resting and the body temperature is
lower, by 3 or 4" C.
b. Body Nourishment in Relation to Testicular Function
The testis is a part of, and therefore dependent upon, the well-being of
the body as a whole. However, as observed in the preceding pages the inter-
stitial cells and their activities in the production of the male sex hormone
are less sensitive to the internal environment of the body than are the seminif-
erous tubules.
The separation of these two phases of testicular function is well demon-
strated during starvation and general inanition of the body as a whole. A
falling off of sperm production is a definite result of starvation diets, although
the germinative cells do not readily lose their ability to proliferate even after
prolonged periods of starvation. But the interstitial cells and the cells of
Sertoli are not as readily affected by inadequate diets or moderate starvation
periods. Sex drive may be maintained in a starving animal, while his ability
to produce mature, healthy sperm is lost. On the other hand, long periods
of inanition also affect sex hormone production and the sexual interests of
the animal.
Aside from the abundance of food in a well-rounded dietary regime, ade-
quate supplies of various vitamins have been shown to be essential. Vitamin
Bi is essential to the maintenance of the seminiferous tubules in pigeons.
Pronounced degenerative changes in the seminiferous tubules of rats and
other mammals occur in the absence of vitamins A and E (Mason, '39).
Prolonged absence of vitamin E produces an irreparable injury to the testis
of rats; injury produced by vitamin A deficiency is reparable. The B-complex
of vitamins seems to be especially important for the maintenance of the
accessory reproductive structures, such as the prostate, seminal vesicles, etc.
The absence of vitamin C has a general body effect, but does not influence
FACTORS INFLUENCING ACTIVITIES OF THE TESTIS 39
the testis directly. Some of these effects may be mediated through the pituitary
gland. As vitamin D is intimately associated with the mineral metabolism of
the body, it is not easy to demonstrate its direct importance.
c. The Hypophysis and Its Relation to Testicular Function
The word "hypophysis" literally means a process extending out below.
The early anatomists regarded the hypophysis cerebri as a process of the
brain more or less vestigial in character. It was long regarded as a structure
through which waste materials from the brain filtered out through supposed
openings into the nasal cavity. These wastes were in the form of mucus or
phlegm, hence the name "pituitary," derived from a Latin word meaning
"mucus." The word pituitary is often used synonymously with the word
hypophysis.
The hypophysis is made up of the pars anterior or anterior lobe, pars
intermedia or intermediate lobe, and a processus infundibuli or posterior
lobe. The anterior lobe is a structure of great importance to the reproductive
system; its removal (ablation) results in profound atrophic changes through-
out the entire reproductive tract.
The importance of the pituitary gland in controlling reproductive phe-
nomena was aroused by the work of Crowe, Gushing, and Homans ('10)
and by Aschner ('12) who successfully removed the hypophysis of young
dogs. One of the first fruits of this work was a demonstration of the lack of
genital development when this organ was removed. Since that time many
of the other cohabitants of man — rats, mice, cats, rabbits, etc. — have been
hypophysectomized, and in all cases a rapid involution and atrophy of the
genital structures results from pituitary removal. The testis undergoes pro-
found shrinkage and regression following hypophysectomy, the degree of
change varying with the species. In the rooster and monkey, for example,
regressive changes are more marked than in the rat. (Consult Smith, '39, for
data and references. )
A striking demonstration of the influence of the hypophysis upon the
genital tract is the result of its removal from a seasonal-breeding species,
such as the ferret. Ablation of the pituitary in this species during the non-
breeding season causes slight if any change in the testis and accessory repro-
ductive organs. However, when it is removed during the breeding season,
a marked regression to a condition similar to that present during the non-
breeding season occurs (Hill and Parkes, '33).
The experimental result of hypophysectomy on many animal species thus
points directly to this structure as the site of hormonal secretion, particularly
to the anterior lobe (Smith, '39). The initial work on the relation of pituitary
hormones and the gonad was done upon the female animal. The results of
these studies aroused the question whether one or two hormones were re-
40 THE TESTIS AND ITS RELATION TO REPRODUCTION
sponsible. The latter alternative was suggested by the work of Aschheim and
Zondek ('27) and Zondek ('30) who concluded that two separate substances
appeared to be concerned with the control of ovarian changes.
Nevertheless, for a time the concept of only one gonad-controlling (gona-
dotrophic) hormone was produced by the pituitary, continued to gain atten-
tion, and some workers suggested that the two ovarian effects of follicular
growth and luteinization of the follicle were due to the length of time of
administration of one hormone and not to two separate substances. How-
ever, this position soon was made untenable by research upon the gonado-
trophic substances derived from the pituitary gland. Studies along this line
by Fevold, Hisaw, and Leonard ('31) and Fevold and Hisaw ('34) reported
the fractionation, from pituitary gland sources, of two gonadotrophic sub-
stances, a follicle-stimulating factor or FSH and a luteinization factor or LH.
This work has been extensively confirmed. It should be observed in passing
that the male pituitary gland contains large amounts of FSH, although, as
mentioned below, the function of the testis and the male reproductive system
relies to a great extent upon the luteinizing factor. Some investigators refer
to the LH factor as the interstitial-cell-stimulating hormone, ICSH. (See Evans,
'47; and also Evans and Simpson in Pincus and Thimann, '50.)
The action of these two hormones upon testicular tissue, according to
present information, is somewhat as follows: If pure follicle-stimulating hor-
mone, FSH, which produces only FSH effects in the female, is injected in
low doses into hypophysectomized male rats, the seminiferous tubules are
stimulated and spermatogenesis occurs. Under these conditions, the interstitial
tissue remains unstimulated and the accessories continue in an atrophic state.
It has further been demonstrated that slight amounts of the luteinizing gona-
dotrophic hormone, LH (ICSH), added to the above injections of FSH,
effects a much better stimulation of the spermatogonial tissue, and the inter-
stitial tissue also develops well.
On the other hand, when pure LH (ICSH) is given alone in small doses,
spermatogenesis is stimulated with slight or no effect upon the male accessory
structures. However, when larger doses of the LH (ICSH) factor alone are
injected, the interstitial tissue is greatly stimulated, and the testicular weight
increases much more than when FSH alone is given. Furthermore, the acces-
sory reproductive structures are stimulated and become well developed, sug-
gesting the elaboration of the male sex hormone. In agreement with these
results, the administration alone of testosterone, the male sex hormone, in-
creases the weight and development of the accessory structures in hypophy-
sectomized animals and it also maintains spermatogenesis. It appears, there-
fore, that the effects of the LH substance upon the seminiferous tubules and
the accessory organs occur by means of its ability to arouse the formation of
the male sex hormone.
FACTORS INFLUENCING ACTIVITIES OF THE TESTIS 41
A summary of the actions of the pituitary gonadotrophic hormones upon
testicular tissue may be stated as follows:
(1) Pure FSH in small doses stimulates the seminiferous tubules and
spermatogenesis with little or no effect upon the interstitial tissue or
the accessory reproductive structures, such as the seminal vesicles or
prostate gland;
(2) Small doses of pure LH also stimulate spermatogenesis with little
or no stimulation of the accessory structures;
(3) Pure LH (ICSH) in larger doses stimulates the development of the
interstitial tissue with the subsequent secretion of the male sex hor-
mone and hypertrophy of the accessory reproductive organs;
(4) The male sex hormone in some way aids or stimulates the process
of spermatogenesis, suggesting that the action of LH occurs through
the medium of the sex hormone (fig. 22).
(Consult Evans and Simpson in Pincus and Thimann, '50, for data and
references; also Turner, C. D., '48.)
The foregoing results of the action of the FSH and LH upon testicular
function might suggest that the LH substance alone is essential in the male
animal. However, it should be observed that without the presence of FSH,
LH is not able to maintain the tubules in a strictly normal manner, the
tubules showing a diminution of size. Also, in extreme atrophic conditions
of the tubules, pure FSH stimulates spermatogenesis better than similar quan-
tities of LH. It is probable that FSH and LH (ICSH) work together to effect
complete normality in the male. This combined effect is known as a syner-
gistic effect. It also is of interest that the injection of small doses of testosterone
propionate into the normal male, with the pituitary gland intact, results in
inhibition of the seminiferous tubules, probably due to the suppression of
pituitary secretion by the increased amount of the male sex hormone in the
blood. However, high doses, while they likewise inhibit the pituitary, result
in a level of androgen which stimulates the seminiferous tubules directly
(Ludwig, '50).
Aside from the above actions upon testicular tissue by the luteinizing hor-
mone (LH;ICSH) certain other functions of this substance should be men-
tioned (see fig. 22). One of these is the apparent dependence of the SertoU
cells upon the presence of the interstitial cells (Williams, '50). Interstitial
tissue behavior and development in turn relies mainly upon LH (ICSH)
(Fevold, '39; Evans and Simpson in Pincus and Thimann, '50). As the sperm
are intimately associated with the Sertoli elements during the latter phases
of spermatogenesis in which they transform from the spermatid into the form
of the adult sperm, a very close association and reliance upon the presence
of the luteinizing hormone thus appears to be established in sperm development.
A further study of the LH factor is associated with the maintenance of
Fig. 22. (See facing page for legend.)
42
FACTORS INFLUENCING ACTIVITIES OF THE TESTIS 43
the seminiferous tubules themselves. In aged males, the interstitial tissue and
the seminiferous tubules normally involute and regress with accumulation
of large amounts of connective tissue material. In testicular grafts made into
the rabbit's ear, Williams ('50) found, when interstitial tissue was present
in the grafts, the seminiferous tubules were more nearly normal; when absent,
the tubules underwent fibrosis.
Another function of the LH substance apparently is concerned with release
of the sperm from the Sertoli cells. De Robertis, et al. ('46), showed that
anterior pituitary hormones possibly cause release of sperm from the Sertoli
cells in the toad by the production of vacuoles and apical destruction of
the cytoplasm of the Sertoli elements. In testicular grafts Williams ('50) ac-
cumulated evidence which suggests that vacuoles and secretion droplets in
the Sertoli cells occurred as a result of LH administration. The combined
results of these investigators suggest that sperm release from the Sertoli cell
is dependent, in some way, upon LH (ICSH) activity.
A final function is concerned with the physiological maturing of sperm
in the reproductive duct, at least in many vertebrate species. The well-being
of the epididymis and vas deferens is dependent upon the presence of the
male sex hormone (Creep, Fevold, and Hisaw, '36). As the male sex hor-
mone results from stimulation of the interstitial cells by the interstitial-cell-
stimulating substance, LH (ICSH), the connection between this substance
and the physiological maturation of the sperm cell is obvious.
2. External Environmental Factors and Testis Function
As we have seen above, the anterior lobe of the hypophysis acts as the
main internal environmental factor controlling the testes and, through them,
the reproductive ducts. It has been observed also that food, vitamins, and
anatomical position of the testis are important influences in regulating tes-
ticular function. Furthermore, general physiological conditions such as health
or disease have an important bearing upon the gonads (Mills, '19). All of
Fio. 22. Chart showing the effects of the hypophyseal anterior lobe upon the devel-
oping gametes. It also suggests the various factors influencing pituitary secretion of the
gonadotrophic hormones. FSH and LH. Observe that the primitive gamete in the cortex
of the ovary is subjected to the cortical environment and develops into an oocyte, whereas
in the medullary or testicular environment it develops into a spermatocyte. Experiments
upon sex reversal have demonstrated that the medullary and cortical portions of the
gonad determine the fate of the germ cell. In the male area or medulla, the germ cell
differentiates in the male direction, while in the cortex, the differentiation is in the
direction of the female gamete or oocyte, regardless of the innate sex-chromosome con-
stitution of the primitive germ cell. The fate of the germ cell thus is influenced by four
main sets of factors: ( 1 ) Internal and external environmental factors, controlling the
secretions of the pituitary body, (2) Environment of the testicular tissue (medulla) and
possible humoral substances produced in this tissue, (3) Environment of the ovarian
tissue (cortex) and possible humoral substances elaborated there, and (4) Secretions of
the anterior lobe of the pituitary body.
44 THE TESTIS AND ITS RELATION TO REPRODUCTION
the above conditions are contained witiiin tiie body of the organism, and as
such represent organismal conditions.
The following question naturally arises: Do factors or conditions external
to the body impinge themselves in such a way as to control pituitary and
gonadal function?
a. Light as a Factor
Aside from the supply of nutritive substances or the collision of the many
nervous stimuli with the individual which may arouse or depress the sexual
activities, two of the most important obvious external factors are temperature
and light. Research on the reproductive behavior of many animal species,
during the past twenty years, has shown that both of these factors have great
significance on the reproductive activities of many vertebrate species. Bisson-
nette ('30, '32, '35, a and b) has accumulated evidence which demonstrates
that light is a potent factor in controlling the reproductive behavior of the
European starling (Sturnus vulgaris) and also of the ferret (Putorius vulgaris).
In the starling, for example, the evidence shows that green wave lengths of
the spectrum inhibit testicular activity, while red rays and white light arouse
the reproductive function (fig. 23). The addition of electric lighting to each
day's duration produced a total testis size in midwinter which surpassed the
normal condition in the spring. In the ferret artificially increased day length
beginning at the first part of October brings the testis to maximum size and
activity coupled with a normal mating impulse as early as November and
December (fig. 24). Under normal conditions the male ferret is able to breed
only during February and early March.
These findings relative to the influence of light on the reproductive perio-
dicity of animals confirm a fact which has been known for a long time,
namely, that seasonal breeders brought from the northern hemisphere to the
southern hemisphere reverse their breeding season. For example, ferrets which
normally breed from spring to summer in the northern hemisphere shift their
breeding habits to the September-February period when moved to the southern
hemisphere. Inasmuch as the hypophysis is instrumental in bringing about
secretion of the gonadotrophic hormones responsible for the testicular activity,
it is highly probable that light coming through the eyes (see Hill and Parkes,
'33) influences the nervous system in some way arousing the hypophysis and
stimulating it to secrete these substances in greater quantity. However, one
must keep in mind the caution given by Bissonnette, that light is not the only
factor conditioning the sexual cycles of ferrets and starlings.
While numerous animals, such as the migratory birds, ferret, mare, many
fish, frogs, etc., normally are brought into a breeding condition during the
period of light ascendency, a large number of animals experience a sexual
resurgence only during the time of year when the light of day is regressing
in span. This condition is found in some sheep, goats, buflfalo in nature,
Fig. 23. Sections of testis of the starling (Sturnus vulgaris), showing the effect of
electric lighting added to the bird's normal daily duration of light during the autumn.
(After Bissonnette, Physiol. Zool., 4.) (A) Inside young control bird — no light added
— kept inside as control for (B) from November 9 to December 13. (B) Inside young
experimental bird, receiving additional light from "25 watt" bulb from November 9 to
December 13. Total treatment, 34 davs.
S?"^>>.«<^' *,^v v'^ A.
s?^^Ml
... . . i « i^'v^' «*.-
:.?>'
v>..
Fig. 24. Sections of testis and epididymis, showing modification of sexual cycle in the
ferret, Putorius vulgaris, by exposure to increasing periods of light. (After Bissonnette.
'35b.) (A) Seminiferous tubules from normal male over 1 year old, made on October
3, no lighting. (B) Epididymis of normal male on October 3, no lighting. (C) Seminif-
erous tubules of experimental male on November 7, 36 days of added lighting. (D)
Epididymis of experimental males on Nov mber 7, 36 days of added lighting.
45
46 THE TESTIS AND ITS RELATION TO REPRODUCTION
deer, some fish, etc. Bissonnette ('41 ) working with goats found that: "In-
creasing daily light periods from January 25 to April 5 — followed by diminish-
ing periods until July 5, while temperatures remained normal for the seasons,
with four Toggenburg female goats and one male Toggenburg and one Nubian
female — led to cessation of breeding cycles in February instead of March,
followed by initiation of breeding cycles in May and June instead of Sep-
tember." In the ewe, Yeates ('47) also found that a change from increasing
daylight to decreasing length of day induced reproductive activity. In a similar
manner, Hoover and Hubbard ('37) were able to modify the sexual cycle
in a variety of brook trout which normally breeds in December to a breeding
season in August.
b. Temperature Influences
In the case of the animals mentioned above, temperature does not appear
to be a major factor in inducing reproductive activity. However, in many
animals temperature is vitally influential in this respect. For example, in the
thirteen-lined spermophile (ground squirrel) Wells ('35) observed that breed-
ing males kept at 40' F. continued in a breeding condition throughout the
year. Under normal conditions this rodent hibernates during the winter months
and comes forth in the spring ready to breed; sperm proliferation and general
reproductive development take place during the period of hibernation. As
the temperature rises during the spring and summer, testicular atrophy ensues,
followed by a period of spermatogenesis and reproductive activity when the
lowered temperatures of autumn and winter come again. Light, seemingly, is
not a factor in this sexual cycle. Another instance of temperature control
occurs in the sexual phase of the common red newt, Triturus viridescens. Here
it is the rising temperature of the summer which acts as the inducing agent,
and sperm thus produced are discharged into the accessory ducts during the
fall and winter to be used when copulation occurs in early spring. However,
if this species is kept at a relatively low temperature of 8 to 12° C. during
the summer months, spermatogenesis is inhibited and the testis regresses. In
the stickleback, Gasterosteus aculeatus, as reported by Craig-Bennett ('31),
spermatogenesis occurs during July to early September and appears to be
conditioned by a rising temperature, whereas the interstitial tissue and the
appearance of secondary sexual features reach their greatest development
under increased light conditions and slowly rising temperatures (fig. 15).
Bissonnette, in his work on ferrets, also observed a difference in the behavior
of these two testicular components; the interstitial tissue responds to large
increases of daily light periods, whereas the seminiferous tubules are stimu-
lated by small, gradually increasing periods of light.
The above examples emphasize the importance of a single environmental
factor on the pituitary-gonadal relationship. However, in the hedgehog,
Allanson and Deansley ('34) emphasize temperature, lighting, and hormone
INTERNAL FACTORS AND TESTICULAR FUNCTION 47
injections as factors modifying tiie sexual cycles, while Baker and Ransom
('32, '33, a and b) show that light, food, temperature, and locality affect
the sexual cycles and breeding habits of the field mouse. In some vertebrates,
therefore, a single factor may be the dominant one, whereas in others, numer-
ous factors control the action of the pituitary and reproductive system.
E. Internal Factors Which May Control Seasonal and Continuous Types
of Testicular Function
In endeavoring to explain the differences in response to external environ-
mental factors on the part of seasonal and continuous breeders, one must
keep in mind the following possibilities:
(1) The anterior lobe of the hypophysis in some forms (e.g., ferret)
cannot be maintained in a secretory condition after it has reached its
climax; that is, it apparently becomes insensitive to the light factor. As a
result, regression of the pituitary and testis occurs (Bissonnette, '35b).
(2) In the starling, the anterior hypophysis may be maintained by the
lighting, but the testis itself- does not respond to the presence of the
hypophyseal hormones in the blood (Bissonnette, '35b). The possi-
bility in this instance may be that testicular function wanes because
the body rapidly eliminates the hormone in some way (see Bachman,
Collip, and Selye, '34).
(3) Consideration also must be given to the suggestion that the activities
of the sex gland by the secretion of the sex hormone may suppress
anterior lobe activity (Moore and Price, '32).
We may consider two further possibilities relative to continuous testicular
function:
(4) If the "brake actions" mentioned above are not present or present
only in a slight degree, a degree not sufficient to interrupt the activities
of the anterior lobe or of the sex gland, a more or less continuous
function of the testis may be maintained.
(5) When several or many environmental factors are concerned in pro-
ducing testicular activity, a slight altering of one factor, such as light,
may prove insufficient to interrupt the pituitary-germ-gland relation-
ship, and a continuous breeding state is effected in spite of seasonal
changes.
Underlying the above possibilities which may control testicular function is
the inherent tendency or hereditary constitution of the animal. In the final
analysis, it is this constitution which responds to environmental stimuli, and
moreover, controls the entire metabolism of the body. In other words, the
above-mentioned possibilities tend to oversimplify the problem. The organism
48 THE TESTIS AND ITS RELATION TO REPRODUCTION
as a whole must be considered; reproduction is not merely an environmental-
pituitary-sex gland relationship.
F. Characteristics of the Male Reproductive Cycle and Its Relation to
Reproductive Conditions in the Female
As indicated above, reproduction in the male vertebrate is either a con-
tinuous process throughout the reproductive life of the individual or it is a
discontinuous, periodic affair. In the continuous form of reproduction the
activities of the seminiferous tubules and the interstitial or hormone-producing
tissues of the testis function side by side in a continuous fashion. In the
discontinuous, periodic type of testicular function, the activities of the semi-
niferous tubules and of the interstitial tissue do not always coincide. The
activities of the seminiferous tubules, resulting in the production of sperm
for a particular reproductive cycle, tend to precede, in some species by many
months, the activities of the sex-hormone-producing tissue. Evidently, the
output of the FSH and LH substances from the pituitary gland are spread
out over different periods of the year to harmonize with this activity of the
testicular components.
It will be seen in the next chapter that a continuous breeding faculty is
not present in the female comparable to that of the male. All females are
discontinuous breeders. In some species, the cycles follow each other with
little rest between each cycle unless the female becomes pregnant or "broody."
Some have a series of cycles over one part of the year but experience sexual
quiescence over the remaining portion of the year. However, in most female
vertebrates there is but one reproductive cycle per year.
In harmony with the above conditions, the continuous variety of testicular
function is always associated with the condition in the female where more
than one reproductive cycle occurs per year. Continuous reproductive con-
ditions in the male, therefore, are adapted to serve one female two or more
times per year or several different females at intervals through the year.
Furthermore, the complicated, highly glandular, greatly extended type of
male-reproductive-duct system is adapted to conditions of ( 1 ) continuous
breeding, or (2) service to more than one female during one breeding season
of the year, whereas the simple type of reproductive duct is adapted to the
type of service where all or most of the genital products are discharged during
one brief period. In other words, the entire male reproductive system and repro-
ductive habits are adapted to the behavior of female reproductive activities.
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Tne Verteorate Ovary and Its Relationship
to Reproduction
A. The ovary and its importance
B. Preformationism, past and present
C. General structure of the reproductive system of the vertebrate female
1. General structure of the ovary
2. General structure of the accessory reproductive organs
D. Dependency of the female reproductive system on general body conditions
1. Inanition
2. Vitamins
a. Vitamin A
b. Vitamin B
c. Vitamin C
d. Vitamin E
3. The hypophysis (pituitary gland)
E. Activities of the ovary in producing the reproductive state
1. The ovary as a "storehouse" of oogonia
2. Position occupied by the primitive female germ cells in the ovarian cortex
3. Primary, secondary, and tertiary follicles of de Graaf
4. Hormonal factors concerned with the development of egg follicles
a. Effects produced by the gonadotrophic hormones of the development of the
mammalian egg follicle
b. Stimulating effects of the gonadotrophins on the ovaries of other vertebrates
5. Structure of the vertebrate, mature egg follicle
a. Structure of the mature follicle in metatherian and eutherian mammals
b. Structure of the prototherian egg follicle
c. Egg follicles of other vertebrates
6. Ovulatory process; possible factors controlling ovulation
a. Process of ovulation in higher mammals
1) Changing tissue conditions culminating in egg discharge from the ovary
2) Hormonal control of the ovulatory process
b. Ovulation in vertebrate groups other than the higher mammals
1) Hen
2) Frog
3) Hormonal control of ovulation in lower vertebrates
c. Comparison of the immediate factors affecting egg discharge in the vertebrate
group
7. Internal conditions of the ovary as an ovulatory factor
52
i
THE OVARY AND ITS IMPORTANCE 53
8. Number of eggs produced by different vertebrate ovaries
9. Spontaneous and dependent ovulation in the mammals and in other vertebrates
10. Egg viability after discharge from the ovary
11. History of the egg follicle after ovulation
a. Follicles which do not develop a post-ovulatory body
b. Follicles which develop a post-ovulatory body; formation of the corpus luteum
12. Hormones of the ovary and their activities in effecting the reproductive condition
a. Estrogenic hormone
1) Definition and source of production
2) The ovary as the normal source of estrogen in the non-pregnant female
3) Pituitary control of estrogen formation
4) Effect of estrogen upon the female mammal
5) Effects of estrogen in other vertebrates
b. Progesterone — the hormone of the corpus luteum
1 ) Production of progesterone
2) Effects of progesterone
F. Reproductive state and its relation to the reproductive cycles in female vertebrates
1. Sexual cycle in the female mammal
a. Characteristics and phases of the reproductive cycle
b. Relation of estrus and ovulation in some common mammals
1) Spontaneously ovulating forms (Sexual receptivity of male occurs at or
near time of ovulation)
2) Dependent ovulatory forms (Sexual receptivity [heat] occurs previous to
time of ovulation)
c. Non-ovulatory (anovulatory) sexual cycles
d. Control of the estrous cycle in the female mammal
e. Reproductive cycle in lower vertebrate females
G. Role of the ovary in gestation (pregnancy)
1. Control of implantation and the maintenance of pregnancy in mammals
2. Gestation periods, in days, of some common mammals
3. Maintenance of pregnancy in reptiles and other vertebrates
H. Role of the ovary in parturition or birth of the young
I. Importance of the ovary in mammary-gland development and lactation
J. Other possible developmental functions produced by the ovary
K. Determinative tests for pregnancy
A. The Ovary and Its Importance
One of the editions of the treatise on development, "Exercitationes de
Generatione Animalium," by William Harvey (1578-1657) contains a pic-
ture of Jupiter on a throne opening an egg from which various animals,
including man, are emerging (fig. 25). Upon the egg (ovum) are engraved
the words "ex ovo omnia." At the heading of chapter 62 of this work Harvey
placed a caption which explains the phrase ex ovo omnia more explicitly.
This heading reads: "Ovum esse primordium commune omnibus animalibus"
— the egg is the primordium common to all animals. Published in 1651, this
statement still maintains its descriptive force.
Many individual animals arise by asexual reproduction, that is, through
a process of division or separation from a parent organism. In the phylum
Chordata asexual reproduction is found among the Urochordata, where new
Fig. 25. Copy of the engraved title appearing in one edition of Harvey's dissertation
on generation as shown on p. 139 of Early Theories of Sexual Generation by E. J. Cole.
Observe the words "ex ovo omnia" upon the egg which Jupiter is opening. Various animals
are emerging from the egg.
Fig. 26. Copy of Hartsoeker's figure of human spermatozoan. containing the homon-
culus or "little man," published in 1694. This figure represents a marked preformationist
conception of development. However, it is to be noted that Hartsoeker later abandoned
the preformationist concept as a result of his studies on regeneration.
54
THE OVARY AND ITS IMPORTANCE
55
individuals may arise by budding from a stolon-like base of the parent (fig.
27). This process often is called gemmation, the formation of a new indi-
vidual by a protrusion of a mass of cells from the parental body followed by
its partial or complete separation. It is a prominent method of reproduction
among the lower Metazoa, particularly the coelenterates and sponges. Never-
theless, all animal species among the Metazoa ultimately utilize an egg as
the primordium from which the new individual arises. Sexual reproduction,
generally associated with the fertilization of an egg by a sperm element, appears
to be a needful biological process.
True as the general statement made by Harvey may be, it is not clear what
is meant by the word ovum or egg. We know certain of its characteristics,
but, for the most part, it must be accepted as an accomplished fact enshrouded
in mystery. To Harvey the egg was an indefinite, unorganized association of
substance plus a "primordial generative principle" (see Cole, F. J., '30, p.
1 40 ) . Other minds have conceived of other meanings. Nevertheless, descriptive
and experimental embryology has forced the conclusion that the egg, during
its development within the ovary, experiences a profound process of differ-
entiation, resulting in the formation of an invisible organization. Although
STOMACH
ESOPHAGE AL
1 E N T 18 2
A R Y N X 2
H A GUS 2
—STOMACH
Fig. 27. Forms of asexual reproduction in the subphylum Urochordata of the phylum
Chordata. (From MacBride: Textbook of Embryology, Vol. 1, London. Macmillan.)
(A) Budding from 'stolon of Perophora listen, from MacBride after Hjort. (B), (C)
Two stages of budding in an ascidian, from MacBride after Pizon.
56 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
this organization is invisible, it is imbued with an invincibility which, when
set in motion at the time of fertilization, drives the developmental processes
onward until final fulfillment is achieved in the fully formed body of the
adult organism.
Beyond the fundamental changes effected in the developing egg while in
the ovary, the latter structure has still other roles to maintain. Through the
mediation of the hormones produced within the confines of the ovarian sub-
stance, the female parent is prepared to assume the responsibilities of repro-
duction. In addition, in many vertebrates the further responsibility of taking
care of the young during the embryonic period stems from the hormones
produced in the ovary. In some vertebrates, the instinct of parental care of
the young after hatching or after birth indirectly is linked to ovarian-pituitary
relationships. Because of these profound and far-reaching influences which
the ovary possesses in producing the new individual, it must be regarded as
the dynamic center of reproduction for most animal species.
B. Preformationism, Past and Present
The above statement relative to the importance of ovarian influences and
of the female parent is a position far removed from that held by some in the
past. An ancient belief elevated the male parent and his "seed" or semen.
As Cole, F. J., '30, p. 38, so aptly places the thinking of certain learned
sources during the 16th century: "The uterus is regarded as the 'till'd ground
for to sow the seeds on' — a popular idea, based obviously on the analogy
with plants, which prevailed long before and after this period. The seed of
the male is therefore the chief agent in generation, but cannot produce an
embryo without the cooperation of the female, and whether the result is male
or female depends on which side of the uterus the seed falls, the time of the
year, temperature, and the incidence of menstruation." Or, in reference to
the Leeuwenhoek's belief in an intangible preformationism. Cole, F. J., '30,
p. 57, states: "He asserts that every spermatic animalcule of the ram contains
a lamb, but it does not assume the external appearance of a lamb until it has
been nourished and grown in the uterus of the female." This statement of
A. van Leeuwenhoek (1632-1723) was made as a criticism of N. Hartsoeker
(1656-1725) whose extreme adherence to a seminal preformationism led
him to picture the preformed body of the human individual, the homonculus,
encased within the head of the spermatozoon (fig. 26). Hartsoeker, however,
later abandoned this idea.
In fairness it should be observed that the egg during these years did not
lack champions who extolled its importance. While the Animalculists consid-
ered the sperm cell as the vital element in reproduction, the Ovists, such as
Swammerdam (1637-80), Haller (1708-77), Bonnet (1720-93) and Spal-
lanzani (1729-99) believed that the pre-existing parts of the new individual
were contained or preformed within the egg.
REPRODUCTIVE SYSTEM OF THE FEMALE 57
An extreme form of preformationism was advocated by certain thinkers
during this period. For example. Bonnet championed the idea of encasement
or "emboitement." To quote from Bonnet:
The term "emboitement" suggests an idea which is not altogether correct. The
germs are not enclosed Hke boxes or cases one within the other, but a germ forms
part of another germ as a seed is a part of the plant on which it develops. This
seed encloses a small plant which also has its seeds, in each of which is found a
plantule of corresponding smallness. This plantule itself has its seeds and the latter
bears plantules incomparably smaller, and so on, and the whole of this ever
diminishing series of organized beings formed a part of the first plant, and thus
arose its first growths. (Cole, '30, p. 99.)
On the other hand, there were those who maintained that for some animals,
neither the sperm nor the egg were important as "many animals are bred
without seed and arise from filth and corruption, such as mice, rats, snails,
shell fish, caterpillars, moths, weevils, frogs, and eels" (Cole, '30, p. 38).
This concept was a part of the theory of spontaneous generation of living
organisms — a theory ably disproved by the experimental contributions of
three men: Redi (1626-97); Spallanzani; and Louis Pasteur (1822-95).
Modern embryology embraces a kind of preformationism, a preforma-
tionism which does not see the formed parts of the new individual within
the egg or sperm but which does see within the egg a vital, profound, and
highly complex physiochemical organization capable of producing a new in-
dividual by a gradual process of development. This organization, this self-
determining mechanism, is resident in the nucleus with its genes and the
organized cytoplasm of the fully developed oocyte or egg. However, as shown
later, this organization is dependent upon a series of activating agencies or
substances for its ultimate realization. Some of these activating substances
come from without, but many of them are produced within the developing
organism itself.
C. General Structure of the Reproductive System of the Vertebrate Female
1. General Structure of the Ovary
Morphologically, the ovary presents a series of contrasts in the different
vertebrate classes. In teleost fishes the size of the ovary is enormous compared
to the body of the female (fig. 28), while in the human (fig. 29), cow, sow,
etc., it is a small structure in comparison to the adult body. Again, it may
contain millions of mature eggs in the ling, cod and conger, during each breed-
ing season, wherons only a single egg commonly is matured at a time in the
cow, elephant, or human. During the reproductive season the ovary may
assume a condition of striking colored effects as in the bird, reptile, shark,
and frog, only to recede into an appearance drab, shrunken, and disheveled
in the non-breeding season.
58
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
ROCE SS
MUSCLE
RIAN SINU S
IRI 4N OR TAIL COE LOM
Fig. 28. Dissection of female specimen of the common flounder, Limanda jerruginea.
It particularly shows the ovary with its laterally placed ovarian sinus. Observe that the
ovary, during the breeding season, is an elongated structure which extends backward into
the tail. There are two ovaries, one on either side of the hemal processes of the caudal
vertebrae.
Its shape, also, is most variable in different species. In mammals it is a
flattened ovoid structure in the resting condition, but during the reproductive
phase it may assume a rounded appearance, containing mound-like protrusions.
In birds and reptiles it has the general form of a bunch of grapes. In the
amphibia it may be composed of a series of lobes, each of which is a mass
of eggs during the breeding season, and in teleost and ganoid fishes it is
an elongated structure extending over a considerable area of the body.
Regardless of their many shapes and sizes, the ovaries of vertebrates may
be divided morphologically into two main types, namely, compact and saccular
forms. The compact type of ovary is foimd in teleost, elasmobranch, cyclo-
stome, ganoid, and dipnoan fishes, as well as in reptiles, birds and mammals.
It has the following regions (figs. 30, 31 ):
( 1 ) the medulla, an inner zone containing relatively large blood and lymph
vessels;
(2) the cortex, an area outside of and surrounding the medulla (except
at the hilus), containing many ova in various stages of development;
(3) a tunica albuginea or connective-tissue layer surrounding the cortex;
and
(4) the germinal epithelium or the covering epithelium of the ovary.
The germinal epithelium is continuous with the mesovarium, the peritoneal
support of the ovary, and the particular area where the mesovarium attaches
to the ovary is known as the hilus. Within the mesovarium and passing
REPRODUCTIVE SYSTEM OF THE FEMALE
59
through the hilus are to be found the blood and lymph vessels which supply
the ovary (fig. 30).
The ovary of the teleost fish is a specialized, compact type of ovary adapted
to the ovulation of many thousands, and in pelagic species, millions of eggs
at one time. It has an elongate hilar aspect which permits blood vessels to
enter the ovarian tissue along one surface of the ovary, whereas the opposite
side is the ovulating area. In many teleosts the ovulating surface possesses
a special sinus-like space or lumen (fig. 28) which continues posteriad to
join the very short oviduct. At the time of ovulation the eggs are discharged
into this space and move caudally as the ovarian tissue contracts. In other
teleosts this ovulatory space is not a permanent structure but is formed only
at the time of ovulation. In Tilapia macrocephala, for example, the ovulatory
lumen is formed on the side of the ovary opposite the area where the blood
vessels enter. The formation of this space at the time of ovulation is described
by Aronson and Holz-Tucker ('49) as a rupture of the elastic follicles during
ovulation whereupon the follicle walls shrink toward the ovarian midline,
l-r— / — U T E RU
INFUNDIBULUM
FALLOPIAN TUBE
OVARY
RECTUM
URINARY BLADDER
BIG BONE
URETHRA
VA G I N A
CLITORIS
LABIUM MINUS
LABIUM MAJUS
Fig. 29. Diagrammatic representation of a midsagittal section of the reproductive
organs of the human female. (Slightly modified from Morris: Human Anatomy, Phila-
delphia, Blakiston.)
60
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUC 1 ION
SECONDARY FOLLICLE
PRIMARY FOLLICLE
GERMINAL EPITHELIUM
wyEGG NEST
MESOVARIUM-
MEDULLA "^ (i "
ANTRAL VACUOLE
TERTIARY FOLLICLE
TUNICA ALBUGINEA
MATURE
FOLLICLE
CORP
ALBICANS
FLUID- FILLED
ANTRUM
CORPUS LUTEUM
OVUM
CONNE CT VE TISSUE
RUPTURED FOLLICLE
OVUM WITH CUMULUS
CELLS
FOLLICULAR FLUID
Fig. 30. Schematic three-dimensional representation of the cyclic changes which occur in
the mammalian ovary.
carrying the interstitial tissue and immature ova. This shrinking away of the
tissues of the ovary leaves a space between these tissues and the outside
ovarian wall. A lumen thus is formed along the lateral aspect of the ovary
which is continuous with the oviduct. Many teleosts have two ovaries (e.g.,
flounder); in others there is but one (e.g., perch).
The amphibia possess a true saccular ovary (fig. 32). It has a cortex and
germinal epithelium somewhat similar to the compact ovarian variety, but
the area which forms the medulla in the compact ovary is here represented
by a large lymph space. During early development, the amphibian ovary is
a compact structure, but later there is a hollowing out and disappearance of
the compact medullary portion, and the cortical area remains as a relatively
thin, peripheral region (Burns, '31; Humphrey, '29).
Histologically the vertebrate ovary is composed of two general cellular
groups, namely:
( 1 ) germ cells, and
(2) general tissue cells of various kinds, such as epithelium, connective
tissue, smooth muscle fibers, and the complex of elements compris-
REPRODUCTIVE SYSTEM OF THE FEMALE
61
ing the vascular system of the ovary (figs. 30, 32). Some of the general
cells form the so-called interstitial tissue of the ovary.
The germ cells differ from the general cells in that each of them has a
latent potency for developing a new individual. This latent condition is con-
verted into active potentiality during the differentiation of the primitive germ
cell into the mature egg or ovum.
2. General Structure of the Accessory Reproductive Organs
The accessory reproductive structures of the female vertebrate may be
separated into three general types, viz.:
( 1 ) the total absence of or the presence of a pair of short funnel-like
structures which convey the eggs from the peritoneal cavity through
MESOVARIUM
STALK (PEDIC
OP FOLLIC
VASCULAR AREA
OF FOLLICLE
COLLAPSED EG
FOLLICLE FROM
WHICH EGG H
BEEN OVULATE
CONNECTIV E
INTERSTITIAL T I
RUPT URED CIC
CONNECTIVE
INTERSTITIAL
Fig. 31. Three-dimensional representation of the bird ovary together with the funnel
portion (infundibulum) of the oviduct. Recently ovulated egg is shown in the process of
engulfment by the infundibulum. Various stages of developing eggs are shown.
62
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
AVASCULAR AREA OF FOLLICLE
VASCULAR AREA OF FOLLICLE
3,£/^ ^IMMATURE FOLLICLE
"^ ' ~^ '' ' ^~" \RIAN
CAVITY
BLOOD
SUPPLY
HI LUS
Fig. 32. Anterior half of the saccular ovary of Nectiirus maculosus.
an opening into the urogenital sinus and thence to the outside as in
cyclostome fishes,
(2) a short sinus-like tube attached to each ovary and to the urogenital
sinus or to a separate body opening as in many teleost fishes (fig.
28), and
(3) two elongated oviducal tubes variously modified (figs. 29, 33, 34,
35, 36, 37).
Except in the teleost fishes the cephalic end of each oviduct generally is
open and is placed near the ovary but not united directly with it (figs. 29,
33) although in some species, such as the rat, it is united with an ovarian
capsule (fig. 37). In some vertebrates the anterior orifice of the oviduct may
be located a considerable distance from the ovary, as in frogs, toads, and
salamanders. In many vertebrates, as in birds and snakes, there is but one
oviduct in the adult.
In some vertebrates the oviduct is an elongated glandular tube, as in certain
urodele amphibia (fig. 33) and in ganoid fishes; in others, such as frogs,
birds or mammals, it is composed of two main parts: ( 1 ) an anterior glandular
structure and (2) a more caudally placed uterine portion. The latter may
unite directly with the cloaca, as in the frog (fig. 38) or by means of a third
portion, the vaginal canal or vagina located between the uterus and the
cloaca, as in elasmobranch fishes, reptiles, and birds, or between the uterus
and the external urogenital sinus, as in mammals (figs. 35, 36, 37). The
vaginal canal may be single, as in eutherian mammals, or double, as in meta-
therian mammals (figs. 35, 36). In metatherian (marsupial) mammals it
appears that a third connection with the oviducts is made by the addition
of a birth passageway. This birth canal represents a secondary modification
of a portion of the vaginal canals and associated structures (figs. 34, 35, 114).
(See Nelsen and Maxwell, '42.) One of the main functions of the vagina or
vaginal canal is to receive the intromittent organ of the male during copulation.
REPRODUCTIVE SYSTEM OF THE FEMALE
63
The anterior opening of the oviduct is the ostium tubae abdominale, a
funnel-shaped aperture generally referred to as the infundibulum. In the
transport of the egg from the ovary to the oviduct the infundibulum, in
many species, actually engulfs and swallows the egg.
The portion of the oviduct anterior to the uterus often is called the con-
voluted glandular part; it is highly twisted and convoluted in many species.
In amphibians, reptiles, birds, and in some mammals the glandular portion
OUTH OF
VI DUCT
OVIDUCT
OOCYTE
VI DUG T
E CTUM
I NFUND I B U L U M,
OVIDUCT-
URINARY
OVARY
BLO OD URETHRA
S UPPLY
-MEDIAN PORTION OF
LATERAL VAGINAL
CANAL
UROGENITAL SINUS
Fig. 33
Fig. 34
Fig. 33. Diagrammatic representation of the reproductive structures of female urodele,
Nectiirus maculosus.
Fig. 34. Diagrammatic lateral view of female reproductive system of the opossum,
showing pseudo-vaginal birth canal.
64
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
OVIDUCT
L LO P I AN
TUBE)
AT E R AL
AL CANAL
VA6 I N AL
E -SAC
URINARY
BLADDER
UR 0 G EN I TAL SINUS
Fig. 35. Reproductive structures of female opossum shown from the ventral view. Ob-
serve that the ovary and infundibular portion of the Fallopian tube lie dorsal to the horn
of the uterus.
functions to secrete an albuminous coating which is applied to the egg during
its passage through this region. In amphibians, reptiles, and birds it forms
the major portion of the oviduct, but in mammals it is much reduced in size
and extent. In the latter group it is referred to as the uterine or Fallopian tube.
The uterus is a muscular, posterior segment of the oviduct. Like the
anterior glandular portion of the oviduct, it also has glandular functions, but
these are subservient to its more particular property of expanding into an
enlarged compartment where the egg or developing embryo may be retained.
The protection and care of the egg or of the embryo during a part or all of
its development, is the main function of the uterus in most vertebrates. In the
frogs and toads, however, this structure seems to be concerned with a "ripen-
ing" process of the egg. Large numbers of eggs are stored in the uterine sac
of the frog for a period of time before spawning.
Various degrees of union between the uterine segments of the two ovi-
ducts are found in mammals. In the primates they fuse to form a single
uterine compartment with two anterior uterine tubes (fig. 29). In carnivores,
there is a caudal body of the uterus with two horns extending forward to
unite with the uterine tubes (fig. 36). In the rat and mouse, the uterine seg-
ments may be entirely separate, coming together and joining the single vaginal
chamber (fig. 37). In the opossum the uterine segments are entirely separated,
joining a dual vaginal canal system posteriorly (figs. 34, 35, 114).
DEPENDENCY OF FEMALE REPRODUCTIVE SYSTEM ON BODY CONDITIONS
65
D. Dependency of the Female Reproductive System on General Body
Conditions
1. Inanition
In the immature female mammal continued underfeeding results in general
retardation of sexual development. The younger follicles may develop, but
the later stages of follicular development are repressed. In the adult female,
inanition produces marked follicular degeneration and atresia as shown by
many records of retarded sexual development, reduced fertility, even cessation
of the cyclic activities of menstruation and estrus occurring in man and
domestic animals during war-produced or natural famine (Mason in Allen,
Danforth, and Doisy, '39, p. 1153). The ovary thus seems to be especially
susceptible to starvation conditions, even more so than the testis. As the
condition and well-being of the secondary reproductive structures are de-
pendent upon proper ovarian function, this part of the reproductive system
suffers marked changes as a result of ovarian dysfunction during prolonged
starvation.
SUSPENSORY
LI6 A MEN T
HORN OF
UTERUS
BROAD
LIGAMENT
URINARY
BLADDER
OS UTERI
UROGENITAL
SINUS
Fig. 36. Schematic representation of reproductive organs of the female cat. On the left
side of the illustration, the body of the uterus and uterine horn have been cut open, and
the Fallopian tube and ovary are highly schematized. Observe the partial ovarian capsule
around the ovary shown on the right and the relatively fixed condition of the infundibular
opening of the oviduct lateral to the ovary.
66
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
CLITORIDEAN
(PREPUTIAL)
GLAND
EXTERNAL
VAGINAL ORIFICE
(^TiA-OVI DUCT
UTERUS
CLOACA
Fig. 37
Fig. 38
Fig. 37. Diagrammatic representation of the reproductive organs of the female rat,
showing the bursa ovarica around each ovary. Observe that uteri open directly into the
vagina. (Modified from Turner, '48.)
Fig. 38. Diagrammatic representation of reproductive structures of the female frog.
Observe that the ostium of the oviduct is not an open, moulh-like structure. It remains
constricted until the egg starts to pass through.
2. Vitamins
a. Vitamin A
The ovary is not immediately sensitive to a lack in vitamin A in the diet
but general epithelial changes in the reproductive tract occur which may aid
in producing sterility (Mason, '39).
b. Vitamin B
Ovarian and uterine atrophy occur as a result of deficiency of this vitamin
in monkey, rabbit, mouse and rat (Mason, '39). This effect may be mediated,
at least partly, through the effect of B-deficiency upon the pituitary gland.
c. Vitamin C
During the earlier stages reproductive activity is maintained, but advanced
stages of C-deficiency produce regressive effects (Mason, '39).
ACTIVITIES OF THE OVARY 67
d. Vitamin E
E-deficiency in the female rat does not upset the ovarian and general repro-
ductive behavior. However, established pregnancies are disturbed and are
terminated by resorption of the embryo (Mason, '39). In the domestic fowl,
unless sufficient amount of vitamin E is present in the egg, embryonic death
occurs during early incubation periods of the egg.
3. The Hypophysis (Pituitary Gland)
The ovaries experience pronounced atrophy as a result of hypophysectomy
in mammals and non-mammalian species. The earlier stages of follicle for-
mation in the higher mammalian ovary up to the stage of beginning antrum
formation are not so much affected, but later follicular development and inter-
stitial tissue growth are inhibited (Smith, P. E., '39). (See fig. 40.)
E. Activities of the Ovary in Producing tiie Reproductive State
1. The Ovary as a "Storehouse" of Oogonia
The cortex of the ovary contains many young ova in various stages of de-
velopment. In the human ovary shortly after birth, the number of oogonia
in the cortex of each ovary has been estimated to reach a number as high
as 300,000. This figure should not be taken too literally, as the amount of
variability in the ovary from time to time is great and degeneration of ova
is a common episode. Haggstrom ('21 ) estimated that each ovary of a 22-year-
old woman contained 200,000 young ova. In the ovaries of young rats, Aral
('20, a and b) estimated that there were on the average around 5,000 ova
under 20 /x in diameter.
Without entering into the controversy (Chap. 3) relative to the rhythmic
origin of germ cells in the ovary, one must accept the conclusion that the
normal ovary has within it at all times during its reproductive life large num-
bers of oogonia in various stages of development. Thus the ovary, aside from
its other activities, functions as a storehouse and nursery for young oogonia.
Relatively few of these oogonia develop into mature eggs in the mammals.
For example, the reproductive life of the human female occurs from about
the age of 10 or 14 years to about 48 years. If one egg per monthly cycle
is discharged from the ovary which is functional during that cycle, only about
400 eggs would be matured in this way. The number would be less if preg-
nancies intervened. If one accepts the figures given by Haggstrom, an enor-
mous number of eggs of the human ovary never reach their potential goal.
Similarly, according to Corner ('43) : "The most prolific egg producer among
mammals, the sow, might possibly shed a total of 3,000 to 3,500 eggs, allow-
ing ten years of ovarian activity not interrupted by pregnancy, and assuming
the very high average of 20 eggs at each three weekly cycle, but she has vastly
more than this in the ovaries at birth."
68
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
2. Position Occupied by the Primitive Female Germ Cells
IN THE Ovarian Cortex
Within the cortex the definitive germ cells or oogonia are found in or near
the germinal epithelium (figs. 39, 64). Some authors regard the oogonium
as originating from the cells of the germinal epithelium. (See Chap. 3, section
on "germ cell origin.") The definitive germ cell soon becomes associated
with small epithelial cells (fig. 41). This complex of a germ cell with its
associated epithelial cells is found somewhat deeper in the cortex, within or
below the tunica albuginea. As the oogonium begins to experience the changes
propelling it toward a state of maturity, it is regarded as an oocyte (Chap. 3).
Characteristics of the primitive oocyte are:
(1) an enlargement of the nucleus,
(2) changes within the chromatin material of the nucleus pertaining to
meiosis (Chap. 3), and
(3) a growth and increase in the cytoplasmic substances (fig. 41).
PROLIFERATING
GERM CELL
Fig. 39. (A) Diagrammatic representation of portion of the cortex of a young opossum
ovary near the hiius, showing origin of germ ceils from germinal epithelium or from
cells lying in or near the germinal epithelium of the ovary. (After Nelsen and Swain,
J. Morphol., 71.) (B) Young oocytes in rat ovary, lying in or near the germinal epi-
thelium of the ovary. (After Jones. J. Morphol., 84.)
,S(jejs ""e « J, " V c^a o j' ■■J a u ~ ^'.^s * -„<-'?Ai
Fig. 40. Effects produced by hypophysectomy on the rat ovary and of replacement
therapy utilizing injections of pituitary gonadotrophins. (After Evans, Simpson, and
Penchaez: Symposia of Quantitative Biology, Vol. 5, 1937. The Biological Laboratory,
Cold Spring Harbor, L. I., N. Y.) (A) Ovary of hypophysectomized animal. Observe
that Graafian follicles are small. They do not proceed further in their development than
the beginning of antral vacuole formation unless replacement therapy is applied. (B)
Ovarian condition of hypophysectomized animal receiving replacement therapy in the
form of injections of the LH (ICSH) gonadotrophic factor of the anterior lobe of the
hypophysis. Interstitial tissue is well developed. (C) Ovarian condition of hypophysec-
tomized animal receiving the FSH gonadotrophic factor. Note follicular growth and antral
vacuole formation; interstitial tissue between the follicles remains somewhat deficient.
(D) Ovarian condition of hypophysectomized animal receiving injections of FSH plus
LH. Corpora lutea are evident (as well as enlarged follicles not shown in the figure).
Interstitial tissue remains deficient.
69
70
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
(^>
<S>^.^fi' Q
Fig. 41. Development of primary condition of the Graafian follicle in the opossum
ovary. (A) Young oocyte with associated epithelial (granulosa) cells which in (B)
have encapsulated the oocyte. (C) Encapsulating granulosa cells have increased in
number and are assuming a cuboidal shape. (D) Fully developed condition of the
primary Graafian follicle. Cf. secondary condition shown in fig. 42.
Fig. 42. Secondary conditions of the Graafian follicle in the opossum ovary. Cf. that of
the rat ovary in fig. 40.
As these changes are initiated, the associated epithehai cells increase in
number and eventually encapsulate the oocyte (fig. 41B). This complex of
the oocyte with its surrounding layer of follicle cells is known as an egg follicle.
3. Primary, Secondary, and Tertiary Follicles of de Graaf
In the mammalian ovary the developing egg with its associated cells is
called the Graafian follicle, so named after the Dutch scientist, Reinier de
Graaf (fig. 1 ), who first described this structure in mammals in 1672-1673.
De Graaf was in error, partly, for he believed that the whole follicular com-
plex was the egg. The mammalian egg as such was first described in 1827
ACTIVITIES OF THE OVARY
71
by Karl Ernst von Baer (1792-1876). The following statement is taken from
de Graaf relative to egg follicles.
We may assert confidently that eggs are found in all kinds of animals, since
they may be observed not only in birds, in fishes, both oviparous and viviparous,
but very clearly also in quadrupeds and even in man himself. Since it is known to
everyone that eggs are found in birds and fishes, this needs no investigation; but
also in rabbits, hares, dogs, swine, sheep, cows, and other animals which we have
dissected, those structures similar to vesicles exhibit themselves to the eyes of the
dissectors like the germs of eggs in birds. Occurring in the superficial parts of
the testicles, they push up the common tunic, and sometimes shine through it, as
if their exit from the testis is impending. (See fig. 48; also Corner, '43, page 128.)
The mammalian egg with a single layer of epithelial cells surrounding it
is known as a primary Graafian follicle (fig. 41B-D). As the egg and follicle
grow, the number of epithelial cells increase and eventually there are several
AND
THECA
EXTERNA
DEVELOPING EGG
THE CUMULUS OOPHORUS
C.
Fig. 43. Tertiary conditions of the Graafian follicle in the opossum ovary. Similar con-
ditions are found in other mammalian ovaries. (A) Follicle in which the antral vacuoles
are beginning to form. (B) This is a follicle in which the antral vacuoles are more
numerous and are beginning to coalesce. (C) Condition of the Graafian follicle in the
opossum ovary approaching maturity. Observe that the antral space is large and is filled
with fluid, the liquor folliculi, while the egg and its surrounding cumulus cells are located
at one end of the follicle. The thecal tissue around the follicle is well developed.
72
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
CAVITY OF FOLLICLE
GRANULOSA CELLS
BASEMENT MEMBRANE
THECA INTERNA
CAPILLARY
THECA EXTERN A
Fig. 44. Cellular wall of the mature Graafian follicle in the opossum.
layers of epithelial or granulosa cells surrounding the egg. It may now be
regarded as a secondary Graafian follicle (fig. 42A, B). When a stage is reached
where the granulosa cells form a layer five to seven or more cells in thickness
extending outward from the egg to the forming thecal layers, small antral
vacuoles begin to appear among the granulosa cells. The latter follicle, which
is capable of forming antral vacuoles, may be regarded as a tertiary Graafian
follicle (fig. 43 A).
4. Hormonal Factors Concerned with the Development
OF Egg Follicles
The ovary with its contained egg follicles is greatly affected by the gona-
dotrophic hormones produced in the pituitary body. The removal of the pi-
tuitary body (hypophysectomy) causes profound regression of the ovary and
accessory reproductive structures. Accordingly, the response of the ovarian
tissues to these hormonal substances produced by the hypophysis is responsible
for development of the Graafian follicle beyond the early tertiary stage. (See
fig. 40A.) The relationships between the pituitary hormones and the ovary
have been studied most intimately in the mammals; the pituitary and egg-
follicle relationship in lower vertebrates is more obscure, and probably varies
with the particular group.
a. Effects Produced by the Gonadotrophic Hormones on the
Development of the Mammalian Egg Follicle
The follicle-stimulating hormone, FSH, appears to increase the number
of oogonia and to aid the growth and differentiation of the older follicles. It
is possible that some of the effects of FSH upon follicular growth are medi-
ated through its ability, together with small amounts of the luteinizing hor-
mone, LH (ICSH), to cause the formation of estrogen or the female sex
ACTIVITIES OF THE OVARY 73
hormone, although some investigators beUeve that estrogen production de-
pends mainly upon the action of LH (ICSH). (See Evans and Simpson in
Pincus and Thimann, '50, p. 355.) In harmony with the idea that estrogen
is involved in follicular growth there is some evidence which suggests that
introduction of estrogens into the peritoneal cavities of fishes and mammals
results in a stimulation of mitotic activity in the germinal epithelium of the
ovary. It also has been shown that estrogenic substances retard ovarian atrophy
in hypophysectomized immature rats.
When the Graafian follicles of the mammalian ovary reach the proper
morphological and physiological conditions (i.e., when they reach the tertiary
follicular stage) an increased sensitivity of the follicle cells to FSH occurs.
As a result, antral vacuoles filled with fluid appear among the granulosa cells;
these eventually coalesce and form the large antral cavity typical of the
mature Graafian follicle of the metatherian and eutherian mammal (fig. 43).
The presence of LH (ICSH) is necessary to augment the action of FSH
during the latter part of follicle development. The beneficial action of FSH
and LH together in later follicular development is shown by the fact that the
injection of pure FSH alone is incapable of stimulating growth of the follicle
to its full size or to initiate an increased secretion of estrogen. LH aids the
maturing process of the follicle only when present in very minimal amounts
during the early stages of follicle development and in larger amounts during
the later stages of follicular growth. Large amounts of LH in the earlier phases
of the follicle's development bring about a premature luteinization of the
follicle with ultimate atresia. A proper quantitative balance of these hormones,
therefore, is necessary, with FSH being in the ascendency during the earlier
phases of follicle development, followed by increased amounts of LH with
decreasing amounts of FSH as the follicle reaches maturity (figs. 22, 53, 59).
(For references, consult Evans and Simpson, '50; Turner, '48.)
b. Stimulating Effects of the Pituitary Gonadotrophins on the Ovaries
of Other Vertebrates
The hormonal control of. the developing follicle of other vertebrate ovaries
follows similar principles to those outlined above for the mammalian ovary,
although data obtained from studies upon other vertebrates in no way com-
pares with the large quantity of information obtained in mammalian studies.
In the hen, FSH and LH injected together cause a rapid development of the
follicles and premature discharge of the egg from the ovary (Fraps, Olsen,
and Neher, '42). However, in the pigeon. Riddle ('38) reports that another
pituitary hormone, prolactin, appears to decrease the production of these
hormones and stops egg production with a subsequent atrophy of the ovary.
This may be a special means which reduces the number of eggs laid at each
nesting period. In regard to accessory reproductive structures, an estrogenic
hormone is produced in the ovary of the hen which has profound stimulating
74 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
effects upon the growth of the oviduct (Romanoff and Romanoff, '49, pp.
242-244). In the frog, Rana pipiens, mammalian pituitary gonadotrophins
are able to effect ovulation (Wright and Hisaw, '46). Pituitary gonadotrophins
have been shown also to have profound stimulative effects on the ovaries of
fishes, salamanders, and reptiles.
5. Structure of the Vertebrate, Mature Egg Follicle
As a result of the differentiation and growth induced by the gonadotrophic
hormones of the anterior lobe of the hypophysis described in the preceding
paragraphs, the egg follicle reaches a state of maturity (fig. 43C). This state
is achieved when the follicle is about to rupture with the resultant discharge
of the egg. The size of the mature egg follicle varies greatly in different meta-
therian and eutherian mammals, although the size of the follicle is not related
to the size of the egg. On the other hand the size of the mature egg follicle
in prototherian mammals and in other vertebrate species shows great diver-
gences, being dependent in this group upon the size of the egg at the time
of ovulation (fig. 46).
a. Structure of the Mature Follicle in Metalherian and Eutherian
Mammals*
The structural pattern of the mature Graafian follicle in the human is
strikingly similar to the follicles in other members of this group. It is a vesicular
structure with a diameter approximating five millimeters. Externally, the fol-
licle is composed of two connective-tissue layers, an inner cellular layer con-
taining blood capillaries, the theca interna, and an external, fibrous layer,
the theca externa (figs. 43C, 44). These two layers are not clearly separable.
Passing inward from the theca interna is the basement membrane. Resting
upon this membrane are several layers of epithelial cells comprising the
membrana granulosa. The latter membrane borders the cavity or antrum of
the follicle, which is filled with the liquor folliculi. This liquid is under con-
siderable pressure in the follicle at the time of egg discharge or ovulation.
Projecting inward into the antrum on one side is a small, mound-like
mass of granulosa cells, the cumulus oophorus (fig. 43C). Within this hillock
of epithelium, is the egg, which measures in the human about 130 p. to 140 /x
in diameter. In the opossum, the fully developed Graafian follicle is about
1.25 by 2 mm. in diameter, while the slightly oval egg approximates 120 by
135 IX. The egg of the rat and mouse is small, having a diameter of 75 ^i,
while that of the dog is about 140 y^\ sow, 120 to 140 n\ rabbit, 120 to 130 /x;
monkey, 1 10 to 120 ^; deer, 1 15 /x; cat, 120 to 130 /x; mare, 135 /x; arma-
dillo, 80 /x (Hartman, '29).
* According to Strauss, '39, the mature Graafian follicle of Ericulus is not a vesicular
structure, as in other higher mammals, but is filled with a loose meshwork of granulosa
cells.
ACTIVITIES OF THE OVARY
75
While one Graafian follicle in only one ovary is generally developed in
the human, monkey, cow, ewe, elephant, etc., at each reproductive period,
a multiple condition is found in many other mammals. Each ovary in the
opossum may ripen seven or more follicles, in the bitch (female dog) from
2 to 7 follicles, and in the sow from 4 to 10 follicles at each reproductive period.
b. Structure of the Prototherian Egg Follicle
The follicle of the prototherian mammals contains a relatively large egg,
while the surrounding fluid and follicular tissue in comparison is small in
quantity (fig. 46). In these mammals the egg fills most of the follicular cavity,
with the exception of a small fluid-filled space intervening between it and
the zona pellucida which lies contiguous to the granulosa cells. Internal and
external thecal tissues surround the granulosa cells as in the Graafian follicle
of the higher mammals.
c. Egg Follicles of Other Vertebrates
The fully-developed egg follicle in most vertebrates is similar to that found
in the prototherian mammals in that the egg tends to fill the entire follicle.
The general structural relationships also are similar (figs. 45, 47).
6. Ovulatory Process; Possible Factors Controlling Ovulation
The following description of the ovulatory process in the mammal and in
other vertebrates should not be construed as a description of the mechanism,
as the exact mechanism is unknown. However, a certain amount of general
information has been obtained concerning ovulation and the factors involved.
Much of this information has been obtained from studies of the ovulatory
THECA INTERNA
CONTRACTED FOLLICLE
AFTER EGG DISCHARGE
OC ES S
I ON
Fig. 45. (A) Young egg follicle of Cryptohranchus alleganiensis, a urodele. (From
Noble: "Biology of the Amphibia," New York, McGraw-Hill, after Smith.) (B) Dia-
grammatic representation of ovarian events in the frog resulting in egg discharge. (From
Turner: "General Endocrinology," Philadelphia, W. B. Saunders, slightly modified.)
76
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
GERMINAL
VESI CLE
OOPLASMIC
MEMBRANE
PERI VITELLINE
SPACE
ZONA PE LLUCIDA
FO LLICU LAR
EPITHE LIUM
THE C A INTER N A
THECA EXTERNA
Fig. 46. Diagrammatic representation of the egg of the prototherian mammal. Echidna.
GERMINAL VESICLI
NUCLEUS OF PAN DE R
N IC A
ALBUGI N E A
THECA EXTERNA
THECA INTERNA
GRANULOSA
LAYER
ZONA RADIATA
CLEAR CYT OPLASM
YOLK SPHERE
Fig. 47. Diagrammatic drawings of the pendent egg follicle in the ovary of the hen.
(A) Low magnification of the entire egg follicle. (B) More detailed view of the blasto-
disc portion of the egg, nearing maturity, in relation to the pedicle. The latter supports
the follicle and permits the blood vessels to pass into and out of the follicle. Compiled
from sections of the developing ovary of the hen.
process in higher mammals, especially the rabbit. Among other vertebrates
ovulation in the hen and frog have been the objects of considerable study.
a. Process of Ovulation in Higher Mammals
1) Changing Tissue Conditions Culminating in Egg Discharge from the
Ovary. As the Graafian follicle enlarges and matures under the influence of
ACTIVITIES OF THE OVARY
77
the follicle-stimulating and luteinizing hormones, it moves closer to the ovarian
surface (fig. 30). The surface of the ovary over the ripening follicle bulges
outward, forming a mound-like protuberance (fig. 30). In the rabbit as shown
by Walton and Hammond ('28) and Hill, Allen, and Cramer ('35) the cen-
tral part of the original protuberance pushes out still further and forms a
papilla-like swelling (fig. 48A-D). As the papilla develops, it becomes avas-
BULGING WALL OF GRAAFIAN
FOLLICLE FROM OVARIAN SURFACE
ftJf^S.
Fig. 48. Process of ovulation in the rabbit. (A-C) Early external changes of the
surface of the ovary overlying the bulging Graafian follicle. (D) Formation of a sec-
ondary papilla. (E) Rupture of the secondary papilla with discharge of egg and folli-
cular fluid, the latter oozing down over ovarian surface of the follicle. (F) Area of
rupture with oozing follicular fluid and egg greatly magnified. (G) Follicle after egg
discharge. (A-E and G, slightly modified from Walton and Hammond, Brit. J. Exp.
Biol., 6; F, modifiec from Hill, Allen, and Kramer, Anat. Rec, 63.)
78 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
cular, and the underlying tissues become thin and greatly distended. The
tunica albuginea of the ovary and the two thecal layers of the follicle also
are involved in this thinning-out process. As the distended papillary area
continues to grow thinner, a small amount of blood followed by some of the
follicular fluid containing the egg emerges from the follicle and passes into
the surrounding area in close proximity to the infundibulum of the Fallopian
tube (fig. 48E, F). The entire process is a gradual one and may be described
as gently but not violently explosive (Hill, Allen, and Cramer, '35). It is of
interest and significance to observe that Burr, Hill, and Allen (35) were able
to detect a change in electromotive force preceding and during the known
period of ovulation.
The process of papillary rupture in the rabbit occupies about five seconds;
egg discharge with the surrounding liquor folliculi occurs in approximately
30 to 60 seconds. After the egg has emerged, the follicle as a whole may
collapse. The slit-like opening through which the egg and follicular fluid
passed during ovulation soon is filled with a clot composed of coagulated
blood and follicular fluid (fig. 48G).
While the foregoing processes, visible on the ovarian surface, are consum-
mated, certain internal changes occur which form a part of the ovulatory pro-
cedure. These changes are as follows: At about the time the egg is to be
extruded, the follicular fluid reaches its maximum in quantity. This increase
produces considerable follicular turgidity which may be associated with an
endosmotic effect due to an increase in the salt content of the contained fluid.
Shortly before the surface of the follicle ruptures, the cumulus begins to dis-
integrate, and the egg lies free in the antral fluid. At about this time the first
maturation division of the oocyte occurs in the majority of mammals, and
the first polar body is extruded.
Concerning the internal changes accompanying rupture of the mammalian
follicle, passing mention should be made of the theory that bursting blood
vessels discharge their contents into the follicular fluid and thus cause suffi-
cient pressure to rupture the follicle (Heape, '05). Considerable blood dis-
charge into the follicle seems to be present in some forms, e.g., the mare,
quite absent in others such as the human, and present slightly in the opossum.
2) Hormonal Control of the Ovulatory Process. The hormonal mechanism
involved in ovulation in the spontaneously-ovulating mammals probably is as
follows: The follicle-stimulating hormone causes the growth and development
of the follicle or follicles. Estrogen is released by the growing follicles and
possibly by other ovarian tissues due to the presence of small amounts of LH,
and, in consequence, the estrogenic hormone reaches a higher level in the
blood stream (figs. 53; 59).
In the meantime, it is probable that the corpus luteum hormone, proges-
terone, is produced in small amounts. The exact source of this hormone is
not clear. It may be produced by old corpora lutea or by the interstitial tissue
ACTIVITIES OF THE OVARY 79
of the ovary under the influence of luteotrophin, LTH. The presence of
progesterone, in small quantities together with increasing amounts of estro-
gen, stimulates the anterior lobe to discharge increased amounts of the luteiniz-
ing hormone, LH (ICSH). (S,ee figs. 22, 53, 59.) The elevated level of
estrogen, according to this theory also causes a decreased output of FSH until
it reaches a minimal level at the period shortly before egg discharge (figs.
53, 59). As a result, the increased quantity of LH together with FSH has an
added effect upon the follicle which brings about the chain of events leading
to egg discharge. Evans and Simpson in Pincus and Thimann ('50) give the
proportion of 10 parts of FSH to 1 of LH (ICSH) as the proper hormonal
balance in effecting ovulation in the hypophysectomized rat.
In those mammalian species where ovulation is dependent upon the act of
copulation, a nervous stimulus is involved which increases the output from
the pituitary gland of the gonadotrophic factors, particularly LH.
b. Ovulation in Vertebrate Groups Other Than the Higher Mammals
The physical mechanism involved in the ovulatory procedure in the lower
vertebrate classes is different from that found in higher mammals. Two forms,
the hen and the frog, have been studied in detail. These two animals represent
somewhat different types of ovulatory behavior.
1) Hen. As the hen's egg develops in the ovary, it gradually pushes the
ovarian surface outward; it ultimately becomes suspended from the general
surface of the ovary by means of a narrowing stalk, the pedicle (figs. 31, 47).
When the ovulatory changes are initiated, the musculature of the ovarian
wall overlying the outer surface of the egg appears to contract, and an elon-
gated narrow area along this outer surface becomes avascular. This avascular
area represents the place where the ovarian surface eventually ruptures to
permit the egg to leave the ovary; it is called variously, the rupture area,
stigma, or cicatrix. Gradually, the cicatrix widens and finally a slit-like open-
ing is formed by a tearing apart of tissues in the central region of the cicatrix.
Contractions of the smooth muscle fibers appear to be responsible for this
tearing procedure (Phillips and Warren, '37). The egg eventually is expelled
through the opening and in many instances it rolls into the infundibular funnel
of the oviduct which at this time is actively engaged in an endeavor to engulf
or "swallow" the egg (fig. 31).
2) Frog. The egg of the frog projects into the ovarian cavity within the
ovary and is attached to the ovarian wall by means of a broad area or stalk
(fig. 45B). As the egg enlarges, it tends to push the ovarian surface outward,
and the egg and its follicle thus forms a mound-like protuberance from the
ovarian surface (figs. 45A, B; 72F). The egg and the surrounding ovarian
tissue thus lies exposed on one aspect to the outer surface of the ovary. The
outer surface of exposure is the stigma or area of rupture, and in the older
follicles this area does not contain blood vessels (fig. 72F). As ovulation
80 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
approaches, an opening suddenly appears in the area of rupture. The mus-
culature within the theca interna around the foUicle then contracts, and the
egg rolls out through the opening in the rupture area like a big ameba (fig.
45B). As the egg passes through the aperture, it may assume an hourglass
shape (Smith, B. G., '16). After the egg is discharged, the follicle contracts
to a much smaller size (fig. 45B). It has been suggested that the rupture of
the external surface of the follicle might be produced by a digestive enzyme
(Rugh, '35, a and b).
3) Hormonal Control of Ovulation in Lower Vertebrates. The hormonal
mechanism regulating ovarian rupture and egg discharge in the lower verte-
brate groups has not been as thoroughly explored in all of the vertebrate
groups as it has in the mammals. However, sufficient work has been done to
demonstrate that pituitary hormones are responsible in all of the major verte-
brate groups, including the fishes. Amphibian pituitary implants under the
skin or macerated anterior-lobe pituitary tissue injected into the peritoneal
cavity of various amphibia have been effective in producing ovulatory phe-
nomena (Rugh, '35a). More recently, purified mammalian follicle-stimulating
hormone, FSH, and luteinizing hormone, LH, have been used to stimulate
egg discharge in frog ovarian fragments, as well as in normal and hypophysec-
tomized females. However, the follicle-stimulating hormone alone will not
elicit ovulation (Wright, '45; Wright and Hisaw, '46). Accordingly, both
factors are necessary in the frog, as in mammals. In the hen, these two pi-
tuitary hormones have been shown to bring about ovulation when injected
intravenously (Fraps, Olsen, and Neher, '42; Romanoff and Romanoff, '49,
pp. 208-215). Also, Neher and Fraps ('50) present evidence which suggests
that progesterone plays a part in the physiological chain which elicits ovulation
in the hen. A close relationship between the physiological procedures effecting
ovulation in the hen and the mammal thus appears to exist.
c. Comparison of the Immediate Factors Effecting Egg Discharge in the
Vertebrate Group
In the vertebrates thus far studied contraction of muscle tissue of the fol-
licle following the rupture of surface tissues presumably is the main factor
which brings about egg expulsion. In higher mammals, associated with muscle
contracture, there also may be an increase in follicular turgidity due to endos-
motic phenomena associated with the contained follicular fluid (Walton and
Hammond, '28). In the frog, hen, and mammal the changes involved in the
surface tissues leading to their rupture are associated with the following se-
quence of events:
( 1 ) avascularity of the surface tissues,
(2) a thinning of the surface tissues, and finally
(3) a rupture of these tissues.
ACTIVITIES OF THE OVARY 81
7. Internal Conditions of the Ovary as an Ovulatory Factor
Internal conditions of the ovary undoubtedly are important in controlling
follicular growth and ovulation. For example, in the Northern fur seal,
Callorhinus ur sinus, the female begins to breed at the age of two years. These
seals travel north once a year to the Pribilof Islands in the Bering Sea where
they go on land to give birth to the single young and also to breed. Most of
the cows arrive between the middle of June and the middle of July. Heavy
with young, the females give birth to their offspring within a few hours or
days after their arrival. Breeding again takes place about six days after par-
turition. However, lactation continues, and the young are taken care of during
the summer months.
Accordingly, these seals mate each year and it appears that for any par-
ticular year the mating behavior and ovulation of the egg are controlled by
the ovary, which does not have a corpus luteum. As the corpus luteum, which
forms after ovulation in the site of the Graafian follicle, from which the egg
is discharged, remains intact for a considerable portion of the year, the ovary
which does not have the corpus luteum develops the Graafian follicle for the
next summer period. The following year the other ovary will function, and
so on, alternating each year (Enders, et al., '46). Thus, the corpus luteum
appears to function as a suppressor of follicular growth within the ovary in
which it lies. In the human female, one ovary functions to produce an egg
one month, while the following month the other ovary ovulates its single egg.
It is possible that here also the large corpus luteum suppresses follicular growth
within the particular ovary concerned.
During gestation, the presence of the corpus luteum and its hormone,
progesterone, suppresses follicle growth and ovulation in most of the mam-
malian group. (The placenta may be the source of progesterone during the
later phases of pregnancy in forms such as the human.) On the other hand,
in the mare, according to Cole, Howell, and Hart (31), ovulation may occur
during pregnancy. Species differences, therefore, exist relative to the control
of ovulation by the corpus luteum and its hormone, progesterone.
8. Number of Eggs Produced by Different Vertebrate Ovaries
The number of eggs produced during the lifetime of the female varies with
the species and is correlated generally with the amount of care given to the
young. In many fishes which experience little or no parental care, enormous
numbers of eggs may be produced, as for example, in the cod where several
millions of eggs are spawned in one season. However, in many of the elas-
mobranch fishes (i.e., the shark group) the eggs develop within the oviduct,
and the young are born alive. Therefore, only six to a dozen eggs produced
each reproductive period is sufficient to keep the shark species plentiful. In
the hen, where careful breeding and selection have been carried out with a view
to egg production, a good layer will lay from 250 to 300 eggs a year. The
82 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
deer, moose, fur seal, etc., ovulate one egg per year over a life span of a
few years. As stated previously, the human female might ovulate as many
as 400 eggs in a lifetime. In some species the reproductive life is brief. For
example, in the Pacific salmon (Oncorhynchus) females and males die after
their single spawning season, and a similar demise occurs in the eel (Anguilla).
9. Spontaneous and Dependent Ovulation in the Mammals
AND IN Other Vertebrates
Spontaneous ovulation without apparent stimulation from external sources
occurs commonly throughout the vertebrate series. However, dependent ovu-
lation conditioned by psychic or other nervous stimuli also is found exten-
sively. In certain mammals ovulation has been shown to be dependent upon
the stimulus induced by copulation, as, for example, the ferret, mink, rabbit,
cat, shrew, etc. The stimulus, carried through the nervous system, affects in
some way the anterior lobe of the pituitary gland which then produces in-
creased amounts of LH in addition to FSH. These females experience estrus
spontaneously, but later follicle growth and egg discharge are dependent upon
the added stimulation afforded by copulation.
The element of nervous stimulation has a fundamental relationship to the
ovulatory phenomena in the vertebrates. Dependent ovulation occurs in cer-
tain birds, such as the pigeon, where mating provides a psychic or nervous
stimulation which effects ovulation. The presence of two eggs in the nest tends
to suppress ovulation. The removal of these eggs will arouse the ovulatory
procedures. However, the pigeon may sometimes lay eggs without the presence
of a male. In wild birds in general, the mating reaction is linked to the
stimulus for egg laying. The hen, on the other hand, is not dependent upon
copulation, but in many of the domestic varieties the presence of a number
of eggs in the nest appears to suppress egg laying. In the lower vertebrates
nervous stimuli also appear to have an influence upon ovulation. The mating
antics of many fish and amphibia may be connected with ovulatory phenomena.
10. Egg Viability after Discharge from the Ovary
The length of time that the egg may survive and retain its capacity for
fertilization after leaving the ovary depends upon the nature of the egg and
its membrane and the surrounding environment. In the urochordate, Styela,
the egg may remain for 3 to 4 hours after it is discharged into the sea water
and still be capable of fertilization. In the elasmobranch fishes, reptiles, and
birds the conditions of the oviduct are such that fertilization must take place
in the upper part of the oviduct within a few seconds or minutes after the
egg reaches the infundibular portion. In Fundulus heteroclitus and possibly
many other teleost fishes, the egg must be fertilized within 15 to 20 minutes
after spawning. In the frog, the egg passes to the uterus at the lower end of
the oviduct shortly after it leaves the ovary. Under ordinary reproductive tem-
ACTIVITIES OF THE OVARY 83
peratures which obtain in the spring, the egg may remain there for 3 to 5
days without producing abnormahties. If kept at very cool temperatures,
the period may be extended. Among the mammals the viability after ovulation
varies considerably. In the mare, fertilization must occur within about 2 to 4
hours; rabbit, 2 to 4 hours (Hammond and Marshall, '25); rat, about 10
hours; mouse, 12 to 24 hours (Long, '12; Charlton, '17); opossum, probably
within the first hour or so because of the deposition of the albuminous coating
in the oviduct; fox, probably only a few hours; sow, about 24 hours or less;
man, probably 24 hours or less. In the guinea pig, functional degeneration
may begin within 4 to 8 hours after ovulation (Blandau and Young, '39).
11. History of the Egg Follicle after Ovulation
a. Follicles Which Do Not Develop a Post-ovulatory Body
The changes which occur within the egg follicle after the egg has departed
are most variable in different vertebrate species. In most of the fish group
the ovary as a whole shrinks to a fraction of its previous size, and many
very small, immature eggs, interstitial tissue, and collapsed,, contracted, empty
follicles make up its composition. Similarly, in frogs, toads, and salamanders
the collapsed follicle which follows ovulation does not develop an organized
structure. The thecal tissue contracts into a small rounded form within which
are a few follicle cells (fig. 45B). These bodies soon disappear.
In many snakes and in turtles, the follicle collapses after ovulation, and it
is questionable whether organized bodies develop in the site of the ovulated
follicle. A similar condition appears to be the case in birds. However, Pearl
and Boring ('18) described an abbreviated form of a corpus luteum in the
hen in both discharged and atretic follicles. Also, Rothschild and Fraps ('44)
found that the removal of the recently ruptured follicle or of this follicle to-
gether with the oldest maturing follicle, at a time when the egg which origi-
nated from the ruptured follicle is in the oviduct, retarded the laying of the
egg from 1 to 7 days. Removal of other portions of the ovary in control
hens "practically never" resulted in egg-laying retardation. The ruptured fol-
licle, therefore, is believed, by these investigators, to have some influence on
the time of lay of the egg. Whether the hormone progesterone or something
similar to it may be produced by the ruptured follicle of the hen is ques-
tionable, although present evidence appears to suggest that it does (Neher
and Fraps, '50).
b. Follicles Which Develop a Post-ovulatory Body; Formation of the
Corpus Luteum
Post-ovulatory bodies or corpora lutea (yellow bodies) develop in the
ovaries of elasmobranch fishes which give birth to their young alive. Also
in viviparous snakes of the genera Matrix, Storeria, and Thamnophis, it has
84
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
been shown that the removal of the ovaries with their corpora lutea invariably
results in resorption of the young during the first part of gestation and abortion
of the young during the midgestational period, while their removal during
the close of gestation permits normal birth to occur (Clausen, '40). The
differentiation of the corpus luteum in the snake involves the granulosa cells
of the follicle and possibly the theca interna. The differentiated organ appears
similar to that of the mammal (Rahn, '39).
The function of the corpus luteum which develops in the site of the rup-
tured follicle in all mammals, including the Prototheria (fig. 49), has been
the subject of a long series of studies. (See Brambell, '30, Chap. 9; Corner,
'43, Chap. V.) Its function during the reproductive period of the female
mammal is described below under the section of the ovarian hormones.
The events leading to the formation of the corpus luteum in the mammalian
ovary may be described as follows: After the discharge of the egg, the follicle
collapses. The opening of the follicle at the ovarian surface through which
the egg emerged begins to heal. A slight amount of blood may be deposited
within the antrum of the follicle during the ovulation process in some mam-
mals. If so, the follicle in this condition is known as the corpus hemorrhagicum.
BLOO D VE S SELS
OUTER L AYE R
OF THECA
PROLIFERATING
CELLS OF
Y E R
E C A
LUTEAL CELLS
VASCULAR
SPACE
MITOCHONDRIA
CENTRAL CORE
^ .« '<^
1% I .RvC f ^^
'Of
Fig. 49. (A) Luteal cells of the corpus luteum of the opossum. The cellular conditions
in other higher mammals are similar. The central core has not yet been invaded and re-
sorbed by the phagocytes accompanying the ingrowing luteal cells and blood vessels. This
central core is composed of coagulated blood, blood cefls, and connective tissue fibrils.
(B) Corpus luteum of the platypus {Ornithorhynchus).
ACTIVITIES OF THE OVARY 85
Then, under the influence of the hiteinizing hormone, LH, the granulosa cells
of the follicle and also cells from the theca interna, together with blood capil-
laries, proliferate and grow inward into the antral space (figs. 22, 30, 49).
Phagocytes remove the blood clot within the antral space if present, during
the inward growth of these structures. As the ingression of cells and capil-
laries into the follicle continues, the granulosa cells begin to form large, poly-
hedral lutein cells, while the epithelioid cells of the theca interna form a
mass of smaller cells which resemble the true lutein cells; the latter are formed
in the peripheral area of the corpus luteum and are called paralutein cells.
The small spindle-shaped cells of the theca interna, together with blood capil-
laries, become dispersed between the lutein cells, forming a framework for
the latter.
If the egg is fertilized, the corpus luteum persists and is known as the
corpus luteum of pregnancy; if fertilization does not take place, it is called
the corpus luteum of ovulation. The latter body soon degenerates. Histologi-
cally, both types of corpora are identical when first formed. Eventually the
corpus luteum undergoes involution, and its site becomes infiltrated with
connective tissue. The latter structure is sometimes referred to as the corpus
albicans.
12. Hormones of the Ovary and Their Activities in Effecting
THE Reproductive Condition
The ovary produces two important hormones which have a profound effect
upon the reproductive process. These two hormones are the female sex hor-
mone, estrogen, and the gestational hormone, progesterone.
a. Estrogenic Hormone
1) Definition and Source of Production. The induction of estrus (see p.
93) or conditions simulating this state is a property of a relatively large number
of organic compounds. Because of this estrus-inducing power, they are spoken
of as estrogenic substances or estrogens. Estrogens are widely distributed in
nature. Two of the most potent natural estrogens are estradiol and estrone
(theelin). Both have been extracted from the mammalian ovary and are
regarded as primary estrogenic hormones. The most powerful estrogen is
estradiol, and it is regarded at present as the compound secreted by the ovary.
During pregnancy it also is found in the placenta. These structures are not
the only sources of estrogens, however, for it is possible to extract them from
urine after ovariectomy, and they occur in the urine of males as well as that
of females. The urine of the stallion is one of the richest sources of estrogens,
and the testis contains a high estrogenic content (Pincus and Thimann, '48,
p. 381 ). Estrogens are found also in various plants, such as the potato, pussy
willow, etc.
86 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
The structural formulae of estradiol and of estrone are as follows:
HO'
Estradiol Estrone
2) The Ovary as the Normal Source of Estrogen in the Non-pregnant
Female. Aside from the fact that estradiol and estrone are readily extracted
from the ovary, certain experiments tend to focus attention on the ovary as
an important site of estrogen production. For example, the removal of the
ovaries of a normal, adult female mammal causes the accessory reproductive
organs to undergo profound atrophy. The administration of appropriate
amounts of estrogen will restore the accessories of such a female to the con-
dition normal for the resting state. (Consult Pincus, '50, in Pincus and
Thimann, Chap. I.) The injection of follicle-stimulating hormone with small
amounts of the luteinizing hormone into the diestrous (i.e., sexually-resting)
female with intact ovaries results in follicular development within the ovaries,
accompanied by hypertrophy of the accessory reproductive organs to the full
estrous condition ( Nelsen and White, '4 1 ; Pincus, '50, in Pincus and Thimann ) .
These and similar experiments point to the ovary as the main site of estrogen
formation in the body of the non-pregnant female.
The exact structures of the ovary responsible for estrogen elaboration are
not easily determined. Estrogen is found in all parts of the ovary, but certain
observations and experimental results suggest that it is formed in relation
to the follicular tissues and also by the so-called interstitial tissue of the
ovary. For example, when tumors occur within the thecal tissue of the egg
follicle in women who have experienced the menopause, there is often an
accompanying hypertrophy of the accessory organs. This relationship suggests
that thecal gland tissue of the follicle may have the ability to elaborate estro-
gen (Geist and Spielman, '43). On the other hand, the normal hypertrophy
of the granulosa cells of the egg follicle during the normal reproductive cycle,
with the presence of follicular fluid containing estrogen in the antral space
of the follicle, points to the granulosa cells as a possible source of estrogen.
Also, it has been observed that tumorous growths of the granulosa cells of
the follicle produce an excess of estrogenic substance (Geist and Spielman,
'43). Thus, these observations point to the granulosa cells of the egg follicle
of the ovary as being capable of estrogen formation. Another possible source
of estrogen secretion in the ovary is the interstitial cells, derived in part
from theca interna tissue and atretic follicles. These cells are large polyhedral
epithelioid cells scattered between the follicles. Their growth appears to be
directly stimulated by the injection of pure luteinizing hormone (LH; ICSH)
ACTIVITIES OF THE OVARY 87
in hypophysectomized rats (fig. 40). A rapid production of estrogen results
from sucii injections and this may mean tiiat these cells are involved in
estrogen production within the ovary (Evans and Simpson in Pincus and
Thimann, '50).
In the pregnant female mammal the placenta appears to be a source of
estrogen production (Pincus and Thimann, '48, p. 380; Turner, '48, p. 422).
This is suggested by the successful extraction of estrogen from the placenta
of the human and the mare and also by the fact that in these females removal
of the ovaries during the middle or latter phase of gestation does not result
in estrogen diminution in urinary excretion.
3) Pituitary Control of Estrogen Formation. The removal of the anterior
lobe of the pituitary gland of the female results in marked atrophy of ovarian
structures (figs. 40, 50) and of the accessory reproductive organs. Replace-
ment therapy (i.e., the injections of the pituitary gonadotrophins, FSH and
LH) produces a normal reconstitution of the ovarian and reproductive duct
tissues, effecting a normal appearance and functioning of these structures
mill .
Fig. 50. Follicular atresia in guinea pig ovary. (Redrawn from Asdell, '46.) This atresia
is a sporadic but not uncommon event in the normal ovary of the mammal. However,
after removal of the pituitary gland, marked atresia and degeneration of the more mature
follicles occur. (A) Fragmentation of granulosa cells is shown. (B) Beginning inva-
sion of the antral space by theca interna tissue is depicted. (Cf. fig. 40A.) (C) Late
stage of atresia with invasion of the antral space by internal thecal cells.
88
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
Fig. 51. Effects of estradiol (estrogen) upon the female genital tract of the opossum.
(After Risman, J. Morphol., 81.) (A) Reproductive tract of an ovariectomized female.
(B) Hypertrophied condition of a female experiencing the normal estrous changes. (C)
Reproductive tract of an ovariectomized female injected with estradiol (0.9 mm.) 36
days after the ovaries were removed.
(fig. 40). This evidence suggests that the pituitary gonadotrophins, FSH and
LH, control the development of the ovary and, through their influence upon
the ovarian tissues, promote the secretion of estrogen with the subsequent
hypertrophy of the female accessory reproductive structures. It is to be ob-
served that it is not at all clear that FSH in pure form is able to elicit estrogen
production without the presence of LH (ICSH). (See Evans and Simpson
in Pincus and Thimann, '50, p. 355.)
4) Effect of Estrogen upon the Female Mammal. The changes in the
mammalian accessory reproductive organs produced by estrogen are marked.
An increase in vascularity and great hypertrophy of the accessory structures
result from its injection into ovariectomized females. (See figs. 51, 52, 53.)
Increased irritability and activity of the accessory structures also occur. This
increased activity appears to be an important factor in the transportation of
sperm upward within the female accessory organs to the region where the
egg awaits the sperm's arrival.
The alterations in behavior of the female as a result of estrogen stimulation
may be considerable. Females actually seek the presence of a male during
the period of strong estrogenic influence. The long journey of the female fur
seal to the mating grounds in the Bering Sea, the bellowing and tireless search
of the cow moose, the almost uncontrollable demeanor of seeking the male
on the part of the female dog or of the cow in "heat" — these are a few illus-
trations of the regnant power of this stimulant upon the female mammal.
ACTIVITIES OF THE OVARY
89
The culmination of these changes in behavior, resulting in a receptive attitude
toward the male, is reached at about the time when the egg is discharged
from the ovary in many mammalian species. In certain other mammals the
period of heat may precede the ovulatory phenomena.
5) Effects of Estrogen in Other Vertebrates. In the hen, estrogenic hor-
mone causes enlargement and functional activity of the oviduct. Estrogenic
substance, when injected into female chicks from the eighteenth to the fortieth
day, causes an enlargement of the oviduct to about 48 times the natural size.
Estrogen also has a profound effect upon the activities of the full-grown hen
and aids in egg production (Romanoff and Romanoff, '49; Herrick, '44).
Estrogen has a pronounced effect upon the oviducts of other vertebrate forms.
b. Progesterone — The Hormone of the Corpus Luteum
1) Production of Progesterone. The luteinizing hormone, LH, of the an-
terior lobe of the pituitary gland is concerned not only with the development
Fig. 52. Characteristic histological changes in the female reproductive tract under the
influence of estrogen and progesterone. (A-C) Vaginal cyclic changes in the rat. In
(A) is shown the condition of the vaginal wall in the diestrus (resting) condition; (B)
shows changes in vaginal wail structure during estrus. Observe cornification of outer layer
of cells; (C) shows vaginal wall tissue immediately following estrus, i.e., during metestrus.
The presence of progesterone tends to suppress the action of estrogen. (After Turner:
General Endocrinology, Philadelphia, Saunders.) (D, E) Cyclic changes of the Fallo-
pian tube of the human female during the reproductive cycle. In (D) is shown the mid-
interval of the cycle, i.e., at a time paralleling estrus in mammals in general: (E) shows
the cellular condition of the lining tissue of the Fallopian tube just before menstruation.
In (D) the tissue has responded to the presence of estrogen; (E) effect of progesterone
is shown. (After Maximow and Bloom: A Textbook of flistology, Philadelphia, Saunders.)
(F, G) Cyclic changes in the uterine-wall tissue during the reproductive cycle in the
human female. In (F) is shown general character of the uterine wall during the follicular
phase, i.e., responses to estrogen; (G) shows the general condition of the uterine wall
following ovulation. The uterus is now responding to the presence of progesterone added
to the follicular or estrogenic stimulation. (After Maximow and Bloom: A Textbook of
Histology, Philadelphia, Saunders.)
Fig. 53. {See facing page for legend.)
90
ACTIVITIES OF THE OVARY 91
of the egg follicle, but also, after ovulation or the discharge of the egg from
the egg follicle, the remaining granulosa cells, and also, some of the theca
interna cells of the follicle are induced by the LH factor to form the corpus
luteum (figs. 30, 49). Corpora lutea also may be induced by estrogens. This,
however, appears to be an indirect stimulus aroused through estrogenic stimu-
lation of the pituitary gland to secrete added amounts of the LH factor (Evans
and Simpson in Pincus and Thimann, '50, p. 359).
A further pituitary principle, however, seems to be involved in the func-
tional behavior of the corpus luteum. This principle, referred to as luteotrophin
(LTH), is associated with the lactogenic-hormone complex produced by the
anterior lobe of the pituitary body; it induces the morphologically developed
corpus luteum to secrete progesterone. (Consult Evans and Simpson in Pincus
and Thimann, '50, pp. 359, 360; Turner, '48, p. 379, for references.)
The structural formula of progesterone is as follows:
^/X/X/
2) Effects of Progesterone. Progesterone reduces the irritability of the ac-
cessory structures and stimulates the mucosa of the uterus to undergo further
development. This increased developmental and functional condition of the
Fig. 53. Relationship of the pituitary gonadotrophins and ovarian hormones to the de-
veloping Graafian follicle and reproductive-duct change in a polyestrous female mammal.
The Graafian follicle responds to the pituitary gonadotrophins. FSH and LH, with the
subsequent growth and ultimate rupture of the follicle and ovulation. Ovulation termi-
nates the follicular phase of the cycle. Under the influence of the LH factor the corpus
luteum is established. The latter becomes functional as a result of stimulation by the
luteotrophic (lactogenic) hormone. The progestational hormone (progesterone) then is
elaborated by the luteal cells. The activity of the latter together with estrogen controls
the luteal phase of the cycle.
The rising level of estrogen in the blood suppresses FSH secretion, and together pos-
sibly with small amounts of progesterone stimulates LH secretion. Estrogen and small
amounts of progesterone also probably stimulate the secretion of large quantities of LTH,
and the latter stimulates the secretion of progesterone from the recently formed corpus
luteum. When the estrogen level falls, FSH again is secreted.
When the estrogen level rises, the endometrium of the uterus and vaginal mucosa are
stimulated. The presence of progesterone suppresses vaginal development, but the uterine
mucosa is stimulated to greater activity. Observe that the involution of the endometrial
lining in most mammals is gradual but in primates it is precipitous and violent, resulting
in menstruation (Cf. fig. 59). (The diestrous period on this chart is shown as a rela-
tively brief period compared to the other aspects of the reproductive cycle. However,
it may be very long in females which do not experience a polyestrous condition and in
some species it may last a good portion of a year.) (Compiled from various sources in
the literature. The portion of the chart showing pituitary and gonadal hormonal rela-
tionships is based on data obtained from The Schering Corporation, Bloomfield, N. J.)
92 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
accessory reproductive structures added normally to the estrogenic effects
during the reproductive cycle constitutes the luteal phase of the cycle. In this
phase of the cycle the uterine glands elongate and begin secretion, and the
uterus as a whole is prepared for gestation as a result of the action of the
progestational hormone, progesterone, associated with estrogen. (See figs.
53, 59.)
F. Reproductive State and Its Relation to the Reproductive Cycle in
Female Vertebrates
The changes in the female reproductive organs resulting in structural growth
and development referred to above (70-74, 85-88) are consummated in the
ability of the female to fulfill the reproductive functions. The phase of the repro-
ductive events characterized by the ability to reproduce is known as the repro-
ductive climax. This period of culmination remains for a brief period, to be
followed by recession and involution once again to a resting condition. This
developmental progression to a state of reproductive climax followed by re-
gression to a resting condition constitutes a cycle of changing events. When
conditions again are right, the cycle is repeated. Each of these cyclic periods
is known as a reproductive or sexual cycle (figs. 53-59). The reproductive
life of all female vertebrates is characterized by this series of cyclic changes.
In most vertebrate species, the female experiences one sexual cycle per
year, which corresponds to the seasonal cycle in the male. However, in various
mammals and in certain birds, such as the domestic hen, several or many
reproductive cycles may occur during the year. The male, under these con-
ditions, is a continuous breeder; that is, he produces sperm continuously
throughout the year.
1. Sexual Cycle in the Female Mammal
a. Characteristics and Phases of the Reproductive Cycle
The estrous cycle in mammals is a complex affair composed of a number
of integrated subcycles. The changes occurring in the ovary are called the
ovarian cycle; the cellular changes in the uterine (Fallopian tube) form a
cycle; the responses in the mammary glands constitute the mammary cycle;
the cyclic events in the uterus make up the uterine cycle, while those in the
vagina form the vaginal cycle (figs. 53, 54, 57).
The entire estrous cycle may be divided by ovarian changes into two main
phases: the follicular phase and the luteal phase (fig. 53). The former is under
the immediate influence of the enlarging Graafian follicle, which in turn is
stimulated by the follicle-stimulating and luteinizing hormones of the pituitary
gland, with the subsequent production of estrogen. It is probable that the
luteinizing hormone, LH, is mainly responsible for estrogen secretion. (See
Evans and Simpson in Pincus and Thimann, '50, p. 355.) The luteal phase
REPRODUCTIVE CYCLE IN FEMALE VERTEBRATES 93
on the other hand is controlled by the activities of the corpus luteum, which
has replaced the Graafian follicle under the influence of the luteinizing hor-
mone. The production of progesterone by the corpus luteum is eff'ected as
stated previously by the pituitary hormone, luteotrophin (LTH). OVulation
is the pivotal point interposed between these two phases. The follicular phase
may occur without ovulation, but the true luteal phase of a normal or fertile
reproductive cycle is dependent upon the ovulatory phenomena Certain luteal
conditions may be elaborated in an anovulatory cycle, but we are here con-
cerned with the normal events of the fertile reproductive cycle.
The follicular phase includes that portion of the reproductive cycle known
as proestrus and a considerable part of estrus. Proestrus is the period of
rapid follicular growth and elaboration of the estrogenic substance which
precedes the period of estrus. Estrogen stimulates developmental changes in
the cellular structure of the accessory reproductive organs, particularly the
vagina and the uterus (figs. 52, 53). Estrus represents the climax of the fol-
licular phase. As such, it is a period of sexual receptivity of the male, and,
in spontaneously ovulating forms, of ovulation. During other periods of the
cycle the female is indiff'erent or even antagonistic to the male. The period
of estrus is often called period of heat, or period of rut. Estrus is followed
by pregnancy if mating is allowed and is successful, or, in many species, by
a period of pseudopregnancy if mating is not permitted or if the mating is
sterile (figs. 53-57). In some animals, such as the dog, pseudopregnancy is
a prolonged normal event even if mating does not occur, continuing over a
period almost as long as that of normal pregnancy (fig. 54). In other animals,
such as the opossum, pseudopregnancy forms but a brief episode.
Pseudopregnancy is, generally speaking, intermediate in duration between
that of a normal luteal phase of the cycle and that of gestation. In those female
mammals where it does not occur normally, it is aroused by such procedures
as sucking of the nipples, stimulation of the vagina and cervix by the natural
mating process, or by artificially stimulating these structures. In some forms,
such as the rabbit, pseudopregnancy is aroused by mere handling or even by
sight of a male. (For discussion, see Selye, '48, p. 813.)
The general changes of growth and development of the accessory organs
which occur during pregnancy and pseudopregnancy are controlled largely
by the secretions of the corpus luteum. The conditions thus imposed by the
corpus luteum comprise the luteal or progestational phase of the cycle (fig. 57).
In most mammals, if pregnancy does not occur, the ovary and acces-
sory organs again gradually return to the sexually-resting condition known
as diestrus (fig. 53). In man and other primates the changes within the uterus
are not gradual but are precipitous, and most of the endometrial lining, to-
gether with considerable amounts of blood, is discharged to the outside (figs.
53, 59). This phenomenon is called menstruation. The causes of menstruation
are largely problematical; it is related to the fall of the level of either or both
94 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
of the ovarian hormones, progesterone and estrogen. Why certain mammals
should experience violent endometrial changes evident in menstruation and
others a gradual involution and resorption is a question for the future. The
generaPperiod of change following estrus in a non-fertile cycle is known as
metestrus (fig. 53). In the rat and mouse, metestrus is short, about one or
two days; in the human and opossum it occupies approximately ten days to
two weeks of the cycle; in the dog, about 40 to 50 days, depending upon
the pseudopregnant conditions experienced in different females. The word
anestrus is applied to a prolonged diestrus or sexual quiescence between two
sexual cycles. However, the involution experienced by the sexual organs in
anestrus is somewhat more profound than that prevailing during a brief
diestrus. The term lactational diestrus is used to refer to the prolonged dies-
trous condition in forms such as the rat, wherein estrus is suppressed in the
mother while suckling the young.
The length of the sexual cycle varies with the species. When females of
the rat or mouse are kept away from a male, the estrous or sexual cycle will
repeat itself every 4 to 5 days. In the sow it occurs every 17 to 20 days. In
the opossum there is a prolonged anestrous period during the summer and
autumn months followed by a polyestrous period during the winter and spring
when the estrous cycle reoccurs about every 28 days. In the human female,
the sexual cycle occupies about 28 days, and there are probably about ten
normal ovulatory cycles in a year. Some human females may have more,
while others experience a slightly smaller number of true ovulatory cycles
per year.
Many mammals have one estrous cycle per year. This condition, known as
monestrus, is true of most wild mammals, such as the deer, wolf, fox, moose,
and coyote. In the shrew, mink, and ferret the monestrous period may be
prolonged if the female is kept away from the male.
Various types of polyestrous conditions exist. In the female dog, for ex-
ample, there are two or three estrous periods per year about 4 to 6 months
apart. In the cat there are several cycles about two weeks apart during the
autumn, winter, and spring. In the domestic sheep there is a polyestrous period
from September to February in which the cycles occur about every 17 days,
followed by an anestrous period from early March to September. In the mare
in North America, estrous cycles of about 19 to 23 days occur from March
to August. In South America the breeding season is reversed, corresponding
to the reversed seasonal conditions south of the equator. In England many
mares breed in autumn and winter (Asdell, '46).
In some mammals estrus may follow immediately after parturition or birth
of the young. This may occur occasionally in the rat. Under normal conditions
in the fur seal, the female lactates and gestates simultaneously. It is not a
common procedure.
It should be observed that there are two aspects of the female reproductive
REPRODUCTIVE CYCLE IN FEMALE VERTEBRATES
95
cycle of the mammal relative to fertilization or the bringing together of the
male and female reproductive cell. One aspect is the sexual receptivity of
the female; the other is the time of ovulation of the egg. In most female mam-
mals sexual receptivity and ovulation are intimately associated and occur
spontaneously in the cycle; in others the two events may be separated. In
the former group, the development of "heat" and the maturing of the egg
follicle are closely associated, while in the latter the conditions favoring sexual
receptivity or heat are developed considerably in advance of the maturation
of the follicle, as noted in the table below.
b. Relation of Estrus and Ovulation in Some Common Mammals
1) Spontaneously Ovulating Forms (Sexual Receptivity of Male Occurs at
or near Time of Ovulation):
Length of Estrus or Period
of Heat
Time of Ovulation
Dog
Guinea pig
Man
True period of heat about
5-10 days in the middle of
a 21 -day estrous period
6-11 hrs.
Receptivity not always related
to cyclic events
Mare
2-11 days; average length 5-6
days
Sheep
About 36 hrs.
Sow
1-5 days
Silver fox
1-5 days; occurs once a year
in February
Rat
One determination estimates
estrus to be 9-20 hrs.; most
receptive to male about first
3 hrs. of heat. Another de-
termination estimates estrus
to be 12-18 hrs.
Variable: 1st day; 2nd day; 5th day;
etc., of true period of heat
Views vary: 1-2 hrs. after heat or
estrus begins; 10 hrs. after; at end
of estrus
12-17 days after onset of preced-
ing menstruation; average around
14th day
About 1-2 days before end of es-
trus; best breeding about 3 days
after heat begins
Late in estrus or just after estrus
ends; presumably about 20-36
hrs. after estrus begins
About 1-3 days after onset of estrus
1st or 2nd day of estrus
8-11 hrs. after beginning of heat
2) Dependent Ovulatory Forms (Sexual Receptivity [Heat] Occurs Pre-
vious to Time of Ovulation):
Length of Estrus or Period
of Heat
Time of Ovulation
Cat
2-3 days
Time of ovulation uncertain but is
dependent upon copulation
96
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
Length of Estrus or Period
of Heat
Time of Ovulation
Rabbit (tame)
Shrew
Ferret
Estrus prolonged indefinitely
during the breeding season
from spring to summer; a
series of different sets of
egg follicles matured; each
series lasts about a week,
then becomes atretic
Estrus prolonged
Estrus prolonged
Ovulation 10-14 hrs. after mating
About 55-70 hrs. after mating
About 30 hrs. after mating
If ovulation and subsequent pregnancy are not permitted by mating, ovarian
involution occurs, and an anestrous interlude is established. Anestrus in the
common rabbit, Oryctolagus cuniculus, occurs from October to March, but
is not absolute.
c. Non-ovulatory (Anovulatory) Sexual Cycles
Not all of the cyclic changes referred to above in those species which nor-
mally experience spontaneous ovulation are related to definite egg discharge.
Some cycles occur, more or less abortively, without ovulation of the egg. This
may happen in the human or in other mammals, such as the dog and monkey.
Cycles without ovulations are called non-ovulatory cycles. Menstruation may
follow non-ovulatory cycles in the human female.
d. Control of the Estrous Cycle in the Female Mammal
In the control of a reproductive cycle in the vertebrate animal, three main
categories of factors appear to influence its appearance and course. These are:
(1) external environmental factors, such as light and temperature,
(2) external factors governing food supply, and
(3) internal factors resulting from an interplay of the activities of the pi-
tuitary gland, the ovary, general body health, and of the particular
hereditary constitution of the animal.
These factors should be considered not alone in terms of the immediate
production of fertile conditions in the parent, but rather, in view of the total
end to be achieved, namely, the production of a new individual of the species.
For example, the reproductive cycle in the deer reaches its climax or estrus
in the autumn after a long period of lush feeding for the mother. The young
are born the next spring amid favorable temperatures, followed by another
period of bountiful food supply for the mother during lactation and for the
fawn as it is weaned. A receding light factor in the late summer and early
fall thus may be correlated with the period of heat, which in turn proves to
be an optimum time of the year for conception with the resulting birth the
following spring. Similarly, light ascendency is a factor in producing fertility
REPRODUCTIVE CYCLE IN FEMALE VERTEBRATES 97
in many birds. Here the incubation period for the young is short and a
plentiful supply of food awaits the parents and young when it is needed. In
other words, the factors which induce the onset of the reproductive state
are correlated with the conditions which enhance the end to be achieved,
namely, the production of a new individual.
Let us consider next the internal factors which induce the breeding state
in the female mammal. The commonly held theory regarding the pituitary-
ovarian relationship governing the control of the reproductive periods in the
mammal which ovulates spontaneously is as follows (figs. 53 and 59):
( 1 ) FSH of the pituitary gland stimulates later follicular growth. This factor
probably is aided by small, amounts of the luteinizing factor, LH, to
effect an increased production by the ovarian tissues of the estrogenic
hormone. Early follicle growth probably occurs without FSH.
(2) Estrogen output by the ovary rises steadily during the period previous
to ovulation.
(3) Old corpora lutea or other ovarian tissue possibly secrete minimal
amounts of progesterone under the influence of luteotrophin, LTH.
(4) As the quantity of estrogen rises in the blood stream, it inhibits the
production of FSH and together with small quantities of progesterone,
increases the output of LH from the pituitary gland. This combination
also may cause an increased outflow of the luteotrophic factor.
(5) An increased amount of LH aids in eff'ecting ovulation and the sub-
sequent luteinization of the follicle. As the follicle becomes converted
into the corpus luteum, the presence of the luteotrophic factor brings
about the formation of increased quantities of progesterone and main-
tains for a time the corpus luteum and the functional luteal phase of
the cycle.
(6) In those mammals possessing a series of repeating sexual cycles, it
is assumed that the fall of estrogen in the blood stream after ovulation
suppresses the LH outflow and permits a fresh liberation of FSH
from the anterior lobe of the pituitary gland, thus starting a new cycle.
The lowering of the estrogen level may be particularly and immedi-
ately effective in forms such as the rat and mouse, which have a
short metestrus or luteal phase in the estrous cycle.
e. Reproductive Cycle in Lower Vertebrate Females
While the words estrus, heat, or rut are generally applied to the mammalian
groups, the recurrent periods of sexual excitement in lower vertebrates are
fundamentally the same sort of reaction, although the changes in the repro-
ductive tract associated with ovarian events are not always the same as in
mammals. However, similar cyclic changes in the ovary and reproductive tract
are present in the lower vertebrates, and their correlation with the activities
98 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
of the pituitary gland is an established fact. Consequently, the words estrus,
rut, sex excitement, and heat basically designate the same thing throughout
the vertebrate series — namely, a period during which the physiology and
metabolism of the parental body is prepared to undertake the reproductive
functions. In this sense, the words estrus, anestrus, heat, etc. also may be
applied to the male as well as to the female when the male experiences peri-
odic expressions of the sexual state.
Although the reproductive cycle in all vertebrates represents basically a
periodic development of the reproductive functions, there is a marked dif-
ference between the estrous cycle in the female mammal and the reproductive
cycle in most of the other female vertebrates with the exception of viviparous
forms among the snakes, lizards, and certain fishes. This difference is due
to the absence of a true luteal phase in the cycle. The follicular phase and
elaboration of estrogen appears to be much the same in birds, amphibia, and
fishes as in the mammals, but the phase of the cycle governed by progesterone
secretion, associated with a gestational condition in the accessory reproductive
organs, is found only among those vertebrates which give birth to their
young alive.
The reproductive cycles in certain vertebrates may be changed by selective
breeding and domestication. For example, the domestic hen is derived from
the wild jungle fowl. The jungle fowl conform to the general stimuli of nature
as do most wild birds, and the reproductive cycle is associated with a par-
ticular season of the year. However, domestication and selection by man of
certain laying strains have altered the original hereditary pattern of seasonal
laying. Consequently, good layers will lay eggs over an extended period of
the year, although there is a strong tendency to follow the ancestral plan by
laying most of the eggs during the spring and summer months; during the
fall and winter months, a smaller number of eggs are laid. Some of the vari-
eties of the domestic hen conform more closely to the ancestral condition
than do other strains. Similar changes may be produced in the buffalo, which
in nature breeds in middle to late summer but in captivity has estrous periods
three weeks apart throughout the year (Asdell, '46).
G. Role of the Ovary in Gestation (Pregnancy)
1. Control of Implantation and the Maintenance of
Pregnancy in Mammals
The ruling power of the ovary over the processes involved in pregnancy
is absolute, particularly during its earlier phases. In the first place, the corpus-
luteum hormone, progesterone, is necessary to change the uterus already con-
ditioned by the estrogenic hormone into a functionally active state. The latter
condition is necessary for the nutrition and care of the embryo. A second
change which the gestational hormone imposes upon the genital tract of the
ROLE OF THE OVARY IN GESTATION 99
female is to quiet the active, irritable condition aroused by the estrogenic
factor. Progesterone thus serves to neutrahze or antagonize the effects of
the estrogenic hormone. A placid condition of the uterus must be maintained
during the period immediately following copulation if the fertilized egg is to
be cared for within the uterine structure. Large doses of estrogens injected
into mammals shortly after copulation prevent implantation of the embryo
in all species thus far studied. (See Selye, '48, p. 822.)
A third effect of the presence of progesterone is the inhibition of the
copulatory responses. Immediately following estrus and ovulation, the female
dog will fight off the aggressiveness of the male — an aggressiveness which
she invited a day or two previously. This change in behavior is introduced
by the development of the corpora lutea and the initiation of the luteal phase
of the reproductive cycle. Similar anaphrodisiac changes are sometimes men-
tioned in the behavior of the human female during the luteal phase of the
cycle. Progesterone injections also inhibit the copulatory responses in the
ferret (Marshall and Hammond, '44). All of the above-mentioned activities
of progesterone thus inhibit or antagonize the condition aroused by estrogenic
stimulation.
However, aside from these immediate metestrous and post-ovulatory changes
in behavior induced by progesterone, one of its most essential activities is
concerned with the maintenance of gestation or pregnancy. Ovariectomy or
the removal of the ovaries at any time during the gestational period in the
rat, mouse, and goat results in death and abortion of the embryo. During
the first part of pregnancy in the rabbit, the ovaries must be left intact but
may be removed in the closing phase without endangering the gestational
process. In the human female, and also in the mare, cat, dog, guinea pig,
and monkey, the ovaries may be removed during the latter half of pregnancy
without danger to the offspring. However, ovariectomy performed in the
early stages of pregnancy in these animals, as well as in all other mammals
thits far studied, produces abortion (Pincus, '36; Selye, '48, p. 820). The
corpus luteum hormone, therefore, is essential in the early phases of gestation
in all mammals, and it appears to be necessary during most of the pregnant
period in many other mammals.
It is highly probable that the placenta takes over the elaboration of proges-
terone in those mammals where ovariectomy is possible after the first part
of pregnancy has elapsed. In the human female the corpus luteum normally
involutes at about the third month of pregnancy, but progesterone may be
extracted from the placenta after this period.
Although certain effects of the estrogenic hormone appear to be neutralized
(or antagonized) by progesterone during the early phases of reproduction,
other effects of estrogen in relation to progesterone are important for the
maintenance of the pregnant condition. In this connection the estrogenic
hormone appears to suppress some of the growth-promoting effects of proges-
100 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
terone. The two hormones thus work together to promote a gradual devel-
opment of the uterine tissue and maintain a regulated, balanced condition
throughout pregnancy. The placenta, through its ability to elaborate proges-
terone and estrogen during the latter phases of pregnancy, is an important
feature regulating pregnancy in some mammals.
It should be emphasized in connection with the above statements that the
presence of the fertilized egg and its subsequent development in some manner
affects the maintenance of the corpus luteum. The mechanism by which this
influence is conveyed to the ovary is unknown.
2. Gestation Periods, in Days, of Some Common Mammals*
* Adapted from Asdell, '46; Cahalane, '47; Kenneth, '43.
Armadillo (Dasypus novemcinctus) 150
Bear, black (Ursiis americanus) 210
Bear, polar (Thalarctos maritimiis) 240
Beaver, Canadian (Castor canadensis) 94-100
Bison (Bison bison) 276
Cat, domestic (Felis catiis) 60
Cattle (Bos taiirus) 282
Chimpanzee (Pan satyrus) 250
Deer, Virginian (Odocoileus virginianits) 160-200
Dog, domestic (Canis familiar is) 58-65
Donkey, domestic (Equus asinus) 365-380
Elephant (Elephas africanus) 641
Elephant (Elephas indiciis) 607-641
Elk (A Ices alces) 250
Ferret (Putoriiis furo) 42
Fox, arctic (Alopex lagopus) 60
Fox, red (Vulpes vulpes and V. fulva) 52-63
Giraffe (Giraffa camelopardalis) 450
Goat, domestic (Capra hi reus) 140-160
Guinea pig (Cavia porcellus) 68-71
Horse (Equus caballus) 330-380
Man (Homo sapiens) 270-295
Lion (Felis leo) 106
Lynx (Lynx canadensis) 63
Marten, American (Martes americana) 267-280
Mink (Mustela vison) 42-76
Mole (Talpa europaea) 30
Monkey, macaque (Macaca mulato) 160-179
Mouse, house (Mus musculus) 20-21
Opossum (Didelphis virginiana) 13
V\g (Sus scrofa) 115-120
Rabbit (Lepus: Sylvilagus; Oryctolagus) 30-43
Rats (Various species) 21-25
Seal, fur (Callorhinus sp.) 340-350
Sheep, domestic (Ovis aries) 144-160
Skunk, common (Mephitis mephitis) 63
Squirrel, red (Tamiasciurus sp.) 30-40
ROLE OF THE OVARY IN PARTURITION 101
Tiger (Felis tigris) 106
Whale (Various species) 334-365
Wolf (Canis lupus) 63
Woodchuck (Marmota monax) 35-42
Zebra, mountain (Equus zebra) 300-345
3. Maintenance of Pregnancy in Reptiles and Other
Vertebrates
In certain viviparous species of the genera Storeria, Matrix and Thamnophis,
Clausen ('40) reports that ovariectomy during gestation results in resorption
of the embryo when performed during the earlier phases of gestation and
abortion during the middle of gestation, but during the terminal portion of
pregnancy the process is unaffected and the young are born normally. These
results are similar to those obtained from the rabbit as noted previously.
While experimental evidence is lacking in other vertebrate groups which
give birth to the young alive, the evidence obtained from reptilian and mam-
malian studies suggests that hormones are responsible for the maintenance
of pregnancy. In harmony with this statement, it may be pointed out that
in the viviparous elasmobranch fishes (e.g., sharks) corpora lutea are de-
veloped in the ovaries.
H. Role of the Ovary in Parturition or Birth of the Young
The real factors bringing about parturition are not known, and any ex-
planation of the matter largely is theoretical. However, certain aspects of
the subject have been explored. For example, it was observed above that
progesterone appears to antagonize the action of estrogen with the result
that the uterus stimulated to irritability and contractility under the influence
of estrogen is made placid by the action of progesterone. In harmony with
this action studies have shown that estrogen tends to increase during the
final stages of normal gestation, while progesterone appears to decrease, ac-
companied by an involution of the corpora lutea. Consequently, the foregoing
facts have suggested the "estrogen theory," which postulates that activities
of the uterine musculature are increased by the added amounts of estrogen
in the presence of decreasing amounts of progesterone during the latter phases
of pregnancy. In confirmation of this theory, it has been shown that proges-
terone injected into a pregnant rabbit near the end of the gestation period
will tend to prolong gestation. A second theory of parturitional behavior
assumes that the posterior lobe of the pituitary gland elaborates oxytocin
which induces increased uterine activity, resulting in birth contractions
(Waring and Landgrebe in Pincus and Thimann, '50). Again, a third concept
emphasizes the possibility that the placenta may produce substances which
bring about contractions necessary for the expulsion of the young (Turner,
'48, p. 428). Oxytocic substances have been extracted from the placenta,
which suggests the validity of this theory.
NON-PREGNANT CYCLE
PREGNANT CYCLE
COPULATION NOT PERMITTED COPULATION PERMITTED
i PROESTRUSi-
ESTRUS METESTRUS ANESTRU.S -f^ESTRUS PRE G N A N C Y
VA G I N A L
CYCLE
ERYTHROCYTES
CORNIFIED EPITHELIAL CELLS
LEUKOCYTES
LINING EPITHELIUM
MAMMARY
GLAND
CYCLE
DAYS 0 9
Fig. 54. Changes occurring in the reproductive organs and mammary glands of the
bitch during the reproductive cycle. The student is referred to Asdell ('46), pp. 150-156
and Dukes ('43), pp. 678 682, for detailed description and references pertaining to the
data supporting this chart. The gestation period is based upon data supplied by Kenneth
('43) and the author's personal experience with dogs.
NON-PREGNANT CYCLE PREGNANT CYCLE
COPULATION NOT COPULATION PERMITTED
i PERMITTED i
ESTRUS ESTRUS
115 TO 120
Fig. 55. Reproductive and pregnancy cycles in the sow. (Modified from data supplied
by Corner, Carnegie Inst., Washington, pub. 276, Contrib. to Embryol., 13; the parturition
data derived from Kenneth, '43.)
102
THE OVARY IN MAMMARY-GLAND DEVELOPMENT
103
The specific functions of the ovary in parturition probably are more pro-
nounced in those forms where it is essential throughout most of the gesta-
tional period, such as the viviparous snakes, and among the mammals, such
forms as the opossum, rat, mouse, and rabbit. The waning of corpus-luteum
activity in these species may serve to lower the level of progesterone in the
body and thus permit some of the other factors, such as estrogen or the
pituitary principle, to activate the uterus.
Another factor associated with the ovary and parturition is the hormone
relaxin. This substance was first reported by Hisaw and further studied by
this investigator and his associates (Hisaw, '25, '29; and Hisaw, et al., '44).
NON-PREGNANT CYCLE
PREGNANT CYCLE
COPULATION NOT COPULATION
4- PERMITTED J.
ES TRUS ES T RUS
PERMITTED
304 TO 371
AV 330 TO 345
Fig. 56. Reproductive and pregnancy cycles in the mare. (Parturition period based
upon data supplied by Kenneth ('43); other data supplied by Asdell ('46) and Dukes
('43).) It is to be noted that the first corpus luteum of pregnancy degenerates after
about 35 days; the second "crop of corpora lutea" (Asdell) degenerate by 150 days. The
ovaries may be removed after 200 days of pregnancy without causing abortion of young.
Relaxin aids in the production of a relaxed condition of the pelvic girdle, a
necessity for the formation of a normal birth passageway for the young.
Relaxin somehow is associated in its formation with the presence of proges-
terone in the blood stream and also with the intact reproductive system.
Relaxin together with estrogen and progesterone establishes a relaxed con-
dition of the tissues in the pubic area of the pelvic girdle.
I. Importance of the Ovary in Mammary-Gland Development
and Lactation
Estrogen and progesterone together with the lactogenic hormone, luteo-
trophin, of the pituitary gland are necessary in mammary-gland development.
The entire story of the relationship of these and of other factors in all mam-
mals or in any particular mammal is not known. However, according to one
theory of mammary-gland development and function, the suggestive roles
played by these hormones presumably are as follows (fig. 58): Estradiol and
other estrogens bring about the development of the mammary-gland ducts;
as a result a tree-like branching of the ducts is effected from a simple im-
104
THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
NON -PREGNANT CYCLE
PREGNANT CYCLE
COPULATION NOT COPULATION PERMITTED
J PERMITTED ^
ESTRUS ESTRUS
UTERINE
CYCLE
HEMOR RHAGE
HEMORRHAGE
GLANDS HIGHLY DEVELOPED
PARTURITIOM
r
y
-»-E RYTHROCYTES
^■^^ LEUKOCYTES
i
VAGINAL
CYCLE
^\^ MUCOUS SECRETION '
^^»»..^ EPITHELIUM (^CORNIFICATION)
1
i^
^
1
+
DAYS 0 10 21 10 20 210 TO 335
Fig. 57. Reproductive and pregnancy cycles in the cow. (Parturition period based upon
data supplied by Kenneth ('43), also by Asdell ('46). Other data for chart derived from
Asdell ('46).
Three main characteristics of heat or estrous period are evident: (1) A duration of
heat of only about 10 to 18 hours; (2) abundant secretion (;luring heat of a "stringy
mucus," derived from mucoid epithelium of vagina and from sealing plug of cervix when
cow not in estrus (Asdell); and (3) ovulation occurs from 13'/2 to \SVi hours after ter-
mination of estrus (Asdell). Variation in time of ovulation may be considerable, from
2 hours before end of estrus to 26 hours after (Asdell).
mature pattern established during earlier development (fig. 58A, A', B). The
male mammary gland may remain similar to the condition shown in fig. 58A.
The maturing of the egg follicles within the ovary and the concomitant for-
mation of estrogen which accompanies sexual maturity is linked with the
mere complex state of the mammary-gland system shown in fig. 58B.
The next step of mammary-gland development is carried out under the
influence of progesterone. Progesterone is necessary for the development
of the terminal glandular tissue or alveoli associated with these ducts (fig.
58C, D). Finally, the pituitary lactogenic hormone (luteotrophin [LTH];,
prolactin) stimulates the actual secretion of milk (fig. 58E). Recent research
also has shown that the lactogenic hormone collaborates in some way with
estrogen and progesterone in the development of the mammary-gland tissue.
FIRST PART OF
PREGNANCY
MILK
SECRETION
Fig. 58. Mammary gland changes in relation to reproduction. (Figures are a modifica-
tion of a figure by Corner: Hormones in Human Reproduction, Princeton, Princeton
University Press. The figure in the latter work was based on a figure by C. D. Turner:
Chap. XI of Sex and Internal Secretions, by Allen, et al., Baltimore, Williams & Wilkins,
1939.) Factors involved in mammary gland development and secretion are somewhat as
follows: (A, A') Condition of the young, infantile gland. (B) Development from a
simple, branched, tubular gland of the immature animal (A') into a compound tubular
gland presumably under the direct stimulation of estrogen, according to one theory, or
by the action of estrogen upon the pituitary gland which then releases mammogen I,
producing these changes, according to Turner, et al.: Chap. XI, Sex and Internal Secre-
tions, by Allen, et al., Baltimore, Williams & Wilkins. (C) Transformation of the com-
pound tubular gland into a compound tubulo-alveolar gland under the influence of proges-
terone, during the first part of pregnancy, or, according to Turner, et a!., by the influence
of estrogen plus progesterone which causes the pituitary to release a second mammogen
which produces the alveolar transformation. (D) Effect of the latter part of pregnancy
is to bring about a development of the cells of the acini of the acinous or alveolar system.
The unit shown in (D) represents a simple, branched, acinous gland, in which there are
six alveoli or acini associated with the duct. (E) Affect of parturition is to release the
lactogenic hormone (prolactin; luteotrophin) from the pituitary gland which brings about
milk secretion. During pregnancy the high levels of estrogen presumably inhibit milk
secretion. However, following pregnancy the level of estrogen is lowered permitting
lactogenic-hormone action upon the alveoli of the gland.
The removal of the placenta and embryo at any time during gestation permits milk
flow, provided the mammary glands are sufficiently developed. In the human, any remains
of the placenta after birth inhibit milk secretion, probably because the estrogenic hormone
is elaborated by the placental remnants. (See Selye, '48, p. 829.)
In the rabbit, estrogen and progesterone are necessary for the elaboration of the duct
and secretory acini; in the guinea pig and goat, and to some extent in the primates,
including the human female, estrogen alone is capable of producing the development of
the entire duct and acinous system. (See Turner, '48, p. 430.)
105
Fig. 59. {See facing page for legend.)
106
THE OVARY IN MAMMARY-GLAND DEVELOPMENT 107
During pregnancy, the actual secretion of milk is inhibited by the estrogenic
hormone produced by the ovary and the placenta. The role of estrogen as
an inhibitor of lactation is suggested by the fact that, after lactation has started
following normal parturition, it is possible in the cow and human to suppress
milk flow by the administration of estrogens. After parturition, however,
estrogen is no longer present in sufficient amounts to suppress the secretion
of milk, and the mammary gland begins to function. (In the fur seal a post-
partum estrus with ovulation follows a short time after parturition. However,
the amount of estrogen produced by this reproductive cycle is not sufficient
to curb lactation.) The neurohumoral reflex, or "suckling reflex," produced
by the sucking young appears to maintain the flow of milk over a period of
time. Probably this reflex causes a continuous discharge of the lactogenic hor-
mone from the anterior lobe of the hypophysis.
Another theory of mammary-gland development maintains that estrogen
stimulates the anterior pituitary gland to release mammogen, which causes
development of the duct system, and estrogen plus progesterone induce a
second mammogen which stimulates lobule-alveolar development. The lac-
togenic hormone produces the actual secretion of milk. The ovary thus as-
sumes considerable importance in controlling the (morphological) develop-
ment of the mammary glands in mammals, particularly in those forms in
which the functional condition of the ovary is maintained throughout most
Fig. 59. Stages in the reproductive cycle of the human female and its pituitary-ovarian-
endometrial relationships (Cf. fig. 53). (Compiled from various sources in the literature.)
(a) As shown at the extreme right of the figure, a fall in the level of estrogen and proges-
terone in the blood stream, either or both, is associated with endometrial necrosis, bleed-
ing, and discharge (menstruation), (b) The lowering of the estrogen level is associated
with a new outflow of the follicle-stimulating hormone (FSH), as shown at the right of
the figure, (c) In the left side of the figure, the influence of FSH induces egg follicles,
probably several, to grow. Antral spaces appear and enlarge. The presence of a small
amount of the luteinizing hormone (LH) together with FSH stimulates the secretion of
estrogen by the ovarian tissues, possibly by the follicles and interstitial tissue between
the follicles, (d) In consequence, the estrogen level rises in the blood stream, and
menstruation subsides by the fourth day. (e) The continued influence of estrogen pro-
duces endometrial growth, and probably increases the outflow of LH from the pituitary
(fig. 53). It is probable, also, that the increased estrogen level stimulates a release of
the luteotrophic hormone from the pituitary, which in turn stimulates the formation of
a small quantity of progesterone by either the interstitial tissue of the ovary or in old
corpora lutea. (f) Some of the developing egg follicles degenerate, while one continues
to develop, (g) The elevation of estrogen suppresses the outflow of FSH as indicated
by the heavy broken line to the left, (h) The elevated level of estrogen together possibly
with small amounts of progesterone evokes an increased outflow of LH and LTH as
indicated by the heavy broken line to the right, (i) LH and FSH bring about ovulation
at about the fourteenth day. (j) LH causes development of corpus luteum. (k) LTH
elicits secretion of progesterone by corpus luteum. Possibly some estrogen is secreted
also by corpus luteum. (1) Progesterone and estrogen stimulate added development of
endometrium, (m) In the absence of fertilization of the egg, the corpus luteum regresses,
with a subsequent fall of progesterone and estrogen levels in the blood stream, terminating
the cycle and permitting a new menstrual procedure.
108 THE VERTEBRATE OVARY AND ITS RELATIONSHIP TO REPRODUCTION
of the gestational period, e.g., rat, rabbit, dog, etc. In other species, such as
the human, mare, etc., the placenta through its ability to duplicate the pro-
duction of the ovarian hormones, assumes a role during the latter phase of
pregnancy. (For further details, consult Folley and Malpress in Pincus and
Thimann, '48; Selye, '48, pp. 828-832; and Turner, '48, pp. 428-448.)
In the dog or opossum during each reproductive cycle, the mammary glands
are stimulated to grow and may even secrete milk (dog). These changes
closely parallel the ovarian activities, particularly the luteal phase of the cycle.
In the human, functional growth changes occur in pregnancy, but, pending
the events of the ordinary cycle, alterations in the duct system are slight al-
though the breasts may be turgid due to increased blood flow and connective-
tissue development.
J. Other Possible Developmental Functions Produced by the Ovary
As the eggs of the opossum and rabbit travel through the uterine (Fal-
lopian) tube toward the uterus, they are coated with an albuminous, jelly-like
coating. Similar jelly coatings are added to the eggs of the bird, reptile, frog,
toad, and salamander. These coatings or membranes added to the egg as it
travels through the oviduct are known as tertiary egg membranes.
In the toad, the secretion of the protective jelly by the oviduct can be
elicited by the lactogenic hormone present in beef pituitary glands. The se-
cretion of the albuminous jelly coatings around the eggs of frogs, salamanders,
reptiles, and birds may be related to this hormone. The formation of the
crop milk of pigeons has been shown by Riddle and Bates ('39) to be de-
pendent upon the presence of the lactogenic hormone.
The function of the ovary in influencing the outflow of the lactogenic hor-
mone from the pituitary, if present in the above cases of glandular secretion,
must be an indirect one. Evans and Simpson in Pincus and Thimann ('50)
ascribe the outflow of the "lactogenic hormone (luteotrophic hormone)" of
the mammalian pituitary to estrin produced by the ovary. It is possible that
in the salamanders, frogs, toads, and the birds an indirect ovarian influence
may similarly induce secretion of the lactogenic hormone which in turn gov-
erns the elaboration of the albuminous jelly deposited around the egg in
transit through the oviduct.
K. Determinative Tests for Pregnancy
Various tests have been used to determine the probability of pregnancy
in the human female. These tests are discussed in Chapter 22.
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. 1920b. On the cause of hyper-
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Tne Development or tne Gametes or Sex Cells
A. General considerations
B. Controversy regarding germ-cell origin
C. Maturation (differentiation) of the gametes
1. General considerations
2. Basic structure of the definitive sex cell as it starts to mature or differentiate
into the male meiocyte (i.e., the spermatocyte) or the female meiocyte (i.e., the
oocyte)
3. Nuclear maturation of the gametes
a. General description of chromatin behavior during somatic and meiotic mitoses
b. Reductional and equational meiotic divisions and the phenomenon of cross-
ing over
c. Stages of chromatin behavior during the meiotic prophase in greater detail
1) Leptotene (leptonema) stage
2) Zygotene or synaptene (zygonema) stage
3) Pachytene (pachynema) stage
4) Diplotene (diplonema) stage
5) Diakinesis
d. Peculiarities of nuclear behavior in the oocyte during meiosis; the germinal
vesicle
e. Character of the meiotic (maturation) divisions in the spermatocyte compared
with those of the oocyte
1 ) Dependent nature of the maturation divisions in the female meiocyte
2) Inequality of cytoplasmic division in the oocyte
f. Resume of the significance of the meiotic phenomena
4. Cytosomal (Cytoplasmic) maturation of the gametes
a. General aspects of the cytoplasmic maturation of the gametes
b. Morphogenesis (spermiogenesis) (spermioteleosis) of the sperm
1 ) Types of sperm
2) Structure of a flagellate sperm
a) Head
b) Neck
c) Connecting body or middle piece
d) Flagellum
3) Spermiogenesis or the differentiation of the spermatid into the morphologi-
cally differentiated sperm
a) Golgi substance and acroblast; formation of the acrosome
b) Formation of the post-nuclear cap
c) Formation of the proximal and distal centrioles; axial filament
112
GENERAl. CONSIDERATIONS 113
d) Mitochondrial material and formation of the middle piece of the sperm
e) The cytoplasm, axial filament, mitochondria, and tail formation
c. Cytoplasmic differentiation of the egg
1 ) Types of chordate eggs
a) Homolecithal (isolecithal) eggs
b) Telolecithal eggs
2) Formation of the deutoplasm
3) Invisible morphogenetic organization within the cytoplasm of the egg
4) Polarity of the egg and its relation to body organization and bilateral sym-
metry of the mature egg
5) Membranes developed in relation to the oocyte; their possible sources of
origin
a) Chorion in Stye la
b) Egg membranes of Ampltioxiis
c) Vitelline membrane and zona radiata of elasmobranch fishes
d) Zona radiata of teleost fishes
e) Vitelline membrane (zona radiata) in amphibia
f) Zona radiata (zona pellucida) of the reptile oocyte
g) Vitelline membrane (zona radiata) of the hen's egg
h) Membranes of the mammalian oocyte
5. Physiological maturation of the gametes
a. Physiological difi'erentiation of the sperm
b. Physiological ripening of the female gamete
D. Summary of egg and sperm development
A. General Considerations
In the two preceding chapters the conditions which prepare the male and
female parents for their reproductive responsibilities are considered. This
chapter is devoted to changes which the male and female germ cells must
experience to enable them to take part in the processes involved in the repro-
duction of a new individual.
The gamete is a highly specialized sex cell or protoplasmic entity so dif-
ferentiated that it is capable of union (fertilization; syngamy) with a sex cell
of the opposite sex to form the zygote from which the new individual arises.
The process of differentiation whereby the primitive germ cell is converted
into the mature gamete is called the maturation of the germ cell.
The main events which culminate in the fully-developed germ cell are
possible only after the primitive or undifferentiated germ cell has reached a
certain condition known as the definitive state. When this stage is reached,
the germ cell has acquired the requisite qualities which make it possible for
it to differentiate into a mature gamete. Before the definitive state is reached,
germ cells pass through an eventful history which involves:
( 1 ) their so-called "origin" or first detectable appearance among the other
cells of the developing body, and
(2) their migration to the site of the future ovary or testis.
After entering the developing substance of the sex gland, the primitive
germ cells experience a period of multiplication. If the sex gland is that of
114 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
the male, these undifferentiated sex cells are called spermatogonia; if female,
they are known as oogonia.
B. Controversy Regarding Germ-cell Origin
The problems of germ-cell origin in the individual organism and of the
continuity of the germ plasm from one generation to the next have long
been matters of controversy. Great interest in these problems was aroused
by the ideas set forth by Waldeyer, Nussbaum, and Weismann during the
latter part of the nineteenth century. Waldeyer, 1870, as a result of his studies
on the chick, presented the "germinal epithelium" hypothesis, which main-
tains that the germ cells arise from the coelomic epithelium covering the
gonad. Nussbaum, 1880, championed the concept of the extra-gonadal origin
of the germ cells. According to this view, derived from his studies on frog
and trout development, the germ cells arise at an early period of embryonic
development outside the germ-gland area and migrate to the site and into
the substance of the germ gland.
At about this time the speculative writings of August Weismann aroused
great interest. In 1885 and 1892 Weismann rejected the popular Darwinian
theory of pangenesis, which held that representative heredity particles or
"gemmules" passed from the body cells (i.e., soma cells) to the germ cells
and were there stored in the germ cells to develop in the next generation
(Weismann, 1893). In contrast to this hypothesis he emphasized a complete
independence of the germ plasm from the somatoplasm. He further suggested
that the soma did not produce the germ plasm as implied in the pangenesis
theory, but, on the contrary, the soma resulted from a differentiation of the
germ plasm.
According to the Weismannian view, the germ plasm is localized in the
chromosomal material of the nucleus. During development this germ plasm
is segregated qualitatively during successive cell divisions with the result that
the cells of different organs possess different determiners. However, the nuclear
germ plasm (Keimplasma) is not so dispersed or segregated in those cells
which are to become the primitive sex cells; they receive the full complement
of the hereditary determiners for the various cells and organs characteristic
of the species. Thus, it did not matter whether the germ cells were segregated
early in development or later, so long as the nucleus containing all of the
determinants for the species was kept intact. In this manner the germ plasm,
an immortal substance, passed from one generation to the next via the nuclear
germ plasm of the sex or germ cells. This continuity of the nuclear germ
plasm from the egg to the adult individual and from thence through the
germ cells to the fertilized egg of the next generation, constituted the Weismann
"Keimbahn" or germ-track theory. The soma or body of any particular gen-
eration is thus the "trustee" for the germ plasm of future generations.
The Weismannian idea, relative to the qualitative segregation of the chro-
GERM-CELL ORIGIN
115
matin materials, is not tenable for experimental and cytological evidence sug-
gests that all cells of the body contain the same chromosomal materials.
However, it should be pointed out that Weismann was one of the first to
suggest that the chromosome complex of the nucleus acts as a repository
for all of the hereditary characteristics of the species. This suggestion relative
to the role of the nucleus has proved to be one of the main contributions to
biological theory in modern times.
Fig. 60. Representation of the concept of the early embryonic origin of the primordial
germ cells and their migration into tiie site of the developing germ gland. (A-C are
adapted from the work of Allen, Anat. Anz. 29, on germ cell origin in Chrysemys; D-F
are diagrams based on the works of Dustin, Swift, and Dantschakoff, etc., referred to
in the table of germ-ceil origins included in the text.) (A-C) Germ cells arising within
the primitive entoderm and migrating through the dorsal mesentery to the site of the
primitive gonad, shown in (D), where they become associated in or near the germinal
epithelium overlying the internal mesenchyme of the gonad. (E, F) Increase of the
primitive gonia within the developing germ gland, with a subsequent migration into the
substance of the germ gland of many germ cells during the differentiation of sex.
116
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
GEHM- CELL
LINE I ECTO DER M I ENTODE R M V STOMODAEAL
MESODERM MESODERM MESODERM
ECTODERM
Fig. 61 Diagrammatic representation of the process of chromatin diminution in the
nematode worm, Ascaris equorum {A. megalocephala), and of the "Keimbahn" (in
black, E). One daughter cell shown by the four black dots of each division of the germ-
cell line (i.e., the stem-cell line) is destined to undergo chromatin diminution up to the
16-cell stage. At the 16-cell stage, the germ-cell line ceases to be a stem cell (e.g., P,),
and in the future gives origin only to sperm cells (E). (A-D, copied from King and
Beams ('38); E, greatly modified from Diirkin ('32).)
Animal pole of the cleaving egg (A) is toward the top of the page. (B) Metaphase
conditions of the second cleavage. Observe the differences in the cleavage planes of the
prosomatic cell, S,, and that of the stem cell, P,. (C) Anaphase of the second cleavage
of S,. Observe that the ends of the chromosomes in this cleaving cell are left behind on
the spindle. (D) It is to be noted that the ends of the chromosomes are not included
in the reforming nuclei of the two daughter cells of S,, thus effecting a diminution of
the chromatin substance. In P,, P,, and E.M. ST. of (D), the chromosomes are intact.
E.M. ST. = second prosomatic cell. MST = mesoderm-stomodaeal cell.
A second contributory concept to the germ-cell (germ-plasm) theory was
made by Nussbaum, 1880; Boveri, 1892, '10, a and b, and others. These
investigators emphasized the possibility that a germinal cytoplasm also is
important in establishing the germ plasm of the individual. A considerable
body of observational and experimental evidence derived from embryological
studies substantiates this suggestion. Consequently, the modern view of the
germ cell (germ plasm) embodies the concept that the germ cell is composed
GERM-CELL ORIGIN 117
of the nucleus as a carrier of the hereditary substances or genes and a pecuHar,
speciaHzed, germinal cytoplasm. The character of the cytoplasm of the germ
cell is the main factor distinguishing a germ cell from other soma cells.
The matter of a germinal cytoplasm suggests the necessity for a segregation
of the germinal plasm in the form of specific germ cells during the early
development of the new individual. As a result, great interest, as well as con-
troversy, has accumulated concerning this aspect of the germ-cell problem:
namely, is there a separate germinal plasm set apart in the early embryo which
later gives origin to the primordial germ cells, and the latter, after migration
(fig. 60), to the definitive gonia; or according to an alternative view, do some
or all of the definitive germ cells arise from differentiated or relatively undif-
ferentiated soma cells? The phrase primary primordial germ cells often is
used to refer to those germ cells which possibly segregate early in the embryo,
and the term secondary primordial germ cells is employed occasionally to
designate those which may arise later in development.
The dispute regarding an early origin or segregation of the germinal plasm
in the vertebrates also occurs relative to their origin in certain invertebrate
groups, particularly in the Coelenterata and the Annelida (Berrill and Liu,
'48). In other Invertebrala, such as the dipterous insects and in the ascarid
worms, the case for an early segregation is beyond argument. An actual dem-
onstration of the continuity of the Keimbahn from generation to generation
is found in Ascaris megalocephala described by Boveri in 1887. (See Hegnc,
'14, Chap. 6.) In this form the chromatin of the somatic cells of the body
undergoes a diminution and fragmentation, whereas the stem cells, from which
the germ cells are ultimately segregated at the 16-cell to 32-cell stage, retain
the full complement of chromatin material (fig. 61). Thus, one cell of the
16-cell stage retains the intact chromosomes and becomes the progenitor of
the germ cells. The other 15 cells will develop the somatic tissues of the body.
The diminution of the chromatin material in this particular species has been
shown to be dependent upon a certain cytoplasmic substance (King and
Beams, '38).
In some insects the Keimbahn also can be demonstrated from the earliest
stages of embryonic development. In these forms a peculiar polar plasm within
the egg containing the so-called "Keimbahn determinants" (Hegner, '14,
Chap. 5) always passes into the primordial germ cells. That is, the ultimate
formation and segregation of the primordial germ cells are the result of nuclear
migration into this polar plasm and the later formation of definite cells from
this plasm (fig. 62). The cells containing this polar plasm are destined thus
to be germ cells, for they later migrate into the site of the developing germ
glands and give origin to the definitive germ cells.
Many investigators of the problem of germ-cell origin in the vertebrate
group of animals have, after careful histological observation, described the
germ cells as taking their origin from among the early entodermal cells (see
118
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
table, pp. 121-124). On the other hand, other students have described the
origin of the germ cells from mesodermal tissue — some during the early period
of embryonic development, while others suggest that the primordial germ cells
arise from peritoneal (mesodermal) tissue at a much later time.
In more recent years much discussion has been aroused relative to the
origin of the definitive germ cells in mammals, particularly in the female.
According to one view the definitive germ cells which differentiate into the
mature gametes of the ovary arise from the germinal epithelium (peritoneal
covering) of the ovary during each estrous cycle (figs. 39A, 63, 64). For
example, Evans and Swezy ('31) reached the conclusion that all germ cells
in the ovaries of the cat and dog between the various reproductive periods
degenerate excepting those which take part in the ovulatory phenomena.
Accordingly, the new germ cells for each cycle arise from the germinal epi-
thelium. A similar belief of a periodic proliferation of new germ cells by the
POLE
PLASM
Fig. 62. Early development of the fly, Miastor. (A) Miaslor metraloas. (B) Miastor
ainerkana. In (A) the division figures I and III (II not shown) are undergoing chromatin
diminution, while nucleus IV divides as usual. In (B) one segregated germ is shown at
the pole of the egg. This cell will give origin to the germ cells. Other division figures
experiencing chromatin diminution.
GERM-CELL ORIGIN
119
PROLI FE RAT IN G
GERMINAL EPITHELIUM
GERM CELLS WITH
FORMING FOLL ICLES
pflUger's cord
Fig. 63
Fig. 64
Fig. 63. Cells proliferating inward from germinal epithelium of the ovary of a one-
day-old rat. Observe cords of cell.s (Pfliiger's cords) projecting into the ovarian substance.
Within these cords of cells are young oogonia. (After Vincent and Dornfeld, '48.)
Fig. 64. Cellular condition near the surface of the ovary of a young female opossum.
This section of the ovary is near the hilar regions, i.e., near the mesovarium. Observe
young oocytes and forming Graafian follicles. Primitive germ cells may be seen near the
germinal epithelium.
germinal epithelium has been espoused by various authors. (See Moore and
Wang, '47; and Pincus, '36, Chap. II.) More recent papers have presented
views which are somewhat conflicting. Vincent and Dornfeld, '48 (fig. 63)
concluded that there is a proliferation of germ cells from the germinal epi-
thelium of the young rat ovary, while Jones ('49), using carbon granules as
a vital-marking technic, found no evidence of the production of ova from
the germinal epithelium in rat ovaries from 23 days until puberty. In the
adult rat, she concedes that a segregation of a moderate number of oogonia
from the germinal epithelium is possible.
Aside from the above studies of carefully-made, histological preparations
relative to the time and place of origin of the primordial and definitive germ
cells, many experimental attacks have been made upon the problem. Using
an x-ray-sterilization approach, Parkes ('27); Brambell, Parkes and Fielding
('27, a and b), found that the oogonia and oocytes of x-rayed ovaries of
the mouse were destroyed. In these cases new germ cells were not produced
from the germinal epithelium. Brambell ('30) believed that the destruction
of the primitive oogonia was responsible for the lack of oogenesis in these
x-rayed ovaries. However, this evidence is not conclusive, for one does not
120 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
know what injurious effects the x-rays may produce upon the ability of the
various cells of the germinal epithelium to differentiate.
An experimental study of the early, developing, amphibian embryo relative
to the origin of the primordial germ cells also has been made by various in-
vestigators. Bounoure ('39) applied a vital-staining technic to certain anuran
embryos. The results indicate that the germinal plasm in these forms is asso-
ciated with the early, entodermal, organ-forming area located at the vegetal
pole of the cleaving egg. This germinal plasm later becomes segregated into
definite cells which are associated with the primitive entoderm. At a later
period these cells migrate into the developing germ gland or gonad. On the
other hand, experimental studies of the urodele embryo indicate that the
early germinal plasm is associated with the mesoderm (Humphrey, '25, '27;
Nieuwkoop, '49 ) . Existence of an early germinal plasm associated with the
entoderm in the Anura and with the mesoderm in the Urodela thus appears
to be well established for the amphibia.
The evidence derived from amphibian studies together with the observa-
tions upon the fish group presented in the table (see pp. 121-124) strongly
suggests that an early segregation of a germinal plasm (germ cells) occurs in
these two major vertebrate groups. Also, in birds, the experimental evidence
presented by Benoit ('30), Goldsmith ('35), and Willier ('37) weighs the
balance toward the conclusion that there is an early segregation of germ cells
from the entoderm. Similar conditions presumably are present in reptiles. In
many vertebrates, therefore, an early segregation of primordial germ cells and
their ultimate migration by: (1) active ameboid movement, (2) by the
shifting of tissues, or (3) through the blood stream (see table, pp. 121-124)
to the site of the developing gonad appears to be well substantiated.
The question relative to the origin of the definitive ova in the mammalian
ovary is still in a confused state as indicated by the evidence presented above
and in the table on pp. 121-124. Much more evidence is needed before one
can rule out the probability that the primordial germ cells are the progenitors
of the definitive germ cells in the mammals. To admit the early origin of pri-
mordial germ cells on the one hand, and to maintain that they later disappear
to be replaced by a secondary origin of primitive germ cells from the germinal
epithelium has little merit unless one can disprove the following position, to
wit: that, while some of the primordial germ cells undoubtedly do degenerate,
others divide into smaller cells which become sequestered within or imme-
diately below the germinal epithelium of the ovary and within the germinal
epithelium of the seminiferous tubules of the testis, where they give origin
by division to other gonial cells. Ultimately some of these primitive gonia
pass on to become definitive germ cells.
However, aside from the controversy whether or not the primordial germ
cells give origin to definitive germ cells, another aspect of the germ-cell
problem emphasizing the importance of the primitive germ cells is posed by
GERM-CELL ORIGIN
121
the following question: Will the gonad develop into a functional structure
without the presence of the primordial germ cells? Experiments performed
by Humphrey ('27) on Ambystoma, and the above-mentioned workers —
Benoit ('30), Goldsmith ('35), and Willier ('37) — on the chick, suggest that
only sterile gonads develop without the presence of the primordial germ cells.
Finally, another facet of the germ-cell problem is this: Are germ cells
completely self differentiating? That is, do they have the capacity to develop
by themselves; or, are the germ cells dependent upon surrounding gonadal
tissues for the influences which bring about their differentiation? All of the
data on sex reversal in animals, normal and experimental (Witschi, in Allen,
Danforth, and Doisy, '39), and of other experiments on the development
of the early embryonic sex glands (Nieuwkoop, '49) suggest that the germ
cells are not self differentiating but are dependent upon the surrounding
tissues for the specific influences which cause their development. Furthermore,
the data on sex reversal shows plainly that the specific chromosome complex
(i.e., male or female) within the germ cell does not determine the differen-
tiation into the male gamete or the female gamete, but rather, that the influ-
ences of the cortex (in the female) and the medulla (in the male) determine
the specific type of gametogenesis.
The table given on pp. 121-124 summarizes the conclusions which some
authors have reached concerning germ-cell origin in many vertebrates. It is
not complete; for more extensive reviews of the subject see Everett ('45),
Heys ('31), and Nieuwkoop ('47, '49).
Species
Place uf Origin, etc.
Author
Entosphenus wilderi
(brook lamprey)
Germ cells segregate early in the em- Okkelberg. 1921.
bryo; definitive germ cells derived J. Morphol. 35
from "no other source"
Petroinyzon muriniis
unicolor (lake
lamprey)
Definitive germ cells derive from:
a) early segregated cells, primor-
dial germ cells, and b) later from
coelomic epithelium. Suggests that
primordial germ cells may induce
germ-cell formation in peritoneal
epithelium
Butcher. 1929.
Biol. Bull. 56
Sqiialus acanthias
Germ cells segregate from primitive
entoderm; migrate via the meso-
derm into site of the developing
gonad
Woods. 1902.
Am. J. Anat. 1
Amia and Germ cells segregate early from en- Allen. 1911.
Lepidosteiis toderm; continue distinct and mi- J. Morphol. 22
grate into the developing gonad
via the mesoderm (see fig. 60)
122
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
Species
Place of Origin, etc.
Author
Lophius piscatoriiis
Germ cells segregate from primitive
entoderm; migrate through meso-
derm to site of gonad; migration
part passive and part active
Dodds. 1910.
J. Morphol. 21
Fundulus Germ cells segregate from peripheral
heteroclitiis entoderm lateral to posterior half
of body; migrate through ento-
derm and mesoderm to the site
of the developing gonad
Cottiis hairdii Primordial germ cells derive from
giant cells, in the primitive ento-
derm; migrate through the lateral
mesoderm into the site of the de-
veloping gonad
Lebistes reticulatus
(guppy)
Germ cells segregate early in devel-
opment; first seen in the entoderm-
mesoderm area; migrate into the
sites of the developing ovary and
testis, giving origin to the defini-
tive germ cells
Richards and Thomp-
son. 1921.
Biol. Bull. 40
Hann. 1927.
J. Morphol. 43
Goodrich, Dee, Flynn,
and Mercer. 1939.
Biol. Bull. 67
Rana temporaria Germ cells segregate from primitive
entoderm; migrate into developing
genital glands
Rana temporaria Primordial germ cells from ento-
derm discharged at first spawning.
Later, the definitive germs cells of
adults originate from peritoneal
cells
Witschi. 1914.
Arch. f. mikr. Anat. 85
Gatenby. 1916.
Quart. J. Micr. Sc. 61
Rana catesbiana Primordial germ cells segregate
from primitive entoderm; defini-
tive germ cell derives from pri-
mordial cells according to author's
view but admits possibility of ger-
minal epithelium origin
Rana temporaria,
Triton alpestris,
Bufo vulgaris
Primordial germ cells segregate from
entoderm
Swingle. 1921.
J. Exper. Zool. 32
Bounoure. 1924.
Compt. rend. Acad. d.
Sc. 178, 179
Rana sylvatica Primordial germ cells originate from
entoderm and migrate into the de-
veloping gonads. They give origin
to the definitive sex cells
Witschi. 1929.
J. Exper. Zool. 52
Hemidactylium Primordial germ cells arise in meso-
scutatum derm between somite and lateral
plate; move to site of gonad by
shifting of tissues
Humphrey. 1925.
J. Morphol. 41
GERM-CELL ORIGIN
123
Species
Place of Origin, etc.
Author
Ambystoma
maculutiim
Most germ cells somatic in origin
from germinal epithelium, al-
though a few may come from pri-
mordial germ cells of entodermal
origin
McCosh. 1930.
J. Morphol. 50
Triton, and
Ambystoma
mexicanum
Germ cells differentiate from lateral
plate mesoderm
Nieuwkoop. 1946.
Arch. Neerl. de zool. 7
Chrysemys
marginata
(turtle)
Primordial germ cells from ento-
derm; most of definitive germ cells
arise from peritoneal cells
Dustin. 1910.
Arch, biol., Paris. 25
Sternotherus
odoratus (turtle)
Primordial cells segregate early from Risley. 1934.
entoderm; later definitive cells de- J. Morphol. 56
rive from these and from peri-
toneal epithelium
Callus (domesticus)
gallus (chick)
Germ cells arise from primitive cells
in entoderm of proamnion area
and migrate by means of the
blood vessels to the site of the
developing gonad. Definitive germ
cells of sex cords and later semi-
niferous tubules derive from pri-
mordial germ cells
Swift. 1914, 1916.
Am. J. Anat. 15, 20
Gallus (domesticus)
gallus (chick)
Primordial germ cells arise from en-
todermal cells
Dantschakoff. 1931.
Zeit. f. Zellforsch.,
mikr. Anat. 15
Chick and albino Early primordial cells degenerate; Firket. 1920.
rat definitive cells from peritoneal Anat. Rec. 18
epithelium
Didelphys
virginiana
(opossum)
Germ cells arise from germinal epi-
thelium
Nelsen and Swain.
1942.
J. Morphol. 71
Mus musculus Oogonia derived from primordial Kirkham. 1916.
(mouse) germ cells; spermatogonia from Anat. Rec. 10
epithelial cells of testis cords
Mus musculus Primordial germ ceils of ovary arise
(mouse) from germinal epithelium during
development of the gonads. These
presumably give origin to the de-
finitive sex cells
Fells domestica Primordial cells segregate early but
(cat) do not give origin to definitive
germ cells which derive from ger-
minal epithelium
Brambell. 1927.
Proc. Roy. Soc. Lon-
don, s.B. 101
de Winiwarter and
Sainmont. 1909.
Arch, biol., Paris. 24
124
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
Species
Place of Origin, etc.
Author
Felis domestica Definitive ova derived from germinal Kingsbury. 1938.
(cat) epithelium of the ovary at an early Am. J. Anat. 15
stage of gonad development
Cavia porcellus Primordial germ cells from entoder-
(guinea pig) mal origin degenerate; the primor-
dial germ cells derived from the
germinal epithelium give rise to
the definitive germ cells in the
testis
Bookkout. 1937.
Zeit. f. Zellforsch,
mikr. Anat. 25
Homo sapiens Primordial germ cells found in en-
(man) toderm of yolk sac; migrate by
ameboid movement into develop-
ing gonad
Witschi. 1948.
Carnegie Inst., Wash-
ington Publ. 575.
Contrib. to Embryol;
32
C. Maturation (Differentiation) of the Gametes
1. General Considerations
Regardless of their exact origin definitive germ cells as primitive oogonia
or very young oocytes are to be found in or near the germinal epithelium
in the ovaries of all vertebrates in the functional condition (figs. 39B, 64).
In the testis, the primitive spermatogonia are located within the seminiferous
tubules as the germinal epithelium, in intimate association with the basement
membrane of the tubule (figs. 65, 66).
The period of coming into maturity (maturation) of the gametes is a com-
plicated affair. It involves profound transformations of the cytoplasm, as
well as the nucleus. Moreover, a process of ripening or physiological ma-
turing is necessary, as well as a morphological transformation. The phrase
"maturation of the germ cells" has been used extensively to denote nuclear
changes. However, as the entire gamete undergoes morphological and physio-
logical change, the terms nuclear maturation, cytosomal maturation, and
physiological maturation are used in the following pages to designate the
various aspects of gametic development.
One of the most characteristic changes which the germ cell experiences
during its maturation into a mature gamete is a reduction of chromatin ma-
terial. Because of this, the germ cell which begins the maturing process is
called a meiocyte. This word literally means a cell undergoing diminution and
it is applied to the germ cell during meiosis or the period in which a reduction
in the number of chromosomes occurs. The word haplosis is a technical
name designating this reduction process.
The word meiocyte thus is a general term applicable to both the developing
MATURATION OF GAMETES
125
BASEMENT MEM
OF SE MINI FE R 0
TUBULE
SPERM ATOGONIUM
INTERSTITIAL CELLS
Fig. 65. Semidiagrammatic representation of a part of the seminiferous tubule of the
cat testis.
male and female germ cells. On the other hand, the word spermatocyte is
given to the developing male gamete during the period of chromosome dimi-
nution, whereas the word oocyte is applied to the female gamete in the same
period. When, however, the period of chromosome diminution is completed
and the chromosome number is reduced to the haploid condition, the devel-
oping male gamete is called a spermatid while the female gamete is referred
to as an ootid or an egg. {Note: the word egg is applied often to the female
gamete during the various stages of the oocyte condition as well as after the
maturation divisions have been accomplished.)
The reduction of chromatin material is not the only effect which the meiotic
process has upon the chromatin material, or possibly upon the developing
cytosomal structures as well. This fact will become evident during the descrip-
tions below concerning the meiotic procedures.
Another prominent feature of the gametes during the meiocyte period is
their growth or increase in size. This growth occurs during the first part of the
meiotic process when the nucleus is in the prophase condition and it involves
both nucleus and cytoplasm. The growth phenomena are much more pro-
nounced in the oocyte than in the spermatocyte. Due to this feature of growth,
the oocyte and spermatocyte also are regarded as auxocytes, that is growing
cells, a name introduced by Lee, 1897. The words meiocyte and auxocyte
thus refer to two different aspects of the development of the oocyte and the
spermatocyte.
126 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
2. Basic Structure of the Definitive Sex Cell as It Starts to
Mature or Differentiate into the Male Meiocyte (i.e., the
Spermatocyte) or the Female Meiocyte (i.e., the Oocyte)
The definitive sex cells of both sexes have a similar cytological structure.
The component parts are (fig. 68) :
(1) nucleus,
(2) investing cytoplasm,
(3) idiosome,
(4) Golgi substance, and
(5) chondriosomes.
The nucleus is vesicular and enlarged, and the nuclear network of chro-
matin may appear reticulated. A large nucleolus also may be visible. The
investing cytoplasm is clearer and less condensed in appearance than that
of ordinary cells. The idiosome (idiozome) is a rounded body of cytoplasm
which, in many animal species, takes the cytoplasmic stain more intensely
than the surrounding cytoplasm. Within the idiosome it is possible to demon-
strate the centrioles as paired granules in some species. Surrounding the
idiosome are various elements of the Golgi substance, and near both the
idiosome and Golgi elements, is a mass of chondriosomes (mitochondria) of
various sizes and shapes. The idiosome and its relationship with the Golgi
material, the mitochondria, and the centrioles varies considerably in different
species of animals.
Much discussion has occurred concerning the exact nature of the idiosome.
Some investigators have been inclined to regard the surrounding Golgi sub-
stance as a part of the idiosome, although the central mass of cytoplasm con-
taining the centrioles is the "idiosome proper" of many authors (Bowen, '22).
Again, when the maturation divisions of the spermatocyte occur, the idiosome
and surrounding Golgi elements are broken up into small fragments. How-
ever, in the spermatids the Golgi pieces (dictyosomes) are brought together
once more to form a new idiosome-like structure, with the difference that the
latter "seems never to contain the centrioles" (Bowen, '22). It is, therefore,
advisable to regard the idiosome as being separated into its various com-
ponents during the maturation divisions of the spermatocyte and to view the
reassemblage of Golgi (dictyosomal) material in the spermatid as a different
structure entirely. This new structure of the spermatid is called the acroblast
(Bowen, '22; Leuchtenberger and Schrader, '50). (See fig. 68B.) A similar
breaking up of the idiosome occurs in oogenesis (fig. 68F, G). However, all
meiocytes do not possess a typical idiosome. This fact is demonstrated in
insect spermatocytes, where the idiosomal material is present as scattered
masses to each of which some Golgi substance is attached.
The various features which enter into the structure of the definitive germ
MATURATION OF GAMETES
127
cell do not behave in the same way in each sex during gametic differentiation.
While the behaviors of the chromatin material in the male and female germ
cells closely parallel each other (fig. 67), the other cytosomal features follow
widely divergent pathways, resulting in two enormously different gametic
entities (fig. 68A-H).
RED BLOOD
CORPUSCLE
INTERSTITIAL
CELL
SPERM ATO-
G ON lU M
LUMEN OF TUBULE
Fig. 66. Section of part of a seminiferous tubule of human testis. (Redrawn from Gatenby
and Beams, '35.)
RESTING
CONDITION
PROPHASE -
STAG E S
PRIMARY
SPERMATOCYTE
■•*•*** homologue
FROM ONE
PARE NT
HOMOLOGUE
FROM OTHER
P4R ENT
FIRST
M EIOTI C
DIVI SION
SECONDAR V
S PE H MATOCT TE
OR OOCYTE
SECOND Jii^
M E I 0 T I C ^"^C»^
DIVI SION ^
(1^ (m f1^ fit:
Fig. 67. Diagrammatic representation of the nuclear changes occiirrmg during meiosis
in spermatocyte and oocyte. Six chromosomes, representing three homologous pairs, are
used. Observe the effects of the crossing over of parts of chromatids. The diplotene con-
dition of oocyte depicted by arrows and tfie enlarged nucleus. 1 he haploid condition is
shown in each of the spermatids or in the egg and its three polocytes.
128
SPE RMflTOG E N ESIS
OOGENESIS
Dl 0 SOME
NUCLEUS BEGINS TO
ENLARGE
GOLGI MATERIAL DISPERSED
MITOCHONDRIA
ARG ING NUCLE US
^ ARI ZE D AT ONE SIDE
OF OOCYTE
GOLGI POSSIBLY
CONCERNED WITH ORIGIN
OF FAT DROPLETS
MITOCHONDRIA BECOME
ISPERSEO IN CYTOPLASM
D IN FORMING YOLK
GERMfNAL VESICLE
ANTERIOR
C E N T R lOLES
MITOCHONDRI
POSTERIOR
■ CENTRIOLE'
GOLG I SU B STAN (
AND CYT OPL AS M
DISCARDED
AXIAL FILAME
THES E
SURROU N D
NUC LE US
DISCARDED
CYTOPLASM AND — rrjc
GOLGI SUBSTANCE|;J
FLAGELLUM WITH
CONTAINE D AX I A I
FILAMENT
FAT DROPLETS FORMED
IN RELATION TO
GOLGI ELEMENT
PRIMARY
^ ;(^ EM8R YON IC
Fig. 68. Possible fate of the primitive meiocyte and its cytoplasmic inclusions when
exposed to testicular or ovarian influences. Particular attention is given to the idiosome.
Under male-forming influences the idiosome components are dispersed during the matura-
tion divisions and are reassembled into three separate component structures, namely,
(I) Acroblast of Golgi substance, (2) centriolar bodies, and (3) mitochondrial bodies
(see B). Each of these structures, together with the post-nuclear granules of uncertain
origin, play roles in spermatogenesis as shown. Under ovarian influences the idiosome
is dispersed before the maturation divisions. The Golgi substance and mitochondria play
(according to theory, see text) their roles in the formation of the deutoplasm.
129
130 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
3. Nuclear Maturation of the Gametes
Most of our information concerning the maturation of the nucleus pertains
to certain aspects of chromosome behavior involved in meiosis, particularly
the reduction of the chromosome number together with some activities of
"crossing over" of materials from one chromosome to another. But our infor-
mation is vague relative to other aspects of nuclear development. For example,
we know little about the meaning of growth and enlargement of the nucleus
as a whole during meiosis, an activity most pronounced in the oocyte. Nor
do we know the significance of nuclear contraction or condensation in the
male gamete after meiosis is completed. Therefore, when one considers the
nuclear maturation of the gametes, it is necessary at this stage of our knowledge
to be content mainly with observations of chromosomal behavior.
a. General Description of the Chromatin Behavior During Somatic and
Meiotic Mitosis
As the maturation behavior of the chromatin components in the spermato-
cyte and oocyte are similar, a general description of these activities is given
in the following paragraphs. Before considering the general features and de-
tails of the actions of the chromosomes during meiosis, it is best to recall
some of the activities which these structures exhibit during ordinary somatic
and gonial mitoses.
Cytological studies have shown that the chromosomes, in most instances,
are present in the nucleus in pairs, each member of a pair being the homologue
or mate of the other. Homologous chromosomes, therefore, are chromosomal
pairs or mates. During the prophase condition in ordinary somatic and gonial
mitoses, the various chromosomal mates do not show an attraction for each
other. A second feature of the prophase stage of ordinary cell division is that
each chromosome appears as two chromosomes. That is, each chromosome
is divided longitudinally and equationally into two chromosomes. At the time
when the metaphase condition is reached and the chromosomes become ar-
ranged upon the metaphase plate, the two halves or daughter chromosomes
of each original chromosome are still loosely attached to each other. However,
during anaphase, the two daughter chromosomes of each pair are separated
and each of the two daughter nuclei receives one of the daughter chromo-
somes. Reproduction of the chromatin material and equational distribution
of this material into the two daughter cells during anaphase is a fundamental
feature of the ordinary type of somatic and gonial mitoses. The two daughter
nuclei are thus equivalent to each other and to the parent nucleus. In this
way, chromosomal equivalence is passed on ad infinitum through successive
cell generations.
On the other hand, a different kind of chromosomal behavior is found
during meiosis, which essentially is a specialized type of mitosis, known as
a meiotic mitosis. In one sense it is two mitoses or mitotic divisions with only
MITOSIS OF
SECONDARY
SPERMATO-
GONIUM
MEIOSIS BEGINS
LEPTOTENE
SEX CHROMOSOME
Z YG OTENE
PACHYTENE
Dl PLO T E N E
FIRST
MATURA TION
DIVISION
SECOND
MATURATION
DIVISION
SPERMIOGENESIS
TRA NSFORMATION
OF SPERMATID TO
SPERM Y
PICAL CELL WITH PRIMARY
SPER M ATOGONIA
RIOD OF MULTIPLICATION
SECONDARY SPERMATOGONIA
DIVIDE MITOTIC ALLY
E R lOD OF GROWT H
PRIMARY SPERMATOCYTES
INCREASE IN SIZE WHILE IN
SUCCESSIVE STAGE S OF
PROPHAS E
APICAL OR NURSE
CELL
FIRST MATURATION DIVISION
PRIMARY SPERMATOCYTES FORM
SECONDARY SPERMATOCYTES
SECONDARY MATURATION DIVISION
SECONDARY SPERMATOCYTES
FORM SPER MATI D S
- PERIOD OF TRANSFORMATION
SPERMATID CHANGED TO
SPERM
Fig. 69. Steps in spermatogenesis in the grasshopper. In the center of the chart is
represented a longitudinal section of one of the follicles of a grasshoper testis with its
various regions of spermatogenic activity. In the upper right of the chart the apical-cell
complex is depicted with its central apical cell, spermatogonia, and surrounding epithelial
cells. The primary spermatogonia lie enmeshed between the extensions of the apical cell
and the associations of these extensions with the surrounding epithelial elements of
the complex. (Also see Wenrich, 1916, Bull. Mus. Comp. Zool. Harvard College, 60.)
131
132 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
one prophase; that is, two metaphase-anaphase separations of chromosomes
preceded by a single, pecuUar prophase. The pecuHarities of this meiotic
prophase may be described as follows: As the prophase condition of the
nucleus is initiated, an odd type of behavior of the chromosomes becomes
evident — a behavior which is entirely absent from ordinary somatic mitosis:
namely, the homologous pairs or mates begin to show an attraction for each
other and they approach and form an intimate association. This association
is called synapsis (figs. 67, 69, zygotene stage). As a result, the two homolo-
gous chromosomes appear as one structure. As the homologous chromosomes
are now paired together and superficially appear as one chromosome, the
number of "chromosomes" visible at this time is reduced to one-half of the
ordinary somatic or diploid number. However, each "chromosome"' is in reality
two chromosomes and, therefore, is called a bivalent or twin chromosome.
While the homologous chromosomes are intimately associated, each mate
reproduces itself longitudinally just as it would during an ordinary mitosis
(fig. 67, pachytene stage). (The possibility remains that this reproduction of
chromatin material may have occurred even before the synaptic union.)
Hence, each bivalent chromosome becomes transformed into four potential
chromosomes, each one of which is called a chromatid. This group of chroma-
tids is, collectively speaking, a tetrad chromosome. (As described below,
interchange of material or crossing over from one chromatid to another may
take place at this time.) As a result of these changes, the nucleus now con-
tains the haploid number of chromosomes, (i.e., half of the normal, diploid
number) in the form of tetrads (fig. 67, pachytene stage). However, as each
tetrad represents four chromosomes, actually there is at this time twice the
normal number of chromosomes present in the nucleus (fig. 67; compare
leptotene, pachytene, diplotene and diakinesis).
The next step in meiosis brings about the separation of the tetrad chromo-
some into its respective chromatids and it involves two divisions of the cell.
These divisions are known as meiotic divisions. As the first of these two
divisions begins, the tetrad chromosomes become arranged in the mid- or
metaphase plane of the spindle. After this initial step, the first division of
the cell occurs, and half of each tetrad (i.e., a dyad) passes to each pole of
the mitotic spindle (fig. 67, first meiotic division). Each daughter cell (i.e.,
secondary spermatocyte or oocyte) resulting from the first maturation
(meiotic) division thus contains the haploid or reduced number of chromo-
somes in the dyad condition, each dyad being composed of two chromatids.
A resting or interphase nuclear condition occurs in most spermatocytes, fol-
lowing the first maturation division, but in the oocyte it usually does not
occur (fig. 69, interkinesis).
As the second maturation division is initiated, the dyads become arranged
on the metaphase plate of the mitotic spindle. As division of the cell proceeds,
half of each dyad (i.e., a monad) passes to the respective poles of the spindle
MATURATION OF GAMETES 133
(fig. 67, second meiotic division; fig. 69). As a result of these two divisions,
each daughter cell thus contains the haploid or reduced number of chromo-
somes in the monad (monoploid) condition (fig. 67, spermatid or egg). Meiosis
or chromatin diminution is now an accomplished fact.
It is to be observed, therefore, that the meiotic phenomena differ from
those of ordinary mitosis by two fundamental features:
(1) In meiosis there is a conjugation (synapsis) of homologous chromo-
somes during the prophase stage, and while synapsed together each
of the homologues divides equationally; and
(2) following this single prophase of peculiar character, two divisions
follow each other, separating the associated chromatin threads.
While the meiotic prophase is described above as a single prophase pre-
ceding two metaphase-anaphase chromosome separations, it is essentially a
double prophase in which the process of synapsis acts to suppress one of
the equational divisions normally present in a mitotic division; a synapsed
or double chromosome, therefore, is substituted for one of the longitudinal,
equational divisions which normally appears during a somatic prophase. It is
this substitution which forms the basis for the reduction process, for two
mitotic divisions follow one after the other, preceded by but one equational
splitting, whereas in ordinary mitosis, one equational splitting of the chromo-
somes always precedes each mitotic division.
b. Reductional and Equational Meiotic Divisions and the Phenomenon of
Crossing Over
In the first meiotic division (i.e., the first maturation division), if the two
chromatids which are derived from one homologous mate of the tetrad are
separated from the two chromatids derived from the other homologous mate
the division is spoken of as reductional or disjunctional. In this case the two
associated chromatids of each dyad represent the original chromosome which
synapsed at the beginning of meiotic prophase (fig. 67, tetrads B and C,
first meiotic division). If, however, the separation occurs not in the synaptic
plane but in the equational plane, then the two associated chromatids of each
dyad come, one from one synaptic mate and one from the other; such a
division is spoken of as an equational division (fig. 67, tetrad A, first meiotic
division). There appears to be no fixity of procedure relative to the separation
of the tetrads, and great variability occurs. However this may be, one of the
two meiotic divisions as far as any particular tetrad is concerned is disjunc-
tional (reductional) and the other is equational, at least in the region of the
kinetochore (see p. 135 and fig. 70). If the first division is reductional, the
second is equational and vice versa. Disjunction in the first maturation division
is often referred to as pre-reduction, while that in the second maturation di-
vision is called post-reduction.
N
A'
(•;-.-/nv,-.-.-.-;. ;•.■.••;•:.:/:.•■•.•.•■. ■.•.■:■.•..■..)
A.
8.
A.
B.
A.
B.
flHT'^^^^H'.'V :';-:: ^^^^^^^B
B'
Fig. 70. Some of the various possibilities which may occur as a result of the exchanges
of parts of chromatids during the crossing-over phenomena associated with meiosis. Two
chiasmata (singular, chiasma) are shown in (A), (C), (E). Observe that homologous
chromosome A has split equationaliy into chromatids A and A', while homologous
chromosome B has divided equationaliy into B and B'. The resulting interchanges be-
tween respective chromatids of the original homologous chromosomes are shown in (B),
(D), (F). The kinetochore (place of spindle-fiber attachment) is indicated by the oval
or circular area to the left of the chromatids. (Modified from White: Animal Cytology
and Evolution, London, Cambridge University Press, 1943.)
134
MATURATION OF GAMETES 135
The foregoing statement regarding disjunctional and equational divisions
should be considered in the Hght of the phenomenon of crossing over. In
the latter process, a gene or groups of genes may pass from one chromatid
to the other and vice versa during their association at the four strand stage
(fig. 70). In the region of the centromere or kinetochore (i.e., the point)
of the achromatic, spindle-fiber attachment) and nearby regions, cross overs
are thought not to occur (fig. 70, kinetochore). Consequently, in the regions
of the kinetochore, the statements above regarding disjunctional and equa-
tional divisions of the chromosomes appear to be correct. However, the terms
disjunctional and equational may mean little in other regions of the chromo-
somes of a tetrad during the meiotic divisions. For example, let us assume as
in fig. 70 (see also fig. 67), that we have chromatids A and A', B and B',
A and B representing the original homologues or synaptic chromosomes which
have divided into these chromatids respectively. Then during the tetrad stage
of association or slightly before, let us assume that there has been a crossing
over of genes from chromatid A to chromatid B and from chromatid B to
chromatid A in a particular area (fig. 70A). (It is to be observed that chroma-
tids A' and B' are not involved in this particular instance.) Further, let us
assume that AA' and BB' as a whole are separated at the first maturation
division, the kinetochore and immediate regions would represent a disjunc-
tional division, but for the particular area where crossing over is accomplished,
the division would be equational (fig. 70A, B; central portions of chromatids
A and B in fig. 70B). Thus, it would be for other regions where cross overs
may have occurred. Other cross-over possibilities are shown in fig. 70C-F.
c. Stages of Chromatin Behavior During the Meiotic Prophase in
Greater Detail
The following five stages of chromatin behavior within the prophase nucleus
during meiosis are now in common usage. They are based on the stages
originally described by H. von Winiwarter, '00. The substantive form is pre-
sented in parentheses.
1) Leptotene (Leptonema) Stage. The leptotene stage (figs. 69, 71) repre-
sents the initial stage of the meiotic process and is seen especially well in
the spermatocyte. At this time the nucleus of the differentiating germ cell
begins to enlarge, and the diploid number of very long, slender chromatin
threads make their appearance. (Compare "resting" and leptotene nuclei in
figs. 69, 71.) The chromatin threads may lie at random in the nucleus or
they may be directed toward one side, forming the so-called "bouquet" con-
dition (fig. 69, leptotene stage) . The nucleolus is evident at this time (fig. 7 IB) .
2) Zygotene or Synaptene (Zygonema) Stage. The zygotene stage (figs. 69,
71, 85) is characterized by a synapsis of the chromatin threads. This synapsis
or conjugation occurs between the homologous chromosomes, that is, the
chromosomes which have a similar genie constitution. Synapsis appears to
A. RESTING NUCLEUS
CHROMOSOMES THREAD-LIKE
AND SCATTERED THROUGH-
OUT NUCLEUS, NOT VISIBLE
AS DISTINCT CHROMOSOMES
B. LEPTOTENE NUCLEUS
CHROMOSOMES BECOME
EVIDENT AS DISTINCT
STRUCTURES
NUCLEOLUS
C ZYGOTENE (SYNAPTENE)
NUCLEUS
HOMOLOGOUS CHROMOSOMES
BEGIN TO SYNAPSE,! E,
UNITE IN PAIRS
D PACHYTENE NUCLEUS
CHROMOSOME PAIRS BECOME
CLOSELY ASSOCIATED AND
EACH PAIR SHORTENS AND
APPEARS MUCH THICKER
EACH"CHR0M0S0ME" IS MADE
UP OF FOUR CHROMATIDS,! E .
IT IS A TETRAD DUE TO THE
FACT THAT EACH HOMOLOGOUS
CHROMOSOME WHICH ENTERED
INTO THE ORIGINAL UNION HAS
DIVIDED LONGITUDINALLY INTO
TWO CHROMATIDS
E. DIPLOTENE NUCLEUS
THE CHROMATIDS OF EACH
TETRAD SHOW A REPULSION FOR
EACH OTHER AND THE TETRAD
AS A WHOLE BEGINS TO OPEN
UP THE CHROMATIDS ALSO
BECOME MORE ATTENUANT,
PARTICULARLY IN THE OOCYTE
WHERE THE NUCLEUS EN-
LARGES AND FORMS THE
GERMINAL VESICLE IN WHICH
THERE IS A LARGE AMOUNT
OF NUCLEAR SAP THE
CHROMATIDS BECOME VERY
LONG AND MAY SHOW LATERAL
LOOPS
F GERMINAL VESICLE
YOUNG GERMINAL VESICLE
OF CAT OOCYTE IN TRANSI-
TION FROM THE MORE
TYPICAL DIPLOTENE CONDI-
TION INTO THE MATURE
GERMINAL VESICLE SHOWN
BELOW
G GERMINAL VESICLE
THE GERMINAL VESICLE STAGE
OF THE OOCYTE NUCLEUS OF
THE CAT IN WHICH THE
V/ DIPLOTENE CHROMATIDS ARE
ELOMGATED AND DO NOT
TAKE THE BASIC STAINS
READILY
H GERMINAL VESICLE
GERMINAL VESICLE OF
DEVELOPING SHARK OOCYTE
SHOWING MANY NUCLEOLI
AND "LAMP-BRUSh"cHR0MO-
SOMES
I GERMINAL VESICLE
GERMINAL VESICLE OF
AMPHIOXUS OOCYTE WITH
LARGE NUCLEOLUS!?)
Fig. 71. Certain aspects of the oocyte nucleus during the meiotic prophase. (A-G)
Chromatin and nuclear changes in the oocyte of the cat up to the diplotene condition
when the germinal vesicle is fully developed. (After de Winiwarter and Sainmont, Arch,
biol., Paris, 24.) (H, 1) Germinal vesicle in the dogfish, Scyllium canicula, and in
Amphiuxus. (After Marechal, La Cellule, 24.) Observe the typical "lamp-brush" chromo-
some conditions in the germinal vesicle of the shark oocyte. These lamp-brush chromo-
somes are developed during the diplotene stage of meiosis by great attenuation of the
chromosomes and the formation of lateral extensions or loops from the sides of the
chromosomes.
136
MATURATION OF GAMETES 137
begin most often at the ends of the threads and progresses toward the middle
(fig. 67, zygotene). At this stage the chromatin threads may show a strong
tendency to collapse and shrink into a mass toward one end of the nucleus
(fig. 85C, D). This collapsed condition, when present, is called synizesis.
The zygotene stage gradually passes into the pachytene condition.
3) Pachytene (Pachynema) Stage. Gradually, the synapsis of the homolo-
gous chromosomes becomes more complete, and the threads appear shorter
and thicker. The contracted threads in this condition are referred to as
pachynema (figs. 69, 71, 85E). The nucleus in this manner comes to contain
a number of bivalent chromosomes, each of which is made up of two homolo-
gous mates arranged side by side in synaptic union, known technically as
parasynapsis. (Telosynapsis probably is not a normal condition.) Conse-
quently, the number of chromosomes now appears to be haploid. Each pachy-
tene chromosome (i.e., each of the pair of homologous chromosomes) grad-
ually divides equationally into two daughter thread-like structures, generally
referred to as chromatids. The exact time at which division occurs during
meiosis is questionable. The entire group of four chromatids which arise from
the splitting of the synapsed homologues is called a tetrad.
4) Diplotene (Diplonema) Stage. In the diplotene stage (figs. 67, 69, 71,
85F, G), two of the chromatids tend to separate from the other two. (See
fig. 70A, C, E.) The four chromatids in each tetrad may now be observed
more readily, at least in some species, because the various chromatids of
each tetrad show a repulsion for one another, and the chromatids move
apart in certain areas along their length. This condition is shown in both the
male and female meiocyte, but in the latter, the repulsion or moving apart
is carried to a considerable degree and is associated with a great lengthening
and attenuation of the chromatids. (See fig. 67.) In the female meiocyte at this
stage, the chromosomes become very diffuse and are scattered throughout
the nucleus, somewhat resembling the non-mitotic condition (figs. 71F-I;
72B-E). The peculiar behavior of the chromosomes and nucleus of the oocyte
in the diplotene stage of meiosis is described more in detail on p. 141.
Although there is a tendency for the chromatids to widen out or separate
from each other at this time; they do remain associated in one or more regions.
In these regions of contact, the paired chromatids appear to exchange partners.
This point of contact is called a chiasma (plural, chiasmata). Hence, a
chiasma is the general region where the chromatids appear to have exchanged
partners when the tetrad threads move apart in the diplotene state. (See fig.
70, chiasmata.)
5) Diakinesis. The diplotene stage gradually transforms into the diakinesis
state (figs. 67, 69, 72F, 85H) by a process of marked chromosomal con-
traction. There also may be an opening up of the tetrads due to a separation
of the homologous mates in the more central portions of the tetrad, with the
result that only the terminal parts of the chromatids remain in contact. This
FOLLICLE CELL
Fig 72 Growth of the nucleus during meiosis in the amphibian egg, showing the
enlarged germinal vesicle and diplotene lamp-brush chromosomes with lateral oops.
(A) Early diplotene nucleus of the frog. (B, C, E) Different phases of the diplotene
nucleus in this form. These figures are based upon data provided by Duryee ( 50 and
sections of the frog ovary. (D) Drawing of the unfixed germinal vesicle of Tritums.
Some aspects of the attenuate chromatin threads with lateral loops are shown. The
nucleoli are numerous and occupy the peripheral region of the germinal vesicle. (F)
Semidiagrammatic drawing of the later phases of the developing frog egg. It shows the
germinal vesicle assuming a polar condition, with the initial appearance of germinal
vesicle shrinkage before the final dissolution of the nuclear membrane. Observe that the
chromosomes are contracting and now occupy the center of the germinal vesicle.
138
MATURATION OF GAMETES
139
latter process is called "terminalization." Coincident with this partial separa-
tion, a further contraction of the tetrads may occur. As a result, at the end
of diakinesis the tetrads may assume such curious shapes as loops, crosses,
rings, etc., scattered within the nucleus of the female and male meiocyte (fig.
69, diakinesis). The nuclear membrane eventually undergoes dissolution, and
MITOCHONDRIA
MATERIAL
E R M HEAD
REMNANT
Fig. 73. Various aspects of Sertoli-cell conditions in the fowl. (Redrawn from Zlotnick,
Quart. J. Micr. Sc, 88.) (A) Resting Sertoli cell, showing mitochondria. (B) Sertoli
element at the beginning of cytoplasmic elongation. (C) Sertoli cell with associated
late spermatids.
Fig. 74. Types of chordate sperm. All the chordate sperm belong to the flagellate
variety. {A) Amphioxus iprotochordate). (B) Sa/mo (teleost). (C) Perca (teleost).
(D) Petromyzon (cyclostome). (E) Raja (elasmobranch). (F) Biifo (anuran). (G)
Rana (anuran). (H) Salamandra (urodele). (I) Angiiis (lizard). (J) Crex (bird).
(K) Fringilla (bird). (L) Tardus (bird). (M) Echidna (monotrematous mammal).
(N) Mus (eutherian mammal). (O, P) Man (full view and side view, respectively).
140
MATURATION OF GAMETES
141
the tetrads become arranged on the metaphase plate of the first maturation
division. (See figs. 69, first maturation division; 72F, 119A, B.) This division
is described on pp. 132 and 133.
d. Peculiarities of Nuclear Behavior in the Oocyte During Meiosis;
the Germinal Vesicle
Although the movements of the chromosomes during meiosis in the devel-
oping male and female gamete appear to follow the same general behavior
ACROSOME
Q NUCLEUS
I TOCHONDRIA
Fig. 75
TA I L
Fig. 76
Fig. 75. Non-fiagellate sperm. (A-C) Ameboid sperm of Polyphemus. (After Zach-
arias.) (D) Lobster, Homarus. (After Herrick.) (E) Decapod Crustacea, Galathea
(Anomura). (After Koltzoff.)' (F) Nematode woim, Ascaris.
Fig. 76. Conjugate sperm of grasshopper associated temporarily to form the "sperm boat."
pattern (fig. 67), some differences do occur. For example, in the female when
the diplotene stage is reached, the repulsion of the tetrad threads is greater
(figs. 67, i and $ ; 72). Furthermore, the chromatids elongate and become
very attenuate although they appear to retain their contacts or chiasmata
(fig. 72). Side loops and extensions from the chromatids also may occur,
especially in those vertebrates with large-yolked eggs (e.g., amphibia, fishes,
etc.). (See figs. 71H, 72B-D.) When these lateral extensions are present, the
chromosomes appear diffuse and fuzzy, taking on the characteristics which
142
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
CYTOPLASMIC
MEMBRANE
ACR OSOME
VACUOLE
NUCLEUS
POS T-
UCLEAR CAP
ECK GRANULE
T E R 10 R
CE NT RIOLE
MEMBRANE
OCHONDRIAL
SHEATH
TER 10 R OR
CE NTRIOLE
L FILAMENT
F TAIL
Fig. 77. Spatula-type sperm of various mammals. (Compiled from Bowen; Gatenby
and Beams; Gatenby and Woodger; see references in bibliography.) Observe the vacuole
inside the head of the sperm. Gatenby and Beams found that this vacuole, in some
instances, stains similar to a nucleolus, but suggest it may be a hydrostatic organ, or
respiratory structure. (P. 20, Quart. J. Micr. Sc, 78.)
suggest their description as "lamp-brush" chromosomes. Another difference
of chromatic behavior is manifested by the fact that the chromosomes in the
developing female gamete during the diplotene stage are not easily stained
by the ordinary nuclear stains, whereas the chromosomes in the spermatocyte
stain readily.
Aside from the differences in chromosomal behavior, great discrepancies
in the amount of growth of the nucleus occur in the two gametes during meiosis.
The nucleus of the oocyte greatly increases in size and a large quantity of
nuclear fluid or sap comes to surround the chromosomes (figs. 7 IF, G; 72C,
F, E). Correlated with this increase in nuclear size, the egg grows rapidly,
MATURATION OF GAMETES
143
and deutoplasmic substance is deposited in the cytoplasm (fig. 68F-H). As
differentiation of the oocyte advances, the enlarged nucleus or germinal
vesicle assumes a polar position in the egg (figs. 68H, 70F). When the oocyte
has finished its growth and approaches the end of its differentiation, the
ACROSOME
NUCLEUS
POST ER lOR
NUCLEAR
PLATE
CENTRIOLE
CENTRIOLE
AXIAL
FILAMENT
PROXIMAL
CENTRIOLE
DISTAL
CE N T Rl OLE
Fig. 78
Fig. 79
Fig. 78. Different shapes and positions of the acrosome. (A) Type of acrosome
found in Mollusca, Echinodermata, and Annelida. (B) Reptilia, Aves, and Amphibia.
(C) Lepidoptera. (D) Mammalia. (E) Many Hemiptera and Coleoptera. (After
Bowen, Anat. Rec, 28.) (F) Sperm of certain birds, i.e., finches. (After Retzius, Biol.
Untersuchungen, New Series 17, Stockholm, Jena.) Observe the well-developed acrosome
in the form of a perforatorium. The spiral twist of the acrosome shown in this drawing
is characteristic of passerine birds.
Fig. 79. Sperm of urodele amphibia. (After Meves, 1897, Arch. f. mikr. Anat. u.
Entwichlingsgesch., 50; McGregor, 1899, J. Morphol., 15. (A-E) Stages in the morpho-
genesis of the sperm of Salamandra. (F) Diagram of head, middle piece, etc. of the
sperm of the urodele.
144 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
chromosomes within the germinal vesicle condense once again, decrease in
length (fig. 72F), and assume conditions more typical of the diakinesis stage
(figs. 67; 1 19A). The tetrad chromosomes now become visible. Following the
latter chromosomal changes, the nuclear membrane breaks down (fig. 1 19A),
and the chromatin elements pass onto the spindle of the first maturation divi-
sion (fig. 119B). The nuclear sap, membrane, nucleolus, and general frame-
work pass into the surrounding cytoplasmic substance (figs. 11 9A; 132A-C).
This nuclear contribution to the cytoplasm appears to play an important part
in fertilization and development, at least in some species (fig. 132C; the clear
protoplasm is derived from the nuclear plasm).
e. Character of the Meiotic (Maturation) Divisions in the Spermatocyte
Compared with Those of the Oocyte
1) Dependent Nature of the Maturation Divisions in the Female Meiocyte.
The maturation divisions in the developing male gamete occur spontaneously
and in sequence in all known forms. But in most oocytes, either one or both
of the maturation divisions are dependent upon sperm entrance. For example,
in Ascaris, a nematode worm (fig. 133), and in Nereis, a marine annelid
worm (fig. 130), both maturation divisions occur after the sperm has entered
and are dependent upon factors associated with sperm entrance. A similar
condition is found in the dog (van der Stricht, '23; fig. 115) and in the fox
(Asdell, '46). In the urochordate, Styela, the germinal vesicle breaks down,
the nuclear sap and nucleolus move into the surrounding protoplasm, and
the first maturation spindle is formed as the egg is discharged into the sea
water (fig. 116A, B). Further development of the egg, however, awaits the
entrance of the sperm (fig. 116C-F). Somewhat similar conditions are found
in other Urochordata. In the cephalochordate, Amphioxus, and in the verte-
brate group as a whole (with certain exceptions) the first polar body is formed
and the spindle for the second maturation division is elaborated before normal
sperm entrance (figs. 117C, D; 119D). The second maturation division in
the latter instances is dependent upon the activities aroused by sperm contact
with the oocyte. In the sea urchin, sperm can penetrate the egg before the
maturation divisions occur; but, under these conditions, normal development
of the egg does not occur. Normally in this species both maturation divisions
are effected before sperm entrance, while the egg is still in the ovary. When
the egg is discharged into the sea water, the sperm enters the egg, and this
event affords the necessary stimulus for further development (fig. 131).
2) Inequality of Cytoplasmic Division in the Oocyte. When the first matu-
ration division occurs, the two resulting cells are called secondary spermato-
cytes in the male and secondary oocytes in the female (figs. 67, 69). The
secondary spermatocytes are smaller both in nuclear and cytoplasmic volume.
They also form a definite nuclear membrane. Each secondary spermatocyte
then divides and forms two equal spermatids. In contrast to this condition
MATURATION OF GAMETES 145
of equality in the daughter cells of the developing male gamete during and
following the maturation divisions, an entirely different condition is found
in the developing female gamete. In the latter, one of the secondary oocytes
is practically as large as the primary oocyte, while the other or first polar
body (polocyte) is extremely small in cytoplasmic content although the nuclear
material is the same (fig. 117D). During the next division the secondary
oocyte behaves in a manner similar to that of the primary oocyte, and a
small second polocyte is given off, while the egg remains large (fig. 1 17E, F).
Unlike the secondary spermatocyte, the secondary oocyte does not form a
nuclear membrane. The polar body first formed may undergo a division, re-
sulting in a total of three polar bodies (polocytes) and one egg (ootid).
/. Resume of the Significance of the Meiotic Phenomena
In view of the foregoing data with regard to the behavior of the male and
female gametes during meiosis, the significant results of this process may be
summarized as follows:
( 1 ) There is a mixing or scrambling of the chromatin material brought
about by the crossing over of genie materials from one chromatid to
another.
(2) Much chromatin material with various genie combinations is discarded
during the maturation divisions in the oocyte. In the latter, two polar
bodies are ejected with their chromatin material as described above.
The egg thus retains one set of the four genie combinations which
were present at the end of the primary oocyte stage; the others are
lost. (A process of discarding of chromatin material occurs in the
male line also. For although four spermatids and sperm normally de-
velop from one primary spermatocyte, great quantities of sperm never
reach an egg to fertilize it, and much of the chromatin material is
lost by the wayside.)
(3) A reduction of the number of chromosomes from the diploid to the
haploid number is. a significant procedure of all true meiotic behavior.
(For more detailed discussions and descriptions of meiosis, see De Robertis,
et al., '48; Sharp, '34, '43; Snyder, '45; White, '45.)
4. Cytosomal (Cytoplasmic) Maturation of the Gametes
a. General Aspects of Cytoplasmic Maturation of the Gametes
During the period when the meiotic prophase changes occur in the nucleus
of the oocyte, the cytoplasm increases greatly and various aspects of cyto-
plasmic differentiation are effected. That is, differentiation of both nuclear
and cytoplasmic materials tend to occur synchronously in the developing
146
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
PROACROSOMIC SUBSTANCE
PROACROSOMIC
GRANULES
AXIAL FILAMENT
Fig. 80. Morphogenesis of guinea-pig and human sperm. (A) Spermatocyte of guinea
pig before first maturation division. The Golgi complex with included proacrosomic
granules and centrioles is shown. (After Gatenby and Woodger, '21.) (B) Young sister
spermatids of guinea pig. (C) Later spermatid of guinea pig showing acroblast with
proacrosomic granules. (D) Young human spermatocyte, showing Golgi apparatus with
proacrosomic granules similar to that shown in (A). (After Gatenby and Beams, '35.)
(E) Spermatid of guinea pig later than that shown in (C), showing acroblast with Golgi
substance being discarded from around the acroblast. (F) Later human spermatid,
showing Golgi substance surrounding acroblast with acrosome bead. (After Gatenby and
Beams, '35.) (G) Later human spermatid, showing acroblast, with acrosome bead
within, surrounded by a vacuole. (After Gatenby and Beams, '35.) (H) Later spermatid
of guinea pig, showing outer and inner zones of the acrosome. The inner zone corresponds
somewhat to the acrosome bead shown in (G) of the human spermatid. (After Gatenby
and Wigoder, Proc. Roy. Soc, London. s.B.. 104.)
female gamete. In the male gamete, on the other hand, the meiotic processes
are completed before morphological differentiation of the cytoplasm is initiated.
Another distinguishing feature in the morphogenesis of the sperm relative
to that of the egg is that the cytoplasmic differentiation of the sperm entails
a discarding of cytoplasm and contained cytoplasmic structures, whereas the
oocyte conserves and increases its cytoplasmic substance (fig. 68). In regard
MATURATION OF GAMETES 147
to the behavior of the cytoplasms of the two developing gametes, it is inter-
esting to observe that the idiosome-Golgi-mitochondrial complex behaves very
differently in the two gametes (fig. 68).
A third condition of egg and sperm differentiation involves the possible
function of the "nurse cells." In the vertebrate ovary the follicle cells which
surround the egg have much to do with the conditions necessary for the dif-
ferentiation of the oocyte. The latter cannot carry the processes of differ-
entiation to completion without contact with the surrounding follicle cells.
Spermiogenesis also depends upon the presence of a nurse cell. In the verte-
brate seminiferous tubule, the Sertoli cell is intimately concerned with the
transformation of the spermatid into the morphologically adult sperm, and
a close contact exists between the developing sperm element and the Sertoli
cell during this period (figs. 65, 66, 73). In the discharge of the formed
sperm elements into the lumen of the tubule, the Sertoli cell also is concerned
(Chap. 1).
b. Morphogenesis (Spermiogenesis; Sperrnioteleosis) of the Sperm
1) Types of Sperm. There are two main types of sperm to be found in
animals, namely, flagellate and non-flagellate sperm (figs. 74, 75). Flagellate
sperm possess a flagellum or tail-like organelle; non-flagellate sperm lack this
structure. The flagellate type of sperm is found quite universally among ani-
mals; non-flagellate sperm occur in certain invertebrate groups, particularly
in the nematode worms, such as Ascaris, and in various Crustacea, notably the
lobster, crab, etc. (fig. 75). Flagellate sperm may be either uniflagellate or
biflagellate. Single flagellate sperm occur in the majority of animals, while
a biflagellate form is found in the platode, Procerodes. However, biflagellate
sperm may be found as abnormal specimens among animals normally pro-
ducing uniflagellate sperm.
Conjugate sperm are produced in certain animal species. For example,
two sperm heads adhere closely together in the opossum (fig. 125), also in
the beetle, Dytiscus, and in the gastropod, Turritella. Many sperm heads be-
come intimately associated in the grasshopper to form the so-called "sperm
boat" (fig. 76). However, all conjugate sperm normally separate from each
other in the female genital tract.
2) Structure of a Flagellate Sperm. The flagellate sperm from different
species of animals vary considerably in size, shape, and morphological de-
tails. Some possess long, spear-shaped heads, some have heads resembling
a hatchet, in others the head appears more or less cigar-shaped, while still
others possess a head which resembles a spatula (fig. 74). The spatula-shaped
head is found in the sperm of the bull, opossum, man, etc. The description
given below refers particularly to the spatula-shaped variety. Although all
flagellate sperm resemble one another, diversity in various details is the rule,
148
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
ACROS OME
CAP
POS T -
NUCLEAR
CAP
GOLGf
R E MNANT
Fig. 81
Fig. 82
Fig. 81. Later stages of human spermatogenesis. (Redrawn from Gatenby and Beams,
1935.)
Fig. 82. Stages of guinea-pig spermatogenesis. Observe dual nature of the acrosome;
also, middle-piece bead (kinoplasmic droplet). (A-C redrawn from Gatenby and Beams,
1935; D redrawn from Gatenby and Woodger, '21.)
and the description given below should be regarded as being true of one type
of sperm only and should not be applied to all flagellate sperm.
A fully differentiated spatulate sperm of the mammals possesses the fol-
lowing structural parts (fig. 77).
a) Head. Around the head of the sperm there is a thin, enveloping layer
of cytoplasm. This cytoplasmic layer continues posteriad into the neck, middle
piece, and tail. Within the cytoplasm of the head is the oval-shaped nucleus.
Over the anterior half of the nucleus the apical body or acrosome is to be
found, forming, apparently, a cephalic covering and skeletal shield for the
MATURATION OF GAMETES 149
nucleus. The caudal half of the nucleus is covered by the post-nuclear cap.
This also appears to be a skeletal structure supporting this area of the nucleus;
moreover, it affords a place of attachment for the anterior centrosome and
the anterior end of the axial filament.
In human and bull sperm the acrosome is a thin cap, but in some mam-
malian sperm it is developed more elaborately. In the guinea pig it assumes
the shape of an elongated, shovel-shaped affair (fig. 82), while in the mouse
and rat it is hatchet or lance shaped (fig. 74N). In passerine birds the acrosome
is a pointed, spiral structure often called the perforatorium (fig. 78). On
the other hand, in other birds, reptiles, and amphibia it may be a simple,
pointed perforatorial structure (figs. 74, 78, 79). In certain invertebrate
species, it is located at the caudal or lateral aspect of the nucleus (figs. 75, 78 ) .
b) Neck. The neck is a constricted area immediately caudal to the pos-
terior nuclear cap and between it and the middle piece. Within it are found
the anterior centriole and the anterior end of the axial filament. In this par-
ticular region may also be found the so-called neck granule.
c) Connecting Body or Middle Piece. This region is an important
portion of the sperm. One of its conspicuous structures is the central core
composed of the axial filament and its surrounding cytoplasmic sheath. At
the distal end of the middle piece, the central core is circumscribed by the
distal, or ring centriole. Investing the central core of the middle piece is the
mitochondrial sheath. The enveloping cytoplasm is thicker to some degree
in this area of the sperm than that surrounding the head.
d) Flagellum. The flagellum forms the tail or swimming organ of the
sperm. It is composed of two general regions, an anterior principal or chief
piece and a posterior end piece. The greater part of the axial filament and its
sheath is found in the flagellum. A relatively thick layer of cytoplasm sur-
rounds the filament and its sheath in the chief-piece region of the flagellum,
but, in the caudal tip or end piece, the axial filament seems to be almost
devoid of enveloping cytoplasm. The end piece often is referred to as the
naked portion of the flagellum.
In figure 79 is shown a diagrammatic representation of a urodele amphibian
sperm. Two important differences from the mammalian sperm described above
are to be observed, namely, the middle piece is devoid of mitochondria and
is composed largely of centrioles 1 and 2, and the tail has an elaborate undu-
lating or vibratile filament associated with the chief piece.
3) Spermiogenesis or the Differentiation of the Spermatid into the Mor-
phologically Differentiated Sperm. The differentiation of the spermatid into
the fully metamorphosed sperm is an ingenious and striking process. It involves
changes in the nucleus, during which the latter as a whole contracts and in
some forms becomes greatly elongated into an attenuant structure. (See figs.
79B-F; 85L-P.) It also is concerned with profound modifications of the cyto-
plasm and its constituents; the latter changes transform the inconspicuous
150 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
Spermatid into a most complicated structure. Some of these changes are out-
lined below.
a) GoLGi Substance and Acroblast; Formation of the Acrosome.
The Golgi substance or parts thereof previously associated with the idiosome
of the spermatocyte (fig. 80A) proceeds to form the acrosome of the devel-
oping spermatid as follows: In the differentiating human sperm, the Golgi
substance of the spermatocyte (fig. SOD) becomes aggregated at the future
anterior end of the nucleus, as shown in fig. 80F, where it forms an acroblast
within a capsule of Golgi substance. This acroblast later forms a large vacuole
within which is the acrosomal "bead" (figs. 68B; BOG). The acrosomal bead
proceeds to form the acrosomal cap, shown in figure 81 A, and the latter
grows downward over the anterior pole of the nucleus (fig. 81 A, B). Most of
the Golgi substance in the meantime is discarded (fig. 81 A, B). (See Gatenby
and Beams, '35.)
In the guinea pig the acroblast together with other Golgi substance, mi-
grates around the nucleus toward the future anterior pole of the latter where
the acroblast takes up its new position (fig. SOB, C, E). (See Gatenby and
Woodger, '21.) As shown in figure 80E, the acroblast is composed of inner
and outer acrosomal substances. These inner and outer areas of the acroblast
give origin respectively to the inner and outer zones of the acrosome (fig. 82).
The peripheral or surrounding Golgi material of the acroblast detaches itself
meanwhile from the developing acrosome (fig. SOE, H) and drifts downward
toward the posterior end of the sperm. Eventually it is discarded with the excess
cytoplasm and some mitochondrial material. In some animal species (e.g.,
grasshopper) the acrosomal substance arises from a multiple type of acroblast
(Bowen, '22). (See fig. 83.) Nevertheless, the general process of acrosome
formation is similar to that outlined above.
b) Formation of the Post-nuclear Cap. All spatulate sperm of mam-
NEBENK ERN
( MITOC HONORI A)
Fig. 83. Formation of the acrosome from a multiple acroblast in the grasshopper. (After
Bowen, Anat. Rec, 24.)
MATURATION OF GAMETES
151
MITOCHONDRIAL STRANDS
DERIVED FROM THE
NEBENKERN
Fig. 84. The mitochondrial nebenkern and its elaborate development in Brachynema.
(After Bowen. J. Morphol., 37 and Biol. Bull., 42.) (B~I) Division of the nebenkern
(A) and its elaboration into two attenuant strands extending posteriad into the flagellum.
mals possess a nucleus which has an acrosomal cap over its anterior aspect
and a post-nuclear cap covering its posterior area. Both of these caps tend
to meet near the equator of the nucleus (fig. 77).
The exact origin of the post-nuclear cap is difficult to ascertain. In the
human sperm it appears to arise from a thickened membrane in association
152 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
with centriole 1 (fig. 80G, post-nuclear membrane). This membrane grows
anteriad to meet the acrosomal cap (fig. 81A-C). In the sperm of the guinea
pig, a series of post-nuclear granules in the early spermatid appear to coalesce
to form the post-nuclear cap (fig. 82A-C).
c) Formation of the Proximal and Distal Centrioles; Axial Fila-
ment. While the above changes in the formation of the acrosome are pro-
gressing, the centriole (or centrioles) of the idiosome move to the opposite
side of the nucleus from that occupied by the forming acrosome, and here
in this position the proximal and distal centrioles of the future sperm arise.
In this area the neck granules also make their appearance (figs. 68B; 80F-H).
The axial filament arises at this time and it probably is derived from the two
centrioles simultaneously (fig. 80F, H). The centrioles soon become displaced
along the axial filament, the caudal end of which projects from the surface
of the cell membrane (fig. 80F-H). The axial filament grows outward pos-
teriorly from the cell membrane in line with the two centrioles and the
acrosome-forming material. The anterior-posterior elongation of the sperm
thus begins to make its appearance (fig. 80H). The anterior centriole retains
a position close to the nuclear membrane, but the posterior or ring centriole
moves gradually posteriad toward the cell surface (figs. 81, 82A-C).
d) Mitochondrial Material and Formation of the Middle Piece
OF the Sperm. The behavior of the mitochondria in the formation of sperm
varies greatly. In the spatulate sperm described above, a portion of the mito-
chondrial substance becomes aggregated around the axial filament in the
middle-piece area (figs. 77, 82D). In certain amphibian sperm the middle
piece appears to be formed mainly by centrioles 1 and 2 (fig. 79D-F). In
certain insects the mitochondrial body or nebenkern, divides into two masses
which become extended into elongated bodies associated with the flagellum
(fig. 84). Some of the mitochondrial substance is discarded with the Golgi
substance and excess cytoplasmic materials.
e) The Cytoplasm, Axial Filament, Mitochondria, and Tail For-
mation. Synchronized with the above events, the cytoplasm becomes drawn
out in the posterior direction, forming a thin cytoplasmic layer over the sperm
head, and from thence posteriad over the middle piece and the chief piece
of the flagellum. However, the end piece of the flagellum may be devoid of
investing cytoplasm (fig. 77). As the cytoplasm is elongating posteriorly over
the contained essential structures of the forming sperm, much of the cytoplasm
and Golgi substance and some mitochondria are discarded and lost from the
sperm body. It may be that these discarded bodies form a part of the essential
substances of the spermatic (seminal) fluid. (See Chap. 1.) (See figs. 66;
68B-E; 81; 82; 85M-0.)
The centralized core of the tail is the axial filament which arises in relation
to centrioles 1 and 2 and grows posteriad through the middle piece and tail
MATURATION OF GAMETES
153
(figs. 80F-H; 81A-C; 82A-C; 85M-P). A considerable amount of mitochon-
drial material may also enter into the formation of tail (fig. 84).
A peculiar, highly specialized characteristic of many sperm tails is the
development of a vibratile membrane associated with the axial filament (fig.
79E, F). Its origin is not clear, but it probably involves certain relationships
with the mitochondrial material as well as the cytoplasm and axial filament.
In the formation of the human and guinea-pig sperm, the nucleus experi-
ences only slight changes in shape from that of the spermatid. However, in
many animal species, spermiogenesis involves considerable nuclear metamor-
phosis as well as cytoplasmic change (figs. 69, 79, 85).
In summary it may be stated that while the various shapes and sizes of
mature flagellate sperm in many animal species, vertebrate and invertebrate,
CENTRIOLE
Fig. 85. Spermatogenesis in the common fowl. Observe extreme nuclear metamor-
phosis. (After Miller, Anat. Rec, 70.) (A) Resting spermatocyte. (B) Early leptotene
stage. (C, D) Synaptene stage. (E) Pachytene stage. (F, G) Diplotene stage. (H)
Diakinesis. (I) First division, primary sperm. (J-P) Metamorphosing sperm.
154
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
are numerous, there is a strong tendency for spermiogenesis to follow similar
lines of development. Deviations occur, but the following comparisons between
mammalian and insect spermiogenesis, somewhat modified from Bowen ('22),
illustrate the uniformity of transformation of the basic structures of the
primitive meiocyte:
Mammalian Sperm
Insect Sperm
Nucleus — head
Centrioles — originally double and ar-
ranged in a proximal-distal formation.
The axial filament arises from both
centrioles
Mitochondria — form an elaborate sheath
for the anterior portion of the axial
filament
Idiosome and Golgi apparatus (acroblast
portion) — gives origin to a vesicle
which contains a granule, the acrosome
granule, which is involved in the pro-
duction of the acrosome
Excess Golgi substance — cast off with
excess cytoplasm
Excess cytoplasm — cast off — may be part
of seminal fluid or possibly may be
engulfed by Sertoli cells
Nucleus — head
Centrioles — same as in mammals
Mitochondria — form a somewhat
similar sheath for the axial fila-
ment
Idiosome and Golgi apparatus —
much the same as in mammals
Excess Golgi substance — cast off
with excess cytoplasm
Excess cytoplasm — cast off — may be
part of seminal fluid or possibly
may be engulfed by epithelial cells
of the sperm cyst wall
c. Cytoplasmic Differentiation of the Egg
The cytoplasmic differentiation of the egg involves many problems. These
problems may be classified under three general headings, viz.:
( 1 ) Formation of the deutoplasm composed of fats, carbohydrates and
proteins,
(2) development of the invisible organization within the true protoplasm
or hyaloplasm, and finally,
(3) formation of the vitelline or egg membrane or membranes.
In view of the complexity of these three problems and of their importance
to the egg in the development of the new individual, the mature oocyte or
egg is in a sense no longer a single cell. Rather, it is a differentiated mass
of protoplasm which is capable, after proper stimulation, to give origin to
a new individual composed of many billions of cells. As such, the differen-
tiation of the oocyte within the ovary represents a relatively unknown period
of embryological development.
CENTROSPHERE
CENTROSOME WITH
DIPLOID CENTRIOLE
MITOCHONDRIA
YOLK NUCLEUS
OF BALBIANI
Fig. 86. Young oogonia of the fowl entering the growth (oocyte) stage. (A) Idio-
some from which the Golgi substance has been removed and stained to show the cen-
trosphere (archoplasm). The centrosome has two centrioles. (B) Idiosome with sur-
rounding Golgi substance. The mitochondria surround the Golgi substance and the
nucleus. (After Brambell, '25.)
/ NUCLEUS
MITOCHONDRIAL CLOUD
GOLGI I"
MITOCHONDRIAL YOLK BODY
VACUOLE
GOLG I GRANULES
Fig. 87. The so-called mitochondrial yolk body in the developing egg of the fowl.
(A) Oocyte from 11-week-old chick, showing mitochondrial cloud and Golgi substances
1 and II. (B) Oocyte from ovary of adult fowl, showing both types of Golgi substance
and mitochondrial cloud. (C) Oocyte from ovary of adult fowl, showing the appearance
of the mitochondrial yolk body within the mitochondrial cloud. (D) Oocyte from
ovary of adult fowl, showing fragmentation of Golgi substance I and the association of
the resulting Golgi granules around the mitochondrial yolk body. (After Brambell, '25.)
155
156 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
HECAL LAYER
! — FOLLICLE CELLS
ZONA RADIATA
•PERIPHERAL
MITOCHONDRIAL ZON
• -
^^_,VARIOUS STAGES OF
-^^J?^ T R AN SFOR M ATION OF
'•^MITOCHONDRIA INTO
Fig. 88. Portion of follicle and periphery of oocyte from ovary of the adult bird,
showing the mitochondria and their transformation into the M-yolk spheres of Brambell.
(After Brambell, '25.)
Before considering the various aspects of cytoplasmic differentiation of
the oocyte, it is best for us to review the types of vertebrate and other chordate
eggs in order to be able to visualize the various goals toward which the
developing oocyte must proceed.
1) Types of Chordate Eggs. Eggs may be classified according to the
amount of deutoplasm (yolk, etc.) present in the cytoplasm as follows:
a) HoMOLECiTHAL ( IsoLECiTHAL ) Eggs. Truc homolccithal eggs in the
phylum Chordata are found only in the mammals, exclusive of the Proto-
theria. Here the deutoplasm is small in amount, and is present chiefly in
the form of fat droplets and small yolk spherules, distributed in the cytoplasm
of the egg (figs. 118A, B; 147A).
b) Telolfcithal Eggs. In the telolecithal egg the yolk is present in con-
siderable amounts and concentrated at one pole. Telolecithality of the egg
in the phylum Chordata exists in various degrees. We shall arrange them in
sequence starting with slight and ending with very marked telolecithality as
follows:
(1) Amphioxus and Styela. In Amphioxus and Styela from the subphyla
Cephalochordata and Urochordata, respectively, the yolk present is
centrally located in the egg before fertilization but becomes concen-
trated at one pole at the time of the first cleavage where it is con-
tained for the most part within the future entoderm cells (figs. 132D,
167A).
(2) In many Amphibia, such as the frogs and toads, and also in the
Petromyzontidae or fresh-water lampreys among the cyclostome fishes,
the yolk present is greater in amount than in the preceding eggs. As
such, it is concentrated at one pole, the future entodermal or vegetal
MATURATION OF GAMETES 157
pole, and a greater degree of telolecithality is attained than in the eggs
of Amphioxus or Styela (fig. 141 A).
(3) In many Amphibia, such as Necturus, also in Neoceratodiis and Lepi-
dosiren among the lung fishes, and in the cartilaginous ganoid fish,
Acipenser, yolk is present in considerable amounts, and the cytoplasm
of the animal pole is smaller in comparison to the yolk or vegetal
pole (figs. 150, 151, 152).
(4) In the bony ganoid fishes, Amia and Lepisosteiis, as well as in the
Gymnophiona (legless Amphibia) the yolk is situated at one pole
and is large in quantity (figs. 153B-F; 154).
(5) Lastly, in a large portion of the vertebrate group, namely, in reptiles,
birds, prototherian mammals, teleost and elasmobranch fishes, and
in the marine lampreys, the deutoplasm is massive and the proto-
plasm which takes part in the early cleavages is small in comparison.
In these eggs the yolk is never cleaved by the cleavage processes, and
development of the embryo is confined to the animal pole cytoplasm
(figs. 46, 47).
2) Formation of the Deutoplasm. The cytoplasm of the young oocyte is
small in quantity, with a clear homogeneous texture (figs. 68A; 86A, B). As
the oocyte develops, the cytoplasmic and nuclear volumes increase (fig. 68F),
and the homogeneity of the cytoplasm is soon lost by the appearance of
deutoplasmic substances (fig. 68G, H). In the oocyte of the frog, for example,
lipid droplets begin to appear when the oocyte is about 50 /x in diameter
(fig. 72A). (See Brachet, '50, p. 53.) A little later glycogen makes its
appearance, and finally yolk protein arises.
The origin of fat droplets and yolk spherules has been ascribed variously
to the activities of chondriosomes (mitochondria and other similar bodies),
Golgi substance, and of certain vacuoles. Most observers place emphasis
upon the presence of a so-called "yolk nucleus" or "yolk-attraction sphere"
situated near the nucleus of many oocytes as a structure associated with fat
and yolk formation. In general, two types of yolk bodies have been described.
One is the yolk nucleus of Balbiani and the other the mitochondrial yolk
body of Brambell. The yolk nucleus of Balbiani (fig. 86A, B) consists of
the following:
( 1 ) a central body, the centrosphere or archoplasmic sphere within which
one or more centriole-like bodies are found, and
(2) surrounding this central body, a layer of Golgi substances and chon-
driosomes (i.e., mitochondria, etc.).
This cytoplasmic structure probably is related to the idiosome of the oogonia
(fig. 68A).
The formation of the deutoplasm, according to the theory associated with
the Balbiani type of yolk nucleus is as follows: The surrounding pallial layer
158
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
of Golgi substance and mitochondria moves away from the central portion
(i.e., away from the centrosphere) of the yolk nucleus and becomes scattered
and dispersed as small fragments within the cytosome (fig. 68F, G). The
yolk nucleus as an entity thus disappears, and its fragments become immersed
within the substance of the cytoplasm. Coincident with this dispersion of
yolk nuclear material, rapid formation of small yolk spherules and fat droplets
occur (fig. 68H). It appears thus that the formation of the deutoplasm com-
posed of fat droplets and yolk spherules is directly related to the activities
of the Golgi substance and chondriosomes.
MITOCHONDRIAL
YOLK SPHERES
SECOND TYPE OF
YOLK FORMED
AROUND
MITOCHONDRIAL
YOLK SPHERES
GOLGI APPARATUS
OF FOLLICLE
^%
«-i GOLGI APPARATUS
TYPE 2
B.
Fig. 89. (A) Cytoplasm of oocyte, showing formation of a second kind of yolk (the
M-C-yolk) in a vacuole surrounding the M-yolk sphere. (After Brambell, '25.) (B)
Passing of Golgi substance from the follicle cells mto the ooplasm of developing oocyte
of the fowl. (After Brambell, '25.)
QUANTITATIVE DIFFERENCES
QUALITATIVE DIFFERENCES
Fig. 90. Diagrams showing contrasting theories explaining the organization of polarity
of the cytoplasm of the fully developing egg or oocyte. Diagram at left shows polarity
explained according to quantitative differences, while the diagram to the right shows
qualitative differences. A = animal pole; V = vegetal pole. E represents a substance or
a factor, while EN-1, EN-2, etc., represent different quantities of substance E distributed
from pole to pole. SEC. SEN and SM are different chemical substances assumed to be
responsible for the determination of the ectoderm, entoderm, and mesoderm of the de-
veloping embryo. (After Barth: Embryology, New York, Dryden Press.)
MATURATION OF GAMETES 159
On the other hand, the interpretation and description of the yolk body
and its subsequent activities given by Brambell ('25) present a different
view. According to the latter author, the yolk body is composed entirely of
mitochondria; the Golgi substance and centrosphere are absent. Yolk forma-
tion proceeds as follows: As the young oocyte grows, the mitochondria in-
crease in number and form the mitochondrial cloud (fig. 87A, B). The transi-
tory mitochondrial yolk body differentiates within this cloud (fig. 87C). The
mitochondrial yolk body ultimately breaks up into a mass of mitochondria,
and the latter becomes dispersed in the cytoplasm of the oocyte (figs. 68F,
G; 87D). Some of these dispersed mitochondria transform directly into yolk
spheres (figs. 68H, 88, 89). Following this, another kind of yolk is formed
in vacuoles surrounding these original yolk spheres (figs. 68H, 89A, yolk
spheres plus vacuoles). The fat droplets (C-yolk) within the ooplasm are
formed according to Brambell "possibly under the influence of Golgi elements"
(fig. 68H, fat droplets). Relative to the function of the yolk nucleus and its
mitochondria, Brachet ('50), p. 57, considers it significant at the beginning,
but its real importance is still to be understood.
The relationship, if any, of the oocyte nucleus to the deposition of yolk
materials is not apparent. One must not overlook the real probability that
the germinal vesicle (i.e., the enlarged nucleus of the oocyte) may be related
to the increase and growth of the cytoplasm and to yolk formation, for it is
at this time that the chromatin threads surrender their normal diplotene
appearance and become diffusely placed in the germinal vesicle. They also
lose much of their basic chromatin-staining affinities while the Feulgen reac-
tion is diminished (Brachet, '50, p. 63). With regard to the possible function
of the germinal vesicle in yolk synthesis, the following quotation is taken from
a publication by Brachet ('47):
It is well worth pointing out that Duspiva (1942), using a very delicate and
precise technique, found no correlation between the dipeptidase content of the
nucleus and the onset of vitellus synthesis: such a correlation exists, however, in
the case of the cytoplasm where dipeptidase increases markedly when the first yolk
granules make their appearance. These results suggest that there is not evidence
that the nucleus is the sit,e of an especially active metabolism; cytoplasmic dipep-
tidase probably plays a part in yolk protein synthesis; if the nucleus controls such
a synthesis, it works in a very delicate and still unknown way.
However, the means by which protein synthesis is effected still is a problem
which awaits explanation (Northrop, '50). (The interested student should
consult Brachet, '50, Chap. Ill, for a detailed discussion of the cytochem-
istry of yolk formation.)
Another aspect of the problem of cytoplasmic growth and differentiation
of the oocyte presents itself for further study. Brambell ('25) concluded from
his observations that Golgi substance passes from the follicle cells into the
ooplasm of the growing bird oocyte and contributes to the substance of the
peripheral layer (fig. 89B). Palade and Claude ('49) suggest that at least
160 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
some of the Golgi substance be identified as myelin figures which develop
"at the expense of lipid inclusions." Thus it may be that the Golgi substance
which Brambell observed (fig. 89B) passing from the follicle cells to the
oocyte represents lipid substance. In the growing oocyte of the rat, Leblond
('50) demonstrated the presence of small amounts of polysaccharides in
the cytoplasm of the oocyte, while the surrounding zona pellucida and follicle
cells contained considerable quantities.' These considerations suggest that the
blood stream using the surrounding follicle cells as an intermediary may con-
tribute food materials of a complex nature to the growing cytoplasm of the
oocyte.
The localization of the yolk toward one pole of the egg is one of the move-
ments which occurs during fertilization in many teleost fishes. In these forms,
the deutoplasmic materials are laid down centrally in the egg during oogenesis,
but move poleward at fertilization (fig. 122). A similar phenomenon occurs
also during fertilization in Amphioxus and Styela among the protochordates.
In many other fishes and in the amphibia, reptiles, birds, and monotrematous
mammals, the yolk becomes deposited or polarized toward one pole of the
oocyte during the later stages of oocyte formation, as the cytoplasm and the
germinal vesicle move toward the other pole (figs. 68H, 72F). The polariza-
tion of the deutoplasmic substances thus is a general feature of the organi-
zation of the chordate egg.
3) Invisible Morphogenetic Organization Within the Cytoplasm of the Egg.
Two general categories of substances are developed within the cytoplasm of
the oocyte during its development within the ovary, viz.:
( 1 ) the visible or formed cytoplasmic inclusions, and
(2) an invisible morphogenetic ground substance.
The former group comprises the yolk spherules, fats, and other visible,
often pigmented bodies which can be seen with the naked eye or by means
of the microscope. The morphogenetic ground substance probably is com-
posed of enzymes, hormones, and various nucleocytoplasmic derivatives en-
meshed within the living cytoplasm. However, although we may assume that
the basic, morphogenetic ground substance is composed of enzymes, hor-
mones, etc., the exact nature of the basic substance or its precise relationship
to the various formed inclusions of the cytoplasm is quite unknown (see
Fankhauser, '48, for discussion). More recent experiments demonstrate that
the yolk or deutoplasmic material not only serves as a reservoir of energy
for embryonic development but also is in some way connected with the
essential, basic organization of the egg.
Although we know little concerning the exact nature of the morphogenetic
organization of the egg or how it forms, studies of embryological development
force upon us but one conclusion, to wit, that, during the period when the
oocyte develops in the ovary, basic conditions are elaborated from which the
MATURATION OF GAMETES 161
future individual arises (Fankhauser, '48). Within the cytoplasm of the ma-
ture egg of many chordates, this inherent organization is revealed at the
time of fertilization by the appearance of definite areas of presumptive
organ-forming substances. For example, in the egg of the frog and other
amphibia, the yolk pole is the stuff from which the future entodermal struc-
tures take their origin; the darkly pigmented animal or nuclear pole will
eventually give origin to epidermal and neural tissues; and from the zone
between these two areas mesodermal and notochordal tissues will arise (fig.
119K). Similar major organ-forming areas in the recently fertilized egg have
been demonstrated in other chordates, as in the ascidian, Styela, and in the
cephalochordate, Amphioxus. In the eggs of reptiles, birds, and teleost and
elasmobranch fishes, while the relationship to the yolk is somewhat different,
major organ-forming areas of a similar character have been demonstrated at
a later period of development (Chaps. 6-9). This suggests that these eggs
also possess a fundamental organization similar, although not identical, to
that in the amphibian egg.
4) Polarity of the Egg and Its Relation to Body Organization and Bilateral
Symmetry of the Mature Egg. One of the characteristic features of the ter-
minal phase of egg differentiation in the chordate group is the migration of
the germinal vesicle toward the animal pole of the egg (figs. 72F, 11 9A).
As stated above, in many vertebrate eggs the deutoplasmic material becomes
situated at the opposite pole, known as the vegetal (vegetative) or yolk pole,
either before fertilization or shortly after. The relatively yolk-free protoplasm
aggregates at the animal pole. Consequently the maturation divisions of the
egg occur at this pole (fig. 1 19 A, B, D). The formation of a definite polarity
of the egg, therefore, is one of the main results of the differentiation of the
oocyte.
Various theories have been suggested in an endeavor to explain polarity
in the fully developed egg or oocyte. All these theories emphasize qualitative
and quantitative differences in the cytoplasmic substances extending from
one pole of the egg to the other (fig. 90).
The animal and vegetal poles of the egg have a definite relationship to
the organization of the chordate embryo. In Amphioxus, the animal pole
becomes the ventro-anterior part of the embryo, while in the frog the animal
pole area becomes the cephalic end of the future tadpole, and the yolk pole
comes to occupy the posterior aspect. In teleost and elasmobranch fishes
the yolk-laden pole lies in the future ventral aspect of the embryo, and it
occupies a similar position in the reptile, bird, and prototherian mammal
(see fig. 215). Studies have shown that the early auxiliary or trophoblastic
cells in eutherian mammals lie on the ventral aspect of the future embryo.
Consequently, it is to be observed that the various substances in mature
vertebrate and protochordate eggs tend to assume a polarized relationship
to the future embryonic axis and body organization.
162 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
Many vertebrate and protochordate eggs possess a bilateral symmetry
which becomes evident when the fertilization processes are under way or
shortly after their conclusion. The appearance of the gray crescent in the
frog's egg (fig. 11 9K) and in other amphibian eggs during fertilization and
the similar appearance of the yellow crescent in the fertilized egg of the
ascidian, Styela (fig. 132D) serve to orient the future right and left halves
of the embryo. Conditions similar to that of Styela, but lacking the yellow
pigment, are present in Amphioxus. Similarly, in the chick, if one holds the
blunt end of the egg to the left, and the pointed end to the right, the early
embryo appears most often at right angles, or nearly so, to the axis extending
from the broader to the smaller end of the egg, and in the majority of cases
the cephalic end of the embryo will appear toward the side away from the
body of the observer. There is some evidence that the "yolk" or egg proper
is slightly elongated in this axis. It appears, therefore, that the general plane
of bilateral symmetry is well established in the early chick blastoderm, although
the early cleavages do not occur in a manner to indicate or coincide with
this plane. In prototherian mammals, a bilateral symmetry and an antero-
posterior orientation is established in the germinal disc at the time of ferti-
lization, soon after the second polar body is discharged (fig. 136).
5) Membranes Developed in Relation to the Oocyte; Their Possible
Sources of Origin. A series of membranes associated with the surface of
the oocyte are formed during its development within the ovary. Three general
types of such membranes are elaborated which separate from the oocyte's
surface at or before fertilization, leaving a perivitelline space between the
egg's surface and the membrane. They are:
( 1 ) A true vitelline membrane which probably represents a specialization
or product of the ooplasmic surface. For a time this membrane adheres
closely to the outer boundary of the ooplasm, but at fertilization it
separates from the surface as a distinct membrane.
(2) A second membrane in certain chordates is elaborated by the follicle
cells. It is known as a chorion in lower Chordata but is called the
zona pellucida in mammals.
(3) A zona radiata or a thickened, rather complex, membrane is formed
in many vertebrates; it may be considered to be a product of the
ooplasm or of the ooplasm and the surrounding follicle cells.
All of the above membranes serve to enclose the egg during the early
phases of embryonic development and therefore may be considered as pri-
mary embryonic membranes. As such, they should be regarded as a definite
part of the egg and of the egg's differentiation in the ovary. A description
of these membranes in relation to the egg and possible source of their origin
in the various chordate groups is given below.
a) Chorion in Styela. A previously held view maintained that the chorion
INNER FOLLICULAR
E PI T H E LI UM
OOPLASMIC MEMBRANE
PERIVITELLINE SPACE '^C
Fig. 91. Formation of the chorion in the egg of Styela. (A) Chorion is shown along
the inner aspect of the follicular epithelium. The test cells lie in indentations of the
peripheral ooplasm. (B) Optical section of an ovulated egg. (Redrawn and modified
from Tucker, '42.)
^='1
0 ©mo^
ffi^jjg"^
FOLLICU L A R
E PITHE LIU M
VITELLINE
ME MBRAN E
ZONA R A Dl ATA
YOLK
^ C^C^^^-^ SECONDARY
>/ FOLLIC U LAR
EPITHELIUM
FOLLICULAR
EPITHELIUM
VI TELL I N E
MEMBRANE
YOLK
D.
Fig. 92. Developing vitelline membranes of Scyllium canicula. Observe that two mem-
branes are present in the young egg; later these membranes fuse into one membrane.
(A) Surface area of young oocyte with a vitelline membrane and zona radiata. (B)
Slightly older oocyte with the radiate zone not as prominent. (C) Older oocyte with a
single, relatively thick, vitelline membrane. (D) Nearly mature oocyte with a thin
vitelline membrane. (After Balfour, Plate 25, The Works of Francis Maitland Balfour,
ed. by Foster and Sedgwick, London, Macmillan, 1885.)
163
164
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
and "test" cells of the egg of Styela were ejected from the surface cytoplasm
at the time of ovulation (Conklin, '05). A recent view, however, maintains
that the test cells arise from follicle cells and come to lie in indentations of
the periphery of the egg outside of the thin vitelline membrane (Tucker, '42).
The chorion is formed by the inner layer of follicle cells and comes to lie
between the test cells and the inner layer of follicle cells in the mature egg
(fig. 91 A). At ovulation the chorion moves away from the surface of the
oocyte. At this time also, the test cells move outward from their indentations
in the peripheral ooplasm and come to lie in the perivitelline space between
the egg surface and the chorion (fig. 9 IB). An ooplasmic membrane which
TH EGA FOL LI CULI
GRANULOSA
MICROPYLAR CELL
ZONA RADIATA
YOL K
THECA FOLLIGULI
f^X'^^^ /r^^J^"^ NUCLEI OF
GRAN ULOS A GEL LS
-TUBULES OF
CAPSULAR MEMBRANE
MICROPYLAR CELL
s^^^^S^^^mnmm.
T-ZONA RADIATA
FILAMENTS
PROOUCED BY
FOLLICLE CELLSf?)
MEMBRANE
ZONA RADIATA
Fig. 93. Vitelline membranes of certain teleost fishes. (After Eigenmann. 1890.) (A)
Py^osteus pungtiiis. Radial section through micropyle of egg about 0.4 mm. in diameter.
(B) Radial section through micropyle of egg of Percu, the perch. (C) Vitelline mem-
branes of Fnndiilus hcterodilus about 0.8 mm. in diameter.
MATURATION OF GAMETES 165
represents the thin surface layer of ooplasm is present. However, it does not
separate from the periphery of the egg at fertilization. During its early devel-
opment, the embryo remains within this chorionic shell. The chorion thus
represents the primary embryonic membrane of this species.
b) Egg Membranes of Amphio.xus. Two surface membranes are formed
and eventually separate from the egg of Amphio.xus. The outer vitelline
membrane is elaborated on the surface of the egg and remains in contact
with this surface until about the time of the first maturation division. It then
begins to separate from the egg's surface. (See Chap. 5.) After the sperm
enters and the second maturation division occurs, a second, rather thick,
vitelline membrane also separates from the egg. The first and second vitelline
membranes then fuse together and become greatly expanded to form the pri-
mary embryonic membrane. (See Chap. 5.) A thin ooplasmic membrane
remains at the egg's surface.
c) Vitelline Membrane and Zona Radiata of Elasmobranch Fishes.
In the egg of the shark, Scyllium canicula, two egg membranes are formed,
an outer and an inner membrane. The outer membrane is a homogeneous
vitelline membrane, while the membrane which comes to lie beneath this
outer membrane has a radiate appearance and hence may be called a zona
radiata. This latter membrane soon loses its radiate appearance and becomes
a thin membrane along the inner aspect of the vitelline membrane (fig. 92A,
B). In the mature egg both of these membranes form a thin, composite,
vitelline membrane (fig. 92C, D). At about the time of fertilization the latter
membrane separates from the egg's surface; a perivitelline space then lies
between these structures and the surface ooplasm of the egg.
d) Zona Radiata of Teleost Fishes. The surface ooplasm in teleost
fishes gives origin to a membrane which in many cases has a radiate appearance.
In some species this membrane appears to be composed of two layers. This
radiate membrane which forms at the surface of the egg of teleost fishes
appears to be the product of the ooplasm, and, therefore, should be regarded
as a true vitelline membrane. In the perch a true chorion also is formed
as a gelatinous or filamentous layer produced external to the radiate mem-
brane by the follicle cells (fig. 93B). In Fundulus heteroclitus there are
apparently three distinct parts to the membrane which surrounds the ooplasm
of the egg:
( 1 ) a zona radiata,
(2) a thin structureless membrane external to the zona, and,
(3) the filamentous layer whose filaments are joined to the thin membrane
around the zona (fig. 93C).
These three layers are probably derived from the ooplasm of the egg
(Eigenmann, '90). Consequently, the filamentous chorion or gelatinous layer,
if derived from the egg itself, is not a true chorion in this particular egg.
VITELLINE MEMBRANE
FOLLICLE LAYER
Fig. 94. Vitelline membrane of an almost mature egg of the frog.
YOLK
FOLLICULAR
E PITH ELI U M
PRO LON GAT IONS OF
FOLLICULAR CELL
OUTER
LAYE R
INNER
LAYER
K NO B-Ll KE
ENLARGEMENTS
OF FOLLICULAR
P ROLONGATIONS
FUNDAMEN TAL
SUBSTANCE
Fig. 95. Zona radiata (zona pellucida) or vitelline membrane of Chrysemys picta.
(After Thing, '18.)
FOL LI CLE C ELLS
ZONA R ADI ATA
Fig. 96. Zona radiata of the egg of the fowl. (After Brambell, '25.)
166
MATURATION OF GAMETES 167
At one end of the forming egg, a follicle cell sends an enlarged pseudopodium-
like process inward to the surface of the egg. As a result of this enlarged
extension of the follicle cell to the ooplasmic surface, an enlarged pore-like
opening in the zona radiata is formed. This opening persists as the micropyle
after the egg leaves the ovary (fig. 93A).
As the teleost egg is spawned, the chorionic layer hardens when it comes
in contact with the water. If fertilization occurs, the surface of the egg emits
a fluid and shrinks inward from the zona radiata. In this manner, a peri-
vitelline space is formed between the egg, and the zona is filled with a fluid.
The egg is thus free to revolve inside of the zona (Chap. 5).
e) Vitelline Membrane (Zona Radiata) in Amphibia. In the am-
phibia, a vitelline membrane is formed probably by the surface ooplasm,
although there may be contributions by the follicle cells of the ovary (Noble,
'31, p. 281). This membrane separates from the egg at the time of fertiliza-
tion, forming a perivitelline space (fig. 94). The latter space is filled with
fluid. Later the vitelline membrane expands greatly to accommodate the de-
veloping embryo. A delicate surface layer or membrane forms the outer por-
tion of the ooplasm below the vitelhne membrane. In some amphibia the
vitelline membrane may have a radiate appearance.
f) Zona Radiata (Zona Pellucida) of the Reptile Oocyte. In the
turtle group, the development of the zona radiata (pellucida) appears to be
the product of the follicle cells (Thing, '18). Filamentous prolongations of
the follicle ceUs extend to the surface ooplasm of the developing egg (fig. 95).
A homogeneous substance produced by the follicle cells then fills the spaces
between these prolongations. The filamentous extensions of the follicle cells in
this way produce a radiating system of canals passing through the homogeneous
substance; hence the name, zona radiata. Bhattacharya describes Golgi sub-
stance as passing from the follicle cells through the canals of the zona radiata
into the egg's ooplasm in the developing eggs of Testudo graeca and Uromastix
hardwicki. (See Brambell, '25, p. 147.)
In contradistinction to the above interpretation, Retzius ('12) describes the
homogeneous substance which forms the zona radiata of the lizard, Lacerta
viridis, as originating from the ooplasm of the egg.
g) Vitelline Membrane (Zona Radiata) of the Hen's Egg, The
vitelline membrane, as in the turtle groups, appears to form about the young
oocyte as a result of contributions from the surrounding follicle cells although
the superficial ooplasm of the oocyte may contribute some substance. This
occurs before the rapid deposition of yolk within the developing oocyte. It
is probable that the follicle cells send small pseudopodium-like strands of
cytoplasm through the numerous perforations of the very thin vitelline mem-
brane around the oocyte's surface into the superficial ooplasm in a similar
manner to that which occurs in reptiles. The vitelline membrane (zona radiata)
thus assumes a radiate appearance as it increases in thickness (figs. 47, 96).
168
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
Kl NOPL A S M 1 C
DROPLET
D.
Fig. 97. Kinoplasmic bead or droplet upon the middle piece of mammalian sperm.
(A) Pig sperm. (After Retzius, Biol. Untersuchungen, New Series, 10; Stockholm: Jena.)
(B) Cat sperm. (After Retzius, Biol. Untersuchungen, New Series, 10; Stockholm: Jena.)
(C-D) Dog sperm. (C) Upper part of epididymis. (D) Lower or caudal part of
epididymis.
When the vitelHne membrane thickens, the loci where the cytoplasmic strands
from the follicle cells pass through the membrane become little canals or
canaliculi. As the oocyte increases in size, a thin space forms between the
vitelline membrane or zona radiata and the follicle cells; it is filled with fluid
and forms the follicular space. The egg now is free to rotate within the follicle.
In consequence, the pole of the egg containing the blastodisc always appears
uppermost. Due to the increasing pendency of the egg follicle as the egg ma-
tures, the blastodisc comes to rest, a short while previous to ovulation, at the
base of the pedicle where the blood vessels are most abundant (fig. 47B).
During the latter phases of oocyte development, the vitelline membrane con-
stitutes an osmotic membrane through which all nourishment must pass to
the oocyte, particularly in its later stages of growth. The surface ooplasm forms
a delicate surface membrane beneath the zona radiata.
h) Membranes of the Mammalian Oocyte. All mammalian oocytes
possess a membrane known as the zona pellucida. It is a homogeneous layer
interposed between the ooplasm and the follicle cells. By some investigators
it is regarded as a product of the oocyte, while others regard it as a contri-
MATURATION OF GAMETES 169
bution of the ooplasm and follicle cells. The majority opinion, however, de-
rives the zona pellucida from the follicle cells. In addition to the zona
pellucida, the oocyte of the prototherian mammals has a striate layer lying
close to the surface of the oocyte. This striated layer probably is derived
from the surface ooplasm. This membrane later disappears, and a perivitelline
space occupies the general area between the surface of the oocyte and the
zona pellucida (fig. 46; Chap. 5). The zona pellucida separates from the egg
surface after sperm contact.
5. Physiological Maturation of the Gametes
a. Physiological Differentiation of the Sperm
Added to the nuclear and cytoplasmic transformations of the sperm de-
scribed above, a further process of sperm ripening or maturing appears to
be necessary. In the mammal, for example, the sperm cell must pass through
the epididymis to achieve the ability to fertilize the egg. This is shown by the
fact that sperm taken from the seminiferous tubules will not fertilize, although,
morphologically, two sperm, one from the testis and one from the epididymis
cannot be distinguished other than by the presence in some mammals of the
so-called "kinoplasmic droplet" (figs. 82D, 97). These droplets do not appear
in great numbers upon ejaculated sperm but are found on sperm, particularly
in epididymides. It is possible that these droplets may arise from a secretion
from the epididymal cells (Collery, '44). In the dog, these droplets are at-
tached to the neck of the sperm in the caput epididymidis but are found at
the posterior end of the middle piece of the sperm in the cauda epididymidis
and vas deferens and are probably lost at the time of ejaculation (Collery, '44).
Investigators differ greatly in interpreting the significance of this body. How-
ever, these droplets do seem in some way to be directly or indirectly concerned
with the physiological maturing of the sperm. In this connection Collery ('44)
notes that sperm are probably motile on leaving the seminiferous tubules, but
active forward movement is not seen until the bead has reached the junction
of middle piece and tail.
In the fowl, Domm ('30, p. 318) suggests the probability that the sperm
may undergo an aging or ripening process essential for reproduction some-
where in the reproductive system other than the seminiferous tubules. The
work of Lipsett quoted in Humphrey ('45) suggests that the accessory repro-
ductive system also is necessary for a ripening process of the sperm in urodele
amphibia.
On the other hand, in the frog, sperm taken from the testis have the ability
to fertilize eggs. In this case, the sperm probably undergo a physiological
ripening in the testis along with morphological differentiation.
The foregoing considerations suggest that a physiological maturation of
the sperm is necessary to enable the sperm to take part in the fertilization
process.
170
THE DEVELOPMENT OF THE GAMETES OR SEX CELLS
b. Physiological Ripening of the Female Gamete
The physiological maturing of the oocyte is linked to factors which influence
the developing egg at about the time the maturation divisions occur. Sea-urchin
sperm may penetrate the egg before the maturation divisions occur (Chap. 5).
However, development does not take place in such instances. On the other
hand, sperm entrance after both maturation divisions are completed initiates
normal development. In the protochordate, Styela, marked cortical changes
transpire at about the time the egg leaves the ovary, and as it reaches the
sea water, the germinal vesicle begins to break down. The oocyte becomes
fertilizable at about this time. In Amphioxus, although the first polar body is
given off within the adult body, the egg apparently is not fertilizable until it
reaches the external salt-water environment. The secondary oocyte of the
frog presumably must remain within the uterus for a time to ripen in order
that ensuing development may be normal. These and other instances suggest
that physiological changes — changes which are imperative for the normal
development of the egg — are effected at about the time that the maturation
divisions occur.
D. Summary of Egg and Sperm Development
From the foregoing it may be seen that the development of the gametes in
either sex involves a process of maturation. This maturation entails changes
in the structure and constitution of the nucleus and cytoplasm, and, further,
a functional or physiological ripening must occur. The comparative maturation
events in the egg and sperm may be summarized as follows:
Egg {in Oogenesis)
Sperm (in Spermatogenesis)
Nuclear maturation
a. Homologous chromosomes synapse
and undergo profound changes dur-
ing which parts of homologous chro-
mosomes may be interchanged; ulti-
mately, the chromosome number is
reduced to the haploid number
b. Nucleus enlarges, and contained nu-
clear fluid increases greatly; ulti-
mately the nuclear fluid is contrib-
uted to cytoplasm upon germinal
vesicle break down
c. Nuclear maturation occurs simulta-
neously with cytoplasmic differenti-
ation
1. Nuclear maturation
a. (Similar to the female)
b. Nucleus remains relatively small and
enlargement is slight; nuclear fluid
small in amount; during spermio-
genesis the nucleus may contract into
a compact mass; considerable elon-
gation of nucleus occurs in many
species
c. Nuclear maturation occurs before
spermiogenesis or cytoplasmic differ-
entiation
SUMMARY OF EGG AND SPERM DEVELOPMENT
171
Egg (in Oogenesis)
Sperm (in Spermatogenesis)
2. Cytoplasmic maturation
This involves:
a. Polarization of cytoplasmic materials
and the nucleus in relation to the fu-
ture maturation phenomena; the nu-
cleus becomes displaced toward one
pole, the animal pole, and the yolk,
and other cytoplasmic materials; in
many eggs becomes displaced toward
the opposite or vegetal pole
b. Formation of deutoplasm or stored
food material, varying greatly in
amount in different animal species.
The deutoplasm is composed of
fats, carbohydrates, and protein sub-
stances
2. Cytoplasmic maturation
This involves:
a. Polarization of nucleus and cytoplas-
mic materials along an elongated
antero-posterior axis, with the head,
neck, middle piece, and tail occupy-
ing specific regions along this axis.
The nucleus occupies a considerable
portion of the anterior region or head
b. Little food substances stored within
cytoplasm; food reserve in seminal
fluid
c. Cytoplasm increased in amount; for-
mation of basic organ-forming areas
or cytoplasmic stuffs from which the
future embryo arises
d. Formation of primary embryonic
membranes
3. Physiological maturation or the devel-
opment of a fertilizable stage
This involves:
a. Formation of an organization which
when stimulated by external influ-
ences initiates and carries on the
processes necessary for normal em-
bryonic development
b. Acquisition of ability to enter into
a developmental union with a sperm
c. Development of ability to form and
secrete gynogamic substances which
aid in the fertilization process. (See
Chap. 5)
d. Assumption of an inhibited or dor-
mant condition during which meta-
bolic processes proceed slowly in an-
ticipation of the fertilization event
c. Discarding of a considerable amount
of cytoplasm, some Golgi elements
and mitochondria. Retention of some
Golgi elements, centrioles, mitochon-
dria, etc.
d. No specific membranes formed
around sperm, although elaborate
membranes for motile purposes are
formed in some sperm
3. Physiological maturation or the devel-
opment of the ability to contact and
fertilize the egg
This involves:
a. Development of an organization
which, when stimulated by proper
external substances, responds by a
directed movement resulting in loco-
motion; also capable of being at-
tracted by egg substances
b. Acquisition of ability to fertilize, i.e.,
to enter into a developmental union
with an egg or oocyte
c. Acquisition of ability to produce and
secrete androgamic substances which
aid in the fertilization process
d. Assumption of an active metabolic
state
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PART II
Tne Period or Fertili2;ation
The period of fertilization involves:
(1) The transportation of the gametes to the site normal for the species where en-
vironmental conditions are suitable for gametic union (Chap. 4), and
(2) Fertilization or the union of the gametes (Chap. 5).
The union of the gametes may be divided into two phases, viz.:
( 1 ) The primary phase which is terminated when the sperm has made intimate con-
tact with the egg's surface, and
(2) The secondary phase or the fusion of the two gametes resulting in the initiation
of development.
175
Transportation or tne Gametes (Sperm and E^^) from
tne Germ Glands to tne Site Wnere Fertilization
Normally Occurs
A. Introduction
1. Activities of the male and female gametes in their migration to the site of
fertilization
B. Transportation of the sperm within the male accessory reproductive structures
1. Transportation of sperm from the testis to the external orifice of the genital duct
in the mammal
a. Possible factors involved in the passage of the seminal fluid from the testis to
the main reproductive duct
1) Accumulated pressure within the seminiferous tubules
2) Activities within the efferent ductules of the testis
b. Movement of the semen along the epididymal duct
1 ) Probable immotility of the sperm
2) Importance of muscle contraction, particularly in the vas deferens
3) Summary of factors which propel the seminal fluid from the testis to the
external orifice of the reproductive duct in the mammal
2. Transportation of sperm in other vertebrates with a convoluted reproductive duct
3. Transportation of sperm from the testis in vertebrates possessing a relatively simple
reproductive duct
C. Transportation of sperm outside of the genital tract of the male
1. Transportation of sperm in the external watery medium
2. Transportation of sperm in forms where fertilization of the egg is internal
a. General features relative to internal fertilization
1 ) Comparative numbers of vertebrates practicing internal fertilization
2) Sites or areas where fertilization is effected
3) Means of sperm transfer from the male genital tract to that of the female
b. Methods of sperm transport within the female reproductive tract
1 ) When fertilization is in the lower or posterior portion of the genital tract
2) When fertilization occurs in the upper extremity of the oviduct
3) When fertilization occurs in the ovary
D. Sperm survival in the female genital tract
E. Sperm survival outside the male and female tracts
1. In watery solutions under spawning conditions
2. Sperm survival under various artificial conditions; practical application in animal
breeding
177
178 TRANSPORTATION OF THE GAMETES
F. Transportation of the egg from the ovary to the site of fertilization
1. Definitions
2. Transportation of the egg in those forms where fertilization occurs in the anterior
portion of the oviduct
a. Birds
b. Mammals
3. Transportation of the egg in those species where fertilization is effected in the
caudal portion of the oviduct or in the external medium
a. Frog
b. Other amphibia
c. Fishes
G. Summary of the characteristics of various mature chordate eggs together with the
site of fertilization and place of sperm entrance into the egg
A. Introduction
1. Activities of the Male and Female Gametes in Their
Migration to the Site of Fertilization
The first step in the actual process of fertiHzation and the reproduction of
a new individual is the transportation of the mature gametes from the place
of their development in the reproductive structures to the area or site where
conditions are optimum for their union (fig. 98). This transport is dependent
upon the development of the proper reproductive conditions in the male and
the female parent — a state governed by sex hormones. That is to say, the
sex hormones regulate the behavior of the parents and the reproductive ducts
in such a way that the reproductive act is possible.
The transport of the female gamete to the site of fertilization is a passive
one, effected by the behavior of the reproductive structures. Also, the trans-
portation of the sperm within the confines of the male tract largely is a passive
affair. However, outside of the male reproductive tract, sperm motility is a
factor in eflfecting the contact of the sperm with the egg. Not only is sperm
motility a factor in the external watery medium of those species accustomed
to external fertilization, but also to some degree within the female genital
tract in those species utilizing internal fertilization. However, in the latter
case, sperm transport is aided greatly by the activities of the female genital tract.
B. Transportation of the Sperm Within the Male Accessory
Reproductive Structures
1. Transportation of Sperm from the Testis to the
External Orifice of the Genital Duct in
THE Mammal
Sperm transport within the male genital tract of the mammal is a slow
process. It might be defined better by saying that it is efficiently slow, for the
ripening process of the sperm described in the previous chapter is dependent
WITHIN MALE REPRODUCTIVE STRUCTURES
179
-OVARIAN FOLLICLE
(CERTAIN TELEOST
SUCH AS GAMBUSIA,
PERITONEAL CAVIT
(CERTAIN URODELES)
(OCCASIONALLY IN
BIRDS)
ANTERIOR OVIDUC
(MAMMALS, SHA
BIRDS, REPT
POSTE
a
(URO
(G YMN
(CERT
\ EXTERNAL
(MOST FISH
MOST A N U R Al
Fig. 98. Sites of normal fertilization (x) in the vertebrate group,
below mammals. (B) Mammalia.
(A, C) Vertebrates
upon a lingering passage of the sperm through the epididymal portion of the
male genital tract.
a. Possible Factors Involved in the Passage of the Seminal Fluid from
the Testis to the Main Reproductive Duct
1) Accumulated Pressure Within the Seminiferous Tubules. The oozing
of sperm and seminal fluid from the seminiferous tubules through the rete
tubules into the efferent ductules of the epididymis possibly may be the result
of accumulated pressure within the seminiferous tubules themselves. This
pressure may arise from secretions of the Sertoli cells, the infiltration of fluids
from the interstitial areas between the seminiferous tubules, and by the addi-
180
TRANSPORTATION OF THE GAMETES
tion of sperm to the contents of the tubules. As the seminiferous tubule is
blind at its distal end, increased pressure of this kind would serve efficiently
to push the contained substance forward toward the efferent ductules con-
necting the testis with the reproductive duct.
2) Activities Within the Efferent Ductules of the Testis. The time required
for sperm to traverse the epididymal duct in the guinea pig is about 14 to
16 days. However, when the efferent ductules between the testis and the
epididymal duct are ligated, the passage time is increased to 25 to 28 days
(Toothill and Young, '31). The results produced by ligation of the ductuli
efferentes in this experiment suggest: (a) That the force produced by the
accumulation of secretion within the seminiferous tubules and adjacent ducts
tends to push the sperm solution out of the seminiferous tubules into the
ductuli efferentes and thence along the epididymal duct, and/or (b) at least
a part of the propulsive force which moves the contents of the seminiferous
tubules through the rete tubules and efferent ductules and along the epididymal
duct arises from beating of cilia within the lumen of the efferent ducts. The
tall cells lining the latter ducts possess cilia which beat toward the epididymal
duct. As the sperm and surrounding fluid reach the efferent ductules, the
beating of these cilia would propel the seminal substances toward the epi-
didymal duct.
b. Movement of the Semen Along the Epididymal Duct
1) Probable Immotility of the Sperm. The journey through the epididymal
duct as previously indicated is tedious, and secretion from the epididymal
cells is added to the seminal contents (fig. 99). Sperm motility evidently is
Fig. 99. Human epididymal cells. (Slightly modified from Maximow and Bloom: A
Textbook of Histology, Philadelphia, W. B. Saunders Co.) These cells discharge secretion
into the lumen of the epididymal duct. Observe large, non-motile stereocilia at distal
end of the cells.
WITHIN MALE REPRODUCTIVE STRUCTURES 181
not a major factor in sperm passage along the epididymal portion of the
reproductive duct, as conditions within the duct appear to suppress this mo-
tility. It has been shown, for example (Hartman, '39, p. 681), that sperm
motility increases for trout sperm at a pH of 7.0 to 8.0, in the mammals a pH
of a little over 7.0 seems optimum for motility for most species, while in the
rooster a pH of 7.6 to 8.0 stimulates sperm movements. On the other hand,
an increase of the CO, concentration of the medium raises the hydrogen ion
concentration of the suspension. The latter condition suppresses sperm mo-
tility and increases the life of sea-urchin sperm (Cohn, '17, '18). These facts
relative to the influence of pH on the motility of sperm suggest that motiUty
during the slow and relatively long epididymal journey — a journey which may
take weeks — apparently is inhibited by the production of carbon dioxide by
the large aggregate of sperm within the lumen of the epididymal duct, a con-
dition which serves to keep the spermatic fluid on the acid side. This sup-
pressed activity of the sperm in turn increases their longevity. The matter of
sperm motility within the epididymal duct, however, needs more study before
definite conclusions can be reached relative to the actual presence or absence
of motility.
2) Importance of Muscle Contraction, Particularly of the Vas Deferens.
If sperm are relatively immobilized during their passage through the epididymal
duct by the accumulation of carbon dioxide, we must assume that their
transport through this area is due mainly to the activities of the accessory;
structures together with some pressure from testicular secretion and efferent-
ductule activity as mentioned above. Aside from the forward propulsion re-
sulting from the accumulation of glandular secretion within the epididymal:
duct, muscle contraction appears to be the main factor involved in effecting
this transport. The epididymal musculature is not well developed, and muscle
contraction in this area may be effective but not pronounced. However, added
to the contracture of the epididymal musculature is the contraction of the
well-developed musculature of the vas deferens (fig. 100). During sexual
stimulation this organ contracts vigorously, producing strong peristaltic waves
which move caudally along the duct. The activity of the vas deferens may
be regarded as a kind of "pump action" which produces suction sufficient to
move the seminal fluid from the caudal portions of the epididymis, i.e., from
the Cauda epididymidis into the vas deferens where it is propelled toward the
external orifice. Furthermore, the removal of materials from the cauda epi-
didymidis would tend to aid the movement of the entire contents of the
epididymal duct forward toward the cauda epididymidis. From this point of
view, the vas deferens is an efficient organ for sperm transport, while the
epididymal duct functions as a nursery and a "storage organ" for the sperm
(see Chap. 1). Some sperm also are stored in the ampullary portion of the
vas deferens (fig. 101 ), but this storage is of secondary importance inasmuch
as sperm do not retain their viability in this area over extended periods of time.
182
TRANSPORTATION OF THE GAMETES
EXTERNAL
LONGITUDINAL
MUSCLE
OUTER CIRCULAR
MUSCLE
N T E R N A L
LONGITUDINAL
MUSCLE
TUNICA PROPRIA
EPITHELIU M
LUMEN
EPITHELIUM
TUNICA PROPRIA
INTERNAL
LONGITUDINAL
MUSCLE
OUTER CIRCULAR
MUSCLE
EXTERNAL
LONGITUDINA L
MUSCLE
Fig. 100. Highly muscular character of the ductus deferens. This particular drawing
was made from a longitudinal section of the ductus deferens of a young rat. Observe cilia
(stereocilia?) on inner surface of epithelium, lining the lumen.
3) Summary of Factors Which Propel the Seminal Fluid from the Testis
to the External Orifice of the Reproductive Duct in the Mammal. The fol-
lowing probable influences are at work, propelling sperm from the testis
through the accessory ducts in the mammal:
( 1 ) The pressure of accumulated secretions within the seminiferous tubules
may push the sperm outward toward the accessory ducts;
(2) the beating of cilia and accumulation of secretion within the ductuli
efferentes is another probable force which ushers the sperm and semi-
nal fluid forward;
(3) the secretion from the cells of the anterior epididymis and the body
of the epididymis may serve, together with weak muscle contraction,
to advance the sperm mass toward the posterior epididymis;
WITHIN MALE REPRODUCTIVE STRUCTURES
183
(4) the possibility of a weak sperm motility aiding the advance of the
sperm through the body of the epididymis must not be denied;
(5) the vigorous pumping action of the vas deferens, especially during
the stimulation attending ejaculation, serves to transport the sperm
from the "epididymal well" (the cauda epididymidis) through the vas
deferens to the external areas.
2. Transportation of Sperm in Other Vertebrates with a
Convoluted Reproductive Duct
The transportation of sperm in other vertebrates which possess an extended
and complicated reproductive duct similar to that of the mammal presumably
involves the same general principles observed above (fig. 105A, B). However,
certain variations of sperm passage exist which are correlated with structural
modifications of the accessory reproductive organs. For example, the repro-
ductive duct may be somewhat more tortuous and complicated in some in-
stances, such as in the pigeon, turkey, and domestic cock (figs. 102, 105B).
That is, the entire deferent duct extending from the epididymis caudally to
the cloaca may be regarded as a sperm-storage organ, as sperm may be col-
lected in large numbers all along the reproductive duct. As the cock is
capable of effecting repeated ejaculations over an extended period of time,
LUMEN
GLAND-LIKE
OUT POUCHING S
OF MAIN LUMEN
FOLDS OF
MUCOSA
Fig. 101. Portion of a cross section of the ampullary region of the ductus deferens in
man. Observe gland-like outpouchings of the main lumen and character of mucosal folds.
Surrounding the lumen may be seen the highly muscularized walls of the ampullary area.
184
TRANSPORTATION OF THE GAMETES
POSTERIOR
VENA CAVA
TESTES
ME SORC H I U M
ILIAC VEIN
EPIDIDYMIS
KIDNEY
FEMORAL
VEIN
DORSAL
AORTA
RENAL
PORTA L
VEIN
URETER
VA S
DEFERENS
C LOAC A
Fig. 102. Reproductive and urinary structures of the adult Leghorn cock. Observe that
the vas deferens is a much convoluted structure. (After Domm: In Sex and Internal Se-
cretions, by Allen, et al.,, Baltimore, Williams & Wilkins, 1939.)
each contraction of the caudal portion of the deferential duct during sperm
discharge serves to move the general mass of seminal fluid posteriad in a
gradual manner. The reproductive conditions present in the cock fulfill the
requirements of a continuous breeder capable of serving many individual
Fig. 103. Amplexus in the toad, Bufo fowleri. (Modified from Rugh: The Frog, Its
Reproduction and Development, Philadelphia, Blakiston, 1951.)
^^g34C<.<A,dii^&h.i ^ V , ^
ANAL FIN OF MALE
-CLOACA
N TROMI TTEN
ORGAN
CUT MUSCLE
GROOVE FOR
SPERM PASSAGE
N UPPER SUR FACE
F INTROMITTENT
ORGAN
Fig. 104. Modifications of the fins of male fishes with the resulting elaboration of an
intromittent organ. (A) Gambusia affinis. (B) Ventral view of pelvic fins of Squalus
acanthias. (C) Dorsal view of left fin to show genital groove in intromittent structure.
185
186 TRANSPORTATION OF THE GAMETES
females. It is to be observed in this connection that Mann ('49) gives the
amount of ejaculate in the cock as 0.8 cc, highly concentrated with sperm.
Another variation found in certain birds is the presence of a seminal vesicle
located at the caudal end of the reproductive duct. This outgrowth is a sperm-
storage organ and is not comparable to the secretory seminal vesicle found
in mammals. Such seminal vesicles are found in the robin, ovenbird, wood
thrush, catbird, towhee, etc. These structures enlarge enormously during the
breeding season, but in the fall and winter months they shrink into insignificant
organs (Riddle, '27). It is apparent that the seminal fluid is moved along
and stored at the distal (posterior) end of the reproductive duct in these
species. Other birds, such as the pigeon and mourning dove, lack extensively
developed seminal vesicles, but possess instead pouch-like enlargements of
the caudal end of the reproductive duct when the breeding season is at its
maximum.
In many lower vertebrates which practice internal fertilization, large seminal
vesicles or enlargements of the caudal end of the reproductive duct are present.
Such conditions are found in the elasmobranch fishes. These structures act as
sperm-storage organs during the breeding season.
3. Transportation of Sperm from the Testis in Vertebrates
Possessing a Relatively Simple Reproductive Duct
In forms such as the frog, toad, and hellbender (figs. 9, 105C), the pressure
within the seminiferous tubules of the testis associated with contractions of
the reproductive duct serve to move the sperm along the reproductive duct.
At the time of spawning, a copious discharge of sperm is effected. In teleost
fishes, a general contraction of the testicular tissue and the muscles of the
abbreviated sperm duct propel the sperm outward during the spawning act
(fig. 105D). In teleosts, sperm are stored in the testis, or as in the perch,
large numbers may be accommodated within the reproductive duct (fig. 105D).
Slight motility also may be a factor in effecting sperm transport down the
reproductive duct in the lower vertebrates.
C. Transportation of Sperm Outside of the Genital Tract of the Male
1. Transportation of Sperm in the External Watery Medium
In most teleost fishes and in amphibia, such as the frogs and toads, and
the urodeles of the families Hynobiidae and Cryptobranchidae (possibly also
the Sirenidae), fertilization is external and sperm are discharged in close
proximity to the eggs as they are spawned. Many are the ways by which this
relationship is established, some of which are most ingenious (fig. 103).
Sperm motility, once the watery medium near the egg is reached, brings the
sperm into contact with the egg in most instances. However, exceptional cases
are present where the sperm are "almost completely immobile," such as in
TRANSPORIATION OF SPERM OUTSIDE GENITAL TRACT OF MALE
187
HEAD OF EPIDIDYMIS
WEAK MUSCULAR CONTRACTION
AND ACCUMULATED PRESSURE
STRONG MUSCULAR
CONTRACTION
E FFERENT
DUCTULES
BODY OF EPIDIDYMIS
1 ■»
VAS DEFERENS EjACULATORY
DUCT
sperm storage in
"epididymal well"
(THE CAUDA E Pl Dl D Y M I D I S )
CILIARY ACTION
ACCUMULATED PRESSURE
A
SEMINIFEROUS TUBULES '^'
WEAK MUSCULAR CONTRACTION
STRONG MUSCULAR CONTRACTION
lARY ACT ION
ACCUMULATED PRESSURE
■SEMINIFEROUS TUBULES
B,
EF FERENT
DUCTULES
CILIARY ACTION
STRONG MUSCULAR CONTRACTION
-V
SPERM DUCT
ACCUMULATED PRESSURE
SPERM STORAGE
SEMINIFEROUS TUBULES
SPERM DUCT
RM STORAGE
ULATED PRESSURE
STORAGE
c.
D.
Fig. 105. Various types of reproductive ducts in male vertebrates. The possible activi-
ties which transport the sperm along the ducts are indicated. (A) Mammalian type.
(B) Bird, urodele, elasmobranch fish type. (C) Frog type. (D) Teleost fish type.
the primitive frog, Discoglossus (see Hibbard, '28). Here the sperm must
be deposited in close contact with the egg at the time of spawning. In fishes
which lay pelagic eggs (i.e., eggs that float in the water and do not sink to
the bottom), the male may swim about the female in an agitated manner
during the spawning act. This behavior serves to broadcast the sperm in rela-
tion to the eggs.
EGGS
ANAL
OPENING
TA I L
REGION
BROOD
POUCH
G G
B
BROOD POUCH
OF MALE
Fig. 106. Brood pouch in the male pipefish. (A) Longitudinal view with left flap
pulled aside to show the developing eggs within the pouch. (B) Transverse section to
show relation of eggs to the pouch and dorsal region of the tail.
U
TRANSPORTATION OF SPERM OUTSIDE GENITAL TRACT OF MALE 189
2. Transportation of Sperm in Forms Where Fertilization
OF the Egg is Internal
a. General Features Relative to Internal Fertilization
1) Comparative Numbers of Vertebrates Practicing Internal Fertilization.
Of the 60,000 or more species of vertebrates which have been described, a
majority practice some form of internal fertihzation of the egg. Internal fer-
tilization, therefore, is a conspicuous characteristic of the reproductive phe-
nomena of the vertebrate animal group.
2) Sites or Areas where Fertilization is Effected. The fertilization areas
(fig. 98) for those vertebrates which utilize internal fertilization are:
( 1 ) the lower portions of the oviduct near or at the external orifice,
(2) the oviduct, especially its upper extremity,
(3) possibly the peritoneal cavity,
(4) the follicles of the ovary, and
(5) the brood pouch of the male (figs. 98, 106).
Though the exact place where internal fertilization occurs may vary consid-
erably throughout the vertebrate group as a whole, the specific site for each
species is fairly constant.
3) Means of Sperm Transfer from the Male Genital Tract to That of the
Female. In those fishes adapted to internal fertilization, sperm transport from
the male to the female is brought about by the use of the anal or pelvic fins
which are modified into intromittent organs (fig. 104). In the amphibia two
genera of Anura are known to impregnate the eggs within the oviduct of
the female. In the primitive frog, Ascaphus truei, the male possesses a cloacal
appendage or "tail," used to transport the sperm from the male to the female,
and the oviducts become supplied with sperm which come to lie between the
mucous folds (Noble, '31). (See fig. 107.) In East Africa, in the viviparous
toad, Nectophrynoides vivipara, fertilization is internal, and the young, a hun-
dred or more, develop in each uterus. (See Noble, '31, p. 74.) Just how the
sperm are transmitted to the oviduct and whether fertilization is in the lower
or upper parts of the oviduct in this species is not known.
In contrast to the conditions found in most Anura, the majority of urodele
amphibia employ internal fertilization. In many species the male deposits a
spermatophore or sperm mass (fig. 10). The jelly-Hke substance of the sper-
matophore of the salamanders is produced by certain cloacal or auxiliary
reproductive glands. The spermatophore may in some species be picked up
by the cloaca of the female or in other species it appears to be transmitted
directly to the cloaca of the female from the cloaca of the male. As the sper-
matophore is held between the lips of the cloaca of the female, it disintegrates
and the sperm migrate to and are retained within special dorsal diverticula of
the cloacal wall known as the spermatheca (Noble and Weber, '29) (fig. 108).
Fig. 107. Intromittent organ of the tailed frog of America, Ascaphus truei. (After
Noble, '31.) (A) Cloacal appendage. (B) Ventral view of same. (C) Fully distended
appendage, showing spines on distal end. Opening of cloaca shown in the center.
SPERM ATHECA
. ■•"^,^ DOR SAL
Fig. 108. Diagrammatic sagittal sections of the cloacas of three salamanders, showing
types of spermatheca. (A) Nee turns. (B) Amhystoma. (C) Desmognathus. (Re-
drawn from Noble, '31.)
190
TRANSPORTATION OF SPERM OUTSIDE GENITAL TRACT OF MALE 191
In the male of the gymnophionan amphibia, a definite protrusible copula-
tory organ is present as a cloacal modification, and fertiHzation occurs within
the oviducts (fig. 109). Extensible copulatory organs are found generally in
reptiles and mammals, and are present also in some birds, such as the duck,
ostrich, cassowary, emu, etc. In most birds the eversion of the cloaca with
a slight protrusion of the dorsal cloacal wall functions very effectively as a
copulatory organ.
b. Methods of Sperm Transport Within the Female Reproductive Tract
1) When Fertilization Is in the Lower or Posterior Portion of the Genital
Tract. In many of the urodele amphibia, fertilization is effected apparently
in the caudal areas of the female genital tract or as the egg passes through the
cloacal region. It is probable in these cases that sperm motility is the means
of transporting the sperm to the egg from the ducts of the spermatheca or
from the recesses of the folds of the oviduct.
2) When Fertilization Occurs in the Upper Extremity of the Oviduct. In
several species of salamanders, fertilization of the egg and development of
the embryo occur within the oviduct. Examples are: Salamandra salamandra,
S. atra, Hydromantes genei and H. italicus, all in Europe, and the widely
spread neotropical urodele, Oedipus. The latter contains many species. The
exact region of the oviduct where fertilization occurs is not known, but pre-
sumably, in some cases, it is near the anterior end. Weber ('22) suggests that
fertilization may occur normally in the peritoneal cavity of Salamandra atra.
In these instances, the method by which the sperm reach the fertilization
area is not clear. It is probable that motility of the sperm themselves has
much to do with their transport, although muscular contraction and ciliary
action may contribute some aid.
On the other hand, studies of sperm transport in the female genital tract in
higher vertebrates have supplied some interesting data relative to the methods
and rate of transport. In the painted turtle, Chrysemys picta, sperm are de-
posited within the cloacal area of the female during copulation; from the
cloaca they pass into the vaginal portion of the oviduct and thence into the
uterus. It is possible that muscular contractions, antiperistaltic in nature, propel
the sperm from the cloaca through the vagina and into the uterus. It may be
that similar muscle contractions propel them through the uterus up into the
albumen-secreting portions of the oviduct, or it is possible that sperm motility
is the method of transport through these areas. However, once within the
albumen-secreting section of the oviduct, a band of pro-ovarian ciha (i.e.,
cilia which beat toward the ovary) (fig. IIOA, B) appears to transport the
sperm upward to the infundibulum of the oviduct (Parker, '31). Somewhat
similar mechanisms of muscular contraction, antiperistaltic in nature, and
beating of pro-ovarian cilia are probably the means of sperm transport in
the pigeon and hen (Parker, '31). Antiperistaltic muscular contractions are
Fig. 109. Intromittent organ of male gymnophionan amphibia (Scolecomorphus
uliiguruensis). (After Noble, '31.)
0*^
ALBUMEN-
SECRETING
PORT
OF 0 VI
INFUNDIBULUM
OVARY
URINARY BLADDE
A.
OVA R Y
INFUNDIBULU M
UTERUS
ALBUMEN -SECRETING
PORTION OF OVIDUCT
VA Gl N
R ESPIRATOR
8 L A D 0 E
ISTHMUS
RUDIMENTARY
RIGHT OVIDUCT
UTERUS
VAGINA
CLOACA
c.
INFUNDIBULUM
-^-AL B UM E N-
SECRETING
POR T ION
OF OVIDUCT
0- OVARIAN
ISTHMUS
UDIMENTARY
IG HT OVAR Y
UTERUS
VAGINA
CLOACA D.
Fig. 110. Female reproduction systems of turtle and bird. (Slightly modified from
Parker, '31.) (A) Reproductive organs of the female tortoise, Chrysemys picta. (B)
The same, spread out, showing region of ciliary tract. (C) Reproductive organs of the
female pigeon. (D) The same, spread out, ciliary tract region indicated.
192
TRANSPORTATION OF SPERM OUTSIDE GENITAL TRACT OF MALE 193
known to be possible in the hen (Payne, '14). Active muscular contractions
are suggested, as sperm travel upward to the infundibulum of the oviduct in
about one and one-half hours in the hen.
In the rabbit, antiperistaltic contractions of the cervix and body of the
uterus at the time of copulation pump or suck the sperm through the os uteri
from the vagina and transport them into the uterus at its cervical end (Parker,
'31 ). This transportation occupies about one to three minutes. Passage through
the body of the uterus to the Fallopian tube occurs in one and one-half to
two hours after copulation. It is not clear whether sperm motility alone or
sperm motility plus uterine antiperistalsis effects this transportation. The trans-
port of the sperm upward through the Fallopian tube to the infundibular
region takes about two hours more. The behavior of the uterine (Fallopian)
tube is somewhat peculiar at this time. Churning movements similar to that
of the normal activity of the intestine are produced. Also, temporary longi-
tudinal constrictions of the wall of the tube produce longitudinal compart-
ments along the length of the tube. Within these compartments cilia beat
vigorously in an abovarian direction (i.e., away from the ovary). The general
result of these activities is a thorough mixing and churning of the contents
of the tubes. At the same time these movements succeed in transporting the
sperm up the tube to the infundibular area. The entire journey through the
uterus and Fallopian tube consumes about four hours (Hartman, '39, pp.
698-702; Parker, '31).
Sperm transport through the female genital tract in the rabbit occupies a
relatively long period of time compared to that which obtains in certain other
mammalian species. The journey to the infundibular area of the Fallopian
tube takes only 20 minutes in the majority of cases in the ewe, following
normal service by the ram. The rate of sperm travel toward the ovaries is
approximately four cm. per minute (Schott and Phillips, '41). The passage
time through the entire female duct may be considerably less than this in the
guinea pig, dog, mouse, etc. (Hartman, '39, p. 698). It is probable that the
latter forms experience antiperistaltic muscular contractions of the uterine
cervix, uteri, and Fallopian tubes, which propel the sperm upward to the
infundibular region, the normal site of fertilization.
In the marsupial group the lateral vaginal canals complicate the sperm
transport problem. In the opossum, the bifid terminal portion of the peniai
organ (fig. 114A) probably transmits the sperm to both lateral vaginal canals
simultaneously, where they are churned and mixed with the vaginal contents.
From the lateral vaginal canals the sperm are passed on to the median vaginal
cul-de-sac. From this compartment they travel by their own motive power
or are propelled upward through the uterus and Fallopian tubes to the infun-
dibular area of the latter (figs. 34, 35, 114).
The foregoing instances regarding sperm transport in the female mammal
involve active muscle contractions presumably mediated through nerve im-
SUSPENSORY
LIGAMENT
FA LL 0 P I A N
TUBE
HNFUNDI BULUM
U TE R US
OVARY
LIGAMENT
Fig. 111. Dorsal view of anterior end of uterine horn of the common opossum, Didelphys
virginiana, showing relation of ovary to infundibulum.
CORPUS LUTEUM
CAPSULE
i — H I L U S
)
/— FOLLICLE
^FAT T Y TISSUE
OVARIAN LOBE
Fig. 112. Section through ovary of mature rat, showing lobed condition and ovarian
capsule. (Adapted from Heys, Quart. Rev. Biol., VI.)
194
TRANSPORTATION OF SPERM OUTSIDE GENITAL TRACT OF MALE
195
DORSAL PERITONEUM
LUNG
OVIDUCT
Fig. 113. Open body cavity of adult female of Rana pipiens, showing distribution of
cilia and ostium of oviduct. (Slightly modified from Rugh, '35.)
pulses aroused during the reproductive act or orgasm together with the actual
presence within the reproductive tract of seminal fluid. However, this nerve-
muscular activity is assuredly not the only means of sperm transport although
it may be the more normal and common method. A slower means of trans-
port, that of sperm motility, plays an important role in many instances. This
is suggested by such facts as fertility being equal in women who experience
no orgasm during coitus compared to those who do; proven fertility in rabbits
and dogs whose genital tracts are completely de-afferented by spinal section;
and conception by females artificially inseminated intra vaginum. (See Hart-
man, '39, p. 699.) Moreover, Phillips and Andrews ('37) have shown that
rat sperm injected into the vagina of the ewe along with ram sperm lag behind
the ram sperm in their migration upward in the genital tract. That is, the
abnormal environment of the genital tract of the ewe in which the rat sperm
196
TRANSPORTATION OF THE GAMETES
were placed may have affected their motihty, as well as their ability to survive.
(See Yochem, '29.)
The above data suggest relationships in many of the vertebrates which
doubly assure that sperm will reach the proper site for fertilization in the
oviduct. One aspect of this assurance is the physiological behavior of the
anatomical structures of the oviduct, which may express itself by ciliary beating
in some instances or, in other cases, by muscle contraction. On the other hand,
VAGINAL CANAL
PORTION OF THE
VAGINAL CANAL
VAGINAL PASSAGEWAY
ARY BIRTH CANAL)
UROGENITAL SINUS
Fig. 1 14. Bifid penis of the male opossum; diagram of female reproductive tract. (A)
Extended penis. (After McCrady, Am. Anat. Memoirs, 16. The Wistar Institute of
Anatomy and Biology, Philadelphia.) (B) Female reproductive tract.
SPERM SURVIVAL IN FEMALE GENITAL TRACT 197
if this method fails or is weakened, sperm motility itself comes to the rescue,
and sperm are transported under their own power.
In view of the above-mentioned behavior of the oviduct in transporting
sperm, it is important to observe that the estrogenic hormone is in a large
way responsible for the activities of the oviduct during the early phases of
the reproductive period and, consequently, influences the conditions necessary
for sperm transport. It enhances this process by arousing a state of irritability
and reactivity within the musculature of the uterus and the Fallopian tubes.
It also induces environmental conditions which are favorable for sperm sur-
vival within the female genital tract.
3) When Fertilization Occurs in the Ovary. In certain viviparous fishes
the egg is fertilized in the ovary (e.g., Gambusia affinis; Heterandria formosa).
(See Turner, '37, '40; Scrimshaw, '44.) As the sperm survive for months in
the female tract, sperm transport is due probably to the movements of the
sperm themselves. Motility evidently is a factor in the case of the eutherian
mammal, Ericidus, where ovarian fertilization presumably occurs according to
Strauss, '39.
D. Sperm Survival in the Female Genital Tract
The length of life of sperm in the female genital tract varies considerably
in different vertebrates. In the common dogfish, Squalus acanthias, and also
in other elasmobranch fishes, sperm evidently live within the female genital
tract for several months, and retain, meanwhile, their ability to fertilize. In
the ordinary aquarium fish, the guppy (Lebistes), sperm may live for about
one year in the female tract (Purser, '37). A long sperm survival is true also
of the "mosquito fish," Gambusia. Within the cloacal spermatheca of certain
urodele amphibia, sperm survive for several months. Within the uterus of the
garter snake they may live for three or more months (Rahn, '40), while in
the turtle, Malaclemys centrata, a small percentage of fertile eggs (3.7 per
cent) were obtained from females after four years of isolation from the male
(Hildebrand, '29). Sperm, within the female tract of the hen, are known to
Hve and retain their fertility for two or three weeks or even longer (Dunn, '27 ) .
In the duck the duration of sperm survival is much shorter (Hammond and
Asdell, '26).
Among mammals, the female bat probably has the honor of retaining
viable sperm in the genital tract for the longest period of time, for, while the
female is in hibernation, sperm continue to live and retain their fertilizing
power from the middle of autumn to early spring (Hartman, '33; Wimsatt,
'44). According to Hill and O'Donoghue ('13) sperm can remain alive within
the Fallopian tubes of the Australian native cat, Dasyurus viverrinus, for "at
least two weeks." However, it is problematical whether such sperm are capable
of fertilizing the egg, for motility is not the only faculty necessary in the
fertilization process. In most mammals, including the human female, sperm
198 TRANSPORTATION OF THE GAMETES
survival is probably not longer than 1 to 3 days. In the rabbit, sperm are in
the female genital tract about 10 to 14 hours before fertilization normally
occurs; they lose their ability to fertilize during the early part of the second
day (Hammond and Asdell, '26). In the genital tract of the female rat, sperm
retain their motility during the first 17 hours but, when injected into the
guinea pig uterus, they remain motile for only four and one-half hours. How-
ever, guinea-pig sperm will remain alive for at least 41 hours in the guinea-
pig uterine horns and Fallopian tubes (Yochem, '29).
E. Sperm Survival Outside the Male and Female Tracts
1. In Watery Solutions Under Spawning Conditions
In watery solutions in which the natural spawning phenomena occur, the
life of the sperm is of short duration. The sperm of the frog, Rana pipiens,
may live for an hour or two, while the sperm of Funduliis heteroclitiis probably
live 10 minutes or a little longer. In some other teleost fishes, the fertilizing
abihty is retained only for a few seconds.
2. Sperm Survival Under Various Artificial Conditions;
Practical Application in Animal Breeding
One of the main requisites for the survival of mammalian and bird sperm
outside the male or female tract is a lowered temperature. The relatively high
temperature of 45 to 50° C. injures and kills mammalian sperm while body
temperatures are most favorable for motility of mammalian and bird sperm;
lower temperatures reduce motility and prolong their life. Several workers
have used temperatures of 0 to 2° C. to preserve the life of mammalian and
fowl sperm, but a temperature of about 8 to 12° C. is now commonly used
in keeping mammalian and fowl sperm for purposes of artificial insemination.
Slow freezing is detrimental to sperm, but quick freezing in liquid nitrogen
permits sperm survival even at a temperature of — 195° C. (See Shettles, '40;
Hoaglund and Pincus, '42.)
Another requirement for sperm survival outside the genital tract of the
male is an appropriate nutritive medium. Sperm ejaculates used in artificial
insemination generally are diluted in a nutritive diluent. The following diluent
(Perry and Bartlett, '39) has been used extensively in inseminating dairy
cattle:
Na,SO, 1.36 gr. )
Dextrose 1.20 gr. [ per 100 ml. H,,0.
Peptone 0.50 gr. )
Also, a glucose-sahne diluent has been used with success (Hartman, '39,
p. 685). Its composition is as follows:
Glucose 30.9 gr. \
Na HP0,I2H,0 6.0 gr. I ,^^^ ^, ^.,0.
NaCl 2.0 gr. (
KH.PO, 0.1 gr. ;
TRANSPORTATION OF EGG FROM OVARY TO SITE OF FERTILIZATION 199
Some workers in artificial insemination use one type of diluent for ram
sperm, another for stallion sperm, and still another for bull sperm, etc.
Artificial insemination of domestic animals and of the human female is
extensively used at present. It is both an art and a science. In the hands of
adequately prepared and understanding practitioners, it is highly successful.
The best results have been obtained from semen used within the first 24
hours after collection, although cows in the Argentine have been inseminated
with sperm sent from the United States seven days previously (Hartman, '39,
p. 685).
F. Transportation of the Egg from the Ovary to the Site of Fertilization
1. Definitions
The transportation of the egg from the ovary to the oviduct is described
as external (peritoneal) migration of the egg, whereas transportation within
the confines of the female reproductive tract constitutes internal (oviducal)
migration. It follows from the information given above that the site of fer-
tiUzation determines the extent of egg migration. In those species where ex-
ternal fertilization of the egg is the habit, the egg must travel relatively long
distances from the ovary to the watery medium outside the female body. On
the other hand, in most species accustomed to internal fertilization, the latter
occurs generally in the upper region of the oviduct. Of course, in special
cases as in certain viviparous fishes, such as Gambusia affinis and Heterandria
formosa, fertilization occurs within the follicle of the ovary and migration of
the egg is not necessary. The other extreme of the latter condition is present
in such forms as the pipefishes. In the latter instance the female transfers the
eggs into the brood pouch of the male; here they are fertilized and the embryos
undergo development (fig. 106).
2. Transportation of the Egg in Those Forms Where
Fertilization Occurs in the Anterior Portion
OF THE Oviduct
a. Birds
A classical example of the activities involved in transportation of the egg
from the ovary to the anterior part of the oviduct is to be found in the birds.
In the hen the enlarged funnel-shaped mouth of the oviduct or infundibulum
actually wraps itself around the discharged egg and engulfs it (fig. 31). Peri-
stalsis of the oviduct definitely aids this engulfing process. Two quotations
relative to the activities of the mouth of the oviduct during egg engulfment
are presented below. The first is from Patterson, '10, p. 107:
Coste describes the infundibulum as actually embracing the ovum in its follicle
at the time of ovulation, and the writer [i.e., Patterson] has been able to confirm
his statement by several observations. If we examine the oviduct of a hen that is
laying daily, some time before the deposition of the egg, it will be found to be
200 TRANSPORTATION OF THE GAMETES
inactive; but an examination shortly after laying reveals the fact that the oviduct
is in a state of high excitability, with the infundibulum usually clasping an ovum
in the follicle. In one case it was embracing a follicle containing a half-developed
ovum, and with such tenacity that a considerable pull was necessary to disengage
it. It seems certain, therefore, that the stimulus which sets off the mechanism for
ovulation is not received until the time of laying, or shortly after.
If the egg falls into the ovarian pocket (i.e., the space formed around the
ovary by the contiguous body organs), the infundibulum still is able to engulf
the egg. Relative to the engulfment of an egg lying within the ovarian pocket,
Romanoff and Romanoff, '49, p. 215, states:
The infundibulum continues to advance, swallow, and retreat, partially engulfing
the ovum, then releasing it. This activity may continue for half an hour before the
ovum is entirely within the oviduct.
b. Mammals
In those mammals in which the ovary lies free and separated from the
mouth of the oviduct (figs. 29, 111) it is probable that the infundibulum
moves over and around the ovary intermittently during the ovulatory period.
Also, the ovary itself changes position at the time when ovulation occurs,
with the result that the ovary moves in and out of the infundibular opening
of the uterine tube (Hartman, '39, p. 664). In the Monotremata (prototherian
mammals) during the breeding season, the enlarged membranous funnel
(infundibulum) of the oviduct engulfs the ovary, and a thick mucous-like
fluid lies in the area between the ovary and the funnel (Flynn and Hill, '39).
At ovulation the relatively large egg passes into this fluid and then into the
Fallopian tube. In the rat and the mouse which have a relatively closed ovarian
sac, the bursa ovarica, around the ovary (figs. 37, 112) contractions of the
Fallopian tube similar to those of other mammals tend to move the fluid and
contained eggs away from the ovary and into the tube. Thus it appears that
the activities of the mouth and upper portions of the oviduct serve to move
the egg from the ovarian surface into the reproductive duct at the time of
ovulation in the mammal and bird. This method of transport probably is present
also in reptiles and elasmobranch fishes. In the mammal this activity has been
shown to be the greatest at the time of estrus. The estrogenic hormone, there-
fore, is directly involved in those processes which transport the egg from the
ovary into the uterine tube.
In women, and as shown experimentally in other mammals, the removal
of the ovary of one side and the ligation or removal of the Fallopian tube on
the other side does not exclude pregnancy. In these cases, there is a transmi-
gration of the egg from the ovary on one side across the peritoneal cavity to
the opening of the Fallopian tube on the other where fertilization occurs.
This transmigration is effected, presumably, by the activities of the intact
infundibulum and Fallopian tube of the contralateral side.
Another aspect of egg transport in the mammal is the activity of the ciHa
TRANSPORTATION OF EGG FROM OVARY TO SITE OF FERTILIZATION 201
lining the fimbriae, mouth, and to a great extent, the ampullary portions of
the uterine (Fallopian) tube itself. The beating of these cilia tend to sweep
small objects downward into the Fallopian tube. However, these activities
are relatively uninfluential in comparison to the muscular activities of the
infundibulum and other portions of the Fallopian tube.
Egg transport between the ovary and the oviduct is not always as efficient
as the above descriptions may imply. For, under abnormal conditions the
egg "may lose its way" and if fertilized, may begin its development within'
the spacious area of the peritoneal cavity. This sort of occurrence is called
an ectopic pregnancy. In the hen, also, some eggs never reach the oviduct
and are resorbed in the peritoneal cavity.
3. Transportation of the Egg in Those Species Where
Fertilization is Effected in the Caudal Portion
OF the Oviduct or in the External Medium
a. Frog
In the adult female of the frog (but not in the immature female or in the
male) cilia are found upon the peritoneal lining cells of the body wall, the
lateral aspect of the ovarian ligaments, the peritoneal wall of the pericardial
cavity and upon the visceral peritoneum of the liver. Cilia are not found on
the coelomic epithelium supporting and surrounding the digestive tract, nor
are they found upon the epithelial covering of the ovary, kidney, lung, bladder,
etc. (fig. 113). (See Rugh, '35.) This ciliated area has been shown to be
capable of transporting the eggs from the ovary anteriad to the opening of
the oviduct on either side of the heart (fig. 113) (Rugh, '35). In this form,
therefore, ciliary action is the main propagating force which transports the
egg (external migration) from the ovary to the oviduct. Internal migration
of the egg (transportation of the egg within the oviduct) also is effected mainly
by cilia in the common frog, although the lower third of the oviduct "is abun-
dantly supplied with smooth muscle fibers," and "shows some signs of peri-
stalsis" (Rugh, '35). The passage downward through the oviduct to the uterus
consumes about two hours at 22° C. and, during this transit, the jelly coats
are deposited around the vitelline membrane. The jelly forming "the innermost
layer" is deposited "in the upper third of the oviduct, and the outermost layer
just above the region of the uterus." The ciliated epithelium, due to the spiral
arrangement of the glandular cells along the oviduct, rotates the egg in a spiral
manner as it is propelled posteriad (Rugh, '35). Once within the uterus, the
eggs are stored for various periods of time, depending upon the temperature.
During amplexus, contractions of the uterine wall together, possibly, with
contractions of the musculature of the abdominal wall, expel the eggs to the
outside. At the same time, the male frog, as in the toad, discharges sperm
into the water over the eggs (fig. 103). In the toad, the eggs pass continu-
202 TRANSPORTATION OF THE GAMETES
ously through the oviduct and are not retained in the uterus as in the frog
(Noble, '31, p. 282).
b. Other Amphibia
The transport of the eggs to the site of fertilization in other anuran amphibia
presumably is much the same as in the frog, although variations in detail
may occur. In the urodeles, however, conditions appear to diverge from the
frog pattern considerably. As mentioned previously, fertilization of the eggs
of Salamandra atra may occur within the peritoneal cavity before the egg
reaches the oviduct, while fertilization in most urodeles occurs internally in
the oviduct, either posteriorly or in some cases more anteriorly. In this am-
phibian group, the ostium of the oviduct is funnel-shaped and is open, whereas
in the frog it is maintained in a constricted condition and opens momentarily
as the egg passes through it into the oviduct. (Compare figs. 34, 113.) The
open condition of the oviducal ostium in the urodeles suggests that the ostium
and anterior part of the oviduct may function as a muscular organ in a manner
similar to that of birds and mammals.
c. Fishes
Egg transport in the fishes presents a heterogeneous group of procedures.
In the cyclostomes the eggs are shed into the peritoneal cavity and are trans-
ported caudally on either side of the cloaca to lateral openings of the uro-
genital sinus. The eggs pass through these openings into the sinus and through
the urogenital papilla to the outside. Contractions of the musculature of the
abdominal wall may aid egg transport.
In most teleost fishes, the contraction of ovarian tissue together with prob-
able contractions of the short oviduct is sufficient to expel the eggs to the
outside (fig. 28). A somewhat similar condition is found in the bony ganoid
fish, Lepisosteus, where the ovary and oviduct are continuous. However, in
the closely related bony ganoid, Amia, the eggs are shed into the peritoneal
cavity and make their way into an elongated oviduct with a wide funnel-
shaped anterior opening and from thence to the outside. A similar condition
is found in the cartilaginous ganoid, Acipenser. In the latter two forms, the
anatomy of the reproductive ducts in relation to the ovaries suggests that the
egg-transport method from the ovary to the ostium of the duct is similar to
that found in birds and mammals. Muscular contractions of the oviduct
probably propel the egg to the outside where fertilization occurs. This may
be true also of the salmon group of fishes, including the trout, where a short,
open-mouthed oviduct is present. In the lungfishes (Dipnoi) the anatomy of
the female reproductive organs closely simulates that of urodele amphibia.
It is probable that egg transport in this group is similar to that of the urodeles,
although fertilization in the Dipnoi is external.
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Bibliography
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the hydrogen ion concentration of sperm
suspensions and their fertilizing power.
Anat. Rec. 11:530.
. 1918. Studies in the physiology
of spermatozoa. Biol. Bull. 34:167.
Dunn, L. C. 1927. Selective fertilization
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Flynn, T. T. and Hill, J. P. 1939. The de-
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5
Fertilization
A. Definition of fertilization
B. Historical considerations concerning gametic fusion and its significance.
C. Types of egg activation
1. Natural activation of the egg
2. Artificial activation of the egg
a. Object of studies in artificial parthenogenesis
b. Some of the procedures used in artificial activation of the egg
c. Results obtained by the work on artificial parthenogenesis
D. Behavior of the gametes during the fertilization process
1. General condition of the gametes when deposited within the area where fertiliza-
tion is to occur
a. Characteristics of the female gamete
1 ) Oocyte stage of development
2) Inhibited or blocked condition
3) Low level of respiration
4) Loss of permeability
b. Characteristics of the male gamete
2. Specific activities of the gametes in effecting physical contact of the egg with the
sperm
a. Activities of the female gamete in aiding sperm and egg contact
1) Formation of egg secretions which influence the sperm
a) Fertilizin complex
b) Spawning-inducing substances
b. Activities of the male gamete in aiding the actual contact of the two gametes
1 ) Sperm secretions
a) Secretions producing lysis
b) Secretions related specifically to the fertilization reactions
c) Secretions which induce the spawning reaction in the female
2) Relation and function of sperm number in efi'ecting the contact of the sperm
with the egg
3) Influences of the seminal plasma in eff'ecting sperm contact with the egg
4) Roles played by specific structural parts of the sperm in effecting contact
with the egg
a) Role of the flagellum
b) Role of, the acrosome in the egg-sperm contact
5) Summary of the activities of the egg and sperm in bringing about the pri-
mary or initial stage of the fertilization process, namely, that of egg and
sperm contact
210
DEFINITION
211
3. Fusion of the gametes or the second stage of the process of fertilization
4. Detailed description of the processes involved in gametic union as outlined above
a. Separation and importance of a protective egg membrane, exudates, etc.
b. Fertilization cone or attraction cone
c. Some changes in the physiological activities of the egg at fertilization
d. Completion of maturation divisions, ooplasmic movements, and copulatory
paths of the male and female pronuclei in eggs of various chordate species
1) Fertilization in Styela (Cynthia) partita
a) Characteristics of the egg before fertilization
b) Entrance of the sperm
c) Cytoplasmic segregation
d) Copulatory paths and fusion of the gametic pronuclei
2) Fertilization of Amphioxus
3) Fertilization of the frog's egg
4) Fertilization of the teleost fish egg
5) Fertilization in the egg of the hen and the pigeon
6) Fertilization in the rabbit
7) Fertilization in the Echidna, a prototherian mammal
E. Significance of the maturation divisions of the oocyte in relation to sperm entrance
and egg activation
F. Micropyles and other physiologically determined areas for sperm entrance
G. Monospermic and polyspermic eggs
H. Importance of the sperm aster and the origin of the first cleavage amphiaster
I. Some related conditions of development associated with the fertilization process
1. Gynogenesis
2. Androgenesis
3. Merogony
J. Theories of fertilization and egg activation
A. Definition of Fertilization
The union or fusion (syngamy) of the oocyte or egg (female gamete) with
the sperm (male gamete) to form a zygote is known as fertilization. From
this zygotic fusion the new individual arises. Strictly speaking, the word fer-
tilization denotes the process of making the egg fruitful (i.e., develop) by
means of the sperm's contact with the egg, and as such may not always imply
a fusion of the sperm with the egg. In certain types of hybrid crosses, such
as in the toad egg (Bufo) inseminated with urodele sperm (Triton), egg acti-
vation may occur without fusion of the sperm nucleus with the egg nucleus.
Ordinarily, however, the word fertilization denotes a fusion of the two gametes
(see Wilson, '25, pp. 460-461).
The word zygote is derived from a basic Greek word which means to join
or yoke together. The word is particularly appropriate in reference to the
behavior of the nuclei of the two gametes during fertilization. For, during
gametic union, the haploid group of chromosomes from one gamete is added
to the haploid group from the other, restoring the diploid or normal number
of chromosomes. In most instances, each chromosome from one gamete has
a mate or homologue composed of similar genes in the other gamete. There-
212 FERTILIZATION
fore, the union of the two haploid groups of chromosomes forms an integrated
association in which pairs of similar genes are yolced together to perform their
functions in the development of the new individual.
In most animal species aside from the union of the chromosome groups,
there is a coalescence of most of the cytoplasm of the male gamete with that
of the female gamete as the entire sperm generally enters the egg (figs. 115,
118). However, in some species the tail of the sperm may be left out, e.g.,
rabbit, starfish, and sea urchin, while in the marine annelid. Nereis, the head
of the sperm alone enters, the middle piece and tail being left behind.
The morphological fusion of the two sets of nucleoplasms and cytoplasms
of the gametes during fertilization is made possible by certain physiological
changes which accompany the fusion process. These changes begin the instant
that a sperm makes intimate contact with the surface of the oocyte (or egg).
As a result, important ooplasmic activities are aroused within the egg which
not only draw the sperm into the ooplasm but also set in motion the physico-
chemical machinery which starts normal development. The initiation of normal
development results from the complete activation of the egg. Partial activation
of the egg is possible, and in these instances, various degrees of development
occur which are more or less abnormal. Partial activation of the egg happens
in most instances when the various methods of artificial activation (see p. 217)
are employed.
While the main processes of activation leading to development are con-
cerned with the organization and substances of the egg, one should not over-
look the fact that the sperm also is activated (and in a sense, is fertilized)
during the fusion process. Sperm activation is composed of two distinct phases:
( 1 ) Before the sperm makes contact with the oocyte or egg, it is aroused
by environmental factors to swim and move in a directed manner and
is attracted to the oocyte or egg by certain chemical substances secreted
by the latter; and
(2) after its entrance into the egg's substance, the sperm nucleus begins
to enlarge and its chromosomes undergo changes which make it pos-
sible for them to associate with the egg chromosomes in the first
cleavage spindle. Also, the first cleavage amphiaster in the majority
of animal species appears to arise within the substance of the middle
piece of the sperm as a result of ooplasmic stimulation.
In the process of normal fertilization it is clear, therefore, that two main
conditions are satisfied:
( 1 ) There is a union of two haploid chromosome groups, one male and
the other female, bringing about the restoration of a proper diploid
genie balance; and
(2) an activation of the substances of the fused gametes, both cyto-
HISTORICAL CONSIDERATIONS 213
plasmic and nuclear, is effected, resulting in the initiation of normal
development.
The biochemical and physiological factors which accomplish the union of
the haploid chromosome groups and the activation of the gametes are the
objectives of one of the main facets of embryological investigation today.
B. Historical Considerations Concerning Gametic Fusion and Its
Significance
The use of the word "fertilization" in the sense of initiating development
and the idea of making fertile or fruitful, which the word arouses in one's
mind, reaches back to the dawn of recorded history. The concept of this
fruitfulness as being dependent upon the union of one sex cell with another
sex cell and of the fusion of the two to initiate the development of a new
individual originated in the nineteenth century. However, Leeuwenhoek, in
1683, appears to have been the first to advance the thesis that the egg must
be impregnated by a seminal animalcule (i.e., the sperm) in order to be-
come fruitful, but the real significance of this statement certainly was not
appreciated by him.
Moreover, to Leeuwenhoek, the idea behind the penetration of the egg by
the seminal animalcula was to supply nourishment for the latter, which he
believed was the essential element in that it contained the preformed embryo
in an intangible way. That is, the sperm animalcule of the ram contains a
lamb, which does not assume the external appearance of one until it has
been nourished and grown in the uterus of the female (Cole, '30, pp. 57, 165).
It should be added parenthetically that actual presence of the little animalcules
as living entities had previously been called to Leeuwenhoek's attention in
1677 by a Mr. Ham (Cole, '30, p. 10).
In the years that followed Leeuwenhoek the exact interpretation to be
applied to the seminal animalcules (sperm) was a matter of much debate.
Many maintained that they were parasites in the seminal fluid, the latter being
regarded as the essential fertilizing substance in the male semen. In 1827,
von Baer, who regarded the sperm as parasites, named them spermatozoa,
that is, parasitic animals in the spermatic fluid (Cole, '30, p. 28). Finally,
in the years from 1835-1841, Peltier, Wagner, Lallemand, and Kolliker, es-
tablished the non-parasitic nature of the sperm. Kolliker in 1841 traced their
origin from testicular tissue, and thus settled the argument once and for all
as to the true nature of the seminal animals or sperm.
Various individuals have laid claim to the honor of being the first to de-
scribe the sperm's entry into the egg at fertilization, but the studies of Newport
and Bischoflf (1853, 1854) resulted in the first exact descriptions of the
process. (See Cole, '30, pp. 191-195.) Thus the general proposition set forth
by Leeuwenhoek 170 years earlier became an accepted fact, although the
214 FERTILIZATION
illumination of the details of sperm and egg behavior during fertilization really
began with the studies of O. Hertwig in 1875. The more important studies
which have shed light upon the problems involved in gametic fusion are pre-
sented below:
(1) O. Hertwig, 1875, 1877, in the former paper, described the fusion of
the egg and sperm pronuclei in the Mediterranean sea urchin, Toxo-
pneustes lividus. One aspect of the work published in 1877 was con-
cerned with the formation of the polar bodies in Haemopis and
Nephelis. In a part of the latter publication O. Hertwig presented
descriptions of sperm migration from the periphery of the egg and
the ultimate association of the sperm and egg pronuclei during the
fertilization in the frog, Rana temporaria (fig. 1191, J).
(2) Fol, 1879, contributed detailed information relative to the actual en-
trance of the sperm into the sea-urchin egg and showed that in the
eggs of various animal species only one sperm normally enters. He
also described the formation of the fertilization membrane in the egg
of the sea urchin, Toxopneustes lividus.
(3) Mark, 1881, made important contributions relative to the formation
of the polar bodies in the slug, Deroceras laeve (Limax campestris).
He also presented information which showed that the egg and sperm
pronuclei, although associated near the center of the egg during fer-
tilization, do not actually form a fusion nucleus in this species as de-
scribed for the sea urchin by O. Hertwig. This is an important contri-
bution to the fertilization problem, as fusion nuclei are not formed
in all animal species.
(4) Van Beneden, 1883, in his studies on maturation of the egg and fer-
tilization in Ascaris megalocephala, demonstrated that half of the
chromatin material of the egg nucleus was discharged in the matura-
tion divisions. (He erroneously thought, however, that the female
ejected the male chromosomes at this time, and in the male, the reverse
process occurred.) (See fig. 133C, D.) He demonstrated also that the
two pronuclei in Ascaris do not join to form a fusion nucleus at fer-
tilization. His work revealed further that the male and female pronuclei
each contributes the haploid or half the normal number of chromo-
somes at fertilization and that each haploid group of chromosomes
enters the equatorial plate of the first cleavage spindle as an inde-
pendent unit (fig. 133F-I). Upon the equatorial plate each chromo-
some divides and contributes one chromosome to each of the two
daughter nuclei resulting from the first cleavage division. This contri-
bution of the haploid number (half the typical, somatic number) of
chromosomes from each parent Van Beneden assumed to be a funda-
mental principle of the fertilization process. This principle was defi-
HISTORICAL CONSIDERATIONS 215
nitely established by later workers and it has become known as Van
Beneden's Law.
(5) Boveri, 1887 and the following years, further established the fact of
Van Beneden's Law and also demonstrated that half of the chromo-
somes of the cells derived from the zygote are maternal and half are
paternal in origin. (Fig. 133 is derived from Boveri's study of Ascaris.)
In '00 and '05 he emphasized the importance of the centrosome and
centrioles and presented the theory that the centrosome contributed
by the sperm to the egg at the time of fertilization constituted the
dynamic center of division which the egg lacked; hence, it was a causal
factor in development. This latter concept added new thinking to the
fertilization problem, for O. Hertwig, 1875, had suggested that the
activation of the egg was due to the fusion of the egg and sperm
nuclei. The centrosome theory of Boveri eventually became one of
the foremost theories of egg activation (see end of chapter).
(6) During the last five years of the nineteenth century, intensive studies
on artificial activation of the egg (artificial parthenogenesis) were
initiated. This matter is discussed on page 217 in the section dealing
with artificial activation.
(7) Another attack on the problem of fertilization and its meaning had
its origin in the "idioplasm theory" of Nageli. This theory (1884)
postulated an "idioplasm" carried by the germ cells which formed
the essential physical basis of heredity. A little later O. Hertwig,
Kolliker, and especially Weismann, identified the idioplasm of Nageli
with the chromatin of the nucleus. In the meantime, Roux emphasized
the importance of the chromatin threads of the nucleus and stated
that the division of these threads by longitudinal splitting (separation)
during mitosis implied that different longitudinal areas of these threads
embodied different qualities. (See Wilson, '25, p. 500.) In harmony
with the foregoing ideology and as a result of his own intensive work
on maturation and fertilization in Ascaris and also upon other forms,
Boveri came to the conclusion in 1902 and 1907 (Wilson, '25, p. 916)
that development was dependent upon the chromosomes and further
that the individual chromosomes possessed different qualities.
(8) As a result, the field of biological ideas was at this time well plowed
and ready for another important suggestion. This came in '01 and '02
when McClung ofi'ered the view that the accessory chromosome de-
scribed by Henking (1891) as the x-chromatin-element or nucleolus
was, in the germ cell of the male grasshopper, a sex chromosome
which carried factors involved in the determination of sex. McClung
first made this suggestion and stated a definite hypothesis, immediately
stimulating work by others; in a few years McClung's original sug-
gestion was well rounded out and the dimplete cycle of sex chromo-
216 FERTILIZATION
somes in the life history was formulated. E. B. Wilson led this work,
and the theory that he formulated became the assured basis of cyto-
logical and genetical sex studies constituting one of the greatest present
day advances in zoology. "McClung's anticipation of this theory is a
striking example of scientific imagination applied to painstaking obser-
vation" (Lillie, F. R., '40).
Not only were the sex chromosomes studied, but other chromo-
somes as well, and an intense series of genetical investigations were
initiated by Morgan and his students and others which succeeded in
tying a large number of hereditary traits to individual chromosomes
and also to definite areas or parts of the chromosomes. Thus the
assumptions of Roux and Boveri were amply demonstrated. More-
over, these observations established experimental proof for the con-
cept that in the gametic fusion which occurs during fertilization, the
chromosomes pass from one generation to the next as individual
entities, carrying the hereditary substances from the parents to the
offspring. The heredity of the individual was in this way demonstrated
to be intimately associated with the reunion of the haploid groups of
chromosomes in the fertilization process.
C. Types of Egg Activation
1. Natural Activation of the Egg
Natural parthenogenesis, i.e., the development of the egg spontaneously
without fertilization was suggested by Goedart, in 1667, for the moth, Orgyia
gnastigma, and by Bonnet, in 1745, in his study of reproduction in the aphid.
(See Morgan, '27, p. 538.) Since this discovery by Goedart and Bonnet, many
observations and cytological studies have shown that there are two kinds of
eggs which are capable of natural parthenogenesis:
(1 ) That which occurs in the so-called, non-sexual egg, i.e., the egg which
has not undergone the maturation divisions and, hence, has the diploid
number of chromosomes; and
(2) that which results in the sexual egg, i.e., the egg which has experi-
enced meiosis (Chap. 3) and thus has the reduced or haploid number
of chromosomes (Sharp, '34, pp. 409, 410).
Parthenogenesis from a non-sexual egg is found in daphnids, aphids, flat-
worms, and certain orthopterans. In the case of the sexual egg, parthenogenesis
normally occurs in bees, wasps, ants, some true bugs, grasshoppers, and
arachnids. In this type of egg, development may result with or without fer-
tilization. For example, in the honeybee. Apis mellifica, haploid males arise
from eggs which are not fertilized, workers and queens from fertilized eggs.
Extensive studies of the animal kingdom as a whole have demonstrated,
however, that the majority of oocytes or eggs depend upon the fertilization
EGG ACTIVATION 217
process for activation. Consequently, eggs may be regarded in the following
light: Some eggs are self-activating and develop spontaneously, while others
require sperm activation before development is initiated. However, the dif-
ferences between these two types of eggs may not be as real as it appears, for
it is probable that subtle changes in the environment of the so-called self-
activating or parthenogenetic eggs are sufficient to activate them, whereas in
those eggs which require fertilization a strong, abrupt, stimulus is requisite
to extricate them from their blocked condition and to start development. This
idea is enhanced by the information obtained from the methods employed
in the studies on artificial activation of the egg of different animal species.
2. Artificial Activation of the Egg
a. Object of Studies in Artificial Parthenogenesis
"The ultimate goal in the study of artificial parthenogenesis is the discovery
of the chemical and physical forces which are assumed to cause the initiation
of development" (Heilbrunn, '13). A brief resume of some of the results ob-
tained in the studies on artificial activation of the egg is considered in the
following paragraphs.
b. Some of the Procedures Used in Artificial Activation of the Egg
Tichomiroff, 1885 (Morgan, '27, p. 538), stated that eggs from virgin
silkworm moths could be activated by rubbing or by treatment with sulfuric
acid. Somewhat later Mead, 1896-1897, published results of studies on arti-
ficial parthenogenesis in the annelid worm, Chaetopterus. The egg of this
worm has the germinal vesicle intact when it is deposited in sea water. Almost
immediately after entrance into sea water, the germinal vesicle breaks down,
and the chromatin proceeds to form the spindle for the first maturation
division. At this point, however, it stops and awaits the entrance of the sperm
for further activation. Thus, the immersion of the Chaetopterus egg in sea
water under normal spawning conditions partially activates the egg (Mead,
1897). Mead attempted by artificial means to complete this activation initi-
ated by the sea water. In doing so, he took eggs from normal sea water,
which thus had the first polar spindle, and placed them in sea water to which
V4 to Vi per cent of potassium chloride had been added. Eggs thus treated
proceeded to form normal polar bodies and the beginnings of the first cleavage
occurred. Development ceased, however, at this point. These eggs were further
activated, but not completely activated. Two steps in partial activation are
here demonstrated:
( 1 ) When the egg is spawned into sea water, the nuclear membrane breaks
down and the first polar spindle is formed; and
(2) by the immersion in hypertonic sea water the first and second matura-
tion divisions occur, and the first cleavage begins.
218 FERTILIZATION
This experiment by Mead was one of a number of pioneering studies made
during this period in an endeavor to activate artificially the egg. Another such
experiment was reported by R. Hertwig (1896), using eggs of the sea urchin.
In a strychnine solution the nucleus underwent changes preparatory to the
first division. Also, Morgan (1896) found it possible to produce cleavage,
normal and abnormal, if unfertilized eggs of the sea urchin, Arbacia, were
placed in sea water to which certain amounts of sodium chloride had been
added and then were returned to normal sea water. Morgan ('00) later
found that magnesium chloride added to sea water induced cleavage in eggs
treated for various intervals. Loeb, in 1899 (see Loeb, '06), initiated a
series of experiments on activation of the sea-urchin egg. As a result of many
similar experiments, Loeb reached the conclusion that membrane formation
was the essential part of the activation process in that it stimulates the forma-
tion of the membrane by initiating cytolysis of the egg (see Loeb's theory at
end of chapter). Consequently, he sought substances which would elicit
membrane formation and found that monobasic fatty acids, such as butyric,
acetic, formic, or valerianic, would produce membrane formation, and, also,
that ether, bile salts, saponin, etc., would do the same. However, although
these substances aroused certain initial activities of the egg, Loeb found it
necessary to apply a so-called corrective treatment of hypertonic sea water
to arrest the cytolytic effect of the first treatment, which, according to his
belief, restored respiration to its normal level. As a result, Loeb perfected a
treatment as follows which produced development in a considerable number
of sea-urchin eggs so exposed (Loeb, '06):
(1 ) Unfertilized eggs were placed for Vz to 1 Vi min. in 50 cc. of sea water
to which 3 cc. of N/10 solution of butyric or other monobasic fatty
acid had been added. This treatment effected membrane formation
when the eggs were returned to normal sea water, provided the eggs
had been exposed long enough to the acid.
(2) The eggs were allowed to remain in normal sea water for 5 to 10
min. and then were subjected to the corrective treatment in hypertonic
sea water, made by adding 15 cc. of IVi N. NaCl solution to 100 cc.
normal sea water, and allowed to remain for 20 to 50 min. Lesser
times of exposures also were used successfully.
(3) Following this treatment, the eggs were returned to normal sea water.
An example of an entirely different method from the solution technics
employed above on the sea-urchin egg is that of Guyer ('07) and especially
Bataillon ('10, 'II, '13) on the egg of the frog. The method employed by
the latter with success was as follows: Eggs were punctured with a fine glass
or platinum needle and then covered for a time with frog blood. Puncturing
alone is not sufficient; a second factor present in the blood is necessary for
successful parthenogenetic development. The number of actual developments
EGG ACTIVATION 219
procured by this method is small, however. In many cases an early cleavage
or larval stage is reached, but the advanced tadpole state or that of the fully
developed frog is quite rare.
The method introduced by Bataillon is still used extensively in studies on
artificial parthenogenesis in the frog. Recently, Shaver ('49) finds that the
"second factor" is present on certain cytoplasmic granules obtained by cen-
trifugal fractionation. Heat at 60" C. and the enzyme, ribonuclease, destroy
the second-factor activity. Successful second-factor granules were obtained
from blood, early frog embryos, and "extracts of testis, brain, lung, muscle
and liver." This author also reports that heparin suppresses parthenogenetic
cleavage.
In some of these parthenogenetically stimulated eggs of the frog, the diploid
chromosome relationships appear to be restored during early cleavage; in
others both diploid and triploid cells may be present. Some of these tadpoles
may be completely triploid (Parmenter, '33, '40). However, a large percentage
remain in the haploid condition (Parmenter, '33).
A third method of approach in stimulating parthenogenetic development
was used by Pincus ('39) and Pincus and Shapiro ('40) on the rabbit. In
the former work, Pincus reports the successful birth of young from tubal
eggs activated by exposure to a temperature of 47° C. for three minutes. The
treated eggs were transplanted into the oviducts of pseudopregnant females.
In the latter work, eggs were exposed to a cooling temperature in vivo, that
is, the eggs were allowed to remain in the Fallopian tube during exposure to
cold. The birth of one living female was reported from such parthenogenetic
stimulation.
The foregoing experiments illustrate three different procedures used on
three widely separated animal species, namely, changing the external chemical
environment of the egg, a tearing or injuring of the egg's surface followed
by the application of substances obtained from living tissues, and, finally,
changing the physical environment of the egg. To these three general ap-
proaches may be added that of mechanical shaking. For example, Mathews
('01 ) states that mechanical shaking of the eggs of the starfish, Asterias jorbesi,
results in the development of a small percentage of eggs to the free-swimming
blastula stage.
Some of the recent work on the initiation of development and in stimulating
cells to divide has emphasized the importance of cellular injury as a factor.
Little is known concerning the mode of action of the injuring substances.
Harding ('51) concludes that an acid substance is released as the result of
"injury" and that this acid substance causes "an increase in protoplasmic
viscosity and initiates cell division" in the sea-urchin egg. (Cf. theory of R. S.
Lillie at end of chapter.)
That no single method has been found which activates eggs in general is
not surprising. The eggs of different species are not only in different states of
220 FERTILIZATION
maturation (i.e., development) when normally fertilized (fig. 137), but they
behave differently during the normal fertilization process. In some eggs, such
as the egg of Chaetopterus, there is only a slight change within the egg cortex
during fertilization, whereas in the egg of the teleost fish, the egg of the frog,
or in the egg of the urochordate, Styela, marked cortical changes involving
mass movements of protoplasmic materials can be demonstrated.
c. Results Obtained by the Work on Artificial Parthenogenesis
The question naturally arises: What has the work on artificial activation
of the egg contributed to the solution of the problems involved in egg activa-
tion? It has not, of course, solved the problem, but it has contributed much
toward a better understanding of the processes concerned with egg activation
and of the general problem of growth stimulation including cell division. We
may summarize the contributions of this work as follows:
( 1 ) It has demonstrated that the egg in its normal development reaches
a condition when a factor (or factors) inhibits further development.
That is, it becomes blocked in a developmental sense.
(2) It has shown that this inhibited state may be overcome and develop-
ment initiated by appropriate types of treatment.
(3) It has revealed that activation of the egg is possible only at the time
when normal fertilization occurs in the particular species. In other
words, activation is possible only when favorable conditions are de-
veloped in the egg — conditions which enable it to respond to the acti-
vating stimulus.
(4) It has demonstrated that one of the primary conditions necessary for
the initiation of division or cleavage of the egg is an initial increase
in the viscosity of the egg's cytoplasm.
(5) Certain experiments suggest that chemical compounds, such as heparin
or heparin-like substances, may suppress cleavage and cell division,
presumably due to their ability to decrease the viscosity of the egg.
(6) It therefore follows that substances and conditions which tend to in-
crease the egg's viscosity tend to overcome the inhibited state referred
to in ( 1 ) above and thus initiate development.
(7) Recent evidence suggests that substances which produce cell injury
tend to initiate cell division in the egg. As states of injury have been
shown to produce growths of various kinds during embryonic devel-
opment and also during the post-embryonic period, it is probable that
the principles involved in egg activation are similar to those which
cause differentiation and growth in general.
(8) A common factor, therefore, involved in egg stimulation and other
types of growths, including tumor-like growths, is the liberation of
some substance in the egg or in a cell which overcomes an inhibiting
BEHAVIOR OF THE GAMETES 221
factor (or factors) and thus frees certain morphogenetic or develop-
mental conditions within the egg or within a cell. Once the inhibiting
or blocking condition is overcome, differentiation and growth begin.
(9) Finally, the work on artificial parthenogenesis has demonstrated that
the egg is an organized system which, when properly stimulated, is
able to produce a new individual without the aid of the sperm cell.
This does not mean that the sperm is not an important factor in normal
fertilization, but rather, that the egg has the power to regulate its
internal conditions in such a way as to compensate for the absence
of the sperm.
D. Behavior of the Gametes During the Fertilization Process
The activities of the gametes during the fertilization process may be divided
for convenience into two major steps:
( 1 ) activities of the gametes which bring about their physical contact with
each other, and
(2) activities which result in the actual fusion of the gametes after this
contact is made.
Before considering these two major steps, we shall first observe certain of
the characteristics of the two gametes when they are about to take part in
the fertilization process.
1. General Condition of the Gametes when Deposited
Within the Area Where Fertilization Is to Occur
a. Characteristics of the Female Gamete
1) Oocyte Stage of Development. In the case of most animal species, the
female gamete is in the oocyte stage when it enters into the fertilization
process. (See Chap. 3; also fig. 137.) In the dog and fox the female gamete
is in the primary oocyte stage, and both maturation processes happen after
sperm entrance (fig. 115). In the protochordate, Styela, the first maturation
spindle already is formed when the sperm enters (fig. 116A-D), and in
Amphioxus the first polar body has been given off, and the second matura-
tion spindle is developed when the sperm enters (fig. 117A-D). The last
condition probably holds true for most vertebrate species (figs. 1 18B; 1 19D).
However, in the invertebrates, the sea-urchin egg experiences both maturation
divisions normally before sperm entry.
2) Inhibited or Blocked Condition. When the female gamete thus reaches
a state of development determined for the species, its further development
is blocked or inhibited, and its future development depends on the circum-
vention of this state of inhibition. If not fertilized or artificially aroused when
this inhibited state is reached, the oocyte or egg begins cytolysis. Eggs ferti-
222
FERTILIZATION
lized, when these degenerative (cytolytic) conditions are initiated, fail to
develop normally. If allowed to continue, cytolysis soon produces a condition
in which development is impossible, and dissolution of the egg results.
3) Low Level of Respiration. While the egg is in this inhibited or arrested
state awaiting the event of fertilization, respiration is carried on at a steady
but low level. This respiratory level varies in different animal species (fig.
120). That this respiration rate may not be the direct cause of egg inhibition,
is shown by the fact that the rate of respiration does not always increase imme-
FEMALE NUCLEUS
SPERM TAIL
Fig. 115. (A) Early fertilized egg in upper Fallopian tube of the bitch (dog). Observe
the female nucleus before the first maturation division together with the sperm head
and tail. Note that the sperm, as in other mammals, enters the nuclear pole of the egg.
Observe further that the zona pellucida and the ooplasm are contiguous. (B) Section
of the egg of the dog, taken from the upper part of the Fallopian tube. Observe the
following features: (1) The sperm pronucleus is forming; (2) the egg nucleus has now
entered the meta^hase of the first maturation division; (3) the ooplasm of the egg has
shrunk away from the zona pellucida and a space is present between the egg and the
zona. This space is the perivitelline space, containing an ooplasmic exudate. (C) Sec-
tion of the egg in the Fallopian tube of the bitch, showing the formation of the first
polar body.
diately following fertilization in all species (fig. 120). (Consult Brachet, J.,
'50, p. 105.) Among the vertebrates, the low rate of oxygen consumption
of the unfertilized egg has been shown to continue for some time after fertili-
zation in the toad and frog egg and also in the egg of the teleost fish, Fundulus
heteroclitus. However, in the case of the egg of the lamprey the respiration
rate rises after fertilization (Brachet, J., '50, p. 108).
4) Loss of Permeability. A final characteristic of the female gamete im-
mediately before fertilization is the loss of permeability of the egg surface
to various substances. Correlated with this fact is the presence of definite
ooplasmic or other egg membranes associated with the egg surface. The rela-
tionship between the ooplasmic surface of the egg and these membranes is
altered greatly after fertilization when the egg and the membranes tend to
separate. To what extent the loss of permeability of the egg surface is caused
by the intimate association of these membranes with the egg surface is prob-
lematical. The evidence to date suggests that under normal circumstances
BEHAVIOR OF THE GAMETES 223
they are integrated with the conditions which restrict permeability and egg
activation.
b. Characteristics of the Male Gamete
In contrast to the inertia and metaboHc quietude experienced by the female
gamete, the gamete of the male experiences quite opposite conditions. When
the sperm, for instance, is brought into an environment which favors motility,
such as the posterior region of the vas deferens of the mammal, it becomes
highly motile and continues this motility in the female genital tract. Similarly,
the normal sperm of other vertebrate species is a very active cell when placed
in the normal fertilization area (fig. 121, primary phase of fertilization). To
quote from J. Brachet ('50), page 91: "This very active cell has an intense
metabolism and the maintenance of this latter (condition) is indispensable
to the continuance of motility." As mentioned previously (Chap. 1 ), this high
respiratory metabolism at least in some species is supported partially by the
utilization of a simple sugar in the seminal fluid as the sperm "is rich in oxi-
dative enzymes and in hydrogen transporters" (J. Brachet, '50).
2. Specific Activities of the Gametes in Effecting Physical
Contact of the Egg with the Sperm
(Consult fig. 121, primary phase of fertilization.)
While the gametes are in the condition mentioned, they are physiologically
adapted to fulfill certain definite activities which enhance their contact with
each other and bring about actual union in the fertilization process.
a. Activities of the Female Gamete in Aiding Sperm and Egg Contact
1) Formation of Egg Secretions Which Influence the Sperm. The activities
of the female gamete at this time are concerned mainly with the eftusion of
certain egg secretions. These secretions are known as gynogamic substances,
or gynogamones. They are elaborated by the egg when the latter becomes
physiologically mature, i.e., when it becomes fertilizable (fig. 137).
A study of the natural secretions of the egg in relation to the fertilization
process has occupied the attention of numerous investigators. These studies
began during the early part of the twentieth century. In reference to the egg,
two main groups of substances have been recognized:
( 1 ) the fertilizin complex, and
(2) substances which induce the spawning reactions in the male.
a) Fertilizin Complex. Some of the earliest studies upon fertilizin sub-
stances were made by von Dungern in '02, Schiicking in '03, and De Meyer
in '11. In these experiments an egg-water solution was obtained by allowing
ripe eggs of the sea urchin to stand in sea water for a period of time or by
disintegrating the eggs. All of these observers found that some substance from
CHORION
Fig. 116. Fertilization and maturation of the egg in the urochordate, Styela (Cynthia)
partita. (After Conklin, '05.) (A) Egg shortly after spawning but before sperm entrance.
The spindle fibers of the first maturation division are forming, and the nucleoplasm is
located toward the animal pole. (B) Egg showing the spindle for first maturation divi-
sion. Observe the sperm nucleus just inside the ooplasmic membrane near the midvegetal
pole of the egg. The nucleoplasm (karyoplasm) of the female nucleus has spread into a
thin cap at the animal pole. (C) Metaphase of first division spindle (1, D.S.) nearly
parallel to the surface of the egg; no centrosomes are present. (D) Higher powered
representation of sperm a little later than that shown in (B). The aster for the first
cleavage spindle is forming in the middle piece of the sperm. (E) Slightly more ad-
vanced stage than that shown in (B). Collection of yellow-pigmented, peripheral proto-
plasm (PL.) is shown at bottom of the egg. (F) Anaphase of second polar spindle.
Sperm aster enlarging. (See (G) and (H).) (G) Separation of first polar body. (H)
Metaphase of second polar spindle, paratangential in position. (I) First polar body
(1 P.B.) formed; chromatin of second spindle (CHR.). Sperm has revolved 180°; sperm
aster enlarging. (J ) Telophase of second polar spindle. Sperm aster enlarges, and sperm
nucleus assumes vesicular condition. (K) Separation of second polar body. (L) Two
{Continued on facing page.)
224
BEHAVIOR OF THE GAMETES 225
the egg, when present in dilute solution, caused the sperm of the sea urchin to
loose their motility and to become clumped together or agglutinated. A little
later, F. R. Lillie, '13, '14, '15, studied the activity of the egg water of the
sea urchin, Arbacia, extensively. Lillie associated the egg secretion found in
the egg water with a definite theory concerning the mechanism of fertilization.
He called the substance given off when the sea-urchin egg is allowed to stand
in sea water, "fertilizin"; for, according to his results, washed eggs deprived
of this egg secretion fail to fertilize. Only ripe eggs give of! fertilizin according
to his observations. Lillie found further that the activities of the sperm, intro-
duced by means of a pipette into the egg-water solution are changed greatly.
At first they are activated, to be followed by an agglutination. Moreover, a
drop of egg water introduced into a sperm suspension activates the sperm
and appears to influence them chemically, causing them to be attracted to
the drop. Lillie therefore concluded that fertilizin has a threefold action upon
the sperm:
(1) that it activates the sperm (that is, stimulates their movement),
(2) attracts the sperm by a positive chemotaxis, and
(3) agglutinates the sperm, that is, causes the sperm to associate in clumps.
The agglutination effect F. R. Lillie found is reversible in most sea-urchin
sperm, providing the egg water containing fertilizin is not allowed to act too
long. On the other hand, in the annelid, Nereis, agglutination of the sperm is
"essentially permanent" (Lillie, F. R., '13). Lillie placed most emphasis upon
the "agglutinin" factor in the egg water. He further postulated that fertilizin
not only affected the sperm, but also activates the egg to cause its development
(see theory at end of chapter). Lillie also obtained another substance from
crushed or laked eggs which combines "with the agglutinating group of fer-
tilizin, but which is separate from it as long as the egg is inactive." This
substance present within the egg he called "antifertilizin."
Since the time of F. R. Lillie's original contribution, the subject of fertilizin
and antifertilizin has been actively investigated by various students of the
problem. Some investigators criticized the conclusions drawn by Lillie, but
more recent work substantiates them. For example, M. Hartmann, et al. ('40),
working on the sea urchin, Arbacia pustulosa, and Tyler and Fox ('40) and
Tyler ('41 ), using eggs from Strongylocentrotus purpuratus, find that fertilizin
Fig. 116 — {Continued}
polar bodies (P.B.); fusion of chromosomal vesicles to form egg pronucleus (E.N.).
(M) Movement of sperm nucleus, aster, and of surrounding yellow-pigmented and clear
protoplasm to the posterior pole of the egg. The copulation path of egg pronucleus
(E.N.) to meet the sperm nucleus is in progress. (N) Sperm aster has divided; egg
pronucleus progresses along its copulation path toward posterior pole of egg to meet the
male pronucleus. (O) Egg and sperm pronuclei are making contact with each other.
(P) Pronuclei associate and begin to form early prophase conditions of the first cleavage.
(Q) Metaphase of first cleavage. (R) Anaphase of first cleavage. (S) Late anaphase
of first cleavage.
Fig. 1 17. (See facing page for legend.)
116
BEHAVIOR OF THE GAMETES 227
is present and that it is associated with the jelly layer around the egg. Tyler
('41 ) concludes:
( 1 ) When fertilizin is present in the form of a gelatinous coat around the
egg, it enhances fertilization;
(2) when present only in solution around the egg after the gelatinous coat
is removed, it hinders fertilization by agglutinating the sperm; and
(3) that fertilizin is not entirely essential since eggs can be fertilized when
the jelly coat is removed, but a greater number of sperm are needed
under these circumstances.
Tyler also has detected antifertilizin below the surface of the egg and by
crushing the eggs was able to show that antifertilizin from the interior of the
egg is able to neutralize the fertilizin of the jelly coat surrounding the egg
(Tyler, '40, '42). In Germany, Hartmann and his associates (see Hartmann,
M., et al., '39, a and b, '40) have demonstrated that by exposing fertilizin
to heat or light one may separate the "agglutinating factor" from the "acti-
vating factor." Heat at 95° C. destroys the "agglutinating factor," while ex-
posure to bright light causes the "chemotactic" and "activating" factors to
disappear. The factual presence of the egg products, fertilizin and antifertilizin,
postulated by Lillie thus is well established.
Fertilizin appears to be widely distributed as an egg secretion among ani-
mals, invertebrate and vertebrate. Among the latter it has been identified in
cyclostomes, certain teleost fishes, and in the frog, Rana pipiens (Tyler, '48).
Moreover, it is becoming increasingly clear that the term, fertilizin, as em-
ployed originally by F. R. Lillie, includes more than one secretion. How many
separate enzymes or other substances may be included under the general terms
of fertilizin and antifertilizin remains for the future to determine. Moreover,
the exact presence of particular gynogamic substances in the egg secretions
of different animal species may vary considerably. For example, the sperm-
activating principle may not be present in all animal species. In fact, there
is good evidence to show that it is not present, for example, in all species
of sea urchins.
Fig. 117. Fertilization and maturation of the egg in Amphioxus. (A, B, H after Cer-
fontaine, '06; C-1 after Sobotta, 1897.) (A) Metaphase of first maturation division
before sperm entrance. (B) Anaphase of first maturation division before sperm entrance.
(C) First polar body and metaphase of second maturation division befoie sperm entrance.
Observe the first or primary fertilization membrane. (D) Sperm has entered near vegetal
pole of egg. (E) Outer egg membrane has enlarged and is now much thinner; the
second egg membrane is separating from the egg, and the second polar body is forming.
(F) Outer and inner egg membranes have fused and expanded; pronuclei of sperm and
egg are evident; the sperm aster is to be observed in connection with the sperm nucleus.
(G) Meeting of the two pronuclei between the developing amphiaster. (H) Fusion
nucleus complete. (1) Diploid chromosomes now evident preparatory to the first
cleavage of the egg.
228 FERTILIZATION
The general term "gamones"' (Hartmann, M., '40) has been applied to the
substances produced by the gametes at the time of fertilization. The Hartmann
school further has identified the factor responsible for chemotaxis and acti-
vation as "echinochrome A," that is, the bluish-red pigment of the egg, and
have called it "Gynogamone I." This factor will attract sperm and stimulate
their movements "at the enormous dilution of 1 part in 2,500,000,000 parts
of water" (Brachet, J., '50, p. 96). However, Tyler has not been able to
detect echinochrome in the egg of the Pacific coast sea urchin, Strongylocen-
trotus. But the egg water of this species does activate the sperm of this species,
which suggests that the activating factor may be something else than echino-
chrome. To the agglutinating factor, M. Hartmann and his associates have
given the name "Gynogamone II."
The exact identity of these gamones with particular chemical substances
present in the egg water at the present time is impossible. To quote from
J. Brachet, '50, p. 99:
It is clear that research in this field is complicated by the fact that a number of
agents activate the movements of sperm (alkalinity, glutathione, echinochrome (?),
etc.) . . . There is strong evidence in favor of the protein nature of agglutinin,
while the sperm-activating principle is probably a substance with a small molecule,
its identity with echinochrome being doubtful at the present time.
b) Spawning-inducing Substances. In addition to the fertilizin sub-
stances which act in effecting the actual contact of the sperm with the egg,
a spawning-inducing agent is present in the egg water of certain species. In
the annelid. Nereis, for example, there is something present in the egg water
which induces spawning in the males. Townsend ('39) suggested that this
substance may be glutathione, but Tyler ('48) does not readily concur with
this conclusion. A spawning-inducing agent is found also in the egg water of
oysters (Galtsoff, '40). Among the vertebrates, the spawning behavior of the
female appears to be the important factor in inducing the male reaction.
b. Activities of the Male Gamete in Aiding the Actual Contact
of the Two Gametes
The activities of the male gamete, including those of seminal fluid, are
much more complicated and devious than those of the female gamete. These
activities entail:
( 1 ) production of certain sperm secretions,
(2) activities of large sperm numbers,
(3) presence of a healthy seminal plasma or protective substance for the
sperm, and
(4) physical movements and functioning of specific parts of the sperm cell
itself.
(1) Sperm Secretions. The sperm secretions are known as androgamic sub-
stances or androgamones. These substances have been the object of much
BEHAVIOR OF THE GAMETES 229
Study since the initial endeavors of Fieri in 1899. In recent years, three gen-
eral types of substances have come to be recognized in relation to the sperm
of different species. These three groups of substances are:
( 1 ) secretions which cause lysis,
(2) a substance or substances related specifically to the fertilization reac-
tion (i.e., egg and sperm contact), and
(3) substances which bring about the spawning reaction in the female.
a) Secretions Producing Lysis. To cite the importance of lytic sub-
stances produced by the sperm, reference is made first to the situation in the
amphibian, Discoglossus pictus. In this primitive anuran, the sperm, although
about 2 mm. long, are almost incapable of motility. However, they do ac-
cumulate in the region of a thickened portion of the egg capsule which overlies
a depressed area of the egg. They are capable of passing through this thickened
area of jelly by the aid of a digestive enzyme probably associated with the
acrosome (Hibbard, '28). Hibbard also suggests that "nuclear fluids" accu-
mulate in the bottom of the egg depression and these fluids attract the sperm
to the thickened area of the capsule. If so, here is an example of two chemical
substances, one elaborated by the egg and the other by the sperm, both work-
ing together to bring about fertilization. In substantiation of Hibbard's views
of the presence of a lytic enzyme associated with the sperm of this species,
Wintrebert ('29) found that extracts from the sperm contained an enzyme
which is capable of digesting the inner jelly coat of the egg.
More recently, Tyler ('39) has found that sea-water extracts of frozen
and thawed sperm of two mollusks (Megathura crenulata and Haliotis crache-
rodii) were able to dissolve the egg membranes of the respective species.
Cross-species reactions were not obtained, however. Strong extracts of con-
centrated sperm suspensions bring about egg-membrane disappearance in less
than one-half minute, but with the jelly coat present around the egg it takes
about three minutes. Also, Runnstrom and his collaborators ('44, '45, a and
b, '46) made methanol extracts of sea-urchin sperm which were able to liquefy
the superficial cortical area of the egg.
A most interesting enzyme, known as hyaluronidase, has been extracted
from mammalian testes and from mammalian sperm. This substance is capable
of dispersing the follicle cells of the corona radiata present around most mam-
malian eggs when discharged from the ovary. (Sheep and opossum eggs as
well as those of the monotremes do not possess a layer of follicle cells around
the newly ovulated egg.) This dispersing effect aids fertilization, for it enables
sperm to reach the egg surface before degeneration processes occur in the
egg. Rowlands ('44) eff'ected artificial insemination in the rabbit with dilute
sperm solutions by adding the enzyme hyaluronidase from other sperm to the
dilute suspensions. Without the addition of hyaluronidase, fertilization did not
result. In certain cases in women where artificial insemination was tried but
230 FERTILIZATION
failed when semen alone was used, the addition of hyaluronidase from bull testis
to the semen produced successful fertilization (Leonard and Kurzrok, '46).
b) Secretions Related Specifically to the Fertilization Reac-
tions. The substances mentioned in the preceding paragraphs are related to
the general fertilization process, but they may not be related specifically to
the reactions which bring the sperm in direct contact with the egg. In the egg
we have observed the presence of fertilizin which stimulates a series of sperm
activities directed to this end. Similarly, in the male gamete, sperm of various
species seem capable of producing "androgamic substances which neutralize,
in part, the action of the gynogamic substances and thus assure the precise
mechanism necessary for precise fusion of the gametes" (J. Brachet, '50).
An introductory study by Frank ('39) suggests the presence of a sperm
substance which reacts directly with the fertilizin complex of the egg. It was
shown by this investigator that an extract from the sperm of the sea urchin,
Arbacia, is:
( 1 ) able to destroy the sperm agglutinating factor when added to a solution
of fertilizin derived from the sea-urchin egg, and
(2) possesses the power to agglutinate eggs of the same species.
Other students of the problem have found a similar substance associated
with the sperm. (See Hartmann, Schartau, and Wallenfels, '40; Southwick, '39;
Tyler, '40.) The general term "sperm antifertilizin" has been given to this
substance (or substances) by Tyler and O'Meiveney ('41). Sperm antiferti-
lizin unites with fertilizin produced by the egg, with the result that the sperm
is entrapped at the egg's surface. Tyler and O'Meiveney ('41) regard the
reaction between antifertilizin of the sperm and fertilizin of the egg to be the
"initial (perhaps essential) step in the union of the gametes whereby the
spermatozoon is entrapped by the . . . fertilizin, on the egg."
c) Secretions Which Induce the Spawning Reaction in the Female.
Galtsoff ('38) has shown that the presence of sperm of the oyster, Ostrea
virginica, "easily induces spawning in oysters." He also found that the spawn-
ing reaction is specific in that sperm of different species cannot provoke it.
The active principle of the sperm suspension is thermolabile and insoluble
in water. However, it may be readily extracted in 95 per cent ethyl alcohol
and benzene.
To what extent spawning-inducing substances may be present in other animal
species is questionable, but it may not be an uncommon phenomenon, espe-
cially in sedentary species, such as the oyster and other mollusks. In the ver-
tebrate group, surface contact of the male and female bodies is an important
factor in many cases.
2) Relation and Function of Sperm Number in Effecting the Contact of
the Sperm with the Egg. In the preceding chapter, sperm transport is con-
sidered. This transportation journey is an efficient one with regard to the end
BEHAVIOR OF THE GAMETES 231
achieved, namely, contact of a single sperm with an egg, but from the view-
point of sperm survival it may appear as waste and caprice. This fact is espe-
cially true in those forms utilizing fertilization where only one or a very few
eggs are fertilized. It has been shown by Walton ('27) in experiments dealing
with artificial insemination in the rabbit, when dilution of the sperm is such
that the number falls below 3,000 to 4,000 per cc, fertilization does not take
place. Recent observations by Farris ('49) on the human suggest that num-
bers of sperm below 80,000,000 per cc. are precarious when conception is
the end to be achieved. (For the total number of sperm ejaculated by certain
males during a single copulation, see Chap. 1.) Although exceedingly large
numbers of sperm are deposited in the posterior area of the female repro-
ductive tract, the number becomes less and less as the ovarian end of the
duct is reached. The ability of effective sperm transport within the female
tract probably varies considerably in different species and with different fe-
males in the same species. The rat and the dog appear to be more efficient
in this respect than the rabbit.
The relation of sperm numbers to the efficiency of the fertilization process
is not to be considered merely as a mechanical hit and miss device, whereby
the presence of a greater number of sperm may assure an accurate "hit" or
sperm-egg collision (Rothschild, Lord, and Swann, '51). Hammond ('34)
has shown in the rabbit that fertilization is not effected by the few sperm
which reach the region of the egg first, but by the later aggregations of num-
bers of sperm. The work on hyaluronidase mentioned on page 229 suggests
strongly that one object of the excess sperm is to transport hyaluronidase to
the vicinity of the egg. The presence of this enzyme close to the egg possibly
facilitates the passage through the cells of the corona radiata and also through
the zona pellucida of the single sperm which makes contact with the egg in
the process of fertilization (Tyler, '48). The general result should be regarded
as the working of a cooperative enterprise, where many sperm aid in the dis-
solution of the interference in order that one sperm may reach the egg's surface.
3) Influences of the Seminal Plasma in Effecting Sperm Contact with the
Egg. The importance of the seminal plasma (i.e., the fluid part of the semen;
see Chap. 1) cannot be overestimated (Mann, '49). It is, to a great extent,
the natural environment and at the same time the nutritive medium for the
sperm during the transport from the male ducts through the external medium
or within the lower region of the female genital tract. Its functions may be
stated as follows:
(1) It increases the motiUty of the sperm;
(2) it has a high buffering capacity, which protects the sperm from in-
jurious acids or other injurious substances; and
(3) it is a vehicle for nutritive substances, such as fructose, vitamin C,
and the B complex which provide nourishment for the sperm.
232 FERTILIZATION
The B group of vitamins may be directly related to sperm motility. Other
substances, such as iron, copper, etc., are present. One should consider the
seminal plasma, therefore, as a most important association of substances which
aids in producing a protective environment for the sperm while the latter is
in migration to the egg.
The importance of the environment of the sperm and also that of the egg
cannot be overemphasized. If normal fertilization is to be effected, optimum
conditions for both sperm and egg must be present. An example of this fact
is shown by the observations of Reighard on fertilization of the walleyed pike.
(See Morgan, '27, p. 18.) The best results with the eggs of this teleost fish
were obtained when the eggs were fertilized as soon as they entered the water
from the female genital tract. After two minutes only 40 per cent of the eggs
segment, and after ten minutes no eggs segment. For many fish, "dry fertili-
zation" gives the best results. Dry fertilization consists in stripping the female
to force out the eggs into a dry container and then stripping the milt (seminal
fluid) from the male directly over the eggs. The eggs are then placed in water
after a few minutes. This work suggests strongly that a deleterious environ-
ment for either the egg or the sperm is disturbing to the fertilization process.
4) Roles Played by Specific Structural Parts of the Sperm in Effecting
Contact with the Egg: a) Role of the Flagellum. As indicated in the
foregoing paragraphs, when the sperm cells have reached the normal fertili-
zation site, the activities which bring about actual contact of the sperm with
the egg largely is a sperm problem. Aside from enzymes elaborated by the
sperm, sperm motility is extremely important in achieving this end. Although
sperm may appear to swim rather aimlessly, vigorous, healthy sperm do lash
forward more or less in a straight line for some distance; ill-developed or
otherwise impaired sperm may simply swim round and round or move forward
feebly. In the case of flagellate sperm, the structure which makes the forward
swimming movement possible is the flagellum or tail (figs. 74, 77, 78, 79).
A two-tailed sperm or one in which the flagellate mechanism is not well de-
veloped would be at a disadvantage in this race to reach the confines of the
egg. Brachet ('50) considers the rate of metabolism necessary to support the
activities of the tail or flagellum in sperm movement as directly comparable
to that of muscle.
An interesting peculiarity of a different type of sperm mechanism useful
in achieving contact with the egg's surface is that of the so-called "rocket
sperm" of certain decapod Crustacea described by Koltzoff (fig. 75). After
attachment of the sperm to the egg by its tripod-like tips, the caudal compart-
ment, containing a centriole and the acrosome, explodes. "Koltzoff considers
that the force of the explosion drives the sperm upon, or even into, the egg"
(Wilson, '25, p. 299).
b) Role of the Acrosome in the Egg-sperm Contact. The acrosome
of the sperm (fig. 78) has long been regarded as a structure which has a
BEHAVIOR OF THE GAMETES 233
function in the reactions involved in fertilization. The older conception of
Waldeyer that the acrosome was a perforating device which enabled the sperm
to pass through the egg membranes and thus to enter the egg is untenable in
the light of later observation. Many years ago Bowen ('24) though admitting
a minor mechanical role for the acrosome, emphasized that the acrosome
essentially is a secretory product whose principal function is to initiate the
physicochemical reactions of fertilization. It should be recalled in this con-
nection that Hibbard ('28) and also Parat ('33, a and b) have attributed to
the acrosome of the anuran, Discoglossus, the ability of carrying or producing
an enzyme which enables it to reach the egg's surface through the jelly sur-
rounding the egg. Parat further suggested that the acrosome in this species
contains a "proteolytic enzyme" which, when introduced into the egg, results
in development.
The concept of a proteolytic enzyme associated with the acrosome of Dis-
coglossus is interesting in the light of the suggestion by Leuchtenberger and
Schrader ('50) that the mucolytic enzyme, hyaluronidase, in the bull sperm
may be associated with the acrosome. Both of the above suggestions need
more work before it can be stated with certainty that the acrosome is con-
nected with either of these enzymes in the above species. However, these sug-
gestions do serve to emphasize the possibility that the acrosome may be an
enzyme-producing or enzyme-carrying device which enables the sperm to
make its way through the egg's surroundings to the egg surface, and also, that
it may play a part in egg activation.
5) Summary of the Activities of the Egg and Sperm in Bringing About
the Primary or Initial Stage of the Fertilization Process, Namely, that of Egg
and Sperm Contact.
a) The secretion of fertilizin by the egg:
( 1 ) activates the sperm to increased motility, and
(2) through chemotaxis, entices the sperm to move in the direction of the
egg-
b) In moving toward the egg the lytic substances elaborated by the sperm
enable it to "plow" through the gelatinous envelopes and cellular barriers to
the surface of the egg. This movement undoubtedly is aided by movement of
the flagellum in some species, but not in all (see Discoglossus). The acrosome
of the sperm may function at this time either as an instrument carrying lytic
substances or as one which actually manufactures these substances. The pres-
ence of large numbers of sperm near the egg may aid sperm penetration to
the egg's surface by contributing lytic substances to the environment around
the egg which aid in the removal of membranes and other barriers surrounding
the egg.
c) The antifertilizin of the sperm may then unite with the fertilizin of the
234 FERTILIZATION
egg (probably with the agglutinin factor) ; this reaction presumably agglutinates
the sperm to the egg's surface.
d) An egg-surface, liquefying factor, androgamone III, has been isolated
by Runnstrom, et al. ('44), from sea-urchin sperm (Runnstrom, '49, p. 270).
A similar "sperm lysin" has been isolated also from mackerel testes. This work
suggests that a specific sperm lysin may be involved in the activation processes
within the egg cortex. (See theory of fertilization according to J. Loeb at end
of chapter.)
e) Lastly, in certain animal species, substances may be present in the
seminal fluid which induce the spawning reaction in the female, while in the
egg secretion of certain species, a factor may be present which induces spawn-
ing in the male.
3. Fusion of the Gametes or the Second Stage of the
Process of Fertilization
The actual fusion or union phase of fertilization begins once the sperm has
made contact with the egg (fig. 12 IB). From this instant the rest of the fer-
tilization story becomes essentially an egg problem. The egg up to the time
of sperm contact literally has been waiting, discharging fertilizin substances
into the surrounding medium. However, when a sperm has made successful
contact with the surface of the egg, the waiting period of the egg is over, its
work begins, the fusion of the two gametes ensues, and the drama of a new
life is initiated!
The following events of the fusion process may be listed — events which
occur quite synchronously, once the mechanisms involved in egg activation
and gametic fusion are set in motion:
(a) The separation of an egg membrane (fertilization membrane, vitelline
membrane, chorion, zona pellucida, etc.) from the egg's surface and
the exudation of fluid-like substances from the egg's surface.
(b) A fertilization cone may be elaborated in some species.
(c) Changes in the physicochemical activities of the egg.
(d) The maturation division (or divisions) is completed in most eggs.
(e) Profound cytoplasmic movements occur in many eggs which bring
about various degrees of localization of cytoplasmic substances; these
substances orient themselves into a pattern definite for the species.
In some species a cytoplasmic pattern composed of future, organ-
forming substances is rigidly established and definitely correlated with
the first cleavage of the egg (Styela); in others it is less rigid (frog);
and in still others it appears gradually during cleavage of the egg
(teleost fishes).
(f ) The sperm nucleus enlarges, and the middle-piece area in most animal
species develops a cleavage aster.
BEHAVIOR OF THE GAMETES 235
(g) The copulation movements of the egg and sperm pronuclei take place.
These movements bring about the association of the two pronuclei
near the center of the protoplasm of the egg which is actively con-
cerned with the cleavage phenomena.
(h) The pronuclei may fuse to form a fusion nucleus or they may associate
less intimately. Regardless of the exact procedure of nuclear behavior,
the female and male haploid chromosome groups eventually become
associated in the first cleavage spindle to form one harmonious diploid
complex of chromosomes, composed (in most cases) of paired chromo-
somal mates or homologues.
(i) The first cleavage plane is established.
4. Detailed Description of the Processes Involved in
Gametic Union as Outlined Above
a. Separation and Importance of a Protective Egg Membrane,
Exudates, etc.
The term "fertilization membrane" is applied to the egg (vitelline) mem-
brane which, in many species, becomes apparent only at the time of fertili-
zation. In many other eggs a definite and obvious vitelline membrane is present
before the egg is fertilized and in many respects functions similarly to the
more dramatically formed fertilization membrane. Both types of membrane
fulfill definite functions during fertilization and early development. The fer-
tilization membrane which forms only as a distinct membrane during fertili-
zation was observed first by Fol, in the autumn of 1876, in the starfish egg
(Fol, '79). In the cephalochordate, Amphioxus, two definite membranes sep-
arate from the egg's surface. One membrane forms just before the sperm enters
the egg, while the second membrane separates from the egg after the sperm
enters. Both membranes soon fuse and expand to a considerable size, leaving
a perivitelline space between them and the egg; the latter space is filled with
fluid, the perivitelline fluid (fig. 117B-F, I). In the urochordate, Styela, no
such membrane arises from the egg's surface, but the chorion previously
formed by the follicle cells serves to fulfill the general functions of a fertiliza-
tion membrane (figs. 9 IB, 116). In teleost fishes, the egg emits a considerable
quantity of perivitelline fluid at the time of fertilization, effecting a slight
shrinkage in egg size with the production of a space filled with this fluid be-
tween the egg's surface and the zona radiata (fig. 122A-C). The zona radiata
thus functions as a fertilization membrane. In the gobiid fish, Bathygobius
soporator, the chorion and/or vitelline membrane expands greatly after the
egg is discharged into sea water, and an enlarged capsule is soon formed
which assumes the size and shape of the future embryo at the time of hatch-
ing (fig. 123). (See Tavolga, '50.) In the brook lamprey, according to
Okkelberg ('14), shrinkage of the egg at fertilization is considerable, amount-
ing to about 14 per cent of its original volume. A slight egg shrinkage with
236
FERTILIZATION
DEUTOPLASM
PERIVITELLiN E
Fig. 118. Fertilization in the guinea pig. (After Lams, Arch. Biol., Paris, 28, figures
slightly modified.) (A) Spindle of first maturation division. (B) Second maturation
division completed; head of sperm in cytoplasm beginning to swell. (C) Sperm pro-
nucleus, with tail still attached, greatly enlarged; female pronucleus small. (D) Pronuclei
ready to fuse; chromatin material (chromosomes) evident within. (E) First cleavage
spindle. (F) First cleavage completed. Observe deutoplasmic and cytoplasmic globules
which have been exuded into the space between the blastomeres and the zona pellucida.
(G) Four-cell cleavage stage. Observe that the zona pellucida encloses the four blasto-
meres and the cytoplasmic globules which have been exuded. The zona functions to
keep the entire mass intact.
the emission of fluid is present in the amphibia and the egg thus is enabled
to revolve within a relatively thick vitelline membrane. The latter membrane
expands gradually during development, and is associated intimately with the
surrounding jelly membranes secreted by the oviduct. In the reptiles and birds,
the separation of the egg from the vitelline membrane or zona radiata and
BEHAVIOR OF THE GAMETES 237
the formation of the perivitelline space is less precipitous. In the egg of the
bird (e.g., pigeon or hen) (fig. 126), a vitelline space filled with fluid appears
during the latter phase of oocyte growth in the ovary which separates the
surface ooplasm of the egg from the vitelline membrane. The egg is free to
revolve in this vitelline space. In the prototherian mammals, the zona pellucida
evidently functions in a manner similar to that of the bird or reptile (figs.
46, 127). However, in the metatherian and eutherian mammalia, the zona
pellucida becomes separated from the ooplasm of the egg's surface with the
subsequent development of a perivitelline space at fertilization or during early
cleavage (figs. 115, 118, 124, 125).
It is to be observed, therefore, that there are two general groups of egg
or vitelline membranes in the phylum Chordata which assume an important
role at fertilization and during the earlier part of embryonic development:
( 1 ) those membranes which become separated from the egg surface in a
somewhat dramatic manner at fertilization, and
(2) membranes which separate gradually during the late phases of ovarian
development and during early embryonic development.
In the former group are to be found the egg membranes of the eggs of
Amphioxus, teleost and many other fishes, and the amphibia; in the latter
group are the membranes of eggs of Styela, elasmobranch fishes, reptiles, birds,
and prototherian mammals. The higher mammalian eggs appear to occupy
an intermediate position.
The separation of the so-called fertilization membrane has been most in-
tensively studied in certain invertebrate forms. As a matter of interest, some
of the processes involved in membrane elevation in various invertebrate eggs
are herewith described briefly.
In the nematode, Ascaris, the egg exudes a jelly-like substance after the
sperm has entered. This substance hardens to form a thin, tough membrane
which later thickens and expands. The egg also appears to shrink, leaving an
enlarged perivitelline space between the egg surface and the outer hardened
membrane (figs. 128, 133,C-E).
The formation of the fertilization membrane in Echinarachnius, a genus
of sea urchins, was the subject of intensive study by Just ('19). In this species
the egg is larger than that of the sea urchin, Arbacia. According to Just's
account, the fertilization membrane starts as a "blister" at the point of sperm
contact; from this area it spreads and rapidly becomes lifted off from the
general surface of the egg. Heilbrunn ('13) studied the fertilization membrane
of the sea urchin's egg before fertilization and describes it as a vitelline mem-
brane, "probably a gel or semi-gel" which is present at the surface of the
egg. It becomes visible as a distinct membrane when lifted off from the egg's
surface after fertilization. As this elevation occurs, according to Runnstrom,
cortical granules are exuded from the surface of the egg, accompanied by a
ANIMAL
POLE
V E G E T A L
POLE
Fig. 119. (See facing page for legend.)
238
BEHAVIOR OF THE GAMETES 239
general contraction of the egg surface. These cortical granules later become
merged with the vitelline membrane to form a relatively thick structure
(fig. 129). (See Runnstrom, '49.) Fluid collects between the egg surface
and the fertilization membrane.
On the other hand, in the annelid worm, Nereis, there is a complicated
reaction at the egg's surface at the time of fertilization (Lillie, F. R., '12).
In this egg a definite membrane is present around the newly laid egg. When
a sperm has made an intimate contact with the egg's surface, the cortical
layer of the egg exudes a substance which passes through the membrane to
the outside; this substance turns into jelly on coming in contact with sea water
(fig. 130B). The jelly layer carries away the excess sperm from the egg's
surface. A striated area then appears between the vitelline membrane and
the surface of the egg. This area, shown in fig. 130B as the cortical layer,
represents the collapsed walls of small spaces of the superficial layer of the
cortex of the egg which exude their contents through the vitelline membrane
to form the surrounding jelly. The egg then forms a new ooplasmic surface
beneath the collapsed walls of the small spaces of the original cortex (fig.
130B, ooplasmic membrane).
All of these changes and reactions, namely, the formation of the fertiliza-
tion membrane, the exudation of cortical granules, and the emission of a fluid
or jelly together with the shrinkage of the egg result from changes which
occur in the outer layer of the egg's protoplasm or cortex, and consequently
may be classified as cortical changes. The activation of the egg at the time
of fertilization or during artificial stimulation thus appears to be closely inte-
grated with cortical phenomena. It is debatable whether these changes are
the result of activation or are a part of the "cause" of activation.
The particular activity of egg behavior at the time of fertilization which
Fig. 119. Fertilization phenomena in the egg of Rana pipiens. (Drawings B, D-G
made from prepared slides by the courtesy of Dr. C. L. Parmenter.) (A) Semidia-
grammatic representation of the egg shortly before ovulation. The germinal vesicle has
broken down, and the chromosomes in diakinesis have migrated toward the apex of the
animal pole preparatory to the first maturation spindle formation shown in (B). (B)
First polar spindle. Tetrad condition of chromosomes in process of separation into the
respective dyads. (C) Polar view of egg after first maturation division. Compare with
(D), which represents a section of a comparable condition. (D) Lateral view of spindle
of second maturation division. First polar body present in a slight depression at animal
pole. The egg is spawned in this condition. (E) Second polar body shown in a depres-
sion of the animal pole. Within the superficial ooplasm of the egg, the reorganized female
pronucleus is shown. (F) Meeting of the two pronuclei is shown in this section of the
egg at the bottom of the female copulation path or "egg streak," E.S. (G) Two pronuclei
in contact (shown in F) under higher magnification. (H) Entrance and copulation
paths of sperm nucleus. (Modified from Rugh: The Frog, Philadelphia, The Blakiston
Co., 1951.) (I) Sperm-entrance path, copulation path, and meeting of pronuclei. (From
O. Hertwig, 1877.) (J) First cleavage path, showing daughter nuclei. (From O. Hertwig,
1877.) (K) External, lateral view of the egg just before first cleavage. Arrows show
direction of pigment migration with resulting formation of gray crescent.
240
FERTILIZATION
appears to be common to the eggs of many species (sea urchin, cyclostomatous
and teleost fishes, frog, and mammal) is the contraction of the egg's surface,
together with the exudation of various substances from the egg. (See, in this
connection, the fertihzation theory of Batailion at the end of this chapter.)
It is this behavior of the egg's surface which makes the fertihzation membranes
and other egg membranes more apparent; it represents one of the essential
and immediate activities associated with egg activation. Separation of the
various egg membranes at the time of fertilization appears to be secondary
to this primary activity.
Aside from the immediate functions at the time of fertilization, the activities
of the various types of vitelline membranes are concerned mainly with nutri-
tional, environmental, and protective conditions of the early embryo. The
presence of a fluid in the perivitelline space between the membrane and the
developing egg affords a favorable environment for early developmental proc-
esses. Moreover, it permits the egg to rotate when its position is disturbed,
a proper developmental orientation being maintained. A further accommo-
dation is evident in that it permits the developing egg to exude substances,
including yolk, into the surrounding area, which may be retained in the im-
mediate environment of the egg and later utilized in a nutritional way. If the
surrounding vitelline membrane were not present, this material, solid or fluid,
would be dissipated. For example, in the early cleavage stage of the opossum
or guinea-pig egg, yolk material is discharged into the area surrounding the
early blastomeres (figs. 118, 125). The exuded yolk and dissolved substances
later come to lie in the cavity within the blastomeres and, thereby, may be
used for nutritional purposes. Also, in some forms, such as the opossum, the
early blastomeres utilize the zona pellucida as a framework upon which they
arrange themselves along its inner aspect during the development of the early
CUMINGIA 21° WHITAKER-
CHAETOP. 21° WHITAKER
SA8ELLARIA 20° F.-FREMIET-
NERElS 2 1° WHITAKER-
ARBACIA PUNC 2 I ° W H I T A K E R. _
PARACENTROTUS 2I°WARBURG:
ECHINUS 21° SHEARER'
UNFERTILIZED EGGS
^PARACENTROTUS 2I°RUNNSTR0M
^'SABELLARIA 20°FAURE-FREMIET
^'/,ARBACIA PUNC 21° WHITAKER
= ''',-PAR A CENT ROT US 21° WARBURG
"'',ARBACIA PUST 205° WARBURG
=c--AMOEBA PROTEUS 20° EMERSON
I"^~ ECHINUS MILIARIS 2 1° SHEARER
-^^^C NEREIS 2 1° WHITAKER
""^^"CUM INGIA 21° WHITAKER
VfROG skin 20° ADOLPH
CHAETOPTERUS 21° WHITAKER
FERTILIZED ERGS
Fig. 120. Effects of fertilization on oxygen consumption in various marine eggs. (After
J. Brachet, '50; data supplied by Whitaker.)
BEHAVIOR OF THE GAMETES 241
blastula. This apparent independence of the early cleavage blastomeres in the
opossum and their lack of cohesiveness is evident in other mammals, also.
The tendency of the blastomeres in mammals in general to separate from
each other emphasizes the importance of the zona as a capsule which func-
tions to hold the blastomeres together.
The surrounding egg membrane, in many cases, may act osmotically to
permit a nice balance between the developing egg and the substances outside
of the membrane. For example, in birds, the egg and its contained embryo
together with its immediate environment are largely maintained as a physico-
chemical system due to the osmotic properties of the zona radiata or vitelline
membrane. This membrane separates the watery albumen from the nutritive
yolk material. These two substances have different osmotic conditions. Con-
sequently, the vitelline membrane must maintain the proper conditions be-
tween these two general areas, and it performs this function in an admirable
fashion. It should be emphasized further that the viteUine membrane in the
chick's egg is a living membrane, and consequently its osmotic properties are
different from that of a non-living membrane, such as a collodion membrane.
If the egg and albumen of the hen's egg are separated by a thin collodion
membrane, for example, they will reach an osmotic equilibrium more rapidly
than when separated by the thin vitelline membrane. If, however, the vitelline
membrane is isolated from its normal relationships in the egg, it behaves
similarly to a collodion membrane. It is best to regard the vitelline membrane,
the yolk, and the albumen of the bird's egg as forming an harmonious system,
in which all parts are responsible for the maintenance of the necessary condi-
tions for development. (Consult Romanoff and Romanoff, '49, pp. 388-391.)
Undoubtedly in other eggs, such as that of the frog, the delicate relation-
ship existing between the egg, the perivitelline fluid, the vitelline membrane,
and the surrounding external medium forms a complete unit for the proper
maintenance of developmental conditions. In most eggs the vitelline or similar
membranes maintain the protective function until a relatively late period in
development.
b. Fertilization Cone or Attraction Cone
The fertilization cone results from specialized activity of the surface of the
egg (egg cortex) at the point of sperm contact (fig. 130). This structure has
been described in various invertebrate eggs, such as those of the sea urchins,
annelid worms, mollusks, and in some of the ascidians among the proto-
chordata. In the annelid worm. Nereis virens, as the sperm makes its way
through the egg membrane, a cone of cortical ooplasm flows out to meet the
sperm, making an intimate contact with the perforatorium (acrosome) of the
sperm (fig. 130B, C). When this contact is made, the extended cone with-
draws again gradually, and appears to pull the sperm head into the egg's
substance (fig. 130D-G). In the egg of the sea urchin, Toxopneustes varie-
OVULATION AND
MIGRATION OF EGG
TO AREA WHERE
FERTILIZATION
IS TO OCCUR
SPERM DISCHARGED
FROM T ESTI S AND
MIGRATE TO AREA
WHERE FERTTLI Z ATION
IS TO OCCUR
PRIMARY ^^
PHASE OF
FERT ILIZATION,
I E .ACTIVATION
PROCESSES
AROUSED IN
EGG AND
SPERM WHICH
BRING ABOUT
THEIR CONTACT
GYNOGAMIC
SUBSTANCES
SECRETED INTO
SURROUNDING
FLUIDS FROM
THE EGG.
MATURATION
DIVISIONS
INITIATED IN
CERTAIN EGGS
MOTILITY
STIMULATED
ANDROGAMIC
SUBSTANCES
SECRETED
/
>
SPERM STIMULATED
TO GREATER
ACTIVITY AND
ATTRACTED TO
EGG BY GYNOGAMIC
SUBSTANCES
SPERM SECRETIONS
AID SPERM IN
REACHING EGG
SECONDARY
PHASE OF "~
FERTILIZATION
IE, ACTIVATION
PROCESSES
AROUSED
IN EGG AND
SPERM WHICH
RESULT IN
FUSION OF THE
GAMETES
MATURATION DIVISIONS OF EGG
COMPLETED AND SPERM DRAWN
INTO EGG;SPERM NUCLEUS EN-
LARGES AND ASTER FORMS IN
MIDDLE PIECE OF SPERM
FERTILIZATION MEMBRANE DEVE LOPS
AND PERIVITELLINE SPACE AND
FLUID APPEARS BETWEEN EGG
AND MEMBRANE IN SOME SPECIES
(AMPHIOXUS) , IN OTHER SPECIES
PERIVITELLINE SPACE AND FLUID
FORMS BETWEEN EGG AND
PREVIOUSLY FORMED MEMBRANE
(FISH, FROG, MAMMALS) , EGG MAY
CONTRACT SLIGHTLY WHEN
PERIVITELLINE FLUID FORMS;
OOPLASMIC SUBSTANCES
MIGRATE TOWARD POINT WHERE
SPERM HAS ENTERED (FROG,
AMPHIOXUS , AND STYELA)
FUSION OF PRONUCLEI
AND ESTABLISHMENT OF
NEW DIPLOID CHROMOSOMAL
COMPLEX ; CLEAVAGE
AMPHIASTER FORMS
COMBINATION OF ACTION
OF GYNOGAMIC SUBSTANCE
AND ANDROGAMIC
SUBSTANCE POSSIBLY
BINDS SPERM TO EGG
SURFACE AND IMMOBILIZES
SPERM
GAMETES NOW READY TO
BEGIN SECOND OR FUSION
STATE OF FERTILIZATION
OOPLASMIC MOVEMENTS
OCCUR, RESULTING IN
REORIENTATION AND
SEGREGATION OF DEFINITE
OOPLASMIC SUBSTANCES
CLEAVAGE INITIATED
Fig. 121. (See facing page for legend.)
242
BEHAVIOR OF THE GAMETES 243
gatus, a protoplasmic prominence appears only after a sperm begins to pass
into the egg. It persists until about the time that the pronuclei unite (Wilson
and Mathews, 1895). (See fig. 131B-F.) A prominent fertilization cone is
found also in the starfish, Asterias jorbesi (Wilson and Mathews, 1895).
In the vertebrate group, fertilization cones are not generally observed, but
the protoplasmic bridge from the egg membrane to the ooplasmic surface in
Petromyzon evidently fulfills the functions of a cone (fig. 134C).
The formation of the fertilization cone and its withdrawal again, suggests
that ooplasmic movements are concerned mainly with the sperm's entry into
the interior of the egg. These movements appear to be aroused by some stimulus
emanating from the sperm as it contacts the egg's surface. That is to say, al-
though the sperm becomes immobile once it has touched the egg's surface,
various stimuli, chemical and/or physical, issue from the sperm into the egg
substance. Here these stimuli inaugurate movements in the ooplasm which draw
the sperm into the egg. This modern view thus emphasizes motility of the
cortical area of the egg as the factor which conveys the sperm into the interior
of the egg. It suggests further that the older view of sperm entry which was
presumed to result from sperm motility alone does not agree with the actual
facts demonstrated by observation.
c. Some Changes in the Physiological Activities of the Egg at Fertilization
The separation of the egg membrane from the egg surface, the emission
of fluid substances from the egg's surface into the perivitelline space, and
contraction of the egg's surface have been noted above. Associated with these
immediate results of sperm contact with the egg, a pronounced movement
of cytoplasmic substances within the egg can be demonstrated in many species.
Examples of cytoplasmic movements within the ooplasm of the egg are given
below in the descriptions of the fertilization processes which occur in various
chordate species.
Accompanying the above-mentioned activities, pronounced changes of a
metabolic nature occur. In the egg of the frog and toad, for example, there
is little change in the oxygen consumption during fertilization, although there
Fig. 121. Two stages of fertilization in animals. (A) In the primary phase of fer-
tilization ("external fertilization" of F. R. Lillie), the sperm is activated to greater
motility by the environmental factors encountered at the fertilization site, including the
gynogamic substances secreted by the egg. It is also drawn to the egg by a positive
chemotaxis. The lytic substances (androgamic substances) enable the stimulated sperm
to make its way more easily through the jelly membranes and ooplasmic membranes
surrounding the egg to the egg's surface. At the egg's surface the interaction of gynogamic
and androgamic substances brings about the agglutination of the sperm to the egg's
surface. This initiates stage B, on the secondary phase of the fertilization process ("in-
ternal fertilization" of F. R. Lillie). (B) Secondary phase of fertilization or fusion of
the gametes. (See text for further description.) This stage begins when the sperm has
made contact with the egg and terminates when the first cleavage spindle has formed.
244
FERTILIZATION
DISAPPEARANCE OF
YOLK PLATES BEG
AROUND MICRO PYL
MICROPYLE
OIL DROPS
YOL K PLATES
CHORION
V ITEL LINE
MEMBRANE
PER (VITELLINE S PAC
C R 0 PY L E
0 F
CONCENTRATION
P ROTO PLA S M IN
REGION OF MICROPYLE
PERIVITELLINE SPACE
VITELLINE MEMBRANE
Fig. 122. Changes during fertilization in the egg of Fundidus heteroclitus. (A) Egg
before fertilization. (B, C) Changes in the egg shortly after sperm entrance into the
egg. In (B) is shown the contraction of the egg from the vitelline membrane, the disap-
pearance of the yolk plates, and the formation of the perivitelline space. In (C) is shown
the migration of the peripheral cytoplasm toward the point where the sperm has entered
the egg, forming a cytoplasmic or polar cap.
is a pronounced drop in the respiratory quotient, presumably indicating a
change in the character of oxygen consumption (Brachet, J., '50, p. 106).
Fertihzation does not change the rate of oxygen consumption in the teleost
fish, Fundulus heteroclitus, but, in the laiTiprey, oxygen consumption is in-
creased (Brachet, J., '50, p. 108). Also, in the egg of the sea urchin, fol-
lowing artificial activation or normal fertilization, there is a considerable
increase in oxygen consumption (fig. 120). In the unfertilized and fertilized
egg of the starfish (Asterias) apparently there is no change in the rate of oxygen
metabolism. In the eggs of certain sea urchins it has been shown by Runnstrom
and co-workers (Runnstrom, '49, p. 306) that acid formation occurs follow-
ing fertilization. It is of brief duration. (Consult also Brachet, J., '50, p. 120,
for references.) Other changes have been described, such as an increase in
viscosity of the egg (Heilbrunn, '15), and an increase in permeability of the
egg membrane (Heilbrunn, '15). Fertilization may produce a higher dispersity
of the egg colloidal material, at least in some species. Changes of a metabolic
nature, therefore, are a part of the fertilization picture. (The reader should
consult Brachet, J., '50, Chap. 4, for a thorough discussion of physiological
changes at fertilization.)
BEHAVIOR OF THE GAMETES 245
d. Completion of Maturation Divisions, Ooplasmic Movements, and
Copulatory Paths of the Male and Female Pronuclei in Eggs
of Various Chordate Species
A description of the maturation processes, ooplasmic movements, and the
behavior of the male and female pronuclei in the fertilization processes of
various chordate species is given below. It should be observed that all of
these events occur rather synchronously in the urochordate, Styela, and in
the egg of the frog, while in others, such as the prototherian mammal, Echidna,
they may come to pass in sequence.
1) Fertilization in Styela (Cynthia) partita: a) Characteristics of the
Egg Before Fertilization. The living, fully formed, primary oocyte of the
urochordate, Styela (Cynthia) partita, is about 150 /x in diameter. It possesses
at this time three areas which can be distinguished with clearness, namely, a
peripheral transparent layer which contains a sparsely distributed yellow pig-
ment, a central mass of gray-appearing yolk, and the area of the germinal
vesicle, located near the future animal pole of the egg (fig. 132A).
The first steps leading to the maturation divisions of the chromatin ma-
terial take place before sperm entrance, at the time the egg is spawned or
shortly before. At this time the wall of the germinal vesicle (i.e., the nuclear
membrane) breaks down, and the contained clear cytoplasm moves up to the
animal pole of the egg where it spreads out to form a disc. The chromosomes
then line up on the metaphase plate of the first maturation spindle; they re-
main thus in the metaphase of the first maturation until the sperm enters
(fig. 116A, B).
b) Entrance of the Sperm. The sperm enters the egg (i.e., the primary
oocyte) at the future vegetal (vegetative) pole, either exactly at the pole or
a little to one side (fig. 116B). Sperm entrance at this pole probably is due
to a fundamental structural and physiological condition which in turn reflects
a definite polarity of the egg. Only one sperm normally enters the egg, but
several sperm may penetrate through the chorion into the perivitelline space.
c) Cytoplasmic Segregation. A striking series of changes appear within
the cytoplasm of the egg, immediately following sperm entrance. The yellow-
pigmented, peripheral layer of protoplasm flows toward the point of sperm
entrance (i.e., the vegetal pole) and collects into a "deep, orange-yellow
spot" which surrounds the sperm (fig. 132B, C, peripheral protoplasm). It
later spreads again and then covers most of the lower or vegetal pole of the
egg. Accompanying the flow of yellow peripheral protoplasm toward the vegetal
egg pole, most of the clear protoplasm of the germinal vesicle (i.e., the nuclear
plasm mentioned above) flows with the yellow protoplasm toward the vegetal
pole. The clear protoplasm, to some extent, tends to mingle with the yellow-
pigmented, peripheral protoplasm. In figure 132C, the clear protoplasm may
be observed as a clear area above the yellow-pigmented protoplasm.
The sperm pronucleus next moves upward away from the vegetal pole and
246 FERTILIZATION
toward one side of the egg to a point which marks the posterior pole of the
egg and future embryo (fig. 116M). The clear protoplasm and the yellow-
pigmented protoplasm move upward with the sperm (fig. 132D). The yellow-
pigmented protoplasm at this time forms a yellow crescent just below the
egg's equator, and the middle point of this crescent marks the posterior end
of the future embryo (fig. 132D, E). A distinct crescent of clear protoplasm
appears just above the yellow crescent at this time (fig. 132D-F). The crescent
substance is therefore plainly differentiated at once into clear and yellow proto-
plasm, which remain distinct throughout the entire development (ConkUn,
'05, p. 21).
The yolk material, which at first is centrally located in the egg, moves
toward the animal pole when the clear and yellow-pigmented protoplasms
migrate to the vegetal pole. As the yellow and clear protoplasmic crescents
are formed, the yolk material moves to occupy its ultimate position at the
vegetal pole of the egg (fig. 132D). Later when the first cleavage division
occurs, another crescentic area, the gray crescent, appears on the side of the
egg opposite the yellow crescent.
As a result of the segregation of ooplasmic materials, four definite areas
are localized:
( 1 ) a vegetal, yolk-laden area,
(2) a gray crescent,
(3) the yellow and clear protoplasmic crescents opposite the latter, and
finally
(4) the more or less homogeneous cytoplasm at the animal pole of the egg.
The movements of cytoplasmic materials in the cephalochordate, Amphi-
oxus, are similar to those in Styela (Conklin, '32).
d) CopuLATORY Paths and Fusion of the Gametic Pronuclei. The
entrance of the sperm into the egg substance, its migratory movements in the
ooplasm, its meeting with the egg pronucleus, and final fusion or association
of the pronuclei afford an interesting problem. The factors governing the
movements of the female and male pronuclei are unknown, although the move-
ments in many eggs are spectacular. The movements of the pronuclei in
Styela partita offer an excellent illustration of the copulatory migrations of
the pronuclei within the cytoplasm of the egg.
The sperm enters the egg of Styela partita, as stated previously, at the
vegetal pole near the midpolar area or a little to one side (fig. 116B). The
sperm moves inward through the yellow-pigmented protoplasm and even-
tually becomes surrounded with the yellow and clear protoplasms (figs. 132C;
116B-F). This initial pathway through the superficial protoplasm of the egg
constitutes the penetration path of the sperm (Wilhelm Roux). The sperm
head in the meantime begins to swell and becomes vesicular (figs. 116F, J;
133B-G, Ascaris). The nucleus and the middle piece of the sperm with its
BEHAVIOR OF THE GAMETES 247
forming aster now rotate 180 degrees, so that the aster hes anterior to the
nucleus as it migrates within the egg (fig. 116F, I, J). The sperm aster thus
precedes the pronucleus as the latter moves through the cytoplasm (fig. 1 16M) .
With the movement of the clear and pigmented protoplasmic substances
upward toward the equator and to the point marking the future posterior
end of the embryo, the sperm pronucleus and aster move upward. This latter
movement of the sperm constitutes the copulation path, and it is formed at
a sharp angle to the penetration path (figs. 1 16M, 139B). The egg chromatin
in the meantime undergoes its first and second maturation divisions (fig.
116F-L). After the second polar body has been formed, the haploid number
of chromosomes reform the egg nucleus, now called the female pronucleus
(fig. 116L, M). The latter then moves downward through the yolk along its
copulation path to meet the sperm pronucleus near the posterior pole of the
egg (figs. 1 16M-P; 139B). The actual meeting place in the clear cytoplasm is
about halfway between the posterior pole and the center of the egg (fig. 139B).
Shortly before the pronuclei meet, the sperm aster divides, each aster
moving to opposite poles of the sperm pronucleus (fig. 116N). The two
pronuclei now meet between the amphiaster of the first cleavage (fig. 1 160, P)
and thus become enclosed by the amphiaster spindle (fig. 116P). Following
this association, the entire complex migrates toward the center of the egg
together with a mass of clear cytoplasm. Some of the yellow protoplasm also
migrates slightly centerward. The latter movement of the pronuclei toward
the center of the egg is called the cleavage path. In the new position, slightly
posterior to the egg's center, the pronuclei form an intimate association (figs.
11 6P, 139). The chromosomes then make their appearance, the nuclear mem-
branes disappear, and the chromosomes line up in the metaphase plate of the
first cleavage spindle preparatory to the first cleavage (fig. 116Q). The first
cleavage plane always bisects the midplane of the future embryo and hence
bisects the yellow and clear protoplasmic crescents (figs. 116R, S; 132F, G).
2) Fertilization of Amphioxus. The fertilization stages of Amphioxus are
shown in figures 117A-I; 139C. The general process of fertilization in this
species appears much the same as in Styela. However, in Amphioxus the fer-
tilization phenomena cannot be studied as readily for a pigmented material
is not formed in the peripheral cytoplasm. According to Conklin ('32), the
general movements of the cytoplasmic substances resemble those of Styela. It
is to be observed, however, that the copulation paths of the sperm and egg
pronuclei, and also the cleavage path of the two pronuclei, are different slightly
in Amphioxus from those present in Styela (fig. 139B, C).
3) Fertilization of the Frog's Egg. The egg of Rana pipiens is spherical
and approximately 1.75 mm. in diameter as it hes in the uterine portion of
the oviduct just before spawning. The size, however, may vary considerably.
It has a darkly pigmented animal pole and a lightly colored vegetal pole. The
first maturation division occurs when the egg is ovulated or shortly after
248 FERTILIZATION
ovulation during its passage through the peritoneal cavity en route to the
oviduct (fig. II 9B, C). The secondary oocyte then enters the oviduct, and
during its passage posteriad in the latter, the maturation spindle of the second
maturation division is formed (fig. 1 19D). The egg is in this condition when
it is spawned. Immediately upon its entrance into the water, it is fertilized by
the sperm from the amplectant male.
The sperm enters the egg at a point about 20 to 30 degrees down from the
midregion of the animal pole. As it penetrates through the cortex of the egg,
a trail of dark pigment from the egg's periphery flows in after the sperm
(fig. 11 9H, I). This initial entrance path of the sperm constitutes the pene-
tration path. After making its initial entrance, the sperm begins to travel
toward its meeting place with the female pronucleus. This secondary path is
the copulation path of the sperm (fig. 1191). If the sperm should continue
more or less in a straight line toward the egg pronucleus, the penetration path
and copulation path would be continuous. However, if the sperm should veer
away at an angle from the original penetration path in its journey to meet
the female pronucleus, the copulation path would be at an angle to the pene-
tration path.
The second maturation division of the oocyte occurs in about 20 to 30
minutes after sperm entrance with a surrounding temperature approximating
22° C. After the female pronucleus is organized, it migrates along its copu-
lation path toward the meeting place with the sperm pronucleus, located near
the center of the animal pole cytoplasm of the egg (fig. 1 19F, G).
Shortly after the sperm penetrates the egg, it revolves 180 degrees, and
the middle-piece area travels foremost. This revolving movement, whereby the
middle-piece area assumes a foremost position, is similar to that which occurs
in the protochordates, Styela and Amphioxm. This revolving movement ap-
pears to be characteristic of all sperm after entering the egg. (See figs. 116,
117, 131.) The sperm pronucleus gradually enlarges as it continues along the
copulation path, and the first cleavage amphiaster arises in relation to the
middle-piece region.
Fusion of the two pronuclei occurs at about one and one-half to two hours
after fertilization at a normal room temperature of about 22 "^ C. (fig. 1 19G).
At about two and three-quarter hours after fertilization the first cleavage
furrow begins (figs. 119J; 142A).
As stated above, the peripheral egg cytoplasm with its pigment tends to
flow into the interior of the egg, following the trail of the sperm and thus
forms a pigmented trail. The migration of the superficial cytoplasm with its
pigmented granules is general over the upper pole of the egg and its direction
of flow is toward the point of sperm penetration (see arrows, fig. 119K).
Consequently, ,at a point on the egg's surface opposite the point of sperm
entrance, the peripheral area of the egg becomes lighter in color and assumes
BEHAVIOR OF THE GAMETES 249
a gray appearance. This area is crescentic in shape and is known as the gray
crescent (fig. 1 lOK).
The formation of the gray crescent occurs in the cytoplasmic area just
above the margin where the yellow-white vegetal pole material merges with
the darkly pigmented animal pole. The gray crescent is continuous with the
lighter vegetal pole material and is seen most clearly during the first cleavage
of the egg. The plane which bisects the gray crescent into two equal halves
represents the future median plane of the embryo.
In the frog, Rana jusca, Ancel and Vintemberger ('33) have shown that
extensive movements of egg-surface materials accompanies the formation of
the gray crescent. Sperm contact with the egg's surface thus appears to set
in motion ooplasmic substances which fix the final symmetry of the egg and
the future embryo.
4) Fertilization of the Teleost Fish Egg. When the egg of the teleost fish
is spawned, the yolk lies near the center of the egg, and its yolk-free cyto-
plasm forms a peripheral layer. Around the egg the yolk-free cytoplasm is
somewhat more abundant in the region where the egg nuclear material is situ-
ated. This concentration of the peripheral cytoplasm at the nuclear pole is
more evident in the eggs of some species than in others. The area of nuclear
residence is situated near the micropyle in many teleost eggs, but not in all.
For example, the concentration of cytoplasm with the contained nuclear ma-
terial is located in Bathygobius soporator at the opposite end to the micropyle
(Tavolga, '50). (See fig. 123A.)
The sperm enters the egg through the micropyle (figs. 122, 123, 134A),
and the actual processes of fertilization are initiated when the sperm makes
contact with the peripheral ooplasm near the point where the egg nuclear
material is located. This normally occurs in about a minute or less after the
egg reaches the water. Within a few minutes the second polar body is given
off. Meanwhile, the peripheral cytoplasm flows toward the area where the
sperm has made contact, and a protoplasmic cap forms at this pole (figs.
122C; 123B-D). The remainder of the egg, with the exception of a thin
layer of surface protoplasm, contains the deutoplasmic or yolk material. The
egg is converted in this manner from a more or less centrolecithal egg into a
strongly telolecithal egg. (Compare with Stye la and Amphio.xus.)
While these events are progressing, the egg as a whole contracts slightly,
and a fluid is given off into the forming perivitelline space between the egg's
surface and the vitelline membrane (fig. 122B, C). (However, a space be-
tween the egg membrane and the egg is evident to some extent in certain
teleost eggs before the sperm enters the egg (fig. 123A).) The egg is now
free to rotate vvithin the perivitelline space, being cushioned and bathed by
the perivitelline fluid.
The expansion of the vitelline membrane of the egg in certain teleosts is
both dramatic and prophetic of the future shape of the embryo (fig. 123B-H).
Fig. 123. Development of the gobiid fish, Bathygobius soporator. (After Tavolga, '50,
slightly modified.) (A) Freshly stripped egg. Adherent filaments at proximal end of
chorion; micropyle at distal end. Peripheral cytoplasm partly concentrated at the pole of
the egg containing the female nucleus. (B) Fifteen minutes after fertilization; cyto-
plasm concentrating at nuclear pole of the egg; chorion expanding into shape of future
embryo. (C) Twenty minutes after fertilization; second polar body given off. (D)
Twenty-five minutes after fertilization. (E) Ninety minutes after fertilization. (F)
Seventeen hours after fertilization. (G) Twenty-four hours after fertilization. (H)
Thirty hours after fertilization. (I) Thirty-six hours after fertilization. (J) Ninety-
six hours after fertilization. (K) Ninety-six hours after fertilization. Hatching. (L)
Three days after hatching, temperature 27 to 29°.
250
BEHAVIOR OF THE GAMETES
251
In demersal eggs, that is, eggs which sink to the bottom, the protoplasmic cap
tends to assume an uppermost position. In pelagic eggs, i.e., eggs which float
in the water, the protoplasmic disc turns downward since it is the heaviest
part of the egg.
After the polar bodies are given off, the egg-chromatin material reforms
the female pronucleus. The latter and the sperm pronucleus migrate to a
position near the center of the protoplasmic disc. The first cleavage plane is
established within thirty minutes to an hour following sperm entrance.
5) Fertilization in the Egg of the Hen and the Pigeon. Fertilization in the
hen's egg occurs without any demonstrable movement of cytoplasmic mate-
rials, as manifested in the eggs of Styela, Amphioxus, frog, and teleost fish.
The egg is strongly telolecithal, and the true protoplasm or blastodisc, which
takes part in active development, is a flattened mass about 3 mm. in width.
The germinal vesicle in the mature egg is approximately 350 ^u in diameter
and about 90 /x in thickness (fig. 126A). Approximately 24 hours before
ovulation occurs, the wall of the germinal vesicle begins to break down, and
the contained nuclear sap spreads in the form of a thin sheet below the
ooplasmic membrane overlying the blastodisc (fig. 126B). (See Olsen, '42.)
Changes in the chromatin material of the germinal vesicle are synchronized
with the breakdown of the membranous wall of the vesicle. The chromatin
material, extremely diffuse during the period when the yolk material was
formed and the egg as a whole was growing rapidly, contracts and assumes
the character of thickened chromosomes in the tetrad condition. The diffuse
FALLOPIAN TUBE
Fig. 124. Fertilization stages in the rabbit egg. (A, B after Pincus, '39.) (A) Second
polar body exuded; male and female pronuclei. (B) Twenty-two hours after copulation,
showing two pronuclei close together. (C) Coagulated plug in infundibular portion of
Fallopian tube, containing eggs. This plug is dissolved by sperm during fertilization
process.
ii
A
4®^'"^""-=- v( ZONA PELLUCIDA
-^ Sir''
YOLK
GLOBULES
\<fit
■'■'■\
H^
(tap ^^iffr-
Fig. 125. Fertilization in the opossum. (A after McCrady, '38, from Duesberg; B-F
after Hartman, '16.) (A) Conjugate sperm of opossum. (B) Ovarian egg showing
discus proiigerus around the egg; first polar body extruded; chromosomes of egg nucleus
evident. (C) Tubal ovum. (D) Uterine ovum with pronuclei near center of the egg.
(E) First cleavage spindle of uterine egg. (F) Two-cell stage, showing zona pellucida
and exuded yolk material lying in perivitelline space.
GERMINAL VESICLE
FOLLICLE CELLS
VITELLINE
MEMBRANE
CHROMATIN
P V SPACE
*:■•■> -sJii' E.
Fig. 126. Maturation and fertilization in the hen's egg. (Drawings from photomicro-
graphs by Olsen, '42.) (A) Cross section of germinal vesicle of almost mature egg,
showing the general position and condition of the intact germinal vesicle. (B) Egg just
prior to ovulation. Germinal vesicle spreading laterally as a thin layer below the ooplasmic
membrane. (C) Chromatin material near center of disintegrating germinal vesicle
(G.V.) of an egg estimated to be one hour prior to ovulation. (D) First polar body
(I P.B.) of recently ovulated egg. (E) Cross section of blastodisc of recently ovulated
egg showing male pronucleus ( i }, female pronucleus ( ? ), and second polar body
(2 P.B.).
252
BEHAVIOR OF THE GAMETES 253
diplotene state thus passes into the diakinesis stage (figs. 126C; 135 A, show
the breakdown of the nuclear wall and appearance of chromosomes in the
pigeon).
The first maturation division occurs and the first polar body is extruded
shortly before ovulation (fig. 126D). The second maturation spindle is then
formed. In this state the egg is ovulated. From four to six sperm penetrate
into the egg shortly after it enters the infundibulum of the oviduct. The latter
events are consummated within fifteen minutes after ovulation. The second
maturation division then occurs, followed by the discharge of the second polo-
cyte, which becomes manifest about the time of, or shortly before, the fusion
of a single male pronucleus with the female pronucleus (fig. 126E). Thus,
although polyspermy is the rule, only one sperm pronucleus takes part in the
syngamic process.
After the two pronuclei become closely associated, the chromosomes be-
come evident, the nuclear membranes disintegrate, and the first cleavage spindle
is formed in about five and one-quarter hours after the sperm enters the egg
(Olsen, '42).
In the egg of the pigeon, according to Harper ('04), the germinal vesicle
breaks down, and the first polar spindle forms in the egg just before ovulation
(fig. 135A, B). Fertilization then occurs just as the egg (in reality the primary
oocyte) enters the oviduct. Normally from 15 to 20 sperm enter the blastodisc
of the pigeon's egg. However, only one sperm pronucleus associates with the
female pronucleus. Consequently, unlike the condition in the hen's egg, both
maturation divisions occur and the first and second polar bodies are given
off after sperm entrance (fig. 135C, D). Following the maturation divisions,
the two pronuclei proceed to associate (fig. 135E, F). The first cleavage
nucleus is shown in fig. 135G with two accessory sperm nuclei shown to the
extreme left of the figure.
6) Fertilization in the Rabbit. In the rabbit, ovulation occurs around 10
to 1 1 hours after copulation. It takes about four hours for the sperm to travel
to the upper parts of the Fallopian tube. (See Chap. 4.) The sperm thus lie
waiting for about six to seven hours before the eggs are ovulated. When the
eggs are discharged frorii the ovary, each egg is surrounded by its cumulus
cells. The latter form the corona radiata, surrounding the zona pellucida
(fig. 124A). As the eggs are discharged from their follicles, an albuminous
substance from the follicles forms a clot, and several eggs are included within
this clot (fig. 124C). A sperm, therefore, must make its way through the sub-
stance of the clot, as well as between the cells of the corona radiata, and then
through the zona pellucida to reach the egg. This feat is accomplished partly
by its own swimming efforts and partly also by means of an enzyme (or
enzymes) which dissolves a pathway for the sperm. (See hyaluronidase, etc.,
mentioned on pp. 229.) The ferment hyaluronidase, associated with the sperm,
frees the eggs from the albuminous clot and aids in the dissolution of the
Fig. 127. Maturation and fertilization in Echidna. (Courtesy, Flynn and Hill, '39.)
(A) Oocyte, diameter 3.9 by 3.6 mm. Section. of upper pole of egg showing saucer-shaped
germinal vesicle lying in the germinal disc. (B) First polar spindle of egg just previous
to ovulation. (C) First polar body and chromatin of female nucleus just previous to
formation of second polar spindle shown in (D). (D) Second polar spindle of newly
ovulated egg. Sperm presumably enters germinal disc at this time but possibly may wait
until condition shown in (E) in some instances. (E) Second polar body and female
pronucleus. (F) Male and female pronuclei. (G, H) Fusion stages of pronuclei.
254
BEHAVIOR OF THE GAMETES 255
corona radiata cells, so that each egg lies free in the Fallopian tube, sur-
rounded by the zona pellucida. It may be that some other lytic substance asso-
ciated with the sperm also is active in aiding the sperm to reach the egg's
surface.
The first maturation division of the egg occurs as the egg is being ovulated.
The egg remains in this condition until the sperm enters, which normally
occurs within two hours after ovulation. Thus, sperm entrance into the rabbit's
egg presumably is much slower than in the case of the hen's egg, possibly due
to the albuminous and cellular barriers mentioned above. Several sperm may
penetrate through the zona pellucida into the perivitelline space, but only one
succeeds in becoming attached to the egg's surface (Pincus, '39). The sperm
tail is left behind in the perivitelline fluid, and the sperm head and middle
piece "appear to be drawn into the egg cytoplasm rather rapidly" (Pincus
and Enzmann, '32). The second polar body is then extruded, a process which
ordinarily is completed about the thirteenth hour following copulation (fig.
124A). About three or four hours later (that is, about 17 hours after copu-
lation) the two pronuclei are formed and begin to approach one another,
and at 20 to 23 hours after copulation the pronuclei have expanded to full
size and come to lie side by side (fig. 124B). The migration of the pronuclei
to the center of the egg thus consumes about four to six hours. The spindle
for the first cleavage division generally is found from 21 to 24 hours after
copulation (Pincus, '39). (Consult also Gregory, '30; Lewis and Gregory, '29.)
7) Fertilization in the Echidna, a Prototherian Mammal. The egg of the
Tasmanian anteater. Echidna, when it reaches the pouch is about 15 by 13
mm. in diameter. This measurement, of course, is only approximate, and it
includes the egg proper plus its external envelopes of albumen and the leathery
shell. (The egg of Ornithorhynchus is slightly larger, approximating 17 by 14
mm.) At the time of fertilization in the upper portion of the Fallopian tube,
the fresh ovum of Echidna without its external envelopes measures about four
to 4.5 mm. in diameter.
The fully developed eggs of the monotreme (prototherian) mammals are
strongly telolecithal, with a small disc of true protoplasm situated at one pole
as in the bird or reptile egg. In Echidna aculeata this disc measures about
0.7 mm. in diameter during the maturation stages. Just before the onset of
the maturation divisions of the nucleus, the germinal vesicle is saucer-shaped
and lies in the midportion of the upper part of the disc (fig. 127A). The first
maturation division (fig. 127B, C) occurs before ovulation, while the second
maturation division (fig. 127D, E) occurs after ovulation. There is some evi-
dence that the second maturation division, in some cases, may precede the
actual entrance of the sperm into the germinal disc (Flynn and Hill, '39). In
figure 127F-H, the stages in the association of the male and female pronuclei
are shown.
As fertilization is accomplished, a rearrangement and movement of the
256
FERTILIZATION
ooplasmic substances of the germinal disc occur. As a result, the blastodisc,
circular during the maturation period, becomes transformed into an oval-
shaped affair with the polar bodies situated on one end (fig. 136). The first
plane of cleavage is indicated by numerals I-I, and the second plane of cleavage
by numerals II-II. A distinct bilateral symmetry thus is established by the
rearrangement of ooplasmic materials during the fertilization process, (Com-
pare with Styela, Amphioxus, and frog.)
E. Significance of the Maturation Divisions of the Oocyte in Relation
to Sperm Entrance and Egg Activation
As indicated in the foregoing pages, the maturation divisions of the oocyte
vary greatly in different animal species. Figure 137 shows that the time of
sperm entrance in the majority of eggs occurs either before or during the
maturation divisions, that is, when the female gamete is in the primary or
secondary oocyte condition. In some animals, however, the sperm enters
normally after the two maturation divisions are completed.
The correlation between the maturation period of the egg and sperm entrance
indicates that the breakdown of the germinal vesicle and the accompanying
maturation divisions has a profound effect upon the egg. This conclusion is
substantiated by experimental data. For example, A. Brachet ('22) and
Runnstrom and Monne ('45, a and b), working on the sea-urchin egg, found
PERIVITELLINE
MEMBRANE
SPACE
Fig. 128. Formation of the vitelline membrane in the egg of Ascaris after fertilization.
(After Collier, '36.) (A) Heavy cell wall (vitelline membrane) is beginning to thicken.
(B) Cell wall is reaching condition of maximum thickness. (C) Egg contracts away
from vitelline membrane, leaving perivitelline space filled with fluid-like substance, form-
ing the typical fertilized egg of Ascaris as ordinarily observed.
MICROPYLES 257
that several sperm enter the egg in the sea urchin if insemination is permitted
before the maturation divisions occur. The immature egg, therefore, lacks the
mechanism for the control of sperm entrance. Moreover, A. Brachet ('22)
and Bataillon ('29) demonstrated that the sperm nuclei and asters behave
abnormally under these conditions, and normal development is impossible.
Runnstrom and Monne have further shown for the sea-urchin egg that the
normal fertilization process, permitting the entrance of but one sperm, requires
a mechanism which is built up gradually by degrees during the time when
the maturation divisions of the egg occur, even extending to a necessary short
period after the divisions are completed. Not only is the mechanism which
permits but one sperm to enter the egg established at this time in the sea-
urchin egg, but Runnstrom and Monne further conclude, p. 25, "that the
cytoplasmic maturation" which occurs at the period of the maturation divi-
sions, "involves the accumulation at the egg surface of substances which
participate in the activating reactions." It appears, therefore, that the break-
down of the germinal vesicle together with the phenomena associated with the
maturation divisions is an all-important period of oocyte development, con-
trolling sperm entrance on the one hand and, on the other, presumably being
concerned with formation of substances which permit egg activation.
F. Micropyles and Other Physiologically Determined Areas for
Sperm Entrance
A micropyle is a specialized structural opening in the membrane or mem-
branes surrounding many eggs which permits the sperm to enter the egg.
For example, in the eggs of teleostean fishes or in the eggs of cyclostomatous
fishes, a small opening through the vitelline membrane (or chorion) at one
pole of the egg permits the sperm to enter (figs. 93 A; 134A-F). On the
other hand, many chordate eggs do not possess a specialized micropyle through
the egg membrane. The latter condition is found in the protochordates, Styela
and Amphioxus, and in vertebrates in general other than the fish group. In
Styela and Amphioxus the sperm enters the vegetal pole of the egg, i.e., the
pole opposite the animal or nuclear pole. In most of the vertebrate species
the sperm enters the animal or nuclear pole of the egg usually to one side of
the area where the maturation divisions occur (figs. 115, 118, 1 191). In urodele
amphibians, the passage of several sperm into the egg at the time of fertili-
zation complicates the picture. However, the sperm which finally conjugates
with the egg pronucleus is the one nearest the area where the egg pronucleus
is located. The several sperm entering other parts of the egg ultimately de-
generate (fig. 138). Presumably this condition is present in reptiles and birds
where many sperm normally enter the egg at fertilization.
In conclusion, therefore, it may be stated that the point of sperm entrance
in chordate eggs in general appears to be definitely related to one area of
the egg, either by the presence of a morphologically developed micropyle or
VITELLINE MEMBRANE
-CORTICAL LAYER
— EN OOPLASM
VITELLINE
MEMBRANE
CORTICAL
GRANULE
PLASMA
SURFACE
_,_____^^VITE LLI NE
\ -X^" MEMBRANE
^^^-'"^==^0 0 R T I C A L
_^^^^ GRANULES
" P L A S M A
2 S U R FACE
FE RT IL IZ AT ION
MEMBRANE
PLASMA
SURFACE
Fig 129 Formation of the fertilization membrane in the mature egg of the sea urchin.
(A) Surface of the egg and surrounding jelly coat before fertilization. (After Runnstrom,
'49 p 245 ) (Bl 2 3 4, 5) Point x marks the point of sperm contact. The fertilization
membrane separates 'from the egg at the point of sperm contact and spreads rapidly
around the egg from this point. (After Just, '39, p. 106.) (C) Membrane forma ion m
greater detail (After Runnstrom, '49, p. 276.) ( 1) As the vitelline membrane is l.f ed
off from the plasma surface of the egg, cortical granular material is exuded from the
egg cortex and passes out across the perivitelline space toward the fertilization mem-
brane (2) Cortical granules begin to consolidate with the vitelline membrane. (3)
Fully developed fertilization membrane is formed by a union of the vitelline membrane
with the cortical granules derived from the egg cortex.
258
IMPORTANCE OF THE SPERM ASTER 259
by some physiological condition inherent in the organization of the egg. In
the majority of chordate eggs, the place of sperm penetration is at that pole
of the egg which contains the egg nuclear material, although in some, such
as in the gobiid fish (fig. 123), the micropyle, permitting the sperm to get
through the egg membrane, may be situated at a point opposite the nuclear
pole of the egg.
G. Monospermic and Polyspermic Eggs
In the eggs of most animal species only one sperm normally enters the egg.
Such eggs are known as monospermic eggs. Among the chordates, the eggs
of Styela, Amphioxus, frog, toad, and mammals are monospermic. Abnormal
cleavage and early death of the embryo is the general result of dispermy and
polyspermy in frogs (Brachet, A., '12; Herlant, '11). In those chordates
whose eggs possess much yolk, the eggs are normally polyspermic, and several
sperm enter the egg at fertilization, although only one male pronucleus enters
into syngamic relationship with the egg pronucleus; the other sperm soon de-
generate and die in most cases (fig. 138). (See Fankhauser, '48.) In some
urodele amphibia, it appears that syngamic conjugation of more than one
sperm pronucleus with the egg pronucleus may occur in certain instances and
may give origin to heteroplcidy, and development may be quite normal
(Fankhauser, '45). Examples of normal polyspermic eggs are: birds, reptiles,
tailed amphibia, elasmobranch fishes, and possibly some teleost fishes. Among
the invertebrates, polyspermy is found in some insects and in the Bryozoa.
In the sea urchin, polyspermy may occur, but abnormal embryos are the rule
in such cases as indicated above. Similar conditions are found in certain other
invertebrates (Morgan, '27, pp. 84-86).
Two explanations of normal polyspermy are suggested:
( 1 ) The presence of a superabundance of yolk hinders the operation of
the mechanism whereby the egg inhibits the entrance of extra sperm;
the egg, therefore, falls back upon a second line of defense within its
own substance which excludes the sperm from taking part in or hin-
dering the normal functioning of the syngamic nucleus in its relation
to development; and
(2) a large amount of yolk makes it advantageous to the egg for extra
sperm to enter, as they may contribute enzymes or other substances
which enable the egg better to carry on the metabolism necessary in
utilizing yolk material.
H. Importance of the Sperm Aster and the Origin of the First
Cleavage Amphiaster
One of the older views of fertilization maintained that the egg possessed
the cytoplasm but lacked a potent centrosome or "cell center" capable of
Fig. 130. Fertilization in Nereis. (After F. R. Lillie, '12.) (A) Sperm of Nereis,
entire. (B) Egg of Nereis, 15 minutes after insemination. The fertilization cone is evi-
dent below point of sperm contact. Observe that the intact germinal vesicle is present
in the center of the egg. It will break down as the sperm enters the egg (G). The cortical
substance from the empty cortical compartments in the cortical layer shown around the
periphery of the egg has passed out through the vitelline membrane to form the jelly
layer around the egg. (C~G) Entrance of the sperm head into the ooplasm of the egg
as the fertilization cone substance is withdrawn inward from the vitelline membrane.
(C) Fifteen minutes after insemination. (D) Thirty-seven minutes after insemination.
(E) Forty-eight and one-half minutes after insemination. (F) Fifty-four minutes after
insemination. (G) Sperm head has completed penetration. Observe that the middle
piece of the sperm remains outside, attached to the vitelline membrane. Anaphase of
first maturation division.
260
IMPORTANCE OF THE SPERM ASTER 261
giving origin to the first cleavage amphiaster, whereas the sperm possessed a
dynamic centrosome with its included centriole but lacked sufficient cytoplasm
for division or cleavage. Consequently, fertilization brought together a rela-
tionship necessary for cleavage and development. This idea was first set forth
by Boveri (see fertilization theories at the end of this chapter).
In the majority of animals, the central body (i.e., the centrosome) with
its surrounding aster, which ultimately divides and gives origin to the first
cleavage amphiaster, does not arise until after the sperm has entered the egg.
In these cases the aster complex arises in the middle piece of the sperm in
close proximity to the nucleus. These facts are well illustrated in figures 116,
117, and 131. Many studies of the fertilization process and early cleavage
bolster this general conclusion. There are some exceptions, however, to this
rule. For example, Wheeler ( 1 895 ) in his studies of fertilization in Myzostoma
glabrum demonstrated that the centrioles of the egg near the germinal vesicle
give origin to the amphiaster concerned with polar body formation. Following
the maturation divisions, the female pronucleus with its centrioles and forming
amphiaster, migrates along the copulation path to meet the sperm pronucleus.
The amphiaster and centrioles are closely adherent to the egg pronucleus
during the migration of the latter. In the honeybee, Nachtsheim ('13) found
a similar situation, while in the mollusk, Crepidula plana, Conklin ('04) found
evidence which suggests that one aster of the cleavage amphiaster arises from
the egg, whereas the other aster arises from the sperm, "although there is
not positive evidence that they are directly derived from egg and sperm
centrosomes."
Where the egg develops as a result of artificial stimulation the first cleavage
spindle arises without the aid of the sperm middle piece. In these instances
the amphiaster probably is derived from the central body of the last matura-
tion division, or, it may be, from certain asters or cytasters artificially induced
in the egg cytoplasm by the activation process. The production of numerous
asters in the cytoplasm of the egg by artificial stimulation has long been
known (Mead, 1897; Morgan, 1899, '00).
The general conclusion to be extracted from the evidence at hand, there-
fore, suggests that the central body from the last maturation spindle or other
artificially induced asters in the egg cytoplasm may form the amphiaster of
the first cleavage spindle in the case of an emergency. Such an emergency
arises in normal parthenogenesis or in cases of artificial activation (artificial
parthenogenesis) of the egg. However, under the conditions of normal fer-
tilization the sperm aster fulfills the role of developing the first cleavage
amphiaster.
Regardless of the fact that the first cleavage amphiaster appears to be de-
rived from the middle piece of the sperm, the influence of the egg protoplasm
is undoubtedly an important factor in its formation. In the normal polyspermy
of the newt, Triton (fig. 138; Fankhauser, '48), the sperm aster nearest the
262 FERTILIZATION
egg pronucleus enlarges and develops the amphiaster, whereas the more dis-
tantly located sperm asters fade and disintegrate. This fact suggests that some
influence from the egg pronucleus stimulates the further development of the
amphiaster in the sperm nearest the egg pronucleus. In experiments on in-
semination of egg fragments in the urodele, Triton, Fankhauser ('34) found
that the sperm aster in that fragment which did not contain the egg nucleus
failed to reach the size of the aster in the fragment containing the egg nucleus.
He concludes, p. 204, "The interactions between the sperm complex and the
cytoplasm of the egg seem, therefore, to be stimulated in the presence of the
egg nucleus."
On the other hand, the experiments on androgenesis by Whiting ('49) in
Habrobracon, and the insemination of the "red halves" of the sea-urchin egg
by Harvey ('40) demonstrate that the sperm aster can, without the egg
pronucleus, produce the first cleavage amphiaster. However, the presence of
a nucleoplasmic substance in both of these cases cannot be ruled out. For
example, A. Brachet ('22) and Bataillon ('29), the former working on the
sea-urchin egg and the latter on the eggs of two amphibian species, demon-
strated that large, normal sperm asters and large vesicular sperm nuclei do
not form until after the germinal vesicle breaks down and the egg becomes
mature. Premature fertilization results in polyspermy, small sperm nuclei, and
small sperm asters. In normal fertilization, therefore, it is very probable that
the development of the sperm aster into a normal cleavage amphiaster is
dependent:
( 1 ) upon the egg cytoplasm, and
(2) upon some factor contributed to the egg cytoplasm by the nuclear sap
or from the chromosomes of the female nucleus at the time of the break-
down of the germinal vesicle or during the maturation divisions.
I. Some Related Conditions of Development Associated with the
Fertilization Process
1. Gynogenesis
The word gynogenesis means "female genesis." Therefore, gynogenesis is
the development of the egg governed by the female pronucleus alone. The
male gamete may enter the egg but plays no further role (Sharp, '34, p. 406;
Wilson, '25, p. 460). In the nematode, Rhabdites aberrans, the egg produces
but one polar body, and diploidy is retained. The egg is penetrated by the
sperm which takes no part in later development, as it degenerates upon entering
the egg.
In the above instance, it is doubtful whether or not sperm is necessary to
activate the egg. However, in the nematode, Rhabdites pellio, the egg is pene-
trated by the sperm which plays no further role in development. Nevertheless,
in the latter instance, sperm entrance appears to be necessary for egg acti-
CONDITIONS ASSOCIATED WITH FERTILIZATION PROCESS
263
MIDDLE FERTILIZATION
PIECE ^ CONE
\4,HEAD .M^
Fig. 131. Fertilization in the sea urchin, Toxopneustes variegatus. (After Wilson and
Mathews, 1895.) (A) Sperm head and middle piece. (B) Fertilization cone (attrac-
tion cone; "cone of exudation" of Fol). The fertilization cone forms after the sperm
head and middle piece have entered the egg, and persists through (F) when the pronuclei
begin to come together. (C-J) Different stages in the fusion of the pronuclei. Observe
that the sperm rotates at about 180° and that the sperm aster appears near base of nucleus
(D, E). The aster grows rapidly (F, G) as the sperm pronucleus advances toward the
female pronucleus, and appears between the two pronuclei in (G). In (H) the aster has
divided, and the daughter asters are found at either end of the two fusing pronuclei.
In (I, J) the two asters are at either end of the fusion nucleus. (J) Fusion nucleus
between the amphiaster of the first cleavage.
vation. A somewhat similar phenomenon may also occur in other animal
species taking part in hybrid crosses, where some or all of the paternal chromo-
somes may be eliminated; activation normally occurs in these instances, and
development results. Gynogenesis is experimentally produced in amphibia by
radiating the sperm before fertilization. Development is carried on by the
female pronucleus in the latter instance, although it may produce larvae which
ultimately die. Parthenogenesis, natural and artificial, in all its essential fea-
tures in a sense may be regarded as gynogenesis.
2. Androgenesis
This form of development is experimentally produced by removal of egg
pronucleus with a small pipette before nuclear syngamy occurs (Porter, '39)
or by treating the egg with x-rays before fertilization (Whiting, '49). The
male pronucleus seems incapable of bringing about normal and full develop-
ment in amphibia, but in wasps, where the egg pronucleus has been destroyed
by radiation, it has been successful (Whiting, '49).
GERMINAL VESICLE
(NUCLEAR MATERIAL)
YELLOW
PIGMENTE D
CYTOPLASM
(PERIPHERAL
, PROTOPLASM
TEST CELLS
SP E R M,
CLEAR PROTOPLASM
YOLK MATERIAL
VAGE PLANE
OLAR BODIES
Fig. 132. Movement of ooplasmic substances in the egg of Styela (Cynthia) partita
at the time of fertilization. (All figures after Conklin, '05.) (A) Unfertilized egg after
the disappearance of the nuclear membrane of the germinal vesicle. The gray yolk is
shown in the center of the egg. surrounded by the yellow-pigmented cytoplasm. The test
cells and chorion surround the egg. (B) Egg five minutes after fertilization, showing
the streaming of the peripheral protoplasm indicated by arrows toward the vegetal pole,
where the sperm has entered. The gray yolk is shown in the upper part of the egg below
the nuclear material. The clear protoplasm, derived from the nuclear sap of the germinal
vesicle, also flows down with the peripheral protoplasm. (C) Side view of an egg after
peripheral protoplasm has migrated to the vegetal pole of the egg. The clear protoplasm
is shown at the upper edge of the yellow cap. The polar bodies are forming in the
midpolar area (MP.) at the animal pole. (D) Side view of the egg showing the yellow
crescent and the area of clear protoplasm above the yellow crescent. The yolk material
is shown at the vegetal pole below and to one side of the yellow crescent. (E) Yellow
crescent and clear protoplasm viewed from the posterior pole of the egg. Animal pole
is above the crescent; yolk material is below. (F) Same view as (E) a little later,
showing the external beginnings of the first cleavage. The polar bodies are shown above
the crescent material. (G) View similar to that of (E, F) a little later. The first cleavage
is complete. Observe that the clear protoplasm and the yellow crescent have been bisected
equally. The cleavage plane corresponds to the median axis of the embryo.
264
CONDITIONS ASSOCIATED WITH FERTILIZATION PROCESS
3. Merogony
265
Merogony is the development of part of the egg. that is, an egg fragment.
Egg fragments are obtained by shaking the egg to pieces, by cutting with a
sharp instrument, or by the use of centrifugal force. Andromerogony is the
development of a non-nucleate egg fragment after it has been fertilized by
Fig. 133. Stages in the fertilization of Ascaris. (After Boveri, 1887.) (A) Ameboid
sperm on the periphery of the egg; germinal vesicle in center of primary oocyte. (B)
Sperm entered egg substance; germinal vesicle broken down and tetrads becoming evident.
(C) Sperm in center of the egg; first maturation division, showing tetrads on spindle.
(D) Second maturation division; first polar body at egg's surface. (E) First and second
polar bodies shown; sperm aster forming in relation to sperm. (F) Second polar body;
male and female pronuclei. (G) Male and female haploid chromosomes evident;
amphiaster forming. (H) Chromosomes distinct, showing haploid condition. Observe
amphiaster. (I) Amphiaster complete; metaphase of first cleavage.
266
FERTILIZATION
CERMINAL VESICLE
MICROPYLAR CANAL
I V L LOUS LAYE R
I ZONARAOATA
-y^
.^dihjiuiM
Fig. 134. Micropyle and egg membranes of certain fishes. (A) Micropyle, egg mem-
branes, and germinal vesicle in Lepidostcus. (Modified from three figures drawn by
E. L. Mark, Bull. Mus. Comp. Zool. at Harvard College, 19: No. 1.) (B-F) Micropyle
and egg membranes of the cyclostome, Petromyzon planeri. (Slightly modified from
Calberia, Zeit. Wiss. Zool., 30.) (B) Mature, unfertilized egg. (C) Sperm passes
through the micropyle and enters the protoplasmic strand, P.S. (D) Higher power view
of sperm in protoplasmic strand; also observe that the egg is shrinking away from the
egg membrane, forming the perivitelline space. (E, F) Egg contracts away from the
egg membrane, leaving the egg free to revolve within the membrane.
sperm. Development is not normal and does not go beyond the larval con-
dition. Parthenogenetic merogony is the development of non-nucleate parts
of the egg which have been artificially activated. Artificial activation of non-
nucleate parts of the egg of the sea urchin, Arbacia, is possible by immersion
of these parts of the egg for 10 to 20 minutes in sea water, concentrated to
about one half of the original volume, or by the addition of sodium chloride
to sea water to bring it to a similar hypertonicity (Harvey, '36, '38, '51).
THEORIES OF FERTILIZATION
267
These parthenogenetic merogons develop to the blastula stage only. Gyno-
merogony is the parthenogenetic development of an egg fragment containing
the egg pronucleus.
J. Theories of Fertilization and Egg Activation
Boveri, T., 1887, 1895. In somatic mitoses, the division center or centro-
some is handed down from cell to cell. In the development of the female
gamete, the division center degenerates or becomes physiologically incapable
of continuing the division of the egg either before or after the maturation
divisions. The mature egg thus contains all the essentials for development
other than a potent division center. The sperm, on the other hand, lacks the
REFRACTIVE SUBSTANCE CHROMOSOMES
WALL OF NUCLEUS
SUPERNUMERARY SPERM
•■^v,-.- •;^;;:-; R
G.
Fig. 135. Fertilization phenomena in the pigeon. (After Harper, '04.) (A) Germinal
vesicle of late ovarian egg. The chromatin material is shown in the center of the vesicle;
the nuclear wall is beginning to break down. (B) Spindle of first maturation division.
Egg just ovulated and entering the oviduct. Sperm enters the egg at this time. (C)
Second polar spindle and first polar body. (D) First and second polar bodies; egg
pronucleus reorganizing. (E) Two pronuclei approaching, preparatory to fusion. Sperm
nucleus to the left. (F) Two pronuclei fusing. (G) Accessory sperm nuclei to the
left of this figure; fusion nucleus to the right.
268 FERTILIZATION
H CLEAVAGE
--.•,'■:• . •. U'\:'!llJ^^ POLAR BODIES
\* • * .• • . • • •^^ • ^, • . • -
">•,•• . • • • *^"; — : — ; • • * • . . . .'/
^s; '.'••*.•:.*•.•*•'•*.•.* • •.•■> •. •.; '^ — M ARG I NAL ZON
N • * , • . ■
^••» -• .-
^. •••*•• • V^
n
Fig. 136. Organization of germinal disc of the Echidna egg following fertilization.
(After Flynn and Hill, '39.)
cytoplasmic conditions necessary for development, but possesses an active
division center which it introduces into the egg at fertilization. Fertilization,
therefore, restores the diploid number of chromosomes to the egg and intro-
duces an active division center.
Loeb, J., '13. Loeb believed that two factors were involved in egg activa-
tion: (a) Superficial cytolysis of the egg cortex which leads to a sudden in-
crease in the oxidation processes of the egg, and (b) a factor which corrects
cytolysis and excess oxidation, thus restoring the egg to normal chemical
conditions. He placed great emphasis on superficial cytolysis of the cortex
with the resultant elevation of the fertilization membrane.
Loeb suggested that in normal fertilization the sperm brings in a lytic prin-
ciple which brings about cortical cytolysis, and a second substance which
regulates oxidation.
For discussion of this theory, consult J. Brachet, '50, p. 138.
Bataillon, E., '10, '11, '13, '16. Like Loeb, Bataillon emphasized two
steps in the activation process of the egg: (a) First treatment, whether it is
the puncture of the frog's egg by a fine needle or the butyric acid treatment
of the egg of the sea urchin, according to the method of Loeb, causes; ( 1 ) ele-
vation of fertilization membrane and the excretion of toxic substances from
the egg, and (2) the formation of a monaster, (b) Second treatment, whether
THEORIES OF FERTILIZATION
269
it is blood, in the case of the frog, or hypertonic sea water, as used by Loeb
in the sea-urchin egg, introduces a catalyzer which converts the monaster
into an amphiaster, and in this way renders the egg capable of cleavage.
Bataillon placed great emphasis upon the exudation (excretion) of sub-
stances into the perivitelline space and the elevation of the fertilization mem-
brane. He believed that the unfertilized egg was inhibited because of an
accumulation of metabolic products and that activation or fertilization led
to a release of these substances to the egg's exterior.
For discussion, consult Wilson, '25, p. 484; J. Brachet, '50, p. 144.
Lillie, F. R., '14, '19. This author postulated that a substance, fertilizin,
carried in the cortex of the egg, exerts two kinds of actions in the activation
process: (1 ) An activating, attracting, and agglutinating action on the sperm,
and (2) an activating effect on the egg itself. In essence, the egg is self-
fertilizing, for the fertilizing substance is present in the egg. The procedure
is somewhat as follows: At the period optimum for fertilization, inactive
fertilizin (i.e., inactive from the viewpoint of possessing the ability to activate
the egg) is produced by the egg. Released into the surrounding water, it
activates, attracts, and agglutinates the sperm at the egg's surface. As the
sperm touches the egg, it unites with a part of the fertilizin molecule. The
ENTRANCE
OF SPERMJ
FIRST
MATURATION
DIVISION-
SECOND
MATURATION
DIVISION —
FIRST
MATURATION
DIVISION —
ENTRANCE
OF SPERM
SECOND
MATURATION
DIVISION — '
FIRST
MATURATION
D IVISION-
SEGOND
MATURATION
DIVISION-
ENTRANCE
OF SPERM
Fig. 137. Maturation divisions of the oocyte relative to time of sperm entrance. (A)
Sperm enters the primary oocyte before maturation divisions. In some, e.g., Nereis,
Thalassema, Ascaris, Platynereis, Myzostoma, etc., the sperm enters before the germinal
vesicle breaks down; in Styela, Chaetopterus, pigeon, etc., the first maturation spindle is
formed or forming; in the dog, the condition is somewhat similar to Nereis, Ascaris, etc.
(B) Sperm enters the egg after first maturation division, i.e., in secondary oocyte stage
{Asterias (starfish), Amphioxus, hen, rabbit, man, frog, salamander, newt, most verte-
brates). (C) Sperm enters the egg after maturation divisions are completed, i.e., in
the mature egg (Arbacia and other sea urchins; possibly in monotreme. Echidna, on
occasion).
270
FERTILIZATION
Fig. 138. Polyspermy in the European newt, Triton, f After Fankhauser, '48.) (A)
Ten minutes after insemination at 23° C. Metaphase of second maturation division; four
sperm have entered the egg, one of which is at the vegetal pole of the egg, and another
between the two poles of the egg. (B) One hour and 30 minutes; second polar body
given off; small egg pronucleus moves toward nearest sperm nucleus. The latter will
become the principal sperm nucleus. Observe that accessory sperm nuclei are enlarging
and a sperm aster is developed relative to each. (C) Two hours and 30 minutes. Egg
and principal sperm pronuclei in contact; maximum development of sperm asters. (D)
Three hours. Fusion of egg pronucleus and principal sperm pronucleus. Accessory sperm
nucleus nearest to fusion nucleus shows signs of degeneration. Accessory sperm asters
remain undivided, while principal sperm aster has formed an amphiaster. (E) Three
hours and 30 minutes. Metaphase of first cleavage; all accessory sperm nuclei degener-
ating. (F) Four hours. Early telophase of first cleavage; remnant of accessory nuclei
being pushed out of animal pole region by amphiastei and spindle of first cleavage division.
fertilizin molecule plus the sperm then have the ability to unite with an egg
receptor, and the union of the fertilizin-sperm complex with the egg receptor,
releases the activating principle within the egg, which spreads "with extreme
rapidity" around the egg cortex. The activating principle activates the egg as
a whole, setting it in motion toward development. It is thought to work espe-
cially upon the cortex of the egg, producing cortical changes, including the
formation of a fertilization membrane. Further, it agglutinates or immobilizes
all other sperm around the egg. Consequently, polyspermy may be hindered
by this agglutination effect and by the fertilization membrane. In regard to
polyspermy, Lillie also postulated another substance, antifertilizin, within the
egg which unites with the remaining fertilizin molecules in the egg the instant
that one sperm has made successful union with a molecule of fertilizin, thus
preventing other sperm from entering the egg.
For discussion, see J. Brachet, '50, p. 143; Dalcq, '28.
THEORIES OF FERTILIZATION 271
Lillie, R. S., '41. Like Loeb, R. S. Lillie conceived of cortical changes as
being the main aspect of activation, particularly changes such as a decrease
in viscosity which permits interaction of various substances which normally
are kept separated in the unactivated egg. Lillie's hypothesis may be stated
as follows: An activating substance, comparable to a growth hormone or
auxin, is formed in the egg. This substance may be called (A). The forma-
tion of (A) results from the interaction of two substances, (S) and (B),
present in low concentrations in the egg. One of these substances, (S), is
synthesized in the egg by treating the egg in various ways, such as immersion
in sea water in the presence of oxygen. The other substance, (B), is freed
from pre-existing combination by a simple splitting (hydrolytic) process ini-
tiated or catalyzed by acid. This reaction is independent of oxygen. The
union of the two substances, (S) and (B), forms the activating substance,
(A). Lillie thus believes in a single factor as the initiator of development.
Complete activation of the egg results when (A) is produced in adequate
concentration; partial activation occurs when it is present in quantity below
the optimum concentration.
For discussion, see Brachet, J., '50, p. 141.
Heilbrunn, L. V., '15, '28, '43. This author believes that an increase in
viscosity with resultant coagulation or gelation of egg cytoplasm is involved
directly with the initiation of development. Heilbrunn regards this gelation
process to be similar to the clotting of blood. He also regards calcium as
the main agent in bringing about this effect, and therefore believes calcium
to be concerned directly with egg activation. According to this view, calcium
is bound to the proteins localized in the egg cortex. At the time of activation,
artificially or by sperm contact, part of this calcium is liberated which in turn
produces a coagulation of the cytoplasm, initiating development. Dalcq and
his associates also have emphasized the importance of calcium in the acti-
vation process.
For discussion, see Brachet, J., '50, p. 146; Dalcq, '28; Runnstrom, '49.
Runnstrom, J., '49. Runnstrom more recently has contended that an in-
hibitor of proteolytic enzymes may be present in the vitelline membrane and
cortex of the egg. He assumes that the inhibitor, possibly fertilizin, may be
identical with a heparin-like substance. He further assumes that the inhibitor
is bound to a kinase and is released when protein substances associated with
the sperm unite with the inhibitor. "A kinase acting on a proenzyme may then
be released"; the latter, i.e., the kinase, acts upon the proenzyme in the cortex
of the egg, giving origin to an enzyme or enzymes which initiate development.
Runnstrom's position in essence is a modern statement of the inhibition theory
of F. R. Lillie (see J. Morphol., vol. 22).
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PART III
Tne Development or Primitive
EmDryonic Body Form
The general procedures leading to the development of primitive embryonic body form
in the chordate group of animals are:
(1) Cleavage. Cleavage is the division of the egg into progressively smaller cellular
units, the blastomeres (Chap. 6).
(2) Blastulation. Blastulation results in the formation of the blastula. The blastula
is composed of a cellular blastoderm in relation to a fluid-filled cavity, the blaslocoel.
The blastoderm of the late blastula is composed of neural, epidermal, notoohordal,
mesodermal, and entodermal major presumptive organ-forming areas. In the phylum
Chordata, the notochordal area is the central region around which the other areas are
oriented (Chap. 7). The major presumptive organ-forming areas of the late blastula
exist in various degrees of differentiation (Chap. 8).
(3) Castrulation. This is the process which effects a reorientation of the presumptive
organ-forming areas and brings about their axiation antero-posteriorly in relation to the
notochordal axis and the future embryonic body (Chap. 9). During gastrulation the
major organ-forming areas are subdivided into minor areas or fields, each field being
restricted to the development of a particular organ or part. (Pp. 378, 446, 447.
(4) Following gastrulation, the next step in the development of embryonic body form
is tubulation and extension of the major organ-forming areas (Chap. 10).
(5) As tubulation and extension of the organ-forming areas is effected, the basic or
fundamental conditions of the future organ systems are established, resulting in the
development of primitive body form. As the development of various vertebrate embryos
is strikingly similar up to this point, the primitive embryonic body forms of all vertebrates
resemble each other (Chap. 11).
In the drawings presented in Part III, the following scheme for designating the major
organ-forming areas existing within the three germ layers is adhered to:
EPIDERMAL ECTODERM
NEURAL ECTODERM
MESODERM
ENTODERM
NOTOCHORO
PRE-CHORDAL PLATE
PERIBLAST
277
I
cleavage (Segmentation) and Blastulation
A. General considerations
1. Definitions
2. Early history of the cleavage (cell-division) concept
3. Importance of the cleavage-blastular period of development
a. Morphological relationships of the blastula
b. Physiological relationships of the blastula
1) Hybrid crosses
2) Artificial parthenogenesis
3) Oxygen-block studies
4. Geometrical relations of early cleavage
a. Meridional plane
b. Vertical plane
c. Equatorial plane
d. Latitudinal plane
5. Some fundamental factors involved in the early cleavage of the egg
a. Mechanisms associated with mitosis or cell division
b. Influence of cytoplasmic substance and egg organization upon cleavage
1) Yolk
2) Organization of the egg
c. Influence of first cleavage amphiaster on polyspermy
d. Viscosity changes during cleavage
e. Cleavage laws
1 ) Sach's rules
2) Hertwig's laws
6. Relation of early cleavage planes to the antero-posterior axis of the embryo
B. Types of cleavage in the phylum Chordata
1. Typical holoblastic cleavage
a. AmphioxLis
b. Frog (Rana pipiens and R. sylvatica)
c. Cyclostomata
2. Atypical types of holoblastic cleavage
a. Holoblastic cleavage in the egg of the metatherian and eutherian mammals
1 ) General considerations
2) Early development of the rabbit egg
a) Two-cell stage
b) Four-cell stage
c) Eight-cell stage
d) Sixteen-cell stage
279
280 CLEAVAGE (SEGMENTATION) AND BLASTULATION
e) Morula stage
f) Early blastocyst
3) Types of mammalian blastocysts (blastulae)
b. Holoblastic cleavage of the transitional or intermediate type
1) Anihy stoma maciilutuin (punctatum)
2 ) Lepidosiren paradoxa
3 ) Nee turns maculosus
4) Acipenser stiirio
5 ) A mia calva
6) Lepisosteus (Lepidosteus) osseus
7) Gyinnophionan amphibia
3. Meroblastic cleavage
a. Egg of the common fowl
1 ) Early cleavages
2) Formation of the periblast tissue
3) Morphological characteristics of the primary blastula
4) Polyspermy and fate of the accessory sperm nuclei
b. Elasmobranch fishes
1 ) Cleavage and formation of the early blastula
2) Problem of the periblast tissue in elasmobranch fishes
c. Teleost fishes
1) Cleavage and early blastula formation
2) Origin of the periblast tissue in teleost fishes
d. Prototherian Mammalia
e. Cleavage in the California hagfish, Polistotrema (Bdellostoma) stouti
C. What is the force which causes the blastomeres to adhere together during early
cleavage?
D. Progressive cytoplasmic inequality and nuclear equality of the cleavage blastomeres
1. Cytoplasmic inequality of the early blastomeres
2. Nuclear equality of the early blastomeres
E. Quantitative and qualitative cleavages and their influence upon later development
A. General Considerations
1. Definitions
The period of cleavage (segmentation) immediately follows normal fertili-
zation or any other means which activates the egg to develop. It consists of
a division of the entire egg or a part of the egg into smaller and smaller cellular
entities. In some species, however, both chordate and non-chordate, the early
cleavage stages consist of nuclear divisions alone, to be followed later by the
formation of actual cell boundaries (fig. 62). The cells which are formed
during cleavage are called blastomeres.
As cleavage of the egg continues, the blastular stage ultimately is reached.
The blastula contains a cavity or blastocoel together with an associated layer
or mass of cells, the blastoderm. The blastula represents the culmination and
end result of the processes at work during the cleavage period. Certain aspects
and problems concerned with blastulation are considered separately in the
following chapter. However, the general features of blastular formation are
described here along with the cleavage phenomena.
GENERAL CONSIDERATIONS 281
2. Early History of the Cleavage (Cell-division) Concept
An initial appreciation of the role and importance of the cell in embryonic
development was awakened during the middle period of the nineteenth cen-
tury. It really began with the observations of Prevost and Dumas in 1824 on
the cleavage (segmentation) of the frog's egg. The latter observations repre-
sented a revival and extension of those of Swammerdam, 1738, on the first
cleavage of the frog's egg and of Spallanzani's description in 1780 of the first
two cleavage planes, "which intersect each other at right angles," in the egg
of the toad. Other studies on cleavage of the eggs of frogs, newts, and various
invertebrates, such as the hydroids, the starfish, and nematodes, followed the
work of Prevost and Dumas. The first reported cleavage of the eggs of a rabbit
was made in 1838-1839, a fish in 1842, and a bird in 1847. (See Cole, '30,
p. 196.) Newport, in 1854, finally founded the new preformation by showing
that the first cleavage plane in the frog's egg coincided with the median plane
of the adult body (Cole, '30, p. 196).
In the meantime, the minute structures of the bodies of plants and animals
were intensively studied, and in 1838-1839, the basic cellular structure of
living organisms was enunciated by Schleiden and Schwann. Following this
generalization, many studies were made upon the phenomenon of cell division
in plant and animal tissues. These observations, together with those made
upon the cleaving egg, established proof that cells arise only by the division
of pre-existing cells; and that through cell division the new generation is
formed and maintained. Thus it is that protoplasm, in the form of cells, as-
similates, increases its substance, and reproduces new cells. Life, in this
manner, flows out of the past and into the present, and into the future as a
never-ending stream of cellular substance. This idea of a continuous flow
of living substance is embodied well in the famed dictum of R. Virchow,
"Omnis Cellula e Cellula," published in 1858 (Wilson, E. B., '25, p. 114).
The consciousness of life at the cellular level acquired during the middle
period of the nineteenth century thus laid the groundwork for future studies
in cytology and cellular embryology. Much progress in the study of the cell
had been made since R. Hooke, in 1664, described the cells in cork. In
passing, it should be observed, that two types of cell division, direct and
indirect, were ultimately defined. For the latter, Flemming in 1882, proposed
the name mitosis, while the direct method was called amitosis.
3. Importance of the Cleavage-Blastular Period of
Development
The period of cleavage and blastular formation is a time of profound dif-
ferentiation as well as one of cell division. For, at this time, fundamental
conditions are established which serve the purposes of the next stage in de-
velopment, namely, gastrulation. Experimental embryology has demonstrated
282 CLEAVAGE (SEGMENTATION) AND BLASTULATION
that optimum morphological conditions must be elaborated during the cleav-
age phase of development along with the developing physiology of the blastula.
a. Morphological Relationships of the Blastula
There are two aspects to the developing morphology of the blastula, namely,
the formation of the blastoderm and the blastocoel.
During cleavage and blastulation, the structure of the blastoderm is elabo-
rated in such a manner that the major, presumptive, organ-forming areas of
the future embryonic body are segregated into definite parts or districts of
the blastoderm. The exact pattern of arrangement of these presumptive, organ-
forming areas varies from species to species. Nevertheless, for a particular
species, they are arranged always according to the pattern prescribed for that
species. This pattern and arrangement of the major, presumptive, organ-
forming areas permit the ordered and symmetrical migration and rearrange-
ment of these areas during gastrulation.
Similarly, the blastocoel is formed in relation to the blastoderm according
to a plan dictated by the developing mechanisms for the species. One of the
main functions of the blastocoel is to permit the migration and rearrangement
of the major, presumptive, organ-forming areas during gastrulation. Conse-
quently, at the end of the blastular period, the blastoderm and the blastocoel
are arranged and poised in relation to each other in such a balanced fashion
that the dramatic cell movements of gastrulation or the next period of devel-
opment may take place in an organized manner.
b. Physiological Relationships of the Blastula
The development of a normal-appearing, late blastula or beginning gastrula
in a morphological sense is no proof that proper, underlying, physiological
states have been established. A few examples will be given to illustrate this fact:
1) Hybrid Crosses. When the sperm of the wood frog, Rana sylvatica,
are used to fertilize the eggs of the ordinary grass frog, Rana pipiens, cleavage
and blastulation appear normal. However, gastrulation is abortive, and the
embryo soon dies (Moore, '41, '46, '47).
2) Artificial Parthenogenesis. In the case of many embryos, chordate and
non-chordate, in which the egg is stimulated to develop by means of artificial
activation, the end of the blastular stage may be reached, but gastrulative
processes do not function properly. A cessation of development often results.
3) Oxygen-block Studies. In oxygen-block studies, where the fertilized eggs
of Rana pipiens are exposed to increased partial pressures of oxygen from
the time of fertilization to the four- or eight-cell stage, the following cleavages
and the morphology of the blastula may appear normal, but gastrulation does
not occur. Similar oxygen-pressure exposures during the late blastular and
early gastrular stages have no effect upon gastrulation. This fact suggests that
GENERAL CONSIDERATIONS 283
important physiological events accompany the earlier cleavage stages of de-
velopment (Nelsen, '48, '49).
Aside from the foregoing examples which demonstrate that invisible changes
in the developing blastula are associated with morphological transformations
is the fact that experimental research has demonstrated conclusively that an
organization center is present in the very late blastula and beginning gastrula.
The organization center will be discussed later. However, at this point it is
advisable to state that the organization center is the instigator and the con-
troller of the gastrulative processes, and gastrulation does not proceed unless
it is developed.
The above considerations suggest that the period of cleavage and blastu-
lation is a period of preparation for the all-important period of gastrulation.
Other characteristics of this phase of development will be mentioned in the
chapter which follows.
4. Geometrical Relations of Early Cleavage
a. Meridional Plane
The meridional plane of cleavage is a furrow which tends to pass in a
direction which, if carried to completion, would bisect both poles of the egg
passing through the egg's center or median axis. The latter axis theoretically
passes from the midpolar region of the animal pole to the midpolar region
of the vegetal pole. The beginning of the cleavage furrow which follows the
meridional plane may not always begin at the animal pole (fig. 1400) al-
though in most cases it does (figs. 142A-C; 154A-C; 155A).
b. Vertical Plane
A vertical plane of cleavage is a furrow which tends to pass in a direction
from the animal pole toward the vegetal pole. It is somewhat similar to a
meridional furrow. However, it does not pass through the median axis of the
egg, but courses to one side of this axis. For example, the third cleavage planes
in the chick are furrows which course downward in a vertical plane; paral-
leling one of the first two meridional furrows (fig. 155C). (See also figs. 153D;
154E relative to the third cleavage furrows of the bony ganoid fishes, Amia
calva and Lepisosteus (Lepidosteus) osseus.)
c. Equatorial Plane
The equatorial plane of cleavage bisects the egg at right angles to the
median axis and halfway between the animal and vegetal poles. It is never
ideally realized in the phylum Chordata, and the nearest approach to it is
found, possibly, in one of the fifth cleavage planes of the egg of Amby stoma
maculatum (fig. 149F) and the first cleavage plane of the egg of the higher
mammals (fig. 145A).
284 CLEAVAGE (SEGMENTATION) AND BLASTULATION
d. Latitudinal Plane
The latitudinal plane of cleavage is similar to the equatorial, but it courses
through the cytoplasm on either side of the equatorial plane. For example,
the third cleavage planes of the egg of Amphioxus (fig. 1401) and of the frog
(figs. 141 E; 142F) are latitudinal planes of cleavage.
5. Some Fundamental Factors Involved in the Early
Cleavage of the Egg
a. Mechanisms Associated with Mitosis or Cell Division
There are two mechanisms associated with cleavage or cell division:
(1) that associated with the chromosomes and the achromatic (amphias-
tral) spindle, which results in the equal division of the chromosomes
and their distribution to the daughter nuclei, and
(2) the mechanism which enables the cytoplasm to divide.
In ordinary cell division or mitosis these two mechanisms are integrated
into one process. However, in embryonic development they are not always
so integrated. The following examples illustrate this fact: (1) In the early
development of insects, the chromatin materials divide without a correspond-
ing division of the cytoplasm (fig. 62). (2) During the early cleavage phe-
nomena of the elasmobranch fishes, the chromatin material divides before
corresponding cleavages of the cytoplasm appear (fig. 158A). (3) In the
later cleavage stages of teleost fishes, the peripheral cells of the blastoderm
fuse and form a continuous cytoplasm; within this cytoplasm the separate
nuclei continue to divide without corresponding cytoplasmic divisions and in
this way form the marginal syncytial periblast (fig. 159J, L, M).
On the other hand, cytoplasmic division may occur without a correspond-
ing nuclear division. This behavior has been illustrated in various ways but
most emphatically by the work of Harvey ('36, '38, '40, '51) which demon-
strates that non-nucleate parts of the egg may divide for a period without
the presence of a nucleus. (See, particularly, Harvey, '51, p. 1349.) Simi-
larly, in the early development of the hen's egg, a cytoplasmic furrow or
division occurs in the formation of the early segmentation cavity without
involving a nuclear division (fig. 156C, E). This type of activity on the
part of the cytoplasm illustrates the fact that the cytoplasm has a mechanism
for cell division independent of the nuclear mechanism. Lewis ('39) empha-
sizes the importance of the production of a superficial plasmagel "constriction
ring" which constricts the cytoplasm into two parts during ceil division.
b. Influence of Cytoplasmic Substance and Egg Organization upon
Cleavage
1) Yolk. Since the time of Balfour, much consideration has been given to
the presence or absence of yolk as a factor controlling the rate and pattern
GENERAL CONSIDERATIONS 285
of cleavage. Undoubtedly, in many instances, the accumulation of yolk ma-
terials does impede or alter the cleavage furrows, although it does not suppress
mitotic divisions of the nucleus as shown in the early cleavages in many
insects, ganoid fishes, etc. On the other hand, the study of cleavage phe-
nomena as a whole brings out the fact that other intrinsic factors in the
cytoplasm and organization of the egg largely determine the rate and planes
of the cleavage furrows.
2) Organization of the Egg. An illustration of the dependence of the
pronuclei and of the position of the first cleavage amphiaster upon the general
organization of the cytoplasm of the egg is shown in the first cleavage spindle
in Amphioxus and Styela. In the eggs of these species, the amphiaster of the
first cleavage always orients itself in such a way that the first cleavage plane
coincides with the median plane of the future embryonic body. The first
cleavage plane, consequently, divides the egg's substances into two equal
parts, qualitatively and quantitatively. The movements of the pronuclei and
the first cleavage amphiaster are correlated and directed to this end.
Various theories have been offered in the past to account for the migrations
of the pronuclei at fertilization and for the position of the first cleavage
amphiaster. All of them, however, are concerned with the cytoplasm of the
egg or its movements, which in turn are correlated with the organization of
the egg. (See Wilson, E. B., '25, p. 426.)
A second illustration of the dependence of the chromatin-amphiaster com-
plex on conditions in the cytoplasm is afforded by experiments of Hans
Driesch in 1891 on the isolation of the blastomeres of cleaving eggs of the
sea urchin. He found that the first cleavage of the egg occurred from the
animal to the vegetal pole, resulting in two blastomeres. Now, if these blas-
tomeres are shaken apart, the following cleavages in the isolated blastomeres
behave exactly as if the two blastomeres were still intact, indicating a definite
progression of the cleavage planes. That is, there is a mosaic of cleavage
planes determined in the cytoplasm of the early egg.
A third example of the influence of egg organization upon cleavage is
afforded by the egg of higher mammals. In this group, the first cleavage plane
divides the egg in many cases into a larger and a smaller blastomere. The
larger blastomere then begins to divide at a faster rate than the smaller
blastomere. This accelerated division is maintained in the daughter cells re-
sulting from the larger blastomere. Here, then, is an egg whose yolk material
is at a minimum. Nevertheless, the blastomeres which result from the first
cleavage are unequal in size, and the cellular descendants of one of these
blastomeres divide faster than the descendants of the other blastomere. Some
conditioning effect must be present in the egg's cytoplasm which determines
the size of the blastomeres and the rate of the later cleavages. Many other
illustrations might be given from the studies on cell lineage. However, the
conclusion is inevitable that under normal conditions the cause of the cleavage
286 CLEAVAGE (segmentation) and blastulation
pattern and the rate of blastomere formation is an internal one resident in
the organization of the egg and the peculiar protoplasmic substances of the
various blastomeres. This apparent fact suggests strongly that the egg in its
development "is a builder which lays one stone here, another there, each of
which is placed with reference to future development" (F. R. Lillie, 1895,
p. 46).
c. Influence of First Cleavage Amphiaster on Polyspermy
In figure 138 is shown the behavior of the sperm nuclei during fertiliza-
tion in the urodeie, Triton. This figure demonstrates that the developing first
cleavage amphiaster suppresses the development of the accessory sperm nuclei.
Similar conditions appear to be present in the elasmobranch fishes, chick,
pigeon, etc.
d. Viscosity Changes During Cleavage
"The viscosity changes that occur in the sea-urchin egg are probably typical
of mitosis in general. There is marked viscosity increase in early prophase,
then a decrease, and finally an increase just before the cell divides" (Heilbrunn,
'21 ). Similarly, Heilbrunn and W. L. Wilson ('48) in reference to the cleaving
egg of the annelid worm, Chaetopterus, found that during the metaphase of
the first cleavage the protoplasmic viscosity is low, but immediately preced-
ing cell division protoplasmic viscosity increases markedly.
e. Cleavage Laws
Aside from the factors involved in cleavage described above, other rules
governing the behavior of cells during division have been formulated. These
statements represent tendencies only, and many exceptions exist. "The rules
of Sachs and Hertwig must not be pushed too far" (Wilson, E. B., '25, p. 985 ) .
1) Sachs' Rules:
(a) Cells tend to divide into equal daughter cells.
(b) Each new cleavage furrow tends to bisect the previous one at right
angles.
2) Hertwig's Laws:
(a) The typical position of the nucleus tends to lie in the center of the
protoplasmic mass in which it exerts its influence.
(b) The long axis of the mitotic spindle typically coincides with the long
axis of the protoplasmic mass. In division, therefore, the long axis of
the protoplasmic mass tends to be cut transversely.
6. Relation of Early Cleavage Planes to the
Antero-posterior Axis of the Embryo
In the protochordate, Styela, the first cleavage plane always divides the
yellow and gray crescent material and other cytosomal substances into equal
GENERAL CONSIDERATIONS 287
right and left halves; it therefore achieves a sundering of the egg substances
along the future median plane of the embryo. The second cleavage plane
occurs at right angles to the first (Conklin, '05, a and b). This condition ap-
pears to be true of other ascidians, such as Ciona, Clavelina, etc. (Wilson,
E. B., '25, p. 1012). The behavior of the early cleavage planes is similar in
Amphioxus (Conklin, '32). Cleavage planes such as the foregoing, which
always divide the egg in a definite way have been described as "determinate
cleavage" (Conklin, 1897). Study figures 116, 132 and 167.
The first cleavage plane in the eggs of some frogs (e.g., Rana jusca and
Rana pipiens) shows a great tendency to bisect the gray crescent and thus
divide the embryo into right and left halves. However, unlike Styela, and
Amphioxus, the first plane is not definitely fixed; considerable deviation may
occur in a certain percentage of cases in any particular batch of eggs. In the
newt, Triturus viridescens, the first cleavage plane generally is at right angles
to the median plane of the future embryo (Jordan, 1893). In Necturus
maculosus the first cleavage plane may in some eggs coincide with the median
plane of the embryo, while the second cleavage plane may agree with this
plane in other eggs. In some eggs there is no correspondence between the first
two cleavage planes and the median plane of the embryo; however, the planes
always cut from the animal to the vegetal pole of the egg (Eycleshymer, '04).
In the teleost fish, Fundulus heteroclitiis, in the greater percentage of cases,
the long axis of the embryo tends to coincide with either the first or second
cleavage planes (Oppenheimer, '36). Other teleost fishes appear to be similar.
In the hen's egg the first cleavage plane may or may not lie in the future
median plane of the embryo (Olsen, '42).
In some species it appears that the unfertilized egg may possess bilateral
symmetry. For example, in the frog, Rana jusca, the point of sperm entrance
evidently has an influence in orienting the plan of bilateral symmetry, and,
as a result, the gray crescent appears opposite the point of sperm contact with
the egg. However, in Rana esculenta and in Discoglossus pictus, two other
anuran species, there is no constant relationship between the point of sperm
entry and the plan of bilateral symmetry of the egg (Pasteels, '37, '38). In
the latter cases, unlike, that of Rana jusca, the stimulus of sperm entry pre-
sumably does not influence the plan of bilateral symmetry which is determined
previous to sperm entrance.
It is to be noted that there is a strong tendency in many of the above species
for the first cleavage amphiaster to orient itself in such a manner as to coincide
with the median plane oj the embryo or to be at right angles to this plane.
This fact suggests that the first cleavage amphiaster is oriented in terms of
the egg's organization. It further suggests that the copulation paths of the
respective pronuclei, as they move toward each other, together with the re-
sulting cleavage path of the pronuclei (fig. 1 39), are conditioned by the inherent
organization of the egg's cytoplasm.
288 CLEAVAGE (SEGMENTATION) AND BLASTULATION
B. Types of Cleavage in the Phylum Chordata
Cleavage in the phylum Chordata often is classified as either holoblastic or
meroblastic. These terms serve a general approach to the subject but fail to
portray the varieties and problems of cleavage which one finds within the
phylum. Under more careful scrutiny, three main categories of cleavage types
appear with typical holoblastic cleavage occupying one extreme and typical
meroblastic cleavage the other, while between these two are many examples
of atypical or transitional cleavage types. Moreover, the phenomena of cleav-
age are variable, and while we may list the typical cleavage of any one species
as holoblastic, transitional or meroblastic, under certain modifying circum-
stances the cleavage pattern may be caused to vary.
Holoblastic cleavage is characterized by the fact that the cleavage furrows
bisect the entire egg. In meroblastic cleavage, on the other hand, the disc of
protoplasm at the animal pole only is affected, and the cleavage furrows cut
through this disc superficially or almost entirely. Superficial cleavage occurs
typically in certain invertebrate forms, particularly among the Insecta. How-
ever, in a sense, the very early cleavages in elasmobranch fishes, certain teleost
fishes, and in birds may be regarded as a kind of superficial cleavage.
1. Typical Holoblastic Cleavage
In typical holoblastic cleavage, the first cleavage plane bisects both poles
of the egg along the median egg axis, that is, the first plane of cleavage is
meridional. The second cleavage plane is similar but at right angles to the
first, thereby dividing the "germ" into four approximately equal blastomeres.
(See Sachs' rule (a), p. 286.) The third cleavage plane in typical holoblastic
cleavage occurs at right angles to the median axis of the egg and the foregoing
two meridional planes. (See Sachs' rule (b), p. 286.) As it does not cut along
the equatorial plane, but nearer the animal pole, it is described as a latitudinal
cleavage. Two meridional cleavage planes (see definition, p. 283) followed by
a latitudinal plane (see definition, p. 284) is the cleavage sequence charac-
teristic oj the first three cleavage planes of typical holoblastic cleavage. The
following chordate species exemplify typical holoblastic cleavage:
a. Amphioxns
In this cephalochordate there exists as typical a form of holoblastic cleavage
as is found anywhere in the phylum Chordata. The process of cleavage or
segmentation in Amphioxns has been described in the studies of four different
men; as such, these descriptions form four of the classics of embryonic study.
These studies were made by Kowalewski in 1867; Hatschek in 1881, English
translation, 1893; Cerfontaine, '06; and Conklin, '32. With the exception
of certain slight errors of observation and interpretation, Hatschek's work is
a masterpiece.
POLAR BODY MARKS
ANIMAL POLE
Q PRONUCLEUS
—COPULATION PATH
OF EGG PRONUCLEUS
ENTRANCE PATH
or SPERM PRONUCLEUS
COPULATION PATH
OF SPERM PRONUCLEUS
CLE AVA GE PATH
OF SPINDLE AND
PRONUCLEI
ME Dl AN AXIS OF EGG
c.
A M PH I ox us
POLAR BODY
Q PRONUCLEUS
COPULATION PATH
OF EGG PRONUC LEUS
MEDIAN AXIS OF EGG
OLAR VIEW OF
SPINDLE
CL E AVAGE PAT H
OF SPINDLE AND
PRONUCLEI
COPULATION PATH
OF SPERM PRONUCLEUS
ENTRANCE PATH
OF SPERM PRONUCLEUS
POLAR BODY
5 PRONUCLEUS
COPU LATION PATH
OF EGG PRONUCLEUS
COPULATION PATH
OF SPERM PRONUCLEUS
-CLEAVAGE PATH
OF SPINDLE AND
P RO N U C L E I
POLAR VIEW OF
SPINDLE
ENTRAN CE PATH
OF SPERM PRONUCLEUS
Fig 139 Penetration path of the sperm, copulation paths of the pronuclei the cleavage
path of the pronuclei, and first cleavage spindle. (A) Conditions such as fo""d .n the
urodele, Triton. (B) Conditions such as found in the protochordate, Styela. (C) Con-
ditions such as found in the protochordate, Amphioxus.
289
Fig. 140. (See facing page for legend.)
290
TYPES OF CLEAVAGE 291
The first cleavage furrow cuts through the egg along the median axis of
the egg, starting at the postero-ventral side of the egg (fig. 1400, E). It,
therefore, is a meridional plane of cleavage. The second cleavage plane cuts
at right angles to the first plane, producing four equal cells (fig. HOE, F).
The third cleavage involves four blastomeres. Its plane of cleavage is almost
equatorial but slightly displaced toward the animal pole, and therefore, more
truly described as latitudinal plane of cleavage. This cleavage plane divides
each of the four blastomeres into a smaller micromere at the animal pole
and a larger macromere at the vegetal pole. Eight blastomeres are thereby
produced (fig. 140G-I). In certain cases and at least in some varieties of
Amphioxus, the four micromeres may not be placed exactly above the macro-
meres, but may be rotated variously up to 45 degrees, forming a type of
spiral cleavage (fig. 140J). (See Wilson, E. B., 1893.) The fourth cleavage
planes are meridional, and all of the eight cells divide synchronously. The
result is sixteen cells, eight micromeres and eight macromeres (fig. 140J, K).
The fifth planes of cleavage are latitudinal and simultaneous (fig. 140L). The
Fig. 140. Early cleavage and blastulation in Amphioxus. (K after Hatschek, 1893; all
others after Conklin, '32.) (A) Median section through egg in the plane of bilateral
symmetry, one hour after fertilization. Second polar body at animal pole; egg and sperm
pronuclei in contact in cytoplasm containing little yolk. Approximate antero-posterior
axis shown by arrow. D. and V. signify dorsal and ventral aspects of future embryo.
MS. = mesodermal crescent. (B) Sperm and egg nuclei in contact surrounded by
astral rays. Sperm remnant, SR., shown at right. (C) First cleavage spindle in postero-
ventral half of the egg (see fig. 139C). Arrow shows median plane of future embryo
and also the median plane of the egg. Observe that the spindle is at right angles to this
plane of the egg. (D) Egg in late anaphase of first cleavage. Cleavage furrow deeper
at postero-ventral side of the egg. MS. = mesodermal crescent. (E) Two-cell stage.
Arrow shows median plane of embryo. MS. = mesodermal crescent now bisected into
two parts. (F) Four-cell stage at conclusion of second cleavage, IVi hours after fertili-
zation. (G) Four-cell stage at beginning of third cleavage. Posterior cells. P., slightly
smaller than anterior cells. (H) Animal pole above, vegetal pole below. Cell at left is
posterior, the one at right anterior. Spindles show third or horizontal cleavage plane.
(I) Eight-cell stage, IVi hours after fertilization. Posterior cells, below at right, contain
most of mesodermal crescent. Arrow denotes antero-posterior axis of embryo. (J) Late
anaphase of fourth cleavage. (K) Sixteen-cell stage viewed laterally. There are eight
micromeres and eight macromeres. (L) Side view of 32-cell stage, 3'/4 hours after fer-
tilization. Every nucleus in anaphase or metaphase of sixth cleavage. (M) Left side of
64-ceIl stage. Arrow denotes antero-posterior axis of embryo. (N) Blastula, VA hours
after fertilization. Animal pole above, vegetal below. Entoderm cells at vegetative pole
are larger, are full of yolk, and are dividing. Blastocoel is large. (O) Eighth cleavage
period with more than 128 cells, 4 hours after fertilization. Antero-posterior axis of future
embryo shown by arrows. Polar body indicates animal pole of original egg. Dorsal and
ventral aspects indicated by D. and V., respectively. MS. — mesodermal crescent. (P)
Section of blastula, AVi hours after fertilization. Entoderm cells have nuclei shaded with
lines. (Q) Section of blastula, 5'/2 hours after fertilization. (R) Section of blastula,
6 hours after fertilization. Mesoderm cells lighter and on each side of entoderm cells.
Section nearly transverse to embryonic axis. (S) Section of blastula at stage of pre-
ceding but in a plane as in (Q). MS. = mesodermal crescent. (T) Pear-shaped, late
blastula. Pointed end is mesodermal; entoderm cells have cross-lined nuclei. D. and V.
indicate dorsal and ventral aspects of embryo. See also fig. 167.
292 CLEAVAGE (SEGMENTATION) AND BLASTULATION
plane nearest the animal pole divides each of the eight micromeres into an
upper and a lower micromere, while the plane which furrows the eight macro-
meres divides each into upper and lower macromeres. Thirty-two cells are,
thus, the result of the fifth cleavage planes. The lowest of the macromeres
are larger and laden with yolk material (fig. 140L). The sixth cleavage planes
are synchronous and approximately meridional in direction in all of the 32
cells, resulting in 64 cells (fig. MOM). The blastocoelic cavity is a con-
spicuous area in the center of this cell mass and is filled with a jelly-like
substance (fig. 140N). Study also figure 167.
When the eighth cleavage furrows occur, the blastocoel contained within
the developing blastula is large (fig. 140?). As the blastula continues to
enlarge, the blastocoel increases in size, and the contained jelly-like substance
assumes a more fluid condition (fig. 140O-S). The fully formed blastula is
piriform or "pear-shaped" (fig. MOT). (See Conklin, '32.)
The cleavage pattern of the urochordate. Styela partita, is somewhat similar
to that of Amphioxus, but considerable irregularity may exist after the first
three or four cleavages. In Styela the ooplasm of the egg contains differently
pigmented materials, and yellow and gray crescentic areas are visible at the
time of the first cleavage. (See fig. 132.) These different cytoplasmic areas
give origin to cells which have a definite and particular history in the embryo.
Observations devoted to the tracing of such cell histories are grouped under
the heading of "cell lineage." Cell-lineage observations are more easily made
in the eggs of certain species because of definitely appearing cytoplasmic
areas, where colored pigments or other peculiarities associated with various
areas of the egg make possible a ready determination of subsequent cell
histories. The general organization of the egg of Amphioxus, regardless of the
fact that its cytoplasmic stuffs do not have the pigmentation possessed by the
egg of Styela, appears similar to that of the latter (cf. figs. MOA; 167A).
(See Conklin, '32.)
b. Frog (Rana pipiens and R. sylvatica)
The egg of the frog is telolecithal with a much larger quantity of yolk than
is found in the egg of Amphioxus. The pattern of cleavage in the frog, there-
fore, is somewhat less ideally holoblastic than that of Amphioxus.
The first cleavage plane of the frog's egg is meridional (figs. MIC; M2A-C).
It occurs at about three to three and one-half hours after fertilization at ordi-
nary room temperature in Rana pipiens. It begins at the animal pole and
travels downward through the nutritive or vegetal pole substance, bisecting
both poles of the egg. In the majority of eggs, it bisects the gray crescent. (See
p. 287.) The second cleavage plane divides each of the first two blastomeres
into two equal blastomeres; its plane of cleavage is similar to the first cleavage
plane but is oriented at right angles to the first plane (figs. MID; 142D-E).
The upper, animal pole end of each of the four blastomeres contains most
TYPES OF CLEAVAGE
293
of the dark pigment, while in the lower portion of each blastomere the yellow-
white yolk is concentrated. As a rule, the substance of the gray crescent is
found in two of the blastomeres; the four blastomeres under the circumstances
are not qualitatively equal.
The third or latitudinal cleavage plane is at right angles to both of the
foregoing and somewhat above the equator, dividing each of the four blas-
tomeres into an animal pole micromere and a larger vegetal pole macromere
-ANIMAL POLE
Fig. 141. Normal development of Rana sylvatica. (A) Egg at fertilization. (B)
Formation of gray crescent, sharply defined at one hour after sperm entrance. (C) First
cleavage furrow meridional. (D) Second cleavage furrow meridional. (E) Third cleav-
age furrows, latitudinal in position. Four micromeres above and four macromeres below.
(F) Fourth set of cleavage furrows, meridional in position, although some variation
may exist and vertical furrows may occur. (G-1) Later cleavage stages. Pigmented
pole cells become very small, and pigmented cells creep downward over vegetative pole
area. (J) Appearance of dorsal blastoporal lip. (K) Blastoporal lips spread laterally,
forming a broad. V-shaped structure. Pigmented cells proceed toward blastoporal lips.
(L) Yolk-plug stage of gastrulation. (After Pollister and Moore, '37.)
Fig 142 Early development of Rana pipiens. (A) Polar view of first cleavage. The
animal pole is considerably flattened at this time and tension lines are visible extending
outward along either side of the furrow. (About VA hours after fertilization in the
laboratory, room temperature 20 to 22° C.) (B) First cleavage furrow a little later
(C) First furrow proceeds slowly though the yolk of vegetative (vegetal) pole. (U)
Second cleavage furow meridional and at right angles to the first furrow. (E) Four-cell
stage, view from animal pole. Observe short "cross furrow" connecting first and second
cleavage planes. (F) Fourth cleavages meridional or nearly so. Taken from egg spawned
in nature. Considerable variation may exist. Some cleavages may be vertical and not
meridional. (G-1) Later blastula stage. (J) Stage just before appearance of dorsal
lip of blastopore. (K) Dorsal lip of blastopore. (L) Yolk-plug stage of gastrulation.
294
TYPES OF CLEAVAGE
295
Fig.' 143. Stages in formation of the blastocoel in the cleaving egg of Rana pipiens
taken from stained sections. (A) Eight-cell stage; blastocoel appearing particularly
between micromeres. The macromeres form the floor of the developing blastocoel.
(B, C) Later stages of formation of the blastocoel. Blastocoel situated at animal pole.
Yolk-laden, vegetal pole cells form floor of the blastocoel while smaller, animal pole
cells form its sides and roof. (D) Blastocoel at beginning of gastrulation.
(figs. 141 E; 142F). The fourth set of cleavages, both in Rana sylvatica and
Rana pipiens, in eggs that are spawned naturally, are oriented in a meridional
direction (figs. 141F; 142F). These furrows first involve only the animal
pole micromeres, but later meridionally directed furrows begin to develop in
the yolk-laden macromei-es (figs. 141F; 142F).
The cleavage of the various blastomeres of the egg to this point tends to
be synchronous, and is comparable to that of Amphioxus. However, from
this time on asynchronism is the rule and different eggs in a given lot manifest
various degrees of irregularity. Exceptional eggs may occur in which the next
two cleavage planes resemble the fourth and fifth series of planes in Amphioxus.
But, on the whole, the micromeres divide faster than do the macromeres and
thus give origin to many small, heavily pigmented, animal pole cells, while
the macromeres or vegetal pole cells are larger and fewer in number. The
smaller pigmented cells creep downward gradually in the direction of the
larger vegetal pole cells (figs. 141G-I; 142G-K). The latter migration of the
296
CLEAVAGE (SEGMENTATION) AND BLASTULATION
Fig. 144. Cleavage in the rabbit egg. (After Gregory, '30.) (A) One-ceil stage. (B)
Two primary blastomeres, one larger than the other. (C) Eight-cell stage. (D) Sixteen-
cell stage. (E) Morula stage of 32 cells. (F) External view of stage approximating
that in (G). (G) Inner cell mass and blastocoeiic cleft showing in embryo, about IV2
hours after copulation. (H) inner cell mass and blastocoeiic space in embryo, approxi-
mately 90 hours after copulation. Entoderm cells have not yet appeared.
pigment cells is marked toward the end of the blastular period and during
gastrulation. Cf. figs. 141 H-L; 142H-L.
The blastocoel within the mass of blastomeres of the cleaving egg of the
frog forms somewhat differently from that in Amphioxus in that the cavity
arises nearer the animal pole. The smaller micromeres of the animal pole,
therefore, are more directly involved than the macromeres of the vegetal pole.
Beginning at the eight-cell stage, a spatial separation is present between the
four micromeres at the animal pole. The floor of this space or beginning
blastocoel is occupied by the yolk-laden macromeres (fig. 143 A, B). As
development proceeds, this eccentricity of position is maintained, and the
TYPES OF CLEAVAGE 297
blastocoel or segmentation cavity becomes an enlarged space filled with fluid,
displaced toward the animal pole (fig. 143B-D). The contained fluid of the
blastocoel of amphibia is alkaline, according to the work of Buytendijk and
Woerdeman ('27), having a pH of 8.4 to 8.6.
For general references regarding cleavage in the frog, see Morgan (1897);
Pollister and Moore ('37); Rugh ('51); and Shumway ("40).
c. Cyclostomata
Cleavage in the eggs of the genera of the family, Petromyzonidae, resembles
very closely that of the frog. Further description will not be included. How-
ever, in the marine cyclostomes or hagfishes, the cleavage phenomena are
strongly meroblastic. (See description of the marine cyclostomatous fish at
the end of this chapter. )
2. Atypical Types of Holoblastic Cleavage
A variety of cleavage types is found in the eggs of many vertebrate species
which do not follow the symmetrical, ideally holoblastic pattern exhibited in
the egg of Amphioxus or even in the egg of the frog. In all of these atypical
forms the entire egg ultimately is divided by the cleavage furrows with the
possible exception of the eggs of the bony ganoid fishes, Amia calva and
Lepisosteus osseiis (and also in certain of the gymnophionan amphibia). In
the latter species the yolk material at the yolk-laden pole of the egg is invaded
by isolated nuclei which form a syncytium in the yolk material. Eventually
this yolk material is formed into definite cells and incorporated into the gut
area of the embryo.
a. Holoblastic Cleavage in the Egg of the Metatherian and Eutherian
Mammals
1) General Considerations. The eggs of metatherian and eutherian mam-
mals are the most truly isolecithal of any in the phylum Chordata. They have
also a cleavage pattern distinct from other chordate eggs. The first cleavage
plane in the higher mammalian egg very often divides the egg into a larger
and a slightly smaller blastomere (figs. 144B; 145A, F; 1478, J). As shown
by the work of Heuser and Streeter ('41 ) in the pig, the smaller blastomere
is destined to give origin to the formative tissue of the embryo's body, while
the larger blastomere gives rise to auxiliary tissue, otherwise known as the
nourishment-obtaining or trophoblast tissue (fig. 145A-E). The smaller blas-
tomere also contributes some cells to the trophoblast tissue. A similar con-
dition of progressive specialization of the smaller and the larger blastomeres
of the two-cell stage, producing two classes of cells, the one mainly formative
and the other auxiliary or trophoblast, is present in the monkey (Heuser and
Streeter, '41 ) and probably in other higher mammals as well.
If one compares the early history of these two blastomeres with the early
EMBRYONIC DISC
^^^^^^P-^^^^^.
Fig. 145. Early development of the pig. (A-E) Fate of the first two blastomeres.
The larger blastomere of the two-cell stage gives rise to trophoblast tissue, whereas from
the smaller blastomere, formative cells and trophoblast cells arise. (After Heuser and
Streeter, Carnegie Inst., Washington, Contrib. Embryol., 20:3.) (F) Section of two-
cell stage. Specimen secured from oviduct of sow, killed two days, 3'/2 hours after ovula-
tion. (G) Section of four-cell stage. Age is approximately IVi days. (H) Sixteen-cell
stage, drawn from unsectioned specimen, probably 3'/2 days old. (I) Blastular stage.
Specimen secured from sow, 4% days after copulation. (J-L) Stages showing the for-
mation of the blastocoel. (J) About 4^/4 days after copulation. (K) Six days, 1%
hours after copulation. (L) Six days, 20 hours after copulation. (M) Beginning dis-
integration of trophoblast cells over the inner cell mass and separation of entoderm cells
from the inner cell mass. (N) Trophoblast cells over inner cell mass almost absent,
entoderm forms a definite layer below inner cell mass. (O) Trophoblast cells almost
absent over the embryonic disc; entoderm layer continuous. (P-R) Stages shown in
(M), (N), (O) respectively, showing the whole blastocyst. In (Q) the entoderm cells
are shown migrating outward to line the cavity of the blastocyst.
298
TYPES OF CLEAVAGE 299
development of other vertebrate eggs, it is apparent that the nutritive (tropho-
blast) cells are located at one pole, while the formative cells of the embryo
are found toward the opposite pole. The latter condition resembles the ar-
rangement of formative cells and nutritive substances in teleost and elasmo-
branch fishes, in reptiles, birds, and prototherian mammals. This comparison
suggests, therefore, that the first cleavage plane in the higher mammals cuts
at right angles to the true median axis of the egg (cf. fig. 145A-E). If this
is so, the first cleavage furrow should be regarded as latitudinal and almost
equatorial, and the two blastomeres should theoretically be arranged as shown
in figure 145 A.
The determination of the animal and vegetal poles of the egg in this group
of vertebrates is difficult by any other means than that suggested above. In
many lower chordate species the polar bodies act as indicators of the animal
pole, for they remain relatively fixed at this pole of the egg (e.g., Styela,
Amphioxus, etc.). But in higher mammals the polar bodies "are never sta-
tionary, and there is evidently much shifting" (Gregory, 30, relative to the
rabbit), although in the two-cell stage, the polar bodies often appear between
the two blastomeres at one end. It appears in consequence that the fates of
the two blastomeres of the two-cell stage serves as a better criterion of egg
symmetry at this time than is afforded by the polocytes. According to this
view, the smaller blastomere should be regarded as indicating the animal
pole, while the larger blastomere signifies the vegetative pole (fig. 145A).
With respect to the statements in the previous paragraph, it is well to
mention that Nicholas and Hall ('42) reported that two early embryos may
be produced by isolating the blastomeres of the two-cell stage in the rat, and
one embryo is produced as a result of experimental fusion of two fertilized
eggs. These experimental results suggest that the potencies of the two blas-
tomeres are not so rigidly determined that two different kinds of development
result when the blastomeres are isolated. In normal development, however,
it may be that the innate potencies of the two blastomeres are not precisely
the same. The ability to regulate and thus compensate for lost substances
shown by many different types of early embryonic blastomeres, may explain
the production of two early embryos from the separated blastomeres of the
two-cell stage.
The second cleavage divides the larger blastomere into two cells, giving
origin to three cells. Then the smaller blastomere divides, forming four cells.
Cleavage from this time on becomes irregular, and five-, six-, seven-, eight-,
etc., cell stages are formed.
Segmentation of the higher mammalian egg, therefore, is unique in its
cleavage pattern. The synchrony so apparent in the egg of Amphioxus is
lacking. Irregularity and individuality is the rule, with the auxiliary or nutritive
pole cells dividing faster than those of the formative or animal pole cells.
Moreover, the blastomeres not only show their apparent independence of
300 CLEAVAGE (SEGMENTATION) AND BLASTULATION
each other through their irregularity in division but also by their tendency to
shift their position with respect to one another. One function of the zona
pellucida during the early cleavage period appears to be to hold "the blasto-
meres together" (Heuser and Streeter, '29). From the 16-cell stage on, the
trophoblast or auxiliary cells begin to form the blastocoelic space, first by a
flattening process and later by the formation of a cleft among the cells (fig.
145D). The growing presence of the blastocoel consigns the formative or
inner cell-mass cells to one pole of the blastula (fig. 145J-L). A blastocoelic
space thus is formed which is surrounded largely by trophoblast or nutritive
cells (fig. 145K, L). The blastular stage of development of the mammalian
embryo is called the blastocyst.
2) Early Development of the Rabbit Egg. The following brief description
pertains to the early development of the rabbit egg up to the early blastocyst
condition.
a) Two-cell Stage. The two-cell stage is reached about 22 to 24 hours
after mating or 10 to 12 hours after fertilization. One cell has a tendency to
be slightly larger than the other (fig. 144B). (Cf. also figs. 145A, F; 146A;
147B, J.)
b) Four-cell Stage. This stage is present about 24 to 32 hours after
mating or 13 to 18 hours after fertilization. The larger cell divides first, giving
origin to three cells; the smaller cell then divides. (Cf. figs. 145B, C; 146B, C;
147K, L.) The mitotic spindles tend to assume positions at right angles to
each other during these cleavages.
c) Eight-cell Stage. Eight cells are found 32 to 41 hours after mating.
One member of the larger blastomeres of the four-cell stage divides, forming
a five-cell condition, followed by the division of the second larger cell, pro-
ducing six cells. (Cf. figs. 145C; 147M.) After a short period, one of the
smaller cells segments, and thus, a total of seven blastomeres is formed. The
last cleavage is followed by the division of the other smaller cell, producing
eight blastomeres (fig< 144C; compare with fig. 147N). The mitotic spindles
of each of these cleavages form at right angles to one another, thus demon-
strating an independence and asynchrony. The latter conditions are demon-
strated further by the fact that the blastomeres shift their position continually
in relation to each other during these divisions.
d) SiXTEEN-CELL Stage. The mitotic divisions increase in rate, and at
about 45 to 47 hours after mating the 16-cell stage is reached (fig. 144D).
The cells at the future trophoblast pole begin to flatten, and gradually certain
blastomeres are enclosed within. In the macaque monkey, 16 cells are present
at about 96 hours after fertilization.
e) Morula Stage. At about 65 to 70 hours after mating a solid mass of
cells is present. This condition is known as the morula (mulberry-like) stage
(fig. 144E, F). The trophoblast portion of the cell mass is more active in
cell division.
f^-H ,^iV-'
^
fi
c
D
E
F
Fig. 146. Photomicrographs of cleavage in living monkey eggs. (After Lewis and
Hartman, Carnegie Inst., Washington, Contrib. Embryol., 24.) (Figures borrowed from
fig. 33, Patten, '48.) (A) Late two-cell stage. (B) Early three-cell stage. (C) Late
four-cell stage. (D) Five-cell stage. (E) Six-cell stage. (F) Eight-cell stage; next
cleavage beginning.
301
Fig. 147. (See facing page for legend.)
302
TYPES OF CLEAVAGE 303
f) Early Blastocyst. A few hours later or about 70 to 75 hours after
mating, a well-defined cleft within the cells of the trophoblast pole becomes
evident (fig. 144G). (Cf. fig. 145D, J.) This cleft or cavity enlarges, and
the surrounding trophoblast cells lose their rounded shape and become con-
siderably flattened. As the blastocoel gradually increases in size, the forma-
tive tissue or inner cell mass becomes displaced toward one end of the early
blastocyst, as indicated in fig. 144G, H. The blastocoelic space at this time
is filled with fluid, and the blastocyst as a whole completely fills the area
within the zona pellucida (fig. 144G, H). The pig embryo reaches a similar
condition in about 100 hours after fertilization, and that of the guinea pig in
140 hours.
During its passage down the Fallopian tube, the developing mass of cells
continues to be encased by the zona pellucida. The general increase in size
is slight. In the rabbit and in the opossum, as the cleaving egg passes down
the Fallopian tube, an albuminous coating is deposited around the outside
of the zona pellucida (figs. I44G, H; 147A). This albuminous layer forms
an accessory egg membrane or covering similar to the albuminous layers de-
posited around the egg by the oviducal cells in prototherian mammals, birds,
and reptiles. At about 80 to 96 hours after mating, the rabbit blastocyst enters
the uterus and gradually increases in size. Implantation of the mammalian
blastocyst upon the uterine wall will be considered later. (See Chap. 22.)
3) Types of Mammalian Blastocysts (Blastulae). The early blastocyst of
the rabbit described above is representative of the early condition of the
developing blastula of the eutherian (placental) mammal. However, in the
metatherian or marsupial mammals the early blastocyst does not possess a
prominent inner cell mass similar to that found in the eutherian mammals.
Comparing the early blastocysts of the higher mammals, we find, in general,
that there are three main types as follows:
( 1 ) In most of the Eutheria or placental mammals the inner cell mass
(embryonic knob) is a prominent mass of cells located at one pole of
the blastocyst during the earlier stages of blastocyst formation. (See
Fig. 147. Early development of the opossum egg. (A-H after Hartman, '16; I-N
after McCrady, '38.) (A) Unfertilized uterine egg, showing the first polar body; yolk
spherules (in black) within the cytoplasm; zona pellucida; albuminous layer; and the
outer shell membrane. (B) Two-cell stage. Observe yolk spherules discharged into the
cavity of the zona pellucida. (C) Section through three blastomeres of four-cell stage.
Observe yolk within and without the blastomeres. (D) Section through 16-cell cleavage
stage. Observe yolk within blastomeres and also in cavity of the zona between the
blastomeres. (E) Section through early blastocyst showing yolk and cytoplasmic frag-
ments and an included nucleated cell within the blastocoel. (F-H) Early and later
blastocyst of the opossum, showing the formative tissue at one pole of the blastocyst.
(I) Surface view, fertilized egg. (J) Two-blastomere stage. (K) Cell A has divided
meridionally into A, and A,. (L) Cell B has divided into B, and B^. (M) A, and A^
have divided as indicated. (N) B, and Bj divide next as indicated.
304
CLEAVAGE (SEGMENTATION) AND BLASTULATION
figs. 144G, H; 145J-L.) This condition is found in the monkey,
human, pig, rabbit, etc.
(2) On the other hand, in certain marsupials, such as the American opos-
sum, Didelphys virginiana, and the BraziUan opossum, Didelphys
aurita, the inner cell mass is much less prominent during earlier stages
of the blastocyst. In these species it is indicated merely by a thickened
aggregation of cells at one pole of the blastocyst (fig. 147E-G).
(3) In the marsupial or native cat of Australia, Dasyurus viverrinus, cleav-
age results in an early blastocyst in the form of a hollow sphere of
rounded cells. As the blastocyst expands, the cells increase in number
and become flattened to form a thin layer of cells apposed against the
shell membrane without an apparent inner cell mass or embryonic
knob (fig. 148A-C).
A conspicuous feature of cleavage and early blastocyst formation in the
marsupials should be emphasized. For in this group, the early blastomeres
apparently use the framework of the zona pellucida as a support upon which
they arrange themselves. As a result, the blastocoelic space of the blastocyst
Fig. 148. Early blastular conditions of the marsupial cat of Australia, Dasyurus
viverrinus. (After Hill, '10.) (A) Early blastula. (B) External view of blastocyst, 0.6
mm. in diameter. The cells are becoming flattened and finally reach the condition shown
in (C). (C) Section of wall of blastocyst. 2.4 mm. in diameter.
TYPES OF CLEAVAGE 305
forms directly by cell arrangement and not by the development of a cleft
within the trophoblast cells, as in the eutherian mammals. (See fig. 147C-E;
compare with figs. 144G; 145J.)
The descriptions of the mammalian blastocysts presented above pertain
only to the primary condition of the blastocyst. The changes involved in later
development, resulting in the formation of the secondary blastocyst, will be
described in the next chapter which deals specifically with blastulation.
(For more detailed descriptions of early cleavage in the metatherian and
eutherian mammals see: Hartman ('16) on the American opossum; Hill ('18)
on the opossum from Brazil; Hill ('10) on the Australian native cat, Dasyurus
viverrinus; Heuser and Streeter ('29), and Patten ('48) on the pig; Lewis and
Gregory ('29), Gregory ('30), and Pincus ('39) on the rabbit; Huber ('15)
on the rat; Lewis and Wright ('35 ) and Snell ('41 ) on the mouse; Lewis and
Hartman ('41) and Heuser and Streeter ('41) on the Rhesus monkey.)
b. Holoblastic Cleavage of the Transitional or Intermediate Type
Contrary to the conditions where small amounts of yolk or deutoplasm
are present in the egg of the higher mammal or in Amphioxus, the eggs of
the vertebrate species described below are heavily laden with yolk. As the
quantity of yolk present increases, the cleavage phenomena become less and
less typically holoblastic and begin to assume meroblastic characteristics.
Hence the designation transitional or intermediate cleavage.
1) Ambystoma maciilatum ( punctatiim). The newly spawned egg of
Arnbystoma maculatum is nearly spherical and measures about 2 mm. in
diameter, although the egg size is somewhat variable. The animal pole con-
tains within its median area a small depression, the "light spot" or "fovea."
Within the fovea is a small pit harboring the first polar body. (A comparable
pit is shown in the frog's egg, fig. 119C.) After the second polar body is
formed, this pit may appear somewhat elongated, and the light spot disap-
pears. Just before the first cleavage, the animal pole appears flattened similar
to the condition in the frog's egg. The flattened area soon changes to an
elongated furrow which progresses gradually downward toward the opposite
pole (fig. 149A, B). This cleavage furrow is meridional, dividing the egg into
two, nearly equal blastomeres. The second cleavage furrow is similar to the
first but at right angles to the first furrow (fig. 149C). However, considerable
variation may exist, and the second furrow may arise at various angles to the
first, dividing each of the first two blastomeres into two, slightly unequal,
daughter blastomeres. The third set of cleavages is latitudinal, and each
blastomere is divided into a smaller animal pole micromere, and a larger
vegetal pole macromere (fig. 149D). Later cleavages may not be synchronous.
The first three cleavages described above conform generally to the rules
of typical holoblastic cleavage. However, from this time on cleavage digresses
from the holoblastic pattern and begins to assume certain characteristics of
306
CLEAVAGE (SEGMENTATION) AND BLASTULATION
Fig. 149. Early cleavage in Ainbystoma inuculatum (punctatum). (After Eycleshymer,
J. Morphol., 10, and eggs in the laboratory.) (A, B) First cleavage furrow, meridional
plane. (C) Second cleavage furrow at right angles to first furrow, meridional plane.
(D) Third cleavage furrow, latitudinal, forming four micromeres and four macromeres.
(E) Fourth cleavage furrow; mixture of meridional and vertical planes of cleavage. (F)
Fifth cleavage furrows; mixture of latitudinal and vertical planes of cleavage. Observe
equatorial plane cutting the large macromeres. (G-I) Later cleavage stages.
meroblastic cleavage. For example, the fourth set of cleavages may be a
mixture of vertical and meridional furrows, as shown in figure 149E. The fifth
cleavages are a mixture of horizontal (i.e., latitudinal and equatorial, fig.
149F), vertical and meridional furrows. The sixth set of cleavages is made
up of vertical and horizontal cleavage planes of considerable variableness (fig.
149G). From this time on cleavage becomes most variable, with the animal
pole micromeres dividing much more rapidly than the yolk-laden macromeres
at the vegetal pole (figs. 149H, I).
The blastocoel makes its appearance at the eight-cell stage and appears
as a small space between the micromeres and the macromeres, the latter
forming the floor of the blastocoelic space. At the late blastula stage, the
blastocoel is roofed over by the smaller micromeres, and floored by the yolk-
TYPES OF CLEAVAGE
307
VERTICAL FURROW
Fig. 150. Cleavage in the egg of Lepidosiren paradoxa. (After Kerr, '09.) (A) Be-
ginning of first cleavage, meridional in position. (B) Second cleavage planes, approxi-
mately meridional in position. (C) Third cleavage planes vertical in position, demon-
strating a typical meroblastic pattern. (D) Early biastula. (E) Late blastula.
laden macromeres. The blastocoel is small in relation to the size of the egg
(Eycleshymer, 1895).
2) Lepidosiren paradoxa. The egg of the South American lungfish,
Lepidosiren paradoxa, measures about 6.5 to 7 mm. in diameter. Cleavage
of the egg is complete (i.e., holoblastic), and a relatively large blastocoel is
formed. As in Ambystoma, the blastocoel is displaced toward the animal
pole. The floor of the blastocoel is formed by the large, yolk-laden macromeres.
The first two cleavage furrows are approximately meridional (fig. 150A, B).
These two furrows are followed by four vertical furrows, which, when com-
pleted, form eight blastomeres (fig. 150C). The latter cleavages are subject
to much variation. Although cleavage of the egg is complete, a distinct mero-
blastic pattern of cleavage is found, composed of two meridional furrows
followed by vertical furrowing (see Kerr, '09).
3) Necturus maculosus. In this species of amphibia the egg is large and
its contained yolk is greater than that of Ambystoma. It measures about 5 to
6 mm. in diameter. The egg and its envelopes are attached individually by
the female beneath the flattened surface of a stone (Bishop, '26).
Cleavage in this egg proceeds slowly. The first two cleavage furrows tend
to be meridional, but variations may occur in different eggs. Sometimes they
are more vertical than meridional (fig. 151 A). (See Eycleshymer, '04). The
308
CLEAVAGE (SEGMENTATION) AND BLASTULATION
Fig. 151. Cleavage in the egg of Necturus macidosus. (After Eycleshymer and Wilson,
'10.) (A) First two cleavage planes are meridional. (B) Third cleavage planes tend
to be vertical and meridional. (C) Fourth cleavage planes are vertical, meridional, and
irregular. (D H) Following cleavage planes become irregular, offering a mixture of
modified latitudinal, vertical, and meridional varieties.
third cleavage furrows are irregularly vertical (fig. 15 IB), while the fourth are
latitudinal, cutting off four very irregular micromeres at the animal pole. Seg-
mentation then becomes exceedingly irregular. (See Eycleshymer and Wilson,
'10). One characteristic of cleavage in Necturus is a torsion and twisting of
the cleavage grooves due to a shifting in the position of the blastomeres.
As shown in the figures, the first three cleavage planes assume a distinct
meroblastic pattern of two meridional furrows followed by vertical furrows.
The yolk material evidently impedes the progress of the furrows considerably.
4) Acipenser sturio. In the genus Acipenser are placed the cartilaginous
ganoid fishes. Cleavage in Acipenser sturio, the sturgeon, resembles that of
TYPES OF CLEAVAGE
1ST. FURROW NEARLY
MERIDIONAL
MARGINAL CELLS
Fig. 152. Cleavage in the egg of the sturgeon, Acipenser sturio. (After Dean, 1895.)
(A. B) First and second cleavage planes are approximately meridional. (C) Third
cleavage planes are vertical, usually parallel to first cleavage plane. (D) Fourth cleavage
planes are vertical, cutting off four central cells from the 12 marginal cells. ( E. F)
Later cleavage stages.
Necturus, although the furrows in the yolk pole area are retarded more and
are definitely superficial. The third and fourth sets of cleavage furrows are
vertical and succeed in cutting off four central cells from twelve larger mar-
ginal cells (fig. 152). Cleavage in this form is more holoblastic in its essential
behavior than that in the egg of Amia and Lepisosteus described below
(Dean, 1895).
5) Amia calva. Amia calva is a species of bony ganoid fishes, and it repre-
sents one of the oldest living species among the fishes. Its early embryology
follows the ganoid habit, namely, its cleavages adhere to the meroblastic
pattern of the teleost fishes, with the added feature that the furrows eventually
pass distally toward the vegetal pole of the egg. A few yolk nuclei appear to
be formed during cleavage. These nuclei aid in dividing the yolk-filled cyto-
plasm into distinct cells. The latter gradually are added to the early blastomeres
and to the later entoderm cells of the developing embryo. In other words,
cleavage in this species is holoblastic, but it represents a transitional condition
between meroblastic and holoblastic types of cleavage.
The egg of Amia assumes an elongated form, averaging 2.2 by 2.8 mm.
The germinal disc is. a whitish cap in the freshly laid egg, reaching down over
the animal pole to about one third of the distance along the egg's longer axis.
310
CLEAVAGE (SEGMENTATION) AND BLASTULATION
The vegetal pole is gray in color. The egg membrane is well developed, having
a zona radiata and a villous layer. Strands of the villous layer may attach the
egg to the stem of a water weed or other structure (fig. 153A).
The first cleavage plane is meridional and partly cleaves the protoplasmic
disc into two parts (fig. 153B). This cleavage furrow passes slowly toward
the vegetal pole of the egg. The second cleavage is similar to the first furrow
and at right angles to it (fig. 153C). The third cleavage is variable but, in
general, consists of two furrows passing in a vertical plane at right angles to
the first cleavage furrow (fig. 153D). The fourth set of cleavages is hori-
SYNT. YTiai NUCLEI
Fig. 153. Cleavage in the egg of Amia catva. (After Dean, 1896.) (A) Egg mem
branes of Amia, showing the filamentous (villous) layer attaching the egg to the stem
of a water weed. (B) Second cleavage plane shown cutting through the protoplasmic
disc at one pole of the egg. Section made parallel to the first cleavage plane. (C) First
and second cleavage planes seen from above. (D) Third cleavage planes are vertical
in position as indicated. (E) Fourth cleavage, sectioned in a plane approximately parallel
to first (or second) cleavage. (F) Section through protoplasmic disc at eighth cleavage.
(G) Blastular stage. Blastocoel is indistinct and scattered between (?) blastomeres of
blastoderm. The description given by Whitman and Eycleshymer (1897) does not agree
in certain features with the above.
TYPES OF CLEAVAGE
311
BLASTOCOELIC
SPACE
YOLK
4 th. ,:
CLEAVAU E ""^^•i'Sii;^-^'"'
CENTRAL
CELL
'5^ YOLK BED
Fig. 154. Early development of Lepisosteus osseus. (After Dean, 1895.) (A) Un-
cleaved egg, showing germinal disc. (B) First cleavage is trench-like, extending beyond
(i.e., laterally) to the margin of the germinal disc. (C) Transverse section of cleavage
furrows shown in (B). (D) Four-cell stage. (E) Third cleavage planes are vertical
as indicated. (F) Fourth cleavage planes also are vertical. (G) Germinal disc, sec
tioned 25 hours after fertilization. Blastocoelic spaces dispersed.
zontal. While the latter is in progress the fifth cleavages, which are vertical,
begin. As a result of the fourth and fifth sets of cleavages, a mass of eight
central cells and twenty or more marginal cells arises. Horizontal (i.e., lati-
tudinal) cleavages begin among the central cells at this time, and other cells
(see cell A, fig. 153F) appear to be budded off from the yolk floor from this
period on. The latter are contributed to the growing disc of cells above.
Four types of cleavage furrows now appear in the growing blastoderm as
follows:
(1) cleavage among the central cells, increasing their number,
(2) cleavage among the marginal cells, contributing cells to the central
cells,
312 CLEAVAGE (SEGMENTATION) AND BLASTULATION
(3) cleavage of the marginal cells, increasing the number of marginal cells
and contributing syncytial nuclei to the yolk floor,
(4) cleavage within the syncytial mass of the yolk floor, contributing cells
to the central cells, such as cell A, figure 153F.
Eventually a blastular condition is reached as a result of the foregoing
cleavages which does not possess an enlarged blastocoelic space; rather the
blastocoel is in the form of scattered spaces within a loosely aggregated cap
of cells (fig. 153G). This blastula might be regarded as a stereoblastula, i.e.,
solid blastula (Dean, 1896; Whitman and Eycleshymer, 1897).
6) Lepisosteiis (Lepidosteiis) osseiis. The early development of the
gar pike, Lepisosteus osseus, another bony ganoid fish, resembles that of Amia
described above. The disc of protoplasm which takes part in the early cleav-
ages is a prominent mass located at one pole of the egg (fig. 154A). The first
two cleavage furrows appear to be meridional and partly cleave the proto-
plasmic cap of the egg, as indicated in figure 154B-D. The next cleavages
are vertical and somewhat parallel to one of the meridional furrows (fig.
152E). The fourth cleavages are vertical, cutting off four central cells from
the peripherally located marginal cells (fig. 152F). As in Amia, the marginal
cells contribute syncytial nuclei to the yolk bed below the protoplasmic cap,
and these in turn contribute definite cells to the growing blastodisc. The
blastula of Lepisosteus consists of a loosely aggregated cap of cells among
which are to be found indefinite blastocoelic spaces (fig. 152G). (See Dean,
1895.)
7) Gymnophionan Amphibia. Cleavage presumably is holoblastic, result-
ing in a disc of small micromeres at the animal pole, with large, irregular
macromeres, heavily yolk laden, located toward the vegetal pole (fig. 182A).
(See Svensson, '38.) The latter cells become surrounded during gastrulation
by the smaller micromeres (Brauer, 1897). The blastula of the gymnophionan
amphibia essentially is solid and may be regarded as a stereoblastula.
3. Meroblastic Cleavage
The word meroblastic is an adjective which refers to a part of the germ;
that is, a part of the egg. In meroblastic cleavage only a small portion of the
egg becomes segmented and thus gives origin to the blastoderm. Most of the
yolk material remains in an uncleaved state and is encompassed eventually
by the growing tissues of the embryo. A large number of vertebrate eggs
utilize the meroblastic type of cleavage. Some examples of meroblastic cleav-
age are listed below.
a. Egg of the Common Fowl
(Note: As cleavage in reptiles resembles that of birds, a description of rep-
tilian cleavage will not be given. The reader is referred to figure 231, con-
TYPES OF CLEAVAGE
313
cerning the cleavage phenomena in the turtle. The information given below
is to be correlated also with the developing pigeon's egg.)
The germinal disc (blastodisc) of the hen's egg at the time that cleavage
begins measures about 3 mm. in diameter. Its general relationship to the
egg as a whole is shown in figure 157A.
1) Early Cleavages. The first cleavage furrow makes its appearance at
about four and one-half to five hours after fertilization at the time when the
egg reaches the isthmus of the oviduct (figs. 155A; 157C, D). The first
Fig. 155. Cleavage in the chick blastoderm, surface views. (C after Olsen. '42; the
rest after Patterson, MO.) (A) First cleavage is approximately meridional. (B) Second
cleavage is at right angles to first. (C) Third cleavage planes are vertical as indicated
and approximately parallel to one of the other cleavage planes. Considerable inequality
may exist at this time. (This figure slightly modified from original.) (D) Seventeen-
cell stage. Observe central and marginal cells. (E) Stage approximating 32-cell con-
dition. (F) Surface view of 64-cell stage; 41 central and 23 marginal cells. (G) Surface
view of blastoderm in lower portions of oviduct; 31 marginal and 123 central cells. (H)
Later blastoderm, showing 34 marginal and 312 central cells.
A.
a^'^-^t? ..* -*« rt> ^Vl^^;^V/:■{/:S^y;•■^;C\Vv^^•^;v^ .1*^*'^ ^%** '''j'**'' t'l""*'
B.:
SEGMENTATION CAVITY
CENTRAL CELLS
MARGINAL
CELL
SP^^^i
• ^ '•^ .- \\*,. ■*''>*?: **..:l:^^^0S^■^'.'^siL'i§^0h^»^l•. *' -*"
., ^•4>!r';^- ,- --s-^■
E.
Fig. 156. (See facing page for legend.)
314
TYPES OF CLEAVAGE
315
MARGINAL CELLS
SYNCYTIAL PROTOPLASM
.■#■
,«,.;-:>"VJ;.'^i
^j\;.^v
NUCLEUS OF PANDE R
BLASTODI SC CE LLS
K/^W.
ZONE OF JUNCTURE
(SYNCYTIAL GERM WALL)
CENTR A L PERIBLAST
PRIMI T I VE BLASTOCOEL
CE LLULA R GER M WAL L
MARGI NAL PERI BLAST
MARGIN OF OVERGROWTH
Fig. 156. Cleavage in chick blastoderm, sectional views. (After Patterson, '10.) (A)
Median section through blastoderm approximately at right angles to furrow shown in
fig. 155 A. (B) Section through blastoderm of about eight-cell stage. (C) Section
through blastoderm, showing 32 cells, also showing horizontal cytoplasmic cleft (seg-
mentation cavity). (D) Median section through blastoderm similar to that shown in
fig. 155E. (E) Median section through blastoderm similar to that of fig. 155G. (F, G)
Diagrammatic views of developing avian blastoderms. (F) Diagrammatic section and
surface view of chick blastoderm shown in fig. 155G and fig. 156E. (G) Section of
chick blastoderm about time that egg is laid, depicting the primary blastocoel below
the blastoderm and syncytial tissue at the margins. Observe that the syncytial tissue
serves to implant the blastoderm upon the yolk substance.
furrow consists of a slight meridional incision near the center of the blastodisc,
cutting across the disc to an extent of about one half of the diameter of the
latter (fig. 155A). This furrow passes yolkward but does not reach the lower
portion of the disc where the cytoplasm is filled with coarse yolk granules
(fig. 156A). The second cleavage occurs about 20 minutes later and consists
of two furrows, one on either side of the first furrow and approximately at
right angles to the first furrow. These furrows may be regarded as meridional
(fig. 155B). Though both of the second furrows tend to meet the first furrow
at its midpoint, one of the second furrows may be displaced and, hence, may
not contact the corresponding furrow of the other side. The third set of
furrows is vertical, cutting across the second set of meridional furrows, and,
consequently, tends to parallel the first cleavage furrow (fig. 155C). The
fourth set of furrows is also vertical and, although not synchronous, it pro-
ceeds gradually to form eight central cells which are surrounded by twelve
316 CLEAVAGE (SEGMENTATION) AND BLASTULATION
marginal cells. In figure 155D, five central cells are shown, while in figure
157E, eight central cells are present. The central cells do not have boundaries
below and, thus, are open toward the yolk. As a result, their protoplasm is
continuous with the protoplasm in the deeper-lying portions of the disc. The
marginal cells have boundaries only on two sides, and the cleavage furrows
which form the sides of the marginal cells continue slowly to extend in a
peripheral direction toward the margins of the disc (fig. 155D). The egg
is in this stage of development when it leaves the isthmus and enters the uterus
(fig. 157A, F).
Cleavage from this point on becomes very irregular, but three sets of fur-
rows are evident:
(a) There are vertical furrows which extend peripherad toward the margin
of the blastodisc. These furrows meet at various angles the previously
established furrows which radiate toward the periphery of the blasto-
disc (in fig. 155E, see a., b., c). A branching effect of the radiating
furrows, previously established, in this manner may be produced (in
fig. 155E, see c).
(b) Another set of vertical furrows is found which cut across the median
(inner) ends of the radiating furrows. The latter produce peripheral
boundaries for the centrally located cells (see fig. 155E, d., e., f. ). The
central cells thus increase in number as the blastodisc extends periph-
erally. As a result of this set of cleavage furrows, a condition of the
blastodisc is established in which there is a mass of central cells,
having peripheral boundaries, and an area of marginal cells which
lies more distally between the radiating furrows. It is to be observed
that the marginal cells lack peripheral boundaries (fig. 155E, F).
(c) A third and new kind of cleavage, cytoplasmic but not mitotic, now
occurs below the centrally placed cells, namely, a latitudinal or hori-
zontal cleft which establishes a lower boundary for the centrally lo-
cated cells with the subsequent appearance of a blastocoelic space filled
with fluid (fig. 156B, C).
Thus, at the 16- to 32-cell stages (fig. 155D, E) some of the more cen-
trally located central cells have complete cellular boundaries (fig. 156C),
but central cells, located more peripherally, may not have the lower boundary.
The marginal cells also lack a lower boundary.
A little later, at the 60- to 100-cell stages (fig. 155F), the chick blastoderm
presents the following characteristics:
(a) There is a mass of centrally located cells. These cells lie immediately
above the horizontal cleft mentioned above (fig. 156C, D). They are
completely bounded by a surface membrane and represent distinct
cells. These cells continue to increase by mitotic division and, as early
as the 64-cell stage (fig. 155F), the centrally located cells are in the
PITUITARY
S THE RULE
TRANCE OF
SPERM IMMED-
ATELY AFTER
OVULATION
Fig. 157. Chart showing ovary, oviducal. and pituitary relationships in passage of egg
from the ovary down the oviduct. Developing blastodisc shown in (B-G) in relation to
the oviducal journey. This chart shows an egg which has just been ovulated. Ordinarily,
however, this egg would not be ovulated until sometime after the egg shown in the uterus
has been laid.
317
318
CLEAVAGE (segmentation) AND BLASTULATION
PERM
4 CLEAVAGE NUCLEI
"c E N T R A L CELLS 5 P.
B L A S T 0
PERIBLAST
Fig. 158. Early cleavage phenomena in elasmobranch fishes. (A, B, E. F, G after
Ziegler, '02, from Riichert; C, D after Ziegler. ) (A) Germ disc of Torpedo ocellata,
showing four cleavage nuclei, sperm nuclei, and beginning of first cleavage furrow. (B)
Stage of cleavage, possessing 16 cleavage nuclei. Four central cells and ten marginal
cells are evident from surface view. (C) Surface view of blastoderm of Scy Ilium
canicula with 64 cleavage nuclei. Twenty-nine central cells and seventeen marginal cells
are evident from surface view. (D) Later cleavage stage of S. canicula with 145 cells
showing. (E) Transverse section of (B). (F) Transverse section of blastoderm of
T. ocellata with 64 cells. (G) Median section through blastoderm of T. ocellata at
the end of the cleavage period.
form of two layers situated immediately above the horizontal cleft or
segmentation cavity (fig. 156D).
(b) The horizontal cleft or segmentation cavity gradually widens and en-
larges. It separates the central cells above from the uncleaved germinal
disc or central periblast below.
(c) At the margins of the central cells, these cleavages may be found: ( 1 )
Vertical cleavages occur which cut off more central cells from inner
ends of the marginal cells. As a result, there is an increase in the num-
ber of central cells around the periphery of the already-established cen-
TYPES OF CLEAVAGE 319
tral mass of cells. (2) Vertical cleavages arise whose furrows extend
peripherad toward the margin of the disc. These furrows and previously
formed, similar furrows now approach the outer edge of the blasto-
disc (germinal disc). (See figs. 155H; 156D). (3) True latitudinal
or horizontal cleavages occur which serve to provide lower cell bound-
aries for the more peripherally located, central cells (see cell A, fig.
156E), and which also contribute nuclei without cell boundaries to
the disc substance in this immediate area (see cell B, fig. 156E). As
a result, the marginal or peripheral areas of the blastodisc around the
mass of completely formed, central cells are composed of: (a) mar-
ginal cells which appear near the surface of the blastodisc, having
partial boundaries at the blastodisc surface, and {b) a deeper-lying
protoplasm, possessing nuclei without cell boundaries. This deeper-
lying, multinucleated, marginal protoplasm constitutes a syncytium
(fig. 156F).
2) Formation of the Periblast Tissue. As indicated above, the activities
of the blastoderm extend its margins peripherad. In so doing, some of the
mitotic divisions in the peripheral areas contribute nuclei which come to lie
in the deeper portions of the blastodisc. Some of these nuclei wander distally
and yolkward into the more peripherally located, uncleaved portions of the
protoplasm below the enlarging primary segmentation cavity or blastocoel.
A syncytial protoplasm containing isolated nuclei thus arises around the pe-
ripheral margin of the blastoderm in its deeper areas. This entire syncytial
protoplasm, composed of a continuous cytoplasm with many nuclei, is known
as periblast tissue. It is made up of two general areas: ( 1 ) the peripheral
periblast around the margin of the blastodisc and (2) a central periblast
below the primitive blastocoel (fig. 156G). This periblast tissue is a liaison
tissue which brings the yolk and the growing mass of cells of the blastodisc
into nutritive contact.
When this condition is reached, two kinds of embryonic tissues exist:
(a) the formative or embryonic tissue proper, composed of an aggrega-
tion of distinct cells. These cells constitute the cellular portion of the
blastoderm (see blastodisc cells, fig. 156G), and
(b) the peripheral and central periblast tissue (see fig. 156G). The latter
functions as a trophoblast tissue, and it is continuous with the seg-
mented portion of the blastoderm around the peripheral areas of the
blastodisc. Centrally, however, it is separated from the segmented
area of the blastoderm by the primary blastocoelic cavity. The devel-
opmental condition at this time may be regarded as having reached
the primary blastular stage.
3) Morphological Characteristics of the Primary Blastula. This condition
of development is reached while the egg continues in the uterus (fig. 157G).
320 CLEAVAGE (SEGMENTATION) AND BLASTULATION
A transverse section through one of the diameteis of the primary blastula
presents the following features (fig. 156G):
(a) A central mass of cells of two or several cells in depth overlies the
blastocoelic space. This is the central or cellular portion of the
blastoderm.
(b) Underneath this central blastoderm is the primary segmentation cavity
or primary blastocoel.
(c) Below the primary blastocoel is the central syncytial periblast, which
continues downward to the yolk material; many yolk granules are
present in the layer of the central periblast near the yolk. Nuclei are
not present in the central area of the central periblast, but may be
present in its more peripheral portions.
(d) Around the peripheral areas of the central periblast and the cellular
portion of the blastoderm is the marginal periblast tissue which now
is called the germ wall. The germ-wall tissue contains much yolk ma-
terial in the process of digestion and assimilation.
The central mass of cells or cellular blastoderm increases in cell number
and in size by the multiplication of its own cells and by the contribution of
marginal periblast tissue which gradually forms cells with boundaries from
its substance. The germ wall thus may be divided into two main zones: (1)
an inner zone of distinct cells, which are dividing rapidly and, in consequence,
contribute cells to the peripheral portions of the growing cellular blastoderm
and (2) an outer peripheral zone, the syncytial germ wall (zone of junction).
The latter is in intimate contact with the yolk (fig. 156G). The central peri-
blast tissue gradually disappears. At the outer boundary of the peripheral
periblast, there is an edge of blastodermic cells overlying the yolk. These
cells have complete boundaries and are known as the margin of overgrowth
(fig. 156G). A resume of the early development of the hen's egg in relation
to the parts of the oviduct, pituitary control, laying, etc., is shown in figure 157.
4) Polyspermy and Fate of the Accessory Sperm Nuclei. The bird's egg
is polyspermic and several sperm make their entrance at the time of fertili-
zation (see fig. 157B). The supernumerary sperm stimulate abortive cleavage
phenomena in the peripheral area of the early blastodisc (fig. 155D). How-
ever, these cleavage furrows together with the extra sperm nuclei soon
disappear.
(References: Blount ('09);Lillie ('30);OIsen ('42); and Patterson ('10).)
For later stages in the development of the hen's egg, see chapter 7.
b. Elasniobranch Fishes
1) Cleavage and Formation of the Early Blastula. Like the egg of the bird,
the egg of the elasmobranch fishes is strongly telolecithal, and a small disc
of protoplasm at one pole of the egg alone takes part in the cleavage phe-
TYPES OF CLEAVAGE 321
nomena. Cleavage in the majority of these fishes simulates that of the bird,
but certain exceptional features are present. In some, as in Torpedo ocellata,
meroblastic cleavage is present in an extreme form. The zygotic nucleus di-
vides and the two daughter nuclei divide again forming a syncytial state before
the appearance of the first cleavage furrow. The tendency of retardation or
suppression of the cytoplasmic mechanism of cleavage which occurs in the
bird blastoderm thus is carried to an extreme form in the early development
of some elasmobranch fishes.
The first cleavage furrow is meridional or nearly so (fig. 158A), and the
second furrow is similar and at right angles to the first furrow. The third set
of furrows is vertical and meets the previous furrows at various angles. The
fourth set of cleavages is vertical and synchronous, as is the preceding, and
gives origin to three or four central cells, which, on surface viewing, have
complete cell boundaries but below their cytoplasms are confluent with the
cytoplasm of the blastodisc (fig. 158B and E). Around the periphery of these
central cells, are on the average ten marginal cells which have their cytoplasms
confluent below and peripherally with the general cytoplasm of the disc. The
fifth cleavage furrows are mixed. That is, in the central part of the disc the
cleavage furrows are latitudinal, as the mitotic spindles in this area form
perpendicular to the surface. As a result, distinct daughter cells are cut off
above, while the daughter cells below have cytoplasms confluent with the
general cytoplasm of the disc. A blastocoelic cavity appears between these
two sets of central cells. In the marginal areas the fifth set of cleavages is
vertical, cutting off more central cells and giving origin to more marginal cells.
The sixth set of cleavages is a mixture of vertical cleavages at the periphery
and latitudinal cleavages centrally; it produces a condition shown in figure
158F. In surface view, the blastoderm appears as in figure 158C, D.
From this time on cleavage becomes very irregular and a developmental
condition soon is produced which possesses a central blastoderm of many
cells with an enlarged blastocoelic cavity below (fig. 158G). A syncytial peri-
blast tissue is present at the margins of the blastoderm which also extends
centrally below the blastocoelic space where it forms a central periblast (fig.
158G). In this manner, two kinds of cells are produced:
(a) a blastoderm of distinct cells which ultimately produces the embryo
and
(b) a surrounding trophoblast or periblast tissue which borders the yolk
substance peripherally and centrally. As in the chick, the periblast
tissue has nutritive (i.e., trophoblast) functions.
2) Problem of the Periblast Tissue in Elasmobranch Fishes. Two views
have been maintained, regarding the origin of the periblast nuclei in the
elasmobranch fishes. One view maintains that they arise from the accessory
sperm nuclei derived from polyspermy, for polyspermy is the rule here as it
322 CLEAVAGE (SEGMENTATION) AND BLASTULATION
is in reptiles and birds. In the latter groups, these accessory nuclei may divide
for a time but ultimately degenerate, playing no real part in ontogeny. In the
case of the elasmobranch fishes, the accessory nuclei tend to persist somewhat
longer, and accordingly, it is upon this evidence that some have maintained
that the periblast nuclei arise from them. Others hold that the sperm nuclei
degenerate as they do in reptiles and birds, and the periblast nuclei arise as
a result of the regular embryonic process. A third view concedes that both
these sources contribute nuclei.
In view of the origin of the periblast nuclei in teleost fishes, in the ganoid
fishes, Amia and Lepisosteus, and in reptiles and birds, and of the syncytial
tissue of the later mammalian trophoblast, it is probable that embryonic cells
and tissues and not accessory sperm nuclei are the progenitors of the periblast
tissue. This probability is suggested by figure 158F, G. Furthermore, later on
in the development of the elasmobranch fishes, the entoderm appears to con-
tribute nuclei which wander into the periblast tissue which lies between the
entoderm and the yolk material (fig. 213K, L). In later stages the periblast
tissue is referred to as the yolk syncytium. In the yolk syncytium the periblast
nuclei gradually assume a much larger size.
For further details of the early development of the elasmobranch fishes,
consult Ziegler ('02) and Kerr ('19) and Chapter 7.
c. Teleost Fishes
1) Cleavage and Early Blastuia Formation. During the fertilization process
of the egg in teleost fishes, the superficial cytoplasm of the egg migrates toward
the point of sperm entrance and hence a mound-like disc of protoplasm forms
at the pole of the egg where the sperm enters (figs. 122C; 123B, C). It is
this protoplasmic mass which takes part in cleavage (fig. 123E). The cleavage
planes in the teleost fishes manifest great regularity. The early cleavage fur-
rows almost cut through the entire protoplasmic disc in most teleost eggs,
and a mere strand of cytoplasm is left near the yolk which is not cleaved
(fig. 159E).
In the sea bass, Senaniis atrariiis, the first two cleavage planes are merid-
ional and at right angles to each other (fig. 159A); the third planes are ver-
tical and parallel to the first plane. The result is a group of eight cells in two
rows (fig. 159B). The fourth cleavage furrows are vertical and parallel to
the long axis of the eight cells previously established. These furrows divide
each of the eight blastomeres into inner and outer daughter cells. The result
is 16 cells, arranged in parallel rows of four cells each (fig. 159C, D).
As the 16-cell condition is converted into 32 cells, the four inner cells
divide latitudinally, that is, the cleavage spindle forms perpendicular to the
surface, while the twelve surrounding cells divide vertically (fig. 159D, F, G).
From this time on latitudinal and vertical cleavages become mixed, and the
1ST CLEAVAGE FURROW.
■PER BLAST
Fig 1^9. Early development of the sea bass, Serraniis atrarius, and the trout, Salmo
fario. (A-M after Wilson, 1889 and 1891; N-R after Kopsch, '11.) (A) Two-blastomere
stage, showing anaphase of next division. (B) Eight-blastomere stage (slightly modi-
fied). (C) Sixteen-cell blastoderm. (D) Sixteen-cell stage, showing anaphase nuclei
of next division. In the four centrally placed cells, the spindles are at right angles to the
surface, thus forming a latitudinal cleavage furrow in these cells. (E) Section through
center of four-blastomore stage. (F) Section through center of (D). Observe periblast
tissue. (G) Section showing change from 16-cell stage into 32 cells; see (D). (H)
Thirty-two to 64 cells. (1) Late cleavage blastoderm. Observe marginal and central
periblast. (J) Multiplication of periblast nuclei around the margin of the blastoderm.
(K-M) Late blastoderm, showing marginal and central periblast tissue. (N-R) Cleav-
age of the blastodisc of the trout. Observe that periblast tissue is derived from the
blastodisc cytoplasm directly.
323
324
CLEAVAGE (SEGMENTATION) AND BLASTULATION
Fig. 160. Cleaving eggs of Platypus and Echidna. (After Flynn and Hill, '39.) (A)
Egg, shell, and early cleavage in Ornithorhynchus. (B) Early cleavage in Echidna. See
fig. 161D.
synchronization of mitotic division is lost. In certain other teleost fishes, lati-
tudinal cleavages begin as early as the 8-cell stage.
At the 32- to 64-cell stages in Serranus atrarius, the blastoderm presents a
cap-like mass of dividing cells overlying a forming blastocoel (fig. 159H, I).
Between the blastocoel and the yolk, there is a thin layer of protoplasm
connecting the edges of the cap. This thin protoplasmic layer is the forerunner
of the central periblast tissue; at this stage it contains no nuclei (fig. 159F, H).
2) Origin of the Periblast Tissue in Teleost Fishes. In the sea bass and
many other teleost fishes, some of the surrounding cells at the edge of the
blastoderm lose their cell boundaries and fuse together to form a common
syncytial tissue. The nuclei in this tissue continue to divide (fig. 159J) and
eventually migrate into the periblast tissue below the blastocoel (see arrow,
fig. 159L). The latter then becomes the central periblast, while the syncytial
tissue around the edges of the growing blastodisc forms the peripheral or
marginal periblast (fig. 159K-M).
In the trout, the early cleavage furrows of the blastodisc are incomplete,
and the periblast arises from the syncytial tissue established directly below
and at the sides of the protoplasmic cap (fig. 159N-R). This condition re-
sembles the cleavage process in the elasmobranch fishes.
See Kerr ('19); Kopsch ('11); and H. V. Wilson (1889).
d. Prototherian Mammalia
The Prototheria normally are placed in the class Mammalia along with
the Metatheria (marsupials) and Eutheria (true placental mammals). How-
Fig. 161. Early cleavage in Echidna. (Courtesy, Flynn and Hill, '39.) (A) Four-cell
stage. (B) Eight-cell stage. (C) About 16-cell stage. Two meridional and four vertical
cleavages have occurred. (D) About 32-cell stage. Observe marginal and central cells.
(E) Section through blastodisc of 4 to 8-cell stage. (F) Section through blastoderm
of 16-cell stage. See (C). (G) Section through blastoderm of 32-cell stage, showing
central and marginal cells.
325
326 CLEAVAGE (SEGMENTATION) AND BLASTULATION
ever, the prototherian mammals are aberrant, highly specialized animals, whose
general anatomy and embryology delineates a group quite distinct from the
higher mammals. The duckbill or Platypus (Ornithorhynchus) is found only
in Australia. The other species belonging to this group is the spiny anteater
Echidna aculeata found in New Guinea, Tasmania, and Australia. The duck-
bill lays from one to three heavily yolk-laden eggs in an underground chamber
on a nest of weeds and grasses. The eggs have a leathery shell. The young are
hatched naked, and the mother holds them against her abdomen with her
tail, where they feed upon a milk-like substance which exudes from the milk
glands by means of pore-like openings. The Echidna lays two white, leathery
eggs about the size of the eggs of a sparrow which she places in a temporary
pouch or fold of skin on the ventral abdominal wall. They feed similarly to
the duckbill young.
The early cleavages of Echidna and Ornithorhynchus follow different cleav-
age patterns. (See Flynn and Hill, '39, '42.) The cleavage planes of the
Platypus are more regular and symmetrical and resemble to a degree the
pattern of early cleavage in teleost fishes (fig. 160A), whereas the early
cleavage planes in Echidna simulate to some degree those found in reptiles
(fig. 160B). In both species cleavage is meroblastic.
In Echidna the cleavage furrows cut almost all the way through the proto-
plasmic disc (fig. 161E). The second cleavage in this species is at right angles
to the first, and divides the blastodisc into two larger and two smaller cells
(fig. 161A). The third cleavage furrows tend to parallel the first furrow,
forming eight cells (fig. 161B), while the fourth cleavages run parallel to
the second furrow, and 16 cells are formed (fig. 161C). The fifth cleavages
lack the constancy of the first four sets although they continue to be syn-
chronous; they result in the formation of 32 cells (fig. 161D).
In transverse section, the cells of the 32-celI blastoderm appear as rounded
masses, each cell in its upper portion being free from the surrounding cells
but in its lower extremity intimately attached to the yolk substance (fig. 16 IF).
Another feature of the early cleavages in Echidna is the tendency of the cells
to separate from each other; wide spaces consequently appear between the
blastomeres (fig. 161G). This tendency toward independence and isolationism
of the early blastomeres is characteristic of the higher mammals, as previously
observed. After the 32-cell stage, synchronization is lost and cleavage becomes
very irregular. A central mass of blastodermic cells eventually is formed,
surrounded by marginal cells, known as vitellocytes (fig. 175A).
As cleavage and development proceeds, the central blastomeres become
free from the underlying yolk, expand, and form a layer about two cells in
thickness (fig. 175B). The vitellocytes around the periphery of the blastoderm
eventually fuse to form a syncytium or multinucleated cytoplasmic mass inti-
mately associated with the yolk (fig. 175B, C). This marginal mass of syncytial
tissue forms the marginal periblast. Within the central portion of the blasto-
FORCE WHICH CAUSES THE BLASTOMERES TO ADHERE
327
derm itself two types of cells may be observed, namely, a superficial ecto-
dermal cell and a more deeply situated, somewhat vacuolated, smaller ento-
dermal cell (fig. 175B). (For later stages of blastulation, see chapter 7.)
e. Cleavage in the California Hagfish, Polistotrema (Bdellostoma) stouti
The California hagfish spawns an egg which is strongly telolecithal. The
germinal disc (blastodisc) is situated immediately below the egg membrane
at one end of the egg, adjacent to the micropyle and the anchor filaments
(fig. 162A). Cleavage begins in this disc, and the enlarging blastoderm slowly
creeps downward to envelop the massive yolk material. The freshly laid egg
measures about 29 mm. by 14 mm., including the shell. Without the shell,
the egg is about 22 mm. by 10 mm. and is rounded at each end (Dean, 1899).
The first two cleavage planes may be regarded as meridional (or vertical)
(fig. 162B). The third cleavage appears to be a mixture of vertical and hori-
zontal (latitudinal) cleavages, with the former predominating (fig. 162D, E).
Cleavage from this time on becomes irregular, and a typical meroblastic blasto-
derm soon is attained with central and marginal cells (fig. 162F).
C. What is the Force Which Causes the Blastomeres to Adhere
Together During Early Cleavage?
A question naturally arises concerning the force which makes the blasto-
meres of most chordates adhere to one another during the early cleavage
ANCHOR
FILAMENT
MIC ROP YL
BL A STODE
( G ER M I N A
PLASM)
BLASTODERM
Fig. 162. Egg and cleavage in the marine lamprey, Polistotrema (Bdellostoma) stouti.
After Dean, 1899.) (A) Animal pole end of the egg. (B) Surface view of blasto-
dermic hillock, showing first cleavage furrow. (C) Same, second cleavage. (D) Third
cleavages. (E, F) Later cleavages, strongly irregular. (G) Egg with shell removed.
328 CLEAVAGE (SEGMENTATION) AND BLASTULATION
period. This subject was investigated in the amphibian blastula by Holtfreter,
'39. According to this investigator, blastomeres, when isolated by mechanical
means, appear to wander aimlessly about. When contact is made with other
blastomeres during this wandering process the cells stick or adhere together.
As a result, the mass of adhering cells gradually is formed which becomes
rounded into a ball-shaped structure. The results of this work suggest that
the force which draws the cells together is one of thigmotaxis or contact
affinity, aided by a surface stickiness of the cells. This force only becomes
influential when an isolated cell has made contact with another cell or cells.
On the other hand, the early blastomeres of the cleaving mammalian egg
are evidently held together also by the binding influence of the egg membrane
or zona pellucida. An adhering influence is not prominent until later cleavage
stages.
However, one must not be too ready to espouse a single, mechanical factor
as the main binding force which causes the blastomeres to adhere together,
to move in relation to each other, and to form a definite configuration. Factors
tending toward organization are at work during early and late cleavage as
well as in subsequent development. Relative to these matters, it is well to
cogitate upon the statement of Whitman (1893). "Comparative embryology
reminds us at every turn that the organism dominates cell-formation, using
for the same purpose one, several, or many cells, massing its material and
directing its movements, and shaping its organs, as if cells did not exist, or
as if they existed only in complete subordination to its will" (p. 653).
D. Progressive Cytoplasmic Inequality and Nuclear Equality of the
Cleavage Blastomeres
1. Cytoplasmic Inequality of the Early Blastomeres
In harmony with the differences in the location and activities of the various
blastomeres of the cleaving egg, it is apparent that a difference exists in the
ooplasmic substance within the various cells in many species. In the frog,
for example, the quantity of yolk substance present in the cells of the yolk
pole is much greater than that of the animal pole. Similarly in the four-cell
stage the substance of the gray crescent is located in two of the blastomeres,
while the other two blastomeres have little or none of this substance. Two
of these four cells, therefore, are qualitatively different from the other two.
In the ascidian, Styela partita, the presence of the yellow crescent, yolk sub-
stance, and gray crescent materials demonstrates that in the four- or eight-
cell stages there are qualitative differences in the ooplasmic substances which
enter into the composition of the respective blastomeres (Conklin, '05, a
and b). Similar conditions may be demonstrated for Amphioxus although
pigmented materials are not present in the egg (fig. 167). (See Conklin, '32,
'33.) As cleavage continues in the eggs of Styela and Amphioxus, a progres-
MESOMERES
MACROMERES
SENSORY CILIA
OF
APICAL ORGAN
VEGETATIVE II
PLUS MICROMERES
EXOGASTRULA
Fig. 163. Developmental potencies (cell lineage) of isolated blastomeres of the cleav-
ing sea-urchin egg. representing different levels along the egg axis (from Huxley and
DeBeer, '34, after Horstadius). Observe the following: (1) Progressing from the animal
pole to the vegetative pole, the potency for developing the sensory cilia decreases from
animal pole cells I to animal pole cells II. (2) The potency for developing motile cilia
increases from animal pole cell II to vegetative pole cell I. (3) The potency for gastrula-
tion becomes greater from vegetative pole cell I to vegetative pole cell II. (4) In the
development of vegetative pole cell I, shown at the right of vegetative I, if the third
(equatorial) cleavage plane happens to be displaced near the animal pole, an isolated
vegetative cell I has more animal pole potencies and will develop apical cilia; if the
cleavage plane is displaced toward the vegetative pole, the vegetative pole cell I will
attempt to gastrulate. (5) The disc of vegetative cells II plus the micromeres produce a
gut so large it will not invaginate and hence forms an exogastrula.
329
30 CLEAVAGE (SEGMENTATION) AND BLASTULATION
ve difference in the cytoplasmic substances which enter into the various
lastomeres becomes evident.
That the presence or absence of a specific ooplasmic substance within the
lastomeres determines a difference in the developmental history of the cell
r cells has been shown experimentally for many animal species. For example,
1 the amphibian embryo it has been demonstrated both by constriction of
le developing egg and its membranes with hair loops (Spemann, '02, '03)
nd by placing a small glass rod in the cleavage furrow after the egg mem-
ranes have been removed (Ruud, '25) that each of the blastomeres of the
vo-cell stage will develop a complete embryo // the first cleavage plane bi-
dets the gray crescent. If, on the other hand, the first cleavage plane is at
glu angles to the median plane of the embryo, the blastomere which contains
le substance of the gray crescent will develop a complete embryo, whereas
le other one will give origin to a very imperfect form which does not gas-
ulate normally or produce a semblance of a normal embryo.
Similar experiments upon the egg of the newt, Triton pahnatus, indicate
lat a marked difference in the "developmental potencies exists between the
orsal and ventral sides of the egg within a few minutes from fertilization,
he formation of the gray crescent seems to be a secondary phenomenon
hich makes this difference clearly visible in the eggs of some species"
Fankhauser, '48, p. 694).
In Amphioxus, similar evidence is obtained after the blastomeres have been
lechanically isolated. Typical embryos are developed always from the first
vo blastomeres, for unlike the frog or newt, the first cleavage plane con-
\stently furrows the median axis of the embryo. These twin embryos are half
le normal size (Wilson, E. B., 1893; Conklin, '33). Right and left halves
f the four-cell stage also give rise to normal larvae. Moreover, blastulae also
evelop from isolated blastomeres of the eight-cell stage, bitt the blastulae
hich develop from the micromeres are smaller and have only one type of
ell, namely, ectoderm, and they never go further than the blastular stage.
)n the other hand, those from the macromeres are larger and have entoderm,
lesoderm, as well as ectoderm, but they never progress further than the
astrular stage of development (Conklin, '33). Reference should be made to
gure 167B in this connection. It is to be observed that the macromeres contain
otential mesodermal, entodermal, and ectodermal ooplasm, whereas the mi-
romeres lack the mesodermal and entodermal substances and contain only
ctodermal material.
In the protochordate, Styela, a somewhat different condition is found. If
le cleaving egg of this species is separated at the two-cell stage into two
eparate blastomeres, each blastomere develops only one half of an embryo
Conklin, '05b, '06). That is, the right blastomere develops an embryo minus
he left half, while the left blastomere produces the opposite condition. There
s some tendency to develop or regulate into a complete embryo in that the
PRIMORDIAL SOMA CELL (ECTODERM)
ECTODERM CELLS A+B
CHANGES SHAPE
ECTODERM CELLS
•^V"^ STOMODAEAL CELLS
*^''M ENTODERM CELLS
MESODERM
-^rrif — PRIMORDIAL
■■'■^■- GERM CELL
MESODERM
ECTODERM
Fig. 164. Distribution of presumptive organ regions (cell lineage) during cleavage
in Ascaris. (After Durken: Experimental Analysis of Development. New York, W. W.
Norton, based upon figures by Boveri and zur Strassen.) (A) Two-cell stage, showing
primordial soma cell and first stem cell. (B) Two ectodermal cells. A and B. Soma cell,
S,. is a mixture of mesoderm, stomodacum, and entoderm; second stem cell, P,. is a
mixture of mesoderm and germ-cell material. The symbolism used to designate the
various organ-forming substances is shown in (G). The progressive segregation into sep-
arate cells of the substances shown in cells S., and P, is given in (C-G). Cf. also fig. 6 IE.
331
332
CLEAVAGE (SEGMENTATION) AND BLASTULATION
POSIirON OF 2ND. MATURATION SPINDLE
POINT OF SPERM PENETRATION
D.
CLEAVAGE FURROW OF FIRST CLEAVAGE
ON PART OF NUCLEUS WHICH ENTERED " BRIDGE'
Fig. 165. Drawings of cleavage of a partially constricted egg of Triturus viridescens,
illustrating delayed nucleation. (Slightly modified from Fankhauser, '48.) (A) Shows
constricting loop, point of sperm entrance, and second maturation spindle. The constricted
portion to the right will contain the fusion nucleus. (B) First cleavage furrow in right
half of egg. (C) Second cleavage. The nucleus in the "bridge" area has migrated into
the "bridge." (D) Third cleavage. The nucleus in the bridge area has divided and pro-
duced cleavage furrow through the bridge cytoplasm as indicated. One of the daughter
nuclei of this cleavage is now in the constricted part of the egg at the left. (E) Fourth
cleavage = first division of left half. (F) Blastular stage — late blastula at right, middle
blastula at left.
ectoderm grows over the half of the embryo which failed to develop. Also,
the notochord rounds up into a normally shaped notochord but is only half
the normal size. Essentially, however, these separated blastomeres develop
into "half embryos in which some cells have grown over from the uninjured
to the injured side, but in which absolutely no change has taken place in the
potency of the individual cells or of the different ooplasmic substances"
(Conklin, '06). Similarly, at the four-cell stage isolation of anterior and poste-
rior blastomeres gives origin to anterior and posterior half embryos respectively.
The developing sea-urchin egg has been used extensively for experimental
work in the study of isolated blastomeres. In figures 163 and 166A-D are
shown the different developmental possibilities which arise from isolated blas-
tomeres of the early cleavage stages. Also, in cell-lineage studies on the de-
veloping egg of Ascaris, a difference in the developmental potencies of the
blastomeres is evident (fig. 164). (See also fig. 145A-D in respect to the
early development of the pig.)
The foregoing experiments and observations and others of a similar nature
suggest that, during the early cleavage stages of many different animal species,
a sorting-out process is at work which segregates into different blastomeres
CYTOPLASMIC INEQUALITY AND NUCLEAR EQUALITY 333
distinct ooplasmic substances which possess different developmental poten-
cies. This segregation of different substances into separate blastomeric chan-
nels is one of the functions of cleavage.
2. Nuclear Equality of the Early Blastomeres
Another question next arises: Is there a similar sorting out of nuclear sub-
stances during the cleavage period and do the nuclei in certain cells become
different from those of other cells? Or, do all of the nuclei retain an equality
during cleavage and development? Experimental evidence indicates a negative
answer to the former question and a positive one to the latter.
A precise and illuminating experiment demonstrating nuclear equality of
the early blastomeres may be performed by the hair-loop constriction method
(Spemann, '28; Fankhauser, '48). For example, the fertilized egg of the
newt, Triturus viridescens, may be constricted partially by a hair loop so that
the zygotic nucleus is confined to one side (fig. 165A, B). The side possessing
the nucleus divides, but the other side does not divide (fig. 165B, C). By
releasing the ligature between the two sides at various stages of development
of the cleaving side, i.e., 2-, 4-, 8-, 16-, and 32-cell stages, a nucleus is per-
mitted to "escape" into the cytoplasm of the uncleaved side (fig. 165C, E; in
D the escaped nucleus is seen in the blastomere to the left). By tightening
the loop again after the escaping nucleus has entered the uncleaved cytoplasm,
further nuclear "invasion" of the uncleaved part is blocked. If the original
constriction was made so that the plane of constriction coincides with the
plane of bilateral symmetry, i.e., if it constricts the gray crescent into two
halves, the result is two normal embryos. This occurs after the 2-, 4-, 8- and
16-cell stages of the cleaving half of the egg. Nuclei permitted to escape
when the cleaving side has reached the 32-cell stage do not produce normal
embryos in the uncleaved side, probably because of the changes which have
occurred in the meantime in the cytoplasm of the uncleaved side and not to
the qualitative differences in the nuclei at this stage.
Another type of experiment upon the early cleaving blastomeres which
demonstrates nuclear equality may be performed. It has been shown by
Pfluger, Roux, and Driesch (Wilson, E. B., '25, p. 1059) that a cleaving
egg pressed between two glass surfaces will divide parallel to the pressure
surfaces. That is, the mitotic spindle is moved into a position parallel to the
pressure surfaces. Under these circumstances, the spindle obeys the second
law of Hertwig, namely, that the mitotic spindle tends to coincide with the
long axis of the protoplasmic mass. Cleavage under pressure so applied, there-
fore, will result in a series of vertical cleavage planes. In the sea urchin
(fig. 166) if pressure is applied in the four-cell stage, the mitotic spindles
will form in a horizontal position, as shown in figure 166E, instead of in the
vertical position, as indicated in figure 166B, C, where no pressure is applied.
In other words, all of the nuclei shown in white in the upper blastomeres of
?4 CLEAVAGE (SEGMENTATION) AND BLASTULATION
gure 166C will be displaced horizontally by the applied pressure, as shown
1 figure 165F. If pressure is released at this stage, the mitotic spindle again
beys Hertwig's rule and forms in the long axis of the cytoplasm which is
ow vertical in position. As a result, upper and lower cells are formed, as
1 figure 166G. The original destiny of the nuclei in the cells producing ecto-
erm is shown in white circles; that for the cells destined to produce mesen-
lyme, entoderm, and ectoderm is shown in black (figs. 163, mesomeres;
66C, D). As shown in figure 166G, there is a mixture of these nuclei after
le pressure is released. Regardless of this redistribution of nuclei, develop-
lent proceeds almost normally. Development thus appears to be governed
y the presence of special ooplasmic substances contained within the respective
lastomeres (figs. 163; 166A-D).
The evidence from the foregoing experiments suggests the conclusion that
le nuclei in the early blastomeres are qualitatively equal. Consequently, this
ody of experimental evidence is antagonistic to the older view of Weismann,
ho held that differences in the various parts of the developing organism
■e to be attributed to "differential nuclear divisions" whereby different he-
;ditary qualities (i.e., biophors ) are dispersed to different cells. To quote
om Weismann (1893, p. 76):
Ontogeny depends on a gradual process of disintegration of the id of germ-
lasm, which splits into smaller and smaller groups of determinants in the devel-
pment of each individual, so that in place of a million ditlerent determinants, of
hich we may suppose the id of the germ-plasm to be composed, each daughter-cell
I the next ontogenetic stage would only possess half a million, and each cell of
le following stage only a quarter of a million and so on. Finally, if we neglect
assible complications, only one kind of determinant remains in each cell, viz.,
lat which has to control that particular cell or group of cells.
E. Quantitative and Qualitative Cleavages and Their Influence upon
Later Development
One of the earliest students of the problem of the developmental possi-
ilities of isolated blastomeres was Hans Driesch (1891 and 1892). In these
jblications, Driesch offered the results of experiments in which he shook
Dart the early blastomeres of the sea urchin and studied their development,
riesch found that the two blastomeres resulting from the first division con-
nued to divide, and as though the other blastomeres were present. The
rst division of the isolated blastomere was meridional, as if it had retained
Dntact with its mate of the two-cell stage. The next division was latitudinal,
so, as if it had retained contact with its original mate. Ultimately each iso-
ted blastomere developed into swimming blastulae of half the normal size,
he four blastomeres of the four-cell stage were similarly isolated. Here,
Iso, each divides as if it were part of the whole, and free-swimming blastulae
2velop. However, later development is imperfect or definitely abnormal.
QUANTITATIVE AND QUALITATIVE CLEAVAGES
335
Isolation of blastomeres in the eight-cell stage of development, in most cases,
results in abnormal development.
In Amphioxus, as mentioned previously, isolation of the first two blasto-
meres results in the production of twin embryos of half the normal size. In
the eight-cell stage in Amphioxus, the isolated smaller micromeres will de-
velop blastulae of ectoderm only, whereas the macromeres will develop blas-
tulae with developed entoderm, mesoderm, and ectoderm. In the four-cell
stage, if the two posterior blastomeres are separated from the two anterior
blastomeres, the former develop early embryos which have entoderm and
mesoderm together with ectoderm; the latter have notochord and neural plate
together with ectoderm and possibly a little of the mesoderm (Conklin, '33).
Similarly, in the frog or in the newt, when the first cleavage plane bisects
the gray crescent, the isolation of the first luo blastomeres results in the
Fig. 166. Nuclear equality in the sea-urchin egg. (.AD) Normal cleavage. While
nuclei and black nuclei theoretically so designed to show nuclei in animal and vegetal pole
cells respectively. (E) Four-cell stage flattened by pressure, showing position of spindles
for the third cleavage parallel \.o pressure surface. (F) Fight-cell stage under pressure.
Compare with (C). normal. (G) Horizontal cleavage resulting from release of pressure
after eight-cell stage. Note mixed distribution of nuclei. Later development normal, with
cytoplasmic, organ-forming substances determining development as in fig. 163. Thus it
appears that the nuclei are equal within the blastomeres. whereas the cytoplasm is un-
equally (i.e.. qualitatively) distributed to the respective blastomeres, the particular type of
development of the blastomeres being dependent upon the cytoplasmic substance present.
Black cytoplasm = micromeres which form primary mesenchyme. Coarse dotting =
entoderm, secondary mesenchyme and coelomic material. White, light stipple, and vertical
lines = ectodermal cells.
336
CLEAVAGE (SEGMENTATION) AND BLASTULATION
formation of two normal embryos. However, if the first cleavage is at right
angles to the plane of bilateral symmetry of the egg, the blastomere containing
the gray crescent material will develop a normal embryo, but the other blas-
tomere will not do so.
The above results from isolated blastomeres suggest the following: When
the division of the early egg is purely quantitative, so that the resulting blasto-
meres contain all of the cytoplasmic substances equally, as in the first one or
two cleavage planes in the sea urchin (fig. 166A, B) or the first cleavage in
the frog when it bisects the gray crescent, the isolation of the resulting blasto-
meres tends to produce complete embryos. Such blastomeres are known as
totipotent blastomeres. (See Chap. 8.) However, when cleavage is quali-
tative, such as the second cleavage of Amphioxus, the third cleavage of the
sea urchin (fig. 166C), or the first cleavage of the frog when it occurs at right
angles to the median axis of the embryo, the resulting development depends
upon the qualities (that is, ooplasmic substances) resident in the isolated
blastomeres.
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Tne Cnordate Blastula and Its Si^niiicance
A. Introduction
1. Blastulae without auxiliary tissue
2. Blastulae with auxiliary or trophoblast tissue
3. Comparison of the two main blastular types
B. History of the concept of specific, organ-forming areas
C. Theory of epigenesis and the germ-layer concept of development
D. Introduction of the words ectoderm, mesoderm, endoderm
E. Importance of the blastular stage in Haeckel's theory of "The Biogenetic Law of
Embryonic Recapitulation"
F. Importance of the blastular stage m embryonic development
G. Description of the various types of chordate blastulae with an outline of their organ-
forming areas
1. Protochordate blastula
2. Amphibian blastula
3. Mature blastula in birds
4. Primary and secondary reptilian blastulae
5. Formation of the late mammalian blastocyst (blastula)
a. Prototherian mammal, Echidna
b. Metatherian mammal, Didelphys
c. Eutherian mammals
6. Blastulae of teleost and elasmobranch fishes
7. Blastulae of gymnophionan amphibia
A. Introduction
In the previous chapter it was observed that two main types of blastulae
are formed in the chordate group:
(1) those blastulae without accessory or trophoblast tissue, e.g., Amphi-
oxits, frog, etc. and
(2) those possessing such auxiliary tissue, e.g., elasmobranch and teleost
fishes, reptiles, birds, and mammals.
1. Blastulae Without Auxiliary Tissue
The blastulae which do not have the auxiliary tissues are rounded affairs
composed of a layer of blastomeres surrounding a blastocoelic cavity (figs.
340
INTRODUCTION 341
143C). The layer of blastomeres forms the blastoderm. The latter
e one cell in thickness, as in Amphioxiis (fig. MOT), or several cells
kness, as in the frog (fig. 143C). This hollow type of blastula often is
;d to as a coeloblastula or blastosphere. However, in the gymnophionan
bia, the blastula departs from this vesicular condition and appears
>olid. The latter condition may be regarded as a stereoblastula, i.e., a
ilastula. A somewhat comparable condition is present in the bony ganoid
Amia and Lepisosteus.
main characteristic of the blastula which does not possess auxiliary
is that the entire blastula is composed of formative cells, i.e., all the
nter directly into the formation of the embryo's body.
2. Blastulae with Auxiliary or Trophoblast Tissue
examination of those blastulae which possess auxiliary or trophoblast
shows a less simple condition than the round blastulae mentioned above,
first place, two types of cells are present, namely, formative cells which
into the composition of the embryonic body and auxiliary cells con-
mainly with trophoblast, or nutritional, functions. In the second place,
blastula which possesses auxiliary tissue, the latter often develops pre-
sly, that is, in advance of the formative cells of the blastula. As a
the arrangement of the formative cells into a configuration comparable
; of those blastulae without trophoblast cells may be much retarded in
I instances. This condition is true particularly of the mammalian blastula
)cyst).
lerally speaking, the blastulae which possess auxiliary tissue consist in
:arlier stages of a disc or a mass of formative cells at the peripheral
IS of which are attached the non-formative, auxiliary cells (fig. 159,
ierm-formative cells, periblast-non-formative; also figs. 145K, L; 147G,
he blastocoelic space lies below this disc of cells. However, in mammals
sciliary or nourishment-getting tissue tends to circumscribe the blastocoel,
is the formative cells occupy a polar area (fig. 145G, H). Blastulae,
ised of a disc-shaped mass of cells overlying a blastocoelic space, have
lescribed in classical terms as discoblastulae.
3. Comparison of the Two Main Blastular Types
e compare these two types of blastulae in terms of structure, it is evident
comparison is not logical unless the essential or formative cells and
irrangement are made the sole basis for the comparison, for only the
ive cells are common to both types of blastulae. To make the foregoing
ent more obvious, let us examine the essential structure of a typical
iastula, such as found in Amphioxus, as it is defined by the present-
nbryologist.
studies by Conklin, '32 and '33, demonstrated that the fertilized egg
342 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
of Amphioxus possesses five major, presumptive, origan-forming areas (fig.
167A). These areas ultimately give origin to the ectodermal, mesodermal,
entodermal, notochordal, and neural tissues. In the eight-cell stage of cleavage,
the cytoplasmic substances concerned with these areas are distributed in such
a way that the blastomercs have different substances and, consequently, differ
qualitatively (fig. I67B). Specifically, the entoderm forms the ventral part
of the four ventral blastomeres; the ectoderm forms the upper or dorsal portion
of the four micromeres, while the mesodermal, notochordal, and neural sub-
stances lie in an intermediate zone between these two organ-forming areas,
particularly so in the blastomeres shown at the left in figure 167B. In figure
167C and D is shown a later arrangement of the presumptive, organ-forming
areas in the middle and late stages of blastuiar development. These figures
represent sections of the blastulae. Consequently the organ-forming areas are
contained within cells which occupy definite regions of the hlastula. In figure
167E-G are presented lateral, vegetal pole, and dorso-posterior pole views
of the mature blastula (fig. I67D), representing the organ-forming areas as
viewed from the outside of the blastula.
It is evident from this study by Conklin that the organization of the fertilized
egg of Amphio.xus passes gradually but directly through the cleavage stages
into the organization of the mature blastula; also, that the latter, like the egg,
is composed of five, major, presumptive, organ-forming areas. It is evident
further that one of the important tasks of cleavage and blastulation is to de-
velop and arrange these major, organ-forming areas into a particular pattern.
(Note: Later the mesodermal area divides in two, forming a total of six, pre-
sumptive, organ-forming areas. )
If we analyze the arrangement of these presumptive, organ-forming areas,
we see that the mature blastula is composed of a tloor or hypoblast, made
up of potential, entoderm-forming substance, and a roof of potential ectoderm
with a zone of mesoderm and chordoneural cells which lie in the area between
these two general regions. In fact, the mesodermal and chordoneural materials
form the lower margins of the roof of the mature blastula (fig. 167D). Con-
sequently, the mature blastula of Amphioxus may be pictured as a bilaminar
affair composed essentially of a hypoblast or lower layer of presumptive
entoderm, and an upper concave roof or e pi blast containing presumptive
ectoderm, neural plate, notochord, and mesodermal cells. It is to be observed
further that the blastocoel is interposed between these two layers. This is the
basic structure of a typical coeloblastula. Furthermore, this blastula is com-
posed entirely of formative tissue made up of certain definite, potential, organ-
forming areas which later enter into the formation of the body of the embryo;
auxiliary or non-formative tissue has no part in its composition. All coelo-
blastulae conform to this general structure.
If we pass to the blastula of the early chick embryo, a striking similarity
may be observed in reference to the presumptive, organ-forming areas (fig.
ORGAN-FORMING AREAS 343
173). An upper, epiblast layer is present, composed of presumptive ecto-
dermal, neural, notochordal, and mesodermal cells, while a hypoblast layer
of entodermal potency lies below. Between these two layers the blastocoelic
space is located. However, in the chick blastoderm, in addition to the formative
cells, a peripheral area of auxiliary or irophohlasi (periblast) tissue is present.
B. History of the Concept of Specific, Organ-forming .\reas
The idea that the mature egg or the early developing embryo possesses
certain definite areas having different qualities, each of which contributes
to the formation of a particular organic structure or of several structures,
finds its roots in the writings of Karl Frnst von Baer, 182S-1837. \'on Baer's
comparative thinking and comprehensive insight into embryology and its proc-
esses established the foundation for many of the results and conclusions that
have been achieved in this field during the past one hundred years.
Some forty years later, in 1 874. Wilhclm His in his book. I nsere Korperform,
definitely put forth the organ-forming concept relative \o the germ layers of
the chick, staling that "the germ-disc contains the organ-germs spread out
in a flat plate," and he called this the principle of the organ-forming germ-
regions (Wilson, *25, p. 1041 ). Ray l.ankcster. in 1877, advanced views
supporting an early segregation from the fertilized egg of 'already formed
and individualized" substances, as did ('. O. Whitman ( 1878) in his classical
work on the leech, Clepsine. In this work. Whitman concludes that there is
definite evidence in favor of the preformation of organ-forming stuffs within
the egg. Other workers in embryology, such as Rabl, Van Beneden. etc., began
to formulate similar views (Wilson, '25, pp. 1041-1042).
The ideology embodied within the statement of Ra\ 1 ankesier referred
to above was the incentive for considerable research in that branch of em-
bryological investigatiim known as "cell lineage." To quote more fully from
Lankester"s statement in this eonnecluni. p. 410:
1 hough the substance of a cell may appear homogeneous under the most powerful
mieroseopc. excepting for the fine granular matter suspended in it. it is quite pos-
sible, indeed certain, that it may contain, already formed and individualized, various
kinds ot physiological molecules. Ihe visible process of segregation is only the
sequel of a differentiation already established, and not visible.
The studies on cell lineage in many invertebrate forms, such as that of
Whitman ( 1878 ) on Clepsine, of Wilson ( 1892 ) on Nereis, of Boveri (1892)
and zur Strasscn (1896; fig. 163B) on Ascaris, or the work of Horstadius
('28, '37; fig. 163A) on the sea urchin, serve to emphasize more forcefully
the implications of this statement. In these studies the developmental pro-
spective fates of the various early cleavage blastomeres were carefully observed
and followed.
Much of the earlier work on cell lineage was devoted to invertebrate forms.
One of the first students to study the matter in the phylum Chordata was
344 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
E. G. Conklin who published in 1905 a classical contribution to chordate
embryology relative to cell lineage in the ascidian, Styela (Cynthia) partita.
This monumental work extended the principle of organ-forming, germinal
areas to the chordate embryo. However, the significance of the latter obser-
vations, relative to the chordate phylum as a whole, was not fully appreciated
until many years later when it was brought into prominence by the German
investigator, W. Vogt ('25, '29).
Vogt began a series of studies which involved the staining of different parts
of the amphibian blastula with vital dyes and published his results in 1925
and 1929. The method employed by Vogt is as follows:
Various parts of the late amphibian blastula are stained with such vital
dyes as Nile-blue sulfate, Bismarck brown, or neutral red (fig. 168A). These
stains color the cells but do not kill them. When a certain area of the blastula
is stained in this manner, its behavior during later stages of development can
be observed by the following procedure: After staining a particular area, the
embryo is observed at various later periods, and the history of the stained
area is noted. When the embryo reaches a condition in which body form is
fully established, it is killed, fixed in suitable fluids, embedded in paraffin,
and sectioned. Or, the embryo may be dissected after fixation in a suitable
fluid. The cellular area of the embryo containing the stain thus may be de-
tected and correlated with its original position in the blastula (cf. fig. 1 68 A, B ) .
This procedure then is repeated for other areas of the blastula (fig. 168C-E).
Vogt thus was able to mark definite areas of the late blastula, to follow their
migration during gastrulation, and observe their later contribution to the for-
mation of the embryonic body. Definite maps of the amphibian blastula in
relation to the future history of the respective blastular areas were in this
way established (fig. 169C).
This method has been used by other investigators in the study of similar
phenomena in other amphibian blastulae and in the blastulae and gastrulae
of other chordate embryos. Consequently, the principle of presumptive, organ-
forming areas of the blastula has been established for all of the major chordate
groups other than the mammals. The latter group presents special technical
difficulties. However, due to the similarity of early mammalian development
with the development of other Chordata, it is quite safe to conclude that they
also possess similar, organ-forming areas in the late blastular and early gas-
trular stages.
The major, presumptive, organ-forming areas of the late chordate blastula
are as follows (figs. 167, 169, 173, 174, 179, 180, 181):
(1) There is an ectodermal area which forms normally the epidermal
layer of the skin;
(2) also, there is an ectodermal region which contributes to the formation
of the neural tube and nervous system;
EPIGENESIS AND THE GERM-LAYER CONCEPT 345
(3) a notochordal area is present which later gives origin to the primitive
axis;
(4) the future mesodermal tissue is represented by two areas, one on either
side of the notochordal area. In Amphioxus, however, this mesodermal
area is present as a single area, the ventral crescent, which divides
during gastrulation into two areas;
(5) the entodermal area, which gives origin to the future lining tissue of
the gut, occupies a position in the blastula either at or toward the vege-
tative pole;
(6) there is a possibility that another potential area, containing germinal
plasm, may be present and integrated with the presumptive entoderm
or mesoderm. This eventually may give origin to the primitive germ
cells;
(7) the pre-chordal plate region is associated with the notochordal area
in all chordates in which it has been identified and lies at the caudal
margin of the latter. In gastrulation it maintains this association. The
pre-chordal plate material is an area which gives origin to some of
the head mesoderm and possibly also to a portion of the roof of the
foregut. It acts potently in the organization of the head region. Ac-
cordingly, it may be regarded as a complex of entomesodermal cells,
at least in lower vertebrates.
C. Theory of Epigenesis and the Germ-layer Concept of Development
As the three classical germ layers take their origin from the blastular state
(see Chap. 9), it is well to pause momentarily to survey briefly the germ-layer
concept.
That the embryonic body is derived from definite tissue layers is an old
concept in embryology. Casper Friedrich Wolff (1733-94) recognized that
the early embryonic condition of the chick blastoderm possessed certain layers
of tissue. This fact was set forth in his Theoria Generationis, published in
1759, and in De jormatione intestinorum praecipue, published in 1769, de-
voted to the description of the intestinal tract and other parts of the chick
embryo. In these works Wolff presented the thesis that embryonic develop-
ment of both plants and animals occurred by "a host of minute and always
visible elements that assimilated food, grew and multiplied, and thus gradually
in associated masses" produced the various structures which eventually be-
come recognizable as "the heart, blood vessels, limbs, alimentary canal, kid-
neys, etc." (The foregoing quotations are from Wheeler, 1898.) These state-
ments contain the essence of Wolff's theory of epigenesis. That is, that develop-
ment is not a process of unfolding and growth in size of preformed structures;
rather, it is an indirect one, in which certain elements increase in number and
gradually become molded into the form of layers which later give rise to the
organ structures of the organism.
346 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
Two Other men contributed much to the layer theory of development,
namely, Heinrich Christian Pander (1794-1865) and Karl Ernst von Baer
(1792-1876). In 1817, Pander described the trilaminar or triploblast con-
dition of the chick blastoderm, and von Baer, in his first volume (1828) and
second volume (1837) on comparative embryology of animals, delineated
four body layers. The four layers of von Baer's scheme are derived from
Pander's three layers by dividing the middle layer into two separate layers
of tissue. Von Baer is often referred to as the founder of comparative em-
bryology for various reasons, one of which was that he recognized that the
layer concept described by Pander held true for many types of embryos,
vertebrate and invertebrate. The layer concept of development thus became
an accepted embryological principle.
While Pander and von Baer, especially the latter, formulated the germ-
layer concept as a structural fact for vertebrate embryology, to Kowalewski
(1846-1901) probably belongs the credit for setting forth the idea, in his
paper devoted to the early development of Amphioxus ( 1 867 ) , that a primary,
single-layered condition changes gradually into a double-layered condition.
The concept of a single-layered condition transforming into a double-layered
condition by an invaginative procedure soon became regarded as a funda-
mental embryological sequence of development.
Gradually a series of developmental steps eventually became crystallized
from the fact and speculation present during the latter half of the nineteenth
century as follows:
( 1 ) The blastula, typically a single-layered, hollow structure, becomes con-
verted into
(2) the two-layered gastrula by a process of invagination of one wall or
delamination of cells from one wall of the blastula; then,
(3) by an outpouching of a part of the inner layer of the gastrula, or by
an ingression of cells from this layer, or from the outside ectoderm, a
third layer of cells, the mesoderm, comes to lie between the entoderm
and ectoderm; and finally,
(4) the inner layer of mesoderm eventually develops into a two-layered
structure with a coelomic cavity between the layers.
This developmental progression became accepted as the basic procedure
in the development of most Metazoa.
The original concept of the germ layers maintained that the layers were
specific. That is, entodermal tissue came only from entoderm, ectodermal
tissue from ectoderm, etc. However, experimental work on the early embryo
in which cells are transplanted from one potential layer to another has over-
thrown this concept (Oppenheimer, '40). The work on cell lineage and the
demonstration of the early presence of the presumptive, organ-forming areas
BIOGENETIC LAW OF EMBRYONIC RECAPITULATION 347
also have done much to overthrow the concept concerning the rigid specificity
of the three primary germ layers of entoderm, mesoderm, and ectoderm.
D. Introduction of the Words Ectoderm, Mesoderm, Endoderm
Various students of the Coelenterata, such as Huxley (1849), Haeckel
(1866) and Kleinenberg (1872), early recognized that the coelenterate body
was constructed of two layers, an outer and an inner layer. Soon the terms
ectoderm (outside skin) and endoderm (inside skin) were applied to the outer
and inner layers or membranes of the coelenterate body, and the word
mesoderm (middle skin) was used to refer to the middle layer which ap-
peared in those embryos having three body layers. The more dynamic
embryological words epiblast, mesoblast, and hypoblast (entoblast) soon
came to be used in England by Balfour, Lankester, and others for the words
ectoderm, mesoderm, and endoderm, respectively. The word entoderm is used
in this text in preference to endoderm.
E. Importance of the Blastular Stage in Haeckel's Theory of "The
Biogenetic Law of Embryonic Recapitulation"
In 1859, Charles Darwin (1809-82) published his work On the Origin
of Species by Means of Natural Selection. This theory set the scientific world
aflame with discussions for or against it.
In 1872 and 1874, E. Haeckel (1834-1919), an enthusiast of Darwin's
evolutionary concept, associated the findings of Kowalewski regarding the
early, two-layered condition of invertebrate and vertebrate embryos together
with the adult, two-layered structure of the Coelenterata and published the
blastaea-gastraea theory and biogenetic principle of recapitulation. In these
publications he applied the term gastrula to the two-layered condition of the
embryo which Kowalewski has described as the next developmental step suc-
ceeding the blastula and put forward the idea that the gastrula was an em-
bryonic form common to all metazoan animals.
In his reasoning (1874, translation, '10, Chap. 8, Vol. I), Haeckel applied
the word blastaea to a "long-extinct common stem form of substantially the
same structure as the blastula." This form, he concluded, resembled the
"permanent blastospheres" of primitive multicellular animals, such as the
colonial Protozoa. The body of the blastaea was a "simple hollow ball, filled
with fluid or structureless jelly with a wall composed of a single stratum of
homogeneous ciliated cells."
The next phylogenetic stage, according to Haeckel, was the gastraea, a
permanent, free-swimming form which resembled the embryonic, two-layered,
gastrular stage described by Kowalewski. This was the simple stock form for
all of the Metazoa above the Protozoa and other Protista. Moreover, he
postulated that the gastrula represented an embryonic recapitulation of the
adult stage of the gastraea or the progenitor of all Metazoa.
348 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
The assumed importance of the blastula and gastrula thus became the
foundation for HaeckeFs biogenetic principle of recapitulation. Starting with
the postulation that the hypothetical blastaea and gastraea represented the
adult phylogenetic stages comparable to the embryonic blastula and gastrula,
respectively, Haeckel proceeded, step by step, to compress into the embryo-
logical stages of all higher forms the adult stages of the lower forms through
which the higher forms supposedly passed in reaching their present state
through evolutionary change. The two-chambered condition of the develop-
ing mammalian heart thus became a representation of the two-chambered,
adult heart of the fish, while the three-chambered condition recapitulated the
adult amphibian heart, etc. Again, the visceral arches of the embryonic pha-
ryngeal regions of the mammal represented the gill-slit condition of the fish.
Ontogeny thus recapitulates phylogeny, and phylogeny of a higher species is
the result of the modification of the adult stages of lower species in the phylo-
genetic scale. The various steps in the embryological development of any
particular species, according to this reasoning, were caused by the evolutionary
history of the species; the conditions present in the adult stage of an earlier
phylogenetic ancestor became at once the cause for its existence in the em-
bryological development of all higher forms. Embryology in this way became
chained to a repetition of phylogenetic links!
Many have been the supporters of the biogenetic law, and for a long time
it was one of the most popular theories of biology. A surprising supporter of
the recapitulation doctrine was Thomas Henry Huxley (1825-95). To quote
from Oppenheimer ('40): "One wonders how the promulgator of such a
distorted doctrine of cause and effect could have been championed by the
same Huxley who wrote: 'Fact I know and Law I know; but what is this
Necessity save an empty Shadow of my own mind's throwing?'."
The Haeckelian dogma that ontogeny recapitulates phylogeny fell into error
because it was formulated upon three false premises due to the fragmentary
knowledge of the period. These premises were:
( 1 ) That in evolution or phylogeny, recently acquired, hereditary charac-
ters were added to the hereditary characters already present in the
species;
(2) that the hereditary traits revealed themselves during embryonic devel-
opment in the same sequence in which they were acquired in phylogeny;
and
(3) that Darwin's concept of heredity, namely, pangenesis, essentially was
correct.
The theory of pangenesis assumed that the germ cells with their hereditary
factors were produced by the parental body or soma and that the contained
hereditary factors within the germ cells were produced by gemmules which
BIOGENETIC LAW OF EMBRYONIC RECAPITULATION 349
migrated from the various soma cells into the germ cells. This theory further
postulated the inheritance of acquired characters.
If these three assumptions are granted, then it is easy to understand Haeckel's
contention that embryological development consists in the repetition of pre-
vious stages in phylogeny. For example, if we assume that the blastaea changed
into the gastraea by the addition of the features pertaining to the primitive
gut with its enteric lining, then the gastraea possessed the hereditary factors
of the blastaea plus the new enteric factors. These enteric features could
easily be added to the deric (outer-skin) factors of the blastaea, according
to Darwin's theory of pangenesis. Furthermore, according to assumption (2)
above, in the embryonic development of the gastraea, the hereditary factors
of the blastaea would reveal themselves during development first and would
produce the blastaea form, to be followed by the appearance of the specific
enteric features of the gastraea. And so it proceeded in the phylogeny and
embryology of later forms. In this way the preceding stage in phylogeny be-
came at once the cause of its appearance in the development of the next
phylogenetic stage.
These assumptions, relative to heredity and its mechanism of transference,
were shown to be untenable by the birth of the Nageli-Roux-Weismann con-
cept of the germ plasm (see Chaps. 3 and 5) and by the rebirth or rediscovery
of Mendelism during the latter part of the nineteenth century. Studies in em-
bryology since the days of Weismann have demonstrated in many animal
species the essential correctness of Weismann's assumption that the germ
plasm produces the soma during development, as well as the future germ
plasm, and thus have overthrown the pangenesis theory of Darwin. The as-
siduous study of Mendelian principles during the first twenty-five years of the
twentieth century have demonstrated that a fixed relation does not exist be-
tween the original character and the appearance of a new character as implied
in the Haeckelian law (Morgan, '34, p. 148). Furthermore, that "in many
cases, perhaps in most, a new end character simply replaces the original one.
The embryo does not pass through the last stage of the original character
and then develop the new one — although this may happen at times — but the
new character takes the place of the original one" (Morgan, '34, p. 148).
How then does one explain the resemblances of structure to be found
among the embryos at various stages of development in a large group of
animals such as the Chordata? Let us endeavor to seek an explanation.
In development, nature always proceeds from the general to the specific,
both in embryological development and in the development of phylogeny or
a variety of forms. The hereditary factors which determine these generalized
states or structural conditions apparently are retained, and specialized fac-
tors come into play after the generalized pattern is established. Generalized
or basic conditions, therefore, appear before the specialized ones. An example
of this generalized type of development is shown in the formation of the
350 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
blastula in chordate animals. Although many different specific types and shapes
of blastulae are present in the group as a whole, all of them can be resolved
into two basic groups. These groups, as mentioned in the beginning of this
chapter, are:
( 1 ) blastulae without auxiliary, nutritive tissue and
(2) blastulae with auxiliary tissue.
Moreover, if the auxiliary tissue of those blastulae which possess this tissue
is not considered, all mature chordate blastulae can be reduced to a funda-
mental condition which contains two basic layers, namely, hypoblast and epi-
blast layers. The epiblast possesses presumptive epidermal, neural, notochordal,
and mesodermal, organ-forming areas, while the hypoblast cells form the
presumptive entodermal area. The shapes and sizes of these blastulae will,
of course, vary greatly. Moreover, the hypoblast cells may be present in
various positions, such as a mass of cells at the caudal end of a disc-shaped
epiblast (teleost and elasmobranch fishes), an enlarged, thickened area or
pole of a hollow sphere (many Amphibia) , a single, relatively thin layer of
cells, forming part of the wall of a hollow sphere (Aniphioxus), a rounded,
disc-shaped mass of cells overlain by the thin, cup-shaped epiblast (Clavelina),
a thickened mass attached to the underside of the caudal end of the disc-shaped
epiblast (chick; certain reptiles), a thin layer of cells situated below the epiblast
layer (mammals), or a solid mass of cells, lying below a covering of epiblast
cells (gymnophionan Amphibia). Although many different morphological
shapes are to be found in the blastulae of the chordate group, the essential,
presumptive, organ-forming areas always are present, and all are organized
around the presumptive notochordal area.
But the question arises: Why is a generalized blastular pattern developed
instead of a series of separate, distinct patterns? For instance, why should the
notochordal area appear to occupy the center of the presumptive, organ-
forming areas of all the chordate blastulae when this area persists as a promi-
nent morphological entity only in the adult condition of lower chordates?
The answer appears to be this: The notochordal area at this particular stage
of development is not alone a morphological area, but it is also a physiological
instrument, an instrument which plays a part in a method or procedure of
development. The point of importance, therefore, in the late blastular stage
of development is not that the notochordal area is going to contribute to the
skeletal axis in the adult of the shark, but rather that it forms an integral part
of the biolgical mechanism which organizes the chordate embryo during the
period immediately following the blastular stage. Thus, if the notochordal
material can play an important role in the organization of the embryo and
in the induction of the neural tube in the fish or in the frog, it also can fulfill
a similar function in the developing chid or human embryo. Whatever it does
later in development depends upon the requirements of the species. To use
IMPORTANCE OF THE BLASTULAR STAGE 351
a naive analogy, nature does not build ten tracks to send ten trains with dif-
ferent destinies out of a sta:tion when she can use one track for all for at least
part of the way. So it is in development. A simple tubular heart appears in
all vertebrate embryos, followed by a simple, two-chambered* condition, not
because the two-chambered heart represents the recapitulated, two-chambered,
fish heart but rather because it, like the notochord, is a stage in a dynamic
developmental procedure of heart development in all vertebrates. As far as
the fish is concerned, when the common, two-chambered, rudimentary stage
of the heart is reached, nature shunts it off on a special track which develops
this simple, two-chambered condition into the highly muscular and efficient
two-chambered, adult heart adapted to the fish level of existence in its watery
environment. The three-chambered,* amphibian heart follows a similar pattern,
and it specializes at the three-chambered level because it fits into the amphibian
way of life. So it is with the embryonic pharyngeal area with its visceral and
aortal arches which resemble one another throughout the vertebrate group
during early embryonic development. The elaboration of a common, pha-
ryngeal area with striking resemblances throughout the vertebrate group can
be explained more easily and rationally on the assumption that it represents
a common, physiologically important step in a developmental procedure.
This general view suggests the conclusion that ontogeny tends to use com-
mon developmental methods wherever and whenever these methods can be
utilized in the development of a large group of animals. Development or
ontogeny, therefore, recapitulates phylogenetic procedures and not adult mor-
phological stages. One explanation for this conservation of effort may be
that, physiologically speaking, the number of essential methods, whereby a
specific end may be produced, probably is Hmited. Another explanation sug-
gests that an efficient method never is discarded.
F. Importance of the Blastular Stage in Embryonic Development
Superficially in many forms, chordate and non-chordate, the blastula is a
hollow, rounded structure containing the blastocoelic space within. It is tempt-
ing to visualize this form as the basic, essential form of the blastula. How-
ever, the so-called blastular stage in reality presents many forms throughout
the animal kingdom, some solid, some round and hollow, and others in the
form of a flattened disc or even an elongated band. Regardless of their shape,
all blastulae have this in common: they represent an association of pre-
sumptive organ-forming areas, areas which later move to new positions in
the forming body, increase in cellular mass, and eventually become molded
into definite structures. One of the main purposes of blastulation, therefore,
may be stated as the elaboration (or establishment) of the major, presumptive
organ-forming areas of the particular species and their arrangement in a
particular pattern which permits their ready manipulation during the next
* Exclusive of the sinus venosus.
352 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
Step of development or gastrulation. The particular shape of the blastula has
its importance. However, this importance does not lie in the supposition that
it conforms to a primitive spherical type but rather that the various, pre-
sumptive, organ-forming areas are so arranged and so poised that the cell
movements so necessary to the next phase of development or gastrulation
may be properly executed for the particular species. In most species, the
formation of a blastocoelic space also is a necessary function of blastulation.
In some species, however, this space actually is not formed until the next
stage of development or gastrulation is in progress.
In summary, therefore, it may be stated that the importance of the blastula
does not reside in the supposed fact that it is a one-layered structure or
blastoderm having a particular shape. Rather, its importance emerges from
the fact that the blastoderm has certain, well-defined areas segregated within
it — areas which will give origin to future organ structures. Moreover, these
areas foreshadow the future germ layers of the body. In diploblastic Metazoa,
two germ layers are foreshadowed, while in triploblastic forms, three germ
layers are outlined. As far as the Chordata are concerned, the hypoblast is
the forerunner of the entoderm or the internal germ layer; whereas the
epiblast is composed potentially of two germ layers, namely, the epidermal,
neural plate areas which form the ectodermal layer and the chordamesodermal
or marginal zone cells which give origin to the middle germ layer.
In the following pages, the chordate blastula is described as a two-layered
structure composed of various, potential, organ-forming areas. This two-
layered configuration, composed of a lower hypoblast and an upper epiblast,
is used to describe the chordate blastula for the dual purpose of comparison
and analysis of the essential structure of the various blastulae. The bilaminar
picture, it is believed, will enable the student to understand better the changes
which the embryo experiences during the gastrulative period.
G. Description of the Various Types of Chordate Blastulae with an
Outline of Their Organ-forming Areas
1. Protochordate Blastula
The following description pertains particularly to Amphioxus. With slight
modification it may be applied to other protochordates, such as Clavelina,
Ascidiella, Styela, etc.
As noted in the introduction to this chapter, the potential entodermal cells
of Amphioxus lie at the vegetal pole and form most of the floor or hypoblast
of the blastula (fig. 167D). The upper or animal pole cells form a roof of
presumptive epidermal, notochordal, mesodermal, and neural cells arched
above and around the entoderm. The latter complex of organ-forming cells
forms the epiblast. The blastocoelic cavity is large and insinuated between
the hypoblast and epiblast. The presumptive notochordal and mesodermal
Fig 167 Presumptive organ-forming areas in the uncleaved egg and durmg cleav-
age and blastulation in Amphioxus. (Original diagram based upon data obtained from
Conklin '32 '33.) (A) Uncleaved egg. (B) Eight-cell stage. (C) Early blastula m
(D) Late blastula in section. (E) Late blastula, external view from side.
section.
(F) Late blastula, external, vegetal pole view. (G) Late blastula, external, dorso-
posterior view. The localization of cytoplasmic materials in Styela partita is similar to
that of Amphioxus. Observe that the pointed end of the arrow defines the future cephalic
end of the embryo. The position of the polar body denotes the antero-ventral area, while
the position of the notochordal and neural plate material represents the antero-dorsal
region. The "tail end" of the arrow is the postero-ventral area of the embryo.
353
354
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
areas lie at the margins of the entodermal layer and surround it. As such,
some of the cells of these two, organ-forming areas may form part of the
floor of the blastula. The presumptive, notochordal and neural plate cells lie
at the future dorsal lip of the blastopore and form the dorsal crescent, while
the mesodermal area occupies the ventral-lip region as the ventral crescent
(fig. 167F). In Amphioxus, the mature blastula is pear shaped, with the body
Fig. 168. Ultimate destiny within the developing body of presumptive organ-forming
areas of the late amphibian blastula, stained by means of vital dyes. (After Pasteels: J.
Exper. Zool., 89.) (A) Area of blastula, stained. (B) Destiny of cellular area, stained
in (A). (D, E) Ultimate destiny shown by broken lines of cellular areas, stained in
late blastula shown in (C). (E) Anterior trunk segment. (D) Posterior trunk segment.
Fig 169 Presumptive organ-forming areas in the amphibian late blastula and be-
ginning gastrula. (A, B) General epiblast and hypoblast areas of the early and late
blastular conditions, respectively. The hypoblast is composed mainly of entodermal or
gut-lining structures, whereas the epiblast is a composite of ectodermal (i e epiderma
and neural), mesodermal, and notochordal presumptive areas. Observe that the epiblast
gradually grows downward over the hypoblast as the late blastula is formed. (C) Be-
ginning gastrula of the urodele, Triton. (Presumptive areas shown according to Vogt
'29 ) (D) Same as above, from vegetative pole. (Slightly modified from Vogt, 29 J
(E) Lateral view of beginning gastrula of anuran amphibia. (F) Dorsal view of the
same (E F derived from description by Vogt. '29, relative to Rana jiisca and Bom-
binator: also Pasteels: J. Exper. Zool., 89, relative to Discoglossus.) Observe that an
antero-posterior progression of somites is indicated in C and D.
355
356 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
of the mesodermal crescent comprising much of the neck portion of the "pear"
(fig. 167E).
The blastula of Amphioxus thus may be regarded essentially as a bilaminar
structure (i.e., two-layered structure) in which the hypoblast forms the lower
layer while the epiblast forms the upper composite layer.
2. Amphibian Blastula
In the amphibian type of blastula, a spherical condition exists similar to
that in Amphioxus (fig. 169). The future entoderm is located at the vegetative
(vegetal) pole, smaller in amount in the frog, Rana pipiens, and larger in
such forms as Necturus maculosus (fig. 169A, B). The presumptive noto-
chordal material occupies an area just anterior to and above the future dorsal
lip of the blastopore. The dorsal lip of the gastrula, when it develops, arises
within the entodermal area (fig. 169C-F). Extending laterally on either side
of the presumptive notochordal region is an area of presumptive mesoderm
(fig. 169C-F). Each of these two mesodermal areas tapers to a smaller di-
mension as it extends outward from the notochordal region. The presumptive
notochordal and mesodermal areas thus form a composite area or circular
marginal zone which surrounds the upper rim of the entodermal material.
Above the chordamesodermal zone are two areas. The presumptive neural
area is a crescent-like region lying above or anterior to the presumptive
notochord-mesoderm complex. Anterior to the neural crescent and occupying
the remainder of the blastular surface, is the presumptive epidermal crescent
(fig. 169C-F).
In the various kinds of blastulae of this group, the yolk-laden, vegetal pole
cells actually form a mass which projects upward into the blastocoelic space
(fig. 169 A, B). The irregularly rounded, presumptive entodermal, organ-
forming area, therefore, is encapsulated partially by the other potential germinal
areas, particularly by the chordamesodermal zone (fig. 169B). In a sense,
this is true also of the protochordate group (fig. 167D).
The amphibian type of blastula includes those of the petromyzontoid
Cyclostomes, the ganoid fishes with the exception of bony ganoids, the dipnoan
fishes, and the Amphibia with the exception of the Gymnophiona, where a
kind of solid blastula is present.
It is to be observed that the amphibian and protochordate blastulae differ
in several details. In the first place, there is a greater quantity of yolk material
in the blastula of the Amphibia; hence the presumptive entodermal area or
hypoblast projects considerably into and encroaches upon the blastocoel.
Also, in Amphioxus, the presumptive notochordal area forms a distinct dorsal
crescent apart from the presumptive mesodermal or ventral crescent (fig.
167F), whereas, in the Amphibia, the notochordal material is sandwiched
in between the two wings of mesoderm, so that these two areas form one
composite marginal zone crescent (fig. 169D, E).
TYPES OF CHORDATE BLASTULAE 357
As in Amphioxus, the amphibian blastula may be resolved into a two-
layered structure composed of a presumptive entodermal or hypoblast layer
and an upper, epiblast layer of presumptive epidermal, notochordal, meso-
dermal, and neural tissues. Each of these layers, unlike that of Amphioxus,
is several cells in thickness.
3. Mature Blastula in Birds
Development of the hen's egg proceeds rapidly in the oviduct (fig. 157B-G),
and at the time that the egg is laid, the blastodisc (blastula) presents the
following cellular conditions:
( 1 ) a central, cellular blastoderm above the primary blastocoel and
(2) a more peripheral portion, associated with the yolk material forming
the germ-wall tissue (fig. 156G).
The central blastoderm is free from the yolk substance and is known as
the area pellucida, whereas the germ-wall area with its adhering yolk material
forms the area opaca (fig. 170). Around its peripheral margin the area
pellucida is somewhat thicker, particularly so in that region which will form
the posterior end of the future embryo. In the latter area, the pellucid margin
may consist of a layer of three or even four cells in thickness (fig. 172A).
This thickened posterior portion of the early pellucid area forms the embryonic
shield (fig. 170). Anterior to the embryonic shield, the pellucid area is one
or two cells in thickness (figs. 171 A; 172B).
Eventually the pellucid area becomes converted into a two-layered structure
with an upper or overlying layer, the primitive ectoderm or epiblast and a
lower underlying sheet of cells, the primitive entoderm or hypoblast (figs.
171 A; 172A). The space between these two layers forms the true or secondary
blastocoel. The cavity below the hypoblast is the primitive archenteric space.
At the caudal and lateral edges of the pellucid area, cells from the inner zone
of the germ wall appear to contribute to both hypoblast and epiblast.
The two-layered condition of the avian blastula shown in figure 171 A may
be regarded as a secondary or late blastula. At about the time that the sec-
ondary blastula is formed (or almost completely formed), the hen's egg is
laid, and further development depends upon proper incubational conditions
outside the body of the hen. Shortly after the latter incubation period is
initiated, the primitive streak begins to make its appearance in the midcaudal
region of the blastoderrn, as described in Chapter 9.
Much controversy has prevailed concerning the method of formation of
the entoderm and the two-layered condition in the avian blastoderm. Greatest
attention has been given to the origin of the entoderm in the eggs of the
pigeon, hen, and duck. The second layer is formed in the pigeon's egg as it
passes down the oviduct, in the hen's egg at about the time of laying, and
in the duck's egg during the first hours of the external incubation period. The
358
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
ANTERIOR
^t>4
,' -^t
^
It
, ^f-i-
AREA PELLUCIDA
AREA OPACA
EMBRYONIC SHIELD
^1^'^^i^il^^
POSTERIOR
Fig. 170. Early pre-primitive streak blastoderm of the chick. Blastoderm about 3.2 mm.
in diameter at this time. (After Spratt. '42.)
DELAMINATING CELLS
• • •
SECONDARY OR TRUE BLASTOCOEL
GERM WALL
PRIMITIVE ARCHENTERIC SPACE
DELAMINATING CELL VITELLINE MEMBRANE
EP I B L A ST
ANTERIOR
-GERM WALL
c.
SECONDARY BLASTOCOEL
(
P.OSTE RIOR
PRIMITIVE ARCHENTERIC SPACE
HYPOBLAST
Fig. 171. Origin of the hypoblast (entoderm) in the avian blastoderm. (A) Median,
antero-posterior section of chick blastoderm. Entoderm arises by delamination from
upper or epiblast layer; possibly also by cells that grow anteriad from thickened posterior
area. (Based upon data supplied by Peter. '34, '38, and Jacobson, '38.) (B-D) For-
mation of the hypoblast (entoderm) from epiblast by a process of delamination in the
duck embryo. (Based upon data supplied by Pasteels, '45.)
unincubated chick blastoderm is about 3 mm. in diameter, that of the duck,
about 2 to 3 mm.
The most recent observations, relative to the formation of the second or
hypoblast layer, have been made upon the duck's egg (Pasteels, '45). In this
egg, Pasteels found that, at about nine hours after incubation is initiated, a
two-layered condition is definitely formed and that "the primary entoblast of
the duck is the result of a progressive delamination of the segmenting blastodisc
TYPES OF CHORDATE BLASTULAE
359
separating the superficial cells from the deeper ones" (fig. 171B-D). He
further suggests that "the bilaminar embryo of birds is to be homologized
with the blastula of the Amphibia, the cleft separating the two layers being
equivalent to the blastocoele" (p. 13). The formation of the hypoblast (pri-
mary entoderm) by a process of delamination from the upper layer or epiblast
agrees with the observations by Peter ('38) on the developing chick and pigeon
blastoderm (fig. 172) and of Spratt ('46) on the chick. It also agrees with
some of the oldest observations, concerning the matter of entoderm formation,
going back to Ollacher in 1869, Kionka, 1894, and Assheton, 1896. Others,
such as Duval (1884, 1888) in the chick, and Patterson ('09) in the pigeon,
have ascribed the formation of the primary entoderm to a process of invagi-
nation and involution at the caudal margin of the blastoderm, while Jacobson
('38) came to the conclusion that the entoderm of the pellucid area arose in
chick and sparrow embryos through a process of outgrowth of cells from the
primitive plate and from an archenteric canal produced by an inward bend-
ing of the epiblast and primitive plate tissue. The latter author believed that
the entoderm of the area opaca arose by delamination.
The hypoblast of the chick gives origin to most of the tissue which lines
the future gut, and, therefore, may be regarded as the potential entodermal
area. As in the amphibia and Amphioxus, the epiblast is composed of sev-
eral, presumptive organ-forming areas (fig. 173A). (See Pasteels, '36c;
Spratt, '42, '46.) At the caudal part of the epiblast is an extensive region
of presumptive mesoderm bisected by the midplane of the future embryonic
axis. Just anterior to this region and in the midplane is the relatively small,
presumptive notochordal area. Between the latter and the mesodermal area
is located the presumptive prechordal plate of mesodermal cells. Immedi-
ately in front of the notochordal region lies the presumptive neural area in
the form of a crescent with its crescentic arms extending in a lateral direc-
E PI BLAST
'HYPOBLAST
Fig. 172. Delamination of hypoblast (entoderm) cells from upper or epiblast layer
in the chick blastoderm. (A) Posterior end of blastoderm (cf. fig. 171A). (B) Anterior
end of blastoderm.
360
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
TROPHOBLA S T CELLS
EPIDERMAL
ECTODERM
NEURAL ECTODERM
NOTO CHORD
PRE-CHORDAL PLATE
BLASTOCOEL HYPOBLAST
Fig. 173. Presumptive organ-forming areas in the chick blastoderm. (A) Slightly
modified from Spratt, '46. (B) Schematic section of early chick blastoderm passing
through antero-posterior median axis.
tion from the midline of the future embryonic axis. Anterior to the neural
crescent is the presumptive epidermal crescent. Within the area opaca is
found potential blood-vessel and blood-cell-forming tissue, as well as the
extensive extra-embryonic-tissue materials.
The above description of the presumptive organ-forming areas pertains to
the avian blastula just previous to the inward migrations of the notochordal,
pre-chordal plate, and mesodermal areas; that is, just previous to the ap-
pearance of the primitive streak and the gastrulative process.
4. Primary and Secondary Reptilian Blastulae
The primary blastula of turtle, snake, and lizard embryos is akin in essen-
tial features to that of birds. It consists of a central blastoderm or area
pellucida, overlying a primary blastocoelic cavity, and a more distally situ-
ated opaque blastoderm, together with an indefinite periblast syncytium. A
localized region of the central blastoderm, situated along the midline of the
future embryonic axis and eccentrically placed toward the caudal end, is
known as the embryonic shield.
A specialized, posterior portion of the embryonic shield, in which the upper
layer (epiblasi) is not separated from the underlying cells (hypoblast), is
known as the primitive plate (fig. 174A-D). (Consult also Will, 1892, for
PRIMITIVE PLATE
Fig. 174. Formation of hypoblast (entoderm) layer in certain reptiles; major pre-
sumptive organ-forming areas of reptilian blastoderm. (A) Section through blastoderm
of the turtle, Clemmys leprosa. This section passes through the primitive plate in the
region where the entoderm cells are rapidly budded off (invaginated?) from the surface
layer. It presumably passes through (E) in the area marked entoblast. It is difficult to
determine whether the entoderm cells are actually invaginated, according to the view of
Pasteels, or whether this area represents a region where cells are delaminated or budded
off in a rapid fashion frorh the overlying cells. (B) Similar to (A), diagrammatized
to show hypoblast cells in black. (C) Section through early blastoderm of the gecko,
Platydactylus. Epiblast cells are shown above, primitive entoderm cells below. (D) A
later stage showing primitive plate area with the appearance of a delamination or prolif-
eration of entoderm (hypoblast) cells from the upper layer of cells. (E) Presumptive,
organ-forming areas of the turtle, Clemmys leprosa, before gastrulation. (F) Presump-
tive, organ-forming areas of the epiblast of turtle and other reptiles if the hypoblast is
budded off or separated from the underside of the epiblast without invagination. It is to
be observed that B and D represent modifications by the author.
361
362
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
accurate diagrams of the reptilian blastoderm.) Surrounding the primitive
plate, the central blastoderm is thinner and is but one (occasionally two cells)
cell in thickness (see margins of figs. 174A, C). As development proceeds,
a layer of cells appears to be delaminated or proliferated off from the under-
surface of the primitive plate area (fig. 174C, D). This delamination gives
origin to a second layer of cells, the entoderm or hypoblast (Peter, '34).
Some of these entodermal cells may arise by delamination from more pe-
ripheral areas of the central blastoderm outside the primitive plate area. In
the case of the turtle, Clemmys leprosa, Pasteels ('37a) believes that there
is an actual invagination of entodermal cells (fig. 174A-B). More study is
needed to substantiate this view.
Eventually, therefore, a secondary blastula arises which is composed of a
floor of entodermal cells, the hypoblast, closely associated with the yolk, and
an overlying layer or epiblast. The epiblast layer is formed of presumptive
epidermal, mesodermal, neural, and notochordal, organ-forming areas. The
essential arrangement of the presumptive organ-forming areas in the reptiles
is very similar to that described for the secondary avian blastula. The space
between the epiblast and hypoblast layers is the secondary blastocoelic space.
VITELLOCYTES
CENTRAL BLASTODE R M
E N TO D
Fig. 175. Early blastoderms of the prototherian mammal, Echidna. (A) Early blasto-
derm showing central mass of cells with peripherally placed vitellocytes. (B) Later
blastoderm. Central cells are expanding and the blastoderm is thinning out. Smaller
cells (in black) are migrating into surface layer. Vitellocytes have fused to form a
peripheral syncytial tissue. (C) Later blastoderm composed of a single layer of cells
of two kinds. The smaller cells in black represent potential entoderm cells. (D) Increase
of hypoblast cells and their migration into the archenteric space below to form a second
or hypoblast layer.
TYPES OF CHORDATE BLASTULAE
363
FORMATIVE
CELLS
Fig. 176. Early development of blastoderm of the opossum. (Modified from Hartman,
'16.) (A) Blastocyst wall composed of one layer of cells from which entoderm ceils
are migrating inward. (B-D) Later development of the formative portion of the blasto-
derm. Two layers of cells are present in the formative area, viz., an upper epiblast layer
and a lower hypoblast. Trophoblast cells are shown at the margins of the epiblast and
hypoblast layers.
Both hypoblast and epiblast are connected peripherally with the periblast
tissue.
5. Formation of the Late Mammalian Blastocyst (Blastula)
a. Prototherian Mammal, Echidna
In Echidna, according to Flynn and Hill ('39, '42), a blastoderm some-
what comparable to that of reptiles and birds is produced. An early primary
blastular condition is first established, consisting of a mass of central cells
with specialized vitellocytes at its margin (fig. I75A). A little later, an ex-
tension of this blastoderm occurs, and a definite primary blastocoelic space
is formed below the blastoderm (fig. 175B). During this transformation,
small, deeper lying cells (shown in black, fig. 175B) move up to the surface
and become associated with the thinning blastoderm which essentially becomes
a single layer of cells (fig. 175C). The marginal vitellocytes in the meantime
fuse to form a germ-wall syncytium. This state of development may be re-
garded as the fully developed primary blastula. A little later, this primary
condition becomes converted into a two-layered, secondary blastula, as shown
in figure 175D by the secondary multiplication and migration inward of the
small cells to form a lower layer or hypoblast. The latter process may be
364
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
regarded as a kind of polyinvagination. In this manner the secondary blastula
is formed. It is composed of two layers of cells, the epiblast above and the
hypoblast below with the secondary blastocoelic space insinuated between
these two layers.
b. Metatherian Mammal, Didelphys
The opossum, Didelphys virginiana, possesses a hollow blastocyst akin to
the eutherian variety. (See Hartman, '16, '19; McCrady, '38.) As observed
in the previous chapter, it is produced by a peculiar method. The early blasto-
meres do not adhere together to form a typical morula as in most other
forms; rather, they move outward and adhere to the zona pellucida and come
to line the inner aspect of this membrane. As cleavage continues, they even-
tually form a primary blastula with an enlarged blastocoel.
Following this primary phase of development, one pole of the blastocyst
begins to show increased mitotic activity, and this polar area gradually thickens
(fig. 176A). At this time certain cells detach themselves from the thickened
polar area of the blastocyst and move inward into the blastocoel (fig. 176A, B) .
INNER CELL MASS
Fig. 177. Schematic drawings of early pig development. (A) Early developing blasto-
cyst. (B) Later blastocyst, showing two kinds of cells in the inner cell mass. (C)
Later blastocyst, showing disappearance of trophoblast cells overlying the inner cell mass.
(D) Later blastocyst. Two layers of formative cells are present as indicated with tropho-
blast tissue attached at the margins.
TYPES OF CHORDATE BLASTULAE
365
ARCHENTERIC SPACE
TRO PHOBL AST
ARCHENTERIC SPA CE
Fig. 178. Schematic drawings of the developing blastocyst of the monkey. (After
Heuser and Streeter: Carnegie Inst., Washington. Publ. 538. Contrib. to Embryol. No.
181.) (A, B) Early blastocysts showing formative and non-formative cells in the inner
cell mass. (C-E) Later arrangement of the formative cells into an upper epiblast and
lower hypoblast layer.
These cells form the mother entoderm cells, and by mitotic activity they give
origin to an entodermal layer which adheres to the underside of the thickened
polar area (fig. 176B, C). The polar area then thins out to form the expansive
condition shown in figure 176D. A bilaminar, disc-shaped area thus is formed
in this immediate region of the blastocyst, and it represents the area occupied
366
THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
by the formative cells of the blastula. The edge of this disc of formative
cells is attached to the trophoblast or auxiliary cells (fig. 176D). Only the
formative cells give origin to the future embryonic body.
c. Eiitherian Mammals
The eutherian mammals as a whole present a slightly different picture of
blastocyst development from that described above for marsupial species. These
differences may be outlined as follows:
( 1 ) During the earliest phases of blastocyst development in most eutherian
mammals, a distinct, inner cell mass is elaborated at the formative
or animal pole (fig. Ill A, B). This characteristic is marked in some
species (pig, rabbit, man, and monkey) and weaker in others (mink
and armadillo). It may be entirely absent in the early blastula of
the Madagascan insectivore, Hemicentetes semispinosus; however, in
the latter, a thickening corresponding to the inner cell mass later
NEURAL ECTODERM
NOTOCHORD
PRE-CHORDAL PLATE
ENTODERM
Fig. 179. Presumptive organ-forming areas in the blastoderm of the shark embryo.
(A) Median section of the blastoderm of Torpedo ocellata. Hypoblast cells are shown
in black. Caudal portion of the blastoderm is shown at the right. Cf. (B). (This figure
partly modified from Ziegler, '02 — see Chap. 6 for complete reference.) (B) Map of
the presumptive organ-forming areas of the blastoderm of the shark, Scylliutn canicula.
TYPES OF CHORDATE BLASTULAE
367
EP I BLAST
ENTODERM OR
PRIMARY
HYPOBLAST
NEURAL ECTODERM
NOTO CHORD
ENTODERM
DORSAL 8LAST0P0RAL LIP
Fig. 180. Presumptive organ-forming areas of the teleost fish blastoderm. (A)
Median section through the late blastoderm of Fundidus heteroclitus just previous to
gastrulation. Somewhat schematized from the author's sections. Presumptive entoderm
or hypoblast is shown exposed to the surface at the caudal end of the blastoderm and,
therefore, follows the conditions shown in (B). (B) Presumptive organ-forming areas
of the blastoderm of Fiindulus heteroclitus. Arrows show the direction of cell move-
ments during gastrulation. (Modified from diagram by Oppenheimer, '36.)
appears. Within the inner cell mass, two types of cells are present,
namely, formative and trophoblast (figs. 177B; 178A).
(2) Unlike that of the marsupial mammal, an overlying layer of tropho-
blast cells, covering the layer of formative cells, always is present (fig.
177B). In some cases (rabbit, pig, and cat) they degenerate (the
cells of Rauber, fig. 177C), while in others (man, rat, and monkey)
the overlying cells remain and increase in number (fig. 178A-E).
(3) The entodermal cells arise by a separation (delamination) of cells
from the lower aspect of the inner cell mass (figs. 177C; 178A),
with the exception of the armadillo where their origin is similar to
that of marsupials. With these differences, the same essential goal
arrived at in the marsupial mammals is achieved, namely, a bilaminar,
formative area, the embryonic disc, composed of epiblast and hypo-
blast layers (figs. 177D; 178D, E), which ultimately gives origin to
the embryonic body. A bilaminar, extra-embryonic, trophoblast area,
consisting of extra-embryonic entoderm and ectoderm, also is formed
(figs. 177D; 178D, E). The secondary blastocoel originates between
the epiblast and hypoblast of the embryonic disc, while below the
hypoblast layer is the archenteric space (fig. 178E).
368 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
6. Blastulae of Teleost and Elasmobranch Fishes
In the teleost and elasmobranch fishes, the primary blastula is a flattened,
disc-shaped structure constructed during its earlier stages of an upper blasto-
derm layer of cells, the formative or strictly embryonic tissue, and a peripheral
and lower layer of trophoblast or periblast tissues; the latter is closely asso-
ciated with the yolk substance (figs. 179A; 180A; 181A). The primary blasto-
coelic space lies between the blastoderm and the periblast tissue.
That margin of the formative portion of the blastoderm which lies at the
future caudal end of the embryo is thickened considerably, and presumptive
entodermal material or primary hypoblast is associated with this area. Its re-
lationship is variable, however. In some teleost fishes, such as the trout, the
entodermal cells are not exposed to the surface at the caudal portion of the
blastodisc (fig. 181 A; Pasteels, '36a). In other teleosts, a considerable portion
of the entodermal cells may lie at the surface along the caudal margin of the
blastoderm (fig. 180A; Oppenheimer, '36). In the elasmobranch fishes the
disposition of the entodermal material is not clear. A portion undoubtedly
lies exposed to the surface at the caudal margin of the disc (fig. 179A, B;
Vandebroek, '36), but some entodermal cells lie in the deeper regions of the
blastoderm (fig. 179A).
Turning now to a consideration of the other presumptive organ-forming
areas of the fish blastoderm, we find that the presumptive pre-chordal plate
material lies exposed on the surface in the median plane of the future embryo
immediately in front of the entoderm and near the caudal edge of the blasto-
derm. (It is to be observed that, in comparison, the pre-chordal plate lies well
forward within the area pellucida of the bird blastoderm.) This condition is
found in the shark, Scyllium, in Fundulus, and in the trout, Sahno (figs.
179B; 180B). However, in the trout it lies a little more posteriorly at the
caudal margin of the disc (fig. 18 IB). Anterior to the pre-chordal plate is
the presumptive notochordal material, and anterior to the latter is a rather
expansive region of presumptive neural cells. These three areas thus lie along
the future median plane of the embryo, but they exhibit a considerable vari-
ation in size and in the extent of area covered in Scyllium, Fundulus, and
Salmo (figs. 179, 180, 181).
Extending on either side of these presumptive organ-forming areas, is an
indefinite region of potential mesoderm. In Salmo, presumptive mesodermal
cells lie along the lateral and anterior portions of the blastoderm edge (fig.
18 IB). However, in Scyllium and in Fundulus, it is not as extensive (figs.
179B; 180B). In front of the presumptive neural organ-forming area is a
circular region, the presumptive epidermal area.
In their development thus far the three blastulae described above represent
a primary blastular condition, and the cavity between the blastodisc and the
underlying trophoblast or periblast tissue forms a primary blastocoel. This
condition presents certain resemblances to the early blastocyst in the higher
E PI BLAST
PERI BLA ST
ENTODERM OR PRiMARY
HYPOBLAST
MESODERM
EPIDER MAL
ECTODERM
NEURAL ECTODERM
NOTO CHORD
PRE-CHORDAL PLATE
DORSAL BLASTOPORAL LIP
Fig. 181. Presumptive organ-forming areas of the blastoderm of the trout, Salmo
irideus. (A) Schematized section through blastoderm just previous to gastrulation. Pre-
sumptive entoderm (hypoblast) shown in black at caudal end of the blastoderm. Observe
that entoderm is not exposed to surface. Cf. (B). (B) Surface view of presumptive
organ-forming areas of the blastoderm just before gastrulation.
^^cy^^
Fig. 182. Late blastoderms of Gymnophiona. (Modified from Brauer, 1897.) (A)
Late blastoderm of Hypogeophis alternans. Entoderm cells in black lie below. (B) Be-
ginning gastrula of H. rostratus. Observe blastocoelic spaces in white between the ento-
derm cells.
369
370 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE
mammals and the late blastula of birds. In both groups the trophoblast tissue
is attached to the edges of the formative tissue and extends below in such a
way that the formative cells and trophoblast tissue tend to form a hollow
vesicle. In both, the formative portion of the blastula is present as a disc or
mass of cells composed of presumptive, organ-forming cells closely associated
at its lateral margins with the trophoblast or food-getting tissue. A marked
distinction between the two groups, however, is present in that the future
entodermal cells in fishes are localized at the caudal margin of the disc,
whereas in mammals and birds they may be extensively spread along the
under margin of the disc. In reptiles the condition appears to be somewhat
similar to that in birds and mammals, with the exception possibly of the
turtles, where the future entoderm appears more localized and possibly may
be superficially exposed. Therefore, while great differences in particular fea-
tures exist between the fishes and the higher vertebrates, the essential funda-
mental conditions of the early blastulae in teleost and in elasmobranch fishes
show striking resemblances to the early blastulae of reptiles, birds, and
mammals.
The blastulae of teleost fishes remain in this generalized condition until
about the time when the gastrulative processes begin. At that time the noto-
chordal and mesodermal, cellular areas begin their migrations over the caudal
edge of the blastodisc to the blastocoelic space below, where they ultimately
come to lie beneath the epidermal and neural areas. Associated with the mi-
gration of notochordal and mesodermal cells, an entodermal floor or sec-
ondary hypoblast is established below the notochordal and mesodermal cells
by the active migration of primary hypoblast cells in an antero-lateral direction.
In the elasmobranch fishes there is a similar cell movement from the caudal
disc margin, as found in teleost fishes, but, in addition, a delamination of
entodermal (and possibly mesodermal cells) occurs from the deeper lying
parts of the blastodisc.
7. Blastulae of Gymnophionan Amphibia
In the Gymnophiona, nature has consummated a blastular condition dif-
ferent from that in other Amphibia. It represents an intermediate condition
between the blastula of the frog and the blastodiscs of the teleost and elas-
mobranch fishes and of higher vertebrates (fig. 182). In harmony with the
frog blastula, for example, a specialized periblast or food-getting group of
cells is absent. On the other hand, the presumptive entoderm and the pre-
sumptive notochordal, mesodermal, neural, and epidermal cells form a compact
mass at one pole of the egg, as in teleosts, the ohick, and mammal. Similar
to the condition in the chick and mammal, the entodermal cells delaminate
(see Chap. 9) from the under surface of the blastodisc (Brauer, 1897).
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. 1898. Cell-Lineage and ancestral
reminiscence. Biological Lectures, Ma-
rine Biol. Lab., Woods Hole, Mass. Ginn
& Co., Boston.
. 1925. The Cell in Development
and Heredity. 3rd edit. The Macmillan
Co., New York.
Wolff, C. F. 1759. Theoria Generationis.
Halle.
. 1812. De formatione intestinorum
praecipe, etc. Published in Latin in
Vols. 12 and 13 of St. Petersburg Com-
mentaries (Acad. Sci. Impt. Petropol.
1768-69) and translated by J. F. Meckel,
in Uber die Bildung des Darmkanals im
bebriiteten Huhnchen, Halle.
Zur Strassen, O. 1896. Embryonalentwi-
ckelung der Ascaris megalocephala. Arch.
f. Entwicklngsmech. 3:27, 133.
8
Trie Late Blastula in Relation to Certain Innate
Pnysiolo^ical Conditions: Tw^innin^
A. Introduction
B. Problem of differentiation
1. Definition of differentiation; kinds of differentiation
2. Self-differentiation and dependent differentiation
C. Concept of potency in relation to differentiation
1. Definition of potency
2. Some terms used to describe different states of potency
a. Totipotency and harmonious totipotency
b. Determination and potency limitation
c. Prospective potency and prospective fate
d. Autonomous potency
e. Competence
D. The blastula in relation to twinning
1. Some definitions
a. Dizygotic or fraternal twins
b. Monozygotic or identical twins
c. Polyembryony
2. Basis of true or identical twinning
3. Some experimentally produced, twinning conditions
E. Importance of the organization center of the late blastula
A. Introduction
In the preceding two chapters the blastula is defined as a morphological
entity composed of six, presumptive, organ-forming areas — areas which are
poised and ready for the next phase of development or gastrulation. How-
ever, the attainment of this morphological condition with its presumptive,
organ-forming areas is valid and fruitful in a developmental way only if it
has developed within certain physiological conditions which serve as a spark
to initiate gastrulation and carry it through to its completion.
The physiological conditions of the blastula are attained, as are its mor-
phological characteristics, through a process of diiferentiation. Moreover,
during the development of the blastula, different areas acquire different abilities
to undergo physiological change and, hence, possess different abilities or
373
374 THE BLASTULA IN RELATION TO INNATE CONDITIONS
powers of differentiation. To state the matter differently, the various, pre-
sumptive, organ-forming areas of the blastula have acquired different abilities
not only in their power to produce specific organs of the future body of the
embryo, but also in that some presumptive areas possess this propensity in
a greater degree than do other areas. However, at this point, certain terms in
common usage relating to the problem of differentiation are defined in order
that a better understanding may be obtained concerning the ability to dif-
ferentiate on the part of the presumptive, organ-forming areas of the late
blastula.
B. Problem of Differentiation
1. Definition of Differentiation; Kinds of Differentiation
The word differentiation is applied to that phase of development when a
cell, a group of cells, cell product experiences a change which results in a
persistent alteration of its activities. Under ordinary conditions an alteration
in structure or function is the only visible evidence that such a change has
occurred.
To illustrate these matters, let us recall the conditions involved in the
maturation of the egg. A subtle change occurs within the primitive oogonium
which causes it to enlarge and to grow. This growth results in an increase in
size and change in structure of both the cytoplasm and the nucleus. A little
later, as the egg approaches that condition which is called maturity, observable
morphological changes of the nucleus occur which accompany or initiate an
invisible change in behavior. These latter changes make the egg fertilizable.
Here we have illustrated, first of all, a subtle, invisible, biochemical change
in the oogonium which arouses the formation of visible morphological changes
in the oocyte and, secondly, a morphological change (i.e., nuclear maturation)
which accompanies an invisible physiological transformation.
Another illustration will prove profitable. Let us recall the development
of the mammary-gland tissue (fig. 58). Through the action of the lactogenic
(luteotrophic) hormone, LTH, the cells of the various acini of the fully
developed gland begin to secrete milk. The acini, it will be recalled, were
caused to differentiate as a result of the presence of progesterone. Similarly,
the various parts of the complicated duct system were stimulated to differen-
tiate from a very rudimentary condition by the presence of estrogenic hormone.
Earlier in development, however, the particular area of the body from which
the duct rudiments ultimately arose was conditioned by a change which dic-
tated the origin of the duct rudiments from the cells of this area and restricted
their origin from other areas.
In the foregoing history of the mammary gland, various types of differen-
tiation are exemplified. The final elaboration of milk from the acinous cells
is effected by a change in the activity of the cells under the influence of LTH.
The type of change which brings about the functional activities of a structure
is called pfiysiological differentiation. The morphological changes in the cells
PROBLEM OF DIFFERENTIATION 375
which result in the formation of the duct system and the acini are examples
of morphological differentiation. On the other hand, the invisible, subtle
change or changes which originally altered the respective cells of the nipple
area and, thereby, ordained or determined that the cells in this particular
locale should produce duct and nipple tissue is an example of biochemical
differentiation or chemodifferentiation. Chemodiflterentiation, morphological
differentiation, and physiological differentiation, therefore, represent the three
types or levels of differentiation. Moreover, all of these differentiations stem
from a persistent change in the fundamental activities of cells or cell parts.
It should be observed further that chemodifferentiation represents the initial
step in the entire differentiation process, for it is this change which determines
or restricts the future possible activities and changes which the cell or cells
in a particular area may experience. Also, in many cases, differentiation ap-
pears to arise as a result of stimuli which are applied to the cell or cells exter-
nally. That is, internal changes within a cell may be called forth by an
environmental change applied to the cell from without.
In embryological thinking, therefore, the word differentiation implies a
process of becoming something new and different from an antecedent, less-
differentiated condition. But beyond this, differentiation also connotes a cer-
tain suitableness or purposefulness of the structure which is differentiated.
Such a connotation, however, applies only to normal embryonic differentia-
tion; abnormal growths and monstrosities of many kinds may fulfill the first
phase (i.e., of producing something new) of differentiation as defined in
the first sentence of this paragraph, but they do not satisfy the criteria of
purpose and of suitableness within the organized economy of the developing
body as a whole. It is important to keep the latter implications in mind, for
various structures may appear to be vestigial or aberrant during embryonic
development, nevertheless their presence may assume an important, purpose-
ful status in the ultimate scheme which constructs the organization of the
developing body.
2. Self-differentiation and Dependent Differentiation
In the amphibian, very late blastula and beginning gastrula, the presumptive,
chordamesodermal area, when undisturbed and in its normal position in the
embryo, eventually differentiates into notochordal and mesodermal tissues.
This is true also when it is transplanted to other positions. That is, at this
period in the history of the chordamesodermal cells the ability resides within
the cells to differentiate into notochordal and mesodermal structures. Con-
sequently, these cells are not dependent upon surrounding or external factors
to induce or call forth differentiation in these specific directions. Embryonic
cells in this condition are described as self-differentiating (Roux). Similarly,
the entodermal area with its potential subareas of liver, foregut, and intestine
develops by itself and this area does not rely upon stimuli from other con-
376 THE BLASTULA IN RELATION TO INNATE CONDITIONS
tiguous cells to realize a specific potency. On the other hand, the presumptive,
neural plate region at this time is dependent upon the inducing influence of
the chordamesodermal cells during the process of gastrulation for its future
realization as neural tissue. This area has little inherent ability to differentiate
neural tissue and is described, therefore, as being in a state of dependent
differentiation (Roux). Furthermore, the presumptive skin ectoderm (i.e.,
epidermis), if left alone, will proceed to epidermize during gastrulation, but
foreign influences, such as transplantation, into the future neural plate area
may induce neural plate cells to form from the presumptive skin ectoderm
(fig. 183). The differentiation of neural cells from any of the ectodermal cells
of the late blastula thus is dependent upon special influencing factors applied
to the cells from without.
C. Concept of Potency in Relation to Differentiation
1. Definition of Potency
The word potency, as used in the field of embryology, refers to that prop-
erty of a cell which enables it to undergo differentiation. From this viewpoint,
potency may be defined as the power or ability of a cell to give origin to a
specific kind of cell or structure or to various kinds of cells and structures.
h is questionable, in a fundamental sense, whether potency actually is
gained or lost during development. It may be that the expression of a given
kind of potency, resulting in the formation of a specific type of cell, is merely
the result of a restriction imposed upon other potentialities by certain modi-
fying factors, while the total or latent potency remains relatively constant.
All types of differentiated cells, from this point of view, basically are totipotent;
that is, they possess the latent power to give origin to all the kinds of cells
and tissues of the particular animal species to which they belong.
The specific potencies which denote the normal development of particular
organs undoubtedly have their respective, although often quite devious, con-
nections with the fertilized egg. However, one must concede the origin of
abnormal or acquired potency values due to the insinuation of special in-
ductive or modifying factors which disturb the expression of normal potency
value. For example, tumors and other abnormal growths and tissue distortions
may be examples of such special potencies induced by special conditions which
upset the mechanism controlling normal potency expression.
2. Some Terms Used to Describe Different States of Potency
a. Totipotency and Harmonious Totipotency
The word totipotent, as applied to embryonic development, was introduced
into embryological theory by Wilhelm Roux, and it refers to the power or
ability of an early blastomere or blastomeres of a particular animal species
to give origin to the many different types of cells and structures characteristic
of the individual species. Speculation concerning the meaning of totipotency
POTENCY IN RELATION TO DIFFERENTIATION 377
of a single blastomere received encouragement from the discovery by Hans
Driesch, in 1891, that an isolated blastomere of the tWo- or four-cell stage
of the cleaving, sea-urchin's egg could give origin to a "perfect larva." Driesch
described this condition as constituting an equipotential state, while Roux
referred to it as a totipotential condition. As the word totipotential seems
more fitting and better suited to describe the condition than the word equi-
potential, which simply means equal potency, the word totipotency is used
herein. The word omnipotent is sometimes used to describe the totipotent
condition; as it has connotations of supreme power, it will not be used.
The totipotent state is a concept which may be considered in different
ways. In many instances it has been used as described above, namely, as a
potency condition that has within it the ability to produce a perfect embryo
or individual. The word also has been used, however, to describe a condition
which is capable of giving origin to all or nearly all the cells and tissues of
the body in a haphazard way but which are not necessarily organized to produce
a normally formed body of the particular species. Therefore, as a basis for
clear thinking, it is well to define two kinds of totipotency, namely, totipotency
and harmonious totipotency. The former term is used to describe the ability
of a cell or cell group to give origin to all or nearly all the different cells and
tissues of the particular species to which it belongs, but it is lacking in the
ability to organize them into an harmonious organism. Harmonious totipo-
tency, on the other hand, is used to denote a condition which has the above
ability to produce the various types of tissues of the species, but possesses,
in addition, the power to develop a perfectly organized body.
The fertilized egg or the naturally parthenogenetic egg constitutes an har-
monious totipotential system. This condition is true also of isolated blasto-
meres of the two- or four-blastomere stage of the sea-urchin development,
as mentioned above, of the two-cell state of Amphioxus, or of the first two-
blastomere stage of the frog's egg when the first cleavage plane bisects the
gray crescent. However, in the eight-cell stage in these forms, potency be-
comes more limited in the respective cells of the embryo. Restriction of
potency, therefore, is indicated by a restriction of power to develop into
a variety of cells and tissues, and potency restriction is a characteristic of
cleavage and the blastulative process (figs. 61; 163A; 163B). When a stage
is reached in which the cells of a particular area are limited in potency value
to the expression of one type of cell or tissue, the condition is spoken of as
one of unipotency. A pluripotent state, on the other hand, is a condition in
which the potency is not so limited, and two or more types of tissues may
be derived from the cell or cells.
b. Determination and Potency Limitation
The limitation or restriction of potency, therefore, may form a part of the
process of differentiation; as such, it is a characteristic feature of embryonic
378 THE BLASTULA IN RELATION TO INNATE CONDITIONS
development. Potency limitation, however, is not always the result of the dif-
ferentiation process. For instance, in the development of the oocyte in the
ovary, the building up of the various conditions, characteristic of the totipotent
state, is a feature of the differentiation of the oocyte.
The word determination is applied to those unknown and invisible changes
occurring within a cell or cells which effect a limitation or restriction of potency.
As a result of this potency limitation, differentiation becomes restricted to a
specific channel of development, denoting a particular kind of cell or structure.
Ultimately, by the activities of limiting influences upon the resulting blasto-
meres during cleavage, the totipotent condition of the mature egg becomes
dismembered and segregated into a patchwork or mosaic of general areas of
the blastula, each area having a generalized, presumptive, organ-forming po-
tency. As we have already observed, in the mature chordate blastula there
are six of these major, presumptive organ-forming areas (five if we regard
the two mesodermal areas as one). By the application of other limiting in-
fluences during gastrulation or the next phase of development, each of these
general areas becomes divided into minor areas which are limited to a potency
value of a particular organ or part of an organ. The process which brings
about the determination of individual organs or parts of organs is called
individuation.
When potency limitation has reduced generalized and greater potency value
to the status of a general organ system (e.g., nervous system or digestive
system) with the determination (i.e., individuation) of particular organs
within such a system, the condition is described as one of rigid or irrevocable
determination. Such tissues, transplanted to other parts of the embryo favor-
able for their development, tend to remain limited to an expression of one
inherent potency value and do not give origin to different kinds of tissues or
organs. Thus, determined liver rudiment will differentiate into liver tissue,
stomach rudiment into stomach tissue, forebrain material into forebrain
tissue, etc.
In many instances determination within a group of cells is brought about
because of their position in the developing organism and not because of in-
trinsic, self-differentiating conditions within the cells. Because their position
foreordains their determination in the future, the condition is spoken of as
positional or presumptive determination. For example, in the late amphibian
blastula, the composite ectodermal area of the epiblast will become divided,
during the next phase of development, into epidermal and neural areas as a
result of the influences at work during gastrulation, especially the activities
of the chordamesodermal area. Therefore, one may regard these areas as
already determined, in a presumptive sense, even in the late blastula, although
their actual determination as definite epidermal and neural tissue will not
occur until later.
As stated in the preceding paragraphs, determination is the result of potency
POTENCY IN RELATION TO DIFFERENTIATION 379
limitation or inhibition. However, there is another aspect to determination,
namely, potency expression, which simply means potency release or develop-
ment. Potency expression, probably, is due to an activating stimulus (Spemann,
'38). Consequently, the individuation of a particular organ structure within
a larger system of organs is the result of two synchronous processes:
( 1 ) inhibition of potency or potencies and
(2) release or calling forth of a specific kind of potency (Wigglesworth,
'48).
Associated with the phenomenon of potency inhibition or Hmitation is the
loss of power for regulation. Consequently, individuation and the loss of
regulative power appear to proceed synchronously in any group of cells.
c. Prospective Potency and Prospective Fate
Prospective fate is the end or destiny that a group of cells normally reaches
in its differentiation during its normal course of development in the embryo.
The presumptive epidermal area of the late blastula differentiates normally
into skin epidermis. This is its prospective fate. Its prospective potency, how-
ever, is greater, for under certain circumstances it may be induced, by trans-
plantation to other areas of the late blastula, to form other tissue, e.g., neural
plate cells or mesodermal tissues.
d. Autonomous Potency
Autonomous potency is the inherent ability which a group of cells possesses
to differentiate into a definite structure or structures, e.g., notochord, stomach,
or liver rudiments of the late blastula of the frog.
Versatility of autonomous potency is the inherent ability which a group
of cells possesses to differentiate, when isolated under cultural conditions out-
side the embryo, into tissues not normally developed from the particular cell
group in normal development. In the amphibian late blastula this is true of
the notochordal and somitic areas of the chordamesodermal area, which may
give origin to skin or neural plate tissue under these artificially imposed
conditions.
e. Competence
Certain areas of the late amphibian blastula have the ability to differenti-
ate into diverse structures under the stimulus of varied influence. Conse-
quently, we say that these areas have competence for the production of this
or that structure. The word competence is used to denote all of the possible
reactions which a group of cells may produce under various sorts of stimula-
tions. The entodermal area of the late amphibian blastula and early gastrula
has great power for self-differentiation but no competence, whereas the gen-
eral, neural plate-epidermal area has competence but little power of self-
380 THE BLASTULA IN RELATION TO INNATE CONDITIONS
differentiation (see p. 375). On the other hand, the notochord, mesodermal
area possesses both competence and the ability for self-differentiation.
Competence appears to be a function of a developmental time sequence.
That is, the time or period of development is all important, for a particular
area may possess competence only at a single, optimum period of develop-
ment. The word competence is sometimes used to supersede the other terms
of potency or potentiality (Needham, '42, p. 112).
D. The Blastula in Relation to Twinning
1. Some Definitions
a. Dizygotic or Fraternal Twins
Fraternal twins arise from the fertilization of two separate eggs in a species
which normally produces one egg in the reproductive cycle, as, for example,
in the human species. Essentially, fraternal twins are much the same as the
"sibhngs" of a human family (i.e., the members born as a result of separate
pregnancies) or the members of a litter of several young produced during
a single pregnancy in animals, such as cats, dogs, pigs, etc. Fraternal twins
are often called "false twins."
b. Monozygotic or Identical Twins
This condition is known as "true twinning," and it results from the devel-
opment of two embryos from a single egg. Such twins presumably have an
identical genetic composition.
c. Polyembryony
Polyembrony is a condition in which several embryos normally arise from
one egg. It occurs regularly in armadillos (Dasypopidae) where one ovum
gives origin normally to four identical embryos (fig. 186).
2. Basis of True or Identical Twinning
The work of Driesch (1891) on the cleaving, sea-urchin egg and that of
Wilson (1893) on the isolated blastomeres of Amphioxus mentioned above
initiated the approach to a scientific understanding of monozygotic or identical
twinning. Numerous studies have been made in the intervening years on the
developing eggs of various animal species, vertebrate and invertebrate, and
from these studies has emerged the present concept concerning the matter of
twinning. True twinning appears to arise from four, requisite, fundamental,
morphological and physiological conditions. These conditions are as follows:
(1) there must be a sufficient protoplasmic substrate;
(2) the substrate must contain all the organ-forming stuffs necessary to
assure totipotency, that is, to produce all the necessary organs;
THE BLASTULA IN RELATION TO TWINNING
381
(3) an organization center or the ability to develop such a center must
be present in order that the various organs may be integrated into
an harmonious whole; and
(4) the ability or faculty for regulation, that is, the power to rearrange
materials as well as to reproduce and compensate for the loss of sub-
stance, must be present.
3. Some Experimentally Produced, Twinning Conditions
The isolation of the first two blastomeres in the sea-urchin egg and in
Amphioxus with the production of complete embryos from each blastomere
TRANSPLANTED PROSPECTIVE-
EPIDERMAL ECTODERM
Fig. 183. Early gastrula of darkly pigmented Triton taeniatus with a small piece of
presumptive ectoderm of T. cristatus lightly pigmented inserted into the presumptive,
neural plate area shown in (A). (B) Later stage of development. (C) Cross section of the
later embryo. The lighter eye region shown to the right was derived from the original
implant from T. cristatus. (After Spemann, '38.)
Fig. 184. Demonstration that the presence of the organizer region or organization
center is necessary for development. (Redrawn from Spemann, '38.) (A) Hair-loop
constriction isolates the organizer areas in the dorsal portion of the early gastrula. (B)
Later development of the dorsal portion isolated in (A). (C) Later development of
ventral portion of gastrula isolated in (A). (D) Constriction of organizer area of early
gastrula into two halves. (E) Result of constriction made in (D). Constrictions were
made at 2-cell stage.
382
THE BLASTULA IN RELATION TO INNATE CONDITIONS
CENTER OF
(EMBRYO N I
0 R G A N IZ AT
SHIELD)
Fig. 185. Twinning in teleost fishes. (After Morgan, '34; Embryology and Genetics,
Columbia University Press, pp. 102-104. A, B, C from Rauber; D from Stockard.) In
certain teleost fishes, especially in the trout, under certain environmental conditions,
two or more organization centers arise in the early gastrula. (A-C) These represent
such conditions. If they lie opposite each other as in (A), the resulting embryos often
appear as in (D). If they lie nearer each other as in (B) or (C), a two-headed monster
may be produced.
has been described in Chapter 6. In these cases all the conditions mentioned
above are fulfilled. However, in the case of the isolation of the first two blas-
tomeres in Styela described in Chapter 6, evidently conditions (1), (2), and
(3) are present in each blastomere when the two blastomeres are separated,
but (4) is absent and only half embryos result. That is, each blastomere has
been determined as either a right or left blastomere; with this determination
of potency, the power for regulation is lost. In the frog, if the first two blasto-
meres are separated when the first cleavage plane bisects the gray crescent,
all four conditions are present and two tadpoles result. If, however, the first
cleavage plane separates the gray-crescent material mainly into one blastomere
while the other gets little or none, the blastomere containing the gray-crescent
material will be able to satisfy all the requirements above, and it, consequently,
develops a normal embryo. However, the other blastomere lacks (2), (3),
and (4) and, as a result, forms a mere mass of cells. Again, animal pole
blastomeres, even when they contain the gray-crescent material, when sepa-
rated entirely from the yolk blastomeres, fail to go beyond the late blastular
or beginning gastrular state (Vintemberger, '36). Such animal pole blasto-
meres appear to lack requirements (1), (2), and possibly (3) above. Many
other illustrations of embryological experiments could be given, establishing
SEPARATE CENTERS
OF ORGANIZATION
COMMON AMNIOTIC
VESICLE
TUBE- LIKE C AN AL
SE PAR AT E EMBRYOS
SEPARATE CENTERS
OF ORGANIZATION
CENTER OF
ORGANIZATION
Fig 186 Polyembryony or the development of multiple embryos m the armadillo,
Tatusia novemcincta. (After Patterson, '13.) (A) Separate centers of organization in
the early blastocyst. (B) Later stage in development of multiple embryos. Each embryo
is connected with a common amniotic vesicle. (C) Section through organization centers
a and b in (A). The two centers of organization are indicated by thickenings at right
and left. (D) Later development of four embryos, the normal procedure from one
fertilized egg in this species.
383
384 THE BLASTULA IN RELATION TO INNATE CONDITIONS
the necessity for the presence of all the above conditions. Successful whole
embryos have resulted in the amphibia when the two-cell stage and beginning
gastrula is bisected in such a manner that each half contains half of the chorda-
mesodermal field and yolk substance; that is, each will contain half of the
organization center (fig. 184).
Monozygotic twinning occurs occasionally under normal conditions in the
teleost fishes. In these cases, separate centers of organization arise in the
blastoderm, as shown in figure 185. When they arise on opposite sides of
the blastoderm, as shown in figure 185A, twins arise which may later become
fused ventrally (fig. 185D). When the centers of organization arise as shown
in figure 185B, C, the embryos become fused laterally. Stockard ('21) found
that by arresting development in the trout or in the blastoderm of Fundulus
for a period of time during the late blastula, either by exposure to low tem-
peratures or a lack of oxygen, twinning conditions were produced. The arrest
of development probably allows separate centers of organization to arise.
Normally, one center of organization makes its appearance in the late blastula
of these fishes, becomes dominant, and thus suppresses the tendency toward
totipotency in other parts of the blastoderm. However, in the cases of arrested
development, a physiological isolation of different areas of the blastoderm
evidently occurs, and two organization centers arise which forthwith proceed
to organize separate embryos in the single blastoderm. Conditions appear more
favorable for twinning in the trout blastoderm than in Fundulus. After the
late blastular period is past and gastrulation begins, i.e., after one organization
center definitely has been established, Stockard found that twinning could not
be produced.
In the Texas armadillo, Tatusia novemcincta, Patterson ('13) found that, in
the relatively late blastocyst (blastula), two centers of organization arise, and
that, a little later, each of these buds into two separate organization centers,
producing four organization centers in the blastula (fig. 186A-C). Each of
these centers organizes a separate embryo; hence, under normal conditions, four
embryos (polyembryony) are developed from each fertilized egg (fig. 186D).
It is interesting in connection with the experiments mentioned by Stockard
above, that the blastocyst (blastula) in Tatusia normally lies free in the uterus
for about three weeks before becoming implanted upon the uterus. It may
be that this free period of blastocystic existence results in a slowing down of
development, permitting the origin of separate organization centers. In har-
mony with this concept, Patterson ('13) failed to find mitotic conditions in
the blastoderms of the blastocysts during this period.
In the chick it is possible to produce twinning conditions by separating
the anterior end (Hensen's node) of the early primitive streak into two parts
along the median axis of the developing embryo. Twins fused at the caudal
end may be produced under these conditions. In the duck egg, Wolff and Lutz
('47) found that if the early blastoderm is cut through the primitive node
THE BLASTULA IN RELATION TO TWINNING
385
area (fig. 187A), two embryos are produced as in figure 187 A'. However, if
the primitive node and primitive streak are split antero-posteriorly, as indi-
cated in figure 187B, two embryos, placed as in figure 187B', are produced.
It is evident, therefore, that in the production of monozygotic twins, con-
dition (3) or the presence of the abihty to produce an organization center
is of greatest importance. In the case of the separation of the two blastomeres
of the two-cell stage in Amphioxus or of the division of the dorsal lip of the
early gastrula of the amphibian by a hair loop, as shown in figure 184, a
mechanical division and separation of the ability to produce an organization
center in each blastomere (Amphioxus) or of the separation into two centers
of the organization center already produced (Amphibia) is achieved. Once
these centers are isolated, they act independently, producing twin conditions,
providing the substrate is competent. Similar conditions evidently are pro-
duced in the duck-embryo experiments of Wolff and Lutz referred to above.
In some teleost blastulae, e.g., Fundulus and Salmo, during the earlier period
of development, it has been found possible to separate the early biastoderm
into various groups of cells (Oppenheimer, '47) or into quadrants (Luther,
'36), and a condition of totipotency is established in each part. Totipotency
appears thus to be a generalized characteristic in certain teleost blastoderms
during the earlier phases of blastular development. Harmonious totipotency,
however, appears not to be achieved in any one part of the blastodisc of
these species during the early conditions of blastular formation. During the
MODE OF
STREAK
PRIMITIVE STREAK
AND NODE ARE CUT
INTO TWO HALVES
Fig. 187. Isolation of the organization center in the early duck embryo. (From Dalcq,
'49, after Wolff and Lutz.) (A') Derived from blastoderm cut as in (A). (B') Derived
from blastoderm cut as in (B).
386 THE BLASTULA IN RELATION TO INNATE CONDITIONS
development of the late blastula, however, the posterior quadrant normally
acquires a dominant condition together with a faculty for producing har-
monious totipotency. The other totipotent areas then become suppressed.
These basic conditions, therefore, serve to explain the experiments by Stockard
('21 ) referred to above, where two organization centers tend to become domi-
nant as a result of isolating physiological conditions which tend to interfere
with the processes working toward the development of but one center of organi-
zation. This probable explanation of the twinning conditions in the teleost
blastoderm suggests strongly that the separation and isolation of separate
organization centers is a fundamental condition necessary for the production
of monozygotic or true twinning.
It becomes apparent, therefore, that, in the development of the trout blas-
toderm (blastula), the development of an area which possesses a dominant
organization center is an important aspect of blastulation. In other blastulae,
the seat or area of the organization center apparently is established at an
earlier period, as, for example, the gray crescent in the amphibian egg which
appears to be associated with the organization center during the late blastula
state. Similarly, in the teleost fish, Carassins, totipotency appears to be limited
to one part of the early blastula (Tung and Tung, '43).
It also follows from the analysis in the foregoing paragraphs that in the
production of polyembryony in the armadillo or of spontaneous twinning in
forms, such as the trout (Salmo), a generalized totipotency throughout the
early blastoderm is a prerequisite condition. When a single dominant area
once assumes totipotency, it tends to suppress and control the surrounding
areas, probably because it succeeds in "monopolizing" certain, substrate,
"food" substances (Dalcq, '49).
£. Importance of the Organization Center of the Late Blastula
It is also evident that one of the main functions of cleavage and blastulation
is the formation of a physiological, or organization, center which must be
present to dominate and direct the course of development during the next
stage of development. Consequently, the elaboration of a blastocoel with the
various, presumptive, organ-forming areas properly oriented in relation to
it is not enough. A definite physiological condition entrenched within the
so-called organization center must be present to arouse and direct the move-
ment of the major, organ-forming areas during gastrulation.
Bibliography
Butler, E. 1935. The developmental ca-
pacities of regions of the unincubated
chick blastoderm as tested in chorio-
allantoic grafts. J. Exper. Zool. 70:357.
Dalcq, A. M. 1949. The concept of physio-
logical competition (Spiegelman) and
the interpretation of vertebrate morpho-
genesis. Experimental Cell Research,
Supplement 1: p. 483. Academic Press,
Inc., New York.
Driesch, H. 1891. Entwicklungsmechan-
ische Studien I-II. Zeit. Wiss. Zool.
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Holtfreter, J. 1938. Differenzierungspoten-
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Roux' Arch. f. Entwick. d. Organ.
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Morgan, T. H. 1934. Chap. IX in Embry-
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Needham, J. 1942. Biochemistry and Mor-
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Oppenheimer, J. M. 1947. Organization of
the teleost blastoderm. Quart. Rev. Biol.
22:105.
Patterson, J. T. 1913. Polyembryonic de-
velopment in Tatusia novemcincta. J.
Morphol. 24:559.
Spemann, H. 1938. Embryonic Develop-
ment and Induction. Yale University
Press, New Haven.
Stockard, C. R. 1921. Developmental rate
and structural expression: an experi-
mental study of twins, "double mon-
sters" and single deformities, and the
interaction among embyronic organs
during their origin and development.
Am. J. Anat. 28:115.
Tung, T. C. and Tung, Y. F. Y. 1943.
Experimental studies on the development
of the goldfish. (Cited from Oppen-
heimer, '47.) Proc. Clin. Physiol. Soc.
2:11.
Vintemberger, P. 1936. Sur le developpe-
ment compare des micromeres de I'oeuf
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isolement (b) Apres transplantation sur
un socle de cellules vitellines. Compt.
rend. Soc. de Biol. 122:927.
Wigglesworth, V. B. 1948. The role of the
cell in determination. Symposia of the
Soc. for Exper. Biol. No. II. Academic
Press, Inc., New York.
Wilson, E. B. 1893. Amphioxus and the
mosaic theory of development. J. Mor-
phol. 8:579.
Wolff, E. and Lutz, H. 1947. Embryologie
experimentale — sur la production experi-
mentale de jumeaux chez I'embryon
d'oiseau. Compt. rend. Acad. d. Sc.
224:1301.
387
9
Gastrulation
A. Some definitions and concepts
1. Gastrulation
2. Primitive vertebrate body plan in relation to the process of gastrulation
a. Fundamental body plan of the vertebrate animal
b. The gastrula in relation to the primitive body plan
c. Chart of blastula, gastrula, and primitive, body-form relationships (fig. 188)
B. General processes involved in gastrulation
C. Morphogenetic movement of cells
1. Importance of cell movements during development and in gastrulation
2. Types of cell movement during gastrulation
a. Epiboly
b. Emboly
3. Description of the processes concerned with epiboly
4. Description of the processes involved in emboly
a. Involution and convergence
b. Invagination
c. Concrescence
d. Cell proliferation
e. Polyinvagination
f. Ingression
g. Delamination
h. Divergence
i. Extension
D. The organization center and its relation to the gastrulative process
1. The organization center and the primary organizer
2. Divisions of the primary organizer
E. Chemodifferentiation and the gastrulative process
F. Gastrulation in various Chordata
1. Amphioxus
a. Orientation
b. Gastrulative movements
1) Emboly
2) Epiboly
3) Antero-posterior extension of the gastrula and dorsal convergence of the
mesodermal cells
4) Closure of the blastopore
c. Resume of cell movements and processes involved in gastrulation of Amphioxus
1) Emboly
2) Epiboly
388
GASTRULATION 389
2. Gastrulation in Amphibia with particular reference to the frog
a. Introduction
1 ) Orientation
2) Physiological changes which occur in the presumptive, organ-forming areas
of the late blastula and early gastrula as gastrulation progresses
b. Gastrulation
1) Emboly
2) Epiboly
3) Embryo produced by the gastrulative processes
4) Position occupied by the pre-chordal plate material
c. Closure of the blastopore and formation of the neurenteric canal
d. Summary of morphogenetic movements of cells during gastrulation in the frog
and other Amphibia
1) Emboly
2) Epiboly
3. Gastrulation in reptiles
a. Orientation
b. Gastrulation
4. Gastrulation in the chick
a. Orientation
b. Gastrulative changes
1 ) Development of primitive streak as viewed from the surface of stained
blastoderms
2) Cell movements in the epiblast involved in primitive-streak formation as
indicated by carbon-particle marking and vital-staining experiments
3) Cell movements in the hypoblast and the importance of these movements
in primitive-streak formation
4) Primitive pit notochordal canal
5) Resume of morphogenetic movements of cells during gastrulation in the
chick
5. Gastrulation in mammals
a. Orientation
b. Gastrulation in the pig embryo
c. Gastrulation in other mammals
6. Gastrulation in teleost and elasmobranch fishes
a. Orientation
b. Gastrulation in teleost fishes
1) Emboly
2) Epiboly
3) Summary of the gastrulative processes in teleost fishes
a) Emboly
b) Epiboly
4) Developmental potencies of the germ ring of teleost fishes
c. Gastrulation in elasmobranch fishes
7. Intermediate types of gastrulative behavior
G. The late gastrula as a mosaic of specific, organ-forming territories
H. Autonomous theory of gastrulative movements
I. Exogastrulation
J. Pre-chordal plate and cephalic projection in various chordates
K. Blastoporal and primitive-streak comparisons
390 GASTRULATION
A. Some Definitions and Concepts
1. Gastrulation
According to Haeckel, the word gastrula is the name given to "the impor-
tant embryonic form" having "the two primary germ-layers," and the word
gastrulation is appHed to the process which produces the gastrula. Further-
more, "this ontogenetic process has a very great significance, and is the real
starting-point of the construction of the multicellular animal body" (1874, see
translation, '10, p. 123). Others such as Lankester (1875) and Hubrecht
('06) did much to establish the idea that gastrulation is a process during
which the monolayered blastula is converted into a bilaminar or didermic
gastrula. Haeckel emphasized invagination or the infolding of one portion of
the blastula as the primitive and essential process in this conversion, while
Lankester proposed delamination or the mass separation of cells as the primi-
tive process. While it was granted that invagination was the main process of
gastrulation in Amphioxus, in the Vertebrata, especially in reptiles, birds, and
mammals, delamination was considered to be an essential process by many
embryologists. Some, however, maintained that the process of invagination
held true for all the Chordata other than the Mammalia. It may be mentioned
in passing that Lankester conferred the name "blastopore" upon the opening
into the interior of the blastoderm which results during gastrulation. The
words "blastopore" and "primitive mouth" soon were regarded as synonymous,
for in the Coelenterata, the blastopore eventually becomes the oral opening.
The definition of the gastrula as a didermic stage, following the mono-
layered blastula, is a simple concept, easy to visualize, and, hence, may have
some pedagogical value. However, it is not in accord with the facts unearthed
by many careful studies relative to cell lineage and it does not agree with the
results obtained by the Vogt method (see Chap. 7) applied to the process
of gastrulation in the vertebrate group.
One of the first to define gastrulation in a way which is more consonant
with the studies mentioned in the previous paragraph was Keibel ('01). He
defined gastrulation in the vertebrates ('01, p. 1111) as "the process by
which the entodermal, mesodermal and notochordal cells find their way into
the interior of the embryo." It is to be observed that this definition embodies
the concept of migration of specific, organ-forming areas. We may restate the
concept involved in this definition in a way which includes invertebrates as
well as vertebrates as follows: Gastrulation is the dynamic process during
which the major, presumptive organ-forming areas of the blastula (Chaps.
6 and 7) become rearranged and reorganized in a way which permits their
ready conversion into the body plan of the particular species. That is to say,
during the process of gastrulation, the presumptive organ-forming areas of
the blastula undergo axiation in terms of the body organization of the species.
In some animal species, this reorganization of the blastula into the structural
DEFINITIONS AND CONCEPTS 391
pattern of the gastrula results in the production of a two-layered form, for
example, as in Amphioxus; in others (actually in most metazoan species) it
brings about the formation of a three-layered condition. It is apparent, there-
fore, as observed by Pasteels ('37b, p. 464), that "it is impossible to give a
general definition of the gastrula stage." It is obvious, also, that one cannot
define gastrulation in terms of simple invagination, delamination, or the pro-
duction of a two-layered condition. Many processes, involving intricate move-
ments of cell groups, occur as outlined in the succeeding pages of this chapter.
Relative to the process of gastrulation and later development, emphasis
should be placed upon the importance of the blastocoel. The latter takes its
origin largely by the movement of groups of cells in relation to one another
during cleavage and blastulation. Therefore, we may enumerate the follow-
ing events related to the blastocoel during the early phases of embryonic
development:
( 1 ) The blastocoel is associated with those movements in the developing
blastula which produce the specific cellular configuration of the ma-
ture blastula;
(2) during gastrulation, it enables the various, presumptive organ-forming
areas of the blastula to be rearranged and to migrate into the particular
areas which permit their ready organization and axiation into the
scheme of the body form of the particular species; and
(3) in the period of development immediately following gastrulation, it
affords the initial space necessary for the tubulation of the major,
organ-forming areas.
The events mentioned in (3) will be described in Chapter 10.
2. Primitive Vertebrate Body Plan in Relation to the
Process of Gastrulation
In the animal kingdom, each of the major animal groupings has a specific
body plan. In the phylum, Chordata, the cephalochordate, Amphioxus, and
the vertebrates possess such a plan. It is necessary at this point to review
briefly the rudiments of this primitive or basic body plan.
a. Fundamental Body Plan of the Vertebrate Animal
The vertebrate body essentially is a cylindrical structure with a head or
cephalic end, a middle trunk region, and a tail or caudal end. The dorsum
or dorsal region is the uppermost aspect, while the venter or belly lies below.
Also, the body as a whole may be slightly compressed laterally. Viewed in
transverse section, the body is composed basically of five hollow tubes, par-
ticularly in the trunk area. The epidermal tube forms the exterior and within
the latter are placed the neural, enteric, and two mesodermal tubes, all oriented
around the median skeletal axis or notochord as indicated in figures 188C
and 217G and N.
392
GASTRULATION
b. The Gastrula in Relation to the Primitive Body Plan
If one watches a large transport plane preparing to take off at an airfield,
the following events may be observed:
( 1 ) The cargo and passengers are boarded, the engines are warmed, and
the plane is taxied toward the runway.
(2) Upon reaching the starting end of the runway, the engines are ac-
celerated, and the plane is turned around and headed in the direction
of the take-off.
Fig. 188. Relationship between the presumptive organ-forming areas of the blastula
(diagram A) and the primitive tubular condition of the developing vertebrate body
(diagram C). The gastrula (diagram B) represents an intermediate stage. Consult chart
in text.
GENERAL PROCESSES
393
(3) The engines are further accelerated and the plane is moved down the
runway for the take-off into the airy regions.
Similarly, during cleavage and blastulation, the embryonic machine develops
a readiness, elaborates the major, organ-forming areas in their correct posi-
tions in the blastula, and taxies into position with its engines warming up, as
it were. Once in the position of the mature blastula, the various, major, pre-
sumptive organ-forming areas are turned around and reoriented by the gastrula-
tive processes, and thus, each major, organ-forming area of the gastrula is
placed in readiness for the final developmental surge which results in primitive
body formation. During the latter process the major, presumptive organ-
forming areas in the vertebrate group are molded into the form of elongated
tubular structures with the exception of the notochordal area which forms an
elongated skeletal axis. (The latter phenomena are described in Chapter 10.)
c. Chart of Blastula, Gastrula, and Primitive Body-form Relationships
in the Vertebrate Group
(Fig. 188)
The major, presumptive organ-forming areas are designated by separate
numerals.
Blastula
Gastrula
Primitive Body Form
1. Epidermal crescent
2. Neural crescent
3. Entodermal area
4. Two mesodermal
areas
5. Notochordal crescent
1. Part of ectodermal layer
2. Elongated neural plate a
part of ectoderm layer
3. Primitive archenteron in
rounded gastrulae, such as
frog; archenteric layer in
flattened gastrulae, such as
chick
4. Two mesodermal layers on
either side of notochord
5. Elongated band of cells
lying between mesodermal
layers
1. External epidermal tube
2. Dorsally placed neural tube
3. Primitive gut tube
Two primitive mesodermal
tubes; one along either side
of neural tube, notochord,
and gut tube; especially true
of trunk region
Rounded rod of cells lying
below neural tube and
above entodermal or gut
tube; these three structures
lie in the meson or median
plane of the body
B. General Processes Involved in Gastrulation
Gastrulation is a nicely integrated, dynamic process; one which is controlled
largely by intrinsic (i.e., autonomous) forces bound up in the specific, physico-
chemical conditions of the various, presumptive, organ-forming areas of the
late blastula and early gastrula. These internal forces in turn are correlated
394 GASTRULATION
with external conditions. One of the important intrinsic factors involves the
so-called organization center referred to in Chapter 7. However, before con-
sideration is given to this center, we shall define some of the major processes
involved in gastrulation.
There are two words which have come into use in embryology relative to
the process of gastrulation, namely, epiboly and emboly. These words are
derived from the Greek, and in the original they denote motion, in fact, two
different kinds of motion. The word emboly is derived from a word meaning
to throw in or thrust in. In other words, it means insertion. The word epiboly,
on the other hand, denotes a throwing on or extending upon. These words,
therefore, have quite opposite meanings, but they aptly describe the general
movements which occur during gastrulation. If, for example, we consider
figure 169, these two words mean the following: All the presumptive organ-
forming areas below line a-b in (C) during the process of gastrulation are
moved to the inside by the forces involved in emboly. On the other hand,
due to the forces concerned with epiboly, the presumptive organ-forming
materials above line a-b are extended upon or around the inwardly moving
cells.
Associated with the comprehensive molding processes of epiboly and emboly
are a series of subactivities. These activities may be classified under the fol-
lowing headings:
( 1 ) morphogenetic movement of cells,
(2) the organization center and its organizing influences, and
( 3 ) chemodifferentiation.
C. Morphogenetic Movement of Cells
1. Importance of Cell Movements During Development
AND IN Gastrulation
The movement of cells from one place in the embryo to another to establish
a particular form or structure is a common embryological procedure. This
type of cell movement is described as a morphogenetic movement because it
results in the generation of a particular form or structural arrangement. It is
involved not only in the formation of the blastula where the movements are
slow, or in gastrulation where the cell migrations are dynamic and rapid, but
also in later development. (See Chap. 11.) In consequence, we may say that
cell migration is one of the basic procedures involved in tissue and organ
formation.
The actual factors — physical, chemical, physiological, and mechanical —
which effect cell movements are quite unknown. However, this lack of knowl-
edge is not discouraging. In fact, it makes the problem more interesting, for
cells are living entities utilizing physicochemical and mechanical forces peculiar
MOVEMENT OF CELLS 395
to that condition which we call living. The Uving state is a problem which
awaits solution.
At the period when the process of blastulation comes to an end and the
process of gastrulation is initiated, there is an urge directed toward cell move-
ment throughout the entire early gastrula. Needham ('42, p. 145) uses the
term "inner compulsion" to describe the tendency of the cells of the dorsal-lip
area to move inward (invaginate) at this time. Whatever it is called and
however it may be described, the important feature to remember is that this
tendency to move and the actual movement of the cells represent a living
process in which masses of cells move in accordance with the dictates of a
precise and guiding center of activity, known as the primary organizer or
organization center.
2. Types of Cell Movement During Gastrulation
The following types of cell movement are important aspects of the process
of gastrulation.
a. Epiboly
( 1 ) Extension along the antero-posterior axis of the future embryo.
(2) Peripheral expansion or divergence.
b. Emboly
( 1 ) Involution.
(2) Invagination.
(3) Concrescence (probably does not occur).
(4) Convergence.
(5) Polyinvagination.
(6) Delamination.
(7) Divergence or expansion.
(8) Extension or elongation.
(9) Blastoporal constriction.
Note: While cell proliferation is not listed as a specific activity above, it is
an important aspect of gastrulation in many forms.
3. Description of the Processes Concerned with Epiboly
Epiboly or ectodermal expansion involves the movements of the pre-
sumptive epidermal and neural areas during the gastrulative process. The
general migration of these two areas is in the direction of the antero-posterior
axis of the future embryonic body in all chordate embryos. In the rounded
blastula (e.g., frog, Amphioxus, etc.), the tendency to extend antero-posteriorly
produces an enveloping movement in the antero-posterior direction. As a
result, the presumptive epidermal and neural areas actually engulf and sur-
round the inwardly moving presumptive notochordal, mesodermal, and ento-
396 GASTRULATION
dermal areas. (Study fig. 190A-H.) In flattened blastulae the movements of
epiboly are concerned largely with antero-posterior extension, associated with
peripheral migration and expansion of the epidermal area. (See fig. 202.) The
latter movement of the presumptive epidermal area is pronounced in teleost
fishes, where the yolk is engulfed as a result of epidermal growth and expan-
sion (figs. 210B; 21 ID).
The above-mentioned activities, together with cell proliferation, effect
spatial changes in the presumptive epidermal and neural areas as shown in
figures 189, 190, 191, 198, and the left portion of figure 202A-I. It is to be
observed that the epidermal crescent is greatly expanded, and the area cov-
ered is increased; also, that the neural crescent is changed into a shield-shaped
area, extended in an antero-posterior direction (figs. 192A; 2021).
4. Description of the Processes Involved in Emboly
While forces engaged in epiboly are rearranging the presumptive neural
and epidermal areas, the morphogenetic movements concerned with emboly
move the presumptive chordamesodermal and entodermal areas inward and
extend them along the antero-posterior axis of the forming embryo. This in-
ward movement of cells is due to innate forces within various cell groups;
some apparently are autonomous (i.e., they arise from forces within a par-
ticular cell group), while others are dependent upon the movement of other
cell groups.) (See p. 447.) We may classify the types of cell behavior during
this migration and rearrangement of the chordamesoderm-entodermal areas
as follows:
a. Involution and Convergence
Involution is a process which is dependent largely upon the migration of
cells toward the blastoporal lip (e.g., frog, see heavy arrows, fig. 192) or
to the primitive streak (e.g., bird, see arrows, fig. 204C-E). The word involu-
tion, as used in gastrulation, denotes a "turning in" or inward rotation of
cells which have migrated to the blastoporal margin. In doing so, cells located
along the external margin of the blastoporal lip move over the lip to the inside
edge of the lip (see arrows, figs. 191C-E, H; 192B, C). The inturned or in-
voluted cells thus are deposited on the inside of the embryo along the inner
margin of the blastopore. The actual migration of cells from the outside surface
of the blastula to the external margin of the blastoporal lip is called con-
vergence. In the case of the primitive streak of the chick, the same essential
movements are present, namely, a convergence of cells to the primitive streak
and then an inward rotation of cells through the substance of the streak to
the inside (arrows, fig. 204; black arrows, fig. 202). If it were not for the
process of involution, the converging cells would tend to pile up along the
outer edges of the blastoporal lip or along the primitive streak. Involution
MOVEMENT OF CELLS 397
thus represents a small but extremely important step in the migration of cells
from the exterior to the interior during gastrulation.
b. Invagination
The phenomenon of invagination, as used in embryological development,
implies an infolding or insinking of a layer of cells, resulting in the formation
of a cavity surrounded by the infolded cells (figs. 189, 190, the entoderm).
Relative to gastrulation, this process has two aspects:
( 1 ) mechanical or passive infolding of cells, and
(2 ) active inward streaming or inpushing of cells into the blastocoelic space.
In lower vertebrates, the dorsal-lip area of the blastopore is prone to exhibit
the active form of invagination, whereas the entoderm of the lateral- and
ventral-lip regions of the blastopore tends to move in a passive manner. The
notochordal-canal, primitive-pit area of the primitive streak of higher verte-
brates is concerned especially with the active phase of invagination.
c. Concrescence
This term is used in older descriptions of gastrulation. The word denotes
the movement of masses of cells toward each other, particularly in the region
of the blastopore, and implies the idea of fusion of cell groups from two
bilaterally situated areas. It probably does not occur. (However, see develop-
ment of the feather in Chap. 12.)
d. Cell Proliferation
An increase in the number of cells is intimately concerned with the process
of gastrulation to the extent that gastrulation would be impeded without it,
in some species more than in others. Cell proliferation in Amphioxus, for
example, is intimately associated with the gastrulative process, whereas in the
frog it assumes a lesser importance.
e. Polyinvagination
Polyinvagination is a concept which implies that individual or small groups
of cells in different parts of the external layer of the blastula or blastodisc
invaginate or ingress into the segmentation (blastocoelic) cavity. That is,
there are several different and separate inward migrations of one or more cells.
This idea recently was repudiated by Pasteels ('45) relative to the formation
of the entodermal layer in the avian blastoderm. It applies, presumably, to the
ingression of cells during the formation of the two-layered blastula in the
prototherian mammal, Echidna (see p. 364).
/. Ingression
The word ingression is suitable for use in cases where a cell or small
groups of cells separate from other layers and migrate into the segmentation
398 GASTRULATION
cavity or into spaces or cavities developed within the developing body. In the
primitive-streak area of reptiles, birds, and mammals, for example, meso-
dermal cells detach themselves from the primitive streak and migrate into the
space between the epiblast and hypoblast. Also, in Hie formation of the two-
layered embryo in the prototherian mammal, Echidna, the inward migration
of small entodermal cells to form the hypoblast may be regarded as cellular
ingression (fig. 175D). Ingression and polyinvagination have similar meanings.
g. Delamination
The word delamination denotes a mass sunderance or separation of groups
of cells from other cell groups. The separation of notochordal, mesodermal,
and entodermal tissues from each other to form discrete cellular masses in
such forms as the teleost fish or the frog, after these materials have moved
to the inside during gastrulation, is an example of delamination (fig. 210E, F).
h. Divergence
This phenomenon is the opposite of convergence. For example, after cells
have involuted over the blastoporal lips during gastrulation, they migrate and
diverge to their future positions within the forming gastrula. This movement
particularly is true of the lateral plate and ventral mesoderm in the frog, or
of lateral plate and extra-embryonic mesoderm in the reptile, bird, or mammal
(fig. 192B, C, small arrows).
/. Extension
The elongation of the presumptive neural and epidermal areas externally
and of the notochordal, mesodermal, and entodermal materials after they have
moved inward beneath the neural plate and epidermal material are examples
of extension. The extension of cellular masses is a prominent factor in gas-
trulation in all Chordata from Amphioxiis to the Mammalia. In fact, as a
result of this tendency to extend or elongate on the part of the various cellular
groups, the entire gastrula, in many instances, begins to elongate in the antero-
posterior axis as gastrulation proceeds. The faculty for elongation and exten-
sion is a paramount influence in development of axiation in the gastrula and
later on in the development of primitive body form. The presumptive noto-
chordal material possesses great autonomous powers for extension, and hence,
during gastrulation it becomes extended into an elongated band of cells.
D. The Organization Center and Its Relation to the Gastrulative Process
1. The Organization Center and the Primary Organizer
Using a transplantation technic on the beginning gastrula of the newt, it was
shown by Spemann ('18) and Spemann and Mangold ('24) that the dorsal-
lip region of the blastopore (that is, the chordamesoderm-entoderm cells in
this area), when transplanted to the epidermal area of another embryo of the
THE ORGANIZATION CENTER 399
same stage of development, is able to produce a secondary gastrulative process
and thus initiate the formation of a secondary embryo (fig. 193). Because
the dorsal-lip tissue was able thus to organize the development of a second
or twin embryo, Spemann and Mangold described the dorsal-lip region of the
beginning gastrula as an "organizer" of the gastrulative process. In its normal
position during gastrulation this area of cells has since been regarded as the
organization center of amphibian development. It is to be observed in this
connection that Lewis ('07) performed the same type of experiment but failed
to use an embryo of the same age as a host. As he used an older embryo, the
notochordal and mesodermal cells developed according to their presumptive
fate into notochordal and somitic tissue but failed to organize a new embryo.
More recent experiments upon early frog embryos by Vintemberger ('36)
and by Dalcq and Pasteels ('37), and upon early teleost fish embryos by other
investigators (Oppenheimer, '36 and '47) have demonstrated the necessity
and importance of yolk substance in the gastrulative process. This fact led
Dalcq and Pasteels ('37) to suggest a new concept of the organization center,
namely, that this center is dependent upon two factors: "the yolk and some-
thing normally bound to the gray crescent" (i.e., chordamesodermal area).
It was thought at first that the transplanted organizer material actually or-
ganized and produced the new embryo itself (Spemann, '18, p. 477). But this
idea had to be modified in the light of the following experiment by Spemann
and Mangold ('24): Dorsal-lip material of unpigmented Triton cristatus was
transplanted to an embryo of T. taeniatus of the same age. The latter species
is pigmented. This experiment demonstrated that the neural plate tissue of
the secondary embryo was almost entirely derived from the host and not from
the transplanted tissue. Consequently, this experiment further suggested that
the organizer not only possessed the ability to organize but also to induce host
tissue to differentiate. Induction of neural plate cells from cells which ordi-
narily would not produce neural plate tissue thus became a demonstrated fact.
The concept of an organizer in embryonic development had profound im-
plications and stimulated many studies relating to its nature. Particularly,
intensive efforts were made regarding the kinds of cells, tissues, and other
substances which would effect induction of secondary neural tubes. The re-
sults of these experiments eventually showed that various types of tissues and
tissue substances, some alive, some dead, from many animal species, including
the invertebrates, were able to induce amphibian neural plate and tube forma-
tion. (See Spemann, '38', Chap. X and XI; also see fig. 196A, B and compare
with fig. 193.) Moreover, microcautery, fuller's earth, calcium carbonate, silica,
etc., have on occasion induced neural tube formation. However, the mere in-
duction of neural tube development should not be confused with the organizing
action of normal, living, chordamesoderm-entoderm cells of the dorsal-lip
region of the beginning gastrula. The latter's activities are more comprehensive,
for the cells of the dorsal-lip area direct and organize the normal gastrulative
400 GASTRULATION
process as a whole and bring about the organization of the entire dorsal axial
system of notochord, neural tube, somites, etc. In this series of activities, neural
plate induction and neural tube formation merely are secondary events of a
general organization process.
A clear-cut distinction should be drawn, therefore, between the action of
the dorsal-lip organizer, in its normal position and capacity, and that of an
ordinary inductor which induces secondary neural tube development. The
characteristics of the primary organizer or organization center of the early
gastrula are:
(a) its ability for autonomous or self-differentiation (that is, it possesses the
ability to give origin to a considerable portion of the notochord, pre-
chordal plate material, and axial mesoderm of the secondary embryo),
(b) its capacity for self-organization^
(c) its power to induce changes within and to organize surrounding cells,
including the induction and early organization of the neural tube.
As a result of its comprehensive powers, it is well to look upon the organi-
zation center (primary organizer) as the area which determines the main fea-
tures of axiation and organization of the vertebrate embryo. In other words,
// directs the conversion of the late blastula into the axiated gastrular condition
— a condition from which the primitive vertebrate body is formed. Induction
is a tool-like process, utilized by this center of activity, through which it effects
changes in surrounding cells and thus influences organization and differentia-
tion. Moreover, these surrounding cells, changed by the process of induction,
may in turn act as secondary inductor centers, with abilities to organize specific
subareas.
An example of the ability of a group of cells, changed by inductive influence,
to act as an inducing agent to cause further inductive processes is shown by
the following experiment performed by O. Mangold ('32). The right, pre-
sumptive, half brain of a neurula of Ambystoma mexicanum, the axolotl, was
removed and inserted into the blastocoel of a midgastrula of Triton taeniatus.
Eight days after the implant was made, a secondary anterior end of an embryo
was observed protruding from the anterior, ventral aspect of the host larva.
An analysis of this secondarily induced anterior portion of an embryo demon-
strated the following:
( 1 ) The original implant had developed into a half brain with one eye and
one olfactory pit. However,
(2) it also had induced a more or less complete secondary larval head
with a complete brain, two eyes, with lenses, two olfactory pits, one
ganglion, four auditory vesicles, and one balancer. One of the eyes had
become intimately associated with the eye of the implant, both having
the same lens.
THE ORGANIZATION CENTER 401
The series of inductive processes presumably occurred as follows: The im-
planted half brain induced from the epidermis of the host a secondary anterior
end of a neural plate; the latter developed into a brain which induced the
lenses, auditory vesicles, etc. from the host epidermis. Thus, the original
implant, through its ability to induce anterior neural plate formation from
the overlying epidermis, acted as a "head organizer."
The transformation of the late blastula into the organized condition of the
late gastrula thus appears to be dependent upon a number of separate induc-
tions, all integrated into one coordinated whole by the "formative stimulus"
of the primary organizer located in the pre-chordal plate area of entodermal-
mesodermal cells and adjacent chordamesodermal material of the early gastrula.
2. Divisions of the Primary Organizer
The primary organizer is divisible into two general inductor areas as follows:
(a) the pre-chordal plate of entomesodermal material, and
(b) the chordamesodermal cells which come to lie posterior to the pre-
chordal plate area of the late gastrula.
The pre-chordal plate is a complex of entodermal and mesodermal cells
associated at the anterior end of the notochordal cells in the late gastrula. In
the beginning gastrula, however, it lies between the notochordal material and
the dorsal-lip inpushing of the entoderm in amphibia, and just caudal to the
notochordal area in teleosts, elasmobranch fishes, reptiles, and birds (figs.
169; 173 A; 179B; 180B). The chordamesodermal portion of the primary
organizer is composed of presumptive notochordal cells and that part of the
presumptive mesoderm destined to form the somites. The pre-chordal plate is
known as the head organizer, because of its ability to induce brain structures
and other activities in the head region. (The use of the phrase head organizer
as a synonymous term for pre-chordal plate is correct in part only, for a
portion of the anterior notochord and adjacent mesoderm normally is con-
cerned also with the organization of the head.) On the other hand, the pre-
sumptive notochord with the adjacent somitic (somite) material is described
as the trunk or tail organizer (fig. 191G) because of its more limited inductive
power. For example, Spemann ('31) demonstrated that the head organizer
transplanted to another host embryo of the same age produced a secondary
head with eye and ear vesicles when placed at the normal head level of the
host. Also when placed at trunk level, it induced a complete secondary embryo
including the head structures. However, the trunk organizer is able to induce
head and trunk structures at the head level of the host; but in the trunk region
it induces only trunk and tail tissues. (See Holtfreter, '48, pp. 18-19; Needham,
'42, pp. 271-272; Spemann, '31, '38. The student is referred also to Huxley
and De Beer, '34, Chaps. 6 and 7; and Lewis, '07.)
402 GASTRULATION
E. Chemodifferentiation and the Gastrulative Process
In the previous chapter it was observed that certain areas of the amphibian
blastula are foreordained to give origin to certain organ rudiments in the future
embryo because of their position and not because of their innate physiological
condition. This condition is true of the future neural plate ectoderm and epi-
dermal ectoderm. During the conversion of the late blastula into the late gas-
trula, these areas become changed physiologically, and they no longer are
determined in a presumptive sense but have undergone changes which make
them self-differentiating. This change from a presumptively determined con-
dition to a self-differentiating, fixed state is called determination and the
biochemical change which effects this alteration is known as chemodifferen-
tiation (see Chap. 8).
Chemodifferentiation is an important phenomenon during gastrulation. As
a result of the physiological changes involved in chemodifferentiation, re-
strictive changes in potency are imposed upon many localized cellular areas
within the major, organ-forming areas. In consequence, various future organs
and parts of organs have their respective fates rigidly, and irrevocably deter-
mined at the end of gastrulation. The gastrula thus becomes a loose mosaic
of specific, organ-forming areas (figs. 194, 205). Consequently, the areas of
the beginning gastrula which possess competence (Chap. 8) become more and
more restricted as gastrulation proceeds. ChemodifTerentiation apparently oc-
curs largely through inductive (evocative) action.
F. Gastrulation in Various Chordata
1. Amphioxus
a. Orientation
Consult figures 167, 189, and 190 and become familiar with the animal-
vegetal pole axis of the egg, the presumptive organ-forming areas, etc.
b. Gastrulative Movements
1) Emboly. As gastrulation begins, a marked increase in mitotic activity
occurs in the cells of the dorsal crescent, composed of presumptive noto-
chordal and neural plate cells, and also in the cells of the ventral crescent
or future mesodermal tissue. The general ectodermal cells or future epidermis
also are active (figs. 167, 189, 190B). The entodermal cells, however, are
quiescent (Conklin, '32). Accompanying this mitotic activity, the entodermal
plate gradually invaginates or folds inwardly into the blastocoel (figs. 189,
190). In doing so, the upper portion of the entodermal plate moves inward
more rapidly and pushes forward toward a point approximately halfway be-
tween the polar body (i.e., the original midanimal pole of the egg) and the
point which marks the anterior end of the future embryo (observe pointed
end of arrow, fig. 189). Shortly after the inward movement of the entodermal
GASTRULATION IN VARIOUS CHORDATA 403
plate is initiated, notochordal cells in the middorsal region of the blastopore
involute, move inward along with the entoderm, and come to occupy a position
in the middorsal area of the forming archenteron (fig. 190C-E). Similarly,
mesodermal cells in the upper or dorsal ends of the mesodermal crescent
gradually converge dorso-mediad and pass into the roof of the forming gas-
trocoel (archenteron) on either side of the median area occupied by the
notochordal cells (fig. 190F, G). Thus the roof of the gastrocoel is composed
of notochordal and mesodermal cells (fig. 195A, B),
2) Epiboly. As the above events come to pass, the potential epidermal
and neural cells proliferate actively, and both areas gradually become extended
in an antero-posterior direction. In this way the neural ectoderm becomes
elongated into a median band which lies in the middorsal region of the gastrula
(figs. 190A-H; 247B-F), while the epidermal area covers the entire gastrula
externally with the exception of the neural area.
Thus, the general result of this proliferation, infolding, and involution of
the presumptive entodermal, notochordal, and mesodermal cells, together
with the extension and proliferation of the ectodermal cells is the production
of a rudimentary double-layered embryo or gastrula (figs. 189, 190). Ecto-
dermal cells (epidermal and neural) form the external layer (fig. 190G).
The internal layer is composed of notochordal cells in the dorso-median area
with two narrow bands of mesodermal cells lying along either side of the
median notochordal band of cells while the remainder of the internal layer
is composed of entodermal cells (figs. 190G; 195A, B). At the blastoporal
end of this primitive gastrula are to be found proliferating notochordal, meso-
dermal, entodermal, and ectodermal cells.
3) Antero-posterior Extension of the Gastrula and Dorsal Convergence of
the Mesodermal Cells. The processes associated with epiboly bring about an
antero-posterior extension of the ectodermal layer of cells. Similarly, the cells
which are moved inward by embolismic forces are projected forward toward
the future cephalic end of the embryo and become extended along the median
embryonic axis. Epiboly and emboly, accompanied by rapid cell proliferation
at the blastoporal-lip area, thus effect an antero-posterior elongation of the
developing gastrula (figs. 189H; 190H).
As the gastrula is extended in the antero-posterior direction, a shift occurs
in the position of the mesodermal cells which form the ventral or mesodermal
crescent. The ventral crescent becomes divided ventrally into two halves, and
each half gradually moves dorsalward along the inner aspect of the lateral
blastoporal lips as gastrulation is accomplished. Each arm of the original
crescent in this manner converges dorso-mediad toward the median noto-
chordal cells of the dorsal blastoporal lip, and a mass of mesodermal cells
comes to lie along either side of the notochordal cells. As a result of this
converging movement, entodermal cells of the blastoporal area converge dorso-
404
GASTRULATION
mediad and come to occupy the ventral lip of the blastopore, together with
the externally placed, epidermal cells (fig. 190G, arrow). The blastopore as
a whole grows smaller and moves to a dorsal position during the latter changes
(fig. 247 A-C).
4) Closure of the Blastopore. See Chapter 10, neuralization in Amphioxus.
Fig. 189. Gastrulation in Amphioxus. (Modified from Conklin, '32.) (A) Beginning
gastrula. (B) Observe that entodermal (hypoblast layer) is projected roughly in direc-
tion of future cephalic end of embryo. (C-G ) Observe continued projection of entoderm
toward cephalic end of future embryo. Note also position of polar body. In (F), (G),
and (H) the gastrula begins to elongate along the antero-posterior axis of the developing
embryo. (H) End of gastrular condition. Blastopore is closed by epidermal overgrowth,
and neurenteric canal is formed between archenteron and forming neural tube.
GASTRULATION IN VARIOUS CHORDATA
405
FUTURE
ANTERIOR
POLAR
BOD
Fig. 190. Similar to fig. 189, showing the presumptive organ-forming areas of the
blastula and their position during gastrulation. The position of the respective organ-
forming areas in a transverse section through the midregion of gastrula shown in G is
depicted in fig. 195A. (Based upon data obtained from Conklin, '32.)
c. Resume of Cell Movements and Processes Involved in Gastrulation
of Amphioxus
1) Emboly:
(a) Invagination. The entodermal plate of cells gradually invaginates and
folds inward into the blastocoel.
(b) Proliferation of cells. This is true particularly of notochordal, meso-
dermal, and neural cells near the blastoporal area during initial stages
406 GASTRULATION
of gastrulation, although mitoses occur in other regions as well. During
later stages of gastrulation, the entire complex of cells around the
blastoporal region divides actively.
(c) Involution. Notochordal cells converge to the midregion of the dorsal
blastoporal lip and then turn inward (involute) over the lip area to
the inside.
(d) Extension. General elongation of the embryonic rudiment as a whole
occurs, including the neural plate area.
(e) Convergence. Mesodermal cells converge toward the middorsal area
of the blastopore. The path of this convergence is along the lateral
lips of the blastopore, particularly the inner aspects of the lips. This
movement is pronounced toward the end of gastrulation when each
half of the mesodermal crescent moves dorsad toward the middorsal
area of the blastopore. The mesoderm thus comes to lie on either
side of the notochordal material at the dorsal lip of the blastopore.
(f) Constriction of the blastopore. During later phases of gastrulation, the
blastopore grows smaller (fig. 247A-D), associated with a constriction
of the marginal region of the blastoporal opening, particularly of the
entodermal and epidermal layers. The movement of the mesoderm de-
scribed in (e) above plays a part in- this blastoporal change.
2) Epiboly. The caudal growth of the entire ectodermal layer of cells, epi-
dermal and neural, and their antero-posterior extension is a prominent feature
of gastrulation in Amphioxus.
(Further changes in the late gastrula, together with the closing of the
blastopore, are described in Chapter 10. See tubulation of neural plate, etc.)
2. Gastrulation in Amphibia with Particular Reference
TO THE Frog
a. Introduction
1) Orientation. A line drawn from the middle region of the animal pole to
the midvegetal pole constitutes the median axis of the egg. In the anuran
Amphibia the embryonic axis corresponds approximately to the egg axis. That
is, the midanimal pole of the egg represents the future anterior or antero-
dorsal end of the embryo, while the midvegetal pole area denotes the posterior
region.
As indicated previously (Chap. 7), the very late blastula is composed of
presumptive organ-forming areas arranged around the blastocoelic space. The
yolk-laden, future entodermal cells of the gut or digestive tube form the
hypoblast and are concentrated at the vegetal pole. Presumptive notochordal
and mesodermal cells constitute a marginal zone of cells which surrounds
the upper region of the presumptive entodermal organ-forming area (fig.
169C-F). The presumptive notochordal area is in the form of a crescent.
GASTRULATION IN VARIOUS CHORDATA 407
whose midportion is located just above the future dorsal lip of the early gas-
trula, while the mesoderm lies to each side of the notochordal cells, extending
along the margin of the entoderm toward the corresponding mesodermal zone
of the other side (fig. 169D, F). The presumptive neural crescent occupies
a region just dorsal and anterior to the notochordal area. The remainder of
the animal pole is composed of presumptive epidermis. The presumptive
notochordal, neural plate, and epidermal areas are oriented along the general
direction of the future antero-posterior embryonic axis, the notochordal tissue
being the more posterior. Moreover, the midregion of the notochordal and
neural crescents at this time lies at the dorsal region of the future embryo
(fig. 194A). The presumptive entodermal area, on the other" hand, does not
have the same orientation as that of the above areas. In contrast, its axiation
is at right angles to the future embryonic axis (fig. 194A). If one views a
very early gastrula of the anuran amphibian in such a way that the beginning
blastoporal lip is toward the right (fig. 194A), then:
( 1 ) The foregut material lies toward the right at the region of the forming
blastoporal lip;
(2) the stomach material is slightly to the left of this area; and
(3) the future intestinal area lies to the left and toward the vegetal pole.
Therefore, one aspect of the gastrulative processes is to bring the ento-
dermal area into harmony with the future embryonic axis and, in doing so,
to align its specific, organ-forming subareas along the antero-posterior axis
of the embryo. In other words, the entodermal material must be revolved
about 90 degrees in a counterclockwise direction from the initial position
occupied at the beginning of gastrulation (compare fig. I94A, B).
2) Physiological Changes Which Occur in the Presumptive Organ-forming
Areas of the Late Blastula and Early Gastrula as Gastrulation Progresses.
A striking physiological change is consummated in the presumptive organ-
forming areas of the epiblastic portion of the late blastula during the process
of gastrulation. This change has been demonstrated by transplantation ex-
periments. For example, if presumptive epidermis of the very late blastula
and early gastrula is transplanted by means of a micropipette to the pre-
sumptive neural area and vice versa, the material which would have formed
epidermis will form neural tissue, and presumptive neural cells will form epi-
dermis (fig. 196C, D). (See Spemann, '18, '21; Mangold, '28.)
The experiment pictured in figure 196 involves interchanges between two
presumptive areas within the same potential germ layer, i.e., ectoderm. How-
ever, Mangold ('23) demonstrated that presumptive epidermis transplanted
into the dorsal-lip area, i.e., into the presumptive mesodermal area, may in-
vaginate and form mesodermal tissue. The converse of this experiment was
performed by Lopaschov ('35) who found that presumptive mesoderm from
the region of the blastoporal lip transplanted to the neural plate area of a
408 GASTRULATION
somewhat older embryo becomes, in some cases, normally incorporated in
the neural tube of the host. Similar interchanges of cells of the late blastula
have demonstrated that almost any part, other than the presumptive entoderm,
can be interchanged without disturbing the normal sequence of events. How-
ever, as gastrulation progresses, interchange from epidermal to neural areas
continues to be possible during the early phases of gastrulation (fig. 196C, D)
but not at the end of gastrulation. Similar changes occur also in the mesodermal
area. Pronounced physiological changes thus occur in the presumptive organ-
forming aicas of the entire epiblastic region during gastrulation.
b. Gastrulation
1) Emboly. As gastrulation begins, a small, cleft-like invagination appears
in the entodermal material of the presumptive foregut area. This invagination
is an active inpushing of entodermal cells which fold inward and forward
toward the future cephalic end of the embryo (fig. 191B-E). The upper or
dorsal edge of the cleft-like depression visible at the external surface forms
the dorsal lip of the blastopore (fig. 19 IB). In this connection, study dia-
grams in figure 197. The pre-chordal plate cells are associated with the form-
ing dorsal roof of the archenteron and, therefore, form a part of the invagi-
nated material shortly after this process is initiated.
As the entodermal material migrates inward and the initial dorsal lip is
formed, notochordal cells move posteriad to the dorsal lip and involute to
the inside in close association with the pre-chordal plate cells. Also, the more
laterally situated, notochordal material converges toward the dorsal lip and
gradually passes to the inside, as gastrulation progresses, where it lies in the
mid-dorsal region of the embryo. (See arrows, figs. 188A; 19 IC, D). Here it
begins to elongate antero-posteriorly (i.e., it becomes extended) and forms a
narrow band of cells below the forming neural plate (fig. 191C-G).
With the continuance of gastrulation, the entodermal material moves more
extensively inward (cf. fig. 191C-E) and the entodermal mass of yolk-laden
cells below the site of invagination begins to sink or rotate inwardly. The
dorsal blastoporal lip, therefore, widens considerably (fig. 197 A, B). In many
Amphibia the inner surface of the entoderm, as it progresses inward, forms
a cup-like structure which actually engulfs the blastocoelic fluid (fig. 191 B-D ) .
It is not clear whether this cup-like form is produced by active inward mi-
gration of entodermal cells or whether it may be due in part, at least, to
constrictive forces at the blastoporal lip.
Synchronized with the events described above, the presumptive somitic
mesoderm, located externally along either side of the notochordal area of the
early gastrula, migrates (converges) toward the forming dorso-lateral lips of
the blastopore (fig. 197A, B, broken arrows). Upon reaching the blastoporal
edge, the mesoderm moves over the lip (involutes) to the inside. However,
the mesoderm does not flow over the lip to the inside as a part of the entoderm
GASTRULATION IN VARIOUS CHORDATA
409
in a manner similar to the pre-chordal and notochordal cells; rather, upon
reaching the edge of the blastopore, it involutes over the lip, then insinuates
itself between the inside entoderm and the external surface layer of cells, and,
in this position, passes inward and forward between the entoderm and the
external layer of cells (figs. 19 IH; 198 A).
YOLK PLUG
Fig. 191. Migration of the presumptive organ-forming areas of the blastula during
gastrulation in the amphibia (with reference particularly to the frog). (See fig. 192.)
(A) Late blastula, sagittal section through midplane of future embryo. (B-F) Observe
processes of epiboly and emboly. In epiboly, the black (neural) and white (epidermal)
areas become extended and gradually envelop (fig. 192A) the inward moving notochord,
entoderm, and mesoderm. The processes concerned with emboly bring about the inward
migration of the latter presumptive areas. (G) Late gastrular condition, with neural
area and upper portion of the epidermal area removed to show relationships of the middle
germ layer of chordamesoderm. (H) Horizontal section of middle gastrular condition,
showing involution of mesoderm between entoderm and ectoderm. (I) Late gastrula,
horizontal section, showing yolk plug, mesoderm, and final engulfment of blastocoelic
space by entoderm.
410 GASTRULATION
Coincident with the lateral extensions of the original dorsal lip of the blasto-
pore to form the lateral lips, a more extensive convergence and involution of
presumptive mesoderm located in the lateral portions of the mesodermal cres-
cent occurs (fig. 197A, B). The latter mesoderm eventually forms the lateral
area of the hypomeric mesoderm of the future embryo (figs. 19 IG; 198B,
C). As the lateral lips of the blastopore continue to form in the ventral direc-
tion, they eventually reach a point where they turn inward toward the median
axis and thus form the ventral lip of the blastopore (fig. 197C). A rounded
blastopore, circumscribing the heavily, yolk-laden, entodermal cells, thus is
formed. Associated with the formation of the ventro-lateral and ventral blasto-
poral lips is the convergence and involution of the ventro-lateral and ventral
mesoderm of the gastrula (fig. 191D-F). Accompanying the inward migration
of the entoderm in the region of the dorso-lateral lip of the blastopore, there
is, presumably, an inward rotation of the entodermal mass which lies toward
the ventral blastoporal area. The result of this entodermal movement is the
production of a counterclockwise rotation of the entodermal, organ-forming
rudiments, as indicated in figure 194B, compared to their relative positions
at the beginning of gastrulation, shown in figure 194A. (This counterclockwise
rotation is present to a degree also in Amphioxus (fig. 190A-F). In this way,
the particular, organ-forming areas of the entoderm become arranged antero-
posteriorly in a linear fashion along the embryonic axis. The foregut material
now is situated toward the anterior end of the developing embryo, while the
stomach, liver, small intestine, and hindgut regions are placed progressively
posteriad with the hindgut area near the closing blastopore (fig. 194B). The
yolk material lies for the most part within the ventral wall of this primitive
archenteron.
Associated with the axiation of the entodermal rudiments is the axiation
of the notochord-mesoderm complex. For example, the anterior segment of
the notochord and the pre-chordal plate (i.e., the head organizer) are lo-
cated anteriorly in the gastrula, while the more posterior portions of the
notochord and adjacent mesoderm (i.e., the trunk organizer) are located in
the developing trunk region (fig. 191G). The mesoderm adjacent to the noto-
chord eventually will form the somites or primitive mesodermal segments of
the embryo (figs. 191G; 217E; 224F). Experimentation, using the Vogt
method of staining with vital dyes, has demonstrated that the future, anterior,
presumptive somites lie closer to the blastoporal lips in the beginning gastrula,
whereas the more posterior, presumptive somites are situated at a greater dis-
tance from the blastoporal area. Because of this arrangement, the first or ante-
rior pair of presumptive somites moves inward first, the second pair next, etc.
The mesoderm of the future somites in this way is arranged along the notochord
in an orderly sequence from the anterior to the posterior regions of the gas-
trula (fig. 169, somites 1, 2, 3, 4, etc.). Consequently, axiation and extension
of the somitic mesoderm occur along with the antero-posterior arrangement of
GASTRULATION IN VARIOUS CHORDATA
411
the notochordal material. A similar distribution is effected in other regions
of the mesoderm. Therefore, axiation and antero-posterior extension of the
entodermal, notochordal, and mesodermal cells are conspicuous results of the
activities which effect emboly.
2) Epiboly. The above description is concerned mainly with emboly, that
is, the inward migration of the notochord-mesoderm-entoderm-yolk complex.
Allied with these active events is the downward or caudal migration of the
blastoporal lips. This migration is illustrated in figure 191B-E. In this figure
it may be observed that, as the marginal zone cells of mesoderm and notochord
Fig. 192. Movements of the parts of the blastula during gastrulation in amphibia. (Cf.
fig. 191.) (A) Results of epiboly. (Cf. fig. 191A-F.) Epidermal and neural areas envelop
the other areas during gastrulation. (B) Movements of the areas of the blastula during
emboly, as seen from the vegetative pole. Heavy arrows, solid and broken, show the
converging movements during emboly; light arrows show the extension and divergence
of cells after involution at the blastoporal margin (cf. fig. 191A-F). (C) Similar to
(B), as seen from the left side.
PRIMARY EMBRYO
MESODERM
NOTOCHOR D
NEURAL
T U B E \ >
SECONDARY EMBRYO
MESODERM
«#«;, *^"*^"~~'\'^?»:i'J^5«i^^ ARCHENTERIC
^'^•^-*^'f^""|^# >(^V-- SPACE
:T-- ■!^eSI«&.\ NOTOCHORD
" V'^-W'^*^''-^ S ' ' * * - -r "^"V^^^^ NEURAL
[ l*SP^V>j.V ^ . . y ■'. * ^■-■^'- *V»^^-f M E S 0 D I
m:.
SECONDARY
EMBRYO
PRIMARY EMBRYO
' A ARCHENTERIC SPACE
Fig. 193. Induction of a secondary embryo. (From Spemann, '38.) (A) Host embryo
shown in this figure is Triton taeniatus. A median piece of the upper lip of the blastopore
of a young gastrula of T. cristatus of approximately the same age as the host was im-
planted into the ventro-lateral ectoderm of the host. The implanted tissue developed into
notochord, somites, etc.; the neural tube was induced from the host ectoderm. (B) Cross
section through embryo shown in (A).
412
GASTRULATION
ANTERIOR
SMALL
INTESTINE
NEURAL ECTODERM
BRAIN AREA
NOTOCHORD
STOMACH
PdTERIOR
l\ FOREGUT
BLASTOPORAL
LIP
ESOPHAGUS
YOLK
^ EPIDERMAL
ECTODERM
V E N T R A L
MESODERM
Fig. 194. Developmental tendencies of entodermal area and their reorientation during
gastrulation. (A) Developmental tendencies of entodermal area of young anuran gas-
trula. (B) Counterclockwise rotation of approximately 90° of the entodermal area
during gastrulation.
together with the entoderm and yolk pass to the inside, the forces involved
in epiboly effect the expansion of the purely ectodermal portion of the epiblast
which gradually comes to cover the entire external surface of the gastrula with
the exception of the immediate blastoporal area (study black and white areas
in fig. 191A-E). It may be observed further that the neural crescent now is
elongated along the antero-posterior, embryonic axis where it forms a shield-
shaped region with the broad end of the shield located anteriorly (fig. I92A).
A study of figure 19 IE and F shows that a rotation of the entire gastrula
occurs in the interim between E and F. This rotation is induced by the inward
movement of the entoderm and yolk, depicted in figure 191C-E, with a
subsequent shift in position of the heavy mass of yolk from the posterior pole
of the embryo to the embryo's ventral or belly region. Most of the blastocoel
and its contained fluid is "engulfed" by the inward moving entoderm, as indi-
cated in figure 191C-E, some of the blastocoelic fluid and blastocoelic space
passes over into the gastrocoel. The region of the entodermal yolk mass shown
to the left in figure 19 IE, therefore, is more dense and heavier than the area
shown to the right. The heavier region of the gastrula seeks the lower level;
hence the rotation of the entire gastrula, and the new position assumed in
figure 19 IF.
As the blastopore progressively grows smaller, it eventually assumes a small,
rounded appearance (fig. 197A-E), and the remnants of the presumptive
mesoderm pass over the lips of the blastopore before it closes. In doing so, the
presumptive tail mesoderm converges dorsally and becomes located inside
the dorso-lateral portion of the closed blastopore near the lateral aspects of the
posterior end of the folding neural plate.
A short while previous to blastoporal closure, the midregion of the neural
plate area begins to fold ventrad toward the notochord, while its margins are
GASTRULATION IN VARIOUS CHORDATA
413
elevated and projected dorso-mediad. The exact limits of the neural plate thus
become evident (fig. 197D).
3) Embryo Produced by the Gastrulative Processes. The general result of
epiboly and emboly in the Amphibia is the production of an embryo of three
germ layers with a rounded or oval shape. The potential skin ectoderm and
infolding, neural plate area form the external layer (fig. 192A). Underneath
this external layer are the following structural regions of the middle or meso-
dermal layer:
(a) Below the developing nerve tube is the elongated band of notochordal
cells;
(b) on either side of the notochord is the somitic (somite) mesoderm;
(c) lateral to the somitic area is the mesoderm of the future kidney system;
and
Fig. 195. Placement of the presumptive, organ-forrning areas in an embryo of Am-
phioxus of about six to seven somites. (Modified from Conklin, '32.) (A) Section
through anterior region. (J) Section through caudal end of embryo. (B-I) Successive
sections going posteriorly at different body levels between (A) and (J).
414
GASTRULATION
SECONDARY NEURALIZATION
EPIDERMAL TRANSPLANT
INTO NEURAL PLATE ^^sff^.
NEURAL Plat
INTO EPIDER
LATE TRANSPLANT
M A L AREA
Fig. 196. Ectodermal potencies of the amphibian gastrula. (A and B from Spemann.
'38, after Fischer; C and D from Spemann. '38, after Spemann, '18.) (A) Induction of
a secondary neural plate in the axoloti gastrula by five per cent oleic acid, emulsified in
agar-agar. (B) Induction of secondary neural plate by nucleic acid from calf thymus.
(C) Formation of neural plate tissue from presumptive epidermal cells transplanted into
neural plate region. (D) Reverse transplant, presumptive neural plate becomes epi-
dermal tissue.
(d) Still more lateral and extending ventraliy are the lateral plate and ventral
mesoderm (figs. 191F-I; 198A-C; 221).
The third or inner germ layer of entoderm is encased within the mesodermal
or middle germ layer. The entodermal layer is an oval-shaped structure con-
taining a small archenteric cavity filled with fluid. Its ventral portion is heavily
laden with yolk substance. Also, the future trunk portion of the archenteric
roof is incomplete, the narrow notochordal band forming a part of its mid-
dorsal area (figs. 19 IF; 194B; 219D). Within each of these germ layers are
to be found restricted areas destined to be particular organs. Each layer may
be regarded, therefore, as a general mosaic of organ-forming tendencies.
4) Position Occupied by the Pre-chordal Plate Material. Another feature
of the late gastrula remains to be emphasized, namely, the pre-chordal plate
composed of entodermal and mesodermal cells integrated with the anterior
end of the notochord. During gastrulation the pre-chordal plate invaginates
with the entoderm and comes to occupy the roof of the foregut, just anterior
to the rod-like notochord (fig. 191D-F). In this position it lies below the
anterior part of the neural plate area; it functions strongly in the induction
and formation of the cephalic structures, including the brain as indicated above.
Because of this inductive ability, it is regarded as a principal part of the head
GASTRULATION IN VARIOUS CHORDATA 415
organizer (fig. 191E-G). Eventually pre-chordal plate cells contribute to the
pharyngeal area of the foregut and give origin to a portion of the head meso-
derm, at least in many vertebrate species (Chap. 11, p. 523).
c. Closure of the Blastopore and Formation of the Neurenteric Canal
The closure of the blastopore and formation of the neurenteric canal is
described in Chapter 10, p. 471.
d. Summary of Morphogenetic Movements of Cells During Gastrulation
in the Frog and Other Amphibia
1) Emboly:
(a) Invagination. Invagination in the Amphibia appears to consist of two
phases: ( 1 ) an active infolding or forward migration of the future
foregut, stomach, etc., areas, and (2) an insinking and inward rotation
of future intestinal and heavily laden, yolk cells.
(b) Convergence. This activity is found in the presumptive, notochordal
and mesodermal cells as they move toward the blastoporal lips. A
dorsal convergence toward the dorsal, blastoporal-lip area is particu-
larly true of the more laterally placed parts of the notochordal crescent
and to some extent also of the somitic and lateral plate mesoderm.
The tail mesoderm tends to converge toward the dorsal blastoporal
area when the blastopore nears closure.
(c) Involution. An inward rolling or rotation of cells over the blastoporal
lips to the inside is a conspicuous part of notochordal and mesodermal
cell migration.
(d) Divergence. After the mesodermal cells have migrated to the inside,
there is a particular tendency to diverge on the part of the lateral plate
and ventral mesoderm. The lateral plate mesoderm diverges laterally
and ventrally, while the ventral mesoderm diverges laterally in the
ventral or belly area of the gastrula.
(e) Extension. The phenomenon of extension or elongation is a charac-
teristic feature of all gastrulative processes in the chordate group. Be-
fore arriving at the blastoporal lips, the converging notochordal and
mesodermal cells may undergo a stretching or extending movement.
That is, convergence and stretching are two prominent movements in-
volved in the migration of the marginal zone or chordamesodermal
cells as they move toward the blastoporal lip. After these materials
have involuted to the inside, the chordal cells stretch antero-posteriorly
and become narrowed to a cuboidal band in the midline, and the
lateral plate mesoderm stretches anteriorly as it diverges laterally.
Antero-posterior extension of the somitic mesoderm also occurs.
(f ) Contractile tension or constriction. A considerable constriction or con-
traction around the edges of the blastopore occurs as gastrulation pro-
416
GASTRULATION
gresses. This particularly is true when the blastopore gradually grows
smaller toward the end of the gastrulative process (Lewis, '49).
2) Epiboly. Intimately associated with and aiding the above processes in-
volved in emboly are the movements concerned with epiboly. These move-
ments result from cell proliferation, associated with a marked antero-posterior
extension and expansion of the presumptive epidermal and neural plate areas.
These changes are integrated closely with the inward migration of cells of
the marginal zone (i.e., chordamesoderm), and the presumptive epidermal
and neural areas approach closer and closer to the blastoporal edge, until
finally, when mesodermal and notochordal cells have entirely involuted, ecto-
dermal cells occupy the rim of the blastopore as it closes (figs. 192A; 220D).
Fig. 197. History of the blastopore and adjacent posterior areas of developing embryo
of the frog, Rana pipiens. (A) Dorsal lip of blastopore. Arrows show direction of
initial invagination to form the dorsal lip. (B) Dorso-lateral and lateral-lip portions of
the blastopore are added to original dorsal-lip area by convergence of mesodermal
cells (arrows) and their involution at the edge of the lip. Entodermal material is invagi-
nating. (C) Blastopore is complete; yolk plug is showing. (D) Toward the end of
gastrulation. Blastopore is small; neural plate area becomes evident as neural folds begin
their elevation. (E) Neural folds are slightly elevated; blastopore is very small; size
of blastopore at this time is quite variable. (F) Blastopore has closed; neural folds are
well developed; neurenteric passageway between neural folds and dorsal evagination of
archenteric space into tail-bud area is indicated by N.C. (G) New caudal opening is
forming, aided by proctodaeal invagination, PR.; tail rudiment elevation is indicated.
(H) Proctodaeal opening and tail rudiment are shown.
GASTRULATION IN VARIOUS CHORDATA
417
E NTODERM
NOTOCHORD
MESODERM
MESODERM
Fig. 198. Anterior extension (migration) of the mesoderm from the blastoporal-lip
area after involution at the lip in the urodele, Plcurodeles. (A-C) Progressive inward
migration of the mantel of mesoderm, indicated by the white area stippled with coarse
dots. (A) Early gastrula. (B) Late gastrula. (C) Beginning neurula.
As a result, the presumptive epidermal and neural plate areas literally engulf
the inwardly moving cells.
3. Gastrulation in Reptiles
a. Orientation
The reptilian blastoderm, as gastrulation begins, is composed of an upper
epiblast and a lower hypoblast as indicated previously in Chapter 7 (fig.
174A-D). The formation of the hypoblast as a distinct layer proceeds in a
rapid fashion and immediately precedes the formation of a large notochordal
canal and subsequent cell migration inward. The two events of entodermal
layer (hypoblast) formation and the inward migration of notochordal and
mesodermal cells thus are closely and intimately correlated in reptiles. This
close relationship is true particularly of the turtle group. The upper layer or
epiblast of the reptilian blastoderm is a composite aggregation of presumptive
epidermal, neural, notochordal, and mesodermal cells (fig. 174E, F), arranged
in relation to the future, antero-posterior axis of the embryo. It is possible
that some entodermal material may be located superficially in the epiblast in
the turtle as gastrulation begins (Pasteels, '37a).
b. Gastrulation
Immediately following the formation of the hypoblast, the gastrulative phe-
nomena begin with a rather large inpushing or invagination involving the
notochordal, mesdoermal areas, particularly the pre-chordal plate and noto-
chordal areas. This invagination extends downward and forward toward the
hypoblast along the antero-posterior embryonic axis, and it produces a pouch-
like structure known variously as the notochordal canal, blastoporal canal, or
chordamesodermal canal (figs. 199A-C; 200A-C). The invaginated noto-
418
GASTRULATION
AREA PELLUCIDA
EMBRYONIC SHIELD
NOTOCHORDAL CANAL
OPENING
^PRIMITIVE STREAK
AREA OPAC A
BEGINNING OF
— HEAD FOLD
NOTOCHORD
^.
INTERNAL OPENING
0 F
NOTOCHORDAL CANAL
EXTERNAL
OF
OPENING
NOTOCHORDAL CANAL^
PRIMITIVE STREAK
c.
Fig. 199. Surface views of blastoderm of the turtle, Chrysemys picta, during gastrula-
tion. Darkened area in the center shows the embryonic shield, the region of the noto-
chordal canal in the area of the primitive plate. (A) Young gastrula. External opening
of notochordal canal is wide. (B) Later gastrula. External opening of notochordal
canal is horseshoe-shaped; internal opening of canal is indicated by small crescentic light
area in front of externa! opening. (C) Very late gastrula. Notochord is indicated in
center; head fold is beginning at anterior extremity of blastoderm.
chordal canal reposes upon the entoderm, and both fuse in the region of con-
tact (fig. 200C). The thin layer of cells in the area of fusion soon disappears,
leaving the antero-ventral end of the flattened notochordal canal exposed
to the archenteric space below. After some reorganization, the notochord
app-'^ars as a band, extending antero-posteriorly in the median line, associated
with the entoderm on either side (fig. 201B-G). However, at the extreme
anterior end of the gastrula, the notochordal material, together with the ento-
derm and to some extent the overlying ectoderm, presents a fused condition.
Within this area the pre-chordal plate or anterior portion of the head organizer
is located. In this general region of the embryo, foregut, brain, and other head
structures eventually arise (fig. 199C). The original, relatively large, noto-
chordal invagination soon becomes a small canal which extends cranio-ventrally
GASTRULATION IN VARIOUS CHORDATA
419
fronj the upper or external opening to the archenteric space which lies below
the notochord and entoderm (fig. 200B, E).
Posterior to the opening of the notochordal canal is the thickened primitive
plate (primitive streak), composed of converged presumptive mesodermal
cells (fig. 199). This converged mass of cells involutes to the inside along the
lateral borders of the notochordal canal and also posterior to this opening.
However, most of the mesoderm of the future body of the embryo apparently
passes inward with the notochordal material during the formation of the noto-
chordal canal, where it comes to lie on either side of the median notochordal
band between the ectoderm and the entoderm. These general relationships of
notochord, ectoderm, mesoderm, and entoderm are shown in figure 201A-H.
The extent to which the original notochordal inpushing is developed varies
in different reptilian species. In lizards and snakes its development is more
pronounced than in turtles (cf. fig. 200A, D).
During emboly, the presumptive neural plate and epidermal areas are
N OTO C HORDAL CANAL
^ PRIM
INTERNAL OPENING OF
NOTOCHORDAL CANAL
MESODERM
BEGINNING
NOTOCHORDAL
/ CANAL
: NT00E^^^<i'^=^^^^"'^^^.'^C.
4i
LATER NOTOCHORDAL CANAL
Fig. 200. Sagittal section of reptilian blastoderms to show notochordal inpushing
(notochordal canal or pouch). (A) Section of early gastrulative procedure in Clemmys
leprosa. (After Pasteels, '36b, slightly modified.) (B) Original from slide, Chrysemys
picta. showing condition after notochordal canal has broken through into archenteric
space. (C) Notochordal canal of the lizard, Platyductylus. (D) Later stage of (C).
(E) After notochordal canal has broken through into archenteric space. (C-E, after
Will. 1892.)
420
GASTRULATION
INTERNAL OPENING OF
NOTOCHORDIL CANAL
EXTERNAL OPENING OF
NOT OCHORDAL CANAL
Fig. 201. Transverse sections of the late turtle gastrula as indicated by lines in fig. 199C.
elongated antero-posteriorly by the forces of epiboly. Meanwhile, the external
opening of the notochordal canal changes in shape and together with the
primitive plate moves caudally (fig. 199). As gastrulation draws to a close,
the neural plate area begins to fold inward, initiating the formation of the
neural tube.
4. Gastrulation in the Chick
a. Orientation
As described in Chapter 7, a two-layered blastoderm (blastula) composed
of an epiblast and a hypoblast is present, with the hypoblast more complete
at the posterior end of the blastoderm than at its extreme anterior and antero-
lateral margins (figs. 171 A; 202A). The epiblast over the posterior half of
the blastoderm is composed of presumptive notochordal and mesodermal cells,
and anteriorly in the epiblast are found the presumptive epidermal and neural
areas (figs. 173A; 202A).
b. Gastrulative Changes
1) Development of Primitive Streak as Viewed from the Surface of Stained
Blastoderms. The formation of the primitive streak is a progressive affair.
Figure 170 pictures a pre-streak blastoderm, and it is to be observed that the
ectodermal layer below the epiblast is present as an irregular area in the
GASTRULATION IN VARIOUS CHORDATA 421
caudal region of the area pellucida. A median, sagittal section through a
comparable stage is shown diagrammatically in figure 171 A. Figure 203 A
illustrates an early beginning streak normally found eight hours after incuba-
tion of the egg is initiated, while figure 203 B presents a medium streak, ap-
pearing after about 12 to 13 hours of incubation. In figure 203C, a definite
primitive streak appears in which the primitive groove, primitive pit, primitive
folds, and Hensen's node (primitive knot) are outlined. This condition occurs
after about 18 to 19 hours of incubation. This may be regarded as the mature
streak. A later streak after about 19 to 22 hours of incubation is indicated
in figure 203D. Observe that the head process or rudimentary notochord
extends anteriorly from Hensen's node, while the mesoderm is a deeper-shaded
area emanating from the antero-lateral aspect of the streak. The clear proam-
nion region may be observed at the anterior end of the area pellucida. In the
proamnion area, mesoderm is not present at this time between the ectodermal
and entodermal layers.
2) Cell Movements in the Epiblast Involved in Primitive-streak Formation
as Indicated by Carbon-particle Marking and Vital-staining Experiments.
Recent experiments by Spratt ('46), using carbon particles as a marking de-
vice, have demonstrated that epiblast cells from the posterior half of the pre-
streak blastoderm gradually move posteriad and mediad as gastrulation pro-
ceeds (figs. 202, 204, black arrows). Before the actual appearance of the
streak, mesodermal cells begin to appear between the epiblast and hypoblast
at the posterior margin of the area pellucida. (See fig. 202B, involuted meso-
derm). As cellular convergence posteriorly toward the median line continues,
the primitive streak begins to form as a median thickening posteriorly in the
peUucid area (fig. 202C, observe posterior median area indicated in white).
The rudimentary primitive streak formed in this manner gradually advances
anteriorly toward the central region of the pellucid area of the blastoderm
(fig. 202D, E). In the thickened area of the developing primitive streak, shown
in white at the posterior median portion of the blastoderm in figure 202C,
there are about three to four cell layers of epiblast together with about the
same number of layers of mesoderm below. At its anterior end the streak is
thinner.
The anterior end of this early streak gradually grows forward as a result
of cell proliferation in situ and by cells added through convergence of cells
from antero-lateral areas (Spratt, '46). Some of the cells at the anterior end
of the forming streak may involute or ingress from the epiblast into the space
between the hypoblast and epiblast and thus come to lie at the anterior end
of the forming streak, while other cells ingress laterally between these two
layers (fig. 202C-E, K-O).
As the streak differentiates anteriorly by addition of cells to its anterior
end, it also elongates posteriorly by cellular additions to its caudal end. The
carbon-marking experiments of Spratt demonstrated further that, during the
I EXTR« - EMBR ION I C MESODERM
Fig. 202. {See facing page for legend.)
All
GASTRULATION IN VARIOUS CHORDATA
423
formation of the streak up to about the condition present at 20 to 22 hours
of incubation (figs. 2021, K; 203D), almost the entire posterior half of the
pellucid area, consisting of presumptive pre-chordal plate, notochord, and
mesoderm, is brought into the streak and involuted to the inside between the
hypoblast and epiblast (figs. 202F-H; 204). This condition of development
is often referred to as the "head-process stage" (stage 5, Hamburger and Ham-
ilton, '51 ). At this stage the approximate, antero-posterior limits of the future
embryonic body of the chick, exclusive of the extra-embryonic tissue, are
shown by the general area beginning just anterior to the head process and ex-
tending for a short distance posterior to Hensen's node (figs. 203D; 205D, E).
As indicated in figure 202, there are two parts to the primitive streak:
( 1 ) the area of Hensen's node and primitive pit concerned with invagi-
native movements of pre-chordal plate mesoderm and notochordal
cells and
(2) the body of the streak.
The former area appears to arise independently in the center of the pellucid
area, while the body of the streak is formed at the median, caudal margin of
the pellucid area, from whence it grows anteriad to unite with Hensen's node.
NOTOCHORD \
PRIMITIVE PIT \
Fig. 202. Migration of cells during gastrulation in the chick. Drawing to the left of
the midline represents a surface view; to the right of the midline the epiblast layer has
been removed. (A-F) To the left of the midline based on data provided by Spratt, '46.
(J) Represents lateral, sectional view of (F)-(G), viewed from the left side. Arrows
indicate direction of cell migration. (K)* Indicates a left lateral view of (I), with the
epiblast cut away midsagitally throughout most of the left side of the blastoderm. (L-O)
Transverse sections of (K). as indicated on (K).
424 GASTRULATION
The body of the streak serves as the "door" through which migrating meso-
dermal cells other than the cells of the pre-chordal plate-notochordal area
pass from the epiblast layer downward to the space between the epiblast and
hypoblast.
Using the Vogt method of vital staining, Pasteels ('37b) was able to demon-
strate morphogenetic movements of cells into the primitive streak area and
thence to the inside similar to that described by Spratt (fig. 202G-I).
The evidence derived from the carbon-particle-marking technic and that of
vital staining, therefore, strongly suggests that the primitive streak of the chick
forms as a result of:
(a) converging movements of the epiblast cells toward the median Une of
the posterior half of the pellucid area and
(b) cell proliferation in situ within the streak.
3) Cell Movements in the Hypoblast and the Importance of Those Move-
ments in Primitive-streak Formation. The hypoblast or entodermal layer of
the blastula appears to play a significant role relative to the formation of the
primitive streak in the bird. Various lines of evidence point to this conclusion.
For example, Waddington ('33) reported the results of experiments in which
he separated the epiblast from the hypoblast of early chick and duck embryos
in the early, primitive-streak stage. He then replaced the two layers so that
their longitudinal axes were diametrically reversed, that is, the anterior part
of the entoderm (hypoblast) lay under the posterior part of the epiblast, while
the posterior part of the entoderm lay below the anterior region of the epiblast.
The following results were obtained:
(1) The development of the original streak was suppressed; or
(2) a new, secondary, primitive streak was induced.
During later development, in some cases, the secondary streak disappeared;
in others, it persisted and a double monster was produced. In other instances
the primary primitive streak disappeared and the secondary streak persisted.
The general conclusion set forth by Waddington is as follows: the entoderm
does not induce the differentiation of a definite tissue, but rather, it induces
the form-building movements which lead to the development of the primitive
streak.
Certain experiments made by Spratt ('46) lend added evidence of the im-
portance of the hypoblast in primitive-streak formation. In eight experiments
in which the hypoblast was removed before streak formation, six cases failed
to produce a streak, whereas in two instances a beginning streak was formed.
It may be that in the latter two cases, the induction of morphogenetic move-
ments within the epiblast cells occurred previous to hypoblast removal. These
experiments are too few to permit a definite conclusion; however, they are
suggestive and serve to bolster the conclusion made by Waddington. In a
CASTRULATION IN VARIOUS CHORDATA
425
second set of experiments performed by Spratt, chick blastoderms in the pre-
streak and early-streak stages were inverted and marked with carbon particles.
The results showed that the hypoblast moves forward in the median line below
the epiblast layer. He also demonstrated that this forward movement of the
hypoblast "precedes the anterior differentiation of the primitive streak." Spratt
further observed that: When the movement of the hypoblast deviated to the
left or to the right, the primitive streak similarly deviated. This evidence
"strongly suggests that the hypoblast influences the development of the primi-
tive streak in the overlying epiblast" (Spratt, '46).
AREA 0 PAC A
PE L LUCIDA
EMBRYONIC
SHIELD
MIDPRIMITIVE STREA
^^;t^. - PROAMNION ARE
,,'''hEAD PROCESS
,-i-J* INOTOCHORD ) ..•^■-.y, . ,i^,'5iVv
'1^ FMHRYONIC AREA ~\LI:MS^k.i'
-PRIMITIVE
.^M— ■i-Ih/W^ EXTRA-EMBRYON
.^^., ,. ^1 . ,9-p R I M I T I V E G R
-'';''~-,'^.' ^^ ■ — PRIMITIVE FO
}'^l-^^.Miii^iai!^^^:i£i
MATURE PRIM
STREAK
E AD-PROC ESS
STAGE
Fig. 203. Surface-view drawings of photographs of developing primitive streak. (From
Hamburger and Hamilton, '51, after Spratt.) (A) Initial streak, short, conical thick-
ening at posterior end of blastoderm. (Hamburger and Hamilton, '51, stage 2.) (B)
Intermediate streak. Thickened streak area approaches center of area pellucida. (Ham-
burger and Hamilton, '51, stage 3.) (C) Definitive streak (average length, 1.88 mm.).
Primitive groove, primitive fold, primitive pit, and Hensen's node are present. (Ham-
burger and Hamilton, '51, stage 4.) (D) Head-process stage (19 to 22 hours of incu-
bation). Notochord or head process visible as area of condensed mesoderm extending
anteriorly from Hensen's node. Proamnion area is indicated in front portion of area
pellucida; head fold is not yet present. (Hamburger and Hamilton, '51, stage 5.)
426
GASTRULATION
Fig. 204. Movements in the epiblast layer of the chick during gastrulation and
primitive-streak formation. (Modified sHghtly from Spratt, '46.) (A) Pre-streak con-
dition. Carbon particles are placed as indicated, at a, b, c. d, e, f, and g. (B G) Observe
migration of carbon particles. (C) Short streak. (E) Medium broad streak. (G) Long
streak. (See fig. 203C.)
4) Primitive Pit and Notochordal Canal. If one compares the notochordal
canal, formed during gastrulation in the reptilian blastoderm, with that of
the primitive pit in the chick, the conclusion is inevitable that the primitive
pit of the chick blastoderm represents an abortive notochordal canal. The
lizard, turtle, and chick thus represent three degrees of notochordal canal
development (figs. 200A, D; 202J). In certain birds, such as the duck, a
notochordal canal very similar to that of the turtle gastrula, is formed.
5) Resume of Morphogenetic Movements of Cells During Gastrulation in
the Chick. In view of the foregoing facts relative to primitive-streak forma-
tion, steps in the gastrulative procedure in birds may be described as follows:
(a) Shortly after the incubation period is initiated, hypoblast material at
the caudal end of the blastula starts to move in the median line toward
the future cephalic end of the embryo. This activity may be regarded
as a gastrulative streaming of the hypoblast. (This streaming move-
ment probably represents the chick's counterpart of the forward move-
ment of the entodermal area in the dorsal-lip region of the frog
embryo. )
(b) After this movement of the hypoblast is inaugurated, cells from the
epiblast layer immediately overlying the moving hypoblast pass down-
ward toward the hypoblast. That is, epiblast cells begin to involute
and come to lie between the epiblast and hypoblast; from this new
GASTRULATION IN VARIOUS CHORDATA 427
position the involuted cells migrate laterally and anteriorly between
the hypoblast and epiblast.
(c) In conjunction with the foregoing activities, epiblast cells (presumptive
mesoderm) from the posterior half of the epiblast of the pellucid area
migrate posteriad, converging from either side toward the median line
(fig. 204A-G).
(d) These converging cells begin to pile up in the posterior median edge
of the pellucid area (fig. 204C), where they produce a raphe-like
thickening which marks the beginning of the primitive streak (fig.
204C-G). The beginning streak first makes its appearance at about
seven to eight hours after the start of incubation in the egg of the
chick (fig. 204C).
(e) Once formed, the initial streak grows anteriad in the median line by:
(1) cell proliferation in situ, and by the addition of (2) converging
cells from the epiblast layer.
(f ) Also, the primitive streak apparently grows posteriad by cell prolifera-
tion and the addition of converging cells.
(g) When the migrating cells of the epiblast reach the primitive streak,
they involute and pass downward to the space between the epiblast
and hypoblast. From this new position they move laterad and anteriad
on either side of the midline, diverging to form a broad, middle layer
of mesodermal cells.
(h) As the primitive streak grows anteriad in the epiblast, it eventually
approaches the presumptive pre-chordal plate and presumptive noto-
chordal areas.
(i) The pre-chordal plate and notochordal cells then invaginate to form
the primitive pit; the latter represents a shallow or vestigial notochordal
canal, a structure strongly developed in reptiles and some birds, and
occasionally in mammals.
(j) Notochordal cells from the notochordal crescent converge to the pit
area and probably pass downward in the walls of the pit, whence
they ingress and move forward in the median line (fig. 202A-G, J, K).
The definitive primitive streak is formed after about 18 to 19 hours
of incubation. At about 20 to 22 hours of incubation, the prospective,
notochordal material (e.g., the head process) has already invaginated.
At this time it represents a mass of cells in the median line intimately
associated with the neural plate ectoderm above the pre-chordal plate
cells and the entoderm below (fig. 2021, K). As the primitive streak
recedes posteriad (see p. 431 ), the notochordal material gradually sep-
arates from the surrounding, pre-chordal plate cells and also from the
neural plate material. Eventually the notochordal cells become a dis-
tinct median mass which elongates rapidly (i.e., undergoes extension)
as the nodal area and the primitive streak recede caudally (Spratt, '47).
Fig. 205. (See facing page for legend.)
428
GASTRULATION IN VARIOUS CHORDATA
429
(k) Somitic mesoderm (i.e., the mesoderm of the future somites) appar-
ently passes inward between the epiblast and hypoblast from the antero-
lateral portions of the primitive streak. It migrates forward and be-
comes extended along either side of the notochordal cells during the
period of primitive-streak recession. The nephric and lateral plate
mesoderm involutes along the middle portions of the streak, and this
mesoderm becomes extended antero-posteriorly. The hypomeric or
lateral plate mesoderm also diverges laterally. The extra-embryonic
mesoderm moves inward along the postero-lateral portions of the
streak; it migrates laterally and anteriorly (fig. 2021, extra-embryonic
mesoderm).
-LIVE R
- HEART
- CHORDA
- THYROID
- NEPHROS
- INTESTINE
-ERYTHROCYTES
- MELANOPHORES
-SKELETAL MUSCLE
-SOMITE
— ENTODERM
-SINUS RHOMBOIDAL
■«■ NEURAL CREST
LP-LATERAL PLATE
M-HEAD MESODERM
Fig. 205. Three-germ-layered blastoderm or late gastrula of chick, showing the mosaic
distribution of developmental tendencies. (AC after Rawles, '36; D and E after Rudnick,
'44, from various sources.) (A-C) The lines transversely placed across embryo are at
levels of 0.3 mm. and 0.7 mm. from the center one, considered as 0.0 mm. (A) Ecto-
dermal or external layer: neural plate area is indicated in black, epidermal area in white.
(B) Mesodermal or middle germ layer. (C) Entodermal or inner germ layer. (D)
Ectodermal layer shown on left, mesodermal and entodermal on right. (E) Superficial
or ectodermal layer shown at left, deeper layer, at right. (Note: These diagrams should
be considered only in a suggestive way; final knowledge relative to exact limits of poten-
cies, especially in the mesodermal layer, should be more thoroughly explored.)
TYPE OF MARKING GENERALIZED RESULTS
NUMBER NUMBER
MARKED
DE V E L.
35
m
rz"
31
21
TOTAL
I 95
Fig. 206. Recession of the primitive streak of the chick and growth of the embryo in
front of Hensen's node. Marked cell groups represented by heavy dots; dashes opposite
these are reference marks placed on the plasma clot to permit orientation. This diagram
based upon 6 different types of carbon-marking experiments with the generalized results
6-15 hours following explantation. In type I, the stippled area is invaginated as indicated.
Observe especially type VI, the history of the three areas marked by the three heavy
dots placed on the blastoderm at the head-process stage. It is to be observed that the
embryo as a whole arises from the area in front of Hensen's node. (After Spratt, '47.)
430
GASTRULATION IN VARIOUS CHORDATA 431
(1) While the above activities take place, the area pellucida becomes
elongated posteriorly. The entire pellucid area thus becomes piriform,
i.e., pear-shaped (figs. 202F-I; 203C).
(m) This change in shape of the pellucid area is associated primarily with
the activities involved in epiboly which accompany the embolic activi-
ties observed above. Epiboly brings about the elongation of the pre-
sumptive neural crescent, converting it into an elongated band of cells.
It also effects the expansion and antero-posterior extension of the
overlying presumptive, neural plate and epidermal cells. The latter
behavior is intimately associated with the antero-posterior extension
of the notochordal and mesodermal cellular areas mentioned in (j)
and (k) on pp. 427, 428.
(n) Most of the gastrulative processes in the chick are completed at about
20 to 22 hours after incubation starts. At this time the blastoderm is
in the head-process stage. The so-called head process or "notochordal
process" represents the rudimentary notochord which projects forward
from the primitive streak. (See (j) on p. 427.) At this time the various,
specific, organ-forming areas appear to be well established (figs. 2021;
205A-E). (See Rawles, '36; Rudnick, '44.) From this time on the
primitive streak regresses caudally, as the embryo and embryonic tis-
sues develop in front of it. The caudal regression of the streak is shown
in figure 206. Spratt ('47) concludes that as the streak regresses, it
becomes shortened by transformation of its caudal end into both em-
bryonic and extra-embryonic ectoderm and mesoderm. Finally, the
anterior end of the streak, that is, the primitive knot or Hensen's node
together with possibly some condensation of adjacent streak tissue
(Rudnick, '44), forms the end bud. The latter, according to Homdahl
('26) gives origin to the posterior portion of the embryo caudal to
somite 27 and to the tail. The remains of the end bud come to a final
resting place at the end of the tail.
5. Gastrulation in Mammals
a. Orientation
In the mammals, the formative area of the blastocyst (blastula) is located
at one pole and is known as the embryonic or germ disc. It consists of a lower
hypoblast and an upper epiblast. This embryonic disc is connected to the
non-formative or trophoblast cells around its edges (figs. 176, 177, 178).
In some species the embryonic disc is superficial and uncovered by trophoblast
cells (pig, cat, rabbit, opossum), while in others, it is sequestered beneath a
covering of trophoblast (human, monkey, rat). (See figs. 177, 178.)
PRE-CHORDAL
.PLATE\ NEURAL PLATE ECTODERM
iL ECTODERM
YOLK SAC
P R I M I T I V E
NODE
■PRIMITIVE
P IT
L A S T 0 PORE)
PRIM I T IV E
STREAK
CHORION
CHORIONIC VILLI
PRIMITIVE "STREAK
ALL A NT OE NTER I C DIVERTICULUM*
BLOOD VESSEL
Fig. 207. Three-germ-layer late gastrula of human embryo (pre-somite stage, after
Heuser: Contrib. to Embryo). Carnegie Inst., Washington, Publ. 138, 23: 1932). (A)
Dorsal view of embryonic disc, amnion removed. (B) Sagittal section through median
area of (A). Observe elongated notochordal canal. (C-G) Transverse sections as
mdicated.
432
GASTRULATION IN VARIOUS CHORDATA 433
b. Gastriilation in the Pig Embryo
In the pig embryo, two centers of activity are concerned with the forma-
tion of the primitive streak, namely, a caudal area of mesodermal proliferation
which forms the body of the primitive streak and an anterior primitive knot
or Hensen's node. The similarity of behavior of these two portions of the
primitive streak in the chick and pig suggests strongly that their formation
by a convergence of superficial epiblast cells occurs in the pig as it does in
the chick. Hensen's node, originally described by Hensen (1876) in the rabbit
and guinea pig, is a thickened area of the epiblast in the midline near the
middle of the embryonic disc. As in the chick, the body of the primitive streak
takes its origin at the caudal end of the embryonic disc, where the first appear-
ance of the streak is indicated by a thickening of the epiblast (fig. 209A, B).
From this thickened region, cells are budded off between the epiblast and
hypoblast, where they migrate distad as indicated by the lightly stippled areas
in figure 208. The streak ultimately elongates, continuing to give origin to cells
between the hypoblast and epiblast. Eventually, the anterior neck region of
the body of the streak merges with Hensen's node (fig. 208E, F). From the
anterior aspects of the primitive (Hensen's) node, cells are proliferated off
between the epiblast and hypoblast, and a depression or pit, the primitive pit,
appears just caudal to the node.
The proliferation of cells from the nodal area deposits a median band of
cells which merges anteriorly with the hypoblast below. More caudally, the
hypoblast becomes attached to either side of the median band of cells (fig.
209C). The median band of nodal cells thus forms part of two regions, viz.,
an anterior, pre-chordal plate region, where the nodal cells are merged with
hypoblast (entoderm), and an elongated notochordal band or rod of cells
extending backward between the hypoblast cells (fig. 209C) to Hensen's node,
where it unites with the hypoblast posteriorly (fig. 209D). Unlike the condi-
tion in the chick, the notochordal rod, other than in the pre-chordal plate
area, is exposed to the archenteric space below (fig. 209C). It simulates
strongly that of the reptilian blastoderm as gastrulation draws to a close.
In the meantime, mesodermal cells from the primitive streak migrate for-
ward between the hypoblast and epiblast along either side of the notochord
in the form of two wing-like areas (figs. 208H, I; 209C). Other meso-
dermal cells migrate posteriad and laterad. Consequently, one is able to dis-
tinguish two main groups of mesodermal cells:
( 1 ) formative or embryonic mesoderm, which remains within the confines
of the embryonic or germinal disc and
(2) distally placed non-formative or extra-embryonic mesoderm.
The former will give origin to the mesoderm, of the embryonic body, while
from the latter arises the mesoderm of the extra-embryonic tissues.
In conclusion, therefore, we may assume that, during gastrulation in the
GERM DISC \ /, '■'-'•'•'.•l o
PRIMITIVE STREAK ' iNj'-l B.
AREA onikjiTi\/i
NOTOCHORO
ENSEN'S NODE
PRE<HORDAL PLATE
NOT OC HORD
HE N sen's nod
PRIMITIVE MESO
NECK
PRIMITIVE STR
MAR ei N OF E MB
DISC
Fig. 208. Development of primitive streak, notocliord, and mesodermal migration in
the pig. (After Streeter, '27.) (A) Primitive streak represented as thickened area at
caudal end of embryonic (germ) disc. Migrating mesoderm shown in heavy stipple.
(B-E) Later stages of streak development. Observe mass migration of mesoderm. The
mesoderm outside the germ disc is extra-embryonic mesoderm. (F) Forward growing,
primitive streak makes contact with Hensen's node. (G-1) Observe elongation of
notochord accompanied by recession of primitive streak shown in (I). Observe in (I)
that an embryo with three pairs of somites has formed anterior to Hensen's node. Com-
pare with Spratt's observation on developing chick, fig. 206, type VI.
434
GASTRULATION IN VARIOUS CHORDATA
435
CEPHALIC MARGIN
^MBRYONIC DISC
AUDAL MARGIN
■'■■^ PRIMITIVE GUT '*'*'..'<*.>_, ~^
T R 0 PHECTO DE RM
, _, %
^ «,^«?«Si3^ *^ EN TODER M PROLIFERATING
><*' -* *^^^ ME SODERM
ROPH ECTODERM
MIGRATING
MESODERM
CELLS
RO PHENTODE R M^jL^. iS
— — ^^rr"T^»:;rr:^Svv=^^^~;:rr-7-->jr--— — —^-^ mitive streak
/ y£^°if^'''^f^"^^\ notochord . '"'^~»*»-^^lfj*^''^^4'r>"2:«i-i,
Vv*^*^--'"*^ t?^ \ NOTOCHORD HENSEN S NODE ^~^— *•'■>.:. ^i'^^A^'-V
I .' J^llff ^ AMNIOTIC •-'^^v^^, n
-^' /* %fi pre-chordal mesoblast ■^•■'^■<c«
/4' plate vitelline '-'
f ".'• MES08LA ST
Fig. 209. Longitudinal and transverse sections of the early embryonic (germ) disc of
the pig. (C and D after Streeter, '27.) (A) Early, pre-streak, germ disc, showing caudal
thickening of epiblast layer. (B) Early streak germ disc, showing thickened caudal edge
of disc and beginning migration of mesodermal cells (see fig. 208A). (C) Transverse
section through late gastrula, showing three germ layers. Observe that entoderm is
attached to either side of median notochordal rod. (D) Longitudinal section through
pre-somite, pig blastoderm, showing the relation of notochord to Hensen's node, entoderm,
and pre-chordal plate.
pig embryo, emboly and epiboly are comparable and quite similar to these
activities in the chick.
c. Gastrulation in Other Mammals
Though the origin of notochordal and pre-chordal plate cells in the pig simu-
lates the origin of these cells in the chick, their origin in certain mammals,
such as the mole (Heape, 1883) and the human (fig. 207), resembles the
condition found in reptiles, particularly in the lizards, where an enlarged
notochordal pouch or canal is elaborated by an invaginative process. Conse-
quently, in reptiles, birds, and mammals, two main types of presumptive pre-
chordal plate-notochordal relationships occur as follows:
( 1 ) In one group an enlarged notochordal canal or pouch is formed which
pushes anteriad in the midline between the hypoblast and epiblast;
and
(2) in others an abortive notochordal canal or primitive pit is developed,
and the notochordal cells are invaginated and proliferated from the
436 GASTRULATION
thickened anterior aspect of the pit, that is, from the primitive knot
or primitive node (Hensen's node).
Another peculiarity of the gastrulative procedure is found in the human
embryo. In the latter, precocious mesoderm is elaborated during blastulation
presumably from the trophoblast. Later this mesoderm becomes aggregated
on the inner aspect of the trophoblast layer, where it forms the internal layer
of the trophoblast. This precocious mesoderm gives origin to much of the
extra-embryonic mesoderm. However, in the majority of mammals, embryonic
and extra-embryonic mesoderm arise from the primitive streak as in the chick.
6. Gastrulation in Teleost and Elasmobranch Fishes
a. Orientation
Gastrulation in teleost and elasmobranch fishes shows certain similarities,
particularly in the fact that in both groups the migrating cells use principally
the dorsal-lip area of the blastopore as the gateway from the superficial layer
to the deeper region inside and below the superficial layer. The lateral and
ventral lips are used to some degree in teleosts, but the main point toward
which the migrating cells move is the region of the dorsal lip of the blastopore.
As previously described (Chap. 7), the late blastular condition or blasto-
disc of elasmobranch and teleost fishes consists of an upper layer of formative
tissue, or blastodisc (embryonic disc) and a lower layer of trophoblast or
periblast tissue. The latter is associated closely with the yolk (figs. 179A;
180A; 181 A; 210A). In teleost fishes much of the presumptive entodermal,
organ-forming area (the so-called primary hypoblast) is represented by cells
which lie in the lower region of the caudal portion of the blastodisc (figs. 180A;
181A; 210C). The exact orientation of the hypoblast appears to vary with
the species. In Fundulus, a considerable amount of the presumptive entoderm
appears on the surface at the caudal margin of the blastodisc (fig. 180A, B).
(See Oppenheimer, '36.) However, in the trout, Salmo, presumptive entoderm
lies in the lower areas of the thickened caudal portion of the disc, and the pre-
chordal plate of presumptive entomesoderm alone is exposed (fig. 181 A, B).
(See Pasteels, '36.) The position of the presumptive entoderm in the shark,
Scyllium (Vandebroek, '36), resembles that of Fundulus (fig. 170A), al-
though some entoderm may arise by a process of delamination from the lower
area of the blastodisc (fig. I79A).
b. Gastrulation in Teleost Fishes
1) Emboly. As the time of gastrulation approaches, the entire outer edge
of the blastodisc begins to thicken and, thereby, forms a ring-like area around
the edge of the disc, known as the germ ring (figs. 2 IOC; 211B). At the
caudal edge of the blastoderm, the germ-ring thickening is not only more pro-
nounced, but it also "extends inward for some distance toward the center of
GASTRULATION IN VARIOUS CHORDATA
437
the blastoderm (fig. 211 A, B). This posterior prominence of the germ ring
forms the embryonic shield.
As gastrulation begins, the entodermal cells of the primary hypoblast at the
caudal edge of the embryonic shield stream forward below the epiblast toward
the anterior end of the blastodisc (figs. 210A, D). Coincident with this for-
ward movement of the primary hypoblast, a small, crescent-shaped opening
EMBRYONIC SHIELD
NEURAL PLATE MATERIAL
^, r-Ti'Ejej-'-E;::^.^
GERM RING
■ E M B RVON I C
Fig. 210. Gastrulation in teleost fishes. (A) Sagittal section of early gastrula. (Modi-
fied slightly from Wilson, 1889.) (B) Midsagittal section through late teleost gastrula.
The dorsal and ventral lips of the blastopore are shown approaching each other. (Modi-
fied slightly from Wilson, 1889.) (C) Beginning gastrula of early blastoderm of brook
trout, Salvelinus. Observe inward (forward) migration of primary hypoblast cells and
thickened mass of cells which arises at posterior margin. (After Sumner, '03.) (D)
Later stage in gastrulation of brook trout. (After Sumner, '03.) (E) Transverse section
of late gastrula of brook trout, showing the three germ layers. (After Sumner, '03.)
(F) Transverse section through late gastrula of sea bass. (After Wilson, 1889.) (G)
Midsagittal section through closing blastopore of sea bass. (After Wilson, 1889.) (H)
Longitudinal section through late gastrula of the brook trout. (After Sumner, '03.)
438 GASTRULATION
appears at the caudal edge of the embryonic shield; this opening forms the
dorsal lip of the blastopore (figs. 210A; 211 A, B).
In teleost fishes with a primary hypoblast arranged as in Fundidus (fig.
180A, B), as the entodermal cells of the hypoblast move anteriad from the
deeper portions of the blastodisc, the entodermal cells exposed at the caudal
edge of the epiblast move over the blastoporal lip (i.e., involute) and migrate
forward as a part of the entoderm already present in the deeper layer. (See
arrows, fig. 180B.) The primary hypoblast thus becomes converted into the
secondary hypoblast. In teleosts with a primary hypoblast or entodermal ar-
rangement similar to Salmo (fig. 181 A), the secondary hypoblast is formed
by the forward migration and expansion of the entodermal mass located in
the caudal area of the embryonic shield. In both Fundulus and Salmo follow-
ing the initial forward movement of the entodermal cells, the pre-chordal plate
cells together with the notochordal cells move caudally and involute over the
dorsal blastoporal lip, passing to the inside. (See arrows, figs. 180B; 18 IB.)
The pre-chordal plate and notochordal cells migrate forward along the midline
of the forming embryonic axis. The pre-chordal plate cells lie foremost, while
the notochordal cells are extended and distributed more posteriorly. The
presumptive mesoderm in the meantime converges toward the dorso-lateral
lips of the blastopore (figs. 180B; 18 IB, see arrows), where it involutes,
passing to the inside between the entoderm or secondary hypoblast and epiblast.
Within the forming gastrula, the mesoderm becomes arranged along the upper
aspect of the entoderm and on either side of the median, notochordal material
(fig. 210E, F). The mesoderm in this way becomes inserted between the
flattened entoderm (secondary hypoblast) and the outside ectodermal layer
(Oppenheimer, '36; Pasteels, 36; Sumner, '03; Wilson, 1889).
During the early phases of gastrulation, the involuted entodermal, noto-
chordal, and mesodermal tissues may superficially appear as a single, thick-
ened, cellular layer. As gastrulation progresses, however, these three cellular
areas separate or delaminate from each other. When this separation occurs,
the notochordal cells make their appearance as a distinct median rod of cells,
while the mesoderm is present as a sheet of tissue on either side of the noto-
chord. The entoderm may form two sheets or lamellae, one on either side
of the notochord and below the mesodermal cellular areas (fig. 21 OF) or it
may be present as a continuous sheet below the notochord and mesoderm
(fig. 210E, H). The entodermal lamellae, when present, soon grow mediad
below the notochord and fuse to form one complete entodermal layer (Wilson,
1889).
2) Epiboly. Emboly involves for the most part the movements of cells in
the caudal and caudo-lateral areas of the blastoderm, i.e., the embryonic
portion of the germ ring. However, while the involution of cells concerned
with the development of the dorsal, axial region of the embryo occurs, the mar-
gins of the blastodisc beyond the dorsal-lip area, that is, the extra-embryonic,
GASTRULATION IN VARIOUS CHORDATA 439
germ-ring tissue, together with the presumptive epidermal area, proceeds to
expand rapidly. This growth and expansion soon bring about an engulfment
of the yolk mass (figs. 210B; 211C-F). The blastoporal-hp area (i.e., edge of
germ ring) ultimately fuses at the caudal trunk region (figs. 210G; 21 IF).
As the blastoporal region becomes narrower, a small vesicular outpocketing,
known as Kupflfer's vesicle, makes its appearance at the ventro-caudal end of
the forming embryo at the terminal end of the solid, post-anal gut (fig. 210G).
This vesicle possibly represents a vestige of the enteric portion of the neuren-
teric canal found in Amphioxus, frog, etc. A certain amount of mesodermal
involution occurs around the edges of the germ ring, in some species more
than in others (fig. 210A, B, peripheral mesodermal involution).
As the cellular dispositions involved in extra-embryonic expansion of the
epidermal and germ-ring areas are established, the presumptive, neural plate
material (figs. 179, 180, 181) becomes greatly extended antero-posteriorly
in the dorsal midline (figs. 210A, H; 211E), where it forms into a thickened,
elongated ridge or keel. The latter gradually sinks downward toward the
underlying notochordal tissue (fig. 210E, F). Also, by the time that the yolk
mass is entirely enveloped, the somites appear within the mesoderm near the
notochordal axis, and the developing body as a whole may be considerably
dehmited from the surrounding blastodermic tissue (fig. 21 IG). Therefore,
if the envelopment of the yolk mass is taken as the end point of gastrulation
in teleosts, the stage at which gastrulation is completed does not correspond
to the developmental condition found at the termination of gastrulation in the
chick, frog, and other forms. That is, the embryo of the teleost fish at the
time of blastoporal closure is in an advanced stage of body formation and
corresponds more truly with a chick embryo of about 35 to 40 hours of incu-
bation, whereas the gastrulative processes are relatively complete in the chick
at about 20 to 22 hours of incubation.
3) Summary of the Gastrulative Processes in Teleost Fishes:
a) Emboly:
( 1 ) Formation of the secondary hypoblast. The secondary hypoblast forms
as a result of the forward migration, expansion, and proliferation of
the entodermal cells lying at the caudal margin of the embryonic shield.
This forward migration of the entoderm (primary hypoblast) occurs
below the upper layer or epiblast and thus produces an underlying
entodermal layer or secondary hypoblast.
(2) Pre-chordal plate and notochordal involution. As the formation of
the secondary hypoblast is initiated, the presumptive pre-chordal plate
and notochordal cells move posteriad and converge toward the dorsal
lip of the blastopore, where they involute and pass anteriad in the
median line between the hypoblast and epiblast. The hypoblast or
entodermal layer may be separated into two flattened layers or lamellae,
one on either side of the notochord in some species. However, there
440
GASTRULATION
GERM RING
EXTRA-EMBRYONIC
BLASTODERM
GERM RING
DORSAL LIP OF BLASTOPORE
CLOSING BLASTOPORAL RING
Fig. 211. Gastrulation in teleost fishes. (A-F after Wilson, 1889; G from Kerr, '19,
after Kopsch.) (A) Sea bass. 16 hours, embryonic shield becoming evident, marks
beginning of germ ring. (B) Germ ring well developed. Surface view of blastoderm
of 20 hours. (C) Side view of blastoderm shown in (B). (D) Side view, 25 hours.
(E) Surface view, 25 hours. (F) Side view, 31 hours. (G) Late gastrula of trout,
Sal mo fario.
(3)
is considerable variation among different species as to the degree of
separation of the entodermal layer; in the sea bass it appears to be
definitely separated, whereas in the trout it is reduced to a single layer
of entodermal cells lying below the notochord. The pre-chordal plate,
entoderm, and anterior notochord merge into a uniform mass below
the cranial end of the neural plate.
Mesodermal convergence and involution. Along with the migration of
notochordal cells, the presumptive mesoderm converges posteriad to
the dorso-lateral lips of the blastopore, where it involutes and moves
GASTRULATION IN VARIOUS CHORDATA 441
to the inside on either side of the median, notochordal mass and above
the forming, secondary hypoblast.
b) Epiboly. The germ-ring tissue and the outer areas of the presumptive
epidermal cells gradually grow around the yolk mass and converge toward the
caudal end of the developing embryo. Associated with this migration of cells
is the anterior-posterior extension of the presumptive neural plate material
to form an elongated, thickened, median ridge.
4) Developmental Potencies of the Germ Ring of Teleost Fishes. The
germ ring or thickened, marginal area of the teleost late blastula and early
gastrula has interested embryologists for many years. It was observed in
Chapter 8 that various regions of the marginal area of the blastoderm of the
teleost fish have a tendency to form embryos. Luther ('36), working on the
trout (Salmo), found that all sectors of the blastula were able to differentiate
all types of tissue, i.e., they proved to be totipotent. However, in the early
gastrula, only the sector forming the embryonic shield and the areas immedi-
ately adjacent to it were able to express totipotency. As gastrulation progresses,
this limitation becomes more marked. In other words, a generalized potency
around the germ ring, present during blastulation, becomes restricted when
the embryonic shield of the gastrula comes into prominence. The evidence set
forth in the previous chapter indicates that the possibility for twinning in the
trout becomes less and less as the gastrular condition nears. The restriction
of potency thus becomes a function of a developmental sequence.
In the case of Fundidus, Oppenheimer ('38) found that various areas of
the germ ring, taken from regions 90 degrees or 180 degrees away from the
dorsal blastoporal lip, were able to differentiate many different embryonic
structures // transplanted into the embryonic shield area. Oppenheimer con-
cludes that: "Since under certain conditions the germ-ring can express poten-
cies for the differentiation of many embryonic organs, it is concluded that its
normal role is limited to the formation of mesoderm by the inhibiting action of
the dorsal lip." The results obtained by Luther serve to support this conclusion.
c. Gastrulation in Elasmobranch Fishes
In figure 179B the presumptive major organ-forming areas of the blasto-
derm of the shark, Scyllium canicnla, are delineated. The arrows indicate the
general directions of cell migration during gastrulation. In figure 212A-G are
shown surface views of the dorsal-lip area of different stages of blastodermic
development in this species, while figure 213A-G presents median, sagittal
sections of these blastoderms during inward migration of the presumptive
organ-forming cells. It is to be observed that the dorsal-lip region of the
blastoderm is the focal area over which the cells involute and migrate to the
inside.
TAI L 0 UT G RO WTHS
Fig. 212. Surface views of developing blastoderms of Scy Ilium canicula.
442
POSTERIOR
ANTERIOR
PRE-CHORDAL PLATE
,N0TOCH0RD NEURAL ECTODERM
Fig. 213. Sagittal sections of blastoderms shown in figure 212A-G, with corresponding
letters, showing migration of presumptive organ-forming areas. (See also fig. 179.) (B)
Dorsal lip is shown to left. (H-M) Transverse sections of embryo of Squalus acanthias,
similar to stages shown in 212F and G, for Scy Ilium. (H) Section through anterior
head fold. (M) Section through caudal end of blastoderm. H-M original drawings
from prepared slides.
443
NOTOCHORD
ECTODERM
BLASTOCOEL
GASTROCOEL
Fig. 214. Gastrulation in the gymnophionan Amphibia and in the bony ganoid, Amia
calva. (A, B, C, after Brauer, 1897; D, E, after Dean, 1896.) Sections A-C through de-
veloping embryo of Hypogeophis alternans. (A) Middle gastrula, sagittal section. Ob-
serve that gastrocoel forms by a separation of the entodermal cells. Blastocoel forms
similarly through delamination of entoderm from the overlying epiblast and by spaces
which appear between the cells in situ. (B) Transverse section through late gastrula.
(C) Sagittal section through late gastrula. (D) Late gastrula of Ahiia. Mass of yolk
in center is uncleaved; cellular organization is progressing peripherally around yolk mass.
(E) Later gastrula of Amia. The blastopore is closing, but a large yolk mass still re-
mains uncleaved.
444
GASTRULATION IN VARIOUS CHORDATA 445
In figure 213A, B, and C, two general areas of entoderm are shown:
(a) that exposed at the surface (cf. fig. 179), and
(b) the entoderm lying in the deeper areas of the blastoderm (cf. fig. 179,
cells in black).
According to Vandebroek, '36, the deeper lying entoderm is extra-embryonic
entoderm (in fig. 213, this deeper entoderm is represented as a black area with
fine white stipple), whereas the entoderm exposed at the caudal portion of
the blastoderm in figure 179 A and B, and figure 213A is embryonic entoderm.
The later distribution of the major presumptive organ-forming areas of
the shark blastoderm is shown in figure 213E-M. In figure 213, observe the
periblast tissue connecting the blastoderm with the yolk substrate.
As the notochordal, entodermal, and mesodermal cells move inward during
emboly, the presumptive epidermal and neural areas become greatly expanded
externally by the forces of epiboly as shown in figures 213B-E, and 213H.
(Compare the positions of these two areas in fig. 179B.)
The general result of the gastrulative processes in the shark group is to
produce a blastoderm with three germ layers similar to that shown in figure
21 3L and M. The notochordal and pre-chordal plate cells occupy the median
area below the neural plate as shown in figure 213E and F; the mesoderm
and entoderm lie on either side of the median notochord as shown in figure
213M. A little later the entoderm from either side of the notochord grows
mediad to establish a complete floor of entoderm below the notochord as repre-
sented in figure 213L.
7. Intermediate Types of Gastrulative Behavior
In certain forms, such as the ganoid fish, Amia, and in the Gymnophiona
among the Amphibia, the gastrulative processes present distinct peculiarities.
In general, gastrulation in the bony ganoid fish, Amia calva, presents a con-
dition of gastrulation which is intermediate between that which occurs in the
teleost fishes and the gastrulative procedures in the frog or the newt. For
example, a blastodisc-like cap of cells is found at the end of cleavage in the
bony ganoid. This cap gradually creeps downward around the yolk masses
which were superficially furrowed during the early cleavages. This process re-
sembles the cellular movement occurring during epiboly in teleost fishes. In
addition, the entodermal, notochordal, and mesodermal materials migrate in-
ward in much the same way as occurs in the teleost fishes, although the forma-
tion of the primitive archenteron resembles to a degree the early invaginative
procedure in the frog. However, a distinctive process of entodermal formation
occurs in Amia, for some of the entodermal cells arise as a separation from
the upper portion of the yolk substance where yolk nuclei are found. (See
fig. 214D, E; consult Eycleshymer and Wilson, '06.)
The gastrulative processes in the gymnophionan Amphibia are most pe-
446 GASTRULATION
culiar, particularly the behavior of the entoderm. But little study has been
devoted to the group; as a result, our knowledge is most fragmentary. Elusive
and burrowing in their habits and restricted to a tropical climature, they do
not present readily available material for study. Brauer, 1897, described blastu-
lation and gastrulation in Hypogeophis alternans. Our information derives
mainly from this source.
In some respects gastrulation in Hypogeophis is similar to that in teleost
and bony ganoid fishes, while other features resemble certain cellular activities
in other Amphibia and possibly also in higher vertebrates. For example, the
blastoderm behaves much like the flat blastoderm of teleost fishes, for a dorsal
blastoporal lip or embryonic portion of the germ ring is formed toward which
the notochordal and mesodermal materials presumably migrate, involute, and
thus pass to the inside below the epiblast layer (fig. 214A, B). Also, the rapid
epiboly of the presumptive epidermal area around the yolk material (or yolk
cells) is similar to that of teleost fishes and of the bony ganoid, Amia (fig.
214C-E). However, the behavior of the entodermal cells differs markedly
from that of teleosts. In the first place, there is a double delamination whereby
the solid blastula is converted into a condition having a blastocoel and a gas-
trocoel (fig. 214A). These processes occur concurrently with the gastrula-
tive phenomena. Blastocoelic formation resembles somewhat the delaminative
behavior of the entoderm in reptiles, birds, and mammals, for the entodermal
layer separates from the deeper areas of the epiblast layer. The formation of
the gastrocoel (archenteron) is a complex affair and is effected by a process
of hollowing or space formation within the entodermal cell mass as indicated
in figure 214A. The arrangement of the entodermal cells during later gastrula-
tive stages resembles the archenteron in the late gastrula of other Amphibia.
The archenteron possesses a heavily yolked floor, with the roof of the foregut
region complete, but that of the archenteron more posteriorly is incomplete,
exposing the notochord to the archenteric space (fig. 214A-C).
G. The Late Gastrula as a Mosaic of Specific, Organ-forming
Territories
It was observed above that the presumptive organ-forming areas of the late
blastula become distributed in an organized way along the notochordal axis
during gastrulation. Further, while an interchangeability of different parts of
the epiblast of the late blastula is possible without upsetting normal develop-
ment, such exchanges are not possible in the late gastrula. For during gastru-
lation, particular areas of the epiblast become individuated by activities or
influences involved with induction or evocation. (The word "evocation" was
introduced by Waddington and it has come to mean: "That part of the mor-
phogenetic effect of an organizer which can be referred back to the action of
a single chemical substance, the evocator." See Needham, '42, p. 42.) As a
AUTONOMOUS THEORY OF GASTRULATIVE MOVEMENTS 4
result, the gastrula emerges from the gastrulative process as a general mos£
of self-differentiating entities or territories. (See Spemann, '38, p. 107.)
It necessarily follows, therefore, that the production of specific areas
territories of cells, each having a tendency to differentiate into a specific stru
tare, and the axiation of these areas along the primitive axis of the embryo a
two of the main functions of the gastrulative process. In figure 205A-E, di
grams are presented relative to the chick embryo showing the results of e
periments made by Rawles ('36), Rudnick ('44), and others. (See Rudnic
'44.) These experiments were made to test the developmental potencies
various limited areas of the chick blastoderm. A considerable overlapping
territories is shown, which stems, probably, from the fact that transplant
pieces often show potencies which are not manifested in the intact embry
Therefore, these maps should be regarded not with finality but merely
suggesting certain developmental tendencies.
H. Autonomous Theory of Gastrulative Movements
Our knowledge concerning the dynamics of gastrulation in the Chorda
is based largely on the classical observations of cell movement made 1
Conklin ('05) in Styela, the same author ('32) in Amphioxus, Vogt ('2^
in various Amphibia, Oppenheimer ('36) in Fundulus, Pasteels ('36, '37b)
trout and chick, Vandebroek ('36) in the shark, and Spratt ('46) in tl
chick. For detailed discussions, concerning the morphodynamics of the g£
trulative period, reference may be made to the works published by Roi
(1895), Spemann ('38), Pasteels ('40), Waddington ('40), and Schechtm;
('42).
The theory popularly held, regarding the movements of the major pr
sumptive organ-forming areas of the late blastula, is that a strict autonon
is present among the various groups of cells concerned with the gastrulati
process. Spemann ('38) p. 107, describes this theory of autonomy as follow
Each part has already previously had impressed upon it in some way or oth
direction and limitation of movement. The movements are regulated, not in
coarse mechanical manner, through pressure and pull of the single parts, but th
are ordered according to a definite plan. . . . After an exact patterned arran^
ment, they take their course according to independent formative tendencies whi
originate in the parts themselves.
There are some observations, on the other hand, which point to an inte
dependence of the various cell groups. For example, we have referred to t
observations of Waddington ('33) and Spratt ('46) which suggest that t
movements of the mesoderm in the bird embryo are dependent upon t
inductive influence of the entoderm. Similarly, Schechtman ('42) points o
that presumptive notochordal material does not have the power to invagin£
ANIMAL POLE
O
ANIMAL POLE
VEGETAL POLE
VEGETAL POLE
ANIMAL POLE
ANIMAL POLE
o
VEGETAL POLE
VEGETAL POLE
Fig. 215. Direction of entodermal projection in relation to egg polarity during gas-
trulation in various Chordata. (A) Amphioxus. (B) Frog. (C) Urodele amphibia.
(D) Chick. For diagrammatic purposes, the positions to the right of the median egg
axis in the diagrams arbitrarily are considered as clockwise positions, whereas those to
the left are regarded as counterclockwise.
Pig. 216. Exogastrulation in the axoiotl (Amphibia). (From Huxley and De Beer, '34,
after Holtfreter: Biol. Zentralbl., 53: 1933.) (A, B) Mass outward or exogastrular
movements of entoderm and mesoderm, resulting in the separation of these organ-
forming areas from the epidermal, neural areas shown as a sac-like structure in upper
part of figure. (C) Section of (B). Exogastrulation of this character results when the
embolic movements of gastrulation are directed outward instead of inward. Observe that
neural plate does not form in the ectodermal area.
448
PRE-CHORDAL PLATE AND CEPHALIC PROJECTION 449
area, but it does possess the autonomous power to elongate into a slender
column of cells.
I. Exogastrulation
It was demonstrated by Holtfreter ('33) and also by others that embryos
may be made to exogastrulate, i.e., the entoderm, notochord, and mesoderm
evaginate to the outside instead of undergoing the normal processes involved
in emboly (fig. 216). For example, in the axolotl, Ambystoma mexicanum,
if embryos are placed in a 0.35 per cent Ringer's solution, exogastrulation
occurs instead of gastrulation, and the entodermal, mesodermal and noto-
chordal areas of the blastula lie outside and are attached to the hollow ecto-
dermal vesicle. The exogastrulated material, therefore, never underlies the
ectodermal cells but comes to lie outside the neural plate and skin ectodermal
areas of the gastrula (fig. 216B).
Therefore, the phenomenon of exogastrulation indicates strongly that the
presumptive, neural plate and epidermal areas of the late blastula and early
gastrula are dependent upon the normal gastrulative process for their future
realization in the embryo. Exogastrulation also clearly separates the parts of
the forming gastrula which are concerned with emboly from those which are
moved by the forces of epiboly. That is, exogastrulation results when the forces
of epiboly are separated from the forces normally concerned with emboly.
Normal gastrulation is concerned with a precise and exact correlation of these
two sets of forces.
J. Pre-chordal Plate and Cephalic Projection in Various Chordates
It is evident from the descriptions presented in this chapter that the initial
invaginative movements in gastrulation begin in the region of the dorsal lip
of the blastopore in Amphioxus, fishes, and Amphibia. This initial movement
of cells in the region of the dorsal lip consists in the projection forward, toward
the future head region of the embryo, of foregut entoderm, pre-chordal plate
mesoderm, and notochordal cells. The foregut entoderm, pre-chordal meso-
derm, and the anterior extremity of the notochord come to lie beneath the
anterior portion of the neural plate. The complex of anterior foregut entoderm
and pre-chordal mesoderm lies in front of the anterior limits of the notochord
— hence, the name pre-chordal plate. As such it represents, as previously
observed, a part of the head organizer (see p. 401 ), the complete organization
of the vertebrate head being dependent upon anterior chordal (notochordal),
as well as pre-chordal, factors.
In higher vertebrates a different situation prevails during gastrulation. As
observed in Chapter 7, the late blastula consists of a lower hypoblast and
an upper epiblast in a flattened condition, the hypoblast having separated
from the lower parts of the epiblast. The separation of the hypoblast occurs
shortly before the gastrulative rearrangement of the major, presumptive, organ-
450 GASTRULATION
forming areas begins. The organization of the blastoderm (blastula) is such
that presumptive pre-chordal plate mesoderm and notochordal areas lie far
anteriorly toward the midcentral part of the epiblast. In other words, a con-
tiguous relationship between presumptive pre-chordal entoderm (i.e., ante-
rior foregut entoderm) and presumptive pre-chordal mesoderm and the pre-
sumptive notochord at the caudal margin of the blastula does not exist.
Consequently, a different procedure is utilized in bringing the foregut ento-
derm, pre-chordal mesoderm, and anterior notochord together. That is, the
head-organizer materials must be assembled together in one area underneath
the cephalic portion of the neural plate. This is accomplished by two methods:
( 1 ) The use of a large invaginative process, the notochordal canal, which
projects pre-chordal plate mesoderm and notochord cranio-ventrad
toward the foregut entoderm in the hypoblast below, as described in
figure 200 relative to the reptiles or in figure 207B of the human
embryo and
(2) the use of another and less dramatic method for getting the head-
organizer materials together, the vestigial invaginative process which
produces the primitive pit and Hensen's nodal area.
The latter mechanism succeeds in getting pre-chordal plate mesoderm and
notochord down between the epiblast and hypoblast and forward to unite with
the anterior part of the foregut entoderm. (See Adelmann, '22, '26; Pasteels,
'37b.)
It is not clear whether the invaginative behavior which produces the primi-
tive pit or notochordal canal is an autonomous affair or whether it may be
dependent upon the inductive activities of the entoderm below. More experi-
mentation is necessary to decide this matter. The work of Waddington ('33),
however, leads one to conjecture that inductive activities may be responsible.
Regardless of the factors involved, cephalogenesis or the genesis of the
head is dependent upon the assemblage of anterior foregut, pre-chordal meso-
derm, and anterior notochordal cells beneath the cephalic portion of the
neural plate as described on page 401.
K. Blastoporal and Primitive-streak Comparisons
From the considerations set forth above, it is clear that the area of the noto-
chordal canal or primitive pit (i.e., Hensen's nodal area) corresponds to the
general region of the dorsal lip of the blastopore of lower vertebrates, whereas
the dorso-lateral and lateral lips of the blastopore of lower forms correspond
to the body of the primitive streak in higher vertebrates (Adelmann, '32).
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isolement. (b) Apres transplantation sur
un socle de cellules vitellines. Compt.
rend. Soc. de biol. 122:127.
Vogt, W. 1929. Gestaltungsanalyse am
Amphibienkeim mil ortlicher Vitalfar-
bung. II. Teil: Gastrulation und meso-
dermbildung bei Urodelen und Anuren.
Roux' Arch. f. Entwick. d. Organ.
120:385.
Waddington, C. H. 1933. Induction by the
entoderm in birds. Arch. f. Entwicklngs-
mech. d. Organ. 128:502.
. 1940. Organizers and genes. Cam-
bridge University Press, London.
Will, L. 1892. Beitriige zur Entwicklungs-
geschichte der Reptilien. Zool. Jahrb.
6:1.
Wilson, H. V. 1889. The embryology of
the sea bass (Serranus atrariiis). Bull.
U. S. Fish Comm. 9:209.
10
TuDulation ana Extension or tne Major Or^an-rormin^
Areas: Development or Primitive Body Form
Introduction
1. Some of the developmental problems faced by the embryo after gastrulation
a. Tabulation
b. Increase in size and antero-posteriOr extension of the tubulated, major organ-
forming areas
c. Regional modifications of the tubulated areas
2. Common, vertebrate, embryonic body form
3. Starting point for tubulation
4. Developmental processes which accomplish tubulation
a. Immediate processes
b. Auxiliary processes
5. Blastocoelic space and body-form development
6. Primitive circulatory tubes or blood vessels
7. Extra-embryonic membranes
Tubulation of the neural, epidermal, entodermal, and mesodermal, organ-forming
areas in the vertebrate group
1. Neuralization or the tubulation of the neural plate area
a. Definition
b. Neuralizative processes in the Vertehrata
1) Thickened keel method
2) Neural fold method
c. Closure of the blastopore in rounded gastrulae, such as that of the frog
d. Anterior and posterior neuropores; neurenteric canal
2. Epidermal tubulation
a. Development of the epidermal tube in Amphibia
b. Tubulation of the epidermal area in flat blastoderms
3. Formation of the primitive gut tube (enteric tubulation)
a. Regions of primitive gut tube or early metenteron
b. Formation of the primitive metenteron in the frog
c. Formation of the tubular metenteron in flat blastoderms
4. Tubulation (coelom formation) and other features involved in the early differen-
tiation of the mesodermal areas
a. Early changes in the mesodermal areas
1) Epimere; formation of the somites
2) Mesomere
3) Hypomere
b. Tubulation of the mesodermal areas
454
INTRODUCTION 455
C. Notochordal area
D. Lateral constrictive movements
E. Tubulation of the neural, epidermal, entodermal, and mesodermal, organ-forming
areas in Amphioxus
1. Comparison of the problems of tubulation in the embryo of Amphioxus with that
of the embryos in the subphylum Vertebrata
a. End-bud growth
b. Position occupied by the notochord and mesoderm at the end of gastrulation
2. Neuralization and the closure of the blastopore
3. Epidermal tubulation
4. Tubulation of the entodermal area
a. Segregation of the entoderm from the chordamesoderm and the formation of
the primitive metenteric tube
b. Formation of the mouth, anus, and other specialized structures of the metenteron
5. Tubulation of the mesoderm
6. Later differentiation of the myotomic (dorsal) area of the somite
7. Notochord
F. Early development of the rudiments of vertebrate paired appendages
G. The limb bud as an illustration of the field concept of development in relation to the
gastrula and the tubulated embryo
H. Cephalic flexion and general body bending and rotation in vertebrate embryos
L Influences which play a part in tubulation and organization of body form
J. Basic similarity of body-form development in the vertebrate group of chordate animals
A. Introduction
1. Some of the Developmental Problems Faced by the
Embryo After Gastrulation
a. Tubulation
One of the main problems, confronting the embryo immediately following
gastrulation, is the tubulation of the major organ-forming areas, namely,
epidermal, neural, entodermal, and the two, laterally placed, mesodermal
areas. The epidermal, neural, and entodermal areas eventually form elon-
gated, rounded tubes, whereas the mesodermal tubes are flattened. The epi-
dermal and neural tubes extend the entire length of the developing embryo
(fig. 217A-C), while the entodermal tube normally terminates at the begin-
ning of the tail (fig. 217B, C), although in some instances it may extend even
to the tail's end (fig. 217A). Anteriorly, the entodermal tube ends along the
ventral aspect of the developing head (fig. 217A, C). The two mesodermal
tubulations are confined mainly to the trunk region of the embryo, but in the
early embryo of the shark they continue forward into the head almost to the
posterior limits of the developing eyes (fig. 217D). The condition of the
mesodermal tubes in the Amphibia resembles to a degree that in the shark
embryo (fig. 217B, E).
An important concept to grasp is that the tubulations of the respective areas
occur synchronously or nearly so. It is true that the initial stages of the epi-
dermal and entodermal tubulations slightly precede the other tubulations in
Fig. 217. Primary tubes (tubulations) of the primitive vertebrate body. (A) Sche-
matic representation of epidermal, neural, and entodermal tubes in the early embryo of
the shark. Observe that a well-developed, post-anal or tail gut continues to the end of
the tail. (B) Gut, neural, and epidermal tubes in the amphibian type. (C) Gut,
neural, and epidermal tubes in the chick and mammal type. (D) Mesodermal tube in
the shark embryo. (E) Mesodermal tube in the amphibian embryo. (F) Mesodermal
condition in the early bird and mammal embryo. (G) Transverse section of shark
embryo, showing tubulations of major organ-forming areas and primary coelomic con-
ditions. (H) Transverse section of frog embryo shortly after closure of neural tube,
showing the five fundamental body tubes oriented around the notochord.
456
INTRODUCTION 457
Amphioxus, in the frog, and in forms having rounded gastrulae, while in the
chick the neural area is precocious. Viewed in their totality, however, the
tubulations of all of the major organ-forming areas are simuhaneous processes
with the exception of the notochord which does not become tubulated but
continues as an elongated rod of cells.
b. Increase in Size and Antero-posterior Extension of the Tubulated,
Major Organ-forming Areas
Another goal to be achieved by the embryo during the immediate, post-
gastrular period is an increase in size, together with an antero-posterior exten-
sion of the major organ-forming areas. These changes are associated with
tubulation, and they aid in producing the elongated, cylindrical form typical
of the chordate body.
c. Regional Modifications of the Tubulated Areas
As tubulation of the various major organ-forming areas progresses, specific,
organ-forming areas or fields (see end of chapter), located along the respec-
tive primitive body tubes, begin to express themselves and develop in a spe-
cialized manner. Thus, regional differentiation of the major organ-forming
areas, comprising each primitive body tube, is another feature of the post-
gastrular period. As a result, localized areas along each of the body tubes
show changes in shape, and specific, individualized structures begin to make
their appearance. For example, the neural tubulation develops the primitive
parts of the brain at its anterior end, while the posterior portion of the neural
tube, caudal to the brain area, begins to form the spinal cord. Thus, the
primitive brain becomes a specific peculiarity of the head region. Also, the
epidermal tubulation at its cranial end contributes definite structures peculiar
to the head. In the pharyngeal region, special developmental features arise in
the entodermal tube together with the epidermal tube and the mesoderm. In
the trunk region, modifications of the entodermal and mesodermal tubes give
origin to many of the structural conditions peculiar to this area, while in the
tail, the neural and epidermal tubulations together with activities of the meso-
derm account for the characterstic structures of the tail appendage. These
special developmental features of the respective, tubulated, organ-forming
areas, which arise in specific areas along the antero-posterior axis of the
embryo, occur in much the same way throughout the vertebrate group with
the result that common or generalized structural conditions of the tubulated
organ-forming areas appear in all vertebrate embryos. That is, the primitive
brains of all vertebrate embryos up to a certain stage of development resemble
each other in a striking manner; the contributions of the epidermal tubulation
to the head also resemble each other, and the early development of the
pharyngeal and trunk regions is similar. As a result, the early morphogenesis
458
DEVELOPMENT OF PRIMITIVE BODY FORM
NEURAL ECTODERM
PIDERMAL
ECTODERM
SU8N0T0CH
EPIDERMAL
ECTODERM
ENTODERM
NEURAL TUBE
PERIBLAST
COELOM
VENTRO-LATERAL
MUSCLE TRACT
WOLFFIAN DUCT
ECTODERM
EPIDERMAL ECTODERM
MESODERM
NOTOCHORD
ENTODERM
Fig. 218. Solid keel of neural ectoderm in teleost and bony ganoid fishes. (A and B
after H. V. Wilson, 1889; C after Dean, 1896.) (A) Neural ectoderm separating from
epidermal ectoderm. (B) Neural tube completely separated from epidermal ectoderm.
(C) Late gastrular condition of Amia calva.
of the respective body tubes tends to follow a similar procedural plan through-
out the entire vertebrate series.
2. Common, Vertebrate, Embryonic Body Form
As a result of the changes outlined above and the tendency to form common,
generalized, structural conditions during the early phases of development, a
common, generalized, primitive embryonic body form is developed in the
embryos of all vertebrate species in which the rudiments of various, future,
INTRODUCTION 459
organ systems conform to generalized, basic plans. After the generalized plan
of a particular system is established, it is modified in later development to fit
the requirements of the habitat in which the particular species lives. In the
cephalochordate, Amphioxus, a similar body form also develops, although it
is considerably modified.
The common, generalized, primitive embryonic body form of all vertebrate
embryos possesses the following characteristics:
( 1 ) It is an elongated structure, cyHndrical in shape, and somewhat com-
pressed laterally.
(2) It is composed of five, basic, organ-forming tubes, oriented around a
primitive axis, the notochord (fig. 217).
(3) It possesses the following regions: (a) head, (b) pharyngeal area,
(c) trunk, and (d) tail (figs. 217, 226, 227, 230, 238, 244, 246).
In Chapter 1 1 and the following chapters, various details of these common
regions and other features will be considered. In this chapter, we are con-
cerned mainly with tubulation and antero-posterior extension of the major
organ-forming areas in relation to body-form development.
3. Starting Point for Tubulation
The starting point for tubulation of the major organ-forming areas and
subsequent, primitive, body formation is the gastrula; which, as observed in
Chapter 9, exists in two forms, namely, rounded and the flattened gastrulae
(figs. 219, 232). Many heavily yolked embryos, such as the embryo of Nec-
turus maculosus, although they form a rounded gastrula, are faced with some
of the problems of the flattened gastrulae (fig. 227). The rounded gastrulae,
found in the frog, Amphioxus, etc., differ from the flattened gastrulae present
in the bird, reptile, mammal, and teleost and elasmobranch fishes, mainly by
the fact that, at the beginning of tubulation and body formation, the epidermal
and gut areas already are partially tubulated in the rounded gastrulae. That is,
in the rounded blastoderm, the initial stages of tubulation occur in these two
major organ-forming areas during gastrulation. This means that the ventral
portion of the trunk area in rounded gastrulae is circumscribed by intact cellular
layers of the embryonic trunk region, with yolk material contained within the
cell layers, while, in flattened gastrulae, the ventro-lateral portions of the
trunk region are spread out flat, the yolk not being surrounded by the future,
ventro-lateral walls of the embryonic trunk region. These conditions are illus-
trated in figures 219B and C and 234A-F.
The developmental problems faced by these two groups of gastrulae, there-
fore, are somewhat different. Moreover, tubulation of the organ-forming areas
and the development of body form in Amphioxus varies considerably from
that of the rounded gastrulae of the vertebrate group. For this reason, tubula-
tion in Amphioxus is considered separately.
460
DEVELOPMENT OF PRIMITIVE BODY FORM
Regardless of differences, however, all vertebrate gastrulae, rounded and
flattened, possess three fundamental or basic regions, to wit, ( 1 ) a cephalic
or head region, containing the rudiments of the future head and pharyngeal
structures, (2) a trunk region, wherein lie the undeveloped fundaments of
the trunk, and (3) an end-bud or tail rudiment, containing the possibilities
of the future tail.
4. Developmental Processes Which Accomplish Tubulation
a. Immediate Processes
The term, immediate processes, signifies the events which actually produce
the hollow tubular condition. In the case of the epidermal, enteric, and neural
tubulations, the immediate process is mainly one of folding the particular,
BRAIN AREA ^^7^^r^&?^^^
HEAD REGION J^^ 7^ *
SENSORY PLATE
GILL - PLATE ■■
AREA
TRUNK REGION ■
NEURAL FO
TAIL REGION
PRE-CHORDAL PLATE
NEURAL ECTODERM
EPIDERMAL
ME30DE RM
ECTODERM
COELOMIC SPACE
ENTODERM
NOTO CHORD
NEURAL
E C T 0 D E R M
VENTRAL
MESODERM
Fig. 219. Relationships of the major presumptive organ-forming areas at the end of
gastrulation in the anuran amphibia. (A) External view of gastrula, showing the ecto-
dermal layer composed of presumptive epidermis (white) and presumptive neural plate
(black), as viewed from the dorsal aspect. (B) Diagrammatic median sagittal section
of condition shown in (A). (C) Same as (B), showing major organ-forming areas.
(D) Section through middorsal area of conditions (B) and (C), a short distance caudal
to foregut and pre-chordal plate region. Observe that the notochord occupies the mid-
dorsal area of the gut roof.
INTRODUCTION 461
organ-forming area into a hollow tubular affair. With respect to the meso-
dermal areas, the immediate process is an internal splitting (delamination),
whereby the mesodermal area separates into an outer and an inner layer
with a space or cavity appearing between the two layers. In the case of the
teleost fishes, a process of internal separation of cells appears to play a part
also in the neural tubulation.
b. Auxiliary Processes
Aiding the above activities which produce tubulation are those procedures
which extend the tubulated areas into elongated structures. These auxihary
processes are as follows:
( 1 ) The cephalic or head rudiment, with its contained fundaments of the
developing head region, grows forward as a distinct outgrowth. This
anterior protrusion is known as the cephalic or head outgrowth (figs.
223A, B; 232I-L).
(2) The trunk rudiments enlarge and the trunk region as a whole under-
goes antero-posterior extension (figs. 225 A; 233).
(3) The tail-bud area progresses caudally as the tail outgrowth and forms
the various rudimentary structures associated with the tail (figs. 225;
230F; 238).
(4) A dorsal upgrowth (arching) movement occurs, most noticeable in
the trunk area. It serves to lift the dorsal or axial portion of the trunk
up above the yolk-laden area below, and the developing body tubes
and primitive body are projected dorsalward (figs. 221, 224, 241 ).
(5) In embryos developing from rounded gastrulae, a ventral contraction
and reshaping of the entire ventro-lateral areas of the primitive trunk
region are effected as the yolk is used up in development. This results
in a gradual retraction of this area which eventually brings the ventro-
lateral region of the trunk into line with the growing head and tail
regions (cf. figs. 220, 223, 225 on the development of the frog, and
227 on the development of Necturus).
(6) In embryos developing from flattened gastrulae, a constriction of the
ventral region of the developing trunk comes to pass. This constriction
is produced by an ingrowth toward the median line of entodermal,
mesodermal, and epidermal cellular layers in the form of folds, the
lateral body folds. Upon reaching the midline, the cellular layers fuse
as follows; The entodermal layer from one side fuses with the ento-
dermal layer of the other; the mesodermal layers fuse similarly; and,
finally, the epidermal layer from one side fuses with the epidermal
layer of the opposite side. The result is a general fusion of the re-
spective body layers from either side, as shown in figure 24 IC and D,
which establishes the ventral region of the trunk. A complete fusion
throughout the extent of the ventral body wall does not take place
462 DEVELOPMENT OF PRIMITIVE BODY FORM
NEURAL PLATE A.B. C D. E F. G.
H.I.J.
LIVER DIVERTICULUM
Fig. 220. Beginning neural fold stage of frog embryo from prepared material. (A)
Beginning neural fold stage as seen from dorsal view. (B) Sagittal section near median
plane of embryo similar to that shown in (A). (C) Same as (B), showing organ-
forming areas. (D) Midsagittal section of caudal end of frog embryo slightly younger
than that shown in fig. 223B. Observe that the blastopore practically is closed, while the
dorsal diverticulum of the hindgut connects with the neurocoel to form the neurenteric
canal. Observe, also, ventral diverticulum of hindgut.
until later in development, and, as a result, a small opening remains,
the umbilicus, where the embryonic and extra-embryonic tissues are
continuous. This discontinuity of the embryonic layers permits the
blood vessels to pass from the embryonic to the extra-embryonic re-
gions. {Note: In the teleost fishes, although a typical, flattened, gas-
trular form is present, the formation of the ventral body wall of the
trunk through a general retraction of tissues resembles that of the
rounded gastrulae mentioned above.)
5. Blastocoelic Space and Body-form Development
During the terminal phases of gastrulation in such forms as Amphioxus
and the frog, the blastocoel, as a spacious cavity, disappears for the most
part. Its general area is occupied by cells which migrated into the blastocoel
INTRODUCTION
463
during gastrulation. However, the disappearance of the blastocoehc space
is more apparent than real. For, while most of the original blastocoelic space
is thus occupied and obliterated, a part of the original blastocoel does remain
as an extremely thin, potential area between the outside ectoderm and the
mesoderm-entoderm complex of cells. In flattened blastoderms, as in the
chick, the actual space between the ectoderm, mesoderm, and entoderm is
considerable (fig. 234E, F). To sum up: Though the blastocoelic space ap-
pears to disappear during the terminal phases of gastrulation, a residual or
potential space remains between the three germ layers, more pronounced in
some species than in others. This residual space gradually increases during
the tubulation processes of the major organ-forming areas. In doing so, it
permits not only the tubulation of these areas within the outside ectoderm,
but it allows important cell migrations to occur between the various body tubes.
6. Primitive Circulatory Tubes or Blood Vessels
Accompanying the tubulations of the epidermal, neural, entodermal, and
the two mesodermal areas on either side of the notochord, is the formation
Fig. 221. Transverse sections through early neural fold embryo of the frog as shown
in fig. 220A and B. (A-J) Sections are indicated in fig. 220B by lines A-J, respectively.
Observe that the dorsal arching (dorsal upgrowth) movement of the dorsally situated
tissues accompanies neural tube formation.
464
DEVELOPMENT OF PRIMITIVE BODY FORM
NEURAL CREST CELLS
VAGUS LATERAL LINE
PLACODES
EPI BRANCHIAL
PLACODES
OF N VAGUS
^OTIC VESICLE
-!V LATERAL
LINE
PLACODE
LONGITUDINAL
ECTODERMAL
THICKENING
ECTODERMAL
THICKENING
HYOMANOIBULAR
CLEFT
EPIBRANCHIAL PLACODE
GLOSSOPHARYNGEUS
FACIALIS
D
SUPRAORBITAL
GROUP OF SENSE
ORGANS
MIOBODY
LINE OF SENSE
ORGANS
Fig. 222. Neural crest cells in Ainby stoma piinctatum. (A and B from Johnston:
Nervous System of Vertebrates, Philadelphia, Blakiston, '06; C-F from Stone: J. Exper.
Zool., '35.) (A) Transverse section of early neural tube of Ambystoma, neural crest
cells located dorsally and darkly shaded. (B) Later stage than (A), showing relation
of neural crest cells, epidermis, and neural tube. (C-F) Neural crest cells stippled,
placodes of special lateral line sense organs and cranial nerve ganglia shown in black.
The neural crest cells arise from dorsal portion of neural tube at points of fusion of
neural folds and migrate extensively. A considerable portion of neural crest cells descends
upon the mesoderm of visceral arches as indicated in (D-F) and contributes mesodermal
cells to these arches, where they later form cartilaginous tissue.
of a delicate system of vessels which function for the transport of the circula-
tory fluid or blood. The formation of these blood vessels begins below the
forming entodermal tube as two, subenteric (subintestinal) tubes or capillaries.
These capillaries grow forward below the anterior portion of the forming
digestive tube. Near the anterior end of the latter, they separate and pass
upward on either side around the gut tube to the dorsal area, where they come
together again below the notochord and join to form the rudiments of the
dorsal aortae. The latter are two delicate supraenteric capillaries which ex-
tend from the forming head area caudally toward the trunk region. In the
TUBULATION OF ORGAN-FORMING AREAS 465
latter region, each rudiment of the dorsal aorta sends a small, vitelline blood
vessel laterally into that portion of the gut tube or yolk area containing the
yolk or other nutritional source. In the yolk area, each joins a plexus of
small capillaries extending over the surface of the yolk substance. These
capillaries in turn connect with other capillaries which join ultimately each of
the original subintestinal blood capillaries. Below the anterior or foregut por-
tion of the entodermal tube, the two subintestinal blood vessels fuse and thus
form the beginnings of the future heart (figs. 234-237; 332). The further
development of this system of primitive vessels is described in Chapter 17.
7. Extra-embryonic Membranes
Associated with the development of body form and tubulation of the major,
organ-forming areas, is the elaboration of the very important extra-embryonic
membranes. As the essential purpose at this time is to gain knowledge of the
changes concerned with tubulation of the major organ-forming areas and the
development of primitive body form, consideration of these membranes is de-
ferred until Chapter 22. The latter chapter is concerned with various activities
relating to the care and nutrition of developing embryos of various vertebrate
species.
B. Tubulation of the Neural, Epidermal, Entodermal, and Mesodermal,
Organ-forming Areas in the Vertebrate Group
1. Neuralization or the Tubulation of the Neural Plate Area
a. Definition
The separation of the neural plate material from the skin ectoderm, its mi-
gration inward, and its formation into a hollow tube, together with the segre-
gation of the accompanying neural crest cells, is called neuralization.
b. Neuralizative Processes in the Vertebrata
Neuralization is effected by two general procedures in the vertebrate
subphylum.
1) Thickened Keel Method. In teleost, ganoid, and cyclostomatous fishes,
the neural plate material becomes aggregated in the form of a thickened,
elongated ridge or keel along the middorsal axis of the embryo (figs. 21 OF;
218C). This keel separates from, and sinks below, the overlying skin ectoderm
(fig. 218A). Eventually the keel of neural cells develops a lumen within its
central area and thus gradually becomes transformed into an elongated tube,
coincident with the tubulations of the other major organ-forming areas (fig.
218B). In the cyclostomatous fish, Petromyzon planeri, although neuraliza-
tion closely resembles the condition in teleost fishes, in certain respects the
behavior of the neuralizative changes represents an intermediate condition
466
DEVELOPMENT OF PRIMITIVE BODY FORM
Fig. 223. Early neural tube stage of the frog, Rana pipiens, IV2 to 3 mm. in length.
(A) Dorsal view. (B) Midsagittal section of embryo similar to (A). (C) Same as
(B), showing organ-forming areas. Abbreviations: V. HD. = ventral hindgut divertic-
ulum; D. HD. = dorsal hindgut diverticulum; PHAR. = pharyngeal diverticulum of fore-
gut. (D) Later view of (A). (E) See fig. 224.
between the keel method of the teleost and neural fold method of other verte-
brates described below (Selys-Longchamps, '10).
2) Neural Fold Method. In the majority of vertebrates, the neural (medul-
lary) plate area folds inward (i.e., downward) to form a neural groove. This
neural groove formation is associated with an upward and median movement
of the epidermal layers, attached to the lateral margins of the neural plate,
as these margins fold inward to form the neural folds. A change of position in
the mesoderm also occurs at this time, for the upper part which forms the
somites shijts laterad from the notochordal area to a position between the
forming neural tube and the outside epidermis. This mesodermal migration
permits the neural tube to invaginate downward to contact the notochordal
area. Also, this change in position of the somitic mesoderm is a most important
factor in neuralization and neural tube development as mentioned at the end
of this chapter. (Note: In this stage of development, the embryo is often de-
PROCTOOAEUM
Fig. 224. Transverse sections through frog embryo shortly after closure of the neural tube,
as indicated in fig. 223E. This embryo is slightly older than that shown in 223A-G.
467
468
DEVELOPMENT OF PRIMITIVE BODY FORM
RHOMBENCEPHALON
MESENCEPHALON
PROSENCEPHALON
NEURAL TUBE
NOTOCHORD
MESENCHYME
AREA
ORAL EVAGINATION
ORAL SUCKER
NEURAL TUBE
HEAD GUT
SUBNOTOCHOROAL
ROD
VENTRAL MESODERM
NOTOCHORD
FOREGUT
MIDGUT
Fig. 225. Structure of 3'/2- to 4-mm. embryo of Rana pipiens (about eight pairs of
somites are present). (See fig. 226A and B for comparable external views of lateral and
ventral aspects of 5-mm., Rana sylvatica embryo.) (A) External dorsal view. (B) Mid-
sagittal view. (C) Same, showing major organ-forming areas.
scribed as a neurula, especially in the Amphibia. However, in the bird and
the mammal, the embryo during this period is described in terms of the number
of somitic pairs present, and this stage in these embryos is referred to as the
somite stage.) Each lateral neural fold continues to move dorsad and mesad
until it meets the corresponding fold from the other side. When the two neural
folds meet, they fuse to form the hollow neural tube and also complete the
middorsal area of the epidermal tube (cf. figs. 221, 224, 233, 234, 236, 237,
242, 245A). As a general rule, the two neural folds begin to fuse in the
anterior trunk and caudal hindbrain area. The fusion spreads anteriad and
posteriad from this point (figs. 223, 229, 233, 235, 242, 245A). It is im-
portant to observe that there are two aspects to the middorsal fusion process:
(a) The lateral edges of the neural plate fuse to form the neural tube; and
( b ) the epidermal layer from ehher side fuses to complete the epidermal
layer above the newly formed neural tube.
Associated with the fusion phenomena of the epidermis and of the neural
tube, neural crest cells are given oflf or segregated on either side of the neural
tube at the point where the neural tube ectoderm separates from the skin
TUBULATION OF ORGAN-FORMING AREAS 469
ectoderm (figs. 221C-E; 234B; 236B). The neural crest material forms a
longitudinal strip of cells lying along either side of the dorsal portion of the
neural tube. As such, it forms the neural or ganglionic crest. In some verte-
brate embryos, as in the elasmobranch fish, Torpedo, and in the urodele,
Ambystoma, the cells of the neural crest are derived from the middorsal part
of the neural tube immediately after the tube has separated from the skin
ectoderm (epidermis). (See fig. 222 A, B.) In other vertebrates, such as the
frog, chick, and human, the neural crest material arises from the general area
of junction of neural plate and skin ectoderm as fusion of the neural folds is
consummated (fig. 234B).
The neural crest gives origin to ganglionic cells of the dorsal root ganglia
of the spinal nerves and the ganglia of cranial or cephalic nerves as described
in Chapter 19. Pigment cells also arise from neural crest material and migrate
extensively within the body, particularly to the forming derma or skin, peri-
toneal cavity, etc., as set forth in Chapter 12. A considerable part of the
mesoderm of the head and branchial area arises from neural crest material
(fig. 222C-F). (See Chapters 11 and 15.)
As the neural plate becomes transformed into the neural tube, it undergoes
extension and growth. Anteriorly, it grows forward into the cephalic outgrowth,
in the trunk region it elongates coincident with the developing trunk, while
posteriorly it increases in length and forms a part of the tail outgrowth.
c. Closure of the Blastopore in Rounded Gastrulae, such as that of
the Frog
Neuralization and the infolding of the neural plate cells begins in the frog
and other amphibia before the last vestiges of the entoderm and mesoderm
have completed their migration to the inside. As mentioned above, the neural
folds begin, and fusion of the neural tube is initiated in the anterior trunk
region. From this point, completion of the neural tube continues anteriad and
posteriad. As the neural tube proceeds in its development caudally, it reaches
ultimately the dorsal lip of the now very small blastopore. As the neural tube
sinks inward at the dorsal blastoporal lip, the epidermal attachments to the
sides of the infolding neural tube fuse in a fashion similar to the fusion of the
edges of the neural tube to complete the dorsal epidermal roof. Associated
with this epidermal fusion at the dorsal lip of the blastopore is the fusion
of the epidermal edges of the very small blastopore. The extreme caudal end
of the archenteron or blastoporal canal in this manner is closed off from the
outside (fig. 220D), and the posterior end of the archenteron (the future
hindgut area), instead of opening to the outside through the blastoporal canal,
now opens into the caudal end of the neural tube. In this way, a canal is
formed connecting the caudal end of the future hindgut with the neural tube.
This neurenteric union is known as the neurenteric canal.
It is to be observed in connection with the closure of the blastopore and
GILL- PLATE AREA
NASAL PIT
Fig. 226. External views of embryos of Rana sylvatica and Rana pipiens. (A to J
after Pollister and Moore: Anat. Rec, 68; K and L after Shumway: Anat. Rec, 78.)
(A, B) Lateral and ventral views of 5-mm. stage. Muscular movement is evident at this
stage, expressed by simple unilateral flexure; tail is about one-fifth body length. (Pollister
and Moore, stage 18.) (C, D) Lateral and ventral views of 6-mm. stage. Primitive
heart has developed and begins to beat; tail equals one-third length of body. (Pollister
and Moore, stage 19.) (E, F) Similar views of 7-mm. stage. Gill circulation is established;
hatches; swims; tail equals one-half length of body. (Pollister and Moore, stage 20.)
(G, H) Ten-mm. stage, lateral and dorsal views. Gills elongate; tail fin is well developed
and circulation is established within; trunk is asymmetrical coincident with posterior
bend in the gut tube; cornea of eyes is transparent; epidermis is becoming transparent.
(Pollister and Moore, stage 22.) (l, J) Eleven-mm. stage, true tadpole shape. Oper-
cular fold is beginning to develop and gradually growing back over gills. (K, L)
Eleven-mm. stage of R. pipiens embryo. Observe that opercular folds have grown back
over external gills and developing limb buds; opercular chamber opens on left side of
body only. Indicated in fig. 257B.
470
TUBULATION OF ORGAN-FORMING AREAS 471
the formation of the neurenteric canal that two important changes occur in
the future hindgut area of the archenteron at this time, namely, the posterior
dorsal end of the archenteron projects dorso-caudally to unite with the neural
tube (fig. 220D), while the posterior ventral end of the archenteron moves
ventrad toward the epidermis where it meets the epidermal invagination, the
proctodaeum (fig. 220D).
d. Anterior and Posterior Neuropores; Neurenteric Canal
The fusion of the neural folds in the middorsal area proceeds anteriad and
posteriad from the anterior somitic and hindbrain region as described above.
At the anterior end of the forebrain when fusion is still incomplete, an opening
from the exterior to the inside of the neural canal is present; it forms the
anterior neuropore (figs. 229D; 23 IL; 235B; 242E-G; 245B). When fusion is
complete, this opening is obliterated. The caudal end of the neural tube closes
in a similar manner, and a posterior neuropore is formed (figs. 242E, G;
245). In the chick, as in the mammal, the posterior neuropore at first is a
wide, rhomboidal-shaped trough, known as the rhomboidal sinus. The an-
terior end of the primitive streak is included within the floor of this sinus
rhbmboidalis (fig. 235 A, B). The point of posterior neuroporal closure is at
the base of the future tail in most vertebrates (fig. 245B), but, in the elasmo-
branch fishes, this closure is effected after the tail rudiments have grown
caudally for some distance (fig. 229B-E).
The vertebrate tail arises from a mass of tissue, known variously as the
tail bud, caudal bud, or end bud, and the posterior end of the neural tube
comes to lie in the end-bud tissues (figs. 225, 238C). The end bud grows
caudally and progressively gives origin to the tail. It consists of the following:
(a) the epidermal tube (i.e., the ectodermal covering of the end bud);
within this epidermal layer are
(b) the caudal end of the neural tube;
(c) the caudal end of the notochord;
(d) mesoderm in the form of a mass of rather compact mesenchyme sur-
rounding the growing caudal ends of the notochord and neural tube;
and
(e) a caudal growth from the primitive intestine or gut.
This extension of the gut tube into the tail is called, variously, the tail gut,
caudal gut or post-anal gut. It varies in length and extent of development in
embryos of different vertebrate species. In some species it is joined to the
neural tube; in others it is not so united. For example, the tail gut is as long
as the trunk portion of the gut in the young shark embryo of 8 to 10 mm. in
length, and at the caudal extremity it is confluent with the neural tube (figs.
21 7A; 229F). The confluent terminal portions of the neural and gut tubes
form the neurenteric canal. This well-developed neurenteric canal extends
Fig. 227. {See facing page for legend.)
All
TUBULATION OF ORGAN-FORMING AREAS 473
around the caudal end or base of the notochord. In the developing frog on the
other hand, the confluence between the neural and gut tubes is present only dur-
ing the initial stages of tail formation, and it thus represents a transient relation-
ship (fig. 223B, C). Consequently, as the tail bud in the frog embryo grows
caudally, the neurenteric connection is obliterated and the tail gut disappears.
On the other hand, in the European frog, Bombinator, the condition is inter-
mediate between frog and shark embryos (fig. 228). True neurenteric canals
within the developing tail are never formed in the reptile, chick, or mammal,
although a tail or post-anal gut, much abbreviated, develops in these forms.
(See paragraph below.) In teleost fishes, Kupffer's vesicle possibly represents
a small and transient attempt to form a neurenteric canal (fig. 210G). How-
ever, the tail gut here, with the exception of the terminally placed Kupffer's
Fig. 227. Stages of normal development of Necturus maculosiis. (Slightly modified
from Eycleshymer and Wilson, aided by C. O. Whitman; Chap. 11 in Entwicklimgs-
geschichte d. Wirbeltiere, by F. Keibel, '10.) (A) Stage 15, 14 days, 19 hours after
fertilization. Blastopore is circular and reduced; neural groove is indicated in center
of figure. (B) Stage 18, 17 days, 2 hours old. Blastopore is an elongated, narrow
aperture between caudal ends of neural folds; neural folds prominent and neural groove
is deeper. (C) Stage 21, 18 days, 15 hours old, 3 or 4 pairs of somites. Neural folds
are widely separated in head region, narrower in trunk, and coalesced in tail area. (D)
Stage 22, 20 days, 10 hours, 6 pairs of somites, length about 6 mm. Observe head has
three longitudinal ridges, the middle one represents developing brain, while lateral ones
are common anlagen of optic vesicles and branchial arches. (E) Stage 23, 21 days, 2
hours, 10 to 12 pairs of somites, 7 mm. long. Head projects forward slightly above egg
contour; end of tail is prominent; large optic vesicles protrude laterally from head
area; branchial arch region is caudal to optic vesicle enlargement; anus is below tip of
tail. (F) Stage 24, 22 days, 17 hours, 16 to 18 pairs of somites, 8 mm. long. Anterior
half of head is free from egg contour; optic vesicles and mandibular visceral arch are
well defined. (G) Stage 25, 23 days, 10 hours, 20 to 22 pairs of somites, 9 mm. long.
Head is free from egg surface; t^il outgrowth is becoming free; mandibular, hyoid, first
branchial and common rudiment of second and third branchial arches are visible. Otic
vesicle lies above hyoid arch and cleft between hyoid and first branchial arches. (H)
Stage 26, 24 days. 22 hours, 23 to 24 pairs of somites, length^ 10 mm. Head and caudal
outgrowths are free from egg surface; heart rudiment is shown as darkened area below
branchial arches; cephalic flexure of brain is prominent. (I) Stage 27, 26 days, 26 to
27 myotomes, length — 11 mm. Outline of body is straighter; nasal pits and mouth are
well defined, mandibular arches are long; heart is prominent below branchial arches;
anterior limb buds are indicated; faint outlines of posterior limb buds are evident. (J)
Stage 28, 30 days, 8 hours, 30 to 31 myotomes, length — 13 mm. Trunk of embryo is
straight, head and tail are depressed; surface of yolk is covered by dense network of
capillaries; vitelline veins are prominent; pigment appears below epidermis; anterior limb
bud projects dorsally; nuchal or neck flexure is prominent above heart and limb-bud
area. (K) Stage 29, 36 days, 16 hours, 36 to 38 myotomes, length — 16 mm. Mandibular
arches are forming lower jaw; nuchal and tail flexures are straightening; eye and lens
are well defined; anlagen of gill filament are present on gill bars; pigment cells are
evident on head areas; vitelline veins are prominent; yolk-laden, ventro-lateral portion of
trunk is becoming elongated and contracted toward dorsal region of embryo. (L) Stage
30, 40 days, 20 hours, 44 to 46 myotomes, length — 18 mm. Fore and hind limb buds are
prominent; nasal openings are small. (M) Stage 31. larva 49 days, 21 mm. (N) Stage
32, larva 61 days, 25 mm. (O) Stage 33, larva 70 days, 28 mm. (P) Stage 34, larva
97 days, 34 mm. (Q) Stage 35, young adult form, 126 days, 39 mm.
474
DEVELOPMENT OF PRIMITIVE BODY FORM
EN0-8UD TISSUE
NEURENTEHIC
OTOCHOHD
BNOTOCHORDAl
LIVER OIVERTICULU
Fig. 228. Sagittal section, showing organ-forming areas of Bombinator embryo. (After
O. Hertwig: Lehrbuch der Entwicklungsgeschichte des Menschen iind der Wirbeltiere.
1890. Jena, G. Fischer.) Observe elongated tail gut.
vesicle, is a solid mass of cells. Thus, the shark and Bombinator embryos, on
the one hand, and the frog, chick, or mammal embryo, on the other, represent
two extremes in the development of the tail gut in the vertebrate group.
In the reptiles, also in some birds, such as the duck, in the human embryo,
and certain other mammals, a transient notochordal-neural canal is present
which connects the enteron or gut tube with the caudal area of the forming
neural tube (figs. 200B, E; 207B; 231G-K). This canal is occasionally referred
to as a neurenteric canal. However, it is best to view this condition as a special
type of development within the above group, for it is not strictly comparable
to the neurenteric canal formed in the developing tail of the embryos of the
frog, shark, etc., where the neurenteric canal is formed by a definite union
between neural and tail-gut tubes as they project caudalward into the tail
rudiment.
2. Epidermal Tubulation
The formation of the external, epidermal, tubular layer of the vertebrate
body is a complex procedure. Its development differs considerably in the
rounded type of gastrula of the Amphibia from that in the flattened gastrula
of the chick or mammal.
a. Development of the Epidermal Tube in Amphibia
In the frog and other Amphibia, tubulation of the epidermal area of the
blastula begins during gastrulation. At the end of gastrulation, the changes
involved in epiboly have transformed the ectodermal area of the blastula into
an oval-shaped structure, surrounding the internally placed mesoderm and
entoderm (fig. 219). The neural plate material occupies the middorsal area
of this oval-shaped, ectodermal layer, while the future epidermal area forms
the remainder. Following gastrulation, the anterior end of this oval-shaped
structure, in harmony with the forming neural tube, begins to elongate and
TUBULATION OF ORGAN-FORMING AREAS
475
grows forward as the head outgrowth (figs. 220, 223, 225). A cylindrical,
epidermal covering for the entire head, in this manner, is produced as the
cranial or brain portion of the neural plate folds inward (invaginates). A
similar outgrowth in the tail area proceeds posteriorly, although here the
neural tube grows caudally by proliferative activity within the epidermal tube
instead of folding into the epidermal tube as it does in the cephalic outgrowth
(figs. 223, 225). Coincident with these two outgrowths, the trunk area, with
its ventral, yolk-filled, entodermal cells, elongates antero-posteriorly as the
neural plate folds inward. It also grows larger in harmony with the head and
tail outgrowths. As these activities continue, yolk substance is used up, and
NEURAL FOLDS
Fig. 229. Early stages of tubulation of neural and epidermal organ-forming areas
with resultant body-form development in the shark, Squalus acanthias (drawn from pre-
pared slides). Neural area shown in black; epidermal area is stippled white; neural folds
are outlined in white around edges of black area. (Consult also fig. 230.) (A) Embryonic
area is raised upward; neural plate is flattened; bilateral tail outgrowths are indicated.
(B) Embryo is considerably elevated from extra-embryonic blastoderm; brain area is
much expanded; trunk region of neural groove is pronounced. (C) Neuralization is
considerably advanced; tail rudiments are converging. (D) Neural and epidermal areas
are well tubulated; tail rudiments are fusing. (E) Young Squalus embryo, lying on left
side; tail rudiments are fused into single caudal outgrowth. The body now consists of
a flexed cephalic outgrowth, trunk region, and tail outgrowth. (F) Squalus embryo of
about 10 mm. in length.
476 DEVELOPMENT OF PRIMITIVE BODY FORM
the ventro-lateral region of the trunk is retracted. A cyHndrical shape of the
trunk region thus is established, bringing the trunk area into harmony with
the head and tail outgrowths. (Study particularly fig. 227.) The epidermal
area of the late gastrula thus becomes converted into an elongated, epidermal
tube which forms the external covering or primitive skin (see Chap. 12) jor
the developing body. In Amphibia, this primitive epidermal tube is two layered,
consisting of an outer epidermal ectoderm and an inner neural ectoderm (figs.
221, 224). (See Chap. 12.) In the newly hatched larva, the epidermis is
extensively ciliated in all anuran and urodele Amphibia.
b. Tubulation of the Epidermal Area in Flat Blastoderms
In the flat blastoderms of the elasmobranch fish, chick, reptile, and mammal,
the formation of the external body tube involves processes more complicated
than that of the frog type. The following steps are involved:
( 1 ) A head fold produces a cephalic epidermal extension above the gen-
eral tissues of the blastoderm. This rudimentary fold of the epidermis
contains within it a similar fold of the entodermal layer, together with
the invaginating, neural plate material. The notochordal rod lies be-
tween the forming entodermal fold and developing neural tube (figs.
213F; 230A; 232I-L; 242B, C). Shortly, the primitive head fold
becomes converted into a cylindrical head outgrowth of the epidermal
and entodermal layers, associated with the forming neural tube and
notochord (figs. 229C, D; 230C; 233). The general process is similar
to that in the frog, but it is more complicated in that the head rudiment
first must fold or project itself up above the extra-embryonic areas,
before initiating the outgrowth process.
(2) A second procedure involved in epidermal tubulation in flattened
blastoderms is the dorsal upgrowth movement of epidermal, meso-
dermal, and entodermal tissues. This activity hfts the trunk region of
the embryo up above the general blastodermic tissues (figs. 213H-J;
234B; 241). In some forms, such as the chick, the dorsal upgrowth
movement is more pronounced in the anterior trunk area at first,
gradually extending caudad to the trunk region later (figs. 233, 235).
However, in the pig, human, and shark embryos, the dorsal elevation
extends along the entire trunk area, coincident with the head out-
growth, and thus quickly lifts the embryonic body as a whole up
above the extra-embryonic tissues (figs. 229, 230, 242, 245).
(3) The tail outgrowth, in reptiles, birds, and mammals, begins in a manner
similar to that of the head region, and a tail fold first is developed
which later becomes a cylindrical projection, bounded externally with
epidermal cells, within which are found the notochord, tail mesoderm,
and tail portions of neural and gut tubes (figs. 238C; 239K, L; 245B).
<EAD OUTGROV
NEURAL FOLD
LLARY GROOVE
NOTOCHORD
EU RENTERIC A
tHTROlD GLAND VENTRAL ACRTAE
Fig. 230. Sagittal sections of early elasmobranch embryos. (Slightly modified from
Scammon. See Chap. 12 in Entwicklungsgeschichte d. Wirbeltiere, by F. Keibel.) (A)
Graphic reconstruction from sagittal sections of embryo of 2 mm., seen from left side
(condition roughly comparable to stage between fig. 229 A and B). Observe that neural
plate is broad and flattened with slight elevation of neural folds. (B) Reconstruction
of embryo of 2.7 mm., viewed from left side, showing mesoderm, forming gut, neural
tubes, etc. (Consult (C) below.) (C) Same as (B) with mesoderm removed. Observe
primitive gut and neural tubes. Note: (B) and (C) are comparable to stage shown in
surface view in fig. 229C. (D, E) Same as (B) and (C), embryo 3.5 mm. in length.
(This embryo is comparable to fig. 229D.) (F) Same as (D) with mesoderm removed,
showing primitive vascular tubes and neural crest cells.
477
478 DEVELOPMENT OF PRIMITIVE BODY FORM
In elasmobranch fishes, two flattened tail outgrowths are present at
first which later fuse into a single cylindrical outgrowth (cf. figs. 229;
230F).
(4) A ventral constriction of the ventro-lateral body areas, involving the
ingrowth of the lateral body folds, occurs in the trunk region as indi-
cated in figure 241. This movement aids the establishment of a cylin-
drical body form in the trunk region. Entodermal and mesodermal
body layers, as well as the epidermal layer, are concerned with the
ventral constrictive movement (fig. 24 IB, C).
As a result of the above activities, an elongated, cylindrical body form is
effected in which the epidermal layer forms the outer covering around the
other body tubes.
3. Formation of the Primitive Gut Tube (Enteric Tubulation)
a. Regions of Primitive Gut Tube or Early Metenteron
The details of formation of the enteric tube vary considerably in different
vertebrate species. However, in all, the archenteric conditions of the gastrula
are converted into a primitive tubular metenteron, having three main regions
as follows: (1) foregut, (2) midgut, and (3) hindgut.
b. Formation of the Primitive Metenteron in the Frog
The formation of the foregut in the frog naturally follows as a result of
the anterior growth and extension of the cephalic portion of the primitive
archenteron present at the end of gastrulation (fig. 220B, C). This outgrowth
accompanies the forward growth of the neural and epidermal tubulations of
the developing head described above. The primitive head outgrowth thus is
composed of the anterior ends of the epidermal, neural, and gut tubes together
with the head mesoderm, all oriented around the median notochordal rod
(figs. 221B, C; 223B, C).
The midgut area of the primitive metenteron forms in relation to changes
in the developing trunk region. At the end of gastrulation, its ventral portion
is filled with yolk-laden cells, while its middorsal area is occupied by the
median notochordal band of cells (fig. 219B, C). This middorsal area is soon
completed by the medial growth of the entoderm which grows inward from
either side below the notochord (fig. 219D). Accompanying the completion
of the roof portion of the midgut, the entire midgut area becomes extended
antero-posteriorly (figs. 220B, C; 223B, C; 225B, C). Associated with these
changes, the middorsal area of the midgut moves dorsad toward the notochord,
forming a dorsal, trough-like region of the gut (fig. 224). It is to be observed
in this connection that the neural tube invaginates toward the notochordal
rod, whereas the roof of the gut evaginates (i.e., in a sense it invaginates)
toward the same notochordal area. This dorsal folding of the gut tube in the
MARGINAL CELLS
CENTRAL CELLS \
EIGHT-CELL STAGE
IRD
EM
ARCINAL CELLS
' -^ / ""
FOURTH CLEAVAGE
EGG
CENTRAL C"ELLS
rHT rFLL STAGE FOLLOWING THIRD CLEAVAGE,
EGG INTACT BLASTODERM HEMOVED FROM
/ CEI
, ?• * "^ yrvw
7^«
FIFTH CLEAVAGE
^a,?*""'''*^'^^
MESODERM
>*^
LATE BLASTODERM NOTOCHORDAL CANAL '^"TERIOR NEUROPORE
(SAME AS E) (EXTE8NAL OPENING) \ NEURAL FOLDS
EAD FOLD
NOTOCHORDAL CANAL EPIMERIC MESODERM EPIDERMAL TUBE^
EPIDERMAL TUBE HEAD OUTGROWTH . NEURAL TUBE
(ECTODERM) ' -^. . .
Fig. 231. Series of diagrams, showing stages in the development of the turtle. (A-F)
Cleavage stages after Agassiz. (G-J) Stages of gastrulation, drawn from slide prepara-
tions. (K-T) Stages during development of body form. f-P, Q, T from Agassiz; the
others are original.) (See L. Agassiz, 1857, Cont. Nat. Hist, of U. S. A., Vol. II.)
479
itvV ''>c,"i^- , -'?.■• -Vt-. EPIDERMAL'fOLD^IflV
SBE GINNING OF HE 40 FOLD '„■,;,* *;■" PROAMNION / "■'■.. •".V nSB \
PR MITIVE STREAK
I
si
>.
1 s
jfe:
\i
.'^
w
^'
II
"
'^■l,
^^k.
J. vy
;
eS^
S^::^iN^t^
Vv-
•"as 1
Fig. 232. Early post-gastrular development in the chick. (A-H represent a late
head-process stage — stage 5 of Hamburger and Hamilton, '51. Compare with figure
203D. I-L show the beginnings of the head fold — intermediate condition between stages
7 and 8 of Hamburger and Hamilton, '51.) (A) Surface view, showing primitive streak,
neural plate, and epidermal areas. (B-F) Cross sections of A at levels indicated on
G. (G) Median sagittal section of (A). (H) Same, showing presumptive, organ-
forming areas of entoderm notochord, pre-chordal plate, neural plate, and primitive-streak
mesoderm. (I) Surface view, demonstrating a marked antero-posterior extension of the
neural plate area and beginnings of neural folds. Observe shortening of primitive streak.
(J) Drawing of stained specimen. (K) Median sagittal section of (J). (L) Same,
showing major organ-forming areas. In (G) and (H) the entoderm, notochord, and
overlying neural ectoderm are drawn as separate layers. Actually, however, at this stage,
the three layers are intimately associated.
480
TUBULATION OF ORGAN-FORMING AREAS
481
't'
AREA
PE LLUCIDA
AREA OPACA
NEURAL TUBE
■^t^*" EPIDERMAL TUBF— 7
GUT TUBE '
NEURAL TUBE
HEART RUDIMENT
-. Y
>y BLOOD ISLAN DS
Fig. 233. Early body-form development in chick of 3 to 4 pairs of somites. (Approxi-
mately comparable to Hamburger and Hamilton, '51, stage 8, 26 to 29 hours of incu-
bation.) (A) Surface view, unstained specimen. (B) Stained, transparent preparation.
Observe blood islands in caudal part of blastoderm. (C) Median sagittal section. (D)
Same as (C), showing organ-forming layers.
direction of the notochord is much more pronounced in the flattened blasto-
derms than in the rounded blastoderms of the frog, salamander, etc. (cf.
figs. 224; 237). {Note: Associated with the dorsal invagination of the roof
of the midgut in the frog, is the detachment of a median rod of entodermal
cells from the middorsal area of the gut. This median rod of cells comes to
lie between the notochord and the roof of the midgut. It is known as the
subnotochordal rod (fig. 225C). (See Chapter 15.)
The development of the rudimentary hindgut is consummated by caudal
482 DEVELOPMENT OF PRIMITIVE BODY FORM
growth and extension of the posterior or tail region of the primitive archen-
teron of the late gastrula. These changes result in an extension of the
archenteron in the direction of the developing tail and the area ventral to the
tail (compare fig. 220B-D with figs. 223B, C; 225B, C).
Three general areas of the primitive gut are thus established:
(a) a tubular enlargement and outgrowth into the developing head, the
primitive foregut,
(b) a tubular extension and growth in the caudal region toward the tail,
the primitive hindgut, and
(c) a midgut area whose ventral wall is filled with yolk substance, while
its roof or dorsal wall assumes a trough-like form extending below
the notochord (figs. 223, 224, 225).
The foregut and hindgut areas at this time present the following special
features:
( 1 ) Two terminal diverticula or evaginations evolve at the extreme anterior
portion of the foregut; and
(2) at the extreme caudal end of the hindgut, similar evaginations occur.
In the foregut region, one of these evaginations projects toward the brain
and anterior end of the notochord, while the second diverticulum, more pro-
nounced than the dorsal evagination, moves ventrad toward the epidermis
underlying the developing brain. The dorsal evagination represents the pre-
oral or head gut. In the frog it is much abbreviated (figs. 220B, C; 225B, C).
On the other hand, the antero-ventrally directed, oral, or pharyngeal, evagi-
nation is relatively large and projects toward the ectoderm underlying the
brain where it forms the future pharyngeal area of the foregut (figs. 220; 223;
225B, C). Ultimately an invagination from the epidermis, the stomodaeum,
becomes intimately associated with the anterior end of the pharyngeal evagi-
nation (see Chap. 13). In the hindgut region, the diverticulum which projects
dorsally into the tail is the tail gut, whereas the ventral evagination toward
the epidermis below the tail represents the future rectal and cloacal areas of
the hindgut (figs. 220; 223; 225B, C). It shortly becomes associated with
an invagination of the epidermis, the proctodaeum (fig. 223B, C). As previ-
ously mentioned, the tail gut may be well developed, as in the European frog,
Bombinator (fig. 228), or quite reduced, as in the frog, Rana (fig. 225).
c. Formation of the Tubular Metenteron in Flat Blastoderms
The development of the cylindrical gut tube in those vertebrate embryos
which possess flattened gastrulae is an involved, complicated affair. The de-
velopmental mechanics are not clearly understood. For example, it is not
clear whether the embryonic layers, lying in front of the head fold in figure
232G and H, are folded slightly backward in figures 232K and L and still farther
MESODERM
Fig. 234. Transverse sections of chick embryo with five pairs of somites. (This em-
bryo is slightly older than that shown in fig. 233; a topographical sketch of this develop-
mental stage is shown at the bottom of the figure with level of sections indicated.) Observe
that a dorsal arching (dorsal upgrowth) movement of the dorsal tissues is associated
with neural tube formation. See A and B.
483
484 DEVELOPMENT OF PRIMITIVE BODY FORM
caudad in figure 233C and D by autonomous activities within this tissue, or
whether the actively growing head outgrowth proceeds so rapidly that it
mechanically causes the area in front of the head fold to rotate backward
under the developing foregut and thus contribute to the foregut floor. It is
obvious, however, that the entodermal material, lying in front of the head
fold of the embryo, is folded backward, at least slightly, and thus becomes
a part of the floor of the foregut. The extent, however, varies considerably in
different species. It appears to be greater in the mammal (fig. 242C) than in
the chick. Another example suggesting the integration of different movements
of cellular layers is presented in the formation of the floor of the hindgut of
the developing pig embryo. In figure 242C, the rudiments of the foregut and
hindgut areas are established. However, in figure 242G, it is difficult to eval-
uate how much of the floor of the hindgut in this figure is formed by actual
ingrowth forward from point "a" and to what extent the floor is formed by
the rapid extension of tissues and backward growth of the caudal region of
the embryo as a whole, including the allantoic diverticulum.
Special processes also aid the formation of foregut and hindgut in many
instances. For example, in the chick, the floor of the foregut is established
in part by a medial or inward growth and fusion of the entodermal folds along
the sides of the anterior intestinal portal, as indicated by the arrows in figure
234C. A similar ingrowth of entoderm occurs in the shark embryo (fig. 213J).
although here the entoderm grows in as a solid layer from either side and is
not present in the form of a lateral fold, as in the chick. However, it should
be observed that the formation of the hindgut in the shark embryo arises by
a most interesting and extraordinary method. In the flattened gastrulae of
reptiles, birds, and mammals, the hindgut is established by the formation of
tail folds, involving entodermal and epidermal layers. In the shark embryo,
on the other hand, an enteric groove with enteric folds is formed, and the
folds eventually move ventrad and fuse to form a hollow tube beneath the
notochord of the developing tail.
Though the rudimentary foregut and hindgut areas of the metenteron arise
almost simultaneously in mammalian embryos, such as in the pig and human
embryos, in the chick a different sequence of procedure is present. In the
latter species the foregut begins its development immediately following gas-
trulation when the first pairs of somites are present (fig. 233). The hindgut,
on the other hand, begins its development at a considerably later period when
the embryo has attained many pairs of somites (fig. 238).
Once the rudimentary, pouch-like, foregut and hindgut areas have been
established in embryos developing from flattened gastrulae, their further de-
velopment assumes morphogenetic features similar to those in the frog embryo.
For example, the foregut possesses an antero-dorsal prolongation toward the
brain, the pre-oral or head gut, while slightly posterior to the pre-oral gut,
the future pharyngeal area makes contact ventrally with the stomodaeai in-
TUBULATION OF ORGAN-FORMING AREAS
485
vagination from the epidermal (ectodermal) tube (fig. 242G). Similarly, the
caudal region of the hindgut rudiment contacts the proctodaeal invagination
of the epidermal tube, while a tail gut extension continues into the tail (fig.
217).
The formation of definitive walls of the midgut area in embryos developing
from the flattened gastrular condition (including the higher mammals which
do not possess large amounts of yolk substance) occurs as follows:
(1) Where the entoderm of the midgut terminates on either side of the
notochord at the end of gastrulation, it grows mesad from either side
»irv'fir:si5S:;:-.-.
Fig. 235. Chick embryo of 9 to 10 pairs of somites. (Approximating Hamburger and
Hamilton, '51, stage 10; 33 to 38 hours of incubation.) (A) Surface view, unstained.
(B) Stained preparation. (C) Median sagittal section. Observe the following: heart
is bent slightly to the right; three primary brain vesicles are indicated; foregut touches
infundibular outgrowth of prosencephalon; first indication of downward bending of the
head outgrowth, i.e., the cephalic (cranial) flexure is evident. (D) Same, showing
major organ-forming areas.
486
DEVELOPMENT OF PRIMITIVE BODY FORM
Fig. 236. Transverse sections through chick embryo of about 12 to 13 pairs of somites,
about 38 hours of incubation. (Approximately between stages 10 to 11 of Hamburger
and Hamilton, '51, slightly older than that shown in fig. 235.) Observe that the optic
vesicles are constricting at their bases; heart is bent slightly to the right; anterior neuropore
is evident. (A) Optic vesicles. (B) Stomodaeal area. (C) Anterior end of develop-
ing heart. (D) Caudal extremity of forming heart. (E) Anterior intestinal portal
and forming caudal portion of the heart. (F) Well-developed somites. (G) Open
neural groove.
below the notochord to complete the roof of the midgut (figs. 201D;
209C; 21 OF; 213). This process is similar to that which occurs in
the Amphibia (cf. fig. 219D).
(2) A dorsal arching or evagination of the entoderm toward the noto-
chordal area, comparable to that found in the frog and other Am-
phibia, is present also. A study of figures 213H-J; 217G; 234B;
237E-G; 241B-D demonstrates the marked dorsal upgrowth of all
the forming body layers in the trunk area. (Note: In the elasmobranch
fishes, a subnotochordal rod of cells of entodermal origin is formed
similar to that in the frog and other Amphibia.)
(3) The ventro-lateral walls of the midgut area, in contrast to those found
in the frog, are established largely by actual ingrowth of the entoderm,
mesoderm, and ectoderm with subsequent fusion in the median line
Fig. 237. Chick embryo of 17 to 19 pairs of somites. (Approximating Hamburger
and Hamilton, '51, stage 13, 48 to 52 hours of incubation, sections indicated on outline
drawing.) Head lies partly on left side; auditory pits are deep; cervical flexure is evident
in region of rhombencephalon; cephalic flexure is marked; stomodaeum is a deep inden-
tation touching foregut between the first pair of aortal arches; head fold of amnion
reaches back to anterior part of rhombencephalon (hindbrain). (A) Anterior (telen-
cephalic) portion of prosencephalon, showing closed neuropore; amnion is indicated.
(B) Optic vesicles. (C) Anterior end of foregut, showing anterior extremity of sto-
modaeal invagination and first (mandibular) pair of aortal arches; notochord ends and
pre-chordal plate area begins at about this section. (D) Anterior end of heart (ventral
aorta); observe thin roof plate of neural tube, characteristic of the later myelencephalic
(medulla) portion of rhombencephalon or hindbrain. (E) Otic (auditory) pits and
anterior region of ventricular portion of heart. (F) Caudal limits of forming heart,
dorsal mesocardium, neural crest cells. (G) Caudal end of heart, showing converging
(vitelline) veins of the heart, sclerotome given off to notochordal area, lateral meso-
cardium forming. (H) Anterior trunk area, showing diff'erentiation of somite and
typically flattened condition of ectoderm, mesoderm, and entoderm. (I) Caudal trunk
area, showing undifferentiated somite (epimeric mesoderm), intermediate mesoderm
(mesomere), and lateral plate mesoderm (hypomere). (J) Similar to (I). (K) Caudal
trunk region, showing closing neural tube. (L) Area of Hensen's node. (M) Primi-
tive streak.
487
488
DEVELOPMENT OF PRIMITIVE BODY FORM
in elasmobranch fishes, reptiles, birds, and mammals. This process in-
volves the formation of lateral body folds which fold mesially toward
the median plane. (Study fig. 241 A-D.) In teleost fishes the process
is different, for in this group the entoderm and mesoderm grow out-
ward beneath the primitive epidermis (ectoderm) and soon envelop
the yolk. Thus, the end result in teleosts is much the same as in the
frog and Nectiirus. It is well to observe, at this point, that a corhplete
retraction of the ventro-lateral walls of the midgut and body-wall
tissues surrounding the yolk or yolk-sac area, as in the frog and
Necturus (fig. 227), does not occur in the higher vertebrates, although
in the elasmobranch and teleost fishes such retraction does occur.
TAIL REGION
Fig. 238. Chick embryo of about 27 to 28 pairs of somites. (Corresponding approxi-
mately to Hamburger and Hamilton, '51, stage 16, 51 to 56 hours of incubation.) Fore-
brain (prosencephalon) is divided into telencephalon and diencephalon; epiphysis is ap-
pearing on roof of diencephalon; cephalic and cervical flexures are pronounced; tail bud
is short; anterior part of body is rotated to the left back to about the thirteenth pair
of somites; amnion now covers anterior three fifths of body; heart shows strong ven-
tricular loop; three pairs of aortal arches can be seen. (A) External view. (B) Trans-
parent wholemount." (C) Sagittal section, diagrammatic.
TUBULATION OF ORGAN-FORMING AREAS
489
BULBUS COnDtS_
MESENCHYMe OF HEAD'
Fig. 239. Sections through chick embryo of age indicated in fig. 238. Level of sections
is shown on diagram.
(See Chap. 22.) In the elasmobranch fishes, this retraction of tissues
contributes Uttle to the formation of the wall of the enteron or to that
of the body. However, in teleosts such contribution is considerable.
At this point reference should be made to figures 238C on the chick, 242C
and G on the pig, and 245B on the early human embryo to gain a visual image
of the developing foregut, midgut, and hindgut areas of the primitive meten-
teron. Compare with the frog (fig. 225C).
490
DEVELOPMENT OF PRIMITIVE BODY FORM
4. TUBULATION (COELOM FORMATION) AND OtHER FEATURES
Involved in the Early Differentiation of the
Mesodermal Areas
The differentiation of the mesodermal areas is an all-important feature of
embryonic development, for the mesoderm contributes much to the substance
of the developing body. (See Chaps. 11 and 15.) While the neural, enteric,
and epidermal tubes are being established, radical changes occur within the
two mesodermal layers on either side of the notochord as follows:
a. Early Changes in the Mesodermal Areas
1) Epimere; Formation of the Somites. The longitudinal mass of paraxial
mesoderm which lies along the side of the notochord forms the epimere
(figs. 221F, G; 234E, F). The two epimeres, one on either side of the noto-
chord, represent the future somitic mesoderm of the trunk area. In the early
post-gastrula, the epimeric mesoderm, together with the notochord, lies im-
mediately below the neural plate. However, as neuralization is effected* the
SCERAL) ARCH
OlO
CONO VISCERAL)
ARCH
MANDIBULAR
PROCESS
MAX I LL ARY
PROCESS
MANOIBUL AR
R FIRST
SCERAL ARCH
STERIOR LIMB BUD
Fig. 240. Chick embryo of about 72 to 75 hours of incubation, about stage 20 of
Hamburger and Hamilton, '51.
TUBULATION OF ORGAN-FORMING AREAS
491
EXTR A- E MBRYON
ENTODERMAL TUBULATION
DORSAL ARCHING MOVEMENT
DERMATOME
MYOTOME
SCLEROTOME _
NEPHROTOME
MESODERMAL TUBULATION
DORSAL EVAGINATION OF ENTODERM
ATERAL FOLDS OF AMNION
EMBRYONIC COELOM
EXTRA-EMBRYONIC COELOM
FUSION OF BODY LAYERS
DISAPPEARANCE OF VENTRAL MESENTERY
OR VENTRAL FUSION OF MESODERMAL
LAYERS BELOW GUT TUBE
Fig. 241. Formation of ventral body wail, differentiation of somites, formation of
dorsal and ventral mesenteries, embryonic and extra-embryonic coelom, etc., in chick
embryo. (A) Dorsal upgrowth is evident as neural tube, somites, and forming ento-
dermal (gut) tube are projected upward above the level of the extra-embryonic tissues.
Observe heavy line at left, denoting general region of demarcation between embryonic
and extra-embryonic tissues. (B) Separation of differentiating somite from nephro-
tome; sclerotomic mesenchyme is migrating from somite to notochordal-neural area;
lateral body folds are migrating mediad to form ventral wall of trunk region; lateral
folds of amnion are migrating dorsad. (C-E) Dorsal upgrowth movement lifts em-
bryonic body above extra-embryonic tissues below; fusion of ventral, body-wall layers
begins. (C) Body layers are meeting in midventral line. (D, E) Fusion of ventral
body-wall layers, disappearance of ventral mesentery.
epimeric mesoderm on either side of the notochord gradually moves laterally
and dorsally and comes to lie along the lateral aspects of the notochord and
neural tube. During this migration, each epimere increases in thickness and
becomes segmented into small oblong blocks of cells called somites (figs. 23 IN;
233B; 234D; 245A). A somite which forms in the epimere on one side of
the notochord always has a corresponding somite in the epimere on the other
side of the notochord. Somites thus form in pairs, each pair representing a
primitive segment of the developing body. This primitive segmentation is a
fundamental characteristic of the vertebrate body. It begins in the general
492 DEVELOPMENT OF PRIMITIVE BODY FORM
area of the anterior trunk and posterior hindbrain region of the embryo. In
the chick embryo (see Patterson, '07), the most anterior segment forms first,
and later segmentation progresses in a caudal direction. This probably holds
true for most other vertebrates. However, in elasmobranch fishes, segmenta-
tion of the epimeric mesoderm also extends forward from the hindbrain
area into the head region presenting a continuous series of somites from the
eye region caudally into the tail (fig. 217D). (Study figs. 217D, 230D.) Seg-
mentation of epimeric mesoderm appears in the head region of Amphibia.
In many higher vertebrates, three pairs of somitic condensations appear in
the area just caudal to the eye but at a slightly later period of development
than that of the elasmobranch fishes (fig. 217D-F).
2) Mesomere. The narrow longitudinal band of mesoderm, adjoining the
lateral border of the epimere, is the mesomere (figs. 22 IF, G; 230D; 234E, F).
This mesoderm ultimately gives origin to much of the excretory (kidney)
tissue and ducts and to certain of the reproductive ducts of many vertebrates.
(See Chap. 18.) Because of the origin of nephric tissue from its substance,
this longitudinal band of mesoderm generally is referred to as the urogenital
or nephrotomic mesoderm. Synonymous terms often used are intermediate
mesoderm or intermediate cell mass. The mesomere undergoes a segmentation
similar to the epimeric area in its more anterior portion where the pronephric
kidney develops in higher vertebrates, while in lower vertebrates, such as the
shark embryo, it may be more extensively segmented.
3) Hypomere. The remainder of the mesoderm which extends latero-
ventrally from the mesomere forms the hypomere or hypomeric mesoderm.
It also is called the lateral plate mesoderm or lateral plate mesoblast. This
portion of the mesoderm does not become segmented in present-day verte-
brates. (Compare with the condition in Amphioxus described on p. 505.)
b. Tabulation of the Mesodermal Areas
Coincident with the formation of the somites, a cavity begins to appear
within the mesoderm. This cavity or primitive coelomic space separates the
mesoderm into two layers, an outer layer near the ectoderm and an inner layer
close to the neural, notochordal, and entodermal cells. This hollowing process
within the mesodermal layer is known as coelom formation or tubulation
of the mesoderm. In many embryos of the lower vertebrates, there is a strong
tendency for the coelomic space to form throughout the entire lateral mass
of mesoderm from the epimeric area ventrad into the lateral plate mesoderm.
For example, in elasmobranch (shark) embryos of about 3 to 4 mm. in length
and also in many early post-gastrular amphibia, the following features of the
primitive coelom are found in the trunk region of each mesodermal mass:
( 1 ) The mesoderm possesses a cavity, continuous dorso-ventrally from
the epimere into the lateral plate (figs. 217G, H; 22 IE). When the
epimere (and to some extent the nephrotomic region as well) under-
LATERAL CONSTRICTIVE MOVEMENTS 493
goes segmentation, the coelomic space within these areas becomes
segregated within the segments and, thus, is present in a discontinuous
condition.
(2) The early coelomic cavity in the shark and amphibian embryo, there-
fore, may be divided into three parts: (a) the myocoelic portion within
the epimeric mesoderm, (b) the nephrocoel within the nephrotomic
mesoderm, and (c) the splanchnocoel contained within the hypomeric
or lateral plate mesoderm. While the myocoelic and nephrocoelic re-
gions of the primitive coelom may become segmented and discontin-
uous, that within the splanchnocoel is continuous antero-posteriorly
in the trunk region.
The coelomic cavities contained within the somites of the shark and am-
phibian embryo are soon lost. The coelomic cavity or nephrocoel within the
nephrotome is concerned with the development of the lumen within the tubules
and ducts of the excretory (urinary) system, while the splanchnocoels give
origin to the coelomic cavity proper of the adult. The lateral wall of the
splanchnocoel near the primitive epidermis is known as the somatopleural
mesoderm, and the inner or medial wall associated with the gut tube and
developing heart tissues constitutes the splanchnopleural layer. The epidermis
and somatopleural mesoderm together form the somatopleure, while the ento-
derm and splanchnopleural mesoderm form the splanchnopleure.
In the embryos of higher vertebrates, the coelomic space of the somitic
portion of the primitive coelom (i.e., the myocoels) is less pronounced and
appears somewhat later in development than in the shark and amphibian
embryo, but it does tend to appear. This is true also of the nephrocoel or
coelomic cavity within the nephrotome. (See Chap. 18.) The coelomic con-
dition or splanchnocoel within the hypomere forms similarly in all vertebrates.
These matters will be described more in detail in Chapter 20.
C. Notochordal Area
The notochord is the elongated, median band of cells of the gastrula which
lies between the two mesodermal areas. The notochord thus may be regarded
as a specialized, median portion of the middle germ layer of mesodermal
tissue. During gastrulation and shortly after, there may be a tendency for the
notochordal material in certain forms to canalize or tubulate. Later, the noto-
chordal material becomes converted into a definite rod of notochordal cells
which represents the primitive skeletal axis of the embryo. The notochord
and its relation to the early skeletal system are discussed in Chapter 15.
D. Lateral Constrictive Movements
While the neural, epidermal, and entodermal tubulations are in progress,
a lateral constriction or invagination of the body wall occurs on either side
in all vertebrate embryos from the fishes to the mammals. These constrictions
494 DEVELOPMENT OF PRIMITIVE BODY FORM
are effected at the level of the notochord and lower margin of the somitic
area from the anterior trunk region caudally into the tail. As a result, a
transverse section of the early vertebrate body appears pyriform or pear
shaped, with the neck of the pear directed dorsally (fig. 241C). The con-
striction line is shown typically in the developing embryo of Necturus (fig.
227) where it extends from the lower aspect of the head outgrowth along the
lower boundary of the somitic area to the base of the tail. A line, drawn
across the body from the general area of the two lateral constrictions and
passing through the notochord, divides the embryonic body into an upper or
epaxial (epiaxial) region above the level of the notochord and a lower or
hypaxial (hypoaxial) region below the level of the notochord.
E. Tubulation of the Neural, Epidermal, Entodermal, and Mesodermal,
Organ-forming Areas in Amphioxus
1. Comparison of the Problems of Tubulation in the Embryo
OF Amphioxus with that of the Embryos in the
SuBPHYLUM Vertebrata
a. End-bud Growth
In Amphioxus, the procedures involved in tubulation of the major organ-
forming areas and development of primitive body form differ from those in
the vertebrate group. For example, in the latter group, the basic rudiments of
the head, pharyngeal, trunk, and tail regions appear to be well established
at the end of gastrulation. During tubulation of the major organ-forming areas,
these subregions become extended in an antero-posterior direction and the
rudiments of specific structures begin to express themselves. This is especially
true of the head, pharyngeal, and trunk regions. The vertebrate tail, however,
arises from an end-bud tissue which progressively lays down the various parts
of the tail by means of a proliferative growth in the caudal direction. On the
other hand, in Amphioxus, only a small portion of the anterior end of
the future body is laid down during gastrulation. Further development of the
epidermal, neural, enteric, and mesodermal cellular areas together with the
notochord are dependent upon cell proliferation at the caudal end of the late
gastrula and later embryo. Much of the body of Amphioxus, therefore, is
formed by a caudal proliferative growth of end-bud cells, somewhat com-
parable to the end-bud growth of the tail in the vertebrate group.
b. Position Occupied by the Notochord and Mesoderm at the End
of Gastrulation
A second feature of difference in the developing embryo of Amphioxus
from that of the vertebrate embryo lies in the arrangement of the notochord-
mesoderm complex of cells in the late gastrula. In the late gastrula of
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS 495
Amphioxus, this potential, third germ layer forms a part of the entodermal
roof, although the studies of Conklin ('32) have demonstrated that notochord
and mesoderm are distinct cellular entities even in the blastula. In contrast
to this condition, the notochord and the mesoderm already are segregated as
a middle germ layer between the ectoderm and the entoderm in the late
vertebrate gastrula. The gastrula of Amphioxus, therefore, has the added
problem of segregating the notochordal and mesodermal cells from the ento-
derm during tubulation of the major organ-forming areas.
2. Neuralization and the Closure of the Blastopore
In the late gastrula of Amphioxus, a longitudinal middorsal plate of cells,
the neural plate, elaborated by cell division and extension during gastrulation,
represents the future central nervous system (fig. 247E). As the period of
gastrulation comes to its end, the blastopore decreases greatly in size (fig.
247A-D). The archenteric opening also moves dorsally, coincident with a
shifting of the caudal end of the archenteron in such a way that it projects
in a dorso-caudal direction (figs. 1 89G, H; 247H). This movement of the
archenteron is associated with the migration of the mass of mesodermal cells
from the two lateral areas of the blastoporal lips (fig. 247A, B) to the dorso-
medial portion of the blastopore (fig. 247C), where the mesoderm comes to
lie on either side of the notochord below the neural plate (fig. 247C). As
these changes occur, the dorsal area of the gastrula near the blastopore be-
comes flattened with a subsequent depression of the neural plate (fig. 247C,
D). In sagittal section, the gastrula now appears oval in shape and consid-
erably elongated in the antero-posterior direction (fig. 189G, H); in trans-
verse view, it is triangular, especially at the caudal end (fig. 247D).
As the above changes are brought about, the ectoderm of the ventral lip
of the blastopore grows dorsad, while that of the lateral lips grows mediad.
In this way, the opening of the blastopore is closed by the coming together
and fusion of these ectodermal (epidermal) growths (fig. 247D-F). How-
ever, the archenteron does not lose its connection, at this time, with the
outside environment of the embryo for two reasons:
( 1 ) As observed above, the caudal end of the archenteron previously had
shifted in such a manner that it now projects dorso-caudally; and
(2) synchronized with the epidermal growth which closes the blastoporal
opening (fig. 248A), the neural plate sinks downward, becoming de-
tached along its margin from the epidermal area (fig. 248B-D).
The downward sinking of the neural plate and its detachment from the
epidermal layer begins at the dorsal lip of the blastopore and spreads anteriad.
(Compare fig. 248D with 248B and C.) Consequently, as the epidermal growth
along the lateral lips of the blastopore reaches the area of the sinking neural
EXTRA - EMBRYONIC TISSUE_
FOREGUT-f- I
ANTERIOR
NTESTINAL
5 PORTAL
MIDGUT-
NOTOCHORD -
NEURAL-
5 ECTODERM !|
■ 'F
II POSTERIOR it
INTESTINAL
PORTAL
A —^. ^ '.V
ANTERIOR NEUROPORE
CAUDAL FOLD OF AMNION
ANTERIOR NEUROPORE
Fig. 242. Early development of the pig embryo (B, C, and G from Patten: Embryology
of the Pig, Philadelphia, Blakiston; A is from Streeter: Carnegie Inst. Publ. No. 380,
Contrib.' to Embryol. 100; D, E, and F from Heuser and Streeter: Carnegie Inst. Publ.
No. 394, Contrib. to Embryol. 109. All figures have been modified). (A) Early, neural
groove stage. Neural area is shown in black; amnion is cut away as indicated. (B)
Four-somite stage. (C) Median sagittal section, approximating the stage of develop-
ment shown in (B). Observe foregut, midgut, and hindgut areas. (D) Embryo of about
six pairs of somites. (E) Embryo of about 7 to 8 pairs of somites. (F) Eighteen
pairs of somites. (G) Sagittal sectional diagram of embryo slightly younger than (F),
showing neural and gut tubes, amnion, allantois, and forming heart.
496
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS
497
plate in the region of the dorsal blastoporal lip, it continues forward along the
epidermal margins of the insinking neural plate, growing mesad and fusing in
the midline over the neural plate (fig. 247E-G). In this way, the epidermal
growth forms a covering for the neural plate. It follows, therefore, that the pos-
terior end of the archenteron will now open into the space between the neural
plate and its epidermal covering. This new passageway between the epidermal-
neural plate cavity and the archenteron is the beginning of the neurenteric
canal (figs. 247H; 248A).
The flattened neural plate, canopied by the epidermal overgrowth, then
begins to fold itself into the form of a tube. In doing so, its lateral edges
swing gradually toward the middorsal line, as shown in figure 195. The actual
grooving and tubulation of the neural plate starts at a point about midway
along the embryo at the stage of development shown in figure 247F. It pro-
ceeds anteriorly and posteriorly from this point. At its extreme anterior end,
the neural tube remains open to the surface as the anterior neuropore (figs.
247H; 249A-D). Eventually the caudal end of the neural plate becomes
tubulated, and a definite canal is formed, connecting neural and enteric tubes.
This canal is the neurenteric canal. The neurenteric canal disappears between
the stage of development shown in figure 249C and that shown in figure 249D.
The continued caudal growth of the neural tube is accomplished by cell
proliferation from the posterior end of the tube and neurenteric canal area.
MESENCHYME OF HEAD
NEUR4L GROOVE
NEURAL GROOVE
L
Fig. 243. Sections of pig embryo of about stage shown in fig. 242 (B) and (C).
(Modified from Patten: Embryology of the Pig. 3d Ed., Philadelphia, Blakiston, '48.)
(A) Line 1, fig. 242C. (B) Line 2, fig. 242C. (C) Line 3, fig. 242C. (D) Line 4,
fig. 242C.
498
DEVELOPMENT OF PRIMITIVE BODY FORM
MAXILLARY PROCESS
Fig. 244. Development of body form in the pig embryo. (A and B from Keibel:
Normentafel zur Entwicklungsgeschichte des Schweines (Sus scrofa domesticus). 1897.
Jena, G. Fischer. C, D, and E slightly modified from Keibel, previous reference, and from
Minot: A Laboratory Text-book of Embryology. 1903. Philadelphia, P. Blakiston's Son &
Co.) (A) About 4 to 5 mm. (B) About 6 mm. (C) Ten mm. (crown-rump meas-
urement). (D) Fifteen mm. (E) Twenty mm.
3. Epidermal Tubulation
After the neural plate sinks downward and becomes separated from the
outside epidermis, the medial growth of the epidermis over the neural plate
completes the middorsal area of the primitive epidermal tube (fig. 247E-H).
It then comes to enclose the entire complex of growing and elongating neural,
UNCLOSED PORTION OF
NEURAL TUBE
CORDIS
PERI CARDIAL
C A V I T '
AMNION
EPIMYOCARDIU
sagittal section of model. ^^' '^^"^ ^^) Dorsal view. (B) Median
499
500
DEVELOPMENT OF PRIMITIVE BODY FORM
ANTERIOR NEUROPORE MANDIBULAR CONTRIBUTION TO FIRST BRANCHIAL GROOVE
^"■''E*"^*'- EAR (EXTERNAL AUDITORY MEATUS)
HYOI D
TRIBUTIONS TO
EXTERNAL EAR
DIFFE RENTI ATING
SOMITES
Fig. 246. Development of body form in human embryo. (C from Keibel and Mall:
Manual of Human Embryology, Vol. I, 1910. Philadelphia and London, Lippincott.
A, B, D, and E from Keibel and Elze: Normentafel zur Entwicklungsgeschkhte des
Menschen. Jena, 1908. G. Fischer.) (A) Early neural fold stage. Somites are beginning
to form; notochordal canal is evident. (B) About nine pairs of somites. (C) His's
embryo M. (D) About 23 pairs of somites, 4-5 mm. long. (E) About 35 pairs of
trunk somites, 12 mm. long.
mesodermal, and entodermal tubes and with them it continues to grow in
length principally by rapid cell proliferation at the caudal end of the embryo.
4. TUBULATION OF THE EnTODERMAL ArEA
The primitive metenteron of Amphioxus is derived from the archenteron
of the late gastrula as follows.
a. Segregation of the Entoderm from the Chordamesoderm and the
Formation of the Primitive Metenteric Tube
The mesoderm and notochord which occupy the roof of the archenteron
of the gastrula evaginate dorsally at the anterior end of the embryo and, thus,
become separated from the entoderm. (Compare fig. 195 with fig. 250A.)
This separation of notochord and mesoderm by dorsal evagination from the
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS
501
entoderm continues slowly in a caudal direction from the anterior end until
an embryonic condition is reached approximating about 13 to 14 pairs of
mesodermal segments. At this level, the notochord and mesoderm become
completely separated from the entoderm. As a result, the enteric or gut tube
from this point in its growth posteriad is a separate entity. (See tubulation
of mesoderm on p. 505. Anterior to the fourteenth somite, after the notochord
and mesoderm separate from the entoderm, the latter grows medially from
either side to complete the entodermal roof below the evaginated notochord
and mesoderm (fig. 250A). A primitive metenteric tube thus is formed, as
shown in figure 249C, whose only opening is that which leads by way of the
neurenteric canal (fig. 249A, C) into the neurocoel of the neural tube and
from thence to the outside through the anterior neuropore.
NEURAL PLATE CELLS
NEURAL PLATE
EPIDERMAL OVERGROWTH OF NEUR
PLATE BEGINS AT LATERAL
BLASTOPORAL LIPS AND GROWS
ANTERIAD AND MEDIAD CLOSING
BLASTOPO R E
FORMING NEURENTERIC CANAL
Fig. 247. Closure of the blastopore and epidermal overgrowth of neural plate in
Amphioxus (original diagrams, based on data supplied by Conklin, '32). (A) Vegetal
pole view of early stage of gastrulation, showing general areas occupied by notochordal,
entodermal, and mesodermal cells. (B) Same view of gastrula, one hour later, showing
triangular form of blastopore. (C) Posterior view of late gastrula. Blastopore is now
ovoid in shape and dorsally placed. Gastrula is triangular in transverse section with
dorsal surface flattened. (D) Same view, later. Slight epidermal upgrowth, indicated
by arrows (a and a') merges with ingrowing epidermal edges along lateral lips of
blastopore (b and b') which spreads along epidermal edges of neural plate. (E) Dorsal
view a brief period later than (D). Epidermal ingrowth from lateral blastoporal lips is
now closing the blastoporal opening, shown in broken lines, and also is proceeding
craniad along edges of sinking neural plate. (See fig. 248.) (F, G) Later stages of
epidermal overgrowth of neural plate. (H) Sagittal section of (G).
502
DEVELOPMENT OF PRIMITIVE BODY FORM
GASTROCOEL
NFURAL ECTODERM— Jl.^S:^ih''£
(NEURAL PLATE)
NOTOCHORD
MESODERM
Fig. 248. Sinking of neural plate and epidermal overgrowth of neural plate in Ainphi-
oxus. (Slightly modified from Conklin, '32.) (A) Sagittal section of embryo comparable
to that shown in fig. 247F. (B, C, D) Sections through embryo as shown by lines B,
C, D, respectively, on (A). Observe that the neural plate begins to sink downward from
region of closed blastopore and proceeds forward from this point.
b. Formation of the Mouth, Anus, and Other Specialized Structures
of the Metenteron
At the anterior end of the metenteron, a broad, dorsal outgrowth occurs
which continues up on either side of the notochord and becomes divided into
right and left dorsal diverticula (fig. 249B, H). The left diverticulum remains
small and thick-walled and later fuses with an ectodermal invagination to
form the pre-oral pit, described as a sense organ. The right diverticulum,
however, increases greatly in size, becomes thin-walled, and gives origin to
the so-called head cavity.
The mouth develops at a time when the larva acquires about 16 to 18 pairs
of mesodermal segments or somites (fig. 249D). It appears when the over-
lying epidermis about halfway up on the left side of the body fuses with the
entoderm, a fusion which occurs just posterior to the forming pre-oral pit
(left diverticulum). (See black oval fig. 249D, and fig. 249F.)
At the time that the mouth forms, the entoderm opposite the first pair of
somites pushes ventrally and fuses with the ectoderm. This area of fusion
finally perforates and forms the first gill slit. The gill slit, once formed, moves
up on the right side of the body (fig. 249E). The entodermal area from
which the first and later gill slits make their appearance is known as the
branchial rudiment (fig. 249D).
At the caudal end of the larva, following the degeneration of the neuren-
teric canal, a small area of entoderm fuses with the ectoderm and forms the
anal opening. The anus is first ventral in position, but later moves up to the
left side as the caudal fin develops (fig. 249E, G).
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS
503
5. TUBULATION OF THE MESODERM
Tubulation of the mesoderm and the formation of a continuous antero-
posterior coelom in Amphioxus differs considerably from that found in the
subphylum Vertebrata. This fact becomes evident in tracing the history of
the mesoderm from the time of its segregation from the entoderm of the late
DORSAL D I V E R T I C UlA)M^«B=ffl^ffl-H''l°W°M--l-r|-|-'i;
Fig. 249. Various stages of development of Amphioxus. (A from Kellicott, '13, and
Conklin, '32; B from Kellicott, '13, slightly modified; C-I, slightly modified from Conklin,
'32.) (A) Six-somite stage, comparable to fig. 247G and H. The animal hatches about
the time that two pairs of somites are present. (B) Nine-somite stage. The larva at this
stage swims by means of cilia which clothe the entire ectodermal surface. (C) About
fourteen pairs of somites are present at this stage. Neurenteric canal is still patent. (D)
About 16 to 18 pairs of somites. Neurenteric canal is degenerating; mouth is formed.
(E) About 20 to 22 pairs of somites. Anal opening is established between this stage and
that shown in (D). (F) Trr.nsverse section, showing oral opening, looking from anterior
end of animal. (G) Same through anal area. (H) Frontal section of a 24-hour larva
near dorsal side showing notochord, somites (S-1, S-8, etc.) and undifferentiated tissue
at caudal end. Neural tube shown at anterior end. Nine pairs of somites are present. (I)
Frontal section of a 38-hour larva at the level of the notochord showing section through
the neural tube at the anterior and posterior ends, i.e., in region where larva bends
ventralwards. Thirteen pairs of somites are present with muscle fibrillae along the mesial
borders of the somites.
504
DEVELOPMENT OF PRIMITIVE BODY FORM
gastrula and later embryo to the stage where a continuous antero-posterior
coelomic space is formed, comparable to that found in the vertebrates.
The mesoderm of the late gastrula of Amphioxus is present as a dorso-
median band of cells on either side of the notochord, and together with the
notochord, occupies the dorsal area or roof of the archenteron as mentioned
previously. In the region of the blastopore, the two mesodermal bands diverge
ventrally and occupy the inner aspect of the lateral walls of the blastopore
(fig. 190F, G; 247B). At about the time of blastoporal closure, the two
mesodermal masses of cells, located along the lateral lips of the blastopore,
are retracted dorsally, where they come to lie on either side of the notochord
(fig. 247C). In this position the two bands of mesoderm and the notochord
continue to form the dorsal region or roof of the archenteron until approxi-
mately the time when the embryo is composed of 13 to 14 pairs of meso-
dermal segments or somites (fig. 249C). (See Hatschek, 1893, pp. '31, 132;
Willey, 1894, p. 115; Conklin, '32, p. 106.) When the embryo reaches a
stage of development wherein 15 to 16 pairs of somites are present, the
notochord and mesoderm have separated entirely from the entoderm (fig.
249D). At about this period the neurenteric canal between the metenteron
and the neural tube disappears (fig. 249C, D).
ENTEROCOEU
MESODERM
NCHNOCOEL
SPLANCHNOCOELS FUSE HORIZONTAL SEPTUM
Fig. 250. Differentiation of somites in Amphioxus. (A and B from Conklin, '32; C,
E, and F after Hatschek, 1888 and 1893; D from MacBride, 1898; all figures are modi-
fied.) (A) Somites shortly after separation from entoderm. (B) Later stage, the
somites grow ventrally. (C) Somitic wall begins to differentiate into a thickened, dorsal,
myotomic area, located near notochord and neural tube, and thinner somatic and visceral
areas. (D) Horizontal septum formed which separates dorso-myotomic portion of somite
from splanchnocoelic area below. (E, F) Later stages in differentiation of myotome and
myocoelic diverticulum. (See text.)
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS 505
The formation of a continuous, antero-posterior, coelomic cavity in Am-
phioxus may be described as follows. The mesodermal bands on either side
of the notochord of the post-gastrular embryo become converted into meso-
dermal grooves as each mesodermal band folds inwards or evaginates into
the residual blastocoelic space between the archenteron and the outside ecto-
derm (fig. 195). Beginning at the anterior end, these longitudinal grooves
of mesoderm soon become divided into distinct segments or somites by the
appearance of transverse divisions (fig. 249 A, B, H). The first and second
pairs of somites are formed at the anterior ends of the mesodermal grooves
at about the time that the embryo hatches and swims about by means of
ciliary action.
Eventually each somite becomes entirely constricted from the notochord
and entoderm. In this segregated condition the somite forms a rounded struc-
ture retaining within itself a portion of the original archenteric cavity (fig.
250A). Hence, the cavity within the somite is called an enterocoel and repre-
sents the beginnings of the coelomic cavity of later development, at least in
the anterior 13 or 14 pairs of somites. {Note: It is to be observed in this
connection that the primitive somite in Amphioxus is not comparable to the
primitive somite of the vertebrate embryo. In the latter, the somite represents
merely a segment of the epimeric mesoderm, whereas in Amphioxus it is the
entire mesoderm in each half of a particular segment of the embryo.)
After hatching, the mesodermal bands continue to form into grooves as the
embryo elongates, and, synchronously, successive pairs of somites are formed.
At about the time 8 to 10 pairs of somites are present (fig. 249B, H), the
enterocoels of the first two pairs of somites have become entirely separated
from the archenteron. The enterocoels of the following six pairs of somites
are small and are not as evident at first as those of the first two pairs. Ulti-
mately a definite enterocoel is found, however, in each somite.
Posterior to the eighth or ninth pairs of somites, the forming mesodermal
grooves do not show the enterocoelic pouches as plainly as the more anterior
somites. Slit-like mesodermal grooves tend to be present, however, and, when
the somite is entirely free from the archenteron, this slit-like cavity expands
into the enterocoelic space of the somite. As the region of the fourteenth
pair of somites is approached, the slit-like mesodermal groove becomes more
and more indefinite. Posterior to the fourteenth or fifteenth pair of somites,
the somites originate from a solid mesodermal band on either side of the
notochord. An enterocoelic origin of the cavity within each somite, therefore,
is not possible caudal to this area, and the coelomic space arises by a
hollowing-out process similar to coelomic cavity formation in the vertebrate
group.
At about the time when eight pairs of somites are established, a shift of
506 DEVELOPMENT OF PRIMITIVE BODY FORM
the mesoderm on either side of the embryo produces a condition wherein the
somites of either side may be slightly intersegmental in relation to the somites
on the other side (fig. 249H).
During its later development, each somite grows ventrally (fig. 250B).
That portion of the somite contiguous to the notochord and neural tube
thickens and forms the myotome. The region of the somite near the epidermal
ectoderm is called the somatic or parietal mesoderm, while that associated
with the entoderm forms the visceral or splanchnic mesoderm (fig. 250C).
As the myotome enlarges, the coelomic space becomes more and more
displaced ventrally, and most of it comes to lie on either side of the enteron
(metenteron). (See fig. 250D.) This ventral coelomic space forms the
splanchnocoel, while the dorsal space, lateral to the myotome, is known as
the myocoel. Eventually, the splanchnocoels of each pair of somites push
ventrally to the lower portion of the enteron, where they ultimately fuse
(fig. 250D-F). Gradually the splanchnocoels of each segment fuse antero-
posteriorly and in this way a continuous, antero-posterior, splanchnocoelic
space below and around the gut tube is formed. Tubulation or the formation
of a continuous, antero-posterior, coelomic cavity thus is effected by fusion
of the splanchnocoels of the respective somites on either side (fig. 250F). A
horizontal septum, the intercoelomic membrane also appears, separating the
myocoels above from the splanchnocoelic cavity below (fig. 250D).
6. Later Differentiation of the Myotomic (Dorsal) Area
OF the Somite
While the above events are taking place in the ventral portion of the somite,
the upper, myotomic region undergoes profound modification.
As shown in figure 250D, the myotomic portion of the somite has two un-
equally developed areas:
( 1 ) a medial muscular portion, the myotome and
(2) the laterally placed, thin-walled, parietal part which surrounds the
coelomic space, or myocoel.
The muscular portion enlarges rapidly and, as seen in figure 250E and F,
forms the muscle plate or myotome of the adult. These muscle plates very
early assume the typical > shape characteristic of the adult. On the other
hand, the myocoelic portion contributes important connective or skeletal
tissue to the framework of the body. In each segment, the wall of the myocoel
gives origin to three diverticula as follows:
(a) a lower sclerotomic diverticulum,
(b) a ventral diverticulum, and
(c) a dorsal sclerotomic diverticulum.
TUBULATION OF ORGAN-FORMING AREAS IN AMPHIOXUS 507
The lower sclerotomic diverticulum (fig. 25 OD, E) extends up between the
myotome and the medially placed notochord and nerve cord, as diagrammed
in figure 250F. Its walls differentiate into two parts:
( 1 ) an inner layer which, together with a similar contribution from the
somite on the opposite side, wraps around the notochord and nerve
cord and, subsequently, gives origin to a skeletogenous sheath of
connective tissue which enswathes these structures; and
(2) an outer layer which covers the mesial (inner) aspect of the myotome
with a fascia or connective tissue covering.
The outer surface of the myotome does not have a covering of fascia.
The ventral diverticulum extends between the lateral wall of the splanchno-
coel and the epidermal layer of the body wall (fig. 250E, F) and separates
the parietal wall of the splanchnocoel from the epidermal wall (fig. 250F).
This ventral diverticulum or dermatomic fold, together with the external or
parietal wall of the myocoel above, forms the dermatome. The inner and
outer layers of the ventral diverticulum gradually fuse to form the cutis or
dermal layer of the integument or skin in the ventro-lateral portion of the
body, whereas the parietal wall of the myocoel above gives origin to the same
dermal layer in the body region lateral to the myotome. The dorsal sclerotomic
diverticula form the fin-ray cavities in the dorsal fin. These cavities become
entirely isolated from the rest of the myocoelic spaces. Several fin-ray cavities
occupy the breadth of a single myotome. The dorsal myotomic portion of the
somite thus differentiates into three main structural parts:
(a) the muscular myotome,
(b) the mesial sclerotome or skeletogenous tissue, and
(c) the latero-ventral dermatome or dermal tissue of the skin.
7. Notochord
The notochord arises as a middorsal evagination of the primitive archen-
teron up to about the stage of about 13 to 14 pairs of somites (fig. 195).
Posterior to this region it takes its origin by proliferative growth from a sep-
arate mass of notochordal tissue, lying above the gut and between the two
mesodermal masses of cells. Its origin posterior to the general area of the
thirteenth to fourteenth body segments, therefore, has no relation to the ento-
derm. It rapidly develops into a conspicuous skeletal rod, lying below the
neural tube and between the mesodermal somites and resting in a slight de-
pression along the dorsal aspect of the metenteron or entodermal tubulation
(fig. 249E, H). It continues forward in the head region, anterior to the brain
portion of the neural tube (fig. 249E).
(The student is referred to the following references for further details rela-
tive to the early development of Amphioxus: Cerfontaine, '06; Conklin, '32;
508 DEVELOPMENT OF PRIMITIVE BODY FORM
Hatschek, 1893; Kellicott, '13; MacBride, 1898, '00, '10; Morgan and Hazen,
'00; and Willey, 1894.)
F. Early Development of the Rudiments of Vertebrate Paired
Appendages
Two pairs of appendages, placed at the anterior (pectoral) and posterior
(pelvic) extremities of the trunk, are common to all vertebrate groups. How-
ever, all vertebrates do not possess two pairs of paired appendages. Certain
lizards of the genera Pygopus and Pseudopus have only a posterior pair of
appendages, while in certain other vertebrates the opposite condition is found,
the anterior pair being present without posterior appendages. The latter con-
dition is found in certain teleost and ganoid fishes; the amphibian, Siren
lacertina; the lizard, Chirotes; and among the mammals, the Sirenia and Ce-
tacea. Again, some vertebrates are entirely apodal, e.g., cyclostomatous fishes
and most snakes, although the boa constrictors and pythons possess a pair of
rudimentary posterior appendages embedded in the skin and body wall. Some
have rudimentary appendages only in the embryo, as the legless amphibians
of the order Gymnophiona, and certain lizards. Consequently, the presence
of embryonic rudiments of the paired appendages is a variable feature when
the entire group of vertebrates is considered.
The rudiments of the paired appendages also are variable, relative to the
time of their appearance in the vertebrate group as a whole. They are more
constant in the Amniota, i.e., reptiles, birds, and mammals, in time of ap-
pearance than in the Anamniota, i.e., fishes and amphibia. In the reptiles,
birds, and mammals, the limb buds arise when primitive body form is being
evolved. In the anuran amphibia, the anterior rudiments may appear and go
on to a high degree of differentiation before the appearance of the posterior
pair of appendages. For example, in the frog, Rana pipiens, the posterior limb
buds first make their appearance a brief period before the beginning of
metamorphosis of the tadpole into the adult form. However, the anterior
limb buds differentiate earlier but remain concealed beneath the operculum
until they become visible during the later stages of metamorphosis. In urodele
amphibia, the fore limb bud is not covered by an operculum, and it is visible
at the time of its initial appearance which occurs before the hind limb rudiment
arises (fig. 227J-L).
In the majority of vertebrates, the hmb rudiment first makes its appearance
as an elongated, dorso-ventrally flattened fold of the epidermis, containing a
mass of mesodermal cells within, as shown, for example, in the chick and
mammalian embryos (figs. 240, 244, 246). The contained mesodermal cells
may be in the form of epitheHal muscle buds derived directly from the myo-
tomes (e.g., sharks) or as a mass of mesenchyme (chick, pig, human). (See
Chap. 16.) The early limb-bud fold may be greatly exaggerated in certain
elasmobranch fishes, as in the rays, where the anterior and posterior fin folds
LIMB BUD AN ILLUSTRATION OF FIELD CONCEPT OF DEVELOPMENT 509
fuse together for a time, forming one continuous lateral body fold. On the
other hand, in the lungfishes (the Dipnoi) and in amphibia (the Anura and
Urodela), the appendage makes its first appearance, not as an elongated fold
of the lateral body wall, flattened dorso-ventrally, but as a rounded, knob-like
projection of the lateral body surface (fig. 227K-M).
G. The Limb Bud as an Illustration of the Field Concept of Development
in Relation to the Gastrula and the Tubulated Embryo
In Chapter 9 it was observed that the major presumptive organ-forming
areas are subdivided into many local, organ-forming areas at the end of gas-
trulation. In the neural and epidermal areas, this subdivision occurs during
gastrulation through influences associated with local inductive action. At the
end of the gastrular period, therefore, each local area within the major organ-
forming area possesses the tendency to give origin to a specific organ or a
part of an organ. The restricted, localized areas within each major organ-
forming area represent the individual, or specific, organ-forming fields. Dur-
ing tubulation, the major organ-forming areas with their individuated,
organ-forming fields are molded into tubes, and, thus, the individual fields
become arranged along each tube. Consequently, each tube possesses a series
of individual, organ-forming areas or fields, distributed antero-posteriorly
along the tube.
As a result of the close association of cells and substances during gastrula-
tion and tubulation, many specific organ-forming fields are related to more
than one of the body tubes. Specific organ-forming fields, therefore, may have
intertubular relationships. For example, the lens field is located in the epi-
dermal tube, but, in many species, its origin as a lens field is dependent
upon influences emanating from the optic vesicle of the neural tube (see
Chap. 19). Another example of an association between the parts of two con-
tiguous tubes is the limb-bud field in the urodele, Amby stoma punctatum. As
the limb-bud field in this species illustrates various aspects and properties of
an organ-forming field, it will be described below in some detail*
The presumptive anterior limb disc or limb field of Ambystoma is deter-
mined as a specific limb-forming area in the middle gastrular stage (Detwiler,
'29, '33). Later on in the embryo, it occupies a circular-shaped area within
trunk segments three to six. According to Harrison ('18) and Swett ('23),
its properties as a field, mainly are resident in the cells of the somatic layer
of the mesoderm in this area. If, for example, the somatic layer of mesoderm
in this area is transplanted to another area, a well-developed limb will result.
Also, the mesoderm of the dorsal half of the field forms a greater part of the
limb than the other parts, with the anterior half of the limb disc next in im-
portance. It appears, therefore, that the limb-forming potencies are greatest
in the dorso-anterior half of the limb field and become less postero-ventrally.
Moreover, not "all of the cells which are potentially limb forming go into
510 DEVELOPMENT OF PRIMITIVE BODY FORM
the limb" (Swett, '23). As demonstrated by Harrison ('18) half discs (half
fields), left intact in the developing embryo or removed and transplanted to
other areas, develop into normal limbs.
The above experiments of Harrison, together with those of Detwiler ('29,
'33) suggest that while the limb field is irreversibly determined at an early
stage to form limb tissue, the exact determination of the various parts within
the field is absent at the earlier phases of development. One kind of precise
determination is present, however, for the first digit-radial aspect (i.e., the
pre-axial aspect) of the limb appears to arise only from the anterior end of
the field, whether the field is allowed to develop intact or is split into two
parts. That is, if it is split into two portions, the anterior extremity of the
posterior portion, as well as the original anterior part of the limb field, de-
velops the pre-axial aspect of the limb. This antero-posterior polarization is
present from the first period of field determination. On the other hand, the
dorso-ventral polarity is not so determined; for if the transplanted limb disc
is rotated 180 degrees (i.e., if it is removed and reimplanted in its normal
place dorsal side down) it will develop a limb with the dorsal side up but
with the antero-posterior axis reversed (Harrison. "21). In these cases the
first digit-radial aspect will appear ventral in position. This result indicates
that the pre-axial aspect of the limb becomes oriented always toward the ventral
aspect of the limb. However, the experiments of Swett ('37, '39, '41 ) tend to
show that the reversal of the dorso-ventral axis occurs only when implanted
below the myotomes; for when the rotated limb field is implanted in the
somitic (myotomic) area, it will remain inverted. Factors other than those
resident within the limb field itself, probably factors in the flank area, appear
thus to induce the normal dorso-ventral axis when the limb disc is implanted
in its normal site.
In the descriptions given above, the importance of the somatic layer of
mesoderm as the seat of the limb-forming factors is emphasized. It is obvious,
however, that the epidermal covering of the limbs derived from the epidermal
tubulation also is important in limb formation. For example, epidermal im-
portance is suggested by the experiments of Saunders ('49) on the developing
limb bud of the chick wherein it was found that the apical ridge of ectoderm,
located at the apex of the early limb bud, is essential for normal limb
development.
Individual, or specific, organ-forming fields which appear in the gastrula
and early tubulated embryo thus are generalized areas determined to form
individual organs. As development proceeds, two main limitations are im-
posed upon the field:
( 1 ) The cellular contribution of the field actually entering into the organ
becomes restricted; and
(2) specific parts of the field become progressively determined to form
specific parts of the organ.
CEPHALIC FLEXION AND GENERAL BODY BENDING 511
It is obvious, therefore, thai the fields of influence which govern the de-
velopment of specific organs may be much more extensive in cellular area
than the actual cellular contributions which take part in the formation of
the specific organ structures. Experiments on the forming limb of Ambystoma
also have demonstrated that a particular area of the field is stronger in its
limb-forming potencies than other regions of the field. This property probably
is true of other fields as well.
(For a detailed discussion of the field concept in embryonic development,
reference should be made to Huxley and DeBeer, "34, Chaps. 8 and 9; Weiss,
'39, p. 289 ff.)
H. Cephalic Flexion and General Rod\ Bending and Rotation in
\ ertebrate tmbr>os
The anterior end of the neural tubulation is prone to assume a bent or
flexed contour whereby the anterior end oi the neural tube is directed down-
ward toward the ventral aspect of the embryo. This general behavior pattern
is strong in vcrlcbrate embryos uiih the exception of the telcost fishes. In
teleost fishes this bending habit is slight. As the later development of the
head progresses in other vertebrate embryos, the neural tube shows a pro-
nounced cephalic (cranial) flexure in the region of the midbrain, in some
species more than in others. (See Chap. 19.) An additional bending occurs
in the posterior hindbrain area. The latter flexure is the cervical or nuchal
flexure (figs. 231. 238, 240, 244, 246).
Aside from the acute bending which takes place in the formation of the
cephalic and the nuchal flexures, there is a definite tendency for many verte-
brate embryos to undergo a general body bending, with the result that the
anterior part of the body and the caudal portion of the trunk and tail may
be depressed in a ventral direction (figs. 222C-E; 227; 229F; 238; 240;
244; 246). In the frog embryo, at hatching, the opposite tendency may
prevail for a brief period, and the dorsal trunk region may appear sagging
or hollowed (fig. 226A, C).
In addition to these bending movements, in the embryos of higher verte-
brates, a rotation or twisting (torsion) of the developing body along the
antero-posterior axis is evident. In the chick embryo, for example, the head
region begins to rotate toward the right at about 38 hours of incubation.
Gradually this torsion continues caudally (figs. 237, 238, 239, 260). At
about 70 to 75 hours, the rotational movement reaches the tail region, and
the embryo then comes to lie on its left side throughout its length (fig. 240).
In exceptional embryos, the rotational movement is toward the left, and the
embryo comes to lie on its right side. Similar movements occur in the pig
and other mammals.
This rotational movement is advantageous, particularly in long-bodied
Amnioia, such as the snakes, where it permits the developing embryo to coil
512 DEVELOPMENT OF PRIMITIVE BODY FORM
in spiral form within the extra-embryonic membranes. The coiUng tendency,
however, is not alone confined to the snake group, for the habits of general
body bending, referred to above, essentially is a coiling tendency. Viewed
thus, the rotation or torsion of the developing body along its median axis is
a generalized behavior pattern which permits and aids the coiling habit so
prevalent among the embryos of higher vertebrates. It may be observed further
that the coiling behavior is a common attitude during rest not only among
snakes but also among the adults of many higher vertebrates.
I. Influences Which Play a Part in Tubulation and Organization of
Body Form
In Chapter 9, it was pointed out that the pre-chordal plate material,
that is, organizer material which invaginates first during gastrulation and
which comes to lie in the future head region, induces the organization of
certain head structures and itself may form a part of the pharyngeal wall
and give origin to head mesoderm, etc. On the other hand, the trunk-organizer
material (notochord and somitic mesoderm) which moves to the inside, fol-
lowing the pre-chordal plate material, organizes the trunk region. The follow-
ing series of experiments based upon work by Spemann, '31, sets forth the
inductive properties of these two cellular areas:
Experiment
1. Head-organizer material, taken from one embryo and placed at head level of a
host embryo, will induce a secondary head, having eyes and ear vesicles
2. Head-organizer material, transplanted to trunk and tail levels in host embryos,
induces a complete secondary embryo, including head
3. Trunk-organizer material (i.e., notochord and somitic mesoderm), placed at head
level in host embryo, induces a complete secondary embryo, including the head
structures
4. Trunk-organizer material, placed at future trunk or tail levels in host embryos,
induces trunk and tail structures only
The many influences which play a part in the organization of the vertebrate
head and body constitute an involved and an unsolved problem. The extreme
difficulty of this general problem has long been recognized. (See Kingsbury
and Adelmann, '24. ) The above-mentioned work of Spemann represents a
beginning attempt to analyze this aspect of development and to understand
the factors involved. It demonstrates that the organization of the neural tube
and other axial areas is dependent upon specific cellular areas which migrate
inward during gastrulation. However, this is but one aspect of the problem.
As observed in the series of experiments above, trunk-organizer material
is able to organize a complete secondary embryo, including the head, when
TUBULATION AND ORGANIZATION OF BODY FORM
513
Fig. 251. Dependency of neural tube formation upon surrounding tissues. (A) Effect
of notochord without myotomes. (B) Effect of myotomes without notochord. (C)
Absence of notochord and myotomes.
placed at head level in the host but can only organize trunk and tail structures
when placed in trunk and tail areas of the host. In other words, there exists
a mutual relationship between the level of the host tissues and the transplanted
organizer material of the trunk organizer in effecting the formation of a head
at the head level.
Another forceful example of the interrelationship of developing parts and
formative expression of body structures is shown by the work of Holtfreter
('33) on the development of the neural tube. This work demonstrates that
the form of the neural tube is dependent upon influences in its environment,
as shown in figure 251. The presence of the later developing notochord de-
termines a thin ventral floor of the neural canal, whereas the contiguous
myotome determines a thick wall of the neural tube. Normally, in development,
the notochord lies below the neural tube, while the somites with their myotomic
parts come to lie lateral to the tube. That is to say, the normal bilateral sym-
metry of the neural tube is dependent upon the relationship, in their normal
positions, of the notochord and the myotomes.
The behavior of the developing neural tube, relative to the notochord and
the myotomes, demonstrates the importance of the migration of the somitic
mesoderm from a position contiguous and lateral to the notochord at the
beginning of neuralization to one which is lateral to the forming neural tube
as neuralization and differentiation of the neural tube progresses.
A further illustration of the probable influence of the notochordal area in
morphogenesis and organization of body form is the behavior of the develop-
ing metenteron or enteric tube. As observed previously, the gut tubulation
tends to invaginate or arch upward toward the notochord not only in embryos
developing from flattened gastrulae but also in amphibia. The movement of
the entoderm toward the notochord strikingly resembles the behavior of the
neural plate ectoderm during the formation of the neural tube. This com-
parison becomes more striking when one considers the manner of enteron for-
514 DEVELOPMENT OF PRIMITIVE BODY FORM
mation in the tail and hindgut regions in the shark embryo, Squalus acanthias,
already mentioned, p. 484. In this species the entoderm of the developing tail
actually invaginates dorsad and closes in a manner similar to the forming
neural tube. That is to say, in the developing tail of the shark, two invagina-
tions toward the notochord are evident, one from the dorsal side, which in-
volves the formation of the neural tube, and the other from the ventral side,
effecting the developing enteric tube.
The above facts suggest, therefore, that one of the main organizing influ-
ences at work during tubulation and primitive body formation emanates from
the pre-chordal plate area, the notochord, and the epimeric portion of the
mesoderm. From this general area or center, a chain of acting and interacting
influences extends outward, one part acting upon another, to effect the forma-
tive expression of the various parts of the developing body.
J. Basic Similarity of Body-form Development in the Vertebrate Group
of Chordate Animals
In the earlier portion of this chapter, differences in the general procedures
concerned with tubulation and primitive body formation in round and flat-
tened gastrulae were emphasized. However, basically all vertebrate embryos
show the same tendency of the developing body to project itself upward and
forward in the head region, dorsally in the trunk area and dorso-posteriad in
the tail region. Literally, the embryonic body tends to lift itself up out of,
and above, the area which contains the yolk and extra-embryonic tissues.
This proneness to move upward and to protrude its developing head end
forward and its caudal end backward is shown beautifully in the development
of the embryos of the shark (figs. 229, 230), the mud puppy (fig. 227), the
chick (fig. 235C), and the pig (fig. 242). The embryo struggles to be free
from its bed of yolk and extra-embryonic tissue, as it were, and it reminds
one of the superb imagery employed by the poet, John Milton, in his im-
mortal poem, Paradise Lost, where he describes the development of the
lion thus:
The grassy clods now calv'd; now half appear'd
The tawny lion, pawing to get free
His hinder parts, then springs as broke from bonds,
And rampant shakes his brinded mane.
In summary, therefore, although it appears that rounded and flattened
gastrulae in the vertebrate group may have slightly different substrative con-
ditions from which to start, they all employ essentially similar processes in
effecting tubulation of the respective, major organ-forming areas and in the
development of primitive body form.
Bibliography
Cerfontaine, P. 1906, Recherches sur le
developpement de rAmphioxiis. Arch,
biol.. Paris. 22:229.
Conklin, E. G. 1932. The embryology of
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Dean, B. 1896. The early development of
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38:413.
Detwiler, S. R. 1929. Transplantation of
anterior limb mesoderm from Amblys-
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stage. J. Exper. Zool. 52:315.
. 1933, On the time of determina-
tion of the antero-posterior axis of the
forelimb in Amblystoma. J. Exper. Zool.
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Harrison, R. G. 1918. Experiments on the
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40:589.
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terpretation of the early ontogenetic proc-
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Patterson, J. T. 1907. The order of appear-
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Biol. Bull. 13:121.
Saunders, J. W. 1949. An analysis of the
role of the apical ridge of ectoderm in
the development of the limb bud in the
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tion et formation des feuillets chez Pe-
Iromvzon planeri. Arch, biol., Paris.
25:1.
Spemann, H. 1931. Uber den Anteil von
Implantat und Wirtskeim an der Orien-
tierung und Beschaffenheit der induzier-
ten Embryonalanlage. Arch. f. Entwick-
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Swett, F. H. 1923. The prospective signifi-
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1939. Further experiments upon
the establishment and the reversal of
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515
11
Basic Features or Vertebrate Morpno^enesis
A. Introduction
1. Purpose of this chapter
2. Definitions
a. Morphogenesis and related terms
b. Primitive, larval, and definitive body forms
1 ) Primitive body form
2) Larval body form
3) Definitive body form
3. Basic or fundamental tissues
B. Transformation of the primitive body tubes into the fundamental or basic condition
of the various organ systems present in the primitive embryonic body
1. Processes involved in basic system formation
2. Fundamental similarity of early organ systems
C. Laws of von Baer
D. Contributions of the mesoderm to primitive body formation and later development
1. Types of mesodermal cells
2. Origin of the mesoderm of the head region
a. Head mesoderm derived from the anterior region of the trunk
b. Head mesoderm derived from the pre-chordal plate
c. Head mesoderm contributed by neural crest material
d. Head mesoderm originating from post-otic somites
3. Origin of the mesoderm of the tail
4. Contributions of the trunk mesoderm to the developing body
a. Early differentiation of the somites or epimere
b. Early differentiation of the mesomere (nephrotome)
c. Early differentiation and derivatives of the hypomere
1) Contributions of the hypomere (lateral plate mesoderm) to the developing
pharyngeal area of the gut tube
2) Contributions of the hypomere (lateral plate mesoderm) to the formation
of the gut tube and heart structures
3) Contributions of the hypomere (lateral plate mesoderm) to the external
(ectodermal or epidermal) body tube
4) Contributions of the hypomere or lateral plate mesoderm to the dorsal body
areas
5) Contributions of the lateral plate mesoderm to the walls of the coelomic
cavity
5. Embryonic mesenchyme and its derivatives
516
INTRODUCTION 517
E. Summary of later derivatives of presumptive, major, organ-forming areas of the late
blastula and gastrula
1. Neural plate area (ectoderm)
2. Epidermal area (ectoderm)
3. Entodermal area
4. Notochordal area
5. Mesodermal areas
6. Germ-cell area
F. Metamerism
1. Fundamental metameric character of the trunk and tail regions of the vertebrate
body
2. Metamerism and the basic morphology of the vertebrate head
G. Basic homology of the vertebrate organ systems
1. Definition
2. Basic homology of vertebrate blastulae. gastrulae. and tubulated embryos
A. Introduction
1. Purpose of This Chapter
In this chapter, the basic morphogenetic features which give origin to the
later organ systems are emphasized. These features arise from the stream of
morphogenetic phenomena which come down from the fertihzed egg through
the periods of cleavage, biastulation, gastrulation, and tubulation. This chapter
thus serves to connect the developmental processes, outlined in Chapters 6
to 10, with those which follow in Chapters 12 to 21. As such, it emphasizes
certain definitions and basic structural features involved in the later morpho-
genetic activities which mold the adult body form.
2. Definitions
a. Morphogenesis and Related Terms
The word morphogenesis means the development of form or shape. It in-
volves the elaboration of structural relationships. The morphogenesis of a
particular shape and structure of a cell is called cytomorphosis or cytogenesis
and is synonymous with the term cellular differentiation, considered from the
structural aspect. In the Metazoa, the body is composed of groups of cells,
each cellular group possessing cells of similar form and function. That is, each
cell group is similarly differentiated and specialized. A cellular group, com-
posed of cells similar in form (structure) and function, is called a tissue.
Histology is the study of tissues, and the word histogenesis relates to that
phase of developmental morphology which deals with the genesis or develop-
ment of tissues. An organ is an anatomical structure, produced by an asso-
ciation of different tissues which fulfills one or several specialized functions.
For example, the esophagus, stomach, liver, etc., are organs of the body.
During development, each of the major organ-forming areas, delineated in
518 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
Chapters 6, 7, 9 and 10, produce several specific organs. Organogenesis is
concerned with the formation of these specific organs. A group of organs
which are associated together to execute one general function form an organ
system. The digestive system, for example, has for its general function that
of obtaining nourishment for the body. It is composed of a series of organs
integrated toward this end. The nervous system, similarly, is an assemblage
of specific organs devoted to the discharge of nervous functions. So it is with
the other systems of the organism. System development is concerned with
the genesis of such systems. The association of various systems, integrated
together for the maintenance of the body within a particular habitat, consti-
tutes the organism. Finally, the organism acquires a particular body form
because of the form, structure, and activities assumed by its organ systems
as a result of their adaptation to the functional necessities of the particular
habitat in which the organism lives. It should be urged further that this nice
relationship between form and structure, on the one hand, and functional
requirements, on the other, is a fundamental principle of development from
the egg to the adult. It is a principle intimately associated with the morpho-
genesis of the organ systems described in Chapters 12 to 21.
During development from the egg to the adult form, three major types of
body form are evolved in the majority of vertebrate species.
b. Primitive, Larval, and Definitive Body Forms (see fig. 255)
1) Primitive Body Form. The condition of primitive or generalized, em-
bryonic body form is attained when the embryo reaches a state in which its
developing organ systems resemble the respective developing organ systems
in other vertebrate embryos at the same general period of development. (See p.
520.) Superficially, therefore, the general structure of the primitive embryonic
body of one species resembles that of the primitive embryonic bodies of other
vertebrate species. Such comparable conditions of primitive, body-form devel-
opment are reached in the 10 to 15-mm. embryo of the shark, Squalus
acanthias, of the frog embryo at about 5 to 7 mm., the chick at about 55 to
96 hrs. of incubation, the pig at 6 to 10 mm., and the human at 6 to 10 mm.
2) Larval Body Form. Following primitive body form, the embryo grad-
ually transforms into a larval form. The larval form is present in the period
between primitive body form and definitive body form. The larval period is
that period during which the basic conditions of the various organ systems,
present in primitive body form, undergo a metamorphosis in assuming the
form and structure of the adult or definitive body form. In other words, during
the larval period, the basic or generalized conditions of the various organ
systems are changed into the adult form of the systems, and the larval period
thus represents a period of transition. Embryos which develop in the water
(most fishes, amphibia) tend to accentuate the larval condition, whereas
those which develop within the body of the mother (viviparous teleosts,
INTRODUCTION 519
sharks, mammals) or within well-protected egg membranes (turtle, chick)
slur over the larval condition.
The larval stage in non-viviparous fishes (see Kyle, '26, pp. 74-82) and in
the majority of amphibia is a highly differentiated condition in which the
organs of the body are adapted to a free-living, watery existence. The tadpole
of the frog, Rana pipiens, from the 6-mm. stage to the 11 -mm. stage, presents
a period during which the primitive embryonic condition, present at the time
of hatching (i.e., about 5 mm.), is transformed into a well-developed larval
stage capable of coping with the external environment. From this time on to
metamorphosis, the little tadpole possesses free-living larval features. Another
example of a well-developed, free-living, larval stage among vertebrates is
that of the eel, Anguilla rostrata. Spawning occurs in the ocean depths around
the West Indies and Bermuda. Following the early embryonic stage in which
primitive body form is attained, the young transforms into a form very unlike
the adult. This form is called the Leptocephalus. The Leptocephalus was for-
merly classified as a distinct species of pelagic fishes. After many months in
the larval stage, it transforms into the adult form of the eel. The latter migrates
into fresh-water streams, the American eel into streams east of the Rockies
and the European eel into the European streams (Kyle, '26, pp. 54-58). The
larval stages in most fishes conform more nearly to the adult form of the fish.
The embryo of Squalus acanthias at 20 to 35 mm. in length, the chick
embryo at 5 to 8 days of incubation, the pig embryo of 12- to 18-mm. length,
and the human embryo of 12 to 20-mm. length may be regarded as being
in the stage of larval transition. The young opossum, when it is born, is in a
late larval state. It gradually metamorphoses into the adult body form within
the marsupium of the mother (Chap. 22).
3) Definitive Body Form. The general form and appearance of the adult
constitute definitive body form. The young embryo of Squalus acanthias, at
about 40 mm. in length, assumes the general appearance of the adult shark;
the frog young, after metamorphosis, resembles the adult frog (Chap. 21),
the chick of 8 to 13 days of incubation begins to simulate the form of the
adult bird; the pig embryo of 20 to 35 mm. gradually takes on the body fea-
tures of a pig, and the human fetus, during the third month of pregnancy,
assumes the appearance of a human being. The transformation of the larval
form into the body form of the adult is discussed further in Chapter 21 in
relation to the endocrine system.
3. Basic or Fundamental Tissues
Through the stages of development to the period when the primitive or
generalized, embryonic body form is attained, most of the cells which take
part in development are closely associated. In the primitive embryonic body,
this condition is found in all the five primitive body tubes and in the noto-
chord. These closely arranged cells form the primitive epithelium. In the de-
520 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
veloping head and tail regions, however, mesoderm is present in the form of
loosely aggregated cells, known as mesenchyme. While the cells of the epi-
thelial variety are rounded or cuboidal in shape with little intercellular sub-
stance or space between the cells, mesenchymal cells tend to assume stellate
forms and to have a large amount of intercellular substance between them.
The primitive vascular or blood tubes are composed of epithelium in the sense
that the cells are closely arranged. However, as these cells are flattened and
show specific peculiarities of structure, this tissue is referred to as endothelium.
Also, while the cells of the early neural tube show the typical epithelial fea-
tures, they soon undergo marked changes characteristic of developing neural
tissue. The primitive or generalized, embryonic body thus is composed of
four fundamental tissues, viz., epithelial, mesenchymal, endothelial, and neural
tissues.
B. Transformation of the Primitive Body Tubes into the Fundamental
or Basic Condition of the Various Organ Systems
Present in the Primitive Embryonic Body
1. Processes Involved in Basic System Formation
As the primitive body tubes (epidermal, neural, enteric, and mesodermal)
are established, they are modified gradually to form the basis for the various
organ systems. While the notochordal axis is not in the form of a tube, it also
undergoes changes during this period. The morphological alterations, which
transform the primitive body tubes into the basic or fundamental structural
conditions of the systems, consist of the following:
(a) extension and growth of the body tubes,
(b) saccular outgrowths (evaginations) and ingrowths (invaginations)
from restricted areas of the tubes,
(c) cellular migrations away from the primitive tubes to other tubes and
to the spaces between the tubes, and
(d) unequal growth of different areas along the tubes.
As a result of these changes, the primitive neural, epidermal, enteric, and
mesodermal tubes, together with the capillaries or blood tubes and the noto-
chord, experience a state of gradual differentiation which is directed toward
the production of the particular adult system to be derived from these re-
spective basic structures. The primitive body tubes, the primitive blood capil-
laries, and the notochord thus come to form the basic morphological condi-
tions of the future organ systems. The basic structural conditions of the various
systems are described in Chapters 12 to 21.
2. Fundamental Similarity of Early Organ Systems
The general form and structure of each primitive embryonic system, as it
begins to develop in one vertebrate species, exhibits a striking resemblance
LAWS OF VON BAER 521
to the same system in other vertebrate species. This statement is particularly
true of the gnathostomous vertebrates (i.e., vertebrates with jaws). Conse-
quently, we may regard the initial generahzed stages of the embryonic or rudi-
mentary systems as fundamental or basic plans of the systems, morphologically
if not physiologically. The problem which confronts the embryo of each
species, once the basic conditions of the various systems have been established,
is to convert the generalized basic condition of each system into an adult
form which will enable that system to function to the advantage of the par-
ticular animal in the particular habitat in which it lives. The conversion of
the basic or primitive condition of the various systems into the adult form of
the systems constitutes the subject matter of Chapters 12 to 21.
The basic conditions of the various organ systems are shown in the structure
of shark embryos from 10 to 20 mm. in length, frog embryos of 5 to 10 mm.,
chick embryos from 55 to 96 hrs., pig embryos from 6 to 10 mm., crown-
rump length, and human embryos of lengths corresponding to 6 to 10 mm.
That is to say, the basic or generalized conditions of the organ systems are
present when primitive or generalized embryonic body form is developed.
It is impossible to segregate any particular length of embryo in the above-
mentioned series as the ideal or exact condition showing the basic condition
of the systems, as certain systems in one species progress faster than those
same systems in other species. However, a study of embryos of these desig-
nations serves to provide an understanding of the basic or fundamental con-
ditions of the various systems (figs. 257-262; also fig. 347A).
C. Laws of von Baer
As indicated above, the species of the vertebrate group as a whole tend to
follow strikingly similar (although not identical) plans of development during
blastulation, gastrulation, tubulation, the development of the basic plan of the
various systems and primitive body form. As observed in the chapters which
follow, the fundamental or basic plan of any particular, organ-forming system,
in the early embryo of one species, is comparable to the basic plan of that
system in other species throughout the vertebrate group. However, after these
basic parallelisms in early development are completed, divergences from the
basic plan begin to appear during the formation of the various organ systems
of a particular species.
The classical statements or laws of Karl Ernst von Baer (1792-1876) de-
scribe a tendency which appears to be inherent in the developmental procedure
of any large group of animals. This developmental tendency is for generalized
structural features to arise first, to be remodeled later and supplanted by fea-
tures specific for each individual species. To interpret these laws in terms of
the procedure principle mentioned in Chapter 7, it may be assumed that
general, or common, developmental procedures first are utilized, followed by
522 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
specific developmental procedures which change the generalized conditions
into specific conditions.
The laws of von Baer ( 1828-1837, Part I, p. 224) may be stated as follows:
(a) The general features of a large group of animals appear earlier in de-
velopment than do the special features;
(b) after the more general structures are established, less general structures
arise, and so on until the most special feature appears;
(c) each embryo of a given adult form of animal, instead of passing
through or resembling the adult forms of lower members of the group,
diverges from the adult forms, because
(d) the embryo of a higher animal species resembles only the embryo of
the lower animal species, not the adult form of the lower species.
D. Contributions of the Mesoderm to Primitive Body Formation and
Later Development
The mesoderm is most important to the developing architecture of the
body. Because the mesoderm enters so extensively into the structure of the
many organs of the developing embryo, it is well to point out further the
sources of mesoderm and to delineate the structures and parts arising from
this tissue.
1. Types of Mesodermal Cells
Most of the mesoderm of the early embryo exists in the form of epithelium
(see p. 519). As development proceeds, much of the mesoderm loses the close
arrangement characteristic of epithelium. In doing so, the cells separate and
assume a loose connection. They also may change their shapes, appearing
stellate, oval, or irregular, and may wander to distant parts of the body. This
loosely aggregated condition of mesoderm forms the primitive mesenchyme.
Though most of the mesoderm becomes transformed into mesenchyme, the
inner layer of cells of the original hypomeric portion of the mesodermal tubes
retains a flattened, cohesive pattern, described as mesothelium. Mesothelium
comes to line the various body cavities, for these cavities are derived directly
from the hypomeric areas of the mesodermal tubes (Chap. 20).
2. Origin of the Mesoderm of the Head Region
The primary cephalic outgrowth (Chap. 10), which later forms the head
structures, contains two basic regions, namely, the head proper and the
pharyngeal or branchial region. During its early development, the heart lies
at the ventro-caudal extremity of the general head region; it recedes gradually
backward as the head and branchial structures develop. The exact origin of
the mesoderm which comes to occupy the head proper and pharyngeal areas
varies in different gnathostomous vertebrates. The general sources of the head
mesoderm may be described in the following manner.
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION 523
a. Head Mesoderm Derived from the Anterior Region of the Trunk
The mesoderm of the branchial area in lower vertebrates, such as the sharks
and, to some degree, the amphibia, represents a direct anterior extension of
the mesoderm of the trunk (figs. 217D, E; 230D; 252E). It is divisible into two
parts: (1) a ventro-lateral region, the hypomeric or lateral plate mesoderm,
and (2) a dorsal or somitic portion. The latter represents a continuation into
the head region of the epimeric (somitic) mesoderm of the trunk. That por-
tion of the mesoderm of the branchial area which may be regarded specifically
as part of the mesoderm of the head proper is the mesoderm associated with
the mandibular and hyoid visceral arches, together with the hyoid and man-
dibular somites located at the upper or dorsal ends of the hyoid and man-
dibular visceral arches (fig. 217D, E).
In the higher vertebrates (reptiles, birds, and mammals), the mesoderm
of the branchial region appears early, not as a continuous epithelium, as in
the shark and amphibian embryo, but as a mass of mesenchyme which wan-
ders into the branchial area from the anterior portion of the developing trunk
region (figs. 217F; 233B; 234B). This mesenchyme assumes branchial region
characteristics, for it later condenses to form the mandibular, hyoid, and more
posteriorly located, visceral arches. Also, mesenchymal condensations appear
which correspond to the pre-otic head somites formed in the early shark
embryo. For example, in the chick, there is an abducent condensation, which
corresponds to the hyoid somite of the shark embryo, and a superior oblique
condensation corresponding probably to the mandibular somite of the shark
embryo (cf. fig. 217D, F). (See also Adelmann, '27, p. 42.) Both of these
condensations give origin to eye muscles (Chap. 16). Somewhat similar con-
densations of mesenchyme which form the rudiments of eye muscles occur in
other members of the higher vertebrate group.
b. Head Mesoderm Derived from the Pre-chordal Plate
The term pre-chordal plate mesoderm signifies that portion of the head
mesoderm which derives from the pre-chordal plate area located at the an-
terior end of the foregut. The pre-chordal plate mesoderm is associated closely
with the foregut entoderm and anterior extremity of the notochord in the late
blastula and gastrula in the fishes and amphibia. However, in reptiles, birds,
and mammals, this association is established secondarily with the foregut ento-
derm by means of the notochordal canal and primitive-pit invaginations during
gastrulation. (See Chap. 9 and also Hill and Tribe, '24.)
{Note: It is advisable to state that Adelmann, '32, relative to the 19-somite
embryo of the urodele Ambystoma punctatum, distinguishes between a pre-
chordal mesoderm, which forms the core of the mandibular visceral arch, and
the pre-chordal plate mesoderm, which earlier in development is associated
with the dorsal anterior portion of the foregut entoderm. See figure 252E.)
During the period when the major organ-forming areas are being tubulated,
NEPHROTOMIC PLiTE
EPITHELIAL CORE / (MESOMERE
DMITF
PRONEPHRIC DUCT
HYPOMERIC ■
MESODERMA
CONTRIBUTIO
TO LiTERA
BOOT WALL
CONTRIBUTION
FROM SPLANCHNIC
LAYER OF HYPOMERE
Fig. 252. Mesodermal contributions to developing body. (A-D) Sections through
developing chick of 48-52 hrs. of incubation. (A) Section through somites of caudal
trunk area showing primitive area of mesoderm and coelomic spaces. (B) Section
through anterior trunk area depicting early differentiation of somite. (C) Section
through trunk area posterior to heart revealing later stage of somite differentiation than
that shown in B. (D) Section through developing heart area. Observe dermomyotome,
sclerotomic mesenchyme, and mesenchymal contributions of hypomere to forming body
substance. (E) Mesodermal contributions to anterior end of developing embryo of
Ambystoma of about 19 somites. (Redrawn and modified from Adelmann: 1932, J.
Morphol. 54.) (F) Frontal section of early post-hatching larva of Rana pipiens show-
ing mass of mesoderm lying between gut, epidermal and neural tubes, together with the
contributions of the mesoderm to the visceral arches.
524
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION 525
the pre-chordal plate mesoderm separates as a mass of mesenchyme from the
antero-dorsal aspect of the foregut, anterior to the cephalic terminus of the
notochord (fig. 232G, H). It migrates forward as two groups of mesenchyme
connected at first by an interconnecting bridge of mesenchyme. Eventually
these two mesenchymal masses become separated and each forms a dense
aggregation of mesodermal cells over the mandibular visceral arch and just
caudal to the eye (fig. 252E). In the shark embryo and in the chick it gives
origin to the pre-mandibular somites (condensations) which probably give
origin to the eye muscles innervated by the oculomotor or third cranial nerves.
In Ambystoma, Adelmann ('32, p. 52) describes the pre-chordal plate meso-
derm as giving origin to "the eye muscles" and "probably much of the head
mesenchyme ahead of the level of the first (gill) pouch, but its caudal limit
cannot be exactly determined." Thus it appears that a portion of the head
mesoderm in the region anterior to the notochordal termination is derived
from the pre-chordal plate mesoderm in all vertebrates.
c. Head Mesoderm Contributed by Neural Crest Material
A conspicuous phase of the development of the head region in vertebrate
embryos is the extensive migration of neural crest cells which arise in the mid-
dorsal area as the neural tube is formed (Chap. 10; fig. 222C-F). Aside
from contributing to the nervous system (Chap. 19), a portion of the neural
crest material migrates extensively lateroventrally and comes to lie within the
forming visceral (branchial) arches, contributing to the mesoderm in these
areas (figs. 222C-F; 230D, F). Also, consult Landacre ('21); Stone ('22,
'26, and '29); and Raven ('33a and b). On the other hand, Adelmann ('25)
in the rat and Newth ('51 ) in the lamprey, Lampetra planeri, were not able
to find evidence substantiating this view. However, pigment cells (melano-
phores) of the skin probably arise from neural crest cells in the head region
of all vertebrate groups.
d. Head Mesoderm Originating from Post-otic Somites
There is good evidence that the musculature of the tongue takes its origin
in the shark embryo and lower vertebrates from cells which arise from the
somites of the trunk area, immediately posterior to the otic (ear) vesicle, from
whence they migrate ventrad to the hypobranchial region and forward to
the area of the developing tongue (fig. 253). In the human embryo, Kingsbury
('15) suggests this origin for the tongue and other hypobranchial musculature.
However, Lewis ('10) maintains that, in the human, the tongue musculature
arises from mesenchyme in situ.
3. Origin of the Mesoderm of the Tail
In the Amphibia, the tail mesoderm has been traced by means of the Vogt
staining method to tail mesoderm in the late blastular and early gastrular
526 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
Stages. At the time of tail-rudiment formation, tliis mesoderm forms two bi-
lateral masses of cells located within the "tail bud" or "end bud." These cellular
masses proliferate extensively as the tail bud grows caudally and give origin
to the mesoderm of the tail. Similarly, in other vertebrates, the mesoderm of
the future tail is present as mesenchyme in the terminal portion of the tail
bud. These mesenchymal cells proliferate, as the tail grows caudalward, and
leave behind the mesoderm, which gradually condenses into the epithelial
masses or segments (myotomes) along either side of the notochord and
neural tube.
4. Contributions of the Trunk Mesoderm to the
Developing Body
The mesoderm of the trunk area contributes greatly to the development of
the many body organs and systems in the trunk region. Details of this con-
tribution will be described in the chapters which follow, but, at this point, it
is well to survey the initial activities of the mesodermal tubes of the trunk
area in producing the vertebrate body.
a. Early Differentiation of the Somites or Epimere
The somites (figs. 217, 237, 252) contribute much to the developing struc-
ture of the vertebrate body. This fact is indicated by their early growth and
differentiation. For example, the ventro-mesial wall of the fully developed
somite gradually separates from the rest of the somite and forms a mass of
mesenchymal cells which migrates mesad around the notochord and also
dorsad around the neural tube (fig. 252A-C). The mesenchyme which thus
arises from the somite is known as the sclerotome. In the somite of the higher
vertebrates just previous to the origin of the sclerotome, a small epithelial
core of cells becomes evident in the myocoel; this core contributes to the
sclerotomic material (fig. 252B). As a result of the segregation of the sclero-
tomic tissue and its migration mesad to occupy the areas around the notochord
and nerve cord, the latter structures become enmeshed by a primitive skele-
togenous mesenchyme. The notochord and sclerotomic mesenchyme are the
foundation for the future axial skeleton of the adult, including the vertebral
elements and the caudal part of the cranium as described in Chapter 15.
After the departure of sclerotomic material, myotomic and dermatomic
portions of the somite soon rearrange themselves into a hollow structure (fig.
252C, D), in which the myotome forms the inner wall and the dermatome
the outer aspect. This composite structure is the dermomyotome, and the
cavity within, the secondary myocoel. In many vertebrates (fishes, amphibia,
reptiles, and birds), the dermatome gives origin to cells which migrate into
the region of the developing dermis (Chap. 12) and contributes to the forma-
tion of this layer of the skin.
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION 527
b. Early Differentiation of the Mesomere (Nephrotome)
The differentiation of the nephrotome or intermediate mesoderm will be
considered later (Chap. 18) in connection with the urogenital system.
c. Early Differentiation and Derivatives of the Hypomere
The lateral-plate mesoderm (hypomere), figure 252A, performs an ex-
tremely important function in embryological development. The cavity of the
hypomere (splanchnocoel) and the cellular offspring from the hypomeric meso-
derm, which forms the wall of this cavity, give origin to much of the struc-
tural material and arrangement of the adult body.
1) Contributions of the Hypomere (I.ateral Plate Mesoderm) to the De-
veloping Pharyngeal Area of the Gut Tube. The developing foregut (Chap.
13) may be divided into four main areas, namely, (1) head gut, (2) pha-
ryngeal, (3) esophageal, and (4) stomach areas. The head gut is small and
represents a pre -oral extension of the gut; the pharyngeal area is large and
expansive and forms about half of the forming foregut in the early embryo;
the esophageal segment is small and constricted; and the forming stomach
region is enlarged. At this point, however, concern is given specifically to the
developing foregut in relation to the early development of the pharyngeal
region.
In the pharyngeal area the foregut expands laterally. Beginning at its an-
terior end, it sends outward a series of paired, pouch-like diverticula, known
as the branchial (pharyngeal or visceral) pouches. These pouches push out-
ward toward the ectodermal (epidermal) layer. In doing so, they separate
the lateral plate mesoderm which synchronously has divided into columnar
masses or cells (fig. 252E, F). Normally, about four to six pairs of branchial
(pharyngeal) pouches are formed in gnathostomous vertebrates, although in
the cyclostomatous fish, Petromyzon, eight pairs appear. In the embryo of the
shark, Squalus acanthias, six pairs are formed, while in the amphibia, four
to six pairs of pouches may appear (fig. 252F). In the chick, pig, and human,
four pairs of pouches normally occur (figs. 259, 261 ). Also, invaginations or
inpushings of the epidermal layer occur, the branchial grooves (visceral fur-
rows); the latter meet the entodermal outpocketings (figs. 252F; 262B).
The end result of all these developmental movements in the branchial area
is to produce elongated, dorso-ventral, paired columns of mesodermal cells
(figs. 252E; 253), th? visceral or branchial arches, which alternate with the
branchial-groove-pouch or gill-slit areas (figs. 252F; 253). The most anterior
pair of visceral arches forms the mandibular visceral arches; the second pair
forms the hyoid visceral arches; and the succeeding pairs form the branchial
(gill) arches (figs. 239C, D; 240; 244; 246; 252E; 253). The branchial arches
with their mesodermal columns of cells will, together with the contributions
from the neural crest cells referred to above, give origin to the connective,
muscle, and blood-vessel-forming tissues in this area.
528
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
2) Contributions of the Hypomere (Lateral Plate Mesoderm) to the For-
mation of the Gut Tube and Heart Structures. Throughout the length of the
forming gut tube, from the oral area to the anal region, the lateral plate meso-
derm (mesoblast) contributes much to the forming gut tube. This is occa-
sioned to a great extent posterior to the pharyngeal area by the fact that the
inner or mesial walls of the two hypomeres enswathe the forming gut tube
as they fuse in the median plane (fig. 241), forming the dorsal and ventral
mesenteries of the gut. However, in the heart area, due to the dorsal dis-
placement of the foregut, the dorsal mesentery is vestigial or absent while
the ventral mesentery is increased in extent. Each mesial wall of the hypomeric
mesoderm, forming the ventral mesentery in the region of the developing
heart, becomes cupped around the primitive blood capillaries, coursing an-
teriad in this area to form the rudiments of the developing heart. The ventral
mesentery in the heart area thus gives origin to the dorsal mesocardium, the
ventral mesocardium, and the rudimentary, cup-shaped, epimyocardial struc-
tures around the fusing blood capillaries (figs. 236C-D; 254A). The primitive
blood capillaries soon unite to form the rudiment of the future endocardium
of the heart, while the enveloping epimyocardium establishes the rudiment of
the future muscle and connective tissues of the heart (Chap. 17).
On the other hand, in the region of the stomach and continuing posteriorly
to the anal area of the gut, the movement mediad of the mesial walls of the
two lateral plate (hypomeric) mesodermal areas occurs in such a way as to
ABDUCENS NERVE
TH CRANIAL NERVE
AUDITORY CAPSULE
3TH CRANIAL NER
PROFUNDUS
(OPHTHALMICUS)
DIVISION OF
TRIGEMINAL NERVE
MANDIBULAR VISCERAL ARCH
HYOID VISCERAL ARCH
BRANCHIAL POUCHES I'Sl
SOMITES 1-8= THEORETICAL
SEGMENTS OF THE HEAD
Fig. 253. Diagram illustrating the basic plan of the vertebrate head based upon the
shark, Scyllium canicula. (Modified from Goodrich: 1918, Quart. Jour. Micros. Sci-
ence, 63.)
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
529
POSTERIOR
PHARYNGEAI
REGION OF GUT
ENDOCAR
EPIMVOCARDUM
NEURAL TUBE
DERMOMrOTOl
NOTOCHORD
INTERIOR CARDINA
ESOPHAGEA
REGION OF G
DORSAL MESOCARDIUM
RT REGION
NEURAL TUBE
DERMOMYOTOME
OTOCHORO
DORSAL AORT
7\\\ POSTERIOR CARDiNAL
DORSAL MESENT
NEPHROTOME
DORSAL PANCRE
LIVER REGION
LANCriNOPLEURE "
EPATIC DIVERTICULUW*
CENTRAL MESENTERY
c.
ABDOMINAL REGION
BLADDER REGION
Fig. 254. Diagrams illustrating the contributions of the mesial or splanchnic layers of
the hypomeres to the developing heart and gut structures in reptiles, birds, and mammals.
Sections are drawn through the following regions: (A) Through primitive tubular heart
anterior to sinus venosus. (B) Through caudal end of sinus venosus and lateral meso-
cardia. (C) Through liver region. (D) Through region posterior to liver. (E)
Through posterior trunk in region of urinary bladder.
envelop or enclose the gut tube. This enclosure readily occurs because in this
region of the trunk, the gut tube lies closer to the ventral aspect of the embryo
than in the heart area. Consequently, a dorsal mesentery above and a ventral
mesentery below the primitive gut tube are formed (fig. 254C). The dorsal
and ventral mesenteries may not persist everywhere along the gut (fig. 254D).
The degree of persistence varies in different vertebrates; these variations will
be mentioned later (Chap. 20) when the coelomic cavities are discussed.
However, there is a persistence of the ventral mesentery below the stomach
and anterior intestinal area of all vertebrates, for here the ventral mesentery
(i.e., the two medial walls of the lateral plate mesoderm below the gut) con-
tributes to the development of the liver and the pancreas. These matters are
discussed in Chapter 13.
Aside from the formation of the dorsal and ventral mesenteries by the in-
ward movement and fusion of the medial walls of the lateral plate mesoderm
above and below the primitive enteron or gut tube, that part of the medial
walls of the lateral plate mesoderm which envelops the primitive gut itself is
of great importance. This importance arises from the fact that the entoderm
of the gut only forms the lining tissue of the future digestive tract and its
various glands, such as the liver, pancreas, etc., whereas mesenchymal con-
tributions from the medial wall of the lateral plate mesoderm around the
530 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
entodermal lining give origin to smooth muscle tissue, connective tissue, etc.
(figs. 254C, D; 258; 260; 262; 278C). It is apparent, therefore, that the gut
throughout its length is formed from two embryonic contributions, namely,
one from the entoderm and the other from the mesenchyme given off by the
medial walls of the lateral plate or hypomeric mesoderm.
(Note: The word splanchnic is an adjective and is derived from a Greek
word meaning entrails or bowels. That is, it pertains to the soft structures
within the body wall. The plural noun viscera (singular, viscus) is derived
from the Latin and signifies the same structures, namely, the heart, liver,
stomach, intestine, etc., which lie within the cavities of the body. It is fitting,
therefore, to apply the adjective splanchnic to the medial portion of the hypo-
mere because it has an intimate relationship with, and is contributory to, the
development of the viscera. The somatic mesoderm, on the other hand, is the
mesoderm of the lateral or body-wall portion of the hypomere. The word
splanchnopleure is a noun and it designates the composite tissue of primitive
entoderm and splanchnic mesoderm, while the word somatopleure is applied
to the compound tissue formed by the primitive lateral wall of the hypomere
(somatic mesoderm) plus the primitive ectoderm overlying it. The coelom
proper or splanchnocoel is the space or cavity which lies between the splanchnic
and somatic layers of the lateral plate or hypomeric mesoderm. During later
development, it is the cavity in which the entrails lie.
3) Contributions of the Hypomere (Lateral Plate Mesoderm) to the Ex-
ternal (Ectodermal or Epidermal) Body Tube. The somatopleural mesoderm
gives origin to a mass of cellular material which migrates outward to lie along
the inner aspect of the epidermal tube in the lateral and ventral portions of
the developing body (fig. 252A, D). In the dorsal and dorso-lateral regions of
the body, contributions from the sclerotome and dermatome apparently aid
in forming this tissue layer. The layer immediately below the epidermis con-
stitutes the embryonic rudiment of the dermis. (See Chap. 12.)
4) Contributions of the Hypomere or Lateral Plate Mesoderm to the Dorsal
Body Areas. Many cells are given off both from splanchnic and somatic layers
of the hypomeric mesoderm to the dorsal body areas above and along either
side of the dorsal aorta (fig. 254), contributing to the mesenchymal "pack-
ing tissue" in the area between the notochord and differentiating somite, ex-
tending outward to the dermis.
5) Contributions of the Lateral Plate Mesoderm to the Walls of the Coe-
lomic Cavities. The pericardial, pleural, and peritoneal cavities are lined, as
stated above, by an epithelial type of tissue called mesothelium (fig. 254A-E).
These coelomic spaces (see Chap. 20) are derived from the fusion of the
two primitive splanchnocoels or cavities of the two hypomeres. External to
the mesothelial lining of the coelomic spaces, there ultimately is developed a
fibrous, connective tissue layer. Thus, mesothelium and connective tissue form.
PRIMITIVE EMBRYONIC BODY FORM LARVAL BODY FORM
DEFINITIVE BODY FORM
Fig. 255. This figure illustrates different types of body form in various vertebrates
during embryonic development. A, D, H, M, and Q show primitive embryonic body
form in the developing shark, rock fish, frog, chick, and human. B, larval form of
shark; E and F, larval forms of rock fish; I and J, larval forms of frog; N and O, larval
forms of chick; R, larval form of human. C, G, K, L, P, and S represent definitive
body form in the above species. (Figures on rockfish development (Roccus saxatilis) re-
drawn from Pearson: 1938, Bull. Bureau of Fisheries, U. S. Dept. of Commerce, vol.
49; figures on chick redrawn from Hamburger and Hamilton: 1951, J. Morphol., vol.
88; figure Q, of developing human embryo, redrawn and modified from model based
upon Normentafein of Keibel and Elze: 1908, vol. 8, G. Fischer, Jena; Dimensions of
human embryos in R and S, from Mall: Chap. 8, vol. I, Human Embryology, by
F. Keibel and F. P. Mall, 1910, Lippincott, Philadelphia.)
531
532 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
in general, the walls of the coelomic spaces. These two tissues arise directly
from the hypomeric mesoderm.
5. Embryonic Mesenchyme and Its Derivatives
The mesenchymal cells given off from the mesodermal tubes of the trunk
area, namely, (1) sclerotomic mesenchyme, (2) dermatomic mesenchyme,
(3) mesenchymal contributions from the lateral plate mesoblast (hypomere)
to the gut, skin, heart, and (4) the mesenchyme contributed to the general
regions of the body lying between the epidermal tube, coelom, notochord,
and neural tube, form, together with the head and tail mesoderm, the general
packing tissue which lies between and surrounding the internal tubular struc-
tures of the embryo (fig. 254). Its cells may at times assume polymorphous
or stellate shapes. This loose packing tissue of the embryo constitutes the
embryonic mesenchyme. (See Chap. 15.)
This mesenchyme ultimately will contribute to the following structures of
the body:
(a) Myocardium (cardiac musculature, etc.) and the epicardium or cover-
ing coelomic layer of the heart (Chap. 17),
(b) endothelium of blood vessels, blood cells (Chap. 17),
(c) smooth musculature and connective tissues of blood vessels (Chaps.
16 and 17),
(d) spleen, lymph glands, and lymph vessels (Chap. 17),
(e) connective tissues of voluntary and involuntary muscles (Chap. 16),
(f ) connective tissues of soft organs, exclusive of the nerve system (Chap.
15),
(g) connective tissues in general, including bones and cartilage (Chap. 15),
(h) smooth musculature of the gut tissues and gut derivatives (Chap. 16),
(i) voluntary or striated muscles of the tail from tail-bud mesenchyme
(Chap. 16),
(j) striated (voluntary) musculature of face, jaws, and throat, derived
from the lateral plate mesoderm in the anterior pharyngeal region
(Chap. 16),
(k) striated (voluntary) extrinsic musculature of the eye (Chap. 16),
(1) intrinsic, smooth musculature of the eye (Chap. 16),
(m) tongue and musculature of bilateral appendages, derived from somitic
muscle buds (sharks) or from mesenchyme possibly of somitic origin
(higher vertebrates) (Chap. 16), and
(n) chromatophores or pigment cells of the body from neural crest mesen-
chyme (Chap. 12).
SUMMARY OF DERIVATIVES OF ORGAN-FORMING AREAS 533
E. Summary of Later Derivatives of the Major Presumptive Organ-
forming Areas of the Late Blastula and Gastrula
1. Neural Plate Area (Ectoderm)
This area gives origin to the following:
(a) Neural tube,
(b) optic nerves and retinae of eyes,
(c) peripheral nerves and ganglia,
(d) chromatophores and chromaffin tissue (i.e., various pigment cells of
the skin, peritoneal cavity, etc., chromaffin cells of supra-renal gland),
(e) mesenchyme of the head, neuroglia, and
(f) smooth muscles of iris.
2. Epidermal Area (Ectoderm)
This area gives origin to:
(a) Epidermal tube and derived structures, such as scales, hair, nails,
feathers, claws, etc.,
(b) lens of the eye, inner ear vesicles, olfactory sense area, general, cu-
taneous, sense organs of the peripheral area of the body,
(c) stomodaeum and its derivatives, oral cavity, anterior lobe of pituitary,
enamel organs, and oral glands, and
(d) proctodaeum from which arises the lining tissue of the anal canal.
3. Entodermal Area
From this area the following arise:
(a) Epithelial lining of the primitive gut tube or metenteron, including:
(1) epithelium of pharynx; epithelium pharyngeal pouches and their
derivatives, such as auditory tube, middle-ear cavity, parathyroids, and
thymus; (2) epithelium of thyroid gland; (3) epithelial lining tissue
of larynx, trachea, and lungs, and (4) epitheUum of gut tube and gut
glands, including liver and pancreas,
(b) most of the lining tissue of the urinary bladder, vagina, urethra, and
associated glands,
(c) Seessel's pocket or head gut, and
(d) tail gut.
4. NOTOCHORDAL ArEA
This area:
(a) Forms primitive antero-posterior skeletal axis of all chordate forms,
(b) aids in induction of central nerve tube,
534 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
(c) gives origin to adult notochord of Amphioxus and cyclostomatous fishes
and to notochordai portions of adult vertebral column of gnathostomous
fishes and water-living amphibia, and
(d) also, comprises the remains of the notochord in land vertebrates, such
as "nucleus pulposus" in man.
5. Mesodermal Areas
These areas give origin to:
(a) Epimeric, mesomeric, and hypomeric areas of primitive mesodermal
tube,
(b) epimeric portion also aids in induction of central nerve tube,
(c) muscle tissue, involuntary and voluntary,
(d) mesenchyme, connective tissues, including bone, cartilage,
(e) blood and lymphoid tissue,
(f) gonads with exception of germ cells, genital ducts, and glandular tis-
sues of male and female reproductive ducts, and
(g) kidney, ureter, musculature and connective tissues of the bladder,
uterus, vagina, and urethra.
6. Germ-cell Area
This area gives origin to:
(a) Primordial germ cells and probably to definitive germ cells of all verte-
brates below mammals and
(b) primordial germ cells of mammals and possibly to definitive germ cells.
F. Metamerism
1. Fundamental Metameric Character of the Trunk and
Tail Regions of the Vertebrate Body
Many animals, invertebrate as well as vertebrate, are characterized by the
fact that their bodies are constructed of a longitudinal series of similar parts
or metameres. As each metamere arises during development in a similar
manner and from similar rudiments along the longitudinal or antero-posterior
axis of the embryo, each metamere is homologous with each of the other
metameres. This type of homology in which the homologous parts are ar-
ranged serially is known as serial homology. Metamerism is a characteristic
feature of the primitive and later bodies of arthropods, annelids, cephalo-
chordates, and vertebrates.
In the vertebrate group, the mesoderm of the trunk and tail exhibits a type
of segmentation, particularly in the epimeric or somitic area. Each pair of
somites, for example, denotes a primitive body segment. The nervous system
Fig. 256. Developmental features of the human face. Modified slightly from models by
B. Ziegler, Freiburg, after Karl Peter.
535
536 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
also manifests various degrees of segmentation (Chap. 19), although the
origin and arrangement of the peripheral nerves in the form of pairs, each
pair innervating a pair of myotomic derivatives of the somites, is the most
constant feature.
In the cephalochordate, Amphioxus, the segmentation of the early meso-
derm is more pronounced than that of the vertebrate group. As observed in
Chapter 10, each pair of somites is distinct and entirely separate from other
somitic pairs, and each pair represents all the mesoderm in the segment or
metamere. That is, all the mesoderm is segmented in Amphioxus. However,
in the vertebrate group, only the more dorsally situated mesoderm undergoes
segmentation, the hypomeric portion remaining unsegmented.
2. Metamerism and the Basic Morphology of the
Vertebrate Head
While the primitive, metameric (segmental) nature of the vertebrate trunk
and tail areas cannot be gainsaid, the fundamental metamerism of the verte-
brate head has been questioned. Probably the oldest theory supporting a
concept of cephalic segmentation was the vertebral theory of the skull, pro-
pounded by Goethe, Oken, and Owen. This theory maintained that the basic
structure of the skull demonstrated that it was composed of a number of
modified vertebrae, the occipital area denoting one vertebra, the basisphenoid-
temporo-parietal area signifying another, the presphenoid-orbitosphenoid-
frontal area denoting a third vertebra, and the nasal region representing a
fourth cranial vertebra. (Consult Owen, 1848.) This theory, as a serious
consideration of vertebrate head morphology was demolished by the classic
Croonian lecture given in 1858 by Huxley (1858) before the Royal Society
of London. His most pointed argument against the theory rested upon the
fact that embryological development failed to support the hypothesis that the
bones of the cranium were formed from vertebral elements.
A factor which aroused a renewal of interest in a segmental interpretation
of the vertebrate head was the observation by Balfour (1878) that the head
of the elasmobranch fish, Scy Ilium, contained several pairs of pre-otic (pro-
otic ) somites (that is, somites in front of the otic or ear region ) . Since Balfour's
publication, a large number of studies and dissertations have appeared in an
endeavor to substantiate the theory of head segmentation. The anterior por-
tion of the central nervous system, cranial nerves, somites, branchial (visceral)
arches and pouches, have all served either singly or in combination as proffered
evidence in favor of an interpretation of the primitive segmental nature of
the head region. However, it is upon the head somites that evidence for a
cephalic segmentation mainly depends.
A second factor which stimulated discussion relative to head segmentation
was the work of Locy (1895) who emphasized the importance of so-called
neural segments or neuromeres (Chap. 19) as a means of determining the
OTIC VE,SICLE
SPINAL CORD
NOTOCHORD
SEPTUM
TRfiNSVERSUM-
LIVER COMPLEX
LEFT DIVISION
BULBUS CORDIS
fl^% ; ?T'"f ^''''y ^'""^ ^^"^P"'^' '^^^'"8 development of early systems.
(A) Frog tadpole (R. pipiens) of about 6^7 mm. It is difficult to determine the exact
number of vitelhne arteries at this stage of development and the number given in the
figure is a diagrammatic representation. (A') Shows right and left ventral aortal divi-
figu"res 28o"ind%T5'"'^''' ^^^ '^"^^""^y of frog tadpole of about 10-18 mm. See also
537
NEuRfiL ECTODERM
D OF GILLS
.MESODERM
Fig. 258. Sections and stereograms of Rana pipiens tadpole of 10 mm.
538
Fig. 258 — (Continued) Sections and stereograms of Rana pipiens tadpole of 10 mm.
539
540 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
primitive segmental structure of the vertebrate brain. It is to be observed that
the more conservative figure 253, taken from Goodrich, does not emphasize
neuromeres, for, as observed by Kingsbury ('26, p. 85), the evidence is over-
whelmingly against such an interpretation. The association of the cranial nerves
with the gill (branchial) region and the head somites, shown in figure 253,
will be discussed further in Chapter 19.
A third factor which awakened curiosity, concerning the segmental theory
of head development, is branchiomerism. The latter term is applied to the
development of a series of homologous structures, segmentally arranged, in
the branchial region; these structures are the visceral arches and branchial
pouches referred to above. As mentioned there, the branchial pouches or out-
pocketings of the entoderm interrupt a non-segmented mass of lateral plate
(hypomeric) mesoderm, and this mesoderm secondarily becomes segmented
and located within the visceral arches. These arches when formed, other than
possibly the mandibular and the hyoid arches (fig. 253), do not correspond
with the dorsal somitic series. Consequently, "branchiomerism does not, there-
fore, coincide with somitic metamerism." (See Kingsbury, '26, p. 106.)
Undoubtedly, much so-called "evidence" has been accumulated to support
a theory of head segmentation. A considerable portion of this evidence ap-
parently is concerned more with segmentation as an end in itself than with a
frank appraisal of actual developmental conditions present in the head (Kings-
bury and Adelmann, '24 and Kingsbury, '26). However, the evidence which
does resist critical scrutiny is the presence of the head somites which includes
the pre -otic somites and the first three or four post-otic somites. While the
pre-otic somites are somewhat blurred and slurred over in their development
in many higher vertebrates, the fact of their presence in elasmobranch fishes
is indisputable and consistent with a conception of primitive head segmentation.
Furthermore, aside from a possible relationship with head-segmentation
phenomena, the appearance of the pre-otic and post-otic head somites coin-
cides with basic developmental tendencies. As observed above, for example,
there is a tendency for nature to use generalized developmental procedures in
the early development of large groups of animals (see von Baer's laws, p. 522,
and also discussion relative to Haeckel's biogenetic law in Chap. 7). Nature,
in other words, is utilitarian, and one can be quite certain that if general
developmental procedures are used, they will prove most efficient when all
factors are considered. At the same time, while generalized procedures may
be used, nature does not hesitate to mar or elide parts of procedures when
needed to serve a particular end. The obliteration of developmental steps
during development is shown in the early development of the mesoderm in
the vertebrate group compared to that which occurs in Amphioxus. In the
vertebrate embryo, as observed previously, the hypomeric mesoderm is un-
segmented except in a secondary way and in a restricted area as occurs in
branchiomerism. However, in Amphioxus, early segmentation of the meso-
METAMERISM
541
derm is complete dorso-ventrally, including the hypomeric region of the
mesoderm. It becomes evident, therefore, that the suppression of segmentation
in the hypomeric area in the vertebrate embryo achieves a precocious result
which the embryo of Amphioxus reaches only at a later period of develop-
ment. Presumably in the vertebrate embryo, segmentation of the epimeric
mesoderm is retained because it serves a definite end, whereas segmentation
of the hypomeric mesoderm is deleted because it also leads to a necessary end
result in a direct manner.
When applied to the developing head region, this procedure principle means
this: A primitive type of segmentation does tend to appear in the pre-otic
area as well as in the post-otic portion of the head, as indicated by the pre-otic
and post-otic somites, and secondarily there is developed a branchial metam-
GASSEF1I4N GSNGLION OF NERVE T
METENCEPHl
GENICULATE GANGLION OF NERVE SH.
ACOUSTIC GANGLION OF NERVE TTTTT
MTELENCEPHAL
OTIC VESICLE
SUPERIOR GANGLION OF NERVE H
JUGULAR GANGLION OF NERVE I
PETROSAL GANGLION (
NERV
NODOSE GANGLION 0
Fig. 259. Chick embryo reconstruction of about 100 hrs. of incubation with special
reference to the nervous and urinary systems. See also fig. 336D.
Fig. 260. Sections and stereograms of c...-
erence should be made also to fig. 33bU
542
hick embryo of about 72 hrs. incubation. Ref-
Fig. 260 — (Continued) Sections and stereograms of chick embryo of about 72 hrs. mcu-
bation. Reference should be made also to fig. 336D.
543
Fig. 260 — (Continued) Sections and stereograms of chick embryo of about 72 hrs. incu-
bation. Reference should be made also to fig. 336D.
544
BASIC HOMOLOGY OF ORGAN SYSTEMS 545
erism (branchiomerism). However, all these segmental structures serve a
definite end. In other areas, head development proceeds in a manner which
obscures segmentation, for the probable reason that segmentation does not fit
into the developmental pattern which must proceed directly and precociously
to gain a specific end dictated by problems peculiar to head development.
(Note: For a critical analysis of the supposed facts in favor of segmentation,
together with a marshaling of evidence against such an interpretation, consult
Kingsbury and Adelmann ('24) and for a favorable interpretation of the seg-
mental nature of the head region, see Goodrich ('18) and Delsman ('22).
Figure 253 is taken from Goodrich ('18), and the various structures which
favor a segmental interpretation of the head region are shown.)
G. Basic Homology of the Vertebrate Organ Systems
1. Definition
Homology is the relationship of agreement between the structural parts of
one organism and the structural parts of another organism. An agreeable
relationship between two structures is established if:
( 1 ) the two parts occupy the same relative position in the body,
(2) they arise in the same way embryonically and from the same rudi-
ments, and
(3) they have the same basic potencies.
By basic potency is meant the potency which governs the initial and funda-
mental development of the part; it should not be construed to mean the
ability to produce the entire structure. To the basic potency, other less basic
potencies and modifying factors may be added to produce the adult form of
the structure.
2. Basic Homology of Vertebrate Blastulae, Gastrulae, and
Tubulated Embryos
In Chapters 6 and 7, the basic conditions of the vertebrate blastula were
surveyed, and it was observed that the formative portion of all vertebrate
blastulae presents a basic pattern, composed of major presumptive organ-
forming areas oriented around the notochordal area and a blastocoelic space.
During gastrulation (,Chap. 9), these areas are reoriented to form the basic
pattern of the gastrula, and although round and flattened gastrulae exist, these
form one, generalized, basic pattern, composed of three germ layers arranged
around the central axis or primitive notochordal rod. Similarly, in Chapter
10, the major organ-forming areas are tubulated to form an elongated embryo,
composed of head, pharyngeal, trunk, and tail regions. As tubulation is ef-
fected in much the same manner throughout the vertebrate series and as the
pre-chordal plate mesoderm, foregut entoderm, notochord, and somitic meso-
546
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
GENICULATE GANGLION OF
ACOUSTIC GANGLION OF
AUDITORY VES
SEVENTH NERVE
JUGULAR GANGLION OF TENTH NERVE
ACCESSORY GANGLION
BASIL A R ARTER
DORSAL ROOT
GANGLION or FIRST
CERVICAL NERVE
AORTA
AORTAL
AORTAL
AORTAL
AORTAL /
PULMONAR^
TRAC
NOTOCH
RIGHT AT
LUNG
RIGHT VENTR
GALL
BLADDER
VENTRAL
PANCREAS
DORSAL AO
OMPHALOMES
ARTER
(FUTURE SU
MESENTERIC
Fig. 261. Drawings of pig embryos of about 9.5 to 12 mm. (A) Reconstruction of about
9.5 to 10 mm. pig embryo with special emphasis on the arterial system.
derm appear to be the main organizing influence throughout the series (Chap.
10), the conclusion is inescapable that the; tubulated embryos of all vertebrates
are homologous basically, having the sarrte relative parts, arising in the same
manner, and possessing the same basic potencies within the parts. To this
conclusion must be added a caution, namely, that, although the main segments
or specific organ regions along each body tube of one species are homologous
with similar segments along corresponding tubes of other species, variations
may exist and non-homologous areas may be insinuated or homologous areas
BASIC HOMOLOGY OF ORGAN SYSTEMS
547
may be deleted along the respective tubes. Regardless of this possibility, a
basic homology, however, appears to exist.
During later development through larval and definitive body-form stages,
a considerable amount of molding or plasis by environmental and intrinsic
factors may occur. An example of plasis is given in the development of the
forelimb rudiment of the fish, frog, bird, and pig. In the definitive form, these
structures assume different appearances and are adapted for different func-
METENCEPHALON
BASILAR ARTERY
NOTOCHORD
ROOT OF TON GUE
THYROID GLAND
DEVELOPING EPIGLO
AORTIC ARCH
TRAC
ESOPHAG
SPINAL CORD
MESENCEPHALON
TUBERCULUM
POSTERIUS
INFUNDIBULUM
DIENCEPHALON
RATHKE'S POCKET
EESSEL'S POCKET
PTIC CHIASMA
CESSUS OPTICUS
LENCEPHALON
INA TERMINALIS
IINUS VENOSUSp^j
DORSAL -l--_i_
PANCREAS
DUODENUM
GALL BLADDER
NOTOCHORD
JORSAL AOR
MESONEPHR
KIDNEY
SEPTUM TRANSVERSUM
DUCTUS VENOSUS
EXTRA-EMBRYONIC COELOM
UMBILICAL CORD
ALLANTOIC DIVERTICULUM
''^> GENITAL EMINENCE
PROCTODAEUM
ALLANTOIC STALK
METANEPHRIC
METANEPHROGENOUS
TISSUE SPINAL GANGLION
Fig. 261 — (Continued) (B) Median sagittal section of 10 mm. embryo.
VEIN OF MAXILLARY RESIGN
(BRANCH OF INTERNAL JUGULARl
OTIC VESICLE
PRIMORDIUM
SUPERIOR SAGITTAL
SINUS
PRIMORDIUM TRANSVERSE
SINUS
PULMONARY ARTERY
RIGHT ATRIUM
VEIN OF
MANDIBULAR REGION
(BRANCH OF EXTERNAL
JUGULAR)
INTERNAL JUGULAR
VEIN
DORSAL
SEGMENTAL VEINS
EXTERNAL
JUGULAR VEIN
RIGHT DUCT
OF CUVIER
SUBCARDINAL
VEIN
POSTER lOR
VENA CAVA
METANEPHRIC
DUCT
UMBILICAL
ARTERY
TRANSVERSE ANASTOMOSIS
OF SUBCAROINALS
POSTERIOR CARDINAL
VEIN (,
I2MM PIG EMBRYO SHOWING RIGHT HALF
OF VENOUS SYSTEM
Fig. 261 — (Continued) (C) Lateral view of 12 mm. embryo showing venous system.
(C is redrawn and modified from Minot: 1903, A Laboratory Text-book of Embryology,
Blakiston, Philadelphia.)
548
Fig. 262. Sections and stereograms of 10 mm. pig embryo.
549
Fig. 262 — (Continued) Sections and stereograms of 10 mm. pig embryo.
550
BIBLIOGRAPHY
551
tional purposes. Basically, however, these structures are homologous, although
plasis produces adult forms which appear to be different.
A further statement should be added, concerning that type of molding or
plasis of a developing structure which produces similar structures from con-
ditions which have had a different genetic history. For example, the bat's fore
limb rudiment is molded to produce a structure resembling superficially that
of the bird, although modern bats and birds have arisen through different hnes
of descent. Similarly, the teeth of certain teleost fishes superficially resemble
the teeth of certain mammals, an effect produced from widely diverging lines
of genetic descent. These molding effects or homoplasy, which produce su-
perficially similar structures as a result of adaptations to certain environmental
conditions, are called convergence, parallelism, and analogy. An example of
experimental homoplasy is the induction of eye lenses in the embryo by the
transplantation of optic-cup material to a place in the epidermis which nor-
mally does not produce a lens.
{Note: For a discussion of homology, homogeny, plasis, convergence, etc.,
see Tait, '28.)
Bibliography
Adelmann, H. B. 1925. The development
of the neural folds and cranial ganglia
of the rat. J. Comp. Neurol. 39:19.
. 1927. The development of the eye
muscles of the chick. J. Morphol. 44:29.
1932. The development of the
prechordal plate and mesoderm of Ani-
blystoma pimctatum. J. Morphol. 54:1.
Baer, K. E. von. 1828-1837. Uber Ent-
wickelungsgeschichte der Thiere. Beo-
bachtung und Reflexion. Erster Theil,
1828; Zweiter Theil, 1837. Konigsberg,
Borntrager.
Balfour, F. M. 1878. Monograph on the
development of elasmobranch fishes. Re-
published in 1885 in The Works of
Francis Maitland Balfour, edited by M.
Foster and A. Sedgwick, vol. 1. The
Macmillan Co., London.
Delsman, H. C. 1922.. The Ancestry of
Vertebrates. ValkofF & Co., Amersfoort,
Holland.
Goodrich, E. S. 1918. On the development
of the segments of the head of Scyllium.
Quart. J. Micr. Sc. 63:1.
Hill, J. P. and Tribe. M. 1924. The early
development of the cat (Felis domestica).
Quart. J. Micr. Sc. 68:513.
Huxley, T. H. 1858. The Croonian lecture
— on the theory of the vertebrate skull.
Proc. Roy. Soc, London, s.B. 9:381.
Kingsbury, B. F. 1915. The development
of the human pharynx. L Pharyngeal
derivatives. Am. J. Anat. 18:329.
. 1924. The significance of the so-
called law of cephalocaudal differential
growth. Anat. Rec. 27:305.
— . 1926. Branchiomerism and the
theory of head segmentation. J. Morphol.
42:83.
and Adelmann, H. B. 1924. The
morphological plan of the head. Quart.
J. Micr. Sc. 68:239.
Kyle, H. M. 1926. The Biology of Fishes.
Sidgwick and Jackson, Ltd., London.
Landacre, F. L. 1921. The fate of the
neural crest in the head of urodeles. J.
Comp. Neurol. 33:1.
Lewis, W. H. 1910. Chapter 12. The de-
velopment of the muscular system in
Manual of Human Embryology, edited
by F. Keibel and F. P. Mall. J. B. Lip-
pincott Co., Philadelphia.
Locy, W. A. 1895. Contribution to the
structure and development of the verte-
brate head. J. Morphol. 11:497.
552
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
Newth, D. R. 1951. Experiments on the
neural crest of the lamprey embryo. J.
Exper. Biol. 28:17.
Owen, R. 1848. On the archetype and
homologies of the vertebrate skeleton.
John Van Voorst, London.
Raven, C. P. 1933a. Zur Entwicklung der
Ganglienleiste. I. Die Kinematik der
Ganglienleistenentwicklung bei den Uro-
delen. Arch. f. Entwlngsmech. d. Organ.
125:210.
. 1933b. Zur Entwicklung der Gan-
glienleiste. III. Die Induktionsfahigkeit
des Kopfganglienleistenmaterials von
Rana fuse a.
Stone, L. S. 1922. Experiments on the de-
velopment of the cranial ganglia and
the lateral line sense organs in Amblys-
torna punctatum. J. Exper. Zool. 35:421.
. 1926. Further experiments on the
extirpation and transplantation of mesec-
toderm in Amblystoma punctatum. J.
Exper. Zool. 44:95.
1929. Experiments showing the
role of migrating neural crest (mesecto-
derm) in the formation of head skele-
ton and loose connective tissue in Rana
pulustris. Arch. f. Entwicklngsmech. d.
Organ. 118:40.
Tait, J. 1928. Homology, analogy and
plasis. Quart. Rev. Biol. Ill: 151.
PART IV
Histogenesis and Morpno^enesis
or tne Or^an-Systems
For definitions of cytogenesis, histogenesis, etc., see Chap. 11; for histogenesis and
morphogenesis of the organ systems, see Chaps. 12-21. The events described in Chapters
12-21 occur, to a great extent, during the so-called larval period or period of transition.
During this period of development, the basic conditions of the various organ-systems
which are present at the end of primitive embryonic body formation are transformed
into the structural features characteristic of definitive or adult body form. In other
words, during this phase of development, the basic, generahzed morphological conditions
of the various organ-systems of the embryo are rearranged and transformed into the
adult form of the systems. As a result, the body as a whole assumes the definitive or
adult form.
553
12
Tne Integumentary System
A. Introduction
1. Definition and general structure of the vertebrate integument or skin
2. General functions of the skin
3. Basic structure of the vertebrate skin in the embryo
a. Component parts of the developing integument
b. Origin of the component parts of the early integument
I ) Origin of the epidermal component
2) Origin of the dermal or mesenchymal component
3) Origin of chromatophores
B. Development of the skin in various vertebrates
1. Fishes
a. Anatomical characteristics of the integument of fishes
b. Development of the skin in the embryo of the shark. Squaliis ucunthius
1 ) Epidermis
2) Dermis
3) Development of scales and glands
c. Development of the skin in the bony ganoid fish. LepisosU'us (LepiJoste
osseus
d. Development of the skin in the tcleost fish
2. Amphibia
a. Characteristics of the amphibian skin
b. Development of the skin in Witiirns nuu ulo.sus
c. Development of the skin in the frog, Runa pipiens
3. Reptiles
a. Characteristics of the reptilian skin
b. Development of the turtle skin
4. Birds
a. Characteristics of the avian skin
1 ) Kinds of feathers
2) General structure of feathers
a) Pluma or contour feather
b) Plumule or down feather
c) Filoplume or hair feather
d) Distribution of feathers on the body
b. Development of the avian skin
1 ) Development of the epidermis, dermis, and nestling dov\'n feather
555
556 INTEGUMENTARY SYSTEM
2) Development of the contour feather
a) Formation of barbs during the primary or early phase of contour-feather
formation
b) Secondary phase of contour-feather formation
c) Formation of the barbules and the feather vane
d) Later development of the feather shaft
3) Formation of the after feather
4) Development of the later down and filoplumous feathers
5. Mammals
a. Characteristics of the mammalian skin
b. Development of the skin
1 ) Development of the skin in general
2) Development of accessory structures associated with the skin
a) Development of the hair
b) Structure of the mature hair and the hair follicle
3) Development of nails, claws, and hoofs
4) Development of horns
5) Development of the skin glands
a) Sebaceous glands
b) Sudoriferous glands
c) Mammary glands
C. Coloration and pigmentation of the vertebrate skin and accessory structures
1. Factors concerned with skin color
2. Color patterns
3. Manner of color-pattern production
a. Role of chromatophores in producing skin-color effects
b. Activities of other substances and structures in producing color effects of the
skin
c. Genie control of chromatophoric activity
d. Examples of hormonal control of chromatophoric activity
e. Environmental control of chromatophoric activity
A. Introduction
1. Definition and General Structure of the Vertebrate
Integument or Skin
The word integument means a cover. The word appHes specifically to the
external layer of the body which forms a covering for the underlying structures.
The integument also includes the associated structures developed therefrom,
such as hair, feathers, scales, claws, hoofs, etc. The latter are important fea-
tures of the body covering. The skin is continuous with the digestive and
urogenital tracts by means of mucocutaneous junctions at the lips, anus, and
external genitalia.
The integument is composed of two main parts, an outer epidermis and
an underlying corium or dermis. Below the latter is a third layer of connective
tissue which connects or binds the corium to the underlying body tissues. This
third layer forms the superficial fascia (tela subcutanea or hypodermis). The
superficial fascia is continuous with the deep fascia or the connective tissue
INTRODUCTION 557
which overlies muscles, bones, and tendinous structures of the body (fig.
272H).
2. General Functions of the Skin
The integument acts as a barrier between other body tissues and the ex-
ternal environment. Modifications of the integument serve also as an external
skeleton or exoskeleton in many vertebrates. In warm-blooded forms, the skin
is associated intimately with the regulation of body temperature. The hypo-
dermal portion of the skin often serves to store reserve fatty substances. The
presence of fat functions as a buffer against mechanical injury from without,
as reserve food, and as an aid in temperature regulation in warm-blooded
species. Still another and very important function of the skin is its intimate
association with the end organs of the peripheral nervous system by means
of which the animal becomes acquainted with changes in the external environ-
ment. (See Chap. 19.)
3. Basic Structure of the Vertebrate Skin in the Embryo
a. Component Parts of the Developing Integument
In all vertebrates, the integument arises from a primitive embryonic integu-
ment which at first is composed of the cells of the epidermal tube only, i.e.,
the primitive epidermis. Later this rudimentary condition is supplemented
by a condensation of mesenchymal cells below the epidermis. Following this
contribution, the primitive skin is composed of two main cellular layers:
(1 ) a primitive epidermal (ectodermal) layer of one or two cells in thick-
ness and
(2) an underlying mesenchymal layer.
The former gives origin to the epidermis, while the latter is the fundament
of the dermis. A little later, chromatophores or pigment cells, presumably of
neural crest origin, wander into the primitive dermis and become a con-
spicuous feature of this layer. In the development of the vertebrate group as
a whole, these two basic layers serve as the basis for the later development of
the integument. As a result, these two layers undergo characteristic modifi-
cations which enable the skin to fulfill its specific role in the various vertebrate
species. The marked differences in later development of these two integumen-
tary components in different vertebrate species are associated with the needs
and functions of the skin in the adult form.
b. Origin of the Component Parts of the Early Integument
1) Origin of the Epidermal Component. The epidermal component de-
scends directly from the primitive epidermal (ectodermal) organ-forming
area of the late blastula, which, as we have seen, becomes greatly extended
558
INTEGUMENTARY SYSTEM
during gastrulation and, in the post-gastrular period, is tubulated into the
elongated, cylinder-like structure. The primitive epidermal tube thus forms
the initial skin or outer protective investment of the developing body.
The wall of the primitive epidermal tube at first may be composed of a
single layer of cells of one cell in thickness, as in the shark, chick, pig, opossum,
or human (figs. 263 A; 269A; 272A). However, in teleost fishes and amphibia,
the primitive epidermal tube is composed of two layers of cells. For example,
in the sea bass, the wall of the primitive epidermal tube is composed of two
layers, the outer layer being thin and made up of much-flattened cells and
the lower layer being two cells in thickness (fig. 264A, B). In the anurans
and urodeles, the wall of the primitive epidermal tube is composed of two
layers, each of one cell in thickness (fig. 267 A, D). The lower layer in the
frog, salamander, anjd teleost often is referred to as the inner ectodermal or
nervous layer. It is the germinative layer and thus forms the inner or lower
portion of the stratum germinativum of the later epidermis (fig. 267 A, D).
The outer layer is densely pigmented and forms the periderm.
In the embryo of the shark, chick, and mammal, the single-layered condition
of the primitive epidermal tube soon becomes transformed into a double-
layered condition, the outer layer or periderm being composed of much-
flattened cells (figs. 263B; 269B; 272B). In all vertebrates, therefore, the
MESENCHyME-4 ?'«><3^^'dj=^ V -t-, c^V^t^^
1 '=W'=>^i.,-^_^-4
W^:^. ^-^
?^^
ENAMEL
DENTINE
ODONTOBLASTS
DENTINE ORGAN)
EP,OERM,sf ^E«'OERM^,^;^;^^p;«lgc%^
I STRATUM --e32fi9§55^j3;^S^- v.^Il'^^r^r^
LGtKMINATIVUM />' ^ '.% 'S^^.' -^V- " , ? Jg^^ .v^ iS^'cTi ^^'
DERMIS
PIGMENT CELL^^^^^"^-*'"''^'--' """
ENAMEL
DENTINE
EPIDERMIS J
1
VASCULAR-
LAYER
— EPIDERM
S
DEF
MIS'
FIBROUS-
LAYER
■VASCULAR
LAYER
^
- FIBROUS
LAYER
DEEP^
FASCIA 1
■DERMIS 1
muscleJ
Fig. 263. Developing skin of Sqiialus acanthias. (A) Section through differentiating
somite and epidermis of 10-mm. embryo. (B) Integument of 34-mm. embryo. (C)
Section of skin, showing beginning of scale formation in 60-mm. embryo. (D) Scale
development in 145-mm. embryo. (E) Later stage of placoid scale, projecting through
epidermal layer of skin.
INTRODUCTION
559
UNICELLULAR GLAND
Fig. 264. Diagrams pertaining to the skin of bony fishes. (A and B after H. V. Wilson:
Bull. U. S. Fish Commission, Vol. 9, 1889, reprint, 1891; C after Kingsley: Comp.
Anat. of Vertebrates, 1912, P. Blakiston's Son & Co., Phila.; F from Reed; Am. Nat.,
41.) (A) Section of ectoderm (primitive epidermis) of 39-hr. embryo of Serranus
atrariiis. the sea bass. (B) Epidermis of sea-bass embryo of 59 hrs. (C) Skin of the
lungfish, Protopterus. (D) Integument of teleost fish with special reference to scales.
(E) Higher power of epidermal and dermal tissue overlying scale in D. (F) Poison
gland along pectoral spine of Schilheodes gyriniis.
primitive epidermal layer of the skin eventually is composed of two simple
cellular layers, an out(.r protective periderm, and a lower, actively proliferat-
ing stratum germinativum. It is to be observed further that the periderm in
the recently hatched frog embryo possesses ciliated cells (fig. 267H, I). These
cilia, as in Amphioxiis (fig. 249B), are used for locomotor purposes, and also
function to bathe the surface with fresh currents of water. As such, they
probably play a part in external respiration.
The periderm forms a protective covering for the actively dividing and dif-
ferentiating cells below. In the mammals, the periderm occasionally is called
the epitrichium, as it eventually comes to rest upon the developing hair. In
Amphioxm, there is no periderm, and the epidermal tube (epidermis) remains
as a single layer of one cell in thickness (fig. 250E, F).
2) Origin of the Dermal or Mesenchymal Component. In Amphioxus, the
thin lateral and ventro-lateral walls of the myotome give origin to the derma-
tome which comes to lie beneath the epidermal wall. From the dermatome
arises the dermis or connective-tissue layer of the skin (fig. 250E, F). The
560 INTEGUMENTARY SYSTEM
origin of the embryonic dermis in the vertebrate group is more obscure than
in Amphioxiis, for in the vertebrates its origin varies in different regions of
the developing body. Moreover, the origin of the dermal mesenchyme is not
the same in all species. For example, in the head region of the frog and other
amphibia, the dermal portion of the skin is derived in part from wandering
mesenchyme of the head area, at least in the anterior extremity of the head
and posteriorly to the otic or ear region, while immediately caudal to this
area the mesenchyme of the dermis is derived from the dermatomic portion
of the somite, together with mesenchymal contributions of the outer wall of
the lateral plate mesoderm. In the trunk region of the body, mesenchyme
from the dermatomic portion of the somite wanders off to form the embryonic
connective-tissue layer of the skin in the dorso-lateral region of the embryo.
In the middorsal region, sclerotomic mesenchyme appears to contribute to
the dermal area. However, the dermal layer in the latero-ventral region of
the body is derived from mesenchymal cells whose origin is the somatopleural
layer of the hypomere (lateral plate mesoderm). The dermal layer in the tail
arises from the mesenchyme within the developing end bud (tail bud).
The embryonic dermis in the head region of the chick arises from mesen-
chyme in the head and pharyngeal areas. In the cervico-truncal region, the
dermatome of the somite contributes mesenchyme to the forming dermis on
the dorso-lateral portion of the body wall (Engert, '00; Williams, '10; fig.
269C), whereas latero-ventrally the mesenchyme of the future dermis springs
from the lateral wall of the hypomere. That portion of the developing dermis
overlying the neural tube appears to receive contributions from the sclerotomic
mesenchyme. The mesenchyme which forms the dermal layer of the skin in
the tail descends from the mesoderm of the end bud (tail bud).
In the shark embryo, the origin of the embryonic dermis is similar to that
of the amphibia. In the mammalian embryo, a small portion of the dermal
tissue may arise from the dermatome; however, the greater part arises in the
head and pharyngeal area from the mesenchyme within these areas, in the mid-
dorsal region of the trunk from sclerotomic mesenchyme, and in the latero-
ventral region of the trunk from the outer wall of the lateral plate. In the
tail region, the tissue of the dermis derives from tail-bud mesoderm. Bardeen
('00) concluded that the dermatome in pig and man gives origin to muscle
tissue. However, Williams ('10) doubted this conclusion. The fact remains
that the exact fate of the dermatome or cutis plate of the somite in mam-
mals, and even in the lower vertebrates, is not clear.
3) Origin of Chromatophores. Chromatophores or pigment-bearing cells
occur in relation to the epidermis and the dermis. Dermal chromatophores are
numerous in vertebrates from man down to the fishes. Pigment also appears
in the epidermal cells, hair, feathers, and certain epidermal scales. This pig-
ment is derived from melanoblasts or chromatophores which lie in the basal
area of the epidermis or in the zone between the epidermis and the dermis
DEVELOPMENT OF THE SKIN
561
(Dushane, '44). Experimental embryology strongly suggests that these chro-
matophores are derived from the neural crest cells which in turn take origin
from the primitive ectoderm in association with the neural tube at the time
of neural tube closure. From the neural crests, the mesenchymal cells, which
later give origin to chromatophores, migrate extensively throughout the body
and to the skin areas (Dushane, '43, '44; Eastlich and Wortham, '46).
B. Development of the Skin in Various Vertebrates
1. Fishes
a. Anatomical Characteristics of the Integument of Fishes
The epidermal layer of the skin of fishes is soft, relatively thin, and com-
posed of stratified squamous epithelium (figs. 263E; 264E; 265). Cornifica-
tion of the upper layers is absent in most instances. However, in those fishes
which come out of the water and spend considerable time exposed to the air,
cornification of the surface cells occurs (Harms, '29). Unicellular mucous
glands are abundant, and multicellular glands also are present (fig. 264C).
A slimy mucous covering overlies the external surface of the epidermis. Poison
glands may occur in proximity to protective spines or other areas (fig. 264F).
UNICELLULSR G L ON 0
LOOD VESSEL
Fig. 265. Development of phosphorescent organ in Porichthys notatiis. (From Greene:
J. Morphol., 15.) (A) Rudiment, separating from epidermis. (B) Section of ventral
organ of free-swimming larva. (C) Section of fully developed ventral organ.
562 INTEGUMENTARY SYSTEM
The dermal layer of fishes is a fibrous structure of considerable thickness.
The layer of dermal tissue, immediately below the epidermis, is composed of
loosely woven, connective-tissue fibers, copiously supplied with blood vessels,
mesenchymal cells, and chromatophores. Below this rather narrow region is
a thick layer, containing bundles of fibrous connective tissue. Between the latter
and the muscle tissue is a thin, less fibrous, subcutaneous layer (fig. 263E).
Scales are present generally throughout the group and are of dermal origin
in most species. However, both layers of the skin contribute to scale formation
in the shark and ganoid groups of fishes. Scales are absent in some fishes as,
for example, in cyclostomes and certain elasmobranchs, such as Torpedo. In
certain teleosts, the scales are minute and are embedded in the skin. This
condition is found in the family AnguiUidae (eels).
Highly specialized, phosphorescent organs are developed in deep-sea fishes
as ingrowths of masses of cells from the epidermis. (Consult Green, 1899.)
These epidermal ingrowths (fig. 265 A) separate from the epidermal layer and
become embedded within the dermis (fig. 265B, C).
b. Development of the Skin in the Embryo of the Shark, Squalus
acanthias
1) Epidermis. In shark embryos up to about the 15-mm. stage, the integu-
ment consists of an epidermis composed of one layer of cells, one cell in
thickness (fig. 263A). The shapes of these cells may vary, depending upon
the area of the body. In some areas, especially the dorso-lateral region of the
trunk, they are flattened, while along the middorsum of the embryo they are
cuboidal. In the pharyngeal area they are highly columnar.
By the time the embryo reaches 25 to 35 mm. in length, two layers of cells
are indicated in the epidermis, an outer periderm of much-flattened cells and
a lower, basal, germinative layer, the stratum germinativum (fig. 263B). The
stratum germinativum retains its reproductive capacity throughout life, giving
origin to the cells which come to lie external to it. Eventually the epidermis
is composed of a layer of cells, several cells in thickness. The outer cells may
form a thin squamous layer, covering the external surface (fig. 263D).
2) Dermis. The dermis gradually condenses from loose mesenchymal cells
which lie below the stratum germinativum of the epidermis (fig. 263B, C).
The dermis gradually increases in thickness and becomes composed of scat-
tered cells, intermingled with connective-tissue fibers. Deeply pigmented chro-
matophores become a prominent feature of the dermal layer, where they lie
immediately below the germinative stratum (fig. 263D, E).
3) Development of Scales and Glands. In the formation of the placoid scale
of the shark, masses of mesenchymal cells become aggregated at intervals
below the stratum germinativum to form scale papillae (fig. 263C). Each
papilla gradually pushes the epidermis outward, especially the basal layer (fig.
263D). The cells of the outer margin of the papilla give origin to odontoblasts
DEVELOPMENT OF THE SKIN 563
or cells which secrete a hard, bone-like substance, resembling the dentine of
the teeth of higher vertebrates (fig. 263D). This substance is closely related
to bone. The cells of the basal epidermal layer, overlying the dentine-like sub-
stance, then form an enamel organ, composed of columnar ameloblasts which
produce a hard, enamel-like coating over the outer portion of the conical mass
of dentine (fig. 263D). As this scale or "tooth-like" structure increases in
size, it gradually pushes the epidermis aside and projects above the surface
as a placoid scale (fig. 263E). Some are small, while others are large and
spine-like. Many different shapes and sizes of scales are formed in different
areas of the body (Sayles and Hershkowitz, '37).
As the epidermis increases in thickness, unicellular glands appear within
the epidermal layer (fig. 263D). These glands discharge their secretion of
mucoid material externally, producing a slimy coating over the surface of the
skin. Multicellular glands appear at the bases of the spines which develop at
the anterior margins of the dorsal fins and in the epidermis overlying the
claspers of the pelvic fins of the male.
c. Development of the Skin in the Bony Ganoid Fish, Lepisosteus
(Lepidosteus) osseus
The development of the epidermis and dermis in Lepisosteus is similar to
that of the shark embryo. Consideration, therefore, is confined to the develop-
ment of the characteristic ganoid scale.
In the formation of the ganoid scale of Lepisosteus, a different mechanism
is involved than in that of the placoid scale of the shark embryo. Most of
the scale is of dermal origin; the epidermal contribution of enamel substance
is small and restricted to the outer surface of the spines of the scale (fig.
266D-F).
The scale first appears as a thin calcareous sheet, secreted by the dermal
cells in the outer portion of the dermis (fig. 266A). Unlike the formation of
dentine in the shark skin, the calcareous material comes to enclose some of
the scleroblasts (osteoblasts) or bone-forming cells (fig. 266B). This process
continues as the scale increases in mass, and the scleroblasts become dis-
tributed as bone cells within the hard, bony substance of the scale. These
cells occupy small spaces or lacunae within the bone-like substance, and small
canals (canaliculi) traverse the hard substance of the scale to unite with similar
canals from neighboring, bone-cell cavities (Nickerson, 1893, p. 123).
Spine-like projections (fig. 266F) appear on the surface of the bony scales.
These spines are secondarily developed and form in a manner similar to the
placoid scale of the elasmobranch fish. That is, a dermal papilla is formed
externally to the already-formed dermal scale. This papilla pushes outward
into the epidermal layer, and a dentine-like substance appears on its outer
surface (fig. 266D). As development of the spine proceeds, this cap of dentine
gradually creeps basalward and unites secondarily with the dentine of the
564
INTEGUMENTARY SYSTEM
Fig. 266. Formation of the scale in Lepisosteus (Lepidosteus) osseiis. (After Nickerson:
Bull. Mus. Comp. Zool. at Harvard College, 24.) (A) Section through posterior end
of scale of fish, 150 mm. long. (B) Section through posterior end of decalcified scale
of fish, 300 mm. long. (C) Section through scale of fish, 300 mm. long. (D) Section
showing developing spine. (E) Outlines of scales viewed from surface. (F) Section of
scale spine attached to scale.
scale (fig. 266F). The papillary cells thus become entirely enclosed within
the spines of dentine, with the exception of a small canal, leading to the ex-
terior, at the base of the spine (fig. 266F). As the dentine-like spine develops,
an enamel-like substance is deposited upon its outer surface by the epidermal
cells.
Another characteristic of scale formation in Lepisosteus is the deposition
of ganoin upon the outer surface of the scale (fig. 266B, C). This ganoin ap-
pears to have many of the characteristics of the enamel. It previously was
considered to have been formed by the lower layer of epidermal cells, but
Nickerson (1893) concluded that it is of dermal origin. The outer, ganoin-
covered surface of the scale eventually lies exposed to the exterior in the adult
condition and, therefore, is not covered by epidermal tissue.
Much of the external surface of the body of the bony ganoid fish, Lepi-
sosteus osseus (common garpike), is covered with these plate-like scales, and,
consequently, the epidermal layer of the skin tends to be pushed aside by this
form of scaly armor. In Amia calva the epithelial (epidermal) covering is
retained, and cycloid scales, similar to those of teleosts, are developed. The
"ganoid" scales of Amia lack ganoin. They protect the head (fig. 316D).
DEVELOPMENT OF THE SKIN 565
d. Development of the Skin in the Teleost Fish
The early development of the epidermis and dermis in the teleost embryo
resembles that of the shark embryo, and a soft glandular epidermis eventually
is formed which overlies a thick, connective-tissue-layered dermis, containing
numerous scale pockets, each containing a scale (fig. 264D, E). Considera-
tion is given next to the development of the teleostean scale.
The development of the scale in teleost fishes is a complicated affair (Neave,
'36, '40). It arises in the superficial area of the dermis in relation to an aggre-
gation of cells. This aggregation of cells forms a dermal pocket or cavity. The
latter contains a fluid or gelatinous substance. The scale forms within this
cavity. A homogeneous scale rudiment of compact, connective-tissue fibers,
the fibrillary plate, is established within the gelatinous substance of the scale
pocket. A little later, calcareous or bony platelets are deposited upon this
fibrous scale plate. The scale continues to grow at its periphery and, thus,
stretches the dermal cavity. At the posterior margins of the scale, the dermal
cavity becomes extremely thin. Further growth of the scale posteriorly pushes
the epidermis outward, but the epidermis and the thin dermal cavity wall
normally retain their integrity (fig. 264D).
The mature scale consists of a hard fibrous substrate, upon the upper pos-
terior margins of which are embedded calcified plates. These calcified plates
fuse together basally as development proceeds. Most of the scale is embedded
deeply in the tissue of the dermal or scale pocket. At the anterior, deeply
embedded end of the scale, small, hook-like, retaining barbs or teeth develop
along the inner margins of the scale which serve to fasten the scale within the
pocket (fig. 264D).
2. Amphibia
a. Characteristics of the Amphibian Skin
The amphibian skin is soft, moist, and slimy. It is devoid of scales, with
the exception of the Gymnophiona which possess patches of small scales em-
bedded within pouches in the dermal layer of the skin (fig. 267J). However,
some of the Gymnophiona lack scales entirely. Unicellular and multicellular
glands of epidermal origin are a prominent feature of the amphibian skin
(fig. 267F, G). Specialized poison glands also are present (Noble, '31, p.
133). Glands are developed in some species which attract the members of the
opposite sex during the breeding season. In Cryptobranchus, the epidermal
layer may be invaded by capillaries which penetrate almost to the surface of
the skin in the region of the respiratory folds, located along the lateral sides
of the body (Chap. 14). Cornification of the outer epidermal cells is the
rule during later stages of development, in some species more than in others.
For example, the development of a cornified layer is characteristic of the
skin of toads, whose wart-like structures on the dorsal surface of the body
566
INTEGUMENTARY SYSTEM
represent areas of considerable cornification. Horny outgrowths of the epi-
dermis are common in certain species.
The dermal layer in general is delicate and characterized by the presence
of many pigment cells (chromatophores) of various kinds. The scales within
the skin of the Gyninophiona are of dermal origin. In frogs, the dermis is
LAND :#-:'^^(#'"";d.
MESENCHYME
PIGMENT CELL
DERMAL CHROMATOPHORE -
E RMIS
DERMAL MESENCH
-^^^^
EPIDERMAL PIGMENT CELLS
V V,r--r^ ^ V /- ^ tJPT'"!- _^^ >; ':5?^^^^0UTER COMPACT 1
PIGMENT CELLS
MUCOUS GLAND
NNER COMPACT LAYER ii^
OF DERMIS ^~3>vf'
SUBCUTANEOUS LAYER
:^f /^INTERMEDIATE SPONGY. ,•»/►« XH L \\
i-L-y^ J.a LAYER OF DERMIS ^s< ■•"^' \/ \-f'r.> r 1
^;^^£:^cAPiLLARY |^\Xk.i vjr -'^^/i
EPIDERMIS
EPIDERMAL GLA
Fig. 267. Developing integument of amphibia. (A after Field: Bull. Mus. Comp. Zool.
at Harvard College, 21; F after Dawson: J. Morphol., 34; H and I after Assheton:
Quart. J. Micr. Sc, 38; J from Kingsley, 1925: The Vertebrate Skeleton, Blakiston, Phila-
delphia, after Sarasins. ) ( A ) Section of skin of frog embryo in neural plate stage. ( B )
Section of skin of 10-mm. frog embryo. (C ) Skin of 34-mm. frog embryo. (D) Skin of
Nectiirus embryo, 6 mm. long. (E) Skin of Necturus embryo, 20 mm. long. (F) Struc-
ture of mature skin of Necturus. (G) Structure of skin of Rana pipiens of section
through head shortly after metamorphosis. (H) Frog embryo, 3 mm. long, showing
water streams produced by cilia. (I) Semidiagrammatic figure through suckers of frog
embryo, 6 to 7 mm. long. (J) Section of skin of the Gymnophionan, Epicrium.
DEVELOPMENT OF THE SKIN 567
separated from the deeper areas of the body along the dorso-lateral region
of the trunk by the presence of large lymph spaces.
b. Development of the Skin in Necturus maculosus
The newly formed, epidermal tube of a 6-mm. embryo of Necturus con-
sists of two layers of epidermal cells, an outer periderm and an inner stratum
germinativum (fig. 267D). In the ventro-lateral region of the trunk, however,
these two layers are flattened greatly and may become so attenuated that only
one layer of flattened cells is present. Unicellular glands appear in the head
region and represent modifications of cells of the outer ectodermal (peri-
dermal) layer.
In larvae of 18 to 20 mm. in length, the epidermis is 3 to 4 cells in thickness,
with the outer layer considerably flattened (fig. 267E). The dermis consists
of a mass of mesenchymal cells, with large numbers of chromatophores lying
near the epidermis. Chromatophores also lie extensively within the epidermal
layer; some even approach the outer periphery. According to Eycleshymer
('06), some of the pigment cells of the epidermis represent modified epithelial
cells, while others appear to invade the epidermis from the dermis. Dawson
('20) believed these epidermal pigment cells to be entirely of an epidermal
origin in Necturus. Dushane ('43, p. 124) considered the origin of epidermal
pigment cells in Amphibia in general to be uncertain but suggested "that
these cells also come from the neural crest" via the dermal mesenchyme.
Later changes in the developing skin consist in an increase in the number
of epithelial cells and in a great increase in the thickness of the dermis, with
the formation of bundles of connective-tissue fibers. Associated with these
changes, two types of multicellular alveolar glands arise as invaginations into
the dermis from the stratum germinativum. One type of gland is the granular
or poison gland, and the other is the mucous gland. The latter type is more
numerous (fig. 267F). Mixed glands, partly mucous and partly granular, also
may appear (Dawson, '20). Large club-shaped cells or unicellular glands
may be observed in the lower epidermal areas, while flattened cornified ele-
ments lie upon the outer surface of the epidermis.
The dermis is arranged in three layers as follows:
(a) a thin, outer, compact layer between the lower epidermal cells and
the dermal chromatophores,
(b) below this outer compact layer, the intermediate spongy layer, con-
taining some elastic, connective-tissue fibers as well as white fibers, and
(c) below the spongy layer, the inner compact layer.
The chromatophores located in the outer part of the dermal layer are of
different kinds (see p. 591).
568 INTEGUMENTARY SYSTEM
c. Development of the Skin in the Frog, Rana pipiens
The development of the skin of the common frog resembles closely that
of Necturus. The primitive epidermal tube consists of two layers of ectodermal
cells, an outer periderm and a lower nervous layer or stratum germinativum
(fig. 267A). The cells of the periderm contain pigment granules, and uni-
cellular glands also are present, particularly in the head region. At the 10-mm.
stage, the outer, pigmented, peridermal layer begins to flatten, while the
stratum germinativum assumes the normal characteristics of the reproductive
stratum of the epidermis (fig. 267B). The cells are cuboidal and closely ar-
ranged. A condensation of mesenchyme, immediately below the thin epidermal
layer, represents the rudiment of the future dermis. Chromatophores are
prevalent in the dermal area. In figure 267C are shown the characteristics of
the skin of the head area of the 34-mm. tadpole, while figure 267G represents
the skin of the head region of the newly metamorphosed frog. In this area
of the body, the dermis is compact and dense, but in the dorso-lateral area of
the trunk, large lymph spaces are present in the dermis.
3. Reptiles
a. Characteristics of the Reptilian Skin
Most reptiles are land-frequenting animals. The land type of habitat dic-
tates the development of a mechanism which keeps the lower layers of the
epidermis soft and moist. The problem of epidermal drying is not encountered
to any great extent in the fishes and most amphibia because of the moist con-
ditions under which they live. To circumvent the drying eff'ects imposed upon
land-living animals, the outer layers of the skin become cornified. A super-
ficial or outer stratum corneum, therefore, becomes a prominent feature of
the epidermis of reptiles, birds, and mammals.
Aside from its role of protecting the lower epidermal layers of cells against
loss of moisture, the cornified layer also functions as a protective mechanism
against mechanical injury. Foot pads, friction ridges, and all calloused struc-
tures are evidence of this function. The cornified stratum represents flattened,
dead, epithelial cells, infiltrated with a protein substance, keratin, present
abundantly in all horny structures, such as claws, scales, etc.
Both epidermal and dermal layers are thickened considerably in reptiles,
while epidermal glands, so prominent in fishes and amphibia, are absent,
with the exception of certain specialized regions in the oral and anal areas,
between the carapace and plastron of some turtles, and between the scales in
certain areas of the skin of crocodiles and alligators.
b. Development of the Turtle Skin
The turtle is an example of an armored animal, possessing a "shell" con-
sisting of a dermal skeleton, the carapace, and the plastron, composed of a
DEVELOPMENT OF THE SKIN
569
^%i^
PRIMITIVE
ERTEBRAL BODY
PERIDERM
ATE EPIDERMIS
NOTOChORD
EPIOER
STRATUM CORNEUM
SCALE
EPIDERMIS
^DERMAL MESENCHYME
^ \5- VERTEBRAL ARCH
EURAL TUBE
DERMIS
VERTEBRAL BODY
Fig. 268. Development of turtle skin. (A) Section through turtle embryo, showing
early division of epidermis into periderm and germinative stratum. (B) Section show-
ing two-layered condition of epidermis in slightly older embryo. (C) Section through
dorsal area of embryo, 1 1 mm. long. ( D) Higher power drawing of epidermis of 1 1-mm.
embryo. (E) Section of skin of turtle, after hatching, to show horny plates. (F)
Higher power sketch of skin shown in square in (E). (G) Section of skin of turtle
just before hatching, showing epidermal scales of carapace, dermal mesenchyme, and
vertebrae.
series of interlocking bony paltes, associated with an outer cover, the epidermal
skeleton, composed of horny scutes. The latter comprises the so-called tortoise
shell of commerce. The dorsal carapace and ventral plastron are united along
their lateral edges by a bony ridge, and the carapace is firmly fused with the
vertebrae and ribs of the endoskeleton. The skin of the head, neck, tail, and
legs is fortified with thick horny plates placed at intervals (fig. 268E). Be-
tween these horny plates, the stratum corneum is highly developed (fig. 268F).
At the 11- to 15-mm. stage, the condensation of dermal mesenchyme already
is thickened greatly in the dorsal region of the embryo in the future carapace
area. This thickened condition and the intimate association of the mesenchyme
with the trunk vertebrae and ribs are shown in figure 268C. The rudiment of
the plastron begins to appear in the ventral region at this time.
After the young hatch from the egg, ossifications occur within the dermal
mesenchyme of the carapace and plastron. The bony ossifications of the
570
INTEGUMENTARY SYSTEM
carapace gradually fuse with the flattened trunk vertebrae and the flattened
ribs. In figure 268G is shown a longitudinal section through a part of the mid-
dorsal area of a turtle just before hatching. It is to be observed that the epi-
dermal horny scales or scutes are well formed, while the dermal mesenchyme
of the carapace is wrapped intimately around the flattened, dorsal, spinous
processes of the vertebrae.
Epidermal scales and thickened horny skin pads, together with an armor
of bone, in turtles, demonstrate the types of dermal and epidermal differen-
tiations which form a protective coat in the reptilian group. The "shed skin"
of the snake represents a sheet of horny epidermal scales which is peri-
odically cast off. New scales are reformed repeatedly throughout the life of
snakes. The rattles on the terminal end of the tail in the rattlesnake represent
horny rings, developed proximal to the horny spine, prevalent as the end piece
of the tail of many serpents. Lizards are well protected with thick epidermal
scales, and in some species these scales are reinforced with dermal bony
plates. The crocodiles are tough-skinned animals, possessing thick epidermal
scales; the dorsal scales are supported underneath by corresponding dermal
Fig. 269. Development of skin in the chick. (C after WiUiams: Am. J. Anat., 11.)
(A) Epidermis of 48-hr. chick. (B) Epidermis of 72-hr. chick. (C) Dermal mesen-
chyme, arising from dermatome of embryo of 40 somites. (D) Skin of chick embryo,
incubation six days. (E) Skin of eight-day embryo, showing beginning of feather rudi-
ment. (F) Eleven-day embryo, feather rudiment. (G) Section of mature skin between
feather outgrowths. Observe that the epidermis is thin, and that the dermis is composed
of two compact layers separated by a vascular layer.
DEVELOPMENT OF THE SKIN 571
bony plates. Horny claws develop upon the digits of the appendages in turtles,
crocodiles, and lizards.
4. Birds
a. Characteristics of the Avian Skin
The skin of the bird is more delicate than that of the reptile. The epidermal
layer is thin with a highly cornified external surface. The dermis is composed
of an outer compact layer below the epidermis, and beneath the latter is a
vascular layer. Below the vascular layer is another compact layer of con-
nective-tissue fibers, and between this layer and the deep fascia is the charac-
teristic adipose (fatty) layer (fig. 269G). Extensive cutaneous glands are not
developed. However, the two uropygial or preening glands at the base of the
tail are common to most birds, although they are not present in the ostriches.
In certain gallinaceous birds, such as the common fowl, modified sebaceous
glands are present around the ear. Scales, resembling the reptilian type, are
developed on the distal parts of the legs, while feathers present a feature
characteristic of the avian skin.
1) Kinds of Feathers. Feathers are of many kinds, but they may be grouped
under three major categories:
(1) plumae (plumous or pennaceous feathers), the most perfectly con-
structed type of feather, tilling the role of contour feathers,
(2) plumules (plumulae or plumulaceous feathers), making up the under
feather coat or down, and
(3) filoplumes or hair feathers.
Of all the epidermal structures developed in the vertebrate group, feathers
appear to be the most ingeniously constructed. They possess to a high degree
the qualities of lightness, strength, and toughness which serve to protect a
delicately constructed skin from cold, moisture, and abrasion.
2) General Structure of Feathers: a) Pluma or Contour Feather. The
plumous feather consists of a rachis (shaft or scape) and a vane. The proximal
portion of the rachis or shaft is the quill or calamus. The latter is hollow but
may contain a small amount of loose pith. It has an opening, the inferior
umbilicus, at its base. The quill resides in a feather follicle, a deep pit sur-
rounded by epidermal tissue projecting downward into the dermal part of the
skin (fig. 270D, E).. Above the quill is the expanded "feathery" portion of
the feather, called the vane. At the junction of the quill and the vane is a
small opening, the superior umbilicus, to which is attached, in some contour
feathers, a secondary, smaller shaft, the aftershaft or hyporachis, together
with a group of irregularly placed barbs.
The shaft of the vane of the feather is semisolid, with its interior filled
with a mass of horny, air-filled cavities. Extending outward from the shaft
in this area are lateral branches or barbs (fig. 270E). The barbs form two
BARB RUDIMENT
\
AREA OF
FORMING
RACHIS BY
CONVERGENCE
DORSO-MEDIAD
COLLAR CELLS
DORSAL
VENTRAL
MIGRATION OF COLLAR DORSALLY
RAPIDLY GROWING AREA OF THE COLLAR
RUDIMENTS
Fig. 270. Diagrams of developing feathers in chick. (A) Nestling, down-feather
rudiment of chick of about 12 days of incubation. (B) Feather rudiment, 12 to 14
days of incubation, showing beginning of definitive feather rudiment. (C) Nestling
down rudiment and definitive feather rudiment of chick shortly before hatching. (D)
Relation of nestling down feather to definitive feather shortly after hatching. (E) Later
stage in definitive feather development; nestling down feather is attached to distal end
of first definitive feather. (F-H) Cross sections of nestling down rudiment diagram-
matically shown in (B). (1) Cross section of definitive feather rudiment shown in
(D). (J) Cross section of definitive rudiment shown in (E). It is to be noted that the
sheath around the developing feather extends for a considerable distance beyond the
surface of the skin during development. This area is shortened considerably in E for
diagrammatic purposes. F-I based on data from Jones ('07).
572
DEVELOPMENT OF THE SKIN 573
rows, one on either side of the shaft. From the barbs, smaller branches ex-
tend outward; the latter are the barbules (fig. 270E). An interlocking system
of hooks, the barbicels, enables the barbule of one barb to connect with a
barbule of the next barb. If these inierlocking hooks are disrupted mechani-
cally, the bird restores them while preening its feathers.
b) Plumule or Down Feather. The plumules or down feathers form
an inner feathery coat which lies below the contour feathers in the adult bird.
They constitute the main insulating portion of the feather coat. In the down
feathers of the adult, the barbs arise in bouquet fashion at the distal end of
the quill. On the other hand, the nestling or first down feathers of the chick
or newly hatched birds of other species do not possess a quill, for the barbs
are attached to the distal ends of the apical barbs of the definitive feather
(fig. 270E). Therefore, two types of down feathers are found:
( 1 ) the nestling down feather without a quill and
(2) the later down feather which possesses a quill.
The barbules in down feathers do not interlock, and a vane is not formed
(fig. 270D, E).
c) FiLOPLUME OR Hair Feather. The filoplume or hair feather possesses
a long slender shaft which generally is deprived of barbs, although a tuft of
barbs may be present at the distal end.
d) Distribution of Feathers on the Body. Feathers are not evenly
distributed over the surface of the body but arise in certain definite areas or
feather tracts, the pterylae. Between the pterylae are the apteria or areas
where the number of feathers are reduced or absent altogether. When feathers
are present in an apterium, they consist mainly of a scanty distribution of
downy and filoplumous feathers.
b. Development of the Avian Skin
1) Development of the Epidermis, Dermis, and Nestling Down Feather.
When the epidermal tube in the chick embryo begins to form, it consists of
a single layer of cells of one cell in thickness. As development proceeds, this
single-layered condition becomes transformed into a double layer, so that at
48 to 72 hours of incubation a two-layered epidermis is realized. This condi-
tion consists of an outer layer or periderm, considerably flattened, and an
inner layer or stratum germinativum (fig. 269A, B). At 96 hours of incu-
bation in most parts of the developing integument, a primitive dermis is present
as a loose aggregate of mesenchyme below the two-layered epidermis. The
origin of a part of this mesenchyme from the dermatome is shown in figure
269C. At six days of incubation, mesenchyme is present as a definite dermal
condensation (fig. 269D).
Between the sixth and eighth days of incubation, the epidermis and dermis
increase in thickness, and small, mound-like protuberances begin to appear
574 INTEGUMENTARY SYSTEM
in certain areas (fig. 269E). Each elevation is produced by a mass of cells,
known as the dermal papilla, which pushes the epidermal layer outward (fig.
269E). The initial dermal papillae represent the beginnings of the feather
rudiments. At eleven days of incubation, many feather rudiments have made
their appearance. Each rudiment consists of a central, mesenchymal (dermal)
core or pulp, surrounded externally by epidermal cells. The dermal pulp is
supplied copiously with small blood vessels (fig. 269F). The epidermal cells
at this time are beginning to be arranged into longitudinal columns of cells.
These longitudinal cellular columns represent the initial stages of barb-
rudiment development (fig. 270A). This condition of the developing feather
marks the beginning of the first or the "nestling down" feathers.
At 12 to 14 days of incubation, the feather rudiment increases considerably
in length and begins to invaginate into the dermal layer at its base (fig. 270B).
This invagination of the base of the feather rudiment marks the beginning of
definitive feather formation (Jones, '07). In the developing feather from 14
to 17 days of incubation, two general regions are indicated. These regions of
the developing feather are:
(a) a region from the surface of the skin to the distal end of the feather
germ where the barbs and barbules of the nestling down are being
formed (fig. 270B) and
(b) a proximal region below the surface of the skin where the barbs and
barbules of the definitive feather begin to differentiate (fig. 270B).
After the seventeenth day, the differentiation of the definitive feather pro-
ceeds rapidly (fig. 270C, D).
From the fourteenth to the seventeenth days, the barbs of the nestling down
feathers elongate slightly by adding new ridge material at the basal end of
each ridge (fig. 270B, C). The length of the barb rudiments of the down
feather thus increases as the feather rudiment grows outward from the surface
of the skin. As the barb rudiments elongate, they differentiate into the barbs
and barbules (fig. 27 IB, C). (See Davies, 1889; Strong, '02.) At about
eighteen days of incubation, such a feather may be removed, and the distal
portion of the horny sheath may be ruptured with a needle. Following the
rupture of the horny sheath, the enclosed barbs will spread out as shown in
the distal part of the developing feather in figure 270D.
At eighteen to twenty days of incubation, feather development in the chick
may be represented as shown in figure 270C and D. A distal or nestling-
down-feather region and a proximal definitive-feather area are present. Barbs
and barbules of the definitive feather differentiate in the proximal area. A
real quill is not established at the base of the nestling down feather, although
a horny cylinder may intervene between the base of the down feather and
the barbs of the definitive feather (fig. 270D). (See Jones, '07.) Thus, in the
chick and most birds, the first or nestling down feather and the succeeding
DEVELOPMENT OF THE SKIN 575
definitive feather are developed as one continuous process, and cannot be
regarded as two separate feather growths (Jones, '07, p. 17). When the chick
hatches, the outer horny sheath around the differentiated down feather dries
and cracks open, and the barbs and barbules of the down feather spread out
into fuzzy tufted structures (fig. 270D). Later, as the definitive feather emerges
from the surface of the skin, the down-feather barbs appear as delicate tufts,
attached to the distal ends of the barbs of the definitive feather (fig. 270E).
2) Development of the Contour Feather. The development of the contour
feather is more complicated than that of the nestling down feather described
above. Its development may be divided into early or primary and later or
secondary phases (Lillie and Juhn, '32). The formation of barbs during the
early phase consists in the elaboration of barb and barbule rudiments without
a shaft rudiment. This type of development resembles somewhat that of the
down feather. The secondary phase of contour-feather development is con-
cerned with the formation of a shaft, as well as the barb and barbule rudiments.
a) Formation of Barbs During the Primary or Early Phase of
Contour-feather Formation. During the first phase of contour-feather
formation, the barbs are formed in two different orders. The first order of
barb rudiments arises more or less simultaneously (Lillie and Juhn, '32); they
are practically of the same size, about equal in number on either side, and
dorsally placed. After this first set of barb rudiments is formed, a second order
of barb rudiments arises in seriatim with the youngest barb rudiments, located
more ventrally. (See first and second sets of barb rudiments in fig. 270D.)
Both of these sets of barb rudiments eventually give origin to the barbs at the
apical or distal end of the feather. As a shaft is not formed during the period
when these two sets of barb rudiments are developing, i.e., during the first
phase of definitive, contour-feather formation, these barbs later become asso-
ciated with the forming shaft as the latter develops during the next or second
phase of feather formation.
b) Secondary Phase of Contour-feather Formation. Following the
formation of the barb rudiments mentioned above, the second phase of feather
formation is initiated. It consists in the formation of the shaft and the further
development of barb ridges and barbules. The development of the shaft is
effected by the migration dorsalward of the collar cells (fig. 270E), which
produces a continuous concrescence and fusion in the middorsal line of the
two dorsal ends of the barb-bearing collar. This fusion of the collar cells
forms the rudiment of the shaft as indicated in figure 270D. This concrescence
of cells, however, establishes only the rudiment of the shaft, for it is apparent
that the development of the shaft results from two sets of processes:
( 1 ) the concrescence of a segment of the shaft rudiment at a particular
point in the middorsal line of the feather rudiment and
(2) the elongation and growth of the rudiment material thus established.
576 INTEGUMENTARY SYSTEM
As the shaft is laid down progressively from apex to base, the continuous
concrescence of the collar cells and gradual formation of the shaft rudiment
along the middorsal plane of the feather germ bring about the formation of
the shaft (Lillie, '40; Lillie and Juhn, '32, '38), beginning at its apex and
progressing baseward.
As the collar material is fed into the developing shaft rudiment dorsally, the
bases of the barbs, which are located in the collar or germinative ring, are
carried continuously dorsalward and eventually become located along the sides
of the shaft (fig. 270E). Also, the first set of barbs, which was formed in the
first phase of contour-feather formation, becomes attached along either side
of the developing shaft in the same way that the later barbs become attached.
In the formation of the barb, the apical or distal end of the barb is laid
down by cellular contributions from the collar. Following this, more basal
or proximal portions of the barb are elaborated by cellular deposition from
the collar cells. The base of the barb thus remains attached to the collar
as the barb rudiment elongates, while the apex maintains its position in the
midventral line. As the base of the barb and the collar material to which it
is attached move dorsalward toward the forming shaft, as observed in the
previous paragraph, the base of the barb comes in contact with and fuses
with the rachis or shaft, whereas the ventral extremity, i.e., the distal end
of the barb, remains associated with the mesodermal pulp along the ventral
aspect of the developing feather (fig. 271 A). The barb thus comes to form
a half spiral around the developing feather within the external horny sheath
(fig. 270E). As successive barb rudiments are laid down, the previously
formed barbs are moved progressively distad along with the mesodermal core.
c) Formation of the Barbules and the Feather Vane. During the
period when the barbs are being formed, the side branches of the barbs or
barbules are developed by the formation of groups of cells along either side
of the barb (fig. 271B, C). Each of these groups of barbule cells differentiates
into a barbule. A barbule thus represents a group of cells, specialized to form
an elongated structure as shown in figure 27 ID. After the distal end of the
feather extends markedly beyond the surface of the skin, the horny sheath
breaks, and the barbs and barbules expand to form the vane of the feather.
In doing so, the barbules interlock by means of barbicels which develop on
the barbules, located on the side of the barbs facing toward the apex of the
feather (fig. 27 ID).
d) Later Development of the Feather Shaft. During its develop-
ment, the shaft gradually enlarges in the direction of the base of the feather.
When the feather approaches its mature length, the shaft has enlarged to
the extent that it comes to occupy the entire basal portion of the feather rudi-
ment. As the last condition develops, barb formation becomes less exact until
finally it is suppressed altogether. When this stage is reached, the contained
dermal pulp within the base of the shaft begins to atrophy, starting at the end
EPIDERMIS OF SKIN
APICAL ARBORIZATION
Fig. 271. Diagrams of feather development. (A from F. R. Lillie: Physiol. Zool.,
13; C and D redrawn from Strong: Bull. Mus. Comp. Zool. at Harvard, '40.) (A)
Semidiagrammatic drawing of the pulp (papilla) of a regenerating feather. The axial
artery of the feather is shown traversing the pulp to the distal end. The veins of the
pulp (not shown) consist of a series of central and peripheral veins which connect with
venous sinuses at the base of the pulp and, from thence, communicate with the cutaneous
veins. (B) Part of transverse section of a feather follicle, showing the developing
barbs and barbules. (C) Transverse section of a feather rudiment of the tern. Sterna
hirundo. Pigment cells, within the barb substance, send out processes which distribute
melanin to the cells of the developing barbule. (D) Middle portion of wing-feather
barbule, showing pigment within individual barbule cells together with the distal barbicels
with their booklets; cornification is not complete.
577
578 INTEGUMENTARY SYSTEM
nearest the proximally placed barbs. As a result, a series of horny, hollow
cells are formed within the base of the developing feather shaft. This hollow,
basal end of the feather shaft forms the quill or calamus. The quill has a
proximal umbilicus or opening through which the dermal pulp extends into
the interior of the quill in the intact feather (fig. 27 lA). A distal umbilicus,
from which the after feather emerges, may also be present in some feathers
at the point where the ventral groove of the shaft meets the upper end of the
quill.
3) Formation of the After Feather. The after feather emerges from the
upper end of the quill of the contour feather. It is well developed in the un-
specialized, contour feather but may be absent or represented merely by a
few barbs in flight and tail feathers of the fowl (Lillie and Juhn, '38). For a
description of the after feather and its distribution in birds, reference may be
made to Chandler ('16).
As observed above, when the rachis or shaft reaches a certain size, the de-
velopment of barbs tends to be suppressed. A stage is reached ultimately
when the barbs are irregular and not well formed. Consequently, the barbs
near the quill lose all tendency to form a vane and are placed iii an irregular
fashion along the shaft. As this distortion of barb development occurs dorsally,
some of the developing barbs on the ventral side of the enlarged shaft become
physiologically and morphologically isolated from those which are moving
dorsad in the normal fashion along the collar. As a result, they remain on
the ventral surface and, in this position, they endeavor to form a twin feather.
In doing so, they become attached in their isolated position to the ventral
aspect of the forming quill. The superior umbilicus marks this point of
attachment.
The degree of development of the after feather varies from the presence
of a few barbs to a condition where a well-formed, miniature, secondary feather
is developed. The secondary or after feather in this condition possesses a
secondary rachis or aftershaft, known as the hyporachis, and is attached to
the main rachis at the superior umbilicus.
4) Development of the Later Down and Filoplumous Feathers. The de-
velopment of the later down or undercoat feather is similar to that of the
nestling down feather, with the exception that a basal shaft or quill is formed
to which the barbs become attached at the distal end of the quill. In the for-
mation of the hair feather or filoplume, an elongated shaft of small diameter
is formed to which a few small barbs may be attached at the distal end.
5. Mammals
a. Characteristics of the Mammalian Skin
The adult skin of mammals is characterized by a highly cornified, outer
layer of the epidermis, together with the presence of numerous glands and
hair. Hair, a distinguishing feature of the mammalian skin, is present in all
DEVELOPMENT OF THE SKIN 579
species, with the exception of the Cetacea (whales) and the Sirenia (sea cows) .
Various types of horny structures are associated with the epidermis, while
the dermis may develop plates of bone in certain instances. Both epidermis
and dermis are of considerable thickness.
b. Development of the Skin
1) Development of the Skin in General. As in other vertebrates, the primi-
tive mammalian integument is formed by the epidermal tube which, when
first developed, consists of a single layer, one cell in thickness (fig. 272A).
Later it becomes double layered, having an external flattened periderm and
an inner stratum germinativum. As in other vertebrates, the germinative
stratum is the reproductive layer. Mesenchyme condenses below the germina-
tive stratum, and the rudiment of the future dermis is formed (fig. 272B).
In the further development of the epidermal layer, a third layer of cells,
the stratum intermedium, appears between the periderm and the stratum ger-
minativum (fig. 272C). The stratum germinativum or deep layer of Malpighi
may appear to be several cells in thickness as development proceeds. The
cells of the germinative stratum, in contact with the dermal surface, are
cuboidal or cylindrical (fig. 272C, D). During later developrpent, the epi-
dermis becomes highly stratified, and the outer or external layer is converted
into a cornified layer, the stratum corneum (fig. 272D). Cornification oc-
curs first on the future contact surfaces of the appendages, such as the volar
surface of the hand, plantar surface of the foot, and foot pads of the cat, dog,
etc. Pigment granules (melanin) appear in the deepest layers of the epidermis
in the region of the basal, cylindrical cells of the stratum germinativum during
later fetal development and after parturition (birth).
In the meantime, the dermal mesenchyme increases in thickness, and vari-
ous types of connective-tissue fibers, white and elastic (see Chap. 15), appear
in the intercellular substance between the mesenchymal cells. Pigment cells
make their appearance in the dermis during later fetal development. These
cells descend, probably, from cells of neural crest origin, although other
mesenchymal cells possibly may contribute to the store of pigment-forming
cells. Fat cells occur in the deeper layers of the dermis.
2) Development of Accessory Structures Associated with the Skin: a)
Development of the Hair. The first indication of hair development is the
formation of a localized thickening and invagination of the epidermal layer,
particularly the germinative stratum (fig. 272E). This thickened mass of epi-
dermal cells pushes inward, accompanied by an increase in the number of
epidermal cells in the area of invagination (fig. 272F). Adjacent mesen-
chymal cells of the dermis respond to this epidermal activity by aggregating
about the invaginating mass (fig. 272E, F). As the germinative stratum with
its central core of cells continues to push downward in tangential fashion
PERIDERM
STRATUM GERMINATIVUM
"^-SINGLE LAYER OF
EPIDERMAL CELLS
* DERMIS-
DERMATOME
SOMITE
MYOTOME
SCLEROTOME
STRATUM CORNEUM-
_PERIDERM
-STRATUM INTERMEDIUM
.-i32~GE RMINATI VE STRATUM
DERMIS
CONNECTIVE- T ISSUE
FIBERS
• PERIDERM (EPITRICHIUM)—
EPIDERMIS
DERMIS
^^', ,-^.
SE BACEOUS-GLAND --<5' 't'^^ A' i '. ,^>^^, V' W/
RUDIMENT — ^ / 'h f '/ |'^^.-)^?CJ-,!. Ifj.
lESENCHYMAL ,' \P - i ■ ' '/'/^l J:^^^ ft '■{
AGGREGATION ' ( \\ ^A ' ' W ^^f^'rU
PERIDERM
EPITHELIAL BED '^/\f-^'\'( '1^/ A ^f'O^ .7i\
-EPIDERMIS j:WWM^M)\S
OUTER SHEATH ts,\'„ I M¥' I^^f'^W //) '
HAIR SHAFT >\ '!^ .Mf^'S^W/^'
HAIR CONE i*.- „ ' fMAi^^/j '< ' ■^' ff,
HAIR CONE
INNER HAIR SHEAT
HAIR BULB
PAPILLA
MESENCHYMAL
AGGREGATION
BLOOD VESSELS
^V G
Fig. 272. Diagrams of developing hair. (A from Johnson: Carnegie Inst., Washington,
Publ. No. 226, Contrib. to Embryol., 6; C and D from Pinkus, Chap. 10, The develop-
ment of the integument, Keibel and Mall, 1910, Vol. I, Lippincott, Phila.) (A) Section
through epidermis of 24-somite human embryo. (B) Section through developing skin
of 15-mm. cat embryo. (C) Section through 85-mm. human embryo, showing three-
layered epidermis. (D) Human skin, eight months, showing well-developed stratum
corneum. (E) Early hair germ in human skin. (F) Later hair germ in human skin.
(G) Still later hair germ, showing hair cone, sebaceous-gland rudiment, and epithelial bed.
Observe that the hair cone arises as a result of the proliferative activity of the cells of
the epithelial or hair matrix which overlies the mesenchymal papilla. Compare with
fig. 273A.
580
DEVELOPMENT OF THE SKIN 581
into the dermis, the surrounding mesenchyme forms a delicate, enveloping,
connective-tissue sheath around the epidermal downgrowth (fig. 272G).
As development continues, the distal portion of the germinative stratum
forms a bulbous enlargement, the hair bulb. The mesenchymal rudiment of
the papilla pushes into this bulb at its distal end to form the beginnings of
the knob-like, definitive papilla of the future hair (fig. 272G). The hair rudi-
ment then is formed by the proliferation of the epidermal cells, immediately
overlying the knob-like papilla. The epithelial cells, overlying the papilla,
form the epithelial matrix of the bulb (fig. 272G). The cells of the matrix
soon produce a central core within the hair follicle, known as the hair cone
(fig. 272G). The latter is a conical mass of cells which extends upward
from the bulb into the center of the cellular material of the epidermal down-
growth. The hair cone thus gives origin to the beginnings of the hair shaft
and the inner hair (epithelial) sheath (fig. 272G). The peripheral cells of the
original epithelial downgrowth, which now surround the hair shaft and inner
hair sheath, form the outer sheath (fig. 272G).
When the growing shaft of the hair reaches the level of the epidermal layer
of the skin, it follows along a hair canal or opening in the epidermal layer
and finally erupts at the surface of the skin.
As the foregoing changes are effected, two epithelial growths appear along
the lower surface of the obliquely placed, hair follicle (fig. 272G). The upper
growth is the rudiment of the sebaceous gland which with certain exceptions
generally is associated with hair development. The lower epithelial outgrowth
forms the epithelial bed. This bed represents reserve epithelial material for
future hair generations. The arrector pili muscle arises from adjacent mesen-
chymal cells and becomes attached to the side of the follicle (figs. 272G; 273).
This muscle functions to make the hair "stand on end," so noticeable in the
neck-shoulder area of an angered dog.
The first hair to be developed is known as the down hair, fine hair or lanugo.
In the human, the body is generally covered with lanugo by the seventh to
eighth fetal month. It tends to be cast off immediately before birth or shortly
thereafter. The lanugo corresponds somewhat to the nestling down of the
chick, for the replacing hairs develop from the same follicles as the down hairs
after the follicles have been reorganized from cells derived from the epithelial
bed. However, some replacing hairs appear to arise from new hair follicles.
The hair on the face of the human female, exclusive of the eyebrows,
nostrils, and eyelids, and also on the neck and trunk is of the fine-haired
variety and resembles the lanugo of the fetus, whereas hair on the face of
the human male is of the fine-haired type, exclusive of the eyebrows, eyelids,
nostrils, and beard. Hair on various other regions of the male body may be
of the fine-haired or lanugo variety.
b) Structure of the Mature Hair and the Hair Follicle. The gen-
eral structure of the mature hair and its follicle is as follows: The hair itself
582
INTEGUMENTARY SYSTEM
consists of a shaft and a root (fig. 273A). The hair shaft is composed, when
viewed in transverse section, of three regions of modified cells or products
(fig. 273B). The innermost, central (axial) portion ofthe shaft is the medulla.
It is composed of shrunken, cornified cells separated by air spaces. Surround-
ing the medulla, is the cortex, constructed of a dense horny substance inter-
spersed with air vacuoles. External to the latter is the cuticle, made up of
thin, cornified, epithelial cells with irregular outlines. The cuticle is trans-
parent and glassy in texture. The pigment or coloring substance is contained
within the cortical and medullary portions of the hair. Hair color is dependent
upon two main factors:
( 1 ) the nature and quantity of pigment present and
(2) the amount of air within the cortex and medulla.
In some hairs, a distinct medullary portion may be absent.
While the shaft of the hair represents a cornified modification of epidermal
EPI DE RMIS
ARY AREA
CORT CAL SUBSTANCE OF
CUTICLE OF HAIR
TERNAL ROOT SHE
CUT CLE
XLEY S LAYER
HENLF S
\_
HYPODERMIS
SUBCUTANEOUS
LAYER OR
SUPERFICIAL
FASCIA
CONTAIN ING
FATTY TISSUE
CONNECT VE TISSUE
GLASSY MEMBRANE
EXTERNAL ROOT SHEATH
Fig. 273. Diagrams of hair and follicle. (B redrawn from Maximow and Bloom,
1942. A Textbook of Histology. Saunders, Phila., slightly modified.) (A) Diagram-
matic representation of the hair shaft and follicle in relation to skin. (B) Transverse
section of hair shaft and follicle in skin of a pig embryo.
DEVELOPMENT OF THE SKIN
583
Fig. 274. Diagrams of nails, claws and hoofs. (A redrawn and modified from Pinkus,
Chap. 10. The Development of the Integument, from Keibel and Mall, 1910. Vol. I,
Lippincott, Phila.) (A) Longitudinal section of index finger of human fetus of 8.5
cm. (B) Longitudinal section of human finger, showing relationships of fully developed
nail plate. (C) Claw of the cat. (D) Cloven hoof of the pig. (E) Developing hoof
of pig. (F) Uncleft hoof of horse, lateral view. (G) Uncleft hoof of horse, ventral view.
cells, the root contains the cells in a viable condition before transformation
into the cornified state. The root of the hair consists of the hair papilla, com-
posed of dermal mesenchymal cells, blood vessels, nerve fibers, and a cup-
shaped epithelial matrix which overlies the papilla (fig. 273 A). The hair
shaft and the internal root sheath are derived from the modification of the
cells of the hair matrix. The internal root sheath is composed of the inner
sheath cuticle, together with Huxley's and Henle's layers (fig. 273B). The
internal sheath disappears in the upper regions of the follicle near the entrance
of the sebaceous gland. External to the internal root sheath is the external
root sheath. The latter represents the wall of the epithelial follicle and is the
downward continuation of the epidermal layer of the skin around the root of
the hair. The external root sheath thus forms a pocket-like structure, extend-
ing from the distal margin of the hair matrix to the epidermis of the surface
skin. A sheath of dermal cells and fibers lies around the external root sheath
and acts as the skeletal support of the hair.
During development, hair first appears in the region of the eyebrows and
around the mouth. Later it develops over the surface of the body in a regular
584 INTEGUMENTARY SYSTEM
pattern. This pattern tends to have a definite relationship to scales when
present.
3) Development of Nails, Claws, and Hoofs. Resembling and closely linked
to epidermal scales are the nails, claws, and hoofs of mammals. The claws
of reptiles and birds belong to the same category of terminal protective de-
vices for the digits. Nails are flattened discs of horny material, placed on the
dorsal surfaces of the terminal phalanges (fig. 274A, B). Claws are similar
and represent thickened, laterally compressed, and pointed nails (fig. 274C).
Hoofs are composite structures on the terminal phalanges of the digits, but,
unlike nails and claws, they are composed of two much-thickened nails, one
dorsal and one ventral.
The distal protective device of the human digit is composed of a dorsal
structure, the nail plate or unguis. A formidable, horny subunguis or ventral
nail plate is absent, although a subungual region, consisting of an area of
extreme cornification of the stratum corneum of the skin, is present (fig.
274B). The claw of the cat or dog is similar, with the nail plate compressed
laterally, and the subungual cornification is greater. On the other hand, hoofs
possess a dorsal nail plate (unguis) and a well-developed ventral nail plate
(subunguis). Hoofs may be further divided into two general groups. In one
group are the hoofs of cows, sheep, deer, etc., which form two, nail-forming
mechanisms at the terminus of the digit, one dorsal and one ventral, from
which the dorsal and ventral nail plates arise. In the other group are the
hoofs of horses, donkeys, zebras, etc., which develop a dorsal, nail-developing
mechanism, forming the dorsal nail plate, and two ventral, nail-producing
structures. One of the latter generative devices gives origin to the frog and
the other to the ventral nail plate. Thus, embryologically, nails and claws
belong to one group, whereas hoofs form another.
A better appreciation of the above-mentioned facts relative to claws, nails,
and hoofs can be gained by considering the development of a relatively simple,
terminal structure of the digit, the human finger nail.
The nails on the terminal digits of the developing human finger begin to
form when the embryo (fetus) is about three months old. In doing so, a
thickened epidermal area arises on the dorsal aspect of the terminal end of
the digit. This general, thickened, epidermal area constitutes the nail field.
The proximal portion of the nail field then invaginates in a horizontal direc-
tion, passing inward into the underlying mesenchyme toward the base of the
distal phalanx. This invaginated epidermal material forms the nail fold or
groove, and it lies within the mesenchyme, paralleling the overlying epidermis
(fig. 274A). The nail fold, when viewed from above, is a crescent-shaped
aff'air with the outer aspect of the crescent facing distally; it may be divided
into a deeper layer, the nail matrix, and a more superficial layer. The nail
matrix is confined almost entirely within the nail fold or groove. The distal
edge of the lunula marks its greatest extension distally along the nail field.
DEVELOPMENT OF THE SKIN 585
At about the fifth month, the upper cells of the nail matrix begin to keratinize,
and the keratinized cells gradually fuse into the compact nail plate. As new
material is added to the nail plate from the cells of the matrix, the distal
portion of the plate is pushed progressively toward the end of the digit (fig.
274A). Although that portion of the nail field between the terminal end of
the digit and the lunula takes no part in the formation of the cornified ma-
terial of the nail plate, the underlying dermis below the nail field does form
elongated ridges which push upward into the epidermis of the nail field. These
ridges secondarily modify the already-formed nail plate by producing fine,
longitudinal lines or ridges.
The claw or nail plate of the cat is compressed laterally to form a narrow,
sickle-shaped structure. Three main factors are responsible for this peculiar
form of the nail plate in the cat. One factor is the laterally compressed form
of the distal phalanx. This condition results in a nail-fold invagination which
is laterally compressed. The nail matrix thus is elliptical in shape, dorso-
ventrally, instead of flattened as in the human finger. A second factor re-
sponsible for the extreme, claw-shaped form of the nail plate in the cat is
the more rapid growth in the middorsal portion than in the lateral areas of
the nail plate. This discrepancy in growth results in the highly pointed mid-
region at the distal end of the nail plate. Ventrally, the two lateral sides of
the nail plate tend to approach each other. The area between these two sides
is filled with a cornified mass of subungual material. A final factor governing
the extreme pointedness of the cat's claw is the fact that the claw-distal-
phalanx arrangement, relative to the middle phalanx and tendons, makes the
claw retractile when not in use, thus preserving its pointed distal end (fig.
274C).
The dog's claw or nail on the ordinary digits is compressed laterally less
than that of the cat, with the result that the subungual cornification is broader
and more pronounced and the distal end of the claw not as pointed. However,
the claws upon the vestigial first digit, the so-called dewclaws, are pointed and
cat-like. The fact that the claw of the dog is non-retractile is a factor in re-
ducing its pointedness, for it, unlike the cat's retractile claw, is worn down
continually.
The cloven hoof of the pig or cow is produced by the formation of two
nail plates, one dorsal and one ventral, around each of the distal phalanges
of the third and fourth digits (fig. 274E). The dorsal nail plate is rounded
from side to side and meets the lower nail plate ventrally, with which it
fuses along the lateral and distal portions of the lower plate. The unsplit hoof
of the horse is produced by a somewhat similar arrangement of dorsal and
ventral nail plates around the hoof-shaped phalanx of the third digit (fig.
274F, G). A third nail plate or growth center produces the frog or cuneus.
4) Development of Horns. The horns of cattle arise as two bony out-
growths, one on either side of the head, from the area of the parietofrontal
586
INTEGUMENTARY SYSTEM
bones of the skull. In most instances the frontal bone alone is involved. Each
bony outgrowth pushes the epidermis before it. The epidermis then responds
by producing a highly keratinized, horny substance around the outgrowing
bone. The result is the formation around the bony outgrowth of an unbranched
cone (or horn) of cornified epidermal material (fig. 275A). This type of
horn grows continuously until the mature size is reached. If removed, this
type of horn will not regenerate. Horns of this structure are found in sheep,
goats, cattle, and antelopes.
The horns of the pronghorn, Antilocapra americana, are somewhat similar
to those of cattle, with the exception that the external, keratinized, slightly
branched, horny covering, overlying the bony core, is shed yearly, to be
replaced by a new horny covering (fig. 275B).
On the other hand, the antlers of the deer offer a different developmental
procedure. A new bony core is formed each spring which grows and forms
the mature antler. As this hard, bony antler matures during late summer
and early autumn, the outside covering of epidermis (i.e., the velvet) even-
tually atrophies and drops off, leaving the very hard, branched, bony core or
antler as a formidable fighting weapon for use during the breeding season
(fig. 275C). When the latter period is past, the level of the male sex hormone
falls in the blood stream, which brings about a deterioration of the bony
tissue of the antler near the skull. This area of deterioration continues until
the connection to the frontal bone becomes most tenuous, and the antlers fall
off, i.e., are shed. (See Chap. 1, p. 27.)
The horns of the giraffe are simple, unbranched affairs which retain the
velvet or epidermal covering around a bony core. The horns of the rhinoceros
are formidable, cone-shaped, median structures (one or two), composed of
a keratinized, hair-like substance. These horns are located on the nasal and
frontal bones. (For a discussion of horns in the Mammalia, see Anthony,
'28, '29.)
OF W \
ment ^^,^ 1
ANTlErIJ \; EPIDERMIS
■HED ' ' '
Fig. 275. Horns of mammals. (A) Cow. (B) Prong-horn antelope.
(C) White-tailed deer.
DEVELOPMENT OF THE SKIN 587
5) Development of the Skin Glands. Three types of glands develop in re-
lation to the skin in mammals:
( 1 ) sebaceous or oil glands,
(2) sudoriferous or sweat glands, and
(3) mammary or milk glands.
a) Sebaceous Glands. Sebaceous glands generally are associated with
the hair follicles (figs. 272G; 273A), but in some areas of the body this
association may not occur. For example, in the human, sebaceous glands
arise independently as invaginations of the epidermis in the region of the
upper eyelids, around the nostrils, on the external genitals, and around the
anus. When the sebaceous gland arises with the hair follicle, it generally takes
its origin from the lower side of the invaginated hair follicle, although this
condition may vary (fig. 272G). The sebaceous-gland rudiment originates as
an outpushing of the germinative stratum and differentiates into a simple or
compound alveolar type of gland. The secretion originates as fatty material
within the more centrally located cells of the gland, with subsequent degen-
eration of these cells and release of the oily substance. Since the secretion
forms as a result of alteration of the gland cells themselves, this type of gland
is classified as an holocrine gland. New cells are formed continuously from
that portion of the gland connected with the germinative stratum. The oil
produced is discharged to the surface of the skin through the opening of the
hair follicle when a relationship with the hair is present. If not connected with
a hair follicle, the gland has a separate opening through the epidermal layer.
b) Sudoriferous Glands. Sweat or sudoriferous glands most often de-
velop independently of hair follicles, but in certain areas they form on the
sides of these follicles. Whenever formed, they represent solid, elongated in-
growths of the epidermis into the dermis. Later these cellular cords coil at
their distal ends to form simple, coiled, tubular glands (fig. 276).
The outer wall of the forming sweat gland develops so-called myoepithelial
cells; the latter presumably have the ability to contract. The cells lining the
lumen of the gland secrete (excrete) the sweat, the distal ends of the cells
being discharged with the exudate. Hence, this type of gland is called an
apocrine gland. The secretion is watery and contains salts, wastes, including
urea, and occasionally some pigment granules and fat droplets. In the cat, dog,
and other carnivores, sweat glands are reduced in number.
c) Mammary Glands. Mammary glands are characteristic of the mam-
mals. The first indication of mammary-gland development is the formation of
the milk or mammary ridges (fig. 24 ID, E). These ridges represent elevations
of the epidermis, extending along the ventro-lateral aspect of the embryo
from the pectoral area posteriad into the inguinal region. The ridges are de-
veloped in both sexes and represent a generalized condition of development.
In the human embryo, the mammary ridge is well developed only in the
588 INTEGUMENTARY SYSTEM
-SWEAT PORE
— EPIDE RMIS
- '-■ "'^ ) vSi!// -' '^''' -•■ J>' nil VyV^ ■■ ~/ BEGINNING
-=: .S> ^ * v^ •.- ^^ ; l\ \M -K/i-'' rf; .' - F P I 0 E R M A L
>^\'SJW0'P*">'"' '-■' (Sa^^'-^-vN'' INVAGINATION
_2_r= FAT CELLS
1^
Fig. 276. Diagram of sudoriferous (sweat) gland.
pectoral region, but it is extensive in the pig, dog, and cat. In the cow, horse,
deer, etc., its greatest development is in the inguinal area.
Only very restricted areas of each mammary ridge on either side are utilized
in mammary-gland development. In the pig or dog embryo, a series of local-
ized thickenings begin to appear along the ridge. In the sheep, cow, and
horse, these thickenings are confined to the inguinal region, whereas in the
primates and the elephant, they are found in the pectoral area. In the human,
one thickening in each ridge generally appears, although occasionally several
may arise. These thickenings represent the beginnings of the nipples and
result from increased proliferations of cells (fig. 277B). Eventually, each
thickened portion of the ridge becomes bulbous and sinks inward into the
dermis (fig. 277C). Gradually, solid cords of cells push out from the lower
rim of the solid epidermal mass into the surrounding dermal tissue (fig. 277D).
These cords of cells represent the rudiments of the mammary-gland ducts.
Secondary outpushings appear at the distal ends of the primary ducts. Later,
lumina appear in the primary ducts. Further development of these ducts, with
the formation of the terminal rudimentary acini, occurs during late fetal stages,
resulting in the formation of an infantile state. This condition is found at
birth in the human, dog, cat, etc. Under the influence of hormones present
in the blood stream of the mother (see Chap. 2, p. 1 03 ) , these acini may secrete
the so-called "witch's" milk in the newborn human male and female. While
the occurrence of this type of milk secretion is not uncommon, the gland as
a whole is in a rudimentary, undeveloped state. It remains in this infantile
condition until the period of sexual development when, in the female, the
mammary-gland ducts and attendant structures begin to grow and develop
under the influence of estrogen, the female sex hormone. (See Chap. 2.) It
should be observed that the rounded condition of the developing breast in
the human female at the time of puberty (fig. 277F) is due largely to the
accumulation of fat and connective tissue and not to a great extension of the
duct system of the glands, although some duct extension does occur at this time.
As the original epithelial thickening of the nipple rudiment sinks inward,
DEVELOPMENT OF THE SKIN
589
the center of the thickened area moves downward to a greater extent than the
margins. Some disintegration of the central cells also occurs. As a result, a
slight cavity or crater-like depression is formed in the middle of the epithelial
mass of the rudiment (fig. 211 C). In the cow and rat, this depressed area
continues in this state, while the edges of the cavity and adjacent integument
grow outward to form the nipple (fig. 277E). This type of nipple is called
an inversion nipple. The ducts of the gland thus open into the bottom of the
nipple (teat or mammilla). In the human, the original depression and the
openings of the primary ducts of the gland gradually are elevated outward
to form the type of nipple or mammilla indicated in figure 211 A. This type
of nipple is called an eversion nipple.
Fig. 277. Diagrams showing mammary-gland development. (A) Human nipple
showing mammary duct openings. (Modified from Maximow and Bloom, A Textbook of
Histology, after Schaffer. 1942, Saunders. Phila.) (B) Transverse section of early nipple
rudiment of 20-mm. pig embryo. (C) Transverse section through developing nipple of
pig embryo of 70 mm., showing epidermal invagination into the dermal area of the skin.
(D) Section through nipple of mammary gland of human male fetus, eight months old.
(After Pinkus, Keibel and Mall: Manual of Human Enihryology. Vol. I, 1910, Lippincott,
Phila.) (E) Section through developing nipple of newborn rat. (Redrawn and modified
from Myers, '16. Am. J. Anat., 19.) (F) Development of human mammary gland from
birth to maturity.
590 INTEGUMENTARY SYSTEM
As indicated above, the distribution of nipples and mammary glands along
the ventral abdominal wall varies greatly in different mammalian species. In
lemurs and fruit bats, the mammary glands are developed in the axillary region;
in the human and in primates, they are pectoral; in the cat, they are best
developed in the pecto-abdominal area; in the dog and pig, they are mainly
well developed in the abdominal and inguinal areas; in the cow and horse,
inguinal nipples only appear; and in whales, the mammary glands are located
near the external genitals.
The development of supernumerary mammary glands, i.e., hypermastia, is
rare, but the formation of extra nipples, i.e., hyperthelia, is common in both
male and female. In female mammals, such as the bitch, it is not uncommon
for the breasts to remain in an undeveloped condition in the pectoral area,
whereas those in the inguinal and abdominal areas are normal. When the
mammary glands continue in an undeveloped or regressed state as, for ex-
ample, in the anterior pectoral region of the bitch, the condition is known as
micromastia. On the other hand, the abnormal development of the mammary
glands to an abnormal size is known as macromastia. The latter condition
often is found in cattle and occasionally in the bitch and human.
C. Coloration and Pigmentation of the Vertebrate Skin and
Accessory Structures
1. Factors Concerned with Skin Color
The color of the skin and its accessory structures is dependent upon five
main factors:
( 1 ) the color of the skin itself,
(2) its opacity or translucency,
(3) the presence of pigment granules and special, pigment-bearing cells,
(4) the capillary bed of blood vessels which lies within the dermal portion
of the skin, and
(5) the color of the accessory structures.
The color of the skin itself varies considerably in different species, but it
tends to be slightly yellow, resulting from the presence of fatty tissue, fat
droplets, and constitutent, connective-tissue fibers in the dermis. The prop-
erty of opacity or translucency is an important factor for upon it depends
transmission of light waves through the skin from deeper lying structures,
such as blood vessels, pigment droplets, pigment-bearing cells, etc. The pres-
ence of definite types of pigment granules within or between the cells of the
epidermis and dermis determines the course and kind of light waves which
are reflected. The richness or paucity of blood vessels, ramifying through the
dermal area, also affects the skin's color in many instances.
The color of the accessory structures, particularly the structures derived
PIGMENTATION OF THE VERTEBRATE SKIN 591
from the epidermis, greatly conditions the color pattern of the species. The
color of these accessory structures is dependent upon three main factors:
( 1 ) presence or absence of pigment,
(2) presence of air, and
(3) iridescence.
Pigment and air are dominant factors, for example, in the color exhibited
by hair and feathers. The presence of air diminishes and distorts the effects of
the pigment which may be present. The property of iridescence is to be dis-
tinguished from the color effects due to the presence of certain pigments; the
latter absorb light rays and reflect them, whereas iridescence is dependent upon
the diffraction of light waves from irregular surfaces. Iridescence is important
in the color effects produced by the plumage of a bird or the skin surface of
many fish, reptiles, and amphibia.
2. Color Patterns
In the vertebrates whose manner of life dictates a close association of the
body with the environmental substrate, the underparts have less color than
the parts exposed to the light rays coming from above. Also, within the general,
colored areas, there are certain spots, lines, bars, and dark and light regions
which follow a definite pattern more or less peculiar to the variety, subspecies,
or species. These color patterns tend to be fixed and are determined by the
heredity of the animal. Consequently, they are related to the genie complex
in some way. However, in many species the tone of the color patterns may
be changed from time to time by changing environmental conditions as men-
tioned on page 594.
3. Manner of Color-pattern Production
a. Role of Chromatophores in Producing Skin-color Effects
Work in experimental embryology has demonstrated fairly conclusively that
the pigments necessary for color formation are elaborated principally by cer-
tain cells known as chromatophores. Chromatophores are pigment-bearing
and pigment-elaborating cells. Various cells may produce pigment, but chro-
matophores are cells specialized in the function of pigment elaboration.
The distribution and activities of chromatophores vary in the different ver-
tebrate groups. For example, in fishes, amphibia, and many reptiles, three or
probably four kinds of chromatophores are present in the dermis, namely,
melanophores, lipophores, guanophores, and (possibly) allophores (Nobel,
'31, p. 141). By their presence and arrangement, the chromatophores pro-
duce specific color patterns. Moreover, the expansion and contraction of the
pigmented cytoplasm of some or all the chromatophores effects changes in
color, for the contracted or expanded state determines the types of light rays
which will be absorbed or reflected. The rapid color changes in certain tree
592 INTEGUMENTARY SYSTEM
frogs and lizards are due to this type of chromatophoric behavior. The slower
changes of color in other amphibia and fishes also are due to this type of
chromatophoric activity. It thus appears that dermal chromatophores are re-
sponsible largely for the color effects found in the lower vertebrates. On the
other hand, in the bird group and in mammals, the chromatophores present
are mainly of one type, known as a melanophore. Melanophores produce pig-
ments, known as melanins (Dushane, '44, p. 102). The melanin granules,
elaborated by the bird melanophore, have a wide range of color from yellow
through orange to reddish-brown to dark brown. The melanophores in the
bird deposit the melanin-pigment granules within the feather as it develops
(fig. 271C). Melanophores also deposit melanins in the bill of the male sparrow
at breeding time under the influence of the male sex hormone (Witschi and
Woods, '36). Hair color in mammals is due, mainly, to pigmented granules
deposited in the hair by melanophores. The skin color of various races of
the human species is determined largely by the amount of melanin deposited
within the lower epidermal layers by melanophores resident in the upper
dermal area. In other words, the color of the skin and its appendages in the
higher vertebrate groups is due, to a considerable extent, to diffuse granules
deposited in the epidermis and epidermal structures by melanophores, whereas,
in lower vertebrates, dermal chromatophores are responsible for color pattern
and color change.
b. Activities of Other Substances and Structures in Producing Color
Effects of the Skin
In the common fowl, the presence of carotenoids (lipochromes) in the
Malpighian layer (stratum germinativum) mainly is responsible for the color
of the face, legs, and feet. Orange-red, lipochromic droplets have been found
in the germinative stratum of the head of the pheasant, and these droplets
plus the capillaries in the dermis produce a brilliant red coloration (Dushane,
'44, p. 102). The color of the combs and wattles of the common fowl is con-
ceded generally to be due to the presence of a rich capillary plexus in the
dermis alone. In the ear regions of the fowl, the blood capillaries are reduced
in the dermis, and the presence of certain crystals of unknown chemical
composition produces a double refraction of the light waves. Hence, the ear
region appears white in reflected light.
c. Genie Control of Chromatophoric Activity
The transplantation of small pieces of epidermis and its adhering mesoderm
from one early chick embryo to another is possible. Under these conditions,
the donor tissue with its donor melanophores governs the color pattern of the
feathers developed in the area of the transplant (Willier and Rawles, '40).
That is, melanophores from a Black Minorca embryo, transplanted to a White
Leghorn embryo, will produce a Black Minorca color pattern, in the White
PIGMENTATION OF THE VERTEBRATE SKIN 593
Leghorn in the area of transplant, at least during the development of nestling
lown and juvenile feathers. Barred Rock melanophores produce barred feather
patterns in White Leghorn, New Hampshire Red, Black Minorca, etc. These
results demonstrate that the introduced melanophore produces the color pat-
tern in the feather in the immediate area of the implant.
Various genetic studies (see Dushane, '44, for references) have demon-
strated that the Barred Rock factor is dominant, and that it is sex-linked. For
example, if a Barred Rock hen is crossed with a Rhode Island Red cock, the
Fi male will contain two sex chromosomes, one from each parent. That
:hromosome from the female parent will have a Barred Rock factor, whereas
that from the male parent will not. The Fi cock, therefore, is heterozygous
for barring, and, as the barring factor is dominant, the Fi cock will show
barred feathers. The Fi female, however, derives its single sex chromosome
from the male parent; as this chromosome does not contain the barring factor,
the Fi female is black.
Willier ('41) presents evidence concerning the transplantation of melano-
phores from Fi heterozygous males and Fi heterozygous females of this Barred
Rock cross. Transplanted melanophores from an Fi male into White Leghorn
hosts always produce barred contour feathers in either sex, whereas Fi female
melanophores transplanted to White Leghorn hosts always produce non-barred
or black regions. Danforth ('29) demonstrated that the barring factor in the
skin of the male donor at hatching, when transplanted to a female host at
hatching which lacked the barring factor, produces barred feathers in the
female host in the area of the transplant. The results obtained by Danforth
suggest that the barring gene acts independently of the sex hormone, although
the feather type present in the graft assumes the female characters of the
host and, hence, is affected by the female sex hormone. The results of these
experiments by Willier and Danforth suggest that the barring gene in poultry
acts directly upon the melanophore and not upon the environment in which the
melanophore functions. (For extensive description, references, and discussion
of these phenomena, consult Danforth, '29; Willier, '41; and Dushane, '44.)
d. Examples of Hormonal Comrol of Chromatophoric Activity
In the indigo bunting, the male resembles the female during the non-breeding
season. During the breeding season, however, the male develops a brilliant,
purple-colored, highly iridescent plumage. Castration experiments and gonado-
trophic hormone administration suggest that this nuptial plumage is dependent,
not upon the male sex hormone, but upon gonadotrophic hormones elaborated
by the pituitary gland in the male. In the female, however, the presence of
the female sex hormone inhibits the effects of the pituitary gonadotrophins;
hence, she retains the sexually quiescent type of plumage (Domm, '39, p.
285). Also, in certain cases where the color of the bird's bill is a sex-dimorphic
character appearing during the breeding season only, it has been shown that
594 INTEGUMENTARY SYSTEM
the pigmentation of the bill is dependent upon the presence of the male sex
hormone (Domm, '39).
e. Environmental Control of Chromatophoric Activity
The above-mentioned instances of color-pattern development are concerned
with the elaboration and deposition of pigment within the epidermis and epi-
dermal structures. On the other hand, other observations demonstrate that
the contraction and expansion of chromatophores and, hence, the production
of different tones of color patterns, may be effected by a variety of environ-
mental stimuli in lower vertebrates. In some cases this may be due to direct
stimulation of the chromatophores by light or darkness or by changes in tem-
perature; in other instances the causative factor is a secretion from certain
glands, such as the pituitary or adrenal glands. The latter secretions in some
forms appear to be aroused by light waves to the eye, from whence the stimu-
lation is relayed through the nervous system to the respective gland or glands.
In still other instances the light waves to the eye may cause a direct stimulation
of the chromatophores by means of nerve fibers which reach the chromato-
phores. Other examples suggest that certain neurohumoral substances, elabo-
rated by the terminal fibers of the nerves some distance away from the chro-
matophore, slowly diffuse to the chromatophore, causing its expansion or
contraction (Noble, '32, pp. 141-147; Parker, '40).
Bibliography
Anthony, H. C. 1928; 1929. Horns and Dushane, G. P. 1943. The embryology of
antlers, their evolution, occurrence, and vertebrate pigment cells. Part I. Am-
function in the Mammalia. Bull. New phibia. Quart. Rev. Biol. 18:109.
York Zool. Soc. 31; 32. . I944. xhe embryology of verte-
Bardeen, C. R. 1900. The development of brate pigment cells. Part II. Birds. Quart,
the musculature of the body wall in the Rev. Biol. 19:98.
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Chandler. A. C. 1916. A study of the An experimental study on the feather-
structure of feathers with reference to pigmenting and subcutaneous melano-
their taxonomic significance. University phores in the silkie fowl. J. Exper. Zool.
of California Publ., Zool. 13:243. 103:233.
Danforth, C. H. 1929. Genetic and meta- Engert, H. 1900. Die Entwicklung der
bolic sex differences. J. Hered. 20:319. ventraien Rumpfmuskulatur bei Vogeln.
Davies, H. R. 1889. Die Entwicklung der Morph. Jahrb. 29:169.
Feder und ihre Beziehungen zu anderen Eycleshymer, A. C. 1906. The develop-
Integumentgebilden. Morph. Jahrb. ment of chromatophores in Necturus.
15:560. Am. J. Anat. 5:309.
Dawson, A. B. 1920. The integument of Greene, C. W. 1899. The phosphorescent
jVec/wrM5 macM/o5M5. J. Morphol. 34:487. organs in the toad-fish, Porkhthys no-
Domm, L. V. 1939. Chap. V. Modifica- '"''"■ Girard. J. Morphol. 15:667.
tions in sex and secondary sexual char- Harms, J. W. 1929, Die Realisation von
acters in birds in Sex and Internal Secre- Genen und die consecutive Adaption. I.
tions by Allen, Danforth, and Doisy. Phasen in der Differenzierung der An-
2d ed. The Williams & Wilkins Co., lagenkomplexe und die Frage der Land-
Baltimore, tier-werdung. Zeit. Wiss. Zool. 133:211.
BIBLIOGRAPHY
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Jones, L. 1907. The development of nes-
tling feathers. Oberlin College Lab. Bull.
No. 13.
Lillie, F. R. 1940. Physiology of develop-
ment of the feather. III. Growth of the
mesodermal constituents and blood cir-
culation in the pulp. Physiol. Zool.
13:143.
and Juhn, M. 1932. The physi-
ology of development of feathers. I.
Growth rate and pattern in the indi-
vidual feather. Physiol. Zool. 5:124.
and
-. 1938. Physiology of
development of the feather. II. General
principles of development with special
reference to the after-feather. Physiol.
Zool. 11:434.
Neave, F. 1936. The development of the
scales of Salmo. Tr. Roy. Soc. Canada.
30:550.
. 1940. On the histology and re-
generation of the teleost scale. Quart. J.
Micr. Sc. 81:541.
Nickerson, W. S. 1893. The development
of the scales of Lepidosteus. Bull. Mus.
Comp. Zool. at Harvard College. 24: 1 15.
Noble, G. K. 1931. The Biology of the
Amphibia. McGraw-Hill Book Co., Inc.,
New York.
Parker, G. H. 1940. Neurohumors as
chromatophore activators. Proc. Am.
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Placoid scale types and their distribution
in Squalus acanthias. Biol. Bull. 73:51.
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13
Tne Digestive System
A. Introduction
1. General structure and regions of the early digestive tube or primitive metenteron
a. Definition
b. Two main types of the early metenteron
2. Basic structure of the early metenteron (gut tube)
a. Basic regions of the primitive metenteron
1 ) Stomodaeum
2) Head gut or Seessel's pocket
3 ) Foregut
4) Midgut
5) Hindgut
6) Tail gut (post-anal gut)
7) Proctodaeum
b. Basic cellular units of the primitive metenteron
3. Areas of the primitive metenteron from which evaginations (diverticula) normally
arise
a. Stomodaeum
b. Pharynx
c. Anterior intestinal or pyloric area
d. Junction of midgut and hindgut
e. Cloacal and proctodaeal area
B. Development of the digestive tube bv metenteron
1. General morphogenesis of the digestive tube
2. Histogenesis and morphogenesis of special areas
a. Oral cavity
1 ) General characteristics of the stomodaeal invagination
2) Rudiments of the jaws
3) Development of the tongue
4) Teeth
a) General characteristics
b) Development of teeth in the shark embryo
c) Development of teeth in the frog tadpole
d) Development of the egg tooth in the chick
e) Development of teeth in mammals
5) Formation of the secondary palate
6) Formation of the lips
7) Oral glands
596
INTRODUCTION 597
b. Development of the pharyngeal area
1 ) Pharyngeal pouches and grooves
2) Pharyngeal glands of internal secretion
3) Other respiratory diverticula
c. Morphogenesis and histogenesis of the esophagus and the stomach region of the
metenteron
d. Morphogenesis and histogenesis of the hepato-pancreatic area
1 ) Development of the liver rudiment
a) Shark embryo
b) Frog embryo
c) Chick embryo
d) Pig embryo
e) Human embryo
2) Histogenesis of the liver
3) Development of the rudiments of the pancreas
a) Shark embryo
b) Frog embryo
c) Chick embryo
d) Pig embryo
e) Human embryo
4) Histogenesis of the pancreas
e. Morphogenesis and histogenesis of the intestine
1 ) Morphogenesis of the intestine in the fish group
2) Morphogenesis of the intestine in amphibia, reptiles, birds, and mammals
3) Torsion and rotation of the intestine during development
4) Histogenesis of the intestine
f. Differentiation of the cloaca
C. Physiological aspects of the developing gut tube
A. Introduction
1. General Structure and Regions of the Early Digestive
Tube or Primitive Metenteron
a. Definition
The word metenteron is applied to the gut tube which is developed from the
archenteric conditions of the gastrula. The term primitive metenteron may be
applied to the gut tube shortly after it is formed, that is, shortly after tubulation
of the entoderm to form the primitive gut tube has occurred, while the word
metenteron, unqualified, is applicable to the tubular gut, generally, throughout
all stages of its development following the gastrular state.
b. Two Main Types of the Early Metenteron
Two types or morphological forms of early vertebrate metenterons are de-
veloped immediately after the gastrular stage. In one type, such as is found
in the frog and other amphibia, ganoids, cyclostomes, and lungfishes, the walls
of the gut tube are complete, and the yolk material is enclosed principally
within the substance of the midgut area of the tube (fig. 217). In the second
598
THE DIGESTIVE SYSTEM
ESOPHAGUS
VERTICULUM
GE INTESTINE
OSTflNAL GUT
MESENCI
CONTRIBUTED
FROM SPLANCHNIC
LAYERS
OF HYPOMERE
Fig. 278. Diagrams showing basic features of digestive-tube development in the verte-
brates. (A) The regions of the primitive gut where outgrowths (diverticula) normally
occur. (B) Basic cellular features of the gut tube. (C) Contributions of the basic
cellular composition to the adult structure of the digestive tract. Consult Fig. 293 for
actual structure of mucous layer in esophagus, stomach, and intestines.
type, on the other hand, most of the yolk material Hes outside the confines of
the primitive gut tube (fig. 217), and the midgut region of the primitive tube
is open ventrally, the ventro-lateral walls of the tube being incomplete. The
latter condition is found in elasmobranch fishes, reptiles, birds, and primitive
mammals. In higher mammals, although yolk substance is greatly reduced,
the arrangement is similar to that of the latter group. The teleost fishes repre-
sent a condition somewhat intermediate between these two major groups.
2. Basic Structure of the Early Metenteron (Gut Tube)
(Consult figs. 278A; 279A; 280A; 281A; and 282B.)
a. Basic Regions of the Primitive Metenteron
The primitive vertebrate metenteron possesses the following regions.
1) Stomodaeum. The stomodaeum lies at the anterior extremity of the gut
tube, and represents an ectodermal contribution to the entodermal portion of
the primitive gut. It results from an invagination of the epidermal tube directed
toward the oral evagination of the foregut. The membrane, formed by the
INTRODUCTION
599
apposition of the oral evagination of the foregut and the stomodaeal invagina-
tion of the epidermal tube, constitutes the oral or pharyngeal membrane. Ecto-
derm and entoderm thus enter into the composition of the pharyngeal mem-
brane. This membrane normally atrophies.
2) Head Gut or Seessel's Pocket. This structure represents the extreme an-
terior end of the foregut which projects forward toward the anterior end of
the notochord and brain. It extends cephalad beyond the region of contact of
the stomodaeum with the oral evagination of the foregut. During its earlier
period, the head gut is intimately associated with the anterior end of the
PHARYNGEAL REGION OF
FOREGUT,
DULT CONDITION '-'■
Fig. 279. Morphogenesis of the digestive structures in the dog fish, Sqiialus acanthias.
See also Figs. 29 IC and 296A.
600 THE DIGESTIVE SYSTEM
notochord and the pre-chordal plate mesoderm. The head gut ultimately de-
generates. Its significance probably lies in its function as a part of the head
organizer.
3) Foregut. The foregut comprises the anterior portion of the primitive
metenteron from the region of the stomodaeum and Seessel's pocket, posteriorly
to the intestinal area where arise the liver and pancreatic diverticula. It is
divisible into four general regions:
( 1 ) pharyngeal area,
(2) esophagus,
(3) stomach, and
(4) hepatopyloric segment.
4) Midgut. The midgut area of the gut tube is the general region lying be-
tween the foregut and hindgut regions. This segment of the primitive gut even-
tually differentiates into the greater part of the small intestine. In the early
metenteron, the midgut area is concerned with the digestion of yolk material
in such forms as the frog or with the elaboration of the yolk sac in the shark,
chick, reptile, and mammalian embryos. In addition, it appears that the primi-
tive blood cells also are elaborated in this area. (See Chap. 17.)
5) Hindgut. This portion of the early gut tube is located posteriorly, imme-
diately anterior to the proctodaeum.
6) Tail Gut (Post-anal Gut). The tail gut represents a dorsal, posterior
continuation of the hindgut into the developing tail. As indicated in Chapter
10, it is extremely variable in the extent of its development. (Consult also
fig. 217.)
7) Proctodaeum. The epidermal invagination, which meets the proctodaeal
or ventral evagination of the hindgut, forms the proctodaeum. The anal mem-
brane results when the proctodaeal inpushing meets the entodermal outpushing
of the hindgut. The anal membrane is double, composed of entoderm and
ectoderm. It is destined to disappear.
b. Basic Cellular Units of the Primitive Metenteron
Most of the lining tissue of the primitive metenteron is derived from the
entoderm of the archenteric conditions of the late gastrula. Associated with
the strictly entodermal portion of the primitive metenteron are two contribu-
tions of the epidermal tube as observed on pages 598 and 600, namely, the
stomodaeum and the proctodaeum. Added to this lining tissue are mesen-
chymal contributions, derived from the medial or splanchnic layers of the
hypomeric mesoderm (fig. 278B).
The glandular structures of the digestive tube are derived as modifications
of the lining tissue of the stomodaeal, entodermal, and proctodaeal portions
of the primitive gut tube, whereas muscular and connective tissues differentiate
from mesenchyme (fig. 278C).
INTRODUCTION 601
3. Areas of the Primitive Metenteron from which
evaginations (diverticula) normally arise
Certain areas of the primitive metenteron tend to produce outgrowths (evag-
inations; diverticula). The following comprise these areas (fig. 278A).
a. Stomodaeum
In the middorsal area of the stomodaeum, a sac-like diverticulum or Rathke's
pouch, invaginates dorsally toward the infundibulum of the diencephalic por-
tion of the brain. It remains open for a time and thus retains its connection
with the oral epithelium. Later, however, it loses its connection with the oral
cavity and becomes firmly attached to the infundibulum of the brain. It even-
tually forms the anterior lobe of the hypophysis or pituitary gland. (See
chapters 1, 2, and 21.) Other diverticula of the oral (stomodaeal) cavity
occur. These evaginations form the rudiment of the oral glands and will be
discussed on page 617.
h. Pharynx
The pharyngeal area or pharynx represents the anterior portion of the fore-
gut, interposed between the stomodaeum or oral cavity and the esophagus.
This general region has four main functions:
(1) external respiration,
(2) food passage (alimentation),
(3) endocrine-gland formation, and
(4) development of buoyancy structures.
In most vertebrates, five or six pairs of lateral outgrowths, known as the
visceral or branchial pouches are formed. A ventral outpocketing or outpocket-
ings also occur in all vertebrates. The thyroid-gland diverticulum is the most
constantly formed ventral outgrowth, but lung and air-bladder evaginations
are conspicuous in most vertebrate species. Dorsal and dorso-lateral air-
bladder evaginations occur in many fishes.
c. Anterior Intestinal or Pyloric Area
The anterior intestinal area of the primitive gut, immediately caudal to the
stomach region, is characterized by a tendency to form diverticula. Various
types of outgrowths occur here, the most constant of which are the hepatic
(liver) and the pancreatic evaginations. In lower vertebrates, such as teleost,
ganoid, and some elasmobranch fishes, blind digestive pockets, the pyloric
ceca, may be formed in this area.
d. Junction of Midgut and Hindgut
At the junction of the developing small and large intestin s, outgrowths are
common in many of the higher vertebrates. The diverticula which occur here
602
THE DIGESTIVE SYSTEM
MESONEPMRIC
/ J<^' ' "'"'"'''Sk. 7\ OIVERTICULUM-..^jQ^
MIDGUT J- ^ "^V \ /n\
/ L' , 'V Y,OIVERTICULUM ( /
/ f> ,' '-^ *5&i °^ HINDGUT I /
\ V.'' ' -ra / VENTRAL / ,
SEE 4LS0 FIGURE 296
aOULT FORM ^
Fig. 280. Morphogenesis of the digestive tract in the frog. Rami pipiens. (See Chap. 10.)
may be large and pouch-like, as in certain mammals, or slender and elongated,
as in birds.
e. Cloacal and Proctodaeal Area
The most prominent cloacal diverticula occur ventrally. Ventral urinary
bladders arise in this area in many vertebrates. The allantoic diverticulum
(Chap. 22) is a prominent outgrowth of the ventral wall of the cloaca. In
the chick, the bursa of Fabricius projects dorsally from the area between the
cloaca proper and the proctodaeum. Dorsal urinary bladders occur in fishes,
arising as dorsal diverticula within this general area. The anal glands of
certain mammals, such as the dog, represent proctodaeal evaginations.
B. Development of the Digestive Tube or Metenteron
The following descriptions pertain mainly to the developing shark, frog,
chick, and human embryos. Other forms are mentioned incidentally to empha-
size certain aspects of digestive-tube development.
1. General Morphogenesis of the Digestive Tube
The general morphological changes of the developing digestive tubes of the
shark, frog, chick, and human are shown in figures 279-282.
DEVELOPMENT OF THE DIGESTIVE TUBE 603
2. Histogenesis and Morphogenesis of Special Areas
a. Oral Cavity
1) General Characteristics of the Stomodaeal Invagination. The oral cavity
arises as a simple stomodaeal invagination in most vertebrates. However, in
the toadfish, Opsanus (Batrachus) tan, two stomodaeal invaginations occur
which later fuse to give origin to a single oral cavity (Piatt, 1891). In
Amphioxus, the mouth originates on the left side of the head as shown in figure
249D and F; later, it migrates ventrally to a median position. In cyclostomes,
the original invagination becomes partly everted secondarily, so that the pi-
tuitary invagination eventually lies on the upper portion of the head (fig.
283A, B).
2) Rudiments of the Jaws. In the shark embryo, the mandibular visceral
arches bend to form U-shaped structures on either side of the forming oral
cavity and thus give origin to the primitive framework of the upper and lower
jaws (fig. 253). This condition holds true for other lower vertebrates, includ-
ing the Amphibia. In the chick, the mandibular arch bends similarly to that
in the shark embryo, but only the proximal portion of the upper jaw is present.
The anterior or distal portion is displaced by mesenchyme from the head area
(fig. 240). The latter condition is true also of the mammals (fig. 261). Re-
gardless of whether or not all the jaw framework on either side of the forming
oral cavity is derived from the original mandibular arch, the fact remains
that in the formation of the jaws, a U-shaped, mesenchymal framework on
either side is established in all the gnathostomous or jaw-possessing vertebrates.
3) Development of the Tongue. The "tongue" of the shark is essentially a
fold of the oral membrane of the floor of the mouth, which overlies the basal
(hypobranchial) portion of the hyoid visceral arch. A true, flexible tongue,
however, is never developed in the shark or other fishes. Flexible, protrusile
tongues are found almost entirely in forms which inhabit the land, where they
are used for the acquisition and swallowing of food. The protrusile tongue,
therefore, is a digestive-tract structure primarily, and its use in communica-
tion in the human and other species is a secondary adaptation.
The tongue generally develops from folds or growths, associated with the
floor of the oral cavity and anterior branchial region. These lingual growths
are associated with the ventral or lower jaw portions of the hyoid and man-
dibular visceral arches and the ventral area between these arches. However,
in the frog, the tongue arises from a mass of tissue at the anterior portion of
the floor of the mouth between the mandibular visceral arches. It is protruded
from the oral cavity largely by the flow of lymph into the base of the tongue.
The tongue of the chick and other birds is developed as a fleshy, super-
ficially cornified structure, overlying the anterior portion of the greatly modi-
fied hyoid apparatus. It arises from the tuberculum impar, a swelling located
in the floor of the pharyngeal area between the first and second visceral arches.
604
THE DIGESTIVE SYSTEM
STOMODAEUM
FOREGUT
STOMODAEUM
ANTERIOR
MTESTINAL
PORTAL
;^^PROCTODAEUM
B
50 - 56 HOURS
SMALL INTESTINE
ARGE INTESTINE
GALL BLADDER
RIGHT LIVER LOBE
Fig. 281. Morphogenesis of the gut structures in the chick, Callus (domesticus) gallus.
and the copula protuberance which forms as a result of swellings on the lower
ends of the second and third visceral arches and the intervening area. The
copula forms the root of the tongue; the tuberculum impar contributes the
middle portion; and the anterior part of the tongue arises from folds which
grow forward from the anterior portion of the tuberculum impar (fig. 284).
DEVELOPMENT OF THE DIGESTIVE TUBE 605
In the human and pig embryos, the anterior portion or body of the tongue
arises through the fusion of two ventro-medial swellings of the mandibular
arches (fig. 285B). The root of the tongue takes its origin from areas of ele-
vated tissue upon the ventral ends of the hyoid arches and in the adjacent
area between the hyoid and first branchial visceral arches (fig. 285B). This
elevated tissue is known as the copula. A small, insignificant area, the tuber-
culum impar, emerges from the medio-ventral area between the mandibular
and hyoid visceral arches (fig. 285B). Stages in tongue development in the
human embryo are shown in figure 285A-E.
4) Teeth: a) General Characteristics. Teeth are of two types:
( 1 ) horny teeth and
(2) bony or true teeth.
Horny teeth are found in cyclostomatous fishes, the larval stages of frogs
and toads, and in the prototherian mammal, Ornithorhynchus.
Most vertebrates possess true or bony teeth, although they are absent in
some fishes (e.g., the sturgeon, pipefishes, and sea horses), turtles, and birds.
Among the mammals, certain whales lack teeth, and, m Ornithorhynchus,
vestigial bony teeth are formed before hatching, to be lost and supplanted by
cornified epidermal teeth. Teeth are lacking also in the edentates, Myrme-
cophaga and Manis.
True or bone-like teeth have essentially the same general structure in all
vertebrates. A tooth possesses three general areas (fig. 286E):
( 1 ) crown,
(2) neck, and
(3) root.
The crown projects from the surface of epithelium overlying the jaw or
oral cavity, while the root is attached to the jaw tissue. The neck is the re-
stricted area lying between the root and the crown.
Teeth generally are composed of two substances, enamel and dentine. Some
teeth, however, lack enamel. Examples of the latter are the teeth of sloths and
armadillos. The tusks of elephants also represent greatly modified teeth with-
out enamel. Some teeth have the enamel only on the anterior aspect, such as
the incisors of rodents.
Teeth may be attached to the jaw area in various ways. In sharks, the teeth
are embedded in the connective tissue overlying the jaws (fig. 287F), whereas
in most teleosts, amphibia, reptiles, birds, and mammals, they are connected
to the jaw itself (fig. 287A-D). In many vertebrates, such as crocodilians and
mammals, the tooth is implanted in a socket or alveolus within the jaw tissue
(fig. 287C, D). In other forms, the tooth is fused (i.e., ankylosed) to the
upper surfaces of the jaw (fig. 287A, B). A tooth inserted within a socket
or alveolus of the jaw is spoken of as a thecodont tooth, while those teeth
HEAD FOLD
hensen's node
Fig. 282. Morphogenesis of the digestive tract in the human. Observe differentiation
of the cloaca in E-G, and the mesenteric supports including the omental bursa in G.
(Based upon data from various sources.)
606
DEVELOPMENT OF THE DIGESTIVE TUBE
607
GILL APERTURE
RESPIRATORY
TUBE
Fig. 283. Partial eversion of the oral cavity during development in the embryo of
Petromyzon. (Left) Longitudinal section of the head region in 19-day embryo. (Redrawn
and modified from Kingsley, 1912, Comparative Anatomy of Vertebrates, Blakiston,
Phila.) (Right) Median longitudinal section of head region of adult Petromyzon. (Redrawn
and modified from Neal and Rand, 1936, Comparative Anatomy, Blakiston, Phila.)
fused to the surface of the jaw are referred to either as acrodont or pleurodont
teeth. If the tooth is ankylosed to the upper edge of the jaw, as in many
teleosts and snakes, it falls within the acrodont group (fig. 287B), but if it is
attached to the inner surface of the jaw's edge, as in the frog and Necturus,
it is of the pleurodont variety (fig. 287A).
In most vertebrates, all the teeth of the dentition are similar and thus form
a homodont dentition. In some teleosts, some reptiles, and in most mammals,
the teeth composing the dentition are specialized in various areas. Such
localized groups of specialized teeth within the dentition assume different
shapes to suit specific functions. Consequently, the conical, canine teeth are
for tearing; the incisor teeth are for biting or cutting; and the flat-surfaced,
lophodont and bunodont teeth are for grinding and crushing. A dentition
composed of teeth of heterogeneous morphology is a heterodont dentition.
b) Development of Teeth in the Shark Embryo. The development
of teeth in the shark embryo is identical with that of the placoid scale previ-
ously described. However, the teeth of the shark are larger and more durably
constructed than the placoid scale and they are developed from a dental lamina
of epithelial cells which grows downward along the inner aspect of the jaw.
From this epithelium, a continuous series of teeth is developed as indicated
in figure 287E and F. Within the oral cavity and pharyngeal area, ordinary
placoid scales are found. Teeth are continuously replaced throughout life in
the shark from the dental lamina. The word polyphyodont is applied to a con-
dition where teeth are replaced continuously.
c) Development of Teeth in the Frog Tadpole. The mouth of the
frog tadpole possesses prominent upper and lower lips (fig. 287H). Inside
these lips are rows of horny epidermal teeth. Three or four rows are inside
the upper lip, and four rows are found inside the lower lip. These horny
teeth represent cornifications of epidermal cells. They are sloughed off and
608
THE DIGESTIVE SYSTEM
TUBERCULUM
FORWARD GROWTH
FROM
TUBERUULUM IMPAR
Fig. 284. Development of the tongue in the chick embryo.
replaced continuously until the time of metamorphosis when they are dispensed
with. The permanent teeth begin to form shortly before metamorphosis from
an epithelial ridge (dental lamina) which grows inward into the deeper tissues
around the medial portion of the upper jaw. The teeth develop from an
enamel organ and dental papilla in a manner similar to that of the developing
shark or mammalian tooth. After the young tooth is partially formed, it moves
upward toward the jaw, where its development is completed and attachment
to the jaw occurs. Teeth are replaced continuously during the life of the frog.
d) Development of the Egg Tooth in the Chick. Modern birds do
not develop teeth. However, an ingrowth of epithelium does occur which
suggests a rudimentary condition of the dental lamina of the shark, amphibian,
and mammahan embryo (fig. 2871). It is possible that this represents the
rudiment of a basic condition for tooth development, one which is never
realized, for the sharp edge of the horny beak takes the place of teeth. The
egg tooth is a conical prominence, developed upon the upper anterior portion
of the upper horny jaw (fig. 287J). It is lost shortly after hatching. It appears
to function in breaking the shell at hatching time.
e) Development of Teeth in Mammals. As the oral cavity in the pig or
DEVELOPMENT OF THE DIGESTIVE TUBE
609
in the human embryo is formed, the external margins or primitive jaw area of
the oral cavity soon become differentiated into three general areas (fig. 288A) :
( 1 ) an external marginal elevation, the rudiment of the labium or lip,
(2) slightly mesial to the lip rudiment, a depressed area, the labial or
labiogingival groove, and
(3) internal to this epithelial ingrowth, the gingiva or gum elevation.
The latter overlies the developing jaw. From the mesial aspect of the labial
groove, an epithelial thickening forms which pushes inward into the tissue of
the gum or gingiva. This thickened ridge of epithelium forms the dental
lamina (ledge). (See fig. 288B, C.)
After the dental ledge is formed, epithelial buds arise at intervals along the
ledge. These epithelial buds form the rudiments of the enamel organs. Each
enamel organ pushes downward into the mesenchyme of the gum and even-
tually forms a cup-shaped group of cells, enclosing a mass of mesenchyme,
ARYTENOID SWELLINGS
FORAMEN CECUM /
YNGO- EPIGLOTTICA
SO -EPIGLOTTIC
FOLD
LATINE rONSlL
FORAMEN CECUM
LINGUAL TONSIL
EPIGLOTTIS \
LARYNGEAL OPENING
ARYEPIGLOTTIC FOLD
ADITUS LARYNGIS
NTERARYTENOID NOTCH
ESOPHAGUS
Fig. 285. Development of the tongue in the human embryo. (A-D drawn and modified
from Ziegler models. (A) Fourth week. (B) About fifth week. (C) 6th to 7th week;
IC mm. (D) 7th week; 14 mm. (E) Adult condition. Observe that the mandibular
lingual swellings give origin to the body of the tongue, while the copula forms the root
of the tongue.
NAMEL
DENTINE
BLAST LAYER
EL ORGAN OF
LK TOOTH )
EPITHELIUM OF GUM
ODONTOBLAST LAYER
(DENTINE ORGAN
OF TOOTH)
CELLS Of INNER
LAYER OF DENTAL
SAC FORM THE
CEMENTOBLAST
LAYER WHICH
DEPOSITS CEMENTUM
PULP
DENTINAL FIBERS
CELLS OF OUTER
LAYER OF DENTAL
SAC FORM BONE
SPACES OF MANDIBLE
BONE OF MANDIBLE
Fio. 286. Development of thecodont teeth. (A) Early stage of developing premolar
of human. (B) Cellular relationships of tooth-forming area greatly magnified. (C)
Later stage in tooth development showing dental sac. (D) Vertical section of erupting
milk tooth. (E) Vertical section of canine tooth, in situ. (Redrawn and modified from
Morris, 1942, Human Anatomy, Blakiston, Phila. After Toldt.)
610
DEVELOPMENT OF THE DIGESTIVE TUBE 611
the dental papilla (fig. 288D, E). The enamel organ differentiates into three
layers (fig. 288E):
( 1 ) an inner enamel layer, surrounding the dental papilla,
(2) an outer enamel layer, and
(3) between these two layers, a mass of epithelial cells, giving origin to
the enamel pulp.
The cells of the enamel pulp eventually form a stellate reticulum.
Development thus far serves to establish the basic mechanisms for tooth
development. Further development of the tooth may be divided into two
phases:
(1) formation of the dentine and enamel and
(2) development of the root of the tooth and its union with the alveolus or
socket of the jaw.
The initial phase of tooth formation begins when the inner cells of the inner
enamel layer of the enamel organ become differentiated into columnar epi-
thelial cells. These cells form the ameloblasts (fig. 288E, F). Following this
change in the cells of the inner enamel layer, the mesenchymal cells, facing
the ameloblasts, become arranged into a layer of columnar odontoblasts (fig.
288F). The odontoblasts then begin to deposit the dentine of the tooth. The
initial phase of formation of dentine consists first in the elaboration of an
organic substance or matrix. The organic matrix then becomes impregnated
with inorganic calcareous materials to form the dentine, a hard, bone-like
substance. As the dentinal layer becomes thicker, the odontoblasts recede
toward the dental pulp of the papilla. However, the odontoblasts do not with-
draw entirely from the dentine already formed, as elongated, extremely fine
extensions from the odontoblasts continue to remain within the dentine to
form the dentinal fibers (fig. 286B).
Dentine is deposited by the odontoblasts; the ameloblasts deposit the enamel
layer in the form of a cap, surrounding the dentine (fig. 286A, B). In doing so,
a slight amount of organic substance is first deposited, and then the ameloblast
constructs in some way a prismatic column of hard calcareous material at
right angles to the dentinal surface (fig. 286B). The columnar prisms thus
deposited around the dentine form an exceedingly hard cap for the dentine.
As in the formation ,of the dentine, the elaboration of enamel begins at the
crown or distal end of the tooth and proceeds rootward.
The development of the root of the tooth and its union with the jaw socket
(alveolus) is a complicated procedure. This phase of tooth development is
accomplished as follows: The mesenchyme, with its contained blood vessels
and nerves of the dental papilla, lies within the developing dentinal layer of
the forming tooth. At the base of the tooth (i.e., the end of the tooth opposite
the crown), the mesenchyme of the dental papilla is continuous with
612
THE DIGESTIVE SYSTEM
EPIDERMAL
DENTAL LEDGE
LABIAL PAPILLAE
Fig. 287. Tooth development and arrangement in various vertebrates. (A-D) Tooth
relationships with the jaw. (Redrawn and modified from Rand, 1950, The Chordates,
Blakiston, Phila. After Wilder.) (E) Dental ledge and developing teeth in the dog shark,
Acanthias. (Redrawn and modified from Rand, 1950, The Chordates, Blakiston, Phila.
After Kingsley. ) (F) Section of the shark's lower jaw indicating a continuous replace-
ment of teeth, i.e., a polyphyodont condition. (Redrawn and modified from Rand, 1950,
The Chordates, Blakiston, Phila.) (G) Incisor tooth of rodent. (Redrawn and modified
from Rand, 1950, The Chordates, Blakiston, Phila. After Zittel.) (H) Horny teeth of
12 mm. frog tadpole. (I) Rudimentary dental lamina in upper jaw of chick. (Redrawn
from Lillie, 1930, The Development of the Chick, Holt & Co., N. Y.) (J) Anterior
portion of upper jaw of 18-day chick showing egg tooth.
the mesenchyme surrounding the developing tooth. Around the base, sides,
and crown of the tooth, this mesenchyme condenses and forms the outer and
inner layers of the dental sac (fig. 286C). The latter is a connective-tissue
sac which surrounds the entire tooth, continuing around the outside of the
outer enamel cells of the enamel organ. As the dentine and enamel are de-
DEVELOPMENT OF THE DIGESTIVE TUBE 613
posited, the process of deposition proceeds downward from the crown toward
the developing root of the tooth. However, in the root area, the cellular layers
of the enamel organ are compressed against the dentine, where they form the
epithelial sheath. The sheath eventually disintegrates and disappears. The for-
mation of enamel thus becomes restricted to the upper or crown part of the
tooth, the root portion consisting only of dentine. As the root area of the
tooth lengthens downward, the tooth as a whole moves upward. Finally, the
crown of the tooth erupts to the outside through the tissues of the gum (fig.
286D). The eruption, completion, and shedding of the milk or deciduous
teeth in the human body occur apparently as shown in the following table.
The Milk Dentition
Median incisors 6th to 8th month
Lateral incisors 8th to 12th month
First molars 12th to 16th month
Canines 17th to 20th month
Second molars 20th to 24th month
The Permanent Dentition
First molars 7th year
Median incisors 8th year
Lateral incisors 9th year
First premolars 10th year
Second premolars 11th year
Canines 13th to 14th year
Second molars 13th to 14th year
Third molars 17th to 40th year
This table is taken from McMurrich, J. Playfair. 1922. Keibel and Mall, Manual of Human
Embryology, page 354, Lippincott, Philadelphia.
At about the time of eruption, the tooth becomes cemented into the alveolus
or socket of the jaw in the following manner:
( 1 ) The inner layer of the dental sac (fig. 286D) forms a layer of cemento-
blasts which deposit a coating of cementum over the dentine of the
root (fig. 286E). This occurs only after the epithelial sheath (enamel-
layer cells around the root) has been withdrawn or otherwise has
disappeared.
(2) The cells of the outer layer of the dental sac become active in forming
spongy bone.
(3) As the tooth reaches maturity, the two bony surfaces, i.e., the cementum
of the root and the spongy bone of the jaw socket, gradually begin to
approach each other. Then, as more cementum is deposited and more
spongy bone is formed, the space between the cementum and the
spongy bone of the alveolus becomes extremely narrow (fig. 286E).
614
THE DIGESTIVE SYSTEM
(4) Finally, the dental-sac tissue between these two bony surfaces forms
the peridental membrane, a thin, fibrous, connective-tissue layer whose
fibers are attached to the cementum and to the spongy bone of the
socket. In other words, the cemental bone of the root and the spongy
bone of the socket become sutured together by means of the inter-
locking fibers of the peridental membrane. This type of suture, which
GINGIVA
LABIOGINGI
GROOVE
GUM
NASAL CARTILAGE OF
HAMBER NASAL SEPTUM
TONGUE ^'♦f -''-• ;°*'i*« •^.* •.•," "NJ Z'^^r'/''^''''-'"''''^
MESENCHYME
TOOTH GER
MILK TOOT
MANDIBLE
BONE FORMATION
MECKEL'S OUTER ENAMEL
CARTILAGE LATER
ENAMEL PULP '^
INNER ENAME
Fig. 288. Tooth development in the pig. (A) Upper and lower jaw region of 18 mm.
pig embryo showing labial and gum areas with the labia! groove insinuated between. (B)
Section through snout and upper and lower jaws of 30-mm. pig embryo showing formation
of nasal passageways, secondary palate, lip, gum, and jaw regions, and ingrowing dental
ledge. (C) High-powered drawing of dental ledge shown in square C in figure B. (D)
Section similar to B in 65-mm. pig embryo. (E) Enlargement of area marked E in D
showing dental papilla and enamel organ. (F) Drawing showing juxtaposition of inner
layer of enamel organ (the anjeloblast layer) and the odontoblast cells which differentiate
from the mesenchyme of the dental papilla.
DEVELOPMENT OF THE DIGESTIVE TUBE
615
Fig. 289. Palatal conditions in frog, chick, and mammal. (A) Frog, adult. (B)
Chick, 16-day embryo. (C) Human adult. (Redrawn and modified from Morris, 1942,
Human Anatomy, Blakiston, Phila.) Only the anterior or hard palate is supported by
bone, the soft palate being a fleshy continuation of the palate caudally toward the
pharyngeal area. (D-F) Stages in development of the palate in the pig. (D) 20.5 mm.
(E) 26.5 mm. (F) 29.5 mm.
is formed between the root of the tooth and the walls of the alveolar
socket, is called a gomphosis (fig. 286E).
The permanent teeth, which supplant the deciduous teeth, develop in much
the same manner as the deciduous teeth. Man, Uke the majority of mammals,
develops two sets of teeth and, consequently, is diphyodont. Some mammals,
such as the mole, Scalopus, never cut the permanent teeth, while the guinea
pig sheds its deciduous teeth in utero.
5) Formation of the Secondary Palate. In the fishes and the amphibia, a
secondary palate, separating the oral cavity from an upper respiratory passage-
way, is not formed. The formation of a secondary palate begins in the turtle
group and is well developed in the crocodilians and mammals. The bird also
616
THE DIGESTIVE SYSTEM
has a secondary palate, but it is built more tenuously than that of the croco-
dilian-mammaHan group (fig. 289A-C).
During secondary-palate formation in the mammal, the premaxillary, maxil-
lary, and palatine bones develop secondary plate-like growths which proceed
medially to fuse in the midline (fig. 289D-F). The secondary palate thus
forms the roof of the oral cavity — the air passageway from the outside to the
pharynx being restricted, when the mouth is closed, to the area above the
secondary palate.
6) Formation of the Lips. Lips are ridge-like folds of tissue surrounding
the external orifice of the oral cavity. They are exceptionally well developed
in mammals, where they are present in the form of fleshy mobile structures.
They are absent in the prototherian mammal, Ornithorhynchus , as well as
in birds and turtles, where the horny edges of the beak displace the fleshy folds
at the oral margin. Lips are much reduced in sharks, where the toothed jaws
merge with the general epidermis of the skin, but are present in most fishes,
amphibia, and most reptiles. In general, lips are immobile or only slightly
mobile structures in the lower vertebrates, although in some fishes they possess
a mobility surpassed only in mammals.
In the formation of the lips, a labial groove or insinking of a narrow ledge
POISON GLAND
DUCT OF PAROTID
SUBMAXI LL ARY
GLAND
DEEP PROCESS OF
SUBMAXILLARY GLAND Q,
Fig. 290. Oral glands. (A) Poison and labial glands of the rattlesnake. Crotalus
horidus. (Redrawn from Kingsley, 1912, Comparative Anatomy of the Vertebrates,
Blakiston, Phila.) (B) Loci of origin of salivary glands in human embryo. (Redrawn
from Arey, 1946, Developmental Anatomy, Saunders. Phila.) (C) Position of mature
salivary glands in human. (Redrawn and modified from Morris, 1942, Human Anatomy,
Blakiston, Phila.)
DEVELOPMENT OF THE DIGESTIVE TUBE
617
RECTAL
PYLORIC INTESTINE RECTUM GLAND
VALVE
GILL OPENINGS
DUODENUM
Fig. 291. Diagrams of intestinal tracts in various fishes. (Redrawn from Dean, 1895,
Fishes, Living and Fossil, Macmillan, N. Y.) (A) Petromyzon, the cyclostome. (B)
Protopterus, the lungfish. (C) The shark.
of epidermal cells occurs along the edge of the forming mouth. The labial
groove then divides the edge of the forming mouth into an outermost lip margin
and the gum or jaw region (fig. 288A). In forms where the lip is mobile, the
lip region becomes highly developed and the muscle tissue which invades this
area comes to form the general mass of the lip.
7) Oral Glands. Mouth glands are present throughout the vertebrate series.
Mucus-secreting glands are the predominant type, but specialized glands, pro-
ducing special secretions, appear in many instances. The cyclostomatous fish,
for example, possesses a specialized gland which secretes an anticoagulating
substance to prevent coagulation and stoppage of blood flow in the host fish
to which it may be temporarily attached by its sucker-like mouth. Mean-
while, it rasps the host's flesh with its horny teeth and "sucks" the flowing
blood. Salivary glands (i.e., glands forming the saliva) make their appearance
in the amphibia. Such glands may be found on the amphibian tongue, where,
as lingual glands, they secrete mucus and a watery fluid. Intermaxillary glands
are present on the amphibian palate. The poison glands of the Gila monster
and of snakes represent specialized oral glands (fig. 290A). Salivary glands
are present also below the tongue and around the lips and palate in snakes.
Birds, in general, possess salivary glands of various sorts. The mammals are
characterized by the presence of highly developed, salivary glands, among
which are the parotid, sublingual, and submaxillary glands. Unlike most of the
salivary glands in other vertebrates, the mammalian salivary glands, in many
species, secrete mucus and a watery fluid, together with a starch-splitting
enzyme, ptyalin.
The submaxillary and sublingual glands in mammals arise as evaginations
of the oral epithelium in the groove between the forming lower jaw and the
618
THE DIGESTIVE SYSTEM
, ESOPHAGUS
Fig. 292. Developing stomach regions of the digestive tract. (A-C) Three stages in
the development of the pig's stomach. Arrows indicate formation of omental bursa which
forms from the pocket-like enlargement of the dorsal mesogastrium and proceeds to the
left forming the omental bursa as the pyloric end of the stomach rotates toward the
right. The ventral aspect of the stomach is indicated by crosses. (D) Diagram of the
ruminant stomach. The abomasum corresponds to the glandular stomach of the pig or
human; the other areas represent esophageal modifications. (Redrawn from Kingsley,
1912. Comparative Anatomy of the Vertebrates, Blakiston, Phila.)
developing tongue. The place of origin is near the anterior limits of the tongue.
Two of these epithelial outpushings occur on either side (fig. 290B). The sub-
maxillary-gland and sublingual-gland ducts open at the side of the frenulum of
the tongue (fig. 290C). The parotid glands arise as epithelial evaginations, at
the angle of the mouth, from the groove which separates the forming jaw and
the lip (fig. 290B, C).
The various oral glands, such as the palatine, labial, tongue, and cheek
glands of mammals and lower vertebrates, the poison glands of snakes, etc.,
arise as epithelial buds which grow out from the developing oral cavity in a
manner similar to those of the parotid, submaxillary, and sublingual glands of
mammals. The original epithelial outgrowths may branch and rebranch many
times to produce large, compound, alveolar glands, as in the parotid, sub-
maxillary, and sublingual glands of mammals and the poison glands of snakes.
b. Development of the Pharyngeal Area
1) Pharyngeal Pouches and Grooves. The pharynx is that region of the
early digestive tube which lies between the oral cavity and the esophagus. In
adult vertebrate species, the pharyngeal area is much modified and diflferen-
tially developed. However, in the early embryo, it tends to assume a generalized
sameness throughout the vertebrate series.
The early formation of the pharynx results from a series of outpocketings
of the entoderm of the foregut, associated with a corresponding series of epi-
dermal inpushings; the latter tend to meet the entodermal outgrowths. As a
result of these two sets of movements, the one outward and the other inward,
the lateral plate mesoderm becomes isolated into dorso-ventral columns, the
branchial or visceral arches, between the series of outpocketings and inpush-
DEVELOPMENT OF THE DIGESTIVE TUBE 619
ings (figs. 252F; 260; 262). The entodermal pouches or outpocketings are
called the branchial, pharyngeal, or visceral pouches, while the epidermal (ec-
todermal) inpushings form the visceral or branchial grooves (furrows). The
mesodermal columns constitute the visceral arches.
The number of branchial pouch-groove relationships, thus established, varies
in different vertebrate species. In the cyclostomatous fish, Petromyzon, there
are seven; in Squalus acanthias, the shark, there are six. The latter number is
present typically in a large number of fishes. In most frogs and salamanders,
there are five, pouch-groove relationships with a vestigial sixth; in the chick,
pig, and human, there are four. (In reptiles, birds, and mammals, the fourth
pouch on either side may represent a fusion of two or three pouches.) The
number of visceral arches, of course, varies with the number of pouch-groove
relationships produced, the first pair of arches being formed just anterior to
the first pair of pouches. The first pair of arches are called the mandibular
visceral arches; the second pair constitute the hyoid visceral arches; and the
remaining pairs form the branchial arches.
Within each visceral arch, three structures tend to differentiate:
( 1 ) a skeletal arch,
(2) a muscle column, associated with the skeletal arch, and
(3) the aortal arch, a blood vessel.
In all water-living vertebrates, including those species which spend the larval
period in the water, the entoderm of the branchial pouch and ectoderm of the
branchial groove tend to fuse intimately and perforate to form the branchial
or visceral clefts, with the exception of the first, pouch-groove relationship.
The latter is variable. In the amphibia, the first pouch does not perforate but
becomes associated with the developing ear. In land forms, on the other hand,
the pouches, as a rule, remain imperforate or weakly so. As a rule, they con-
tinue unperforated in mammals. The ectoderm and entoderm of the branchial-
pouch-groove relationships is very thin in the chick, and openings (?) may
appear in the more anterior pouches. (Note: The relation of these pouches to
respiration is discussed in the following chapter.)
2) Pharyngeal Glands of Internal Secretion. An important developmental
function of the pharynx is the formation of masses of epithelial cells from vari-
ous parts of the entodermal wall which serve as endocrine glands. These glands
are the thyroid, parathyroid, thymus, and ultimobranchial bodies. The places
of origin of these cellular masses and their part in the formation of the endo-
crine system are discussed in Chapter 21.
3) Other Respiratory Diverticula. One of the primary functions of the
pharyngeal area is respiration. In most water-living vertebrates, the pharyngeal
pouches are adapted for respiratory purposes. However, in many water-dwelling
species and in all land forms, a median ventral outpushing occurs which de-
llkilU raS^VS^ I*!''" STRIATED
GERMINAL CENTER
FAT CELLS
SUBMUC0S4
LUMEN OF LARGE
INTESTINE
TRANSVERSE COLON)
Fig. 293. Characteristics of the mucous membrane in different regions of the human
digestive tract: (A and D) redrawn and modified from Maximow and Bloom, A Text-
book of Histology, Saunders, Philadelphia; (B and C) redrawn from Bremer, A Text-
book of Histology, Blakiston, Philadelphia. (A) Esophageal area. Stratified squamous
epithelium together with esophageal and cardiac glands are characteristic. The esophageal
glands are located in the submucous layer and are of the tubulo-alveolar variety. The
cardiac glands are found in the upper and lower esophageal regions and are confined
to the mucous layer. (B) Stomach region. The mucous layer of the stomach is fea-
tured by the presence of many glands composed of simple and branched tubules. These
glands open into the bottom of the gastric pits which in turn form small, circular
openings at the mucosal surface. (C) The mucosal walls of the small intestine present
many finger-like processes, the villi, between the bases of which the intestinal glands or
crypts of Lieberkiihn project downward toward the lamina muscularis mucosae. (D)
The mucosa of the large intestine is devoid of villi, and the glands of Lieberkiihn are
longer and straighter than in the small intestine.
620
DEVELOPMENT OF THE DIGESTIVE TUBE 621
velops into the lungs or into structures which function as air bladders and
lungs. (See Chap. 14.)
c. Morphogenesis and Histogenesis of the Esophagus and the Stomach
Region of the Metenteron
The esophageal and stomach areas of the gut develop from that segment of
the foregut which extends from the pharyngeal area caudally to the area of
the developing gut tube from which the liver and pancreatic diverticula arise.
In Amphioxus and certain of the lower vertebrates, a true stomach is not
differentiated within this portion of the foregut. This condition is found in
the cyclostome, Petromyzon, in the lungfish, Protopterus, and various other
forms (fig. 291 A, B). In these species, this segment of the gut merely serves
to transport food caudally to the intestine, and the histogenesis of its walls
resembles that of the esophagus. On the other hand, a true stomach is de-
veloped in all other vertebrate species. The functions of the stomach are to
store food, to break it up into smaller pieces, and to digest it partially. As such,
the stomach comprises that segment of the digestive tract which lies between
the esophagus and intestine. It is well supplied with muscular tissue, is capable
of great distention, and possesses glands for enzyme secretion.
In development, therefore, the foregut area between the primitive pharynx
and the developing liver becomes divided into two general regions in most
vertebrates:
( 1 ) a more or less constricted, esophageal region, and
(2) a posteriorly expanded, stomach segment (figs. 279-282).
The latter tends to expand and to assume a general, V-shaped form, the
portion nearest the esophagus comprising the cardiac region, and the part
nearest the intestine forming the pyloric end.
Many variations in esophageal-stomach relationships are elaborated in dif-
ferent vertebrate species. In the formation of the stomach of the pig or human,
for example, a generalized, typical, vertebrate condition may be assumed to
exist. In these forms, the stomach area of the primitive gut gradually enlarges
and assumes a broad, V-shaped form, with its distal or pyloric end rotated
toward the right (fig. 292A-C). Eventually, the entodermal lining tissue shows
four structural conditions:
(a) There is an esophageal area near the esophagus, where the character
of the epithelial lining resembles that of the esophagus.
(b) A cardiac region occurs, where the epithelium is simple, columnar in
form, and contains certain glands.
(c) There is a fundic region, capable of being greatly expanded. The in-
ternal lining of the fundic area produces numerous, simple, slightly
branched, tubular glands, wherein pepsin is secreted by the chief cells
and hydrochloric acid by the parietal cells (fig. 293).
622 THE DIGESTIVE SYSTEM
(d) The pyloric area is the last segment of the stomach and is joined to
the intestine. It has numerous glands, producing a mucus-like secretion.
The pig's stomach resembles closely that of the human.
If we compare the general morphogenesis of the stomach in the pig or human
with that of the shark, frog, chick, or the cow, the following diflferences exist.
The shark stomach is composed mainly of fundic and pyloric segments (fig.
279C). The stomach of the frog closely resembles that of the pig (fig. 280F).
Unlike the pig, however, the frog is able to evert the stomach by muscular
action projecting it forward through the mouth to empty its contents. In the
chick (fig. 28 IE), an area of the esophagus expands into a crop which func-
tions mainly as a food-storage organ. A glandular stomach (proventriculus),
comparable to the fundus of the pig, is formed posterior to the crop, while,
still more caudally, a highly muscular gizzard or grinding organ is elaborated.
In the cow or sheep, an entirely diflferent procedure of development pro-
duces a greatly enlarged, distorted, esophageal portion of the stomach. This
esophageal area of the stomach comprises the rumen, the reticulum or honey-
comb stomach, and the omasum (psalterium) or manyplies stomach. The distal
end of the stomach of the cow or sheep is the abomasum or true stomach,
comparable to that of the human or pig described above (fig. 292D).
d. Morphogenesis and Histogenesis of the Hepato-pancreatic Area
The hepato-pancreatic area of the digestive tract is a most important one.
Its importance springs not only from the development of indispensable glands
but also from the relationship of the liver to the developing circulatory system
(Chap. 17) and the division and formation of the coelomic cavity. (See
Chap. 20.)
1) Development of the Liver Rudiment. The liver begins in all vertebrates
as a midventral outpushing of the primitive metenteron, immediately caudal
Fig. 294. Development of the liver and pancreatic rudiments. (Diagrams C-E, re-
drawn from Lillie, 1930, The development of the chick. Holt, N. Y. F redrawn from
Thyng, 1908, Am. J. Anat.) (A) Developing liver rudiment in 10 mm. embryo of
the dogshark, Squalus acanthias. (B) Developing liver in tadpole of Rana pipiens.
(See also Figs. 221, 223, 225, 280.) (C) Developing liver rudiments in the 3rd-day
chick. (D) Developing liver in early 4th-day chick. (E) Developing liver in late
4th-day chick. (F) Hepatic evagination in 7.5 mm. human embryo. (G) Relation of
the fully developed liver to associated structures in various vertebrates. (Gl) Squalus
acanthias. The liver is suspended from the posterior surface of the septum transversum
by the coronary ligament. (G2 and G3) Frog, Rana pipiens. G2 transverse view; G3
sagittal view. (G4 and G5) 16-20 day chick. Callus doinesticus. G4 transverse view.
Observe that the liver lobes and peritoneal cavity have grown forward on either side of
the heart and have separated the heart and pericardial cavity from the ventro-lateral
body walls. G5 is a left ventral view of the heart, pericardial cavity, and liver. Left lobe
of the liver is removed. Observe that the septum transversum is applied to the posterior
wall of the parietal pericardium. G6 Mammal. The septum transversum has been com-
pletely displaced by developing diaphragmatic tissue. The liver is suspended from the
caudal surface of the diaphragm by the coronary ligament.
SEPTUM TRiNSVERSUM
CAUDAL ASPECT OF PARiE
BODY WALL
HEART
°"'?A?-o?,'^ (SEPTUM TRANSVERSUM
PARIETAL COMPONENT SHOWN IN BLACKI
PERICARDIUM
G'
MAMMAL
Fig. 294. (See facing page for legend.)
623
624 THE DIGESTIVE SYSTEM
to the stomach. It originates thus between the foregut and midgut areas of
the developing digestive tube.
a) Shark Embryo. In the 10- to 12-mm. shark embryo, Squalus acanthias,
the liver rudiment arises as a midventral evagination of the gut which pushes
downward and forward between the two parts of the ventral mesentery. It
soon becomes divisible into three chambers, viz., a midventral chamber, the
rudiment of the gallbladder, and two lateral chambers, the fundaments of the
right and left lobes of the liver (figs. 279B; 294A).
b) Frog Embryo. In the frog, the liver rudiment appears as a ventro-
caudal prolongation of the foregut area at the early, neural fold stage (figs.
220B; 223B). Later, the anterior end of the hepatic rudiment diff"erentiates
into the liver substance in close relation to the vitelline veins as the latter enter
the heart, while the posterior extremity of the original hepatic rudiment dif-
ferentiates into the gallbladder (figs. 280; 294B, G2, G3).
c) Chick Embryo. In the chick, two evaginations, one anterior and the
other posterior, arise from the anterior wall of the anterior intestinal portal,
beginning at about 50 to 55 hours of incubation (fig. 294C). These evagina-
tions project anteriorly toward the sinus venosus of the heart, where they
eventually come to surround the ductus venosus as it enters the sinus. (See
Chap. 17.) At the end of the fourth day of incubation, secondary evaginations
from the two primary outgrowths begin to produce a basket-like mass of
tubules which surround the ductus venosus (fig. 294E). The gallbladder
arises from the posterior hepatic outpushing toward the end of the third day
of incubation (fig. 294D).
d) Pig Embryo. The liver diverticulum in the 4- to 5-mm. embryo of the
pig begins as a bulbous outpushing of the foregut area, immediately caudal
to the forming stomach (fig. 295E). This outpushing grows rapidly and sends
out secondary evaginations, including the vesicular gallbladder. The latter is
already a prominent structure in the 5. 5-mm. embryo (fig. 295A).
Fig. 295. Development of liver and pancreatic rudiments {Continued). (A) Diagram
of early hepatic diverticulum in pig embryo of about 5.5 mm. (Redrawn and modified
greatly from Thyng, 1908, Am. J. Anat.) For early growth of liver in pig, see Figs. 261 A
and 262. (B) Hepatic ducts, hepatic tubules, and hepatic canaliculi in relation to blood
sinusoids. It is to be observed that the common bile duct ( I ) gives off branches, the hepatic
ducts (2), from which arise the branches of the hepatic duct (3) which are continuous
with the hepatic tubules or hepatic cord cells (4). Compare with Fig. 295C. (C) A
portion of liver lobule of human. (Redrawn and modified from Maximow and Bloom,
A Text-book of Histology, Saunders. Phiia.) Blood sinusoids are shown in black; liver
cells in stippled white; bile canaliculi shown in either white or black. (D) Section
showing three pancreatic diverticula in 5-day chick embryo. (Redrawn from Lillie, 1930,
The development of the chick. Holt, N. Y. After Choronschitsky.) (E) Pancreatic di-
verticula in 5.5 mm. pig embryo. (Redrawn from Thyng, 1908, Am. J. Anat. 7.) (F)
Pancreatic diverticula in 20 mm. pig embryo. (Redrawn from Thyng, 1908, Am. J. Anat.
7.) (G) Pancreatic acini and islet of Langerhans.
LUNG BUD
DORSAL
MESENTERY
PHARYNGE A
/tiroes..
PANCREAS
DUODENUM
VENTRAL
^ ' ^PANCREAS
-X.^____ GALL
' BLADDER
DORSAL PANCREAS
HEPATIC SINUSOID
HEPATIC CELLS
BILE CANALICULUS
DUODE NUM
PANCREAS
PANCREAS
POSTERIOR HEPATIC
Dl VERTICULUM
LIVER
LEFT
OMPHALOMESENTERIC
VEIN
DUCTUS CH0LED0CHU5
OUCT OF VENTRAL PANCREAS
DUODENAL DIVERTICULUM
SLET OF LANGERHANS
Fig. 295. (See facing page for legend.)
625
626
THE DIGESTIVE SYSTEM
)^\ LUNGS
TOMACH\ I -f^^"/
RIGHT ^fc-:ifi '■ LEFT
AT METAMORPHOSIS
Fig. 296. Development of coils in the digestive tracts in the dog shark, Squalus
acanthias, and in the frog, Rana pipiens. (A) Squalus acanthias embryo of 110 mm.
(B-F) Rana pipiens, digestive tube development, shown from ventral aspect. Arrows in
B and C denote primary movements of the primitive gut tube resulting in condition shown
in D.
e) Human Embryo. In the human embryo, the liver arises in a similar
manner to that of the pig embryo from the ventral wall of the foregut, just
posterior to the forming stomach (fig. 294F). The hepatic outpushing invades
the area of the ventral mesentery and becomes intimately associated with the
substance of the septum transversum (fig. 362H). Secondary evaginations or
liver cords ramify extensively within the mesenchyme of the mesentery, and the
vitelline or omphalomesenteric veins, as in other vertebrates, become broken
up into sinusoids, surrounding the outgrowing hepatic cords. The gallbladder
arises as a secondary outgrowth from the posterior wall of the original hepatic
outgrowth (fig. 294F). The gallbladder rudiment enlarges distally and gives
origin to the cystic duct which joins the common bile duct.
2) Histogenesis of the Liver. As the liver pushes out into the ventral mesen-
DEVELOPMENT OF THE DIGESTIVE TUBE 627
tery, it tends to project forward below the forming stomach and the caudal
limits of the heart (figs. 295A; 362H). Within the ventral mesentery, secondary
evaginations or epithelial cords of entodermal cells sprout from the primary
entodermal evagination of the entodermal lining of the gut (fig. 295A). These
epithelial or liver cords grow in between the paired vitelline veins, and the
veins become changed into a mass of capillary-like sinusoids. The liver cords
come to lie in the interstices between the vitelline sinusoids (fig. 295B).
As the liver cords grow within the ventral mesentery, mesenchymal cells,
given off from the medial surfaces of the mesentery, come to surround the
liver cords and give origin to the connective-tissue substance of the liver. The
outer surface of the ventral mesentery retains its integrity and functions as the
peritoneal covering of the growing liver.
It is apparent that the growth of the epithelial (liver) cords progresses
dichotomously, branching into a tree-like system of branches from the original
hepatic diverticulum of the gut tube, thus forming the parenchyma of the
liver (Bloom, '26). The proximal portion of the original hepatic diverticulum
forms the common bile duct, or ductus choledochus, whereas the larger
branches of the hepatic cords develop lumina and form the duct system. The
gallbladder represents an original diverticulum from the common-bile-duct
rudiment. The liver cords appear to be hollow from the beginning. The bile
capillaries thus apparently develop directly within the liver cords. The liver-
cord cells probably assume their typical cuboidal shape under the influence
of the surrounding young connective tissue and branches from the portal vein
(Bloom, '26). The ultimate relationship between hepatic cell cords, liver
sinusoids, and bile ductules is shown in figure 295C.
In the majority of vertebrates, as the liver substance increases within the
ventral mesentery below the stomach area, it expands the ventral mesentery
enormously until the liver, with its coating of ventral mesentery, fills the
coelomic space below the gut tube and posterior to the heart. The developing
liver thus comes in contact with the ventral and lateral body walls and becomes
fused to these walls. The anterior face of the liver, eventually, forms a par-
tition across the coelomic cavity just caudal to the heart (figs. 261; 295A).
The anterior face of the liver substance gradually separates and forms a
primitive partition across the body cavity. This partition is the primary septum
transversum (fig. 295A). (See also Chap. 20.)
As the liver rudiment develops in the pig embryo, the septum transversum
forms essentially as described above, i.e., it develops as a modification of the
ventral mesentery covering the anterior face of the liver. However, in the
human embryo, the primary septum transversum develops precociously, form-
ing a partition across the ventral area of the coelomic cavity between the
developing heart and liver (fig. 362F-H ) . When the hepatic cords in the human
embryo grow forward within the ventral mesentery, they secondarily become
related to the previously formed, primitive septum transversum along the
628
THE DIGESTIVE SYSTEM
caudal aspect of the septum. The ends achieved in the human and pig embryos
are much the same, therefore, and the anterior face of the developing liver
and the septum transversum are intimately associated.
3) Development of the Rudiments of the Pancreas: a) Shark Embryo.
In the embryo of Squalus acanthias, the shark, the pancreas arises as a dorsal
diverticulum of the gut a short distance posterior to the gallbladder and hepatic
outpushings (fig. 279B). It grows rapidly and, in the 18- to 20-mm. embryo,
it is a much-branched gland with its pancreatic duct entering the duodenum
slightly anterior to the beginning coils of the spiral valve.
b) Frog Embryo. In the frog, the pancreas arises from three diverticula,
one dorsal and two ventral, near the liver rudiment (Kellicott, '13, p. 167).
The dorsal diverticulum is solid and separates from the gut tissue. The two
ventral diverticula arise together from the ventral portions of the gut but soon
branch into two rudiments. As these rudiments enlarge and branch, they
eventually unite with the dorsal diverticulum of the pancreas, and the three
fuse to form one gland. The proximal portion of the original, ventral, pan-
creatic outpushing remains as the pancreatic duct and empties into the
duodenum close to the bile duct.
c) Chick Embryo. As in the frog, three pancreatic diverticula arise in the
Fig. 297. Developing coils in the digestive tube of the pig. (A) 12 mm. embryo.
(B) 24 mm. embryo. (C) 35 mm. embryo. (D) Cecum and large intestine showing
coils in 120 mm. embryo. (E) Coiling of large intestine of young adult pig. Observe
haustra or lateral diverticula of colonic wall. (All figures redrawn and modified from
Lineback, 1916, Am. J. Anat. 16.)
DEVELOPMENT OF THE DIGESTIVE TUBE
629
GLANDS OF BRUNNER
MESENTERY
large gland
;as)
OPENINGS OF
NTESTINAL GLANDS
'<T^^ VALVE OF KERKRING B
VILLI
LYMPHATIC NODULE
Fig. 298. Structural composition of walls of human digestive tract. (A) Diagram-
matic representation of digestive tract structure. (B) Portion of wall of small intestine
showing folds of mucosa. (A and B redrawn from Maximow and Bloom, 1942, A Text-
book of Histology, Saunders, Phila. B after Braus.)
chick. The dorsal one appears first as an outpushing into the dorsal mesentery
at the end of the third and early fourth days of incubation (fig. 295D). The
two ventral diverticula arise during the end of the fourth and early fifth days
of incubation as two lateral diverticula of the posterior hepatic evaginations
close to the latter's origin from the duodenum. The three diverticula fuse into
one pancreatic mass, but tend to retain the proximal portions of the original
outpushings as pancreatic ducts. Two or even all three may persist in the adult.
d) Pig Embryo. Two pancreatic diverticula make their appearance in the
pig embryo. One, the ventral pancreatic diverticulum, arises from the proximal
end of the hepatic evagination, while the other, the dorsal diverticulum,
emerges as a separate dorsal outpushing from the duodenal area approximately
opposite the hepatic diverticulum (fig. 295E). In the 20-mm. embryo of the
pig, these two diverticula proceed in development as shown in figure 295F.
At about the 24-mm. stage, the duct of the ventral pancreas is obliterated,
the dorsal pancreatic duct (duct of Santorini) remaining ordinarily as the
pancreatic duct of the adult (Thyng, '08).
e) Human Embryo. Dorsal and ventral pancreatic evaginations occur in
the human embryo in a manner similar to that in the pig. Both fuse into one
mass, although the dorsal pancreas grows much faster and forms much of the
bulk of the pancreatic tissue. The ventral pancreas swings dorsally as the
stomach and duodenal area of the intestine are rotated toward the right side
of the peritoneal cavity. In doing so, the dorsal pancreas appropriates the duct
of the ventral pancreas proximally toward the intestine, while distally it retains
its own duct. This combined duct, or duct of Wirsung, first observed by
Wirsung in 1642 (see Lewis, '12), is the pancreatic duct of the adult. Occa-
630 THE DIGESTIVE SYSTEM
sionally, two ducts opening into the intestine are retained, the original dorsal
duct, the accessory duct or duct of Santorini, described by Santorini (see
Lewis, '12), and the duct of Wirsung or ventral pancreatic duct. The latter
condition appears to be normal in the dog.
4) Histogenesis of the Pancreas. The original pancreatic diverticula branch,
rebranch, and form an elaborate duct system. The secretory portions of the
pancreas or the acini arise as terminal outgrowths of the distal portions of the
duct system. The pancreas thus is a compound alveolar (acinous) gland. The
loose connective tissue of the pancreas forms the surrounding mesenchyme,
derived from the mesenteric tissue.
Two types of secretory cells bud off from the developing duct system. The
majority form the acini of the pancreatic gland and pour their secretions into
the duct system. This constitutes the exocrine aspect of the pancreas. Other
cell masses bud off from the duct system and give origin to the islets of
Langerhans. The latter form the endocrine portion of the pancreas (fig. 295G).
e. Morphogenesis and Histogenesis of the Intestine
1) Morphogenesis of the Intestine in the Fish Group. In the fishes, the
intestinal rudiment of the digestive tube does not undergo extensive elonga-
tion during development. A relatively short tube is formed as shown in figure
279C, although some coiling of the intestine does occur in teleost fishes. A
distinct, small and large division of the intestine is not formed; intestinal and
rectal areas only are developed. Specialized rectal outgrowths develop in sharks
(fig. 279C), while, in teleost fishes, pyloric evaginations or cecae are formed.
2) Morphogenesis of the Intestine in Amphibia, Reptiles, Birds, and Mam-
mals. The development of the intestine in this group of vertebrates involves
considerable elongation and coiling (figs. 280, 281, 282). Two general divi-
sions of the intestine are formed, a small intestine, developed from the midgut
portion of the primitive metenteron, and a hindgut or colon, derived from the
hindgut portion of the gut tube. A rectal area is formed at the caudal end of
the hindgut. There is a tendency also for enlargements or extensions to occur
in the area of junction between the small intestine and colon in the birds
and mammals.
3) Torsion and Rotation of the Intestine During Development. Twisting
and rotation of the stomach and intestine is a general feature of alimentary-
tract development. In the shark embryo, the stomach is rotated in such a
way that its pyloric end is pulled upward toward the liver, forming a J-shaped
structure (fig. 296A). Also, the duodenal and valvular areas of the intestine
are rotated vertically, and the place of attachment of the dorsal mesentery
moves into a ventro-lateral position.
The developing stomach and intestine of the frog embryo presents a re-
markable and precise rotative procedure. In the early stages, the primitive
metenteron is a simple tube, continuing from the forming stomodaeum caudad
DEVELOPMENT OF THE DIGESTIVE TUBE 631
to the proctodaeum (fig. 280B). At the 6- to 7-mm. stage, the stomach-liver
area begins to rotate toward the right as indicated in figure 296B. At about
7 to 9 mm., the stomach-fiver area is projected to the right and anteriad, while
the midgut and hindgut regions move toward the left (see arrows, fig. 296C).
At the stage of development when the larvae approximate 10 mm. in length,
the stomach and intestinal areas are arranged as in figure 296D. Through the
larval stages to the time of metamorphosis, the midgut or small intestinal area
becomes greatly extended and coiled as shown in figure 296E. At the time
of metamorphosis, the small intestine becomes greatly reduced in relative
length (fig. 296F).
The chick embryo manifests similar gastrointestinal torsion. The duodenal
area of the intestine and the gizzard are pulled forward toward the liver, while
the small intestine becomes coiled and lies to a great extent in the umbilical
stalk, to be retracted later into the abdominal area.
At the 10-mm. stage in the pig, the digestive tract consists of a simple
tubular structure as shown in figure 297 A (Lineback, '16). In this figure,
the pyloric-duodenal area is projected forward toward the liver, where the
pyloric-duodenal area eventually is tied to the liver on the right side of the
peritoneal cavity, with the result that the forming stomach lies transversely
across the upper part of the abdominal cavity. The cecal and large intestinal
areas are rotated around the small intestine (see arrow, fig. 297A), when
the latter lies herniated within the umbilical cord. In figure 297B is shown the
condition in the 24-mm. pig. It is to be observed that there is now a half rota-
tion of the large intestine around the small intestine, the latter being consider-
ably coiled, while in figure 297C a complete rotation of 360 degrees is shown.
Aside from these rotational movements, extensive coiling of the gut tube
occurs, especially in the higher vertebrates. For example, the small intestine of
the frog becomes coiled extensively during the larval period (fig. 296E). Refer-
ence to figure 297D and E shows a similar coiling of the large intestine of
the pig.
Rotational movements of the intestine in the human embryo also occur. For
example, in the human embryo of about 23 mm., a condition is present, com-
parable to that of the pig embryo of 24 mm., and the future large intestine
has been rotated 180 degrees around the small intestine as shown in figure
282F. Unlike the pig, however, a complete rotation of the gut is not effected.
Also, the large intestihe does not later form into a double coil as in the pig.
In the human embryo soon after the intestine is retracted from its herniated
position in the umbilical cord (fig. 282G), the cecal area of the large intestine
becomes fixed to the right side of the peritoneal cavity near the crest of the
ilium (Hunter, '28). The ascending, transverse, and descending portions of
the large intestine are then developed (fig. 364G, H).
4) Histogenesis of the Intestine. During histogenesis of the intestine, two
632 THE DIGESTIVE SYSTEM
prominent modifications of the internal lining or mucous membrane tend to
occur:
(a) Small finger-like projections or villi are formed which project inwardly
into the lumen (fig. 298A); and
(b) the internal lining may project inwardly in the form of extensive elon-
gated folds.
In many fishes, such as the sharks, lungfishes, ganoids, and cyclostomes,
elaborate folds of the mucosa, known as the spiral folds or valves, are formed
(fig. 291C). Similarly, in higher vertebrates, elongated folds may occur, such
as the valves of Kerkring in the human and pig small intestine (fig. 298B).
Another conspicuous feature of the early histogenesis of the entodermal
layer is the formation of epithelial membranes and plugs. The pharyngeal
membrane is formed by the stomodaeal ectoderm and pharyngeal epithelial
layers. The proctodaeal membrane is similarly constructed. This structure
serves as a temporary blocking device between external and internal media.
Under normal conditions these membranes degenerate and disappear, although
occasionally they may persist. Epithelial plugs, temporarily obUterating the
lumen of the digestive tract, appear with regularity in many vertebrates. Such
temporary obstruction, for example, may appear in the developing digestive
tract of the chick or in the human esophagus, duodenum, and other areas of
the digestive tract.
/. Differentiation of the Cloaca
As previously observed, the caudal end of the intestine expands into the
cloaca, an enlarged area which eventually receives the urinary products as well
as the intestinal substances. The differentiation of this area is considered in
Chapter 18.
C. Physiological Aspects of the Developing Gut Tube
Within the developing digestive tubes of the shark, reptiles, birds, and
mammals, a brownish-green, pigmented material appears during the latter
phases of embryonic development. This material is composed of cells, bile
pigments, mucus, etc. It is discharged during the period just before or after
parturition. Fetal swallowing of ammionic fluid, gastrointestinal motility, the
pfesence of enzymes, fetal digestion and absorption, and defecation are well-
established facts in the physiology of the developing digestive tract of the
mammalian fetus (Windle, '40, Chap. VII).
Bibliography
Bloom, W. 1926. The embryogenesis of
human bile capillaries and ducts. Am. J.
Anat. 36:451.
Hunter, R. H. 1928. A note on the devel-
opment of the ascending colon. J. Anat.
62:297.
Kellicott, W. E. 1913. Outlines of Chor-
date Development. Henry Holt & Co.,
New York.
Lewis, F. T. 1912. Development of the
Pancreas. Vol. II. Human Embryology
by Keibel and Mall. J. B. Lippincott Co.,
Philadelphia.
Lineback, P. E. 1916. The development of
the spiral coil in the large intestine of
the pig. Am. J. Anat. 20:483.
Piatt, J. B. 1891. Further contribution to
the morphology of the vertebrate head.
Anat. Anz. 6:251.
Thyng, F. W. 1908. Models of the pan-
creas in embryos of the pig, rabbit, cat
and man. Am. J. Anat. 7:489.
Windle, W. F. 1940. Physiology of the
Fetus. W. B. Saunders Co., Philadelphia.
633
14
Respiratory and Buoyancy Systems
A. Introduction
1. External and internal respiration
2. Basic structural relationships involved in external respiration
a. Cellular relationships
b. Sites or areas where external respiration is accomplished
c. Main types of organs used for respiration
B. Development of bronchial or gill respiratory organs
1. Development of gills in fishes
a. Development of gills in Squalus acanthias
b. Gills of teleost fishes
c. External gills
2. Development of gills in Amphibia
a. General features
b. Development of gills in Necturus niaculosus
c. Development of gills in the larva of the frog, Rana pipiens
1) Development of external gills
2) Formation of the operculum
3) Internal gills
4) Resorption and obliteration of gills
C. Development of lungs and buoyancy structures
1. General relationship between lungs and air bladders
2. Development of lungs
a. Development of lungs in the frog and other Amphibia
h. Lung development in the chick
1) General features of lung development
2) Formation of air sacs
3) Formation of the bronchi and respiratory areas of the chick's lung
4) Trachea, voice box, and ultimate position of the bird's lung in the body
5) Basic cellular composition of the trachea, lungs, and air sacs
c. Development of lungs in the mammal
1) Origin of the lung rudiment
2) Formation of the bronchi
3) Formation of the respiratory area of the lung
4) Development of the epiglottis and voice box
5) Cellular composition
6) Ultimate position of the mammalian lung in the body
3. Development of air bladders
4. Lunglessness
634
INTRODUCTION 635
A. Introduction
1. External and Internal Respiration
Respiration consists of two phases: (1 ) external and (2) internal. External
respiration enables the organism to acquire oxygen from its external environ-
ment and to discharge carbon dioxide into this environment. Internal respira-
tion is the utilization of oxygen and the elimination of carbon dioxide by the
cells and tissues of the organism. The formation of the structural mechanisms
related to external respiration, in many vertebrates, is associated intimately
with buoyancy functions. The development of external respiratory and buoy-
ancy mechanisms is discussed in this chapter.
2. Basic Structural Relationships Involved in External
Respiration
a. Cellular Relationships
In effecting external respiration, it is necessary for blood capillaries to come
into a close relationship with a moist or watery medium containing sufficient
amounts of oxygen and a lowered content of carbon dioxide. The mechanisms
permitting this relationship vary in different vertebrates. In lower vertebrates,
blood capillaries in the gills or in the skin are brought near the watery medium
containing oxygen, while, in higher vertebrates, lungs are used for this purpose.
In lower vertebrates, an epithelial layer of cells is always interposed between
the blood stream and the oxygen-containing fluid. Small amounts of mesen-
chyme or connective tissue may interpose also (fig. 299B & C). However, in
the air capillaries of the lungs of birds (fig. 307C) and in the air cells (alveofi)
of mammalian lungs (figs. 299A; 309G), the surrounding blood capillaries
may be exposed intimately to the air-fluid mixture containing oxygen, and the
barrier of epithelium between the blood capillaries and the air mixture may
be greatly reduced if not entirely absent.
b. Sites or Areas Where External Respiration Is Accomplished
External respiration is achieved in various areas in the embryos and adults
of different vertebrate species. In the early shark embryo, external gill fila-
ments, attached to the pharyngeal area, serve as a mechanism for effecting
external respiration (fig. 299D), whereas, in the chick and reptile embryo,
allantoic contacts with surface membranes of the egg are important (fig. 299E).
In the frog tadpole, the flattened tail region is a factor, as well as the presence
of gills and lungs associated with the pharyngeal area. The embryos of higher
mammals utilize allantoic-placental relationships for this phase of respiration
(see Chap. 22). Similarly, in adult vertebrate species, various areas of the
body are used as respiratory mechanisms, such as a moist skin (fig. 299B),
gills, lungs, vascular villosities, or papillae (fig. 299F). The skin is most im-
Fig. 299. (See facing page for legend.)
636
DEVELOPMENT OF GILL RESPIRATORY ORGANS 637
portant in the amphibian group as a respiratory mechanism (Noble, '31, pp.
162, 174-175). However, considering the vertebrate group in its entirety,
the branchial or pharyngeal area is the particular part of the developing body
devoted to the formation of adult respiratory mechanisms.
c. Main Types of Organs Used for Respiration
Two main types of respiratory organs are developed in the vertebrate group:
( 1 ) branchial organs or gills in water-living forms and
(2) pulmonary organs or lungs in land-frequenting species.
Both of these organs represent pharyngeal modifications.
B. Development of Branchial or Gill Respiratory Organs
As observed in the previous chapter, p. 618, the invaginating branchial
grooves and the outpocketing branchial pouches come together in apposition
in the early embryos of all vertebrate species, and, in water-living forms,
varying numbers of these pouch-groove relationships perforate to form the
gill slits. In cyclostomatous fishes (fig. 301 A, B), the number of perforations
is six or more pairs; in elasmobranch and teleost fishes, there are five or six
pairs (fig. 301C, D); and in amphibia, two or three pairs become perforated.
In general, the first pair of branchial-pouch-groove areas is concerned with
the formation of the spiracular openings or with the auditory mechanisms.
However, in some species it may be vestigial. In water-inhabiting species, the
succeeding pairs of pouch-groove areas and their accompanying visceral arches
may develop gill structures. (See p. 669, visceral skeleton.)
Two types of gill mechanisms are developed in the vertebrate group:
( 1 ) internal gills in fishes and
(2) external gills in amphibia and in lung fishes.
In all cases, gill development involves a modification of visceral-arch struc-
ture. This modification involves the external surface membranes and blood
vessels of the arches. The first two pairs of visceral arches, the hyoid and
mandibular, are utilized generally throughout the vertebrate series in jaw and
tongue formation (see Chap. 13). On the other hand, the third and succeeding
pairs of visceral arches are potentially branchial or gill-bearing arches in
Fig. 299. Structural relationships of respiratory surfaces. (A after Clements, '38; B
after Noble, '31; E after Patten: Am. Scientist, vol. 39, '51; F and G after Noble, '25; C
and D original.) (A) Respiratory surface in air sac of pig, 18 hrs. after birth. Capil-
laries are exposed to air surface. (B) Section through epidermis of respiratory, integu-
mentary folds along the sides of the body of Cryptobranchus alleganiensis. (C) Trans-
verse section of external gill filament of Rana pipiens. (D) External gill filaments of
Squalus acanthias. (E) The ailantoic-egg-surface relationship of the developing chick
embryo. (F) Respiratory villosities or "hair" of Astylosternus robust us, the hairy
frog. (G) Section through skin of vascular villosity shown in (F).
638
RESPIRATORY AND BUOYANCY SYSTEMS
Fig. 300. Respiratory surface relationships in fishes. (A-C original; D and E after
Romer: The Vertebrate Body. 1949, Philadelphia, Saunders.) (A-C) External gill
filaments and developing gill lamellae on gill arch of shark embryo, Squalus acanthias.
(D) Section of gill arch of a shark. (E) Section of gill arch of a teleost fish.
water-living forms. In reptiles, birds, and mammals, the potency for gill for-
mation by these arches ostensibly is lost.
1. Development of Gills in Fishes
a. Development of Gills in Squalus acanthias
As the developing gill arch of Squalus acanthias enlarges, the lateral por-
tion extends outward as a flattened membrane, the gill septum (fig. 300A).
On the posterior surface of the early gill arch, the covering epithelium pro-
duces elongated structures, the external gill filaments. Each gill filament con-
tains a capillary loop which connects with the afferent and efferent branchial
arteries (see Chap. 17). These filaments are numerous and give the branchial
area a bushy appearance when viewed externally (fig. 300B). The epithelial
covering on the anterior face of the gill arch, in the meantime, produces
elongated, lamella-like folds, the gill lamellae or gill plates (fig. 300C). During
later embryonic life, the external gill filaments are retracted and resorbed as
gill lamellae are developed at the basal area of the filaments. The gill arch
thus comes to have a series of gill lamellae or plates developed on anterior
and posterior surfaces, i.e., the surfaces facing the gill-slit passageway. The
gill plates on each surface of the gill arch form a demibranch, and the two
demibranchs constitute a holobranch or complete gill.
Meanwhile, internal changes occur within the branchial arch. The original
aortal (vascular) arch becomes divided into efferent and afferent aortal ar-
teries, with capillaries interposed between the two (fig. 341A-D). Afferent
capillaries bring blood from the afferent portion of the aortal arch to the gill
lamellae, while efferent capillaries return the blood to the efferent segment of
the aortal arch. Associated with these changes, a skeletal support for the gill
arch and gill septum is formed (fig. 315C and D). It is to be observed that
the branchial or gill rays extend outward between the lamellae and thus form
a series of supports for the gill septum and lamellae. Musculature is developed
also in relation to each gill arch (fig. 327B).
DEVELOPMENT OF GILL RESPIRATORY ORGANS
639
b. Gills of Te least Fishes
Gill development in teleost fishes is similar to that of Squalus acanthias,
but the gill septum is reduced, more in some species than in others (fig.
300D, E). An operculum or external covering of the gills, supported by a
bony skeleton, also is developed. The operculum forms an armor-like, pro-
tective door, hinged anteriorly, which may be opened and closed by opercular
muscles (fig. 301D).
c. External Gills
Aside from the formation of external gill filaments as mentioned above
(fig. 300B), true external gills, resembling those of Amphibia, occur in most
of the dipnoan (lung) fishes and Polypterus in the larval stages (fig. 302A).
2. Development of Gills in Amphibia
a. General Features
The gills of Amphibia occur only in the larval condition and in some adults
which retain a complete aquatic existence, such as the mud puppy, Necturus
maculosus, and the axolotl, Ambystoma mexicanum. In other adult amphibia
which have not renounced a continuous watery existence, such as Amphiuma
and Cryptobranchus, the larval gills also are lost. Cryptobranchus relies largely
upon the skin as a respiratory mechanism (fig. 299B). External gills are
formed in the larval stage of all amphibia, and, in some, they present a
bizarre appearance (Noble, '31, Chaps. Ill and VII). In the frog tadpole,
external gills are formed first, to be superseded later by an internal variety.
The amphibian external gill is a pharyngeal respiratory device which differs
Fig. 30 L Gill arrangement in various fishes. (After Dean: Fishes, Living and Fossil,
1895, New York and London, Macmillan and Co.) (A) Polistotrema {Bdellostoma).
(B) Hagfish, Myxine. (C) Shark. (D) Teleost.
640
RESPIRATORY AND BUOYANCY SYSTEMS
Fig. 302. External gills. (A after Kerr: Chap. 9, Entwicklungsgeschichte der Wir-
beltiere, by Keibel, Jena, G. Fischer; B from Noble, '31; C-E original.) (A) Larval
form of Lepidosiren paradoxa. (B) Larval form of Pseiidohranchus striatus. (C, D)
Early developmental stages of Necturus maculosus. (E) Gill filaments on gill of adult
Necturus.
considerably from that found in most fishes. In many species, the gill is a
columnar musculo-connective tissue structure with side branches, projecting
outward from a restricted area of the branchial arch (fig. 302B). Gill fila-
ments or cutaneous vascular villosities extend outward from the tree-like
branches of the central column. The exact pattern differs with the species. In
some amphibian larvae, the gill is a voluminous sac-like affair (see Noble,
'31, p. 61).
As observed in the previous chapter, there are five pairs of branchial-pouch-
groove relationships in frogs and salamanders, although six may occur in the
Gymnophiona (Noble, '31, p. 159). In the Gymnophiona, also, the first pair
of branchial pouches perforates to the exterior for a while during embryonic life
and each perforation forms a spiracle similar to that of the sharks and certain
other fish. Later it degenerates. In other Amphibia, the first pair of branchial
pouches never perforates to the exterior. It is concerned with the formation
of the Eustachian tubes, as in most frogs and toads, or it degenerates and
eventually disappears. The second, third, fourth, and fifth pairs of branchial
pouches perforate variously in different Amphibia. In the frog, Rana pipiens,
the second, third, and fourth branchial-pouch-groove relationships generally
perforate, and sometimes the fifth does also. In Necturus maculosus, the third
and fourth pairs normally perforate.
DEVELOPMENT OF GILL RESPIRATORY ORGANS 641
b. Development of Gills in Necturus maculosus
The gills of Necturus arise at about the 10- to 14-mm. stage as fleshy
columnar outgrowths from a limited region of the third, fourth, and fifth vis-
ceral arches (i.e., the first, second, and third branchial bars or gill arches).
(See fig. 302C.) These outgrowths are at first conical in shape (fig. 227)
but later become compressed laterally. Epidermal outgrowths or gill filaments
arise from the sides of these outgrowing gill columns (fig. 302C, D). (See
Eycleshymer, '06.) As the larva grows and matures, the development of gill
filaments from the sides of the gill columns becomes profuse (fig. 302E).
During the elaboration of the gill column and gill filaments, the original aortal
(vascular) arch becomes separated into two main components, the afferent
artery from the ventral aorta to the gill column and an efferent artery from
the gill column to the dorsal aorta (Chap. 17).
c. Development of Gills in the Larva of the Frog, Rana pipiens
1) Development of External Gills. As stated on p. 639, two types of gills
are developed in the frog larva, external and internal. The external gills are
developed as follows: At about the 5-mm. stage, the gill-plate area on either
side of the embryo begins to be divided into ridges by vertical furrows (fig.
303A). Eventually, three ridges appear. These ridges represent the third,
fourth, and fifth visceral arches (i.e., the first, second, and third branchial
arches) . From the upper external edges of these arches, a conical protuberance
begins to grow outward, beginning first on the first branchial arch. Ultimately,
three pairs of these fleshy columns are formed (fig. 303B). From these gill
columns, finger-hke outgrowths, the gill filaments, arise. An abortive type of
gill may form also in relation to the fourth branchial arch. The gill column and
the filaments possess the ability to expand and contract.
2) Formation of the Operculum. At approximately the 9- to 10-mm. stage,
an oro-pharyngeal opening is formed by rupture of the pharyngeal membrane.
At this time, also, the opercular membranes arise. Each operculum arises as
a fold of tissue along the caudal edge of the hyoid or second visceral arch.
This opercular fold on either side grows backward over the gill area. Even-
tually, the two opercula fuse ventrally and laterally with the body waU to
form a gill chamber for the gills (fig. 303C). On the right side the fusion of
the operculum with the body wall is complete. However, on the left side the
fusion of the operculum in the mid-lateral area of the body wall is incom-
plete and a small opening remains as the opercular opening (fig. 257B').
3) Internal Gills. During the above period of opercular development, the
external gills become transformed into internal gills, and branchial clefts form
between the gill arches. In doing so, the external gill columns gradually shrink,
and small, delicate, gill filaments sprout from the outer edges of the gill arches
(fig. 303D). External respiration is achieved now not by a movement of the
gill in the external medium, as previously, but by the passage of water into
642
RESPIRATORY AND BUOYANCY SYSTEMS
the mouth, through the gill slit, over the gill filament, and, from thence,
through the opercular opening to the exterior. Both types of gill filaments,
external and internal, fundamentally are similar.
4) Resorption and Obliteration of Gills. The resorption of gills is a phe-
nomenon associated with metamorphosis in dipnoan fishes and in Amphibia,
although certain species of Amphibia, as indicated on p. 639, retain certain
larval characteristics in the adult condition. Most species metamorphose into
an adult form which necessitates many changes in body structure (Noble, '31,
p. 102). This transformation has been related to the thyroid hormone (Chap.
21 ). In frogs, toads, and salamanders, the thyroid hormone produces degenera-
tion and resorption of gills, the branchial clefts fuse, and the larval branchial
skeleton is changed into the adult form (fig. 317).
An interesting feature of gill resorption in the anuran tadpole is that the
degenerating gills produce a cytolytic substance which brings about the for-
mation of the hole in the operculum through which the foreleg protrudes
during metamorphosis (Hellf, '24; Noble, '31, p. 103).
C. Development of Lungs and Buoyancy Structures
1. General Relationship Between Lungs and Air Bladders
The functions of buoyancy and external respiration are related closely.
Lungs and air bladders (sacs) constitute a series of pharyngeal diverticula
associated with these functions (fig. 304A-F). (For an historical approach
to the work on developing lungs, see Flint, '06; for studies on air bladders,
consult Goodrich, '30.) Air bladders (sacs) are a characteristic feature of
GILL RUDIMENTS
INTERNAL GILL FILA
EXTERNAL GILL FILAMENTS
Fig. 303. Gill development in the tadpole of Rana pipiens. (All drawings are original.)
(A) Five- to six-mm. tadpole. (B) Frontal section of 7-mm. tadpole. (C) External,
ventral view of 10-mm. tadpole, showing opercular fold covering gill area. (D) Gill
bar, internal and external gill filaments of 10- to ll-mm. stage.
DEVELOPMENT OF LUNGS AND BUOYANCY STRUCTURES
643
STURGEON
AND MANY
TELEOSTS
ERYTHRINUS
CERATOOUS
RETE MIRABILE
■INTESTINAL ARTERY
PORTAL VEIN
HEPATIC VEIN
Fig. 304. Swim-bladder and lung relationships. (A-F slightly modified from Dean:
Fishes, Living and Fossil, 1895, New York and London, Macmillan and Co.; G after
Goodrich, '30.) (A-E) Sagittal and transverse sections of swim-bladder relationships.
(F) Lung relationship of Dipnoi and Tctrapoda. (G) Diagram of physoclistous swim
bladder of teleost fish.
most teleost and ganoid fishes. In elasmobranch and cyclostomatous fishes, the
air bladder is absent. Two main types of air bladders are found:
(1 ) a physoclistous type (fig. 304G), in which a direct connection with the
pharyngeal area is lost (e.g., the toadfish, Opsanus tan), and
(2) a more primitive physostomous variety (fig. 304A-E), retaining a
pharyngeal or pneumatic duct (e.g., the common pike or pickerel,
Esox Indus ) .
One function of the air bladder presumably is to alter the density of the
fish in such a way as to keep its density as a whole equal to the surrounding
water at various levels (Goodrich, '30, p. 586). Buoyancy, therefore, is one
of the main functions of the air bladder.
The air bladders of fishes, in some cases at least, have both respiratory or
lung and buoyancy functions (Goodrich, '30, pp. 578-593). In the bony
ganoid fishes, Amia calva and Lepisosteus osseiis (fig. 304B), the air bladder
apparently has a primary function of external respiration and, therefore, may
644 RESPIRATORY AND BUOYANCY SYSTEMS
be regarded as a lung which secondarily is associated with the function of
buoyancy. The latter condition is found also in the Dipnoi (lungfishes).
The lung of the mud puppy, Necturus maculosus, is capable of considerable
extension, particularly in the antero-posterior direction, is devoid of air cells
within, and, hence, probably serves the buoyancy function as much or more
than that of respiration. The lungs of sea turtles are capable of great distension
and aid the animal in maintaining a position near the surface of the water.
In the bird group, air sacs are united directly to the lungs, as sac-like exten-
sions of the latter.
Thus, the formation of structures which assume the responsibility for the
functions of buoyancy and respiration is a characteristic feature of pharyngeal
development in most vertebrate species.
2. Development of Lungs
a. Development of Lungs in the Frog and Other Amphibia
In the 5- to 6-mm. embryo of Rana pipiens, the lungs arise as a solid evagi-
nation of the midventral area of the pharynx at the level of the fifth branchial
pouches and over the developing heart. At the 7-mm. stage from this evagina-
tion, two lung rudiments begin to extend caudally below the developing
esophagus (fig. 305). In the 10-mm. embryo, the lungs extend backward
from a common tracheal area above the heart and liver area (fig. 258D).
At this time, the entodermal lung buds are surrounded by a mass of mesen-
chyme and coelomic epitheHum. The entodermal lining eventually becomes
folded to form larger and smaller air chambers.
In Necturus, the development of lungs is similar to that of the frog, but the
inner surface of the lungs remains quite smooth. The tracheal area of the
frog and Necturus shows little differentiation and represents a comparatively
short chamber from the lungs to the glottis. In some urodeles, the trachea is well
differentiated, possessing cartilaginous, supporting structures (e.g., Amphiuma,
Siren ) .
NEURAL TUBE
NOT OChORD
E SOPMAGUS
Fig. 305. Lung rudiment of 7-mm. of frog tadpole. (Cf. fig. 258.)
DEVELOPMENT OF LUNGS AND BUOYANCY STRUCTURES
645
Fig. 306. Lung development in the chick. (All figures, with the exception of A, were
redrawn from Locy and Larsell: '16, Am. J. Anat., vols. 19, 20; A original.) (A) Ex-
ternal view of lung rudiment during third day of incubation. (B) Transverse section
through pharynx and lung pouches of embryo of 52 to 53 hrs. of incubation. (C) Sec-
tion slightly anterior to (B), showing laryngotracheal groove. (D) Lateral view of
lung outgrowth of chick at close of fourth day of incubation. (E) Diagram of dissection,
exposing left lung of 9-day embryo. Air sacs are now evident; observe relation of heart
to lungs. (F) Ventral view of lungs and air sacs of 12-day embryo. (G) Diagram
of lateral view of bronchi of 9-day embryo. Four ectobronchi, from which parabronchi
are arising, are shown at right of figure.
b. Lung Development in the Chick
1) General Features of Lung Development. The development of lungs in
the chick differs greatly from that in the Amphibia and other vertebrates.
(For a thorough description of the developing lung of the chick, reference
should be made to Locy and Larsell, '16, a and b.)
Lung development begins during the first part of the third day of incubation
in the form of ventro-lateral, ridge-like enlargements of the pharynx, imme-
diately posterior to the fourth pair of branchial (visceral) pouches. These
evaginations arise from a ventral, groove-like trough of the pharyngeal floor
(fig. 306A). The entire area of the pharyngeal floor, where the lung rudiments
begin to develop, gradually sinks below the pharyngeal-esophageal level, and
its remaining connection with the pharynx proper is the laryngotracheal groove
in the floor of the pharynx (fig. 306B, C).
After the lung and tracheal rudiments are formed, they extend backward
646
RESPIRATORY AND BUOYANCY SYSTEMS
rapidly into the surrounding mesenchyme and they soon project dorsally, as
indicated in figure 306D. The latter figure presents the developmental condi-
tion of the lung rudiments late on the fourth day of incubation. Two areas of
the lung rudiment are evident, namely, the tracheal and lung rudiments proper.
The external appearance of the developing lungs on the ninth day of incuba-
tion is shown in figure 306E, while that of the twelfth day with the forming
air sacs is shown in figure 306F.
2) Formation of Air Sacs. The air sacs arise as extensions from the main
bronchi during the sixth to seventh day of incubation. During the ninth day,
they are present as well-developed structures (fig. 306E). The abdominal
air sac appears as a posterior continuation of the mesobronchus or primary
bronchus of the lung, while the cervical air sac arises from the anterior ento-
Fio. 307. Lung development in the chick. (All figures, after Locy and Larsell: '16,
Am. J. Anat., vols. 19, 20.) (A) Diagram of dissection of lung of 9'/2-day embryo,
designed to show entobronchi and air-sac connections with bronchial tree. (B) Diagram
of mesial aspect of adult lung, showing parabronchial connections between entobronchi
and ectobronchi. Dorsal and lateral bronchi are not shown. (C) Simplified diagram
to show air capillaries in relation to infundibula and parabronchus. (Blood capillaries
added to one sector of figure represent a modification of the original figure.) (D) Dia-
gram of lateral surface of right lung of 15-day embryo, showing recurrent bronchi of
abdominal and posterior intermediate air sacs. Anastomoses of recurrent bronchi are
also shown.
DEVELOPMENT OF LUNGS AND BUOYANCY STRUCTURES
647
ESOBRONCHUS
RECURRENT BRONCHI
ABDOMINAL AIR SA
NTERMEOIATE
Fig. 308. Respiratory structures in adult birds. (A after Kingsley, '12, Comparative
Anatomy of Vertebrates, Philadelphia, P. Blakiston's Son & Co.; B slightly modified from
Goodrich, '30.) (A) Syrinx or voice box of canvasback, Aythya. (B) Diagram of
left side view of lungs and air sacs of an adult bird.
bronchus, an outgrowth of the mesobronchus at the anterior extremity of the
lung. The anterior intermediate, posterior intermediate, and the interclavicular
air sacs take their origins from the ventral surface of the lungs and represent
outgrowths from the entobronchi (figs. 306G, 307A). The interclavicular air
sac arises from the fusion of four moieties, two from each lung. The air sacs
lie among the viscera and send out slender diverticula, some of which may
enter certain bones (fig. 308B).
3) Formation of the Bronchi and Respiratory Areas of the Chick's Lung.
Internally, the primary bronchial division of each lung passes into the lung's
substance where it continues as the mesobronchus. The mesobronchus thus
represents a continuation of the main or primary bronchial stem of the lung
and is a part of the original entodermal outpushing from the pharynx. From
the mesobronchus, the ectobronchi and entobronchi arise as diverticula (fig.
307A, B). The parabronchi or lung pipes develop as connections between the
ectobronchi and entobronchi (fig. 307B). The parabronchi constitute the
respiratory areas of the lung, for the parabronchi send off from their walls
elongated diverticula, the infundibula or vestibules. The vestibules are
branched distally (fig. ,307C) and anastomose with each other to form the
air capillaries. The blood capillaries (fig. 307C) ramify profusely between the
air capillaries. It is not clear that the air capillaries possess definite cellular
walls throughout.
As indicated in figure 307D, other or recurrent bronchi are formed as air
passages which arise from the air sacs and grow back into the lungs, where
they establish secondary connections with the other bronchi. The air sacs thus
represent expanded parts of the bronchial circuits of the kings which not only
648
RESPIRATORY AND BUOYANCY SYSTEMS
■ PMARVNGEAL
LOWER LOBE
Fig. 309. Lung development in the mammal. (A-F modified from Flint, '06; G modi-
fied from Maximow and Bloom, '42, A Textbook of Histology, Philadelphia, Saunders.)
(A-F) Development of the bronchial tree in the pig. (G) Terminal respiratory relation-
ships in the human lung. Respiratory bronchioles arise from terminal divisions of the
terminal bronchiole; from the respiratory bronchiole arise the alveolar ducts which may
terminate in spaces, the atria; from the atrium the alveolar sacs arise; and the side walls
of each alveolar sac contain the terminal air sacs or alveoli.
provide buoyancy but effect a more thorough utilization of the available air
by the respiratory areas of the lungs. That is, all the air passing through the
respiratory parts of the lung is active, moving air. (See Locy and Larsell, 16b,
pp. 42-43; Goodrich, '30, pp. 600-607.)
4) Trachea, Voice Box, and Ultimate Position of the Bird's Lung in the
Body. The trachea of the bird's lung is an elongated structure, reinforced by
cartilage rings or plates in the tracheal wall. The voice box of the bird is de-
veloped at the base of the trachea in the area of the tracheal division into the
DEVELOPMENT OF LUNGS AND BUOYANCY STRUCTURES 649
two major bronchi. It is an elaborate structure, consisting of a number of folds
of the mucous membrane together with an enlargement of this particular area.
This structure is known as the syrinx (fig. 308A). The morphological struc-
ture of the syrinx varies from species to species. The ultimate position of the
bird's lung in the body is shown in figure 308B.
5) Basic Cellular Composition of the Trachea, Lungs, and Air Sacs. It is
obvious from the description above that the entire lining tissue and the res-
piratory membrane of the bird's respiratory and air-sac system are derived
from the original entodermal evagination, whereas the muscle, connective, and
other tissues are formed from the surrounding mesenchyme.
c. Development of Lungs in the Mammal
1) Origin of the Lung Rudiment. The first indication of the appearance
of the lungs in the pig and human embryo is the formation of a midventral
trough or furrow in the entoderm of the pharynx, the laryngotracheal groove.
This groove forms immediately posterior to the fourth branchial (visceral)
pouch, approximately at the stage of 3 to 4 mm. in both pig and human. In
the human, about the fourth week, and 3-mm. pig, the laryngotracheal groove
deepens, and its posterior end gradually forms a blind, finger-like pouch which
creeps posteriorly below the esophageal area as a separate structure (fig.
309A). Thus, the original laryngotracheal groove is restricted to the cephalic
end of the developing lung rudiment, where it forms a slit-like orifice in the
midventral floor of the pharynx at about the level of the fifth visceral (i.e.,
third branchial) arch.
2) Formation of the Bronchi. As the caudal end of the original lung rudi-
ment grows caudad, it soon bifurcates into left and right bronchial stems as
shown in figure 309B. Each primary or stem bronchus is slightly enlarged at
the distal end. As the stem bronchi of the right and left lung buds continue
to grow distally, evaginations or secondary bronchi arise progressively from
the primary bronchi as indicated in figure 309C-E. While this statement
holds true for the human embryo, the apical bronchus (i.e., eparterial bronchus
because this lobe of the lung comes to lie anterior to the pulmonary artery)
in the pig arises directly from the trachea as shown in figure 309D. Each of
these secondary bronchi forms the main bronchus for the upper and middle
lobes of the lungs (fig. 309D, E). From each lobular bronchus, other bronchial
buds arise progressively and dichotomously, with the result that the bronchial
system within each lobe of the lung becomes complex, simulating the branches
upon the limb of a tree. Considerable variation may exist in the formation of
the various bronchi in different individuals.
3) Formation of the Respiratory Area of the Lung. This growth of bronchial
buds of the pulmonary tree continues during fetal life and for a considerable
time after birth. The large bronchi give rise to smaller bronchi, and, from the
latter, bronchioles of several orders originate. Finally, the terminal bronchioles
650
RESPIRATORY AND BUOYANCY SYSTEMS
arise. Fifty to eighty terminal bronchioles have been estimated to be present
for each lobule of the human lung (Maximow and Bloom, '42, p. 465). From
each of the terminal bronchioles, a varying number of respiratory bronchioles
arise, which in turn give origin to the alveolar ducts, and, from the latter,
arise the alveolar sacs and alveoli. Each alveolus represents a thin-walled
compartment of the alveolar sac (fig. 309G). The exact cellular structure of
the terminal air compartments or alveoli is not clear. In the frog lung, a layer
of flattened epithelium is present. However, in the lung of the bird and the
mammal, this epithelial lining may not be complete, and the wall of the alveolus
may be formed, in part at least, by the endothelial cells of the surrounding
capillaries (fig. 299A; Palmer, '36; Clements, '38).
4) Development of the Epiglottis and Voice Box. The epiglottis is the
structure which folds over the glottis and thus covers it during deglutition.
The glottis is the opening of the trachea into the pharynx. An epiglottis is
found only in mammals. It arises as a fold in the pharyngeal floor in the area
between the third and fourth visceral arches. It grows upward and backward
in front of the developing glottis (fig. 3 lOA-C) . In the meantime, the arytenoid
swellings or ridges appear on either side of the glottis.
The larynx or voice box is an oval-shaped compartment at the anterior end
of the trachea in mammals. It is supported by cartilages derived from the
visceral arches (Chap. 15). The vocal cords arise as transverse folds along
the lateral sides of the laryngeal wall.
5) Cellular Composition. The epithelial lining of the larynx, trachea,
bronchi, etc., is derived from the entodermal outpushing, whereas the sur-
PLICA PHftRYNGO- EPIGtOTTICA OR
LATERAL GLOSSO-EPIGLOTTIC FOLD
ROOT OF TONGUE
EPIGLOTTIS
Fig. 310. Development of the epiglottis and entrance into the larynx in the human
embryo. (Consult also fig. 285.) (All figures slightly modified from Keibel and Mall:
Manual of Human Embryology, vol. II, '12, Philadelphia, Lippincott.) (A) About
16-mm., crown-rump length, 7 to 8 weeks. (B) About 40-mm., crown-rump length,
9 to 10 weeks. (C) Late fetal condition.
BIBLIOGRAPHY 651
rounding mesenchyme gives origin to the cartilage, muscle, and connective
tissue present in these structures.
6) Ultimate Position of the Mammalian Lung in the Body. See Chapter 20.
3. Development of Air Bladders
It is difficult to draw a clear distinction between air bladders of Pisces and
the lungs of Tetrapoda. Air bladders and gills appear to be the standard ar-
rangement for most fishes. It is probable, therefore, that the function of
external respiration rests mainly upon the branchiae or gills in all fishes other
than the Dipnoi, while the function of buoyancy is the responsibility of the
air bladder. In some fishes {Dipnoi and ganoids), the functions of buoyancy
and respiration converge into one structure, the air bladder or lung, as they
do in many Tetrapoda.
In development, air bladders, like the lungs of all Tetrapoda, arise as di-
verticula of the posterior pharyngeal area. In most cases, the air bladder arises
as a dorsal diverticulum (fig. 304A, B), while, in other instances, its origin
appears to be from the lateral wall (fig, 304C). In Salmonidae , Siluridae, etc.,
for example, it arises from the right wall, while in Cyprinidae, C haracinidae ,
etc., it takes its origin from the left wall. The air bladder generally is a
single structure (fig. 304A, C, D), but in some cases it is double or bilobed
(fig. 304E).
Generally speaking, the air bladder receives blood from the dorsal aorta
or its immediate branches (fig. 304G), but in Dipnoi and Polypterus, the
blood supply to the air bladder comes from the pulmonary arteries as it does
in Tetrapoda.
4. Lunglessness
Many urodele amphibia have reduced or lost their lungs entirely. In many
cases the reduced condition of the lungs or absence of lungs is compensated
for by the development of buccopharyngeal respiration. The latter type of
respiration depends upon an extreme vascularization of the pharyngeal and
caudal mouth epithelium and rapid throat movements which suck the air in
and then expel it. In Aneides (Autodax) lugubnis, a land form, these throat
movements may reach 120 to 180 movements per minute (Ritter and Miller,
1899). Lungless aquatic salamanders also practice buccopharyngeal respira-
tion, although, in Pseudotriton ruber, cutaneous respiration evidently is re-
sorted to (Noble, '25).
Bibliography
Clements, L. P. 1938. Embryonic develop- Eycleshymer, A. C. 1906. The growth and
ment of the respiratory portion of the regeneration of the gills in the young
pig's lung. Anat. Rec. 70:575. Necturus. Biol. Bull. X: 171.
652
RESPIRATORY AND BUOYANCY SYSTEMS
Flint, J. M. 1906. The development of
the lungs. Am. J. Anat. 6:1.
Goodrich, E. S. 1930. Studies on the Struc-
ture and Development of Vertebrates.
Macmillan and Co., London.
HelfT. O. M. 1924. Factors involved in the
formation of the opercular leg perfora-
tion in anuran larvae during metamor-
phosis. Anat. Rec. 29:102.
Locy. W. A. and Larsell, O. 1916a. The
embryology of the bird's lung. Based on
observations of the domestic fowl. Part
I. Am. J. Anat. 19:447.
and
1916b. The embry-
ology of the bird's lung. Based on obser-
vations of the domestic fowl. Part II.
Am. J. Anat. 20:1.
Maximow, A. A. and Bloom, W. 1942. A
Textbook of Histology. W. B. Saunders
Co., Philadelphia.
Noble, G. K. 1925. The integumentary,
pulmonary and cardiac modifications
correlated with increased cutaneous res-
piration in the Amphibia; a solution to
the "hairy frog" problem. J. Morphol.
& Physiol. 40:341.
. 1931. The Biology of the Am-
phibia. McGraw-Hill Book Co., Inc.,
New York.
Palmer, D. W. 1936. The lung of a human
foetus of 170 mm. C. R. length. Am. J.
Anat. 58:59.
Ritter, W. E. and Miller, L. 1899. A con-
tribution to the life history of Autodax
lugubris Hallow., a Californian salaman-
der. Am. Nat. 33:691.
15
Tne S-Keletal System
A. Introduction
1. Definition
2. Generalized or basic embryonic skeleton; its origin and significance
a. Basic condition of the skeletal system
b. Origin of the primitive ghost skeleton
1) Notochord and subnotochordal rod
2) Origin of the mesenchyme of the early embryonic skeleton
c. Importance of the mesenchymal packing tissue of the early embryo
B. Characteristics and kinds of connective tissues
1. Connective tissue proper
a. Fibrous types
1) Reticular tissue
2) White fibrous tissue
3) Elastic tissue
b. Adipose tissue
2. Cartilage
a. Hyaline cartilage
b. Fibrocartilage
c. Elastic cartilage
3. Bone
a. Characteristics of bone
b. Types of bone
c. Characteristics of spongy bone
d. Compact bone
C. Development of skeletal tissues
1. Formation of the connective tissue proper
a. Formation of fibrous connective tissues
b. Formation of adipose or fatty connective tissue
2. Development of cartilage
3. Development of bone
a. Membranous bone formation
b. Endochondral and perichondrial (periosteal) bone formation
1 ) Endochrondral bone formation
2) Perichondrial (periosteal) bone formation
c. Conversion of cancellous bone into compact bone
D. Development (morphogenesis) of the endoskeleton
1. Definitions
653
654 THE SKELETAL SYSTEM
2. Morphogenesis of the axial skeleton
a. General features of the skeleton of the head
1) Neurocranium or cranium proper
2) Visceral skeleton or splanchnocranium
3) Development of the skull or neurocranium
4) Vicissitudes of the splanchnocranium
b. Ossification centers and the development of bony skulls
c. Development of the axial skeleton
1) Axial skeleton of the trunk
a) Notochord
b) Vertebrae
c) Divisions of the vertebral column
d) Ribs
e) Sternum
2) Axial skeleton of the tail
d. Development of the appendicular skeleton of the paiied appendages
1 ) General features
2) Development of the skeleton of the free appendage
3) Formation of the girdles
e. Growth of bone
f. Formation of joints
1) Definitions
2) Ankylosis (synosteosis) and synarthrosis
3) Diarthroses
4) Amphiarthroses
g. Dermal bones
A. Introduction
1. Definition
The word skeleton is used coinmonly to denote the hard, supporting frame-
work of the body, composed of bone and cartilage. In this restricted sense it
is employed to refer particularly to the internal or endoskeleton (see p. 668).
The word has a broader meaning, however, for the skeletal system includes
not only the bony and cartilaginous materials of the deeper-lying, internal
skeleton but also the softer, pliable connective tissues as well. Thus, the
skeletal tissues in a comprehensive sense may be divided as follows:
(1) the soft skeleton, composed of pliable connective tissues which bind
together and support the various organs of the body and
(2) the hard or firm skeleton, formed of bone, cartilage, and other struc-
tures which protect and sustain, and give rigidity to the body as a
whole. The exoskeletal structures described in Chapter 12 in reality
are a part of the hard, protective skeleton of the vertebrate body.
(Note: Blood and lymph are often classified as a part of the connective
tissues. See Maximow and Bloom, '42, p. 39.)
INTRODUCTION
655
Fig. 311. (A) Diagram showing basic mesenchymal packing tissue around the various
body tubes and notochord. (B) Contribution of embryonic mesenchyme to adult
skeletal tissue.
2. Generalized or Basic Embryonic Skeleton;
Its Origin and Significance
a. Basic Condition of the Skeletal System
The generalized or basic skeleton of the embryo which has achieved primi-
tive body form is composed of the notochord or primitive skeletal axis,
together with the mass of mesenchyme which comes to fill the spaces between
the epidermal, neural, enteric, mesodermal, and primitive circulatory tubes.
Because of the delicate nature of the mesenchymal cells and the coagulable
intercellular substance between them, this primitive skeleton sometimes is re-
ferred to as the "ghost skeleton" (fig. 311 A).
b. Origin of the Primitive Ghost Skeleton
1) Notochord and Subnotochordal Rod. As observed in Chapters 9 and
10, the notochord becomes segregated as a distinct entity during gastrulation
and embryonic body formation. It soon comes to form a rod-like structure,
surrounded by a primitive notochordal membrane. The notochordal axis ex-
tends from the pituitary body (hypophysis) and diencephalic region of the
brain caudally to the end of the tail (fig. 217). In many of the lower verte-
brates, a second rod of cells, the hypochord or subnotochordal rod, evaginates
and segregates from the roof of the gut in the trunk region of the embryo
during tubulation and early body-form development; it comes to lie immedi-
ately below the notochord (fig. 228). The subnotochoral rod soon degenerates.
656 THE SKELETAL SYSTEM
The notochord never extends cranialward beyond the hypophysis and
infundibular downpushing from the diencephalon in any of the vertebrates.
This meeting place of the hypophysis, notochord, and infundibulum is a con-
stant feature of early vertebrate structure from the cyclostomatous fishes to
the mammals. In Amphioxus, however, the notochord projects anteriad be-
yond the limits of the "brain" (fig. 249D, E).
2) Origin of the Mesenchyme of the Early Embryonic Skeleton. The origin
of mesenchyme in the early embryo is set forth in Chapter 1 1, page 520.
c. Importance of the Mesenchymal Packing Tissue of the Early Embryo
The mass of mesenchymal cells which comes to lie between the embryonic
body tubes not only forms the primitive skeletal material of the early embryo
but it also serves as a reservoir from which later arise many types of cells
and tissues, as indicated in the following diagram;
endothelial cells of capillaries and other blood vessels
lipoblasts ►fat cells
^chondrobiasts (cartilage-forming cells) ^chondrocytes
and cartilage
, fibroblasts ►fibrous connective tissue
.osteoblasts^ ►osteocytes and bony substances, including
Mesenchymal -:^^'^^ dermal bones and the dermal substances of scales
cells ^Z;;;;]""-*' macrophages ^-phagocytes
hemocytoblast (free, wandering, mesenchymal cell)
V\\^ erythrocytes
y^monocytes
\ blood platelets
white blood cells
'myoblasts (for smooth, cardiac, and skeletal muscle)
In regard to the skeletal system, it is pertinent to point out the fact that
wherever mesenchyme exists, the possibility for connective tissue develop-
ment also exists.
B. Characteristics and Kinds of Connective Tissues
Connective tissues, other than adipose tissue, are characterized by the pres-
ence of intercellular substances which become greater in quantity than the
cellular units themselves. In consequence, the various types of connective
tissue are classified in terms of the intercellular substance present. Excluding
the blood, three main categories of connective tissues are found:
( 1 ) connective tissue proper,
(2) cartilage, and
(3) bone.
CHARACTERISTICS OF CONNECTIVE TISSUES
1. Connective Tissue Proper
The connective tissues proper may be divided into
(a) fibrous types and
(b) fatty or adipose tissue.
657
a. Fibrous Types
1) Reticular Tissue. This type of connective tissue possesses stellate cells,
between which are found delicate aggregations of fibrils and a fluid-like, inter-
cellular substance (fig. 312B).
2) White Fibrous Tissue. White fibrous tissue contains bundles or sheets
of white, connective-tissue fibers (i.e., collagenous fibers), placed between
the cells. Some elastic fibers may be present (fig. 312C, D). Collagenous
fibers yield gelatin upon boiling with water and are not digested readily by
trypsin (Maximow and Bloom, '42).
3) Elastic Tissue. Elastic connective tissue is similar to the white fibrous
variety but contains a large percentage of elastic tissue fibers which extend
under stress but contract again when tension is released (fig. 312E). Elastic
fibers are resistant to boiling water and are digested readily by trypsin
(Maximow and Bloom, '42). Elastic tissue may have a yellowish tinge when
viewed macroscopically.
COLLAGENOUS FIBER BUNDLES
ON CELT
ELASTIC FIBERS
FAT CELLS
ELASTIC TISSUE
DEVELOPING ADIPOSE TISSUE
Fig. 312. Types of soft connective tissues. (A, D, and E redrawn from Bremer, 1936,
Textbook of Histology, Philadelphia, Blakiston; B and C redrawn from Keibel and Mall,
1910, Manual of Human Embryology, vol. 1, Philadelphia. Lippincott; F redrawn from
Bell, '09.)
658
THE SKELETAL SYSTEM
'i^^&Y\^'f^'^ &)^M^ 1^^ V?^>S1^^- Intercellular (jr,
,0/
=a;M
PHECARTILiGt STAGE
ERiCHONDRiai. (VASCULAR), bud; />/52:' •' l'/*l"" , " ? wh iteO) " V cartTlage -celYs and capsules
FIBROCARTILAGE
GMONDRIN SHOWN IN BLACK
AROUND CARTILAGE CELLS
LASTIC CART
05TIC FIBERS
Fig. 313. Types of cartilaginous tissue. (A-C) Development of hyaline cartilage.
(D) Destruction of cartilage by perichondrial vascular bud preparatory to ossification.
The cartilage spicules may be infiltrated with calcium salt at this period. (Redrawn
from Bremer. 1936, Textbook of Histology, Philadelphia. Blakiston.) (E) Fibrocar-
tilage. from area of tendinous union with bone. (F) Elastic cartilage from human,
larynx. (Redrawn and modified from Bremer. 1936, Textbook of Histology, Philadelphia,
Blakiston.)
b. Adipose Tissue
Adipose tissue contains a fibrous network of white and elastic fibers, be-
tween which fat cells develop. Eventually, the fibrous connective tissue is
displaced and pushed aside by the fat-containing elements (fig. 312F).
2. Cartilage
Cartilage is a type of connective tissue with a solid intercellular substance.
The latter is composed of a fibrous framework filled with an amorphous ground
substance. Unlike bone, the intercellular substance may be readily cut with
a sharp instrument. Three main types of cartilage are found:
( 1 ) hyaline,
(2) fibrous, and
( 3 ) elastic.
a. Hyaline Cartilage
Hyaline cartilage (fig. 313A-C) is the most widespread variety of cartilage.
It is characterized by a solid, amorphous, ground substance, slightly bluish
in appearance, easily bent and capable of being cut with a sharp instrument.
CHARACTERISTICS OF CONNECTIVE TISSUES 659
The amorphous ground substance or chondrin is reinforced by fibers of the
collagenous (white) variety, but the quantity of fiber present is much less
than in fibrous or elastic cartilage. The chondrocytes (i.e., the cartilage cells)
lie within capsules. Canaliculi apparently do not connect one capsule with
another. This type of cartilage forms a considerable part of the temporary
axial and appendicular skeleton of the developing organism and remains as
the adult axial and appendicular skeleton in cyclostomatous and elasmobranch
fishes. In the adults of other vertebrates, it is supplemented to various degrees
by bone.
b. Fibrocartilage
Fibrocartilage (fig. 313E) is a transitional form between white fibrous
connective tissue and cartilage. It contains bundles of collagenous fibers, placed
parallel to each other. Between the fibrous bundles, cartilage capsules are
present, containing cartilage cells (chondrocytes). A small amount of amor-
phous ground substance or chondrin is present, particularly around the cell
capsules. Some types of fibrocartilage contain more of the amorphous ground
substance than other types. Fibrocartilage is found in the intervertebral discs
between the vertebrae, in the area between the two pubic bones in mammals,
and in certain ligaments, such as the ligamentum teres femoris.
c. Elastic Cartilage
Elastic cartilage (fig. 313F) differs from the hyaline variety by the pres-
ence of an interstitial substance which contains branching and interlacing
fibers of the elastic variety. The elastic fibers penetrate through the amorphous
substance in all directions. While hyaline cartilage is bluish in color, the color
of elastic cartilage is yellowish. It is found in the external ear of mammals,
in the mammalian epiglottis. Eustachian tubes, the tubes of the external
auditory meatus, etc.
3. Bone
a. Characteristics of Bone
Bone forms the greater part of the adult skeleton of all vertebrates above
the cyclostomatous and elasmobranch fishes. In teleost fishes and in land-
frequenting vertebrates, it tends to displace most of the cartilaginous sub-
stance of the skeleton. The interstitial substance of bone is composed of a
fundamental fibrous material similar to that of connective tissue. These fibers
are called osteocollagenous fibers. A small amount of amorphous ground
substance also is present. The interstices of this fibrous and amorphous sub-
strate are infiltrated with mineral salts, particularly calcium salts, to form
the bony substance. The latter is formed in layers, each layer constituting a
lamella. The bone cells or osteocytes are present in small cavities or lacunae
between the lamellae. The lacunae are connected with each other by small
Fig. 314. Types and development of bone. (A) Compact and cancellous (spongy)
bone. (B) Diagram showing structure of compact bone. (Redrawn and slightly modi-
fied from Maximow and Bloom, 1942, A Textbook of Histoloi-y, Philadelphia, Saunders.)
(C) Stages in conversion of marrow canal or space of spongy bone into an Haversian
system by deposition of concentric layers of bony lamellae. (D) Haversian systems
of compact bone from thin, ground section. (Redrawn and modified from Bremer, 1936,
Textbook of Histology, Philadelphia, Blakiston.)
660
Fig. 314 — (Continued) Types and development of bone. (E) Diagram showing inva-
sion of cartilage by perichondria! vascular buds, preparatory to deposition of bony sub-
stance on cartilaginous spicules produced by erosion of cartilage (compare with fig. 313,
D). (F) The formation of spongy bone within, by deposition of bony substance on
cartilaginous spicules. See spicule "A." Compact bone is deposited on outer surface of
cartilaginous replica of future bone by periosteal osteoblasts, forming bony cylinder of
compact bone. (Redrawn and modified from Bremer, 1936, Textbook of Histology, Phila-
delphia, Blakiston.) (G) Formation of membrane bone from jaw of pig embryo. (Re-
drawn and modified from Bremer, 1936, Textbook of Histology, Philadelphia, Blakiston.)
(H) Bone destruction and resorption. Observe osseous globules within substance of osteo-
clast. (From Jordan, '21, Anat. Rec, 20.)
661
662 THE SKELETAL SYSTEM
channels or canaliculi which course through the lamellae. Some of the
canaliculi join larger channels within the bony substance which contain blood
vessels. Bony substance in the living animal, therefore, is living tissue, con-
structed of the following features (fig. 314):
( 1 ) Bony layers or lamellae are present, composed of a ground substance
of fibrous and amorphous materials infiltrated with mineral salts, par-
ticularly the salts of calcium (fig. 314A, B);
(2) between the bony layers are small cavities or lacunae, each contain-
ing a bone cell or osteocyte (fig. 314B);
(3) coursing through the lamellae and connecting the various lacunae, are
small channels, known as canaliculi, into which extend processes from
the osteocytes (fig. 314B); and
(4) the canaliculi make contact in certain areas with blood vessels which
lie within small canals coursing through the bony substance or in
larger spaces, called marrow cavities (fig. 314A, B).
b. Types of Bone
From these fundamental structural features, two types of bone are formed:
( 1 ) spongy and
(2) compact.
The difference between these two types of bone rests upon the proportion
of bony substance to blood-vessel area or marrow cavity present, and is not
due to a difference in the character of the bony substance itself.
c. Characteristics of Spongy Bone
Spongy bone differs from compact bone in that large marrow cavities or
spaces are present between an irregular framework of compact bone. The
bony substance present is in the form of a meshwork of irregular columns
or trabeculae between the marrow-filled spaces (fig. 314A).
d. Compact Bone
Compact bone (fig. 314A, B, D) lacks the widespread, marrow-filled cavi-
ties of the spongy variety, the marrow spaces being reduced to a minimum.
This is accomplished by the utilization of a structural unit known as the
Haversian system, named after Clopton Havers, an English anatomist who
discovered the system during the latter part of the seventeenth century while
investigating the blood supply of bone. The bony walls of the shafts of long
bones are composed largely of many Haversian systems, associated side by
side as shown in figure 314D. Irregular layers (lamellae) lie between the
various systems.
The Haversian system is composed of a very narrow canal or lumen, the
Haversian canal, around which are placed concentrically arranged bony plates
DEVELOPMENT OF SKELETAL TISSUES 663
(lamellae) with their associated lacunae, osteocytes, and canaliculi (fig.
314B-D). Blood vessels from the marrow cavity within the bone or from
the surface of the bone via Volkmann's canals (fig. 314D) pass into the
Haversian canals, thus supplying nourishment and other life-maintaining
features to the canaliculi and through the latter to the osteocytes. Compact
bone thus restricts the marrow cavity to a central area, and the Haversian
and Volkmann canals convey the blood supply into the compact bony sub-
stance which surrounds the central marrow cavity. In general, the Haversian
systems are formed parallel with the long axis of the bone. Circumferential
lamellae surround the external surface of the bone around the Haversian
systems. Inner circumferential lamellae also are present lining the marrow
cavities of long bones.
C. Development of Skeletal Tissues
1. Formation of the Connective Tissue Proper
a. Formation of Fibrous Connective Tissues
In the early embryo, following the ghost-skeleton stage, two types of con-
nective tissues are found:
( 1 ) Mucoid or loose connective tissue is located in Wharton's jelly in
the umbilical cord of mammals and in other parts of the embryo. This
embryonic type of connective tissue is characterized by the presence
of large mesenchymal cells whose processes contact the processes of
other surrounding mesenchymal cells (fig. 312A). Within the mesh-
work formed by these cells and their processes, mucus or a jelly-like
substance is present. Very delicate fibrils may lie within this jelly.
(2) A second type of early embryonic connective tissue is reticular tissue.
It contains stellate mesenchymal cells whose processes contact each
other (fig. 312B). Very delicate bundles of fibrils may be present
which are closely associated with the cells.
The foregoing, connective-tissue conditions of the early embryo eventually
are replaced by the mature forms of connective tissue. In this process the
reticular type of connective tissue appears to form an initial or primary stage
of connective-tissue development. For example, in the development of white
fibrous tissue, a delicate network of fine fibrils appears within the ectoplasmic
ground substance between the primitive mesenchymal cells, thus forming a
kind of reticular tissue (fig. 312A, B). With the appearance of fibrils be-
tween the mesenchymal cells, the latter may be regarded as fibroblasts. Fol-
lowing this reticular stage, the ectoplasmic ground substance becomes more
fibrillated and parallel bundles of white fibers arise, probably by the direct
chemical transformation of the earlier fibrils into white or collagenous fibers
(fig. 312C). (See Bardeen, '10, p. 300.) It is probable that the elastic con-
664 THE SKELETAL SYSTENf
nective tissue with its elastic fibers arise in a similar manner, with the excep-
tion that elastic fibers are formed instead of collagenous fibers.
The matter of fiber formation within connective tissues has been the sub-
ject of much controversy. The older view of Flemming (Mall, '02, p. 329)
maintains that the fibers arise within the peripheral area of the cytoplasm of
the cell from whence they are thrown off into the intercellular space where
they continue to grow. However, most observers now agree that the fibrils
arise from an intercellular substance, i.e., from the substance lying between
the fibroblasts, but the manner by which this intercellular substance itself
arises is questionable. Some observers, such as Mall ('02) and Jordan ('39),
set forth the interpretation that the intercellular substance is derived from
a syncytial ectoplasm which becomes separated from the early mesenchymal
cells. Baitsell ('21) and Maximow ('29), however, consider the intercellular
substance to be a secretion product of the mesenchymal cells which have
become fibroblasts. The observations of Stearns ('40) on living material in
a transparent chamber of the rabbit's ear suggest that the ground substance
is exuded by the surface of the fibroblasts and that the fibers then develop
within this exudate.
b. Formation of Adipose or Fatty Connective Tissue
Adipose tissue is fibrous connective tissue which contains certain specialized
cells of mesenchymal origin, the lipoblasts. The latter have the ability to
produce lipoidal substances and to store these substances within the confines
of their own boundaries. Adipose or fatty tissue arises in fibrous connective
tissues in various parts of the body in proximity to blood capillaries.
Lipogenesis or the formation of the fatty substance is an unsolved problem.
Two main types of fat are formed, white and brown. The process of hpo-
genesis in white fat, according to Schreiner ('15) who studied the process in
detail in the hagfish embryo, Myxine glutinosa, consists at first in liberation
of small buds from the nucleolus within the nucleus. These buds pass through
the nuclear membrane into the cytoplasm as granules or chromidia. In the
cytoplasm these granules appear as mitochondria. The latter increase in num-
ber by division. The secondary granules then separate and each gives origin
to a liposome which liquefies and expands into a small fat globule. Regardless
of the exact method by which the small fat globules arise, when once formed,
the small globules coalesce to form the large fat globule, typical of white fat,
which ultimately pushes the nucleus and cytoplasm of the lipoblast to the
periphery (fig. 312F). (See Bell, '09.) Lipoblasts in the mature condition are
fat cells or lipocytes.
The above type of fat-cell formation occurs in the subcutaneous areas of
the embryo. In the human embryo it begins at about the fourth month. How-
ever, aside from the common type or white-fat formation, another kind of
fat-cell development occurs in certain restricted areas of the body in the so-
DEVELOPMENT OF SKELETAL TISSUES 665
called brown fat tissue found in certain adipose glands. It is referred to as
brown fat because a brownish pigment may be present in certain mammals.
During brown-fat formation, mesenchymal cells become ovoid in shape and
develop a highly granular cytoplasm. These granules give origin to small fat
globules which remain distinct for a time and do not readily fuse to form the
large fat globule, characteristic of white fat. However, they ultimately may
coalesce and become indistinguishable from the ordinary lipocyte found in
white fat. In man, this type of fat disappears shortly after birth; in the cat,
it is present until maturity when it transforms into the ordinary type or white
fat; and in the rat, it persists throughout life (Sheldon, '24). In the wood-
chuck, this type of fat forms the hibernating gland (Rasmussen, '23). In
mice and other rodents, the presence of a small amount of brownish pigment
is evident in this type of fat. In the young monkey, hibernating-gland tissue
is found in the cervical, axillary, and thoracic areas (Sheldon, '24).
2. Development of Cartilage
The formation of cartilage is an interesting process. During the initial stage
of cartilage development, mesenchymal cells withdraw their processes, assume
a rounded appearance, and become closely aggregated. This condition is
known as the pre-cartilage stage (fig. 313A). Gradually the pre-cartilage
condition becomes transformed into cartilage by the appearance of the inter-
cellular substance, characteristic of cartilage between the cells (fig. 313B, C).
As in the case of the connective tissues described on page 664, two schools
of thought explain the appearance of this intercellular substance:
(a) as a modification of the ectoplasm which separates from the chondro-
blasts and
(b) as a secretion of these cells.
In hyaline cartilage, the homogeneous, amorphous, ground substance is
predominant, together with a small number of fibrils; in fibrocartilage, a
large number of white, connective-tissue fibers and a smaller amount of the
amorphous substance is deposited; and in elastic cartilage, elastic, connective-
tissue fibers are formed in considerable numbers. The mesenchyme, immedi-
ately surrounding the mass of cartilage, forms the specialized tissue, known
as the perichondrium. The perichondrial layer, as the name implies, is the
tissue immediately surrounding the cartilage. It connects the cartilage with
the surrounding connective tissue and mesenchyme. The inner cells of the
perichondrium transform into chondroblasts and deposit cartilage; in this
manner the cartilage mass increases in size by addition from without. The
latter form of growth is known as peripheral growth. On the other hand, an
increase within the mass of cartilage already formed is the result of interstitial
growth. Interstitial growth is effected by an increase in the number of cells
within the cartilage and by a deposition of intercellular substance between
666 THE SKELETAL SYSTEM
the cells. The increase in the intercellular substance separates the chondro-
blasts from each other, and the mass of cartilage expands as a whole. These
two types of growth are important processes involved in the increase in size
of many body structures. Cartilage formation in the human embryo begins
during the fifth and sixth weeks.
3. Development of Bone
Bone develops as the result of the calcification of previously established
fibrous or cartilaginous connective tissues. The transformation of fibrous con-
nective tissue into bone is called membranous or intramembranous bone for-
mation, and the process which transforms cartilage into bone constitutes
endochondral or intracartilaginous bone development. Membranous bone for-
mation occurs in the superficial areas of the body, particularly in or near the
dermal area of the skin whereas cartilaginous bone formation is found more
deeply within the substance of the body and its appendages.
a. Membranous Bone Formation
Membranous bone formation occurs as follows (fig. 314G): Thin spicules
or bars of a compact intercellular substance, known as ossein, gradually come
to surround collagenous (osteogenic) fibers which lie between fibroblast cells.
Later, these spicules of ossein become calcified by the action of specialized
cells, called osteoblasts, which surround the osseinated fibrils. Osteoblasts may
represent transformed fibroblasts or, more directly, transformed mesenchymal
cells. With the deposition of the bone salts, the tissue is converted from ossein
into bone. Thus, spicules of ossein and connective tissue fibers serve as the
basis for bone deposition and become converted into bony spicules. These
spicules are converted next into bony columns (trabeculae) by the formation
of layers (lamellae) of compact bone around the original bony spicule. Such
bony columns or trabeculae are characteristic of spongy bone (fig. 314A).
Some of the bone-forming cells become enclosed within the lacunar spaces
in the bone during the above process and are left behind as bone cells or
osteocytes (fig. 314A). The osteocytes within their respective lacunae tend
to be located between the layers of bony material (fig. 314A-D).
After the primary trabeculae of spongy bone are formed, the surrounding
mesenchyme, which encloses the site of bone formation, becomes converted
into a membranous structure, known as the periosteum. The cells of the
inner layer of periosteum are transformed into osteoblasts and begin to de-
posit successive layers of compact bone around the initial framework of
spongy bone (peripheral growth). The latter activity results in an increase
in diameter of the bony area.
The first bone thus formed occurs in a restricted area. As the bone grows,
the previously formed bone is torn down and resorbed, while new compact
bone is built up around the area occupied by the spongy bone. Either by the
DEVELOPMENT OF SKELETAL TISSUES 667
formation of new cellular entities or by the fusion of osteoblasts, multinucle-
ated giant cells appear which aid in the dissolution of the previously formed
bone. These multinucleate cells are known as osteoclasts (fig. 314H). The
marrow-filled spaces between the trabeculae of spongy bone contain blood
spaces (sinusoids), developing red blood cells, looser connective tissues, and
fat cells (fig. 314H). When the trabeculae of spongy bone are resorbed, the
marrow-filled area increases in size.
b. Endochondral and Perichondrial (Periosteal) Bone Formation
While membranous bone development utilizes collagenous fibrils and ossein
as a foundation upon which the osteoblasts deposit bone salts, endochondral
that is, intracartilaginous bone development employs small spicules or larger
masses of cartilage as a basis for calcification. The small columns or spicules
of cartilage are produced as a result of erosion and removal of cartilage. This
erosion of cartilage is produced by perichondrial cells and vascular tissue
which invade the cartilaginous substance from the perichondrium.
1) Endochondral Bone Formation. Endochondral bone formation occurs
as follows:
(a) The initial step in erosion of cartilage is the migration within the
cartilage, in a manner not understood, of the scattered cartilage cells.
This migration brings about the arrangement of the cartilage cells and
their capsules into elongated rows (fig. 314F). Some deposition of
calcium within the cartilaginous matrix occurs at this time.
(b) As this realignment of the cartilage cells is effected, vascular buds
from the inner layer of the perichondrium invade the cartilage, eroding
the cartilaginous substance and forming primary marrow cavities (figs.
31 3D; 314E, F). Large multinucleate cells or chondroclasts make
their appearance at this time and aid the process of dissolution of
cartilage.
(c) Following this procedure, osteoblasts arise within the peripheral areas
of each vascular bud and begin to deposit bone matrix upon the
small spicules of calcified cartilage which remain. (See spicule "a,"
fig. 314F.) The continual deposition of bone salts around these spicules
converts the greatly eroded cartilaginous mass into spongy or cancel-
lous bone (fig. 314F).
2) Perichondrial (Periosteal) Bone Formation. As cancellous bone is
formed within the cartilaginous mass, the surrounding perichondrium of the
original cartilage now becomes the periosteum, and the cells of the inner layer
of the periosteum deposit circumferential layers of compact bone (perichon-
drial or periosteal bone formation) around the periphery of the cancellous
bone (fig. 314F). The latter action forms a cylinder of compact bone around
the spongy variety and around the cartilage which is being displaced (fig.
668 THE SKELETAL SYSTEM
314F). The primary marrow spaces, established by the original invasion of
the perichondria! vascular buds, merge to form the secondary marrow areas
of the developing bone. This merging process is effected by the dissolution
of previously formed bony spicules or trabeculae.
c. Conversion of Cancellous Bone into Compact Bone
Spongy or cancellous bone is converted into compact bone by the deposition
of layers of compact bone between the trabeculae or columns of spongy bone,
thus obliterating the marrow cavities around the trabeculae of the cancel-
lous bone and converting the intervening areas into Haversian systems
(fig. 314C, D).
D. Development (Morphogenesis) of the Endoskeleton
1. Definitions
For pedagogical purposes, the hard, skeletal tissues may be divided into
the external skeleton or exoskeleton and the internal skeleton or endoskeleton.
The exoskeleton comprises all the hard, protective structures which are de-
rived from the mesenchyme of the dermis and from the epithelium of the
epidermis, described in Chapter 12. The exoskeleton as a whole will not be
described further.
Excluding the exoskeleton and the softer, connective-tissue portion of the
skeletal tissues, we shall proceed with a description of the morphogenesis of
the main skeletal support of the vertebrate body, the endoskeleton. The endo-
skeleton is composed of the axial skeleton and the appendicular skeleton.
The axial skeleton is composed of the skeleton of the head, the skeleton of
the trunk, and the skeleton of the tail. The skeleton of the appendages is
made up of the pectoral and pelvic girdles and the bony supports for the
appendages.
2. Morphogenesis of the Axial Skeleton
a. General Features of the Skeleton of the Head
The cranium or skeleton of the head comprises:
( 1 ) the protective parts for the special sense organs and the brain, and
(2) the skeleton of the oral area and anterior end of the digestive tract.
That portion of the cranium which protects the brain and its associated,
special sense organs may be called the skull, cranium proper, or neurocranium
(fig. 315D), whereas that which surrounds the anterior portion of the digestive
tract and pharyngeal area is known as the visceral skeleton or splanchno-
cranium (fig. 315D).
DEVELOPMENT OF THE ENDOSKELETON
669
MOUT^
MECKEL'S CARTILAGE
OTIC CAPSULE
<7V^YOI0
K-\ ARCH
^ A.
■PHARYNGOBRANCHIAL
EPIBRANCHIAL
CERATOBRANCHIAL
BRANCHIOSTEGAL
PTERYGOQUAORATE CARTILAGE
ORBIT NEUROCRANIUM
CKEL'S \
!TILAGE \
HYPOBRANCHIAL
YOMANDIBULAR CARTILAGE
BASIBRANCHIAL
SPLANCHNOCRANIUM
Fig. 315. Developmental stages of the chondrocranium in the dogfish, Squalus
acanthias. (A and B redrawn from EI-Toubi, '49. Jour. Morph., 84.) (A) Early de-
velopmental stage, 37-mm. embryo, lateral view. (B) Intermediate stage, 45-mm.
embryo, lateral view. (C) Branchiostegal (gill support) rays attached to ceratobranchial
segment of gill arch. (D) Adult stage of chondrocranium (neurocranium plus splanch-
nocranium), lateral view.
1) Neurocranium or Cranium Proper. The neurocranium is present in
three main forms in the vertebrate group:
( 1 ) a complete cartilaginous cranium without dermal reinforcing bones,
as in cyclostomatous and elasmobranch fishes (fig. 315D),
(2) an inner cartilaginous cranium, associated with an outer or surround-
ing layer of bony plates, as in Amia (fig. 316C, D), the adult skull
of Necturus and the frog being similar but slightly more ossified (fig.
317B, C), and
(3) an almost entirely ossified cranium, in teleosts, reptiles, birds, and
mammals (figs. 318C; 319C, D, E).
Various degrees of intermediate conditions exist between the above
groupings.
2) Visceral Skeleton or Splanchnocranium. The splanchnocranium or vis-
ceral skeleton consists, of a number of cartilaginous or bony arches which tend
to enclose the anterior portion of the digestive tube (fig. 315D). They are
present in pairs, one arch on one side, the other arch on the other side. The
first two pairs are related to the skull in gnathostomes. The succeeding pairs
of visceral arches are associated with the branchial or gill apparatus in fishes
and in certain amphibia, such as Necturus.
3) Development of the Skull or Neurocranium. The neurocranium of all
vertebrates from the fishes to the mammals possesses a beginning cranial con-
BRANCHIOSTEGiL RafS
Fig. 316. Developmental stages of neurocranium of the bowfin, Amia calva. (A and
B redrawn from De Beer, '37, after Pehrson; C and D from Allis, 1897, J. Morph.,
12.) (A) Ventral view of 9.5-mm. stage. (B) Dorsal view of 19.5-mm. stage. (C)
Cartilaginous neurocranium of adult stage. (D) Dermal (membrane) bones overlying
neurocranium of adult stage. Cartilage = coarse stipple; bone = fine stipple.
670
DEVELOPMENT OF THE ENDOSKELETON 671
dition in which dense mesenchyme, the so-called desmocranium, comes to
surround the brain and its appendages. The membranous cranium is more
pronounced in the basal areas of the brain. This pre-cartilage stage is followed
by formation of cartilage which results in the development of a chondric neuro-
cranium. A complete cartilaginous neurocranium is not formed in all verte-
brate groups, although the ventro-lateral areas of all vertebrate skulls are laid
down in cartilage. This basic, chondrocranial condition exists as the first step
in skull formation, and it consists of three main regions, composed of car-
tilaginous rudiments (figs. 316A, 320):
( 1 ) The basal plate area is composed of a pair of parachordal cartilages
on either side of the anterior extremity of the notochord, together
with the otic capsules, surrounding the otic (ear) vesicles.
(2) A trabecular or pre-chordal plate area lies anterior to the notochord.
This area begins at the infundibular-hypophyseal fenestra and extends
forward below the primitive forebrain. Two elongated cartilages, the
trabecula cranii (fig. 320A) or a single elongated cartilage (fig. 320B),
the central stem or trabecular plate, develop in the basal area of this
region. With the trabecular area are associated the sphenolateral,
orbital or orbitosphenoidal cartilages and the optic capsules. The
latter are placed in a position lateral to the orbitosphenoidal cartilages.
(3) A nasal capsular or ethmoidal plate area, associated with the develop-
ing olfactory vesicles, later arises in the anterior portion of the tra-
becular region (figs. 316A, 319A).
This fundamental cartilaginous condition of the vertebrate skull or
neurocranium is followed by later conditions which proceed in three
ways: (a) In the elasmobranch fishes, an almost complete roof of
cartilage is developed, and the various cartilaginous elements fuse to
form the cartilaginous neurocranium (fig. 315). This neurocranium
enlarges but never becomes ossified, (b) In the ganoid fish, Amia,
the frog, Rana, the mud puppy, Necturus, etc., the basic, ventro-
laterally established, cartilaginous neurocranium is converted into a
more or less complete chondrocranium by the formation of a roof
and the complete fusion of the various cartilaginous elements (figs.
316A-C; 317A, B). In these forms, the cartilaginous cranium be-
comes ossified in certain restricted areas. In addition to this cartilagi-
nous neurocranium, superficial, membrane bones (dermal bones) are
added to the partially ossified chondrocranium. These membrane bones
come to overlie and unite with the partly ossified cartilaginous skull
(figs. 316D; 317C). (Consult also Table 1.) The adult skull or neuro-
cranium in these forms thus is composed of a chondrocranial portion
and an osteocranial part, the osteocranial part arising from cartilagi-
nous and membranous sources, (c) In reptiles, birds, mammals, and
672
THE SKELETAL SYSTEM
in many teleost fishes, the basic ventro-lateral regions of the cartilagi-
nous neurocranium only are formed (figs. 318A, B; 319A, B). This
basic chondrocranium undergoes considerable ossification, forming
cartilage bones, which replaces the cartilage of the chondrocranium.
These cartilage bones are supplemented by superficially developed
membrane bones which become closely associated with the cartilage
bones. The adult skulls of these vertebrates are highly ossified struc-
tures, composed of cartilage and membrane bones. (See Tables 2 and
3.) A few cartilaginous areas persist in the adult skull, more in teleost
fishes than in the reptiles, birds, and mammals (Kingsley, '25 and De
Beer, '37).
4) Vicissitudes of the Splanchnocranium. The early visceral skeleton, es-
tablished in the embryo, experiences many modifications in its development
in the diff'erent vertebrate groups.
In the elasmobranch fishes, the first visceral (mandibular) arch on either
side gives origin to an upper jaw element, composed of the palatoquadrate
(pterygoquadrate) cartilage, and a lower jaw element or Meckel's cartilage
angulosple
Fig. 317. Developmental stages of neurocranium in the frog. (A and B redrawn from
De Beer, "?7, after Pusey; C. redrawn and modified from Marshall, 1893, Vertebrate
Embryology, New York, Putnam's Sons.) (A) Intermediate condition between larval
and adult form. (B) Adult form of cartilaginous cranium, present after metamorphosis.
(C) Adult neurocranium composed of membrane and cartilage bones associated with
basic cartilaginous neurocranium (see Table 1). Cartilage = coarse stipple; bone = fine
stipple.
DEVELOPMENT OF THE ENDOSKELETON
673
ENTOGLOSS
Fig. 318. Developmental stages of bird neurocranium. (A and B redrawn from De
Beer, '37, from De Beer and Barrington.) (A) Dorsal view of 8'/2-day stage of Anas
(duck). (B) Lateral view of 14-day stage of Anas. (C) Lateral view, adult stage of
Callus (chick). Cartilage = coarse stipple; bone = fine stipple.
(fig. 315D). Each second visceral (hyoid) arch in the shark forms on each side
an upper hyomandibula, attached to the otic capsule by fibers of connective
tissue, a ceratohyal part, and a lower basihyal element (fig. 315D).
The basihyal portion of the two hyoid arches forms a basis for the so-called
tongue. The succeeding branchial arches form supports for the gills and de-
velop cartilaginous branchial rays which extend out into the gill area (fig.
315C). Each branchial arch on each side divides into four cartilages, namely,
the upper pharyngobranchial, and the lower hypobranchial, the epibranchial
and the ceratobranchial elements. The last two elements lie between the first
two, and the ceratobranchial element is articulated with the hypobranchial
element (fig. 315D).
The visceral skeleton in ganoid and teleost fishes arises similarly to that
in elasmobranchs but becomes largely ossified in the adult (fig. 316).
In the frog, the well-developed, visceral skeleton of the late larva becomes
greatly modified during metamorphosis and the acquisition of adulthood. The
hyoid arch persists in cartilage. The mandibular arch contributes to the forma-
tion of the upper and lower jaws. The lower jaw in the metamorphosed frog
consists of Meckel's cartilages, reinforced by membrane bones, the dentaries
and the angulospenials. The pterygoquadrate cartilages remain as cartilage
and are reinforced by the pterygoid, quadratojugal, squamosal, maxillae and
premaxillae, to form the upper jaw (fig. 317B, C and Table 1).
In birds, the first visceral or mandibular arch contributes to the formation
of the quadrate and articulare at the angle of the jaw. These two bones on
674 THE SKELETAL SYSTEM
either side represent cartilage bones. (See Table 2.) The hyoid and first
branchial-visceral arches form the complicated support for the tongue (con-
sult Table 2).
In mammals, the visceral arches contribute as much to the adult con-
dition as in other higher vertebrates. In the human, the caudal portion of
the vestigial upper jaw rudiment persists as the incus, and the caudal portion
of Meckel's cartilage contributes to the formation of the malleus. The man-
dibular arch thus contributes to the important ear bones (fig. 319C-2). The
upper portion of the hyoid arch probably forms the stapes; the ventral portion
forms one half of the hyoid bone; and the intervening tissue of the primi-
tive hyoid arch contributes to the formation of the stylohyal structures (fig.
319C, D). The third arch on each side forms the greater horn of the hyoid;
the fourth contributes to the thyroid cartilage; the fifth pair forms the arytenoid
and cricoid cartilages (fig. 319C and Table 3).
b. Ossification Centers and the Development of Bony Skulls
The formation of the bony crania of all vertebrates entails the use of
centers of ossification which involve methods of bone formation previously
described. As a rule, one ossification center arises in a single bone, with the
exception of those bones, such as the human frontal, sphenoid, or occipital
bones, which result from the fusion of two or more bones. In these instances
separate centers of ossification are developed in each individual bone. The
exact number of ossification centers in all bones has not been exactly
determined.
c. Development of the Axial Skeleton
1) Axial Skeleton of the Trunk: a) Notochord. The notochord is one
of the basic structural features of the chordate group of animals. It will be
recalled (Chapters 9 and 10) that the primitive notochordal band of cells
is the physiological instrument which effects much of the early organization
of the developing body of the vertebrate embryo. Aside from this basic, ap-
parently universal function in vertebrate development, the notochord later
functions as a prominent feature in the development of the median skeletal
axis. In the cyclostomatous fishes, a persistent, highly developed notochord,
enclosed in elastic, and fibrous, connective-tissue sheaths, is found in the adult.
The enveloping, connective-tissue sheaths establish a covering for the nerve
cord above and for the blood vessels immediately below the notochord. Ver-
tebrae are not developed, but in the cyclostomes (Petromyzontia) paired carti-
laginous rods lie along either side of the nerve cord above (Goodrich, '30,
pp. 27, 28). In the Dipnoi and in the cartilaginous ganoids, such as Acipenser
sturio, the notochord persists unconstricted by vertebral elements although
supplemented by these structures. In the shark group and in teleost fishes in
general, as well as in certain Amphibia, such as Necturus, the notochord
is continuous but constricted greatly by the developing vertebral centra. In
STOID FPNTANEL
Fig. 319. Developmental stages of mammalian neurocranium and splanchnocranium.
(A) Human chondrocranium at end of third month viewed from above (from Keibel
and Mall, 1910, Manual of Human Embryology, vol. I, after Hertwig's model). (B)
Same, lateral view, slightly modified. (C-1) Lateral view of adult skull showing visceral
arch (splanchnocranial) derivatives. {C-1) Auditory ossicles (see fig. 319B). Malleus
derived from caudal end of Meckel's cartilage in lower jaw portion of mandibular visceral
arch; incus from caudal end of maxillary process of mandibular arch; stapes from
upper or hyomandibular portion of hyoid visceral arch. (D) Lateral view of cat skull
and visceral arch (splanchnocranial) derivatives. (E) Human cranium, lateral view,
at birth showing fontanels (from Morris, '42, Human Anatomy, Philadelphia, Blakiston).
Cartilage = coarse stipple; bone = fine stipple.
675
676
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TRABECULA CRANII
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(ORBITAL) CARTILAGE
HYPOPHYSIAL STALK
HYPOPHYSIAL CARTILAGE-..
POLAR CARTILAGE
INTERNAL CAROTID ARTERY
NOTOCHORD
PARACHORDAL CARTILAGE
AUDITORY CAPSULE
Fig. 320. Diagrams of basic cartilaginous underpinning or foundation of the vertebrate
neurocranium. (Somewhat modified from De Beer, '37, after De Beer and Woodger.)
(A) Pisces. (B) Placenta! mammals. It is to be observed that the trabecula cranii in
the fish is represented by the central stem or trabecular plate in the mammal.
most amphibia and in the reptiles, birds, and mammals, the notochord tends
to be entirely displaced by the vertebrae, and its residual remains are restricted
within or between the vertebrae. In mammals, the residual remainder of
the notochord constitutes the nucleus pulposus (pulpy nucleus) near the
center of the fibrocartilage of the intervertebral disc. In the human, according
to Terry, '42, p. 288, the pulpy nucleus forms a "pivot round which the bodies
of the vertebrae can twist or incline."
b) Vertebrae. Vertebrae, the distinct segments of which the spinal column
consists, arise from sclerotomic mesenchyme, derived from the ventro-mesial
aspects of the various somites (fig. 252A-D). Potentially, this sclerotomic
mesenchyme in each primitive segment becomes segregated into eight masses,
four on either side of the notochord. These eight masses or blocks of mesen-
chyme form the arcualia. The arcualia become arranged in relation to the
notochord and the developing intermuscular septa as indicated in figure 321 A.
These masses are designated as basidorsals and basiventrals, interdorsals and
interventrals. Thus there are two basidorsals, two basiventrals, two interdor-
sals, and two interventrals.
During the formation of the vertebra in mammals, the sclerotomic masses
within a primitive body segment become associated about the notochordal
axis as indicated in figure 321J-L. It is to be observed that the arteries from
the dorsal aorta lie in an intersegmental position. This position represents
the area of the myoseptal membrane, shown in figure 321 A. As the scle-
rotomic masses increase in substance, each mass on each side of the noto-
chord becomes divisible into an anterior area, in which the mesenchymal cells
are less dense, and a posterior area, where the cells are closely aggregated
DEVELOPMENT OF THE ENDOSKELETON 683
(fig. 321 J). The less dense mesenchymal mass represents the rudiment of
the interdorsal vertebral element, while the posterior dense mass of mesen-
chyme is the basidorsal element. As development proceeds, the basidorsal
mass of cells from one segment and the interdorsal mass of the next posterior
segment on either side of the notochord move toward each other and align
themselves in the intersegmental area as shown in figure 32 IK, L. The basi-
dorsal element thus comes to lie along the anterior portion of the interseg-
mental area, and the interdorsal rudiment occupies the posterior part of
this area. The four vertebral elements, two on either side of the notochord
in the intersegmental area, form the basic vertebral rudiments, although rudi-
mentary basiventral and interventral elements possibly are present. The inter-
segmental artery eventually comes to lie laterally to the forming vertebra.
Once these basic rudiments of the vertebra are established, the vertebra
begins to form. In doing so, there is an increase in the number of mesen-
chymal cells present, and the sclerotomic masses move toward and around
the notochord in the intersegmental position. The two dense basidorsal ele-
ments from either side expand dorsally around the neural tube as the two
interdorsal rudiments coalesce to form the body of the centrum (fig. 321M).
Laterally, the rudiment of the rib arises as a condensation of mesenchyme con-
tinuous with the forming neural arch and centrum. The rib element continues
to grow ventro-laterally, particularly in the thoracic area (fig. 321N). In the
lateral growth of the rib rudiment, surrounding mesenchyme is organized and
incorporated into the growing structure of the rudiment.
Once the vertebral rudiment is established as a dense mass of mesenchyme,
the pre -cartilage stage of cartilage development occurs (fig. 313A). The pic-
cartilage stage is followed soon by cartilage (fig. 313B, C). Later, centers
of ossification arise as indicated in figure 3210, and the cartilaginous con-
dition becomes converted into a bony condition. Secondary centers of ossifi-
cation, forming bony epiphyses, ultimately arise after birth at the anterior
and posterior ends of each centrum. When the ultimate size of the vertebra
is attained, the epiphyseal cartilages between the epiphyses and the centrum
of each vertebra become ossified, and the epiphyses thus unite with the
centrum. The intervertebral discs of fibrocartilage form in the segmental
position between the vertebrae.
It is to be observed that the intersegmental arrangement of the vertebrae
permits direct passage of the spinal nerves to the developing musculature
within each segment and also permits the musculature of each segment to
attach itself to two successive vertebrae. The latter feature is particularly
advantageous in lateral bending movements, so prominent in the swimming
movements of water-dweUing forms.
See legend, fig. 321, for vertebral development in various vertebrates.
c) Divisions of the Vertebral Column. In fishes, two main divisions
of the vertebral column are recognizable, the caudal region where the ver-
8RANE BONE
Fig. 321. {See facing page for legend.)
684
DEVELOPMENT OF THE ENDOSKELETON 685
Fig. 321. Development of vertebrae. The vertebral column in the phylum Vertebrata
is a variable structure. In the early embryo the primitive notochord serves as the primitive
axis. Later this structure develops fibrous sheaths in fishes and amphibia. The notochord
plus its surrounding sheaths serves as the only axial support in the embryo and adult
stages of Amphioxus and Cyclostomes. However, in all true vertebrates, the notochord
is supplemented during later embryonic stages by vertebral rudiments known as arcualia
(fig. 321, A). Eight arcualia are present typically in each vertebral segment. The arcualia
begin as mesenchymal condensations from the sclerotome (see fig. 252, A-D), and later
are transformed into cartilaginous masses. In the elasmobranch fishes the cartilaginous
arcualia fuse to form the vertebra as described below, but in most vertebrates they
undergo ossification.
I. The Formation of Vertebrae in Fishes. In certain instances among the fishes, the
arcualia are merely saddled on to the notochord and its sheaths. This condition is found,
for example, in the lung fishes and cartilaginous ganoid fishes (fig. 321, E). A vertebral
centrum is not developed in these instances.
In the elasmobranch fishes the vertebra is formed essentially from that group of
arcualia known as the basalia, that is, the basidorsals and basiventrals. These rudi-
ments invade the fibrous sheath from above and below on either side and form the
neural arch and centrum as indicated in fig. 321, C. The interbasalia — that is, the
interdorsals and interventrals— lie between the vertebrae. The notochord is constricted
greatly in the region of the centrum but is disturbed little in the areas between the
centra. That is, the centrum is hollowed out or deeply concave at either end. This form
of centrum is found in all amphicoeious vertebrae (fig. 321, P). In the tail region
(fig. 321, C), there are two vertebrae per muscle segment. This condition is known as
diplospondyly. Other cartilaginous elements may enter into the formation of the centrum
as indicated in fig. 321, C.
The diplospondylous condition in the tail region of Ainia presumably is developed
as indicated in fig. 321, H'. In the trunk region of Ainiu the arcualia associate to form
the vertebrae as in fig. 321, H. A certain amount of membrane bone may enter into the
composition of the centra in Ainiii. In the teleost fishes (fig. 321, I), the basidorsals
form the neural arches, but the centrum is developed almost entirely from the ossifi-
cation of fibrous connective tissue membrane (i.e., membrane bone formation). The
basiventrals form the area of attachment of the pleural ribs and also form the hemal
arches.
II. Development of Vertebrae in Amphibia. In the frog (fig. 321, B), the neural
arch of each vertebra appears to arise as the result of fusion and ossification of two
basidorsal arcualia. Ossificati'on spreads from the neural arch downward into the devel-
oping centrum. The centrum, however, develops as a result of perichordal ossification
which arises within the membranous connective. tissue around the notochord. The rudi-
mentary interdorsals and interventrals probably grow inward into the intercentral spaces
to obliterate the notochord between the centra. The interdorsal-interventral complex
fuses ultimately with the caudal end of the centrum, to form a rounded knob which
articulates with the concave end of the next posterior vertebra. That is, the vertebrae in
the frog are procoelous (fig. 321, Q). The urostyie of the frog probably represents a
fusion of rudimentary vertebrae caudal to the ninth or sacral vertebra. Vestigial noto-
chordal remains may exist in the center of each bony centrum.
The development of the vertebrae in Necturus (fig. 321, D), resembles that of the
frog, with the exception that the bony centrum arises from a perichordal ossification
which is entirely independent of the neural arch. Also, the notochord remains continuous,
being constricted in the region of the bony centrum, but relatively unconstricted in the
area between the centra. That is, the vertebrae are of the amphicoeious type (fig. 321, P).
The basiventral arcualia unite to form the hemal arches in the tail.
III. Development of Vertebrae in the Chick and Mammals. The development of
the vertebra in the chick is a complicated affair, as the vertebra is composed of a complex
of fused arcualia associated with a perichordal ossification (see fig. 321, F). The vertebrae
are heterocoelous, their ends being partly procoelous and opisthocoelous. In mammals
686 THE SKELETAL SYSTEM
tebrae possess hemal arches and the trunk region without hemal arches but
with ribs. The amphibia begin to show a third division, the cervical area or
anterior portion of the trunk region in which the vertebrae do not possess ribs.
This area is Umited to one vertebra, the axis. In the amphibia, also, a sacral
region begins to make its appearance. It is only slightly differentiated in water-
abiding forms but well developed in the Anura. The caudal vertebral area in the
Anura generally is fused to form the coccyx or urostyle. The reptilian vertebral
column manifests great variability in the different orders. The turtles show cer-
vical, trunk, and tail regions, with the trunk vertebrae fused with the bony plates
of the carapace. In snakes, a short cervical area, a greatly elongated trunk
region, and a caudal area are present. Some of the snakes possess the largest
number of vertebrae among verterbates, the number reaching several hundreds.
Sacral vertebrae are absent in snakes. The lizards and crocodilians show condi-
tions closely resembling the amphibia. In the birds, caudal, synsacral, thoracic,
and cervical regions are present, while, in mammals, cervical, thoracic,
lumbar, sacral, and caudal regions exist.
d) Ribs. Ribs are not found in cyclostomatous fishes. In the gnathostomes,
two types of ribs may be present:
( 1 ) dorsal ribs and
(2) ventral or pleural ribs.
Fig. 321 — (Continued)
the vertebra appears to arise from two basidorsal and two interdorsal arcualia as indi-
cated in fig. 321, G. The origin of the basidorsal and interdorsal vertebral rudiments
from the sclerotomic mesenchyme are shown in figure 321, J-M. The vertebrae are of
the acoelous (aniphiplatvan) type (fig. 321, S). The chevron bones and hemal arches
in the tail region of many mammals represent basiventral elements. Fig. 321, M-O, shows
the rib outgrowths from the developing vertebrae. Observe centers of ossification in the
vertebra in fig. 321, O.
Fig. 321, A, presents a lateral view of the so-called arcualia in relation to the notochord
and the myosepta (myocommata). According to this theory of the development of the
vertebrae, the arcualia form the main rudiments from which future vertebrae arise. (B)
The adult frog vertebrae showing probable contributions of arcualia. (C and C) Prob-
able contributions of the arcuaha to trunk and tail vertebrae of Sqiialus acanthias. (D)
The adult vertebrae of Necturiis maculosiis. (E) The role played by the arcualia in
forming the axial supporting structure in Acipenser sturio. (Redrawn and modified from
Goodrich, Vertebrate Craniata, 1909.) (F) The composite origin of the vertebra in
the bird. (Redrawn from Piiper, 1928. Phil. Trans. Series B, 216.) (G) Probable con-
tributions of the arcualia to vertebra formation in man. (H) Probable contributions
of the arcualia in the formation of trunk and caudal vertebrae in Ainia catva. (1) Same
for the teleost. Conodon nohilis. (J-L) The origin and early development of the
sclerotomic mesenchyme in the mammal. (M) shows vertebral and costal development
in a 15-mm. pig embryo. (N) presents vertebral and costal development in a human
embryo of 1 1 mm. The vertebral and rib rudiments are in the mesenchymal stage at
this period. (Redrawn from Bardeen, 1910. Keibel and Mall, Vol. I, Human Embryology,
Lippincott, Phila.) (O) is a drawing of developing vertebra in the 22-mm. opossum
embryo. (P, Q, R, and S) are diagrams of amphicoelous, procoelous, opisthocoelous
and slightly biconcave amphiplatyan (acoelous) vertebrae. (Redrawn and modified from
Kingsley, '25.)
DEVELOPMENT OF THE ENDOSKELETON
687
Ribs develop in relation to the basidorsal and basiventral elements and
extend outward in the myosepta. The dorsal rib appears typically in the
position between the epaxial and hypaxial divisions of the primitive skeletal
musculature, whereas the pleural rib lies in close relationship to the coelomic
cavity (fig. 311B). It is questionable whether or not the hemal arch, when
present, is homologous with the ventral or pleural ribs. The shark, Squalus
acanthias, has dorsal ribs. This condition is true also of all Tetrapoda. In
Amia, the ribs are of the pleural variety, whereas, in most teleosts, pleural
ribs are present, supplemented by dorsal or epipleural ribs.
PRESTERNUM + ANTERIOR STERNUM
MANUBR lUM
COSTAL CONDENSATIONS
CONDENSATION
OF PRESTERNUM
XIPHISTERNUM'
Fig. 322. Development of the sternum in the mammal. (A and C redrawn from
Hanson. '19, Anat. Rec, 17; B redrawn from Kingsley, '25.) (A) Diagrammatic recon-
struction of sternum of '24-mm. pig embryo. The two precartilaginous condensations
of the mesosternum are united anteriorly with the presternal condensation. The rib or
costal condensations are approaching and uniting with the sternal condensations. (B)
Schematic representation of sternal rudiments in the mammal. The mesosternal cartilages
have segmented into cartilaginous segments or sternebrae. Bilateral centers of ossification
arise in each sternebra which later form the bony sternebra. (C) Sternum of old boar,
weight 450 lbs. It is to be observed that the sternebrae have remained distinct, and in
two of the sternal segments anterior to the xiphisternum the two centers of ossification
produce a dual condition within the sternal segment. In the human and certain other
mammals the sternebrae fuse to form the gladiolus or corpus sterni.
688 THE SKELETAL SYSTEM
As indicated above, ribs may be considered as outward extensions or proc-
esses of the vertebrae. In the frog, the much-abbreviated ribs become firmly
ossified to the basidorsal elements of the vertebrae and extend outward as
the transverse processes. However, in most vertebrates, they are articulated
with the vertebrae by means of lateral extensions or processes from the
vertebrae.
Chondrification of the rib occurs separately from the chondrification of
the vertebra, and articulations develop between the rib and the vertebrae
(fig. 3210). Similarly, when ossification develops, a separate center of ossi-
fication arises in the body of the rib (fig. 32 lO). However, epiphyseal cen-
ters arise in the tubercular and capitular heads, which later unite with the
shaft of the rib. The student is referred to Kingsley, '25, for a full discussion
of vertebrae and ribs.
e) Sternum. A sternum connected with the ribs, and thus forming a
part of the protective thoracic basket, is found only in reptiles, birds, and
mammals. A sternum is absent in the gymnophionan Amphibia (Apoda), is
reduced to a midventral cartilaginous series of bars in Nectiirus, and forms
a part of the pectoral girdle in the frog (fig. 323C).
In its formation in the mammal, the sternum begins as a bilateral series
of mesenchymal aggregations between the ventro-mesial ends of the clavicular
and costal concentrations of mesenchyme (fig. 322A). These mesenchymal
aggregations move toward the midline, form pre-cartilage, and then form
cartilage. The median cartilaginous mass at the anterior end forms the pre-
sternum or episternum; the portion between the rib elements forms the meso-
sternum, and the posterior free area is the metasternum or xiphisternum
(fig. 322B). In forms which have a clavicle, the latter articulates with the
episternum. The anterior portion of the mesosternum unites ultimately with
the presternum to form the rudiment of the manubrium. The mesosternum
segments into blocks or sternebrae, while the caudal free end of the sternum
forms the xiphisternum (fig. 322C). Centers of ossification arise in these
areas and convert them to bone. In the human, the sternebrae of the meso-
sternum unite to form the body or corpus sterni, but, in the cat, pig, and
many other mammals, they remain distinct.
2) Axial Skeleton of the Tail. The axial skeleton of the tail is modified
greatly from that of the trunk region. In water-living vertebrates, the tail
forms a considerable portion of the body. As the tail is used for swimming
purposes, the contained vertebrae are developed to serve this end. In con-
sequence, rib processes are reduced or lost entirely, and hemal arches for
the protection of the caudal blood vessels are strongly developed features.
Another feature subserving the swimming function is the tendency toward
diplospondyly, i.e., the development of two vertebral centra per segment (fig.
32 IH'). In land forms, the tail tends to be reduced. However, in the
armadillo, kangaroo, etc., the tail is a formidable structure, and hemal-arch
OMOSTERNUM
Fig. 323. Pectoral and pelvic girdles. (A) Diagrammatic pectoral girdle of Tetrapoda
(modified from Kingsley, '25). (B) Pectoral girdle of Squalus acanthius. (C) Pectoral
girdle of the frog, Rana (redrawn from Kingsley, '25, after Parker). Observe that clavicle
is a small bony bar superimposed upon procoracoid; suprascapula removed on right side.
(D) Pectoral girdle of the bird. Callus. (E) Human pectoral girdle. (F) Diagram-
matic representation of pelvic girdle in Tetrapoda (modified from Kingsley, '25). (G)
Pelvic girdle in Squalus acanthius. (H) Pelvic girdle in Rana cuteshiana. (I) Pelvic
girdle in Callus (chick). (J) Pelvic girdle in human. (K) Pelvic girdle in Didelphys
(opossum). (L) Dorsal view of sacrum and pelvic girdle in the armadillo, Tatusia.
689
690 THE SKELETAL SYSTEM
Structures for the protection of blood vessels are developed in the interver-
tebral area.
d. Development of the Appendicular Skeleton of the Paired Appendages
1) General Features. Two types of appendages are found in the vertebrate
group:
( 1 ) median unpaired appendages which take their origin in the median
plane and
(2) paired bilateral appendages which arise from the lateral surface of
the body.
Median appendages appear in the fishes, aquatic urodeles, and in the larval
form of anuran amphibia. They also occur in the crocodilian and lizard
groups, among the reptiles, and, among mammals, in the whales.
All appendages arise as outgrowths of the body. The median appendages
or fins of fishes possess separate skeletal structures for support, but the
median, fin-like structures in the tails of amphibia, reptiles, and whales do
not acquire a separate internal skeleton. All fishes possess a median caudal
or tail fin at the terminus of the tail, a median anal fin posterior to the anal
area, and one or more median dorsal fins.
Most vertebrates possess two pairs of bilateral appendages (Chap. 10,
p. 508), one pair located anteriorly in the pectoral or breast region and the
other pair situated posteriorly in the pelvic area just anterior to the anus.
Each paired appendage has a skeleton composed of two parts:
( 1 ) a girdle component and
(2) a limb component.
The girdle component of each appendage is associated with the axial skele-
ton of the trunk and also with the girdle component of the appendage on
the contralateral side. The entire girdle of each pair of appendages thus tends
to form a U-shaped structure with the closed portion placed ventrally (fig.
323 A-K). In fishes, the open dorsal area of the U-shaped girdle in the pec-
toral area may be closely associated with the axial skeleton, but, in land
forms, it is the pelvic girdle which joins the axial skeleton. This relationship
is to be expected, for, in fishes, the tail is the more important propulsive
mechanism, the head region being the "battering ram" insinuating itself
through the water. As a result, the skull, anterior vertebrae, and the pectoral
girdle ofttimes form a composite structure as, for example, in many teleost
fishes. In land-living vertebrates, on the other hand, the main propulsive force
is shifted anteriorly from the tail region and is assumed to a great extent by
the posterior pair of appendages. In consequence, the pelvic girdle acquires
an intimate relationship with the axial skeleton, and a fusion of vertebrae to
form the sacrum occurs. The sacrum serves as the point of articulation be-
DEVELOPMENT OF THE ENDOSKELETON 691
tween the pelvic girdle and the axial skeleton and is most highly developed
in those species which use the hind limbs vigorously in support and propul-
sion of the body (fig. 3231, L).
Two main types of bilateral appendages are found in the vertebrate group:
( 1 ) the ichthyopterygium of Pisces and
(2) the cheiropterygium of Tetrapoda.
The former is flattened dorso-ventrally, and assumes the typical flipper or
fin shape, while the latter is an elongated, cylindrical affair.
2) Development of the Skeleton of the Free Appendage. The paired ap-
pendages arise either as a dorso-ventrally flattened fold of the epidermal
portion of the skin, or as a cylindrical outgrowth of the epidermis. (See
Chap. 10.) Within the epidermal protrusion, is a mass of mesenchyme (figs.
262D, E; 324A). As development proceeds, condensations of mesenchyme,
centrally placed, begin to foreshadow the outlines of the future skeletal struc-
tures of the limb (fig. 324A, C, D). This mesenchyme gradually becomes
more compact to form a pre-cartilage stage, to be followed by a cartilaginous
condition.
The pattern, which these cartilages of the limb assume, varies greatly
in the two types of limbs mentioned above. In the ichthyopterygium (fig.
323B, G), they assume a radially arranged pattern, extending out from the
point of attachment to the girdle, whereas, in the cheiropterygium (fig. 323A),
they assume the appearance characteristic of the typical limb of the Tetrapoda.
In the tetrapod limb, such as that of the hog, chick, or human, elongated,
cylindrically shaped bones begin to make their appearance in mesenchyme
(fig. 324A-E). Following the cartilaginous condition, a center of ossification
arises in the shaft or diaphysis of each developing bone, transforming the
cartilage into bone (figs. 314E, F; 324E). Cancellous or spongy bone is
formed centrally within the shaft, while compact bone is deposited around
the periphery of the shaft (fig. 314E, F). Later, the cancellous bone of the
shaft is resorbed, and a compact bony cylinder, containing a relatively large
marrow cavity, is formed. Separate centers of ossification, the epiphyses,
arise in the distal ends of the bones (fig. 3241). Each epiphysis is separated
from the bone of the shaft by means of a cartilaginous disc, the epiphyseal
cartilage (fig. 3241). At maturity, however, the bony epiphysis at each end
of the bone becomes firmly united with the shaft or diaphysis by the appear-
ance of an ossification center in the epiphyseal cartilage (fig. 324J). Inter-
nally, the ends of the long bones tend to remain in the cancellous or spongy
condition, whereas the shaft is composed of compact bone with an enlarged
central marrow cavity (fig. 324J). For later changes of the bony substance
involved in the growth of bone, see growth of bone, p. 693.
3) Formation of the Girdles. The typical tetrapod pectoral girdle (fig.
323 A) is composed of a sternal midpiece, three lateral columns, extending
ZONE OF CARTILAGE I'll
EROSION
ARTICULAR CARTILAGE
Ul APH YSIS
PERIOSTEUM
Fig. 324. Development of long bones of the appendages. (B and E have been modified
to show conditions present in the fore- and hind appendages at about 8 weeks. For de-
tailed description of limb development consult Bardeen, '05, Am. J. Anat., 4; Lewis, '02,
Am. J. Anat., 2.) (A) Forelimb at II mm. (B) Forelimb at about eighth week,
showing centers of ossification in humerus, radius and ulna. (C) Hindlimb at II mm.
(D) Hindlimb at 14 mm. (E) Hindlimb at about eighth week, showing centers of
ossification in femur, tibia, and fibula.
The heavy strippling in A, C, D represent centers of chondrification; the black areas
in B and E portray ossification centers within cartilaginous form of the long bones.
F-J represent stages in joint development.
692
DEVELOPMENT OF THE ENDOSKELETON 693
dorsad from the sternal area on either side, the clavicle, procoracoid, and
coracoid to which is attached dorsally the scapula. Often a suprascapula is
attached to the scapula. The pelvic girdle of the Tetrapoda, on the other
hand (fig. 323F), is composed of two lateral columns on either side. The
anterior column is called the pubis, and the posterior column is the ischium.
An ilium is attached to the dorsal ends of the pubis and ischium on either
side. Epipubic and hypoischial midpieces are sometimes present at the mid-
ventral ends of the pubic and ischial columns in some species.
As in the development of the skeleton of the free appendage, all the rudi-
ments of these structures are laid down in cartilage and later ossify, with the
exception of the clavicle which may be of intramembranous origin (Hanson,
'20a and '20b). The clavicles are more strongly developed in man, whereas
the coracoidal elements are vestigial (fig. 323E). In the cat, the coracoidal
and clavicular elements are reduced. However, in the chick and frog, the
coracoidal elements are dominant (fig. 323C, D). In the pelvic girdle, the
iliac, pubic, and ischial elements are constant features in most Tetrapoda.
In the shark, a single coracoid-scapula unit is present in the pectoral girdle
and the pelvic girdle is reduced to a small transverse bar of cartilage (fig.
323B, G).
e. Growth of Bone
Bone once formed is not a static affair, for it is constantly being remodeled
and enlarged during the growth period of the animal. In this process, bone
is destroyed and resorbed by the action of multinucleate giant cells, called
osteoclasts, or specialized, bone-destroying cells and is rebuilt simultaneously
in peripheral areas by osteoblasts from the surrounding periosteal tissue.
To understand the processes involved in bone growth, let us start with
the conditions found in the primitive shaft of a long bone (fig. 314F). Within
the bony portion of the shaft, there is a network of cancellous bone, and,
peripherally, there are lamellae of compact bone. The following transforma-
tive activities are involved in the growth of this bone:
( 1 ) Within the bone, the cancellous columns of bony substance are de-
stroyed by osteoclasts, the bony substance is resorbed, the marrow
spaces are enlarged, while, peripherally, circumferential lamellae are
deposited around the bones beneath the periosteum.
(2) Distally, cartilage is converted into cancellous bone while outer cir-
cumferential lamellae are fabricated beneath the periosteum. The bony
substance thus creeps distally, lengthening the shaft of the bone.
(3) As the bone increases in length, some of the bony substance, forming
the wall of the shaft or diaphysis is destroyed. This alteration is ef-
fected to a degree by vascular buds which grow into the bony sub-
stance from the periosteum around the outer surface of the bone and
from the endosteum which lines the marrow cavities. These vascular
694 THE SKELETAL SYSTEM
buds erode the bony substance with the aid of osteoclasts and produce
elongated channels in the bone, channels which tend to run length-
wise along the growing bone. Once these channels are made, osteo-
blasts lay down bony lamellae in concentric fashion, converting the
channel into an Haversian system. (Consult Maximow and Bloom,
'42, pp. 141-145.) The Haversian systems thus tend to run parallel
to the length of the bone. The Haversian canals open into the central
marrow cavity of the bone in some of the Haversian systems, whereas
others, through Volkmann's canals, open peripherally.
(4) While the foregoing processes are in progress, circumferential lamellae
are laid down around the bone. The bone's diameter thus grows by
the erosion of its bony walls (including previously established Haver-
sian systems) and by the formation of new bony substance externally
around the diaphysial area which is destroyed and resorbed. New
Haversian systems and new circumferential lamellae in this way super-
sede older systems and lamellae.
At the distal ends of the bone within the spaces of the cancellous bone,
red marrow is found. In the shaft or diaphysis, however, the contained marrow
cavity is filled with yellow bone marrow, composed mainly of fat cells.
The distal growth of elongated, cylindrically shaped bones, such as the
phalanges or the long bones of the limbs, is possible, while epiphyseal carti-
lage remains between the shaft of the bone and the bony epiphysis at the
end of the bone. The maintenance and growth of the epiphyseal cartilage is
prerequisite to the growth of these bones, for the increase in the length of
the bony shaft involves the conversion of cartilage nearest to the bony shaft
into cancellous bone. A bony cylinder of compact bone is then formed
around the cancellous bone. When, however, the epiphyseal cartilage ceases
to maintain itself, and it in turn becomes ossified, uniting the epiphysis to
the bony shaft, growth of the bone in the distal direction comes to an end.
Growth in the length of a vertebra also involves the epiphyseal cartilages
lying between the bony ends of the centrum and the epiphyses. Increase in
size of the diameter of the vertebra results from the destruction and resorption
of bone already formed and the deposition of compact bone around the
periphery.
In the case of flattened bones of cartilaginous origin such as the scapula
or the pelvic-girdle bones, growth in the size of the bone is effected by the
conversion of peripherally situated cartilage into bone, and by the destruction
and resorption of bone previously formed and its synchronous replacement
external to the area of destruction. On the other hand, in the growth of flat
bones of membranous origin, the bone increases in size along its margins
at the expense of the connective tissue surrounding the bone. Growth in the
diameter of membrane bones is similar to that of cartilage bone, namely,
destruction, resorption, and deposition of new bone at the surface.
DEVELOPMENT OF THE ENDOSKELETON 695
/. Formation of Joints
1) Definitions. The word arthrosis is derived from a Greek word meaning
a joint. In vertebrate anatomy, it refers to the point of contact or union of
two bones. When the contact between two bones results in a condition where
the bones actually fuse together to form one complete bone, the condition
is called ankylosis or synosteosis. If, however, the point of contact is such
that the bones form an immovable union, it is called a synarthrosis; if slightly
movable, it forms an amphiarthrosis; and where the contact permits free mo-
bility, it is known as a diarthrosis. Various degrees of rapprochement between
bones, therefore, are possible.
2) Ankylosis (Synosteosis) and Synarthrosis. In the development of the
bones of the vertebrate skull, two types of bone contact are effected:
( 1 ) ankylosis and
(2) synarthrosis.
In the human frontal bone, for example, two bilaterally placed centers of
ossification arise in the connective-tissue membrane, lying below the skin in
the future forehead area. These two centers increase in size and spread
peripherally until two frontal bony areas are produced, which are separated
in the median plane at birth. Later on in the first year following birth, the
two bones become sutured (i.e., form a synarthrosis) in the midsagittal plane.
Beginning in the second year and extending on into the eighth year, the
suture becomes displaced by actual fusion of bone, and ankylosis occurs.
In the cat, however, the two frontal bones remain in the sutured condition
(synarthrosis). The temporal bone in the human and other mammals is a
complex bone, arising by the ultimate fusion (ankylosis) of several bones.
In the human at birth, three separate bones are evident in the temporal bone:
(1) a squamous portion,
(2) a petrous portion, and
(3) a tympanic part.
The squamous and the tympanic bones are of membranous origin, whereas
the petrous portion arises through the ossification of the cartilaginous otic
capsule. The fusion of these three bones occurs during the first year follow-
ing birth. The occipital bone is another bone of complex origin. Five centers
of ossification are involved, viz., a basioccipital, two exoccipitals, a squamous
inferior, and a squamous superior. The last arises as a membrane bone;
the others are endochondral. Ultimate fusion of these entities occurs during
the early years of childhood and is completed generally by the fourth to
sixth years. In the cat, the squamous superior remains distinct as the inter-
parietal bone. Finally, the sphenoid bone in the human represents a con-
dition derived from many centers of ossification. According to Bardeen,
'10, fourteen centers of ossification arise in the sphenoidal area, ten of them
696 THE SKELETAL SYSTEM
arising in the orbitotemporal region of the primitive chondrocranium. At
birth, two major portions of the sphenoid bone are present, the presphenoid
and the basisphenoid, being separated by a wedge of cartilage. Ultimate
fusion of these two sphenoid bones occurs late in childhood (Bardeen, '10).
In the adult cat, they remain distinct. The maxillary bone in the human
arises as a premaxillary and a maxillary portion; later these bones fuse to
form the adult maxilla. In the cat, on the other hand, these two bones re-
main distinct. (Consult also Table 3.)
The history of the human skull, therefore, is one of gradual fusion (anky-
losis) of bones. In many parts, however, fusion does not occur, and definite
sutures (synarthroses) are established between the bones, as in the case of
the two parietals, the parietal and the occipital, the frontal and the parietals, etc.
The formation of the association between the parietal bones and neigh-
boring bones establishes an interesting developmental phenomenon, known
as the fontanels. The fontanels are wide, membranous areas between the de-
veloping parietal and surrounding bones which, at birth, are not ossified.
These membranous areas are the anterior fontanel, in the midline between
the two parietals and two frontal bones, and the posterior fontanel, between
the parietals and the occipital bones. The lateral fontanels are located along
the latero-ventral edges of the parietal and neighboring bones (fig. 319E).
3) Diarthrosis. A diarthrosis or movable joint is established at the distal
ends of the elongated, cylindrically shaped bones of the body. Diarthroses
are present typically in relation to the bones of the appendages. As the bones
of the appendages form, there is a condensation of the mesenchyme in the
immediate area of the bone to be formed. At the ends of the bone, the
mesenchyme is less dense than in the area where the rudimentary bone is
in the process of formation (fig. 324A-E). As a result, the area between
bones is composed of mesenchyme less compact and less dense than in the
areas where bone formation is initiated (fig. 324F, G). This mesenchyme at
the ends of the bones thus forms a delicate membrane, tying the bony rudi-
ments together, and, as such, forms a rudimentary synarthrosis. As develop-
ment proceeds, the miniature bone itself becomes more dense, and, eventually,
cartilage is formed. The latter later is displaced gradually by bone (fig. 324E),
The areas between the ends of the respective developing bones become, on
the contrary, less dense, and a space within the mesenchyme is developed
between the ends of the forming bones (fig. 324H). As this occurs, con-
nective tissue, continuous with the periosteum, forms around the outer edges
of the ends of the bones, tying the ends of the bones together (fig. 324H, I).
A cavity, the joint cavity, thus is formed at the ends of the bones, bounded
by the cartilage at the ends of the bones and peripherally by connective tissues
or ligaments which tie the ends of the bones together along their margins.
The membrane which lines the joint cavity is known as the synovial mem-
BIBLIOGRAPHY 697
brane, and the cartilaginous discs at the ends of the bones form the articular
cartilages (fig. 324H, J).
4) Amphiarthrosis. The term amphiarthrosis refers to a condition inter-
mediate between synarthrosis and diarthrosis. This condition occurs for ex-
ample in the area of the pubic symphysis.
g. Dermal Bones
As observed in figure 3 11 A, the primitive mesenchyme of the ghost skele-
ton of the embryo underlies the epidermal tube, as well as enmeshing the
neural, gut, and coelomic tubes. As mentioned previously, wherever mesen-
chyme exists, a potentiality for bony or bone-like structures also exists. Con-
sequently, it is not surprising that various types of dermal armor or exoskeletal
structures in the form of bone, dermal scales, and bony plates are developed
in various vertebrates in the dermal area, as described in Chapter 12. Aside
from the examples exhibited in Chapter 12, other important bony contribu-
tions to the skeleton of vertebrates may be regarded as essentially dermal
in origin. Among these are the membrane bones of the skull (Tables 1, 2,
and 3). These bones sink inward and become integrated with the basic chon-
drocranial derivatives to form a part of the endoskeleton. Other examples
of membrane bones of dermal origin are the gastralia or abdominal ribs
of the Tuatera (Sphenodon) and the Crocodilia, the formidable, dermal, bony
armor of the Edentata, e.g., the armadillo, and the bony plates on the head,
back, and appendages in certain whales (Kingsley, '25, p. 17). All these
examples of dermal armor or exoskeletal structures form an essential pro-
tective part of the entire hard or bony skeleton of vertebrate animals.
Bibliography
Baitsell, G. A. 1921. A study of the de- Hanson, F. B. 1919. The development of
velopment of connective tissue in the the sternum in Sus scrofa. Anat. Rec.
Amphibia. Am. J. Anat. 28:447. 17:1.
Bardeen, C. R. 1910. Chap. XI. The de- . 1920a. The development of the
velopment of the skeleton and of the shoulder-girdle of Sus scrofa. Anat. Rec.
connective tissues. Human Embryology. 18:1
Edited by Keibel and Mall. J. B. Lip-
pincott Co., Philadelphia.
. 1920b. The history of the earliest
stages in the human clavicle. Anat. Rec.
Bell. E. T. 1909. II. On the histogenesis 19:309.
of the adipose tissue of the ox. Am. J. , , ,, r- ^^o^ a . j r cu n
■ Q-412 Jordan, H. E. 1939. A study of fibnllo-
genesis in connective tissue by the
De Beer, G. R. 1937. The development method of dissociation with potassium
of the vertebrate skull. Oxford Univer- hydroxide, with special reference to the
sity Press, Inc., Clarendon Press, New umbilical cord of pig embryos. Am. J.
York. Anat. 65:229.
Goodrich, E. S. 1930. Studies on the struc- Kingsley, J. S. 1925. The Vertebrate Skele-
ture and development of vertebrates. ton. P. Blakiston's Son & Co., Philadel-
Macmillan and Co., London. phia.
698
THE SKELETAL SYSTEM
Lewis, W. H. 1922. Is mesenchyme a
syncytium? Anat. Rec. 23:177.
Mall, F. P. 1902. On the development of
the connective tissues from the connec-
tive-tissue syncytium. Am. J. Anat.
1:329.
Maximow, A. 1929. Uber die Entwicklung
argyrophiler und koliagener Fasern in
Kulturen von erwachsenem Saugetierge-
webe. Jahrb. f. Morph. u. Mikr. Anat.
Abt. II. 17:625.
and Bloom, W. 1942. A Textbook
of Histology. W. B. Saunders Co., Phila-
delphia.
Rasmussen, A. T. 1923. The so-called hi-
bernating gland. J. Morphol. 38:147.
Shaw, H. B. 1901. A contribution to the
study of the morphology of adipose tis-
sue. J. Anat. & Physiol. 36: (New series,
16) :1.
Sheldon, E. F. 1924. The so-called hiber-
nating gland in mammals: a form of
adipose tissue. Anat Rec. 28:331.
Stearns, M. L. 1940. Studies on the de-
velopment of connective tissue in trans-
parent chambers in the rabbit's ear.
Part II. Am. J. Anat. 67:55.
Schreiner, K. E. 1915. Uber Kern- und
Plasmaveranderungen in fettzellen wahr-
end des fettansatzes. Anat. Anz. 48:145.
Terry, R. J. 1942. The articulations. Mor-
ris' Human Anatomy, Blakiston, Phila-
delphia.
16
The Muscular System
A. Introduction
1. Definition
2. General structure of muscle tissue
a. Skeletal muscle
b. Cardiac muscle
c. Smooth muscle
B. Histogenesis of muscle tissues
1. Skeletal muscle
2. Cardiac muscle
3. Smooth muscle
C. Morphogenesis of the muscular system
1. Musculature associated with the viscera of the body
2. Musculature of the skeleton
a. Development of trunk and tail muscles
1) Characteristics of trunk and tail muscles in aquatic and terrestrial vertebrates
a) Natatorial adaptations
b) Terrestrial adaptations
c) Aerial adaptations
2) Development of trunk and tail musculature
a) General features of myotomic differentiation in the trunk
b) Differentiation of the myotomes in fishes and amphibia
c) Differentiation of the truncal myotomes in higher vertebrates and par-
ticularly in the human embryo
d) Muscles of the cloacal and perineal area
e) Development of the musculature of the tail region
b. Development of muscles of the head-pharyngeal area
1) Extrinsic muscles of the eye
2) Muscles of the visceral skeleton and post-branchial area
a) Tongue and other hypobranchial musculature
b) Musculature of the mandibular visceral arch
c) Musculature of the hyoid visceral arch
d) Musculature of the first branchial arch
e) Muscles of the succeeding visceral arches
f) Muscles associated with the spinal accessory or eleventh cranial nerve
g) Musculature of the mammalian diaphragm
c. Development of the musculature of the paired appendages
d. Panniculus carnosus
699
700 THE MUSCULAR SYSTEM
A. Introduction
1. Definition
The muscular system produces mobility of the various body parts. As such,
it is composed of cells specialized in the execution of that property of living
matter which is known as contractility. Since contractility is a generalized
property of living matter, it may occur without the actual differentiation of
muscular tissue. In the developing heart of the chick, for example, contractures
begin to occur as early as 33 to 38 hours of incubation before muscle cells,
as such, have differentiated (Patten and Kramer, '33).
2. General Structure of Muscle Tissue
Muscle cells are elongated, fibrillated structures, known as muscle fibers.
They contain many elongated fibrils, called myofibrils, extending longitudi-
nally along the muscle fiber. The myofibrils may possess a series of cross
striations in the form of light and dark transverse bands as in skeletal or
striated muscle and cardiac muscle, or the transverse bands may be absent
as in smooth muscle (fig. 325A-C). In smooth muscle, the myofibrils are
extremely fine, whereas in striated muscle they are seen readily under the
microscope.
a. Skeletal Muscle
In skeletal muscle, the muscle fibers are elongated, cylinder-shaped struc-
tures; the ends are rounded; and a row of nuclei extend along the periphery
of the muscle fiber or cell, and are more numerous at the ends of the cell
than in the central portion. The cell, as a whole, is filled with myofibrils,
embedded in a matrix of sarcoplasm. The latter contains fat droplets, gly-
cogen, interstitial granules, amino acids, mitochondria, and Golgi substances.
The surrounding cell membrane is a delicate structure and is known as the
sarcolemma.
The myofibrils are composed of dark and light transverse bands, a dark
band alternating with a light band. The bands are arranged along the myofibrils
in such a manner that the dark band of one fibril is at the same level as
the dark bands of other fibrils. The light bands are arranged similarly. This
arrangement presents the effect shown in figure 325A.
Two types of muscle fibers are found in skeletal muscle. In one type, the
red or dark fiber, there is an abundance of sarcoplasm with fewer myofibrils.
The myofibrils possess weaker transverse markings or striations. In the second
type, the pale or white fiber, there is less of the sarcoplasm present with a
larger number of highly difi'erentiated myofibrils, having well-defined trans-
verse striations. This muscle fiber is larger in transverse diameter than the
red type. In many animals, such as man, these two sets of fibers are inter-
mingled in the various skeletal muscles, but in some, such as the breast
INTRODUCTION 701
muscles of the common fowl, the white fibers constitute most of the muscle.
Also, in the M. quadratus femoris of the cat or the M. semitendinosus of
the rabbit, the red fiber predominates. In general, the more continuously
active muscles contain the greater number of red fibers, while the less con-
tinuously active contain pale fibers. Pale fibers react more quickly and thus
contract more readily than the red fibers. However, they are exhausted more
rapidly.
Connective tissue, mostly of the white fibrous variety, associates the muscle
fibers (cells) into groups called muscles. Muscles, such as the Mm. biceps
brachii, biceps femoris, sartorius, rectus abdominis, etc., are a mass of asso-
ciated muscle fibers, tied together by connective-tissue fibers.
The surrounding connective tissue of a particular muscle is known as the
external perimysium (fig. 325D). The external perimysium extends central-
ward into the muscle and separates it into smaller bundles of fibers, or
fasciculi. Thus each fasciculus is a group of muscle fibers, surrounded by
the internal perimysium. The perimysium around each fasciculus extends into
the fasciculus between the muscle cells, where its fibers become associated
with the sarcolemma of each muscle fiber (cell).
The connection between the muscle fibers and their tendinous attachment
has attracted considerable interest. One view holds that the myofibrils pass
directly into the tendinous fibers. An alternative and more popular view main-
tains, however, that it is the sarcolemma which attaches directly to the ten-
dinous fibers. Hence, the pull of the muscle is transmitted through the sar-
colemmas of the various muscle cells to the tendon.
b. Cardiac Muscle
Cardiac muscle is characterized by the presence of alternating dark and
light bands as in skeletal muscle. The striations are not as well developed,
however, as in skeletal muscle, nor is the sarcolemma around the muscle fibers
as thick. Another distinguishing feature of cardiac muscle is the fact that
the fibers anastomose and thus form a syncytium, although M. R. Lewis ('19)
questions this interpretation. Still another characteristic structure of cardiac
muscle is the presence of the intercalated discs (fig. 325C). These discs are
heavy transverse bands which extend across the fiber at variable distances
from one another. A final feature which distinguishes cardiac muscle is the
central location of the nuclei within the anastomosing fibers.
c. Smooth Muscle
Smooth muscle fibers are elongated, spindle-shaped elements which may
vary in length from about 0.02 mm. to 0.5 mm. The larger fibers are found
in the pregnant uterus. The diameter across the middle of the fiber approxi-
mates A \o 1 IX. This middle area contains the single nucleus. The fiber
702 THE MUSCULAR SYSTEM
tapers gradually from the middle area and may terminate in a pointed or
slightly truncate tip (fig. 325B).
Smooth muscle cells may contain two kinds of fibrils:
( 1 ) fine myofibrils, presumably concerned with contraction phenomena,
within the cytoplasm and
(2) myoglial or border fibrils, coarser than the myofibrils, in the peripheral
areas of the cell.
The myoglial fibrils are not usually demonstrable in adult tissues.
A connective-tissue mass of fibers between the smooth muscle fibers which
binds the fibers into bundles as in skeletal muscle is not readily demonstrated.
It may be that a kind of adhesiveness or stickiness (Lewis, W. H., '22) asso-
ciates these muscle fibers into a mass, within which each muscle cell is a
distinct entity and not part of a syncytium. However, around the muscle
bundles, elastic and white fibers (Chap. 15) seem to hold the muscle tissue
in place and some elastic fibers may be present between the cells, especially
in blood vessels.
B. Histogenesis of Muscle Tissues
1. Skeletal Muscle
The primitive embryonic cell which gives origin to the later muscle cells is
called a myoblast. The myoblasts which give origin to skeletal muscle fibers
are derived from two sources:
( 1 ) mesenchyme and
(2) myotomes.
(See Chap. 11 for origin of m.esenchyme and myotomes; also consult
fig. 252.)
In striated-muscle-fiber formation, the myoblasts begin to elongate and
eventually produce cylinder-like structures. As the cell continues to elongate,
the nuclei increase in number, and, hence, the myoblast becomes converted
into a multinuclear affair in which the nuclei at first lie centrally along the
axis of the cell. Later, the myofibrils increase, and the nuclei move peripherally.
As the myofibrils grow older, dark and light areas appear along the fibrils.
These dark and light bands are shown in figure 325E. Observe that the light
band is bisected by the slender membrane, known as Krause's membrane,
shown in the figure as the dark line, Z., and the dark band is bisected by
Hensen's membrane.
2. Cardiac Muscle
The musculature of the vertebrate heart takes its origin from the two mesial
walls of hypomeric fnesoderm (i.e., the splanchnic layers of mesoderm) which
come to surround the endocardial primordia or primitive blood capillaries
HISTOGENESIS OF MUSCLE TISSUES
703
coursing anteriad below the foregut (Chap. 17). These two enveloping layers
of mesoderm give origin to the epicardiuin and myocardium of the heart, and
in consequence they are referred to as the epimyocardial rudiment. From the
surfaces of the two layers of hypomeric mesoderm which face the primitive
blood capillaries, mesenchymal cells are given off. These mesenchymal cells
constitute the myocardial primordium. The outer wall of each hypomeric
layer of mesoderm, however, retains its epithelial character and eventually
gives origin to the epicardium or coelomic covering of the heart. The mesen-
chymal cells which form the myocardial primordium surround the two endo-
cardial rudiments (blood capillaries) and later form an aggregate of coalesced
cells, i.e., a syncytium. The future heart musculature arises from this syncytium.
As the mass of the myocardial syncytium increases in size, the nuclei be-
come irregularly scattered, and myofibrils make their appearance. The number
of myofibrils rapidly increases, and dark bands of anisotropic substance (i.e.,
substance which is doubly refractive under polarized light) alternate with
lighter bands of isotropic substance. Z lines soon appear which bisect the
lighter segment of the myofibrils.
The myofibrils increase, and the myocardial syncytium gradually becomes
drawn out into elongated strands of cytoplasm which appear to anastomose
(fig. 325C). The nuclei are scattered within these strands. As the myofibrils
nucle;us
NTERCALATED DISC
=^
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':
=H
is?iaiw'.:;.:r«y^i ;;
-tl-
»■■-»■■; \ia
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-^
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DARK BAND
LIGHT BAND
MEDIAN MEMBRANE
N BANDS OF HENSEN
GROUND MEMBRANE OR Z LINEOFKRAUSE
DIAGRAM OF MUSCLE STRIATIONS
Fig. 325. Structure of the three types of muscle tissue. (All figures redrawn from
Bremer (1936), Textbook of Histology, Philadelphia, Blakiston.) (A) Skeletal or stri-
ated muscle fibers. Observe that nuclei lie at the periphery of the muscle fibers. (B)
Smooth muscle fibers. Upper part of figure shows fibers cut transversely, while lower
part represents a longitudinal view of separate fibers. (C) Cardiac muscle. Observe that
the fibers appear to anastomose; intercalated discs shown as dark, transverse bands. (D)
Connective tissue contributions to skeletal muscle tissue. (E) Diagram of muscle stria-
tions. (After Heidenhain.)
704
THE MUSCULAR SYSTEM
MESENCHYME GIVES
ORIGIN TO SMOOT
MUSCLE TISSUE
ENTODERM
VENTRAL SEPTUM
PLEURAL R
MYOSEPTjM
Fig. 326. Arrangement of muscle tissues. (A) Ventricles of alligator heart, ventral
aspect, showing spiral arrangement of superficial muscle layers. (Redrawn from Shaver,
Anat. Rec, 29.) (B) Arrangement of smooth muscle layers of the stomach. (Redrawn
from Bremer, 1936, Textbook of Histology, Philadelphia, Blakiston. after Spalteholz. )
(C) Transverse section of tail of Squalus acanthias showing arrangement of epaxial and
hypaxial muscle groups. (D) Primitive arrangement of myotomes into epaxial and
hypaxial groups in relation to the myocommata or myosepta. Observe that the myoseptum
attaches to the middle of the vertebra. (Redrawn and modified from Goodrich, Vertebrate
Craniata, 1909, New York, Macmillan Co., and Kingsley, Comparative Anatomy of
Vertebrates, 1912, Philadelphia, Blakiston.
continue to increase, they become aggregated into groups and are arranged
in such a manner that the dark and light bands of adjacent fibrils form regular
dark and light bands across the muscular strands. The intercalated discs
finally make their appearance here and there across the muscle strands (fig.
325C). In some areas, there are no nuclei within the muscle strand between
the intercalated discs.
3. Smooth Muscle
Smooth muscle cells arise from mesenchyme. In doing so, the mesenchymal
cells lose their stellate shapes, elongate, and eventually become spindle shaped.
Accompanying these changes, the nuclei experience some extension in the
direction of the elongating cells (fig. 325B). Fibrils appear in the cytoplasm,
first at the periphery in the form of coarse fibers, to be followed somewhat
later by the true myofibrils of finer texture. It is possible that the coarser
fibrils, the so-called myoglial fibers, represent bundles of myofibrils. The
MORPHOGENESIS OF MUSCULAR SYSTEM 705
myofibrils in smooth muscle fibers do not assume anisotropic (dark) and
isotropic (light) bands or cross striations. Increase in the number of muscle
fibers (cells) appears to occur by the mitotic division of existing fibers and
also by the transformation of other mesenchymal cells.
C. Morphogenesis of the Muscular System
1. Musculature Associated with the Viscera of the Body
The musculature associated with the viscera of the body is of the smooth
type with the exception of cardiac muscle and anterior part of the esophagus.
Smooth and cardiac musculature are under involuntary control. The smooth
muscle tissue of the digestive tract is derived from mesenchyme, which arises
from the inner or splanchnic layers of the hypomeres, while that of the urinary
and genital systems takes its origin from nephrotomic mesoderm and contri-
butions from the splanchnic layers of the two hypomeres (fig. 3 11 A, B). The
smooth muscle tissue associated with many of the blood vessels of the body
arises from mesenchymal sources in the immediate area of the blood vessels.
The arrangement of muscle tissue in various parts of the digestive tract,
blood vessels, and urinary and reproductive ducts is generally in the form of
circular and longitudinal layers (fig. 325B). On the other hand, the myo-
cardium or muscle tissue of the heart is an association of layers or sheets
which tend to be wound in complex spirals. Particularly is this true of the
ventricular portion of the heart (fig. 326A). Also, in the stomach, the arrange-
ment of the muscle layers is complex, being composed of an outer longitudinal
layer, a middle circular layer, and an inner, somewhat spirally arranged, obhque
layer (fig. 326B). The general pattern of arrangement of smooth and cardiac
muscle tissues shows much similarity throughout the vertebrate group.
2. Musculature of the Skeleton
The skeletal musculature is striated and under voluntary control. It is that
musculature which moves various parts of the endoskeleton and integumental
structures, enabling the animal to adapt itself to surrounding environmental
conditions. The development of skeletal musculature will be described under
the following headings:
(a) development of trunk and tail muscles,
(b) development of muscles of the head-pharyngeal area,
(c) development of the musculature of the paired appendages, and
(d) development of the panniculus carnosus in Mammalia.
a. Development of Trunk and Tail Muscles
1) Characteristics of Trunk and Tail Muscles in Aquatic and Terrestrial
Vertebrates. In endeavoring to understand the development of the trunk and
706 THE MUSCULAR SYSTEM
tail musculature in the vertebrate group as a whole, it is important that one
consider the environment in which the various species live, for the trunk and
tail musculature is adapted to the general junctions of moving the animal in
its particular habitat. We may recognize three main environmental adaptations:
( 1 ) natatorial,
(2) terrestrial, and
( 3 ) aerial.
a) Natatorial Adaptations. Animals, adapted to swimming, possess a
different arrangement of the musculature of the trunk and tail regions than
do terrestrial and aerial forms. A transverse section through the tail of the
dogfish, Squalus acanthias, demonstrates that the musculature is arranged
around the vertebrae in a definite pattern. A horizontal skeletogenous septum
extends outward from either side, dividing the muscles on each side of the
vertebra into epaxial and hypaxial groups, and dorsal and ventral septa are
present in the middorsal and midventral areas (fig. 326C).
Viewed laterally, the muscles are divided by transverse membranes, the
muscle septa, myosepta, or myocommata (figs. 326D; 327A). The position
of the myocomma corresponds to the intermyotomic (intersegmented) area
observed in Chapter 15. Each myocomma is attached to the vertebral body
(really several vertebral bodies). The myotomes (fig. 326D) lie in the seg-
mented position between the myocommata and are attached to the latter. In
the tail, both these groups of muscles are attached to the myocommata and
the vertebrae, but, farther forward in the trunk, it is the epaxial group which
is associated directly with the myocommata and the vertebrae, the hypaxial
group being less direct in its contact with the vertebral column. (See fig. 311B.)
In figure 327B, the myotomes and myosepta (myocommata) have a Z-shaped
appearance because of a secondary modification during development.
It is evident, therefore, that in the shark, the skeletal muscles of the trunk
and tail exist in the form of segments, each segment being divided into an
upper epaxial and lower hypaxial component. This arrangement of the muscles
and the attachment of the fibers to the myosepta, and thus through the
myoseptum to the vertebra, produces a mechanism exceedingly well adapted
to the side-to-side movement of the vertebral column so necessary during
natation. The conditions present in the sharks are comparable to those of
other fishes, and, in all, the epaxial musculature is exceedingly well developed.
b) Terrestrial Adaptations. In the land-frequenting vertebrates, there
is less development of and dependence upon the tail region and the dorsal
or epaxial musculature for locomotive purposes. In consequence, the epaxial
musculature is segregated on either side of the vertebrae in a dorsal position,
while the hypaxial musculature and its derivatives in the bilateral appendages
are expanded ventrally. The suppression of epaxial muscle development is
carried to an extreme form in the aerial adaptations of the bird. In non-
MORPHOGENESIS OF MUSCULAR SYSTEM 707
aquatic forms the tail musculature is greatly reduced, and in some forms is
almost non-existent.
A consideration of the effect that locomotive habits have upon musculature
development may be shown by a brief comparison of the musculature in a
water-living amphibian, such as Necturus, and in a land-going adventurer,
such as the frog. In Necturus, the dorsal (epaxial) musculature, the primitive
M. dorsalis trunci, is more like that of the fish, with the muscle fibers attached
to the myocommata (fig. 327C), although, contrary to the piscine condition,
the muscle fibers close to the vertebrae are attached directly to the vertebrae,
where they form short bundles. In the frog, the attachment of the epaxial
musculature to the vertebrae is more extensive. Bundles of muscle fibers, the
Mm. intertransversarii, pass between the vertebral transverse processes, while
Mm. intemeurales connect the transverse processes and spinous processes,
respectively, of the vertebrae. A separate muscle, the M. longissimus dorsi,
extending from the head to the urostyle, separates from the above-mentioned
dorsal muscles (fig. 327D). Although a slight suggestion of myocommata
may be present, there is little functional relationship of the myocommata to
the vertebrae. Laterally, Mm. coccygeo-sacralis and coccygeo-iliacus also are
present as differentiations of the dorsal musculature (fig. 327D). Therefore,
a definite formation of special and individual muscles occurs in the dorsal
or epaxial musculature of the frog, whereas in Necturus, the dorsal musculature
tends to resemble the segmental myotomic condition of the fish. It is to be
observed that the dorsal musculature of the frog is adapted to a land-going
existence, while the dorsal musculature of Necturus is suited to swimming
movements.
A further land adaptation is shown in many salamanders, such as the
various species of Desniognathus, where the dorsal trunk musculature differ-
entiates in the neck region into several muscles which insert upon the skull.
The latter muscles permit lateral movements of the head.
Turning to the hypaxial musculature, we find that this musculature in
Necturus also approaches the condition in fishes. Let us examine this mus-
culature in more detail. In the midventral abdominal area, the fibers assume
a primitive, strictly segmental, antero-posterior direction. These muscle bundles
form the M. rectus abdominis. Along the lateral side of the body wall, the
myosepta (myocommata) are retained between the segmented muscles. How-
ever, two layers of muscle fibers are present, an outer thick M. obliquus
externus, whose fibers' run postero-ventrally, and an inner thin layer, the
M. obliquus internus, with fibers coursing antero-ventrally. Turning now to
the frog, we find that a segmented rectus abdominis (M. rectus abdominis)
is present. In each lateral body wall, an outer external oblique muscle (M.
obliquus externus superficialis) runs postero-ventrally, while an internal
transverse muscle (M. transversus) courses antero-ventrally (fig. 327D). In
Necturus and the frog, therefore, the primitive myotomic condition of the
708 THE MUSCULAR SYSTEM
hypaxial musculature of the shark is disrupted, and the myotomes tend to
spHt into layers or sheets of muscles. This splitting is slight in Necturus and
marked in the frog. Also, in the frog, the myocommata are displaced as a
part of the muscular-skeletal mechanism, with the exception of the rectus
abdominis muscle whose segmentation possibly is a secondary development.
In mammals (fig. 327E), the epaxial musculature is differentiated into
a complex of muscles, extending from the sacral area anteriorly into the cervical
region and connecting the various vertebrae with each other and the vertebral
column with the ribs. The epaxial musculature in the trunk area of the bird is
much less developed than it is in the mammal. The hypaxial musculature
in both bird and mammal becomes separated into distinct layers, such as
the external, internal oblique, and transversus muscles. External and internal
intercostal muscles are present between the ribs. In the midventral area, the
rectus abdominis muscle tends to retain its primitive segmentation.
It is noteworthy to observe that the external and internal intercostal muscles
in the mammal appear much the same as the lateral body muscles in Necturus,
particularly if we keep in mind the fact that ribs grow out into the myoseptal
(myocommal) area (fig. 326D). The external intercostal muscles run postero-
ventrally, while the internal intercostals pass antero-ventrally from one rib
to the next (fig. 327E). The intercostal musculature of the mammal thus
retains the primitive, segmented condition.
c) Aerial Adaptations. The musculature of the bird is a highly dif-
ferentiated organization of structures in which the primitive myotomic plan
is greatly distorted. The epaxial musculature is reduced greatly over the trunk
region, although well developed in the cervical area. Hypaxial musculature
is present in the form of external and internal oblique, and transverse muscle
layers. Very short rectus abdominis muscles are to be found. Aside from the
intrinsic muscles of the limbs, a large percentage of the volume of the hypaxial
Fig. 327. Development of branchial and somitic muscles in various vertebrates. (A)
Basic areas of the embryo from which skeletal muscle develops. The skeletal muscles of
the limb buds are portrayed as masses of mesenchyme represented in this figure as stippled
areas in the two limb buds. The origin of this mesenchyme varies in different vertebrates
(see text). (B) Skeletal muscular development in the shark. The muscle tissue derived
from the hyoid visceral arch is shown in black with white lines. Muscle tissue derivatives
from the mandibular visceral arch are shown anterior to the black-white line areas of the
hyoid musculature. (C) Same for Necturus inuculosus. (D) Same for the frog. (E)
Epaxial muscles and intercostal part of hypaxial muscles of cat. External intercostals
mostly removed. The "masseter muscle," a derivative of the mandibular visceral arch tissue
of the embryo, also is shown. (E') Superficial facial and platysma muscle distribution in
the cat. These muscles are derivatives of the hyoid visceral and mesenchyme. (E")
External pterygoid muscle in the cat, another derivative of the branchial arch mesen-
chyme. (F) Anterior muscles of the goose. The muscles derived from the primitive
hyoid visceral arch are shown in black with white lines. (Adapted from Huber, 1930,
Quart. Rev. Biol., vol. 5, and from FiJrbringer, 1888, Morphologic und Systemalik der
Vogel, van Holkema, Amsterdam.) (F') The temporal and masseter muscles in the
common fowl. These muscles are derived from the mandibular visceral arch.
UEV4T0RS
OF THE GILLS
ICUCULLORIS)
M DELTOIDEUS
M PECT0R4LIS
M PTERVGOIDI
Fig. 327. (See facing page for legend.)
709
710
THE MUSCULAR SYSTEM
musculature of the bird is contained within the pectoral muscles (fig. 327F).
As such the pectoral musculature represents an extreme adaptation to the
flying habit. A somewhat similar adaptation is found among mammals, in the
bat group. Myotomic metamerism is much less evident in the bird than in
any other group of vertebrates, and the only remains of it appear in the
intercostal muscles and some of the deeper muscles of the cervical area.
2) Development of Trunk and Tail Musculature: a) General Features
OF Myotomic Differentiation in the Trunk. The muscles of the trunk
are derived from the primitive myotomes. As described previously, Chapters
11, 12, and 15, the primitive body segment or somite differentiates into the
HYOID SWELLING
EXTERNAL E
MANDIBULAR PROCESS
MAXILLARY PROCESS
4TH CERVICAL MYOTOME
ARM MUSCULATURE
PRECARTILAGE
PRIMORDiUM OF
ARM SKELETON
ST THORACIC MYOTOME
MESENCHYMAL
PRIMORDIUM OF
PECTORAL MUSCLES
VENTRAL (HYPAXIAL) REGION
OF 7th myotome
DORSAL (EPAXIALI REGION
OF tth myotome
1ST LUMBAR MYOTOME
SPINAL GANGLION
LUMBOSACRAL PLEXUS
ST SACRAL MYOTOME
9MM
4 1/2 WEEKS
Fig. 328. Muscle development in the human embryo. (A and B redrawn from Bardeen
and Lewis, 1901, Am. J. Anat., 1.) (A) Early division of truncal myotomes into dorsal
(epaxial) and ventral (hypaxial) regions.
MORPHOGENESIS OF MUSCULAR SYSTEM
711
HYOID SWELLINGS
OF EXTERNAL E
TRAPEZIUS
DELTOID (CUT)
LEVATOR SCAPULA
AND
SERRATUS ANTERIO
LATISSIMUS OORSI
AND
TERES MAJOR
EXTERNAL OBLIQU
SACROSPl NA LI
tEPAXIAL MUSCULA
RECTUS FEMO
FIRST LUMBAR
Fig. 328 — (Continued) Muscle development in the human embryo. (A and B redrawn
from Bardeen and Lewis, 1901, Am. J. Anat., 1.) (B) Differentiation of myotomal
derivatives in ll-mm. embryo. Observe that the dorsal division of the spinal nerves is
distributed to the epaxial musculature, while the lateral division of the ventral rami passes
to the intercostal areas.
sclerotome, myotome, and dermatome (fig. 252). After the sclerotome has
departed toward the median plane, the myotome and dermatome reconstruct
the dermo-myotome which has a myocoelic cavity within (fig. 311A). The
inner layer or myotome gives origin to the muscle fibers of the later myotome.
The fate of the dermatome or cutis plate is not definite in all vertebrates. In
lower vertebrates it is probable that most of the dermatome gives origin to
dermal mesenchyme (Chap. 12). However, in mammals, according to Bardeen
('00) in his studies relative to the pig and human, the dermatome or cutis
plate gives origin to muscle cells. On the other hand, Williams ('10) does
712
THE MUSCULAR SYSTEM
not tolerate this view, but believes, in the chick at least, that the dermatome
gives origin to dermal mesenchyme.
The primitive position of the myotome is lateral to the nerve cord and
notochord. As development progresses, the individual myotomes grow ven-
trally toward the midventral line (fig. 327A). As this downgrowth progresses,
each myotome becomes separated into dorsal (epaxial) and ventral (hypaxial)
segments (fig. 328A). As indicated above and in figure 326D, the ribs grow
MANDIBULAR SWE
EXTERNA
HYOID SWELLING
EXTERNAL
EXTERNAL AUD
MEATUS
LATISSIMUS OORSI
EXTERNAL OBLIQUE
RECTUS ABDOMINIS
SACROSPINALIS
BENEATH
LUMBO - DORSAL FASCIA
SARTORIUS
QUADRICEPS FEMORIS
TENSOR FASCIAE LATA
GLUTEUS MEDIUS
BICEPS FEMORIS
GLUTEUS MAXIMUS
20MM
/WEEKS
UMBILICAL
ARTERIES
UMBILICAL VEIN
TIBIALIS ANTICUS
___ EXTENSOR
-^ % H ALLUCIS
LONGUS
^», i-^d EXTENSOR
\ia? ' DIGITORUM LONGUS
Fig. 329. Later development of musculature in human embryo. (A after Bardeen and
Lewis, 1901. Am. J. Anat.. 1.) (A) Limb and superficial trunk musculature of 20-mm
human embryo.
SPINAL GANGLION
NEURAL PROCESS
TRAPEZIUS
BRACHIORADIALIS
EXTENSOR CARPI F
SUPIN
EXTENSOR POLLIC
BREVIS
EXTENSOR POLLICIS
LONGUS
EXTENSOR
INDICIS
EXTENSOR CARPI
ULNARIS
SCAPULA
TER
SERRATUS
CLOSED
URETHRAL
GROOVE
bulbocavernosus
;hiocavernosus
URETHRAL ORIFICE
GLUTEUS MAXIMUS
GLUTEUS MAXIMUS
MALE INDIFFERENT STAGE
Fig. 329 — (Continued) Later development of musculature in human embryo. (B after
Lewis, 1902, Am. J. Anat., 1.) (B) Developing forelimb musculature of human embryo
(lateral aspect of limb). (C) Differentiation of cloacal musculature in human embryo.
713
714 THE MUSCULAR SYSTEM
out in the area occupied by the myocommata or connective tissue partitions
between the myotomes, and thus ribs and myocommata are correlated inti-
mately with myotomic differentiations in all lower vertebrates. However, in
reptiles, birds, and mammals, the outgrowing ribs travel downward within the
connective tissue between the myotomes, but the development of the mycom-
mata are suppressed.
b) Differentiation of the Myotomes in Fishes and Amphibia. In
the fishes, as the ventral myotomic progression occurs, the differentiating
muscle fibers become united anteriorly and posteriorly to the myocommata.
In Necturus and in amphibian larvae, in general, this relationship also is
established, but, in addition, the myotomes become separated into sheets or
layers. In the frog during metamorphosis, this splitting of myotomes and
the segregation of separate layers and bundles of distinct muscles is carried
further. Also in the frog, a marked migration of separate bundles of muscle
fibers occurs, while the fusion of parts of separate myotomes is indicated in
the development of the M. longissimus dorsi which superficially appears to
be segmented (fig. 327D). There is a pronounced tendency, therefore, in
the development of the frog musculature for the primitive myotomic plan
to be distorted and myotomes fuse, split, degenerate or migrate to serve the
required functional purpose of the various muscles.
c) Differentiation of the Truncal Myotomes in Higher Verte-
BRATA and Particularly in the Human Embryo. The principles of myo-
tomic modification by fusion, splitting into separate components, migration
of parts of myotomes away from the primitive position, and degeneration
of myotomic structure as exemplified in the developing musculature of the
frog, are utilized to great advantage in reptiles, birds and mammals. The
end to be served in all instances is the adaptation of a particular muscle or
muscles to a definite function.
In the development of the adult form of the musculature in the human
embryo, the basic division of the primitive myotomes into dorsal (epaxial)
and ventral (hypaxial) regions occurs (fig. 328A). The dorsal region of the
myotomes is located alongside the developing vertebrae, dorsal to the trans-
verse processes. The ventral portions of the myotomes pass ventrally external
to and between the ribs, enclosing the developing viscera.
In a slightly older embryo, the dorsal or epaxial musculature begins to lose
its primitive segmentation, and the myotomes fuse into an elongated myo-
tomic column, extending caudally from the occipital area (fig. 328B). The
deeper portions of the myotomes, associated with the developing vertebrae,
appear to retain their original segmentation, and the Mm. levatores costarum,
interspinales, intertransversarii, and rotatores persist as segmental derivatives
of the myotomes. The outer layer of the dorsal or epaxial musculature splits
lengthwise into an outer muscle group, the dorsally placed Mm. longissimus
dorsi and spinalis dorsi, and a latero-ventral Mm. iliocostalis group (fig.
MORPHOGENESIS OF MUSCULAR SYSTEM 715
328B). (See Lewis, W. H., '10.) Between the above two major groups of
muscles derived from the epaxial muscle column are other epaxial derivatives
such as the semispinalis and multifidus muscles.
The ventral or hypaxial portions of the myotomes overlying the develop-
ing ribs fuse into a continuous mass, while the medial portions of the myo-
tomes lying between the ribs give origin to the Mm. intercostales interni
and externi. The ventral ends of the fused myotomes on either side of the
midventral line split off longitudinally to form the M. rectus abdominis
which becomes an elongated sheet, extending from the anterior pectoral area
caudal to the differentiating pelvic girdle. The tendency toward segmentation
of the two rectus abdominis muscles probably represents a secondary process
in man. Tangential splitting of the fused thoracic and abdominal myotomes
and migration of the fibers give origin to the Mm. obliquus abdominis ex-
ternus, obliquus abdominis internus, transversus abdominis, serratus posterior
superior, and serratus posterior inferior.
The deep or subvertebral muscles below the vertebral column in the dorsal
area are derived from two sources. The Mm. longus colli and longus capitis
arise from the migration of myotomic tissue to the ventral vertebral surfaces
in the neck region, whereas the Mm. iliopsoas appear to be derived from the
musculature of the hind limb (Lewis, W. H., '10).
d) Muscles of the Cloacae and Perineal Area. The muscle tissue of
the cloaca forms a circle of constricting muscular bands which surround the
cloacal opening. These muscular bands are derived from myotomic tissue of
the posterior truncal region.
In the higher mammals, the primitive cloacal opening becomes divided
during development into anterior urogenital and posterior anal openings, and
the cloacal musculature is divided into the musculature associated with the
urethra, external genital structures, and the anal sphincter (fig. 329C).
e) Development of the Musculature of the Tail Region. The
musculature of the tail arises from the tail-bud mesoderm of the early embryo.
This mesenchyme condenses to form myotomic concentrations which later
divide into epaxial and hypaxial segments as in the truncal region of the
body. These myotomic segments are well developed in all fishes and in the
adults of amphibia other than the Anura. In fishes the enlarged condition
of the epaxial and hypaxial muscles of the tail region coincides with the
elongation of neural spines and hemal processes of the tail vertebrae where
they serve the functidn of moving the caudal fin from side to side. Three
main types of caudal fin skeletal arrangement in fishes (see fig. 331B-D)
act as the framework for the fin which serves the relatively enormous pro-
pulsive force generated by the tail musculature.
In Necturus, in Cryptobranchus, and in other water-dwelling amphibians,
and also in crocodilians, whales, etc., the tail musculature is developed to
serve the natatorial function which requires a lateral movement of the tail.
716 THE MUSCULAR SYSTEM
On the other hand, the prehensile or grasping movement of the tail of the
opossum, or the tails of western-hemisphere monkeys necessitates an extreme
adaptation on the part of individual muscle bundles and their attachment to
the caudal vertebrae. Similar specializations are found in the writhing tail of
the cat group. The wagging movement of the tail of the dog or the swishing
motion of the tails of cows, horses and other mammals is the result of the
activities of the Mm. abductor caudae internus and abductor caudae externus
which appear to be derivatives of the hind-limb musculature.
b. Development of Muscles of the Head-pharyngeal Area
1) Extrinsic Muscles of the Eye. The extrinsic muscles of the eyeball are
one of the most constant features of vertebrate morphology. Six muscles for
each eye are found in all gnathostomes, innervated by three cranial nerves
as follows:
( 1 ) M. rectus superior — cranial nerve III,
(2) M. rectus internus or anterius — cranial nerve III,
(3) M. rectus inferior — cranial nerve III,
(4) M. rectus externus (posterius or lateralis) — cranial nerve VI,
(5) M. obliquus superior — cranial nerve IV, and
(6) M. obliquus inferior — cranial nerve III.
To these muscles may be added the Mm. retractor oculi of many mammals
and the Mm. quadratus and pyramidalis of birds.
In the shark group, the muscles of the eye arise from three pre-otic somites
or head cavities, namely, the pre-mandibular, mandibular and hyoid somites
(figs. 253, 327A). The pre-mandibular somite, innervated by the oculomo-
torius or third cranial nerve, gives origin to all of the rectus muscles with the
exception of the Mm. rectus externus. The Mm. obliquus inferior also arises
from the pre-mandibular somite. From the mandibular somite, innervated
by the trochlearis or fourth cranial nerve, arises the Mm. obliquus superior,
while the hyoid somite gives origin to the Mm. rectus externus (Balfour, 1878;
Piatt, 1891; Neal, '18). A derivation of eye muscles from three pre-otic
somites or mesodermal condensations has been described in the gymno-
phionan amphibia by Marcus ('09), in the turtle by Johnson ('13), in the
chick by Adelmann ('26, '27), and in the marsupial mammal, Trichosurus,
by Fraser ('15). For extensive references regarding the eye-forming somites
or mesodermal condensations, see Adelmann ('26, and '27).
Various disagreements, concerning the presence or absence of the various
head somites and the origin of the eye muscles therefrom, are to be found
in the literature. Regardless of this lack of uniformity of agreement, it is
highly probable that the premuscle masses of tissue which give origin to the
eye muscles in the gnathostomous vertebrates, in general, adhere closely to
MORPHOGENESIS OF MUSCULAR SYSTEM 717
the pattern of the eye-muscle development from three pre -otic pairs of somites
as manifested in the shark embryo.
2) Muscles of the Visceral Skeleton and Post-branchial area: a) Tongue
AND Other Hypobranchial Musculature. As indicated in figures 253
and 327A, a variable number of post-otic or met-otic somites are concerned
with the composition of the head of the gnathostomous vertebrate. In the
dogfish, Squalus acanthias, about six pairs of post-otic somites contribute to
the structure of the head (De Beer, '22). For most vertebrates, about three
pairs of post-otic somites, a conservative estimate, appear to enter into the
head's composition. The hypobranchial musculature in the elasmobranch
embryo arises as myotomal buds from the myotomes of posterior head area.
These muscle buds migrate ventrad from these myotomes to the hypobranchial
region as indicated in figure 253. Associated with this migration of myotomal
material is the migration and distribution of the hypoglossal nerve, com-
pounded from the ventral roots of post-otic spinal nerves to this area (fig.
253). In the human, W. H. Lewis ('10) favors the view that the tongue
musculature arises in situ from the hypobranchial mesenchyme, but Kingsbury
('15) suggests the post-otic origin of the tongue musculature for all verte-
brates. Regardless of its origin, the tongue musculature is innervated by
ventral nerve roots of post-otic segments in higher vertebrates, i.e., the
hypoglossal or twelfth cranial nerve. The tongue musculature becomes asso-
ciated with the basihyal portion of the hyoid arch, which acts as its support.
In mammals, the sternohyoid, sternothyroid, and omohyoid muscles are in-
nervated also by the hypoglossal or twelfth cranial nerve. These muscles
probably arise from the post-otic myotomes in a manner similar to the tongue
musculature.
b) Musculature of the Mandibular Visceral Arch. The mesoderm,
associated with this arch, gives origin to the muscles of mastication, and as
a result these muscles are innervated by special visceral motor fibers located
in the trigeminal or fifth cranial nerve. In the shark, the muscles arising from
the mandibular visceral arch tissue are the adductor mandibulae and the
first ventral constrictor muscles (fig. 327B); in the frog, the temporal, mas-
seter, pterygoid, and mylohyoid muscles; in the chick, the pterygotemporal,
temporal, and digastric muscles; and, in mammals, the temporal, masseter,
pterygoid, anterior portion of the digastric, mylohoid, tensor tympani, and
tensor veli palatini muscles (fig. 327D, E', E", F, F').
c) Musculature of the Hyoid Visceral Arch. The musculature, which
develops from mesenchyme associated with the embryonic hyoid arch, be-
comes distributed as indicated in figures 327 and 330. It is to be observed
that, in the adult shark (fig. 327B), this musculature functions in relation
to the hyoid arch. In the adult frog (fig. 327D), it is represented by deep
facial musculature or the depressor mandibulae and subhyoideus muscles. In
the adult goose (fig. 327F), it is present as the M. sphincter colli, which
718 THE MUSCULAR SYSTEM
represents superficial facial musculature, and the M. depressor mandibulae
or deep facial musculature. In mammals (figs. 327E'; 330A-D), the muscles
derived from the hyoid arch is distributed over the cervico-facial area as many
separate muscles. The musculature derived from the hyoid arch is innervated
by the seventh or facial cranial nerve. Reference may be made to the extensive
review of the literature by Huber ('30, a and b), relative to the facial
musculature in vertebrates.
d) Musculature of the First Branchial Arch. The musculature of
the first branchial arch is innervated by the glossopharyngeal or ninth cranial
nerve. In the shark, the muscle tissue arising from the first branchial arch
becomes the constrictor musculature of this arch, but, in the mammal, it
gives origin to the stylopharyngeus muscle and to the constrictors of the
pharynx.
e) Muscles of the Succeeding Visceral Arches. In the shark, these
muscles contribute to the constrictor muscles of the gill arches and are under
the domain of the vagus or tenth cranial nerve. In the mammal, this muscle
tissue becomes associated with the larynx and with the constrictors of the
pharynx.
f) Muscles Associated with the Spinal Accessory or Eleventh
Cranial Nerve. The sternocleidomastoid and trapezius musculature in the
human, according to W. H. Lewis ('10), arises from a premuscle mass as-
sociated at the caudal end of the pharyngeal area below the post-otic myo-
tomes (fig. 336A). With the musculature arising from this premuscle mass,
the spinal accessory or eleventh cranial nerve becomes associated. The tra-
pezius musculature migrates extensively over the scapular area (fig. 329A).
g) Musculature of the Mammalian Diaphragm. The striated mus-
culature of the mammalian diaphragm appears to arise from the ventral por-
tions of the myotomes in the midcervical area. In the human, this diaphragmatic
musculature is innervated by the ventral roots of cervical nerves IV and V,
while, in the cat, cervical nerves V and VI are involved. These ventral rami
give origin to the phrenic nerve, which later migrates posteriad with the
diaphragmatic musculature together with the developing diaphragm during
the division of the coelomic cavities (Chap. 20).
c. Development of the Musculature of the Paired Appendages
Two main theories have arisen relative to the origin of the paired append-
ages. One is the gill-arch theory of Gegenbauer (1876) and the fin-fold or
lateral-fold theory of Balfour ( 1881 ). According to the theory of Gegenbauer,
the limb girdles are modified gill arches, and the limb tissue itself represents
a modification of the gill septa and supporting gill rays. The pelvic limbs were
produced, according to this theory, by a backward migration of the gill arch
involved. The lateral-fold theory, on the other hand, postulated that the paired
hmbs were derived from longitudinal fin folds. The endoskeleton within the
MORPHOGENESIS OF MUSCULAR SYSTEM
719
NUCHAL PORTION OF PLATYSMA AND
POSTAURICULO-OCCIPITAL MUSCLE GROUP
ZYGOMATICUS
ZYGOMATICUS
M. TRIANGULARIS
M. PLATYSMA
Fig. 330. Facial and cervical muscles in mammals derived from the mesoderm of the
hyoid arch. (Redrawn from Huber, 1930, Quart. Rev. Biol., 5.) (A) Opossum (Didel-
phys). (B) Cat (Felis). (C) New-born baby (white) human. (D) Adult (white)
human.
fold arose as a support for the fold in a manner similar to the median fins.
The latter theory has the greatest number of adherents today.
The early development of the rudiments of the paired appendages and the
properties of the limb field are discussed in Chapter 10, page 508. Relative
to the developing limb; the exact origin of the cells which go to make up its
intrinsic musculature has been the object of much study. In the elasmobranch
and teleost fishes, muscle buds from the myotomes in the vicinity of the
developing fin fold unquestionably contribute dorsal and ventral premuscle
masses of cells to the limb, which give origin respectively to
1 ) the dorsal, elevator and extensor muscles, and
2) the ventral depressor and adductor muscles of the fin.
720
THE MUSCULAR SYSTEM
In tetrapod vertebrates, however, the exact origin of the cells which enter
into the formation of the limb's intrinsic musculature is open to question. In
the amphibia, including Urodela and Aniira, Field (1894) described myo-
tomic processes which contribute to the musculature of the anterior limbs.
Byrnes (1898), working experimentally with the same group, and W. H.
Lewis ('10b) deny this conclusion and affirm the somatopleural or in situ
HOMOCERCAL TYPE
( CYPRINUS CARPIO 1
(GERMAN CARPI
ETEROCERCAl type
^ scaphirhyncmus )
Fig. 331. (A) Innervation of premuscle masses in head and pharyngeal areas, and of
myotomes in the cervical and caudal head regions of 7-mm. human embryos. Four
post-otic (occipital) myotomes and the premuscle mass of the trapezius and sterno-
mastoid muscles are shown just back of the tenth cranial nerve. The first cervical myotome
and spinal nerve are shown just posterior to the fourth occipital myotome. (Redrawn from
W. H. Lewis, 1910, chap. 12 in Manual of Human Embryology, vol. I, by F. Keibel
and F. P. Mall, Philadelphia, Lippincott.) (B, C, D) Types of caudal fins in fishes.
MORPHOGENESIS OF MUSCULAR SYSTEM 721
origin of the limb musculature and connective tissues. Similar affirmations
and denials are found in the literature, relative to origin of the intrinsic limb
muscles in higher vertebrates, including man. For example, Ingalls ('07)
described myotomic cell migrations into the developing human limb, whereas
W. H. Lewis ('10a) was not able to subscribe to this view.
Although actual muscle tissue from the myotomes to the limb buds cannot
be traced in all cases, the fact remains that the nerve supply to a myotome
or to a particular group of muscle-forming cells appears to be a constant
feature. For example, the facial musculature, which is derived from the hyoid
arch mesenchyme of the embryo as set forth above, retains its innervation
by the facial or seventh cranial nerve, even though the muscle migrates far
forward from its original site of development. The innervation of the trapezius
muscle by the spinal accessory nerve is another example of this same fidelity
of the nerve supply to the original site of the origin of the muscle-forming
cells. Mall (1898, p. 348) describes this relationship between the nerves and
myotomes as follows: "As the segmental nerves appear, each is immediately
connected with its corresponding myotome, and all of the muscles arising
from a myotome are always innervated by branches of the nerve which orig-
inally belonged to it." (See fig. 33 lA.)
The development of the musculature of the tetrapod limb involves two
main premuscle masses of tissue:
( 1 ) An intrinsic mass of muscle-forming mesenchyme within the develop-
ing limb which condenses to form separate muscle-forming associations
of cells around the developing skeleton of the limb. Each of these
cellular associations then proceeds to differentiate into a particular
muscle or closely integrated group of muscles (figs. 328B; 329A and
B). That is, the intrinsic mass of muscle-forming tissue gives origin
to the intrinsic musculature of the limb.
(2) An extrinsic mass of premuscle tissue which ultimately gives origin
to the musculature which attaches the limb and its girdle to the axial
skeleton. This premuscle tissue arises from two sources:
(a) Premuscle tissue from the limb bud which migrates from the limb bud
proximally toward the axial skeleton. In the forelimb, the pectoral,
latissimus dorsi, and teres major muscles develop from this mass of
tissue, while in the hind-limb the caudo-femoralis, iliopsoas, piriformis,
and certain of the gluteal muscles appear to arise from muscle-forming
tissue which extends axially to unite the limb with the axial skeleton.
(b) Premuscle tissue which arises outside the limb bud mesenchyme. The
muscles which arise from this tissue serve to attach the limb girdle to
the axial skeleton. From premuscle tissue of this type arise the Mm.
trapezius, sternocleidomastoideus, rhomboidei, levator scapulae, ser-
ratus anterior, and omohyoideus.
722
THE MUSCULAR SYSTEM
d. Panniciilus Carnosus
There are two groups of skeletal "skin muscles," that is, muscles under
voluntary control which move the skin and skin structures. One group is the
mimetic or facial musculature, described on page 717 and originating from
the primitive hyoid mesoderm; the other is the panniculus carnosus, found
only in the Mammalia and derived embryologically from the tissue which
forms the pectoral musculature. The facial musculature is innervated by
cranial nerve VII or the facial nerve, while the panniculus carnosus receives
its innervation from the anterior thoracic nerves (fig. 327E').
The panniculus carnosus is highly developed in the guinea pig and porcupine
and, although less developed in the rabbit, cat, dog, and horse, it forms a
prominent muscular layer. The fibers may be divided into two groups:
(a) fibers which arise and insert in the superficial fascia of the skin and
(b) fibers that arise in the superficial fascia of the back and thigh and
converge toward the greater tuberosity of the humerus, where they
insert.
For extensive references and descriptions, see Langworthy ('24 and '25).
Bibliography
Adelmann, H. B. 1926. The development
of the premandibular head cavities and
the relations of the anterior end of the
notochord in the chick and robin. J.
Morphol. 42:371.
. 1927. The development of the
eye muscles of the chick. J. Morphol.
44:29.
Balfour, F. M. 1878. A monograph on the
development of elasmobranch fishes.
Chap. X in The Works of Francis Mait-
land Balfour. Edited by M. Foster and
A. Sedgwick. Vol. 1, 1885. Macmillan
and Co., London.
. 1881. On the development of the
skeleton of the paired fins of elasmo-
branchii, considered in relation to its
bearings on the nature of the limbs of
the Vertebrata. Chap. XX in The Works
of Francis Maitiand Balfour. Edited by
M. Foster and A. Sedgwick. Vol. 1,
1885. Macmillan and Co., London.
Bardeen, C. R. 1900. The development of
the musculature of the body wall in the
pig. Johns Hopkins Hospital Reports.
9:367.
Byrnes, E. F. 1898. Experimental studies
on the development of limb-muscles in
Amphibia. J. Morphol. 14:105.
De Beer, G. R. 1922. The segmentation
of the head in Squalus acanthius. Quart.
J. Micr. Sc. 66:457.
Field, H. H. 1894. Die Vornierenkapsel,
ventrale Musculatur und Extremitate-
nanlagen bei den Amphibien. Anat. Anz.
9:713.
Eraser, E. A. 1915. The head cavities and
development of the eye muscles in Tri-
chosurus vulpecula with notes on some
other marsupials. Proc. Zool. Soc, Lon-
don, sA. 299.
Gegenbaur, C. 1876. Zur morphologic der
Gliedmaassen der Wirbelthiere. Morph.
Jahrb. 2:396.
Huber, E. 1930a. Evolution of facial mus-
culature and cutaneous field of Trigemi-
nus. Part I. Quart. Rev. Biol. 5:133.
. 1930b. Evolution of facial mus-
culature and cutaneous field of Trigemi-
nus. Part L Quart. Rev. Biol. 5:389.
Ingalls, N. W. 1907. Beschreibung eines
menschlichen Embryos von 4:9mm.
Arch. f. mikr. Anat. u. Entwicklngsgesch.
70:506.
BIBLIOGRAPHY
723
Johnson, C. E. 1913, The development of
the prootic head somites and eye mus-
cles in Chelydra serpentina. Am. J. Anat.
14:119.
Kingsbury, B. F. 1915. The development
of the human pharynx. Part 1. The
pharyngeal derivatives. Am. J. Anat.
18:329.
Langworthy, O. R. 1924. The panniculus
carnosus in cat and dog and its genetical
relationship to the pectoral musculature.
J. Mammalogy. 5:49.
. 1925. A morphological study of
the panniculus carnosus and its geneti-
cal relationship to the pectoral muscula-
ture in rodents. Am. J. Anat. 35:283.
Lewis, M. R. 1919. The development of
cross-striations in the heart muscle of
the chick embryo. Johns Hopkins Hosp.
Rep. 30:176.
Lewis, W. H. 1910a. Chap. 12, Develop-
ment of the Muscular System in Human
Embryology. Edited by Keibel and Mall.
J. B. Lippincott Co., Philadelphia.
. 1910b, The relation of the myo-
tomes to the ventrolateral musculature
and to the anterior limbs in Amblystoma.
Anat. Rec. 4:183.
. 1922. The adhesive quality of
cells. Anat. Rec. 23:387.
Mall, F. P. 1898, Development of the ven-
tral abdominal walls in man. J. Morphol.
14:347.
Marcus, H. 1909. Beitriige zur Kenntnis
der Gymnophionen. IH. Zur Entwick-
lungsgeschichte des Kopfes. I Teil.
Morph. Jahrb. 40:105.
Neal, H. V. 1918, The history of the eye
muscles. J. Morphol. 30:433.
Patten, B. M. and Kramer, T. C. 1933,
The initiation of contraction in the
embryonic chick heart. Am. J. Anat.
53:349.
Piatt, J. B. 1891. A contribution to the
morphology of the vertebrate head,
based on a study of Acanthias vulgaris.
J. Morphol. 5:79.
Williams, L. W. 1910. The somites of the
chick. Am. J. Anat. 11:55.
17
Tne Circulatory System
A. Introduction
1. Definition
2. Major subdivisions of the circulatory system
B. Development of the basic features of the arteriovenous system
1. The basic plan of the arteriovenous system
2. Development of the primitive heart and blood vessels associated with the primi-
tive gut
3. Formation of the primitive blood vessels associated with the mesodermal and neural
areas
4. Regions of the primitive vascular system
C. Histogenesis of the circulatory system
1. The heart
2. Formation of the primitive vascular channels and capillaries
3. Later development of blood vessels
a. Arteries
b. Veins
c. Capillaries
4. Hematopoiesis (Hemopoiesis)
a. Theories of blood-cell origin
b. Places of blood-cell origin
1 ) Early embryonic origin of blood cells
2) Later sites of blood-cell formation
3) Characteristics of development of the erythrocyte
4) Characteristics of various white blood cells
a) Granulocytes
b) Lymphoid forms
D. Morphogenesis of the circulatory system
1. Introduction
2. Transformation of the converging veins of the early embryonic heart into the
major veins which enter the adult form of the heart
a. Alteration of the primitive converging veins of the heart in the shark, Squalus
acanthias
b. Changes in the primitive converging veins of the heart in the anuran amphibia
1 ) The vitelline veins
2) Lateral (ventral abdominal) veins
3) Formation of the inferior vena cava
4) Formation of the renal portal system
5) Precaval veins
724
INTRODUCTION 725
c. Changes in the primitive converging veins of the heart in the chick
1 ) Transformation of the vitelline and allantoic veins
a) Vitelline veins
b) Allantoic veins
2) Formation of the inferior vena cava
3) Development of the precaval veins
d. The developing converging veins of the mammalian heart
3. Development of the heart
a. General morphology of the primitive heart
b. The basic histological structure of the primitive embryonic heart
c. Importance of the septum transversum to the early heart
d. Activities of early-heart development common to all vertebrates
e. Development of the heart in various vertebrates
1 ) Shark, Squalus acanthias
2) Frog, Rana pipiens
3) Amniota
a) Heart of the chick
b) Mammalian heart
( 1 ) Early features
(2) Internal partitioning
(3) Fate of the sinus venosus
(4) The division of the bulbus cordis (truncus arteriosus and conus)
f. Fate of embryonic heart segments in various vertebrates
4. Modifications of the aortal arches
5. Dorsal aortae (aorta) and branches
E. Development of the Lymphatic System
F. Modifications of the circulatory system in the mammalian fetus at birth
G. The initiation of the heart beat
A. Introduction
1. Definition
Living matter in its active state depends for its existence upon the beneficent
flow of fluid materials through its substance. This passage of materials con-
sists of two phases:
( 1 ) the inflow of fluid, containing food materials and oxygen, and
(2) the outflow of fluid, laden with waste products.
In the vertebrate grqup as a whole, the inflow of materials to the body
substance occurs through the epithehal membranes of the digestive, integu-
mentary, and respiratory systems, while the outflow of materials is effected
through the epithelial membranes of the excretory, respiratory, and skin sur-
faces. The passage of materials through the substance of the body lying
between these two sets of epithelial membranes is made possible by (a) the
blood and (b) a system of blood-conveying tubes or vessels. These structures
form the circulatory system.
726 THE CIRCULATORY SYSTEM
2. Major Subdivisions of the Circulatory System
The circulatory system is composed of two major subdivisions:
( 1 ) the arteriovenous system, composed of the heart, arteries, and veins
together with the blood vessels and capillaries of smaller dimensions
intervening between the arteries and veins, and
(2) the lymphatic system, made up of lymph sacs, and lymphatic vessels
together with specialized organs such as the spleen, tonsils, thymus
gland, and lymph nodes. In larval and adult amphibia pulsating lymph
hearts are a part of the lymphatic system. Lymph hearts are present
also in the tail region of the chick embryo.
The lymphatic vessels parallel the vessels of the arteriovenous system, and
one of their main functions appears to be to drain fluid from the small spaces
within tissues as well as larger spaces, such as the various divisions of the
coelomic cavity.
The blood within the arteriovenous system is composed of a fluid substance
or plasma together with red blood corpuscles or erythrocytes, white blood
cells of various types, and blood platelets. The latter are small protoplasmic
bodies which may represent cytoplasmic fragments of the giant, bone-marrow
cells or megakaryocytes. The blood within the lymphatic system is composed
of a vehicle the lymph fluid, similar to the plasma of the arteriovenous blood
system, together with various white blood corpuscles.
B. Development of the Basic Features of the Arteriovenous System
1. The Basic Plan of the Arteriovenous System
The primitive circulatory system is constructed of three main parts:
( 1 ) two sets of simple capillary tubes, bilaterally developed on either side
of the median line (fig. 332),
(2) a local modification of these tubes which forms the rudimentary heart,
and
(3) blood cells and fluid contained within the tubes.
2. Development of the Primitive Heart and Blood Vessels
Associated with the Primitive Gut
The primitive vascular tubes or capillaries form below the anterior region
of the developing metenteron or gut tube in relation to the yolk sac or yolk-
containing segment of the gut. Two sets of identical tubes begin to form, one
set on either side of the median plane of the embryo (fig. 332A and B).
Simultaneous with the formation of these primitive, subintestinal blood capil-
laries, the splanchnic layers of the two hypomeric portions of the mesodermal
tubes grow mesiad to cup around the blood capillaries in the area just posterior
DEVELOPMENT OF THE BASIC FEATURES OF THE ARTERIOVENOUS SYSTEM 727
to the forming pharyngeal area of the gut tube (figs. 234; 236D, E; 332F-M).
This encirclement of the primitive blood capillaries by the splanchnic layers
of the hypomeric mesoderm produces the rudimentary tubular heart, com-
posed within of two fused subintestinal capillaries and without of modified
fused portions of the hypomeric mesoderm. The modified portions of the
hypomeric mesoderm form the epimyocardial rudiment of the heart, while the
fused capillaries within establish the rudimentary endocardium (fig. 332F-M).
Proceeding anteriad from the area of primitive tubular heart, the blood
capillaries establish the primitive ventral aortae (fig. 332A).
From the primitive ventral aortae, the two capillaries move forward toward
the anterior end of the foregut where they diverge and pass dorsally, one on
either side of the foregut, as the first or mandibular pair of aortal arches. In
the dorsal area of the foregut the two primitive aortal arches pass inward
toward the median plane and each aortal arch joins a primitive capillary
which runs antero-posteriorly along the upper aspect of the developing gut
tube. These two supraintestinal blood vessels are the rudiments of the future
dorsal aorta and they are known as the dorsal aortae. They lie above the
primitive gut and below the notochord. In the region where the mandibular
pair of aortal arches joins the dorsal aortae, each primitive dorsal aorta sends
a capillary sprout toward the developing eye region and the brain. This
capillary forms the rudiment of the anterior end of the internal carotid artery.
About the midregion of the developing midgut, each of the dorsal aortae
sends off a lateral branch which connects with a series of capillaries in the
yolk or yolk-sac area of the deveoping midgut. The vessels which diverge
from the dorsal aorta to the yolk-sac region form the rudiments of the two
vitelline arteries. The capillary network in the yolk region or yolk-sac area
of the midgut in turn connect with the two subintestinal capillaries, previously
mentioned, which enter the forming heart. The two latter blood vessels con-
stitute the vitelline veins (fig. 332B). Meanwhile, successive pairs of aortal
arches are formed posterior to the first pair, connecting the ventral aortae
with the dorsal aortae (fig. 332D). These aortal arches pass through the
substance of the visceral arches, as mentioned in Chapters 14 and 15.
3. Formation of the Primitive Blood Vessels Associated
WITH THE Mesodermal and Neural Areas
The system of blood vessels described above (fig. 232A) is developed
in relation to the primitive gut tube. Very shortly, however, another system
of vessels is established dorso-laterally to the mesodermal tubes. This second
system of blood capillaries forms the beginning of the cardinal system (fig.
232B). The cardinal system is composed of two anterior cardinal veins
which begin as a series of small capillaries on either side over the forming
brain; from whence these veins proceed backward, one on either side over
the branchial mesoderm, and lateral to the forming somites. These vessels
728 THE CIRCULATORY SYSTEM
eventually proceed latero-ventrad in their development along the outer lateral
aspect of the somatopleural mesoderm to the caudal regions of the forming
heart, where they turn ventrad along the outer aspect of the somatopleural
layer of the hypomere. In the region where the anterior cardinal veins turn
ventrad toward the heart region, each anterior cardinal vein is joined by a
posterior cardinal vein. The latter proceeds forward from the posterior end
of the developing embryo, lying along the outer aspect of the nephrotomic
portion of the hypomere below the primitive epidermal tube (fig. 332B).
The union of the anterior and posterior cardinal veins on either side forms
the common cardinal vein. The latter travels postero-ventrally along the outer
aspect of the somatopleure until it reaches the upper limits of the caudal
region or sinus venosus of the developing heart. In this area, the splanchno-
pleural layer (epimyocardium) and the endocardial layer of the developing
sinus venosus, bulge laterad to fuse with the somatopleural layer of the
hypomere. This area of contact between the epimyocardial layer of the sinus
venosus and somatopleural mesoderm produces a bridge across the coelomic
space. The two posterior, dorso-lateral regions of the sinus venosus thus ex-
tend dorso-laterad on either side across the coelomic space to join the somato-
pleure. Each common cardinal vein perforates through the somatopleure in
this area and empties into the sinus venosus at a point lateral to the entry of
the two vitelline veins (fig. 332C). This bridge established across the coelomic
cavity from the somatopleure of the body wall to the splanchnopleure of the
heart forms a lateral mesocardium on either side. The two lateral mesocardia
Fig. 332. Early development of primitive vascular system including tubular heart.
(The diagrams included in this figure should be studied together with descriptions in
Chapter 10 relative to tubulation of the major organ-forming areas of the early embryo.)
(A) Diagram of the early bilaterally developed vascular tubes (capillaries) which form
in relation to the primitive gut tube. This system of capillaries constitutes the first or
early vitelline system of developing circulatory structures. (B) The cardinal or primary
venous system is added to the primitive vitelline system. (C) The area of union between
the early vitelline and cardinal systems at the caudal end of the heart. (D) The basic
(fundamental) condition of the vascular system. (E) Two diagrams showing the union
of the vitelline and cardinal systems distally between the somites and near the nerve cord.
The three vascular tubules to the left in this drawing show an early relationship of the
intersegmental arteries and veins, and the drawing of the three vascular tubules to the
right depict a later stage of this developmental relationship. (F-M) Stages in the de-
velopment of the early tubular heart in shark, frog, and chick embryos. As the mammal
is similar to the chick it is not included. (F-H) Early development of the heart in
Squalus acanthias. (F) The lower, mesial edges of the hypomeric mesoderm begins to
cup around the primitive subintestinal capillaries. (G) Later stage. (H) A transverse
section through the heart which is now in the form of a straight tube comparable to
that shown in Fig. 339A. (I-K) Early stages in the development of the frog heart.
Observe that the ventral areas of the two hypomeres become confluent and later form
a trough-like cup around the forming subintestinal capillaries below the foregut. (Redrawn
from Kellicott, 1913. Outlines of Chordate Development. Henry Holt, N. Y.) (L-M)
Early development of the chick heart. (L) At about 26 hrs. of incubation. (M) About
30 hrs. of incubation.
Fig. 332. (See facing page for legend.)
729
730 THE CIRCULATORY SYSTEM
represent the initial stages in the development of the various coelomic divisions
of the primitive coelomic space (Chap. 20).
As the cardinal and intestinal systems of the primitive vascular system be-
come joined together centrally via the common cardinal veins, the two systems
become joined peripherally by means of a series of intersegmental blood
vessels. The latter arise from the dorsal aortae and travel dorsally between
the somites and myotomes to the central nerve tube (fig. 232E). In the
nerve-tube area, the primitive intersegmental arteries become continuous with
the rudiments of the intersegmental veins which course laterad to join the
anterior and posterior cardinal veins. When the above vascular channels are
well established, another set of veins is formed between the somatopleural
mesoderm of the hypomere and the developing integument (figs. 332D; 336C,
D). The last veins course along the lateral body wall, arising in the pelvic area
and emptying into the sinus venosus of the heart. In fishes and amphibia,
these veins are called lateral veins, but in reptiles, birds, and mammals,
they are denominated the allantoic or umbilical veins as they drain principally
the allantoic area of the embryo.
4. Regions of the Primitive Vascular System
The primitive morphological plan of the vascular system, as outlined above,
is a basic condition strikingly comparable in all vertebrate embryos. In view
of the later changes of this fundamental vascular plan necessitated by the
adaptation of the vascular system to the environmental conditions existing
within the various habitats of the adult, it is well to demarcate, for the
purposes of later discussion, certain definite regions of the primitive arterio-
venous system. These regions are (fig. 332D):
( 1 ) the converging veins of the heart, composed of the lateral, common
cardinal, anterior and posterior cardinal, and vitelline veins,
(2) the primitive heart, made up of the primitive sinus venosus, atrium,
ventricle, and bulbus cordis,
(3) the branchial area, composed of the ventral aortae, aortal arches, and
adjacent dorsal aortae, and,
(4) the dorsal aortae (later aorta) and efferent branches.
C. Histogenesis of the Circulatory System
1. The Heart
Consult Chap. 16.
2. Formation of the Primitive Vascular Channels and
Capillaries
Two principal theories have emerged to account for the origin of the
primitive blood vessels in the embryo. These theories are the angioblast theory
and the local origin theory.
HISTOGENESIS OF CIRCULATORY SYSTEM 731
The angioblast theory rests upon the assumption that a special vascular
tissue, called the angioblast by Wilhelm His, develops in the area of the yolk
sac. This angioblast tissue, according to the angioblast theory, forms a vas-
cular rudiment within which the endothelium, or flattened epithelial cells
peculiar to blood capillaries, is developed. This endothelium produces the
primitive capillaries of the yolk area, and, further, it grows into the developing
embryo where it forms the endothelium of the entire intra-embryonic vascular
system. That 's, the angioblast in the yolk area provides the source from
which arises the endothelial lining of all the primitive blood vessels of the
embryo and also of all later endothelium of later blood vessels. The endo-
thelium of all blood vessels thus traces its ancestry back to the yolk-sac
angioblast.
The local origin theory may be divided into two schools of thought. One
school espouses the idea that "mesenchyme may, in practically any region
of the body, transform into vascular tissue" (McClure, '21, p. 221). Accord-
ingly, primitive blood capillaries arise in loco from mesenchyme in various
parts of the embryo, and these local vessels sprout, grow, and become united
to form the continuous vascular system. The endothelium which forms the
walls of all capillaries and the lining tissue of all blood vessels of larger
dimensions forms directly from mesenchyme. Addition to this mesenchyme
may occur by proliferation from endothelium already formed or by the con-
version of mesenchyme as single cells or cellular aggregates (McClure, '21;
Reagan, '17).
A second school which advocates the local origin of blood vessels differs
from the view described above principally by the assumption that, while the
endothelium of blood vessels appears to arise in loco from the mesenchyme,
it is not a generalized type of mesenchyme but rather a "slightly modified
mesenchymal cell" (Stockard, '15). Relative to this position, the following
quotation from Stockard, '15, p. 323, is given:
The facts presented seem to indicate that vascular endothelium, erythrocytes and
leucocytes, although all arise from mesenchyme, are really polyphyletic in origin:
that is, each has a different mesenchymal aniage. To make the meaning absolutely
clear, I consider the origin of the liver and pancreas cells a parallel case. Both arise
from endoderm, but each is formed by a distinctly different endodermal aniage,
and if one of these two aniagen is destroyed, the other is powerless to replace its
product.
3. Later Development of Blood Vessels
While the capillary possessing a wall composed of thin, flattened endo-
thelial cells is the basic or fundamental condition of all blood vessels in the
body, it is only of transitory importance in the development of the arteries
and veins. For, in the formation of the arteries and the veins, the primitive
capillary enlarges and its endothelial wall is soon reinforced by the addition
of white and elastic connective tissue fibers and smooth muscle tissue. The
732 THE CIRCULATORY SYSTEM
connective tissue and smooth muscle develop from the adjacent mesenchyme
present in the area in which the capillary makes its appearance.
a. Arteries
The arteries are the system of blood vessels which convey the blood from
the heart to the systemic organs. Most arteries are composed of three coats
of tissue which come to surround the endothelium of the capillary, namely,
an inner tunica intima, a middle tunica media, and an outer tunica adventitia.
The tunica media is composed of smooth muscle fibers and elastic connective
tissue fibers, while the other two coatings are fabricated of connective tissue
fibers.
In the large arteries in the immediate vicinity of the heart, the tunica media
is poorly muscularized but its elastic fibers are plentiful. However, in the
more distally placed arteries, the so-called distributing arteries which include
most of the arteries, the tunica media is supplied copiously with smooth
muscle fibers.
b. Veins
The veins are the vascular tubes which convey the blood from the systemic
organs back to the heart. The walls of the veins are more delicate than those
of the arteries, and the various tunics mentioned above are thinner, especially
the tunica media. The veins of the extremities form internal, pocket-shaped
valves which prevent the blood from moving backward.
c. Capillaries
The capillaries which form the ramifying bed of blood vessels between the
arteries and veins retain the primitive condition, and their walls are composed
of flattened endothelial cells. The size of the arteries and the thickness of the
arterial walls decrease as they approach the capillary bed, while those of the
veins increase as they leave the capillary area.
4. Hematopoiesis (Hemopoiesis)
a. Theories of Blood-cell Origin
Hematopoiesis is the name given to the process which effects the forma-
tion of blood cells. Though it is agreed that blood cells generally arise from
mesenchymal cells, all students of the problem do not concur in the belief
that all arise from a specific type of mesenchymal cell. For example, in the
quotation given above from Stockard, '15, it is stated that one type of mesen-
chymal cell gives origin to the red blood cells, while leukocytes or white
blood cells arise from a slightly different type of mesenchymal cell. This may
be called the dualistic theory of hematopoiesis. The view held today by many
in this field of development is that all blood cells arise from fixed, undif-
HISTOGENESIS OF CIRCULATORY SYSTEM 733
ferentiated, mesenchymal cells which give origin to a mother cell, the
hemocytoblast. From the hemocytoblast, four main stem cells arise, lympho-
blasts, monoblasts, granuloblasts, and erythroblasts, each of which differen-
tiates into the adult type of blood cell as shown in fig. 333A. Such an inter-
pretation is the basis for the monophyletic or unitarian th:;ory of blood cell
origin. Some observers, however, believe that the erythrocyte, granulocyte,
and the monocyte each have a separate stem cell. The latter view is the basis
of the trialistic theory. (Consult Maximow and Bloom, '42, pp. 107-116 for
discussion relative to blood-cell origin.)
b. Places of Blood-cell Origin
1) Early Embryonic Origin of Blood Cells. It long has been recognized
that the yolk-sac area is a region of early blood-cell development. This is
one aspect of the angioblast theory of His, referred to on page 731. In the
teleost fish, Fundulus, Stockard ('15) reports the origin of red blood cells
from two main sources:
( 1 ) an intermediate cell mass or blood string in the vicinity of the noto-
chord and
(2) the blood islands in the yolk sac.
However, the yolk-sac area appears to be the primary source for the early
phases of hematopoiesis in most vertebrate embryos. In the human embryo,
both red and white cells have been described as arising from primitive hemo-
cytoblasts in the yolk sac by Bloom and Bartelmez ('40). These authors
report the origin of primitive erythrocytes as arising primarily intra-vascularly,
although some develop extra-vascularly. Definitive erythrocytes develop, ac-
cording to these authors, in the entoderm and within blood vessels of the
yolk sac (fig. 333B). In the 24-hr. chick embryo, the blood islands in the
area vasculosa of the blastoderm show a direct conversion of mesodermal
cells into primitive blood cells and the endothelium of the forming blood
capillaries (fig. 333C). In the frog, blood islands appear in the mesoderm
and entoderm of the ventro-lateral areas of the body of 3- to 4.5-mm. embryos.
These islands are extensive, extending from the liver area caudally toward
the tail-bud region.
2) Later Sites of Blood-cell Formation. As indicated previously in teleost
fishes, early blood formation occurs in the region of the notochord near the
developing kidney tissue, as well as in the yolk-sac area. During later develop-
ment, hematopoiesis in teleost fishes is centered in the kidney area. The origin
of blood cells from kidney tissue also is true in the amphibian tadpole (Jordan
and Speidel, '23, a and b). The liver also functions in these forms to produce
blood cells. In the developing shark embryo, blood cells appear to be formed
around the heart and later in the esophageal area of the adult. In the adult
frog, the spleen functions as a center of blood-cell formation, although in the
GIANT CELLS Of
MARROW AND SPLEEN
A, MEGAKARYOCYTES
B. POLYKARYOCYTES
MONOCYTE
EOSINOPHILE NEUTROPHILE BASOPHILE " ^ ° BLOOD
GRANULOCYTE G R A N U LOCYT E rran ULO C Y T E CORPUSCLE
(ERYTHROCYTE)
■aV^- ., CENTRAL CELL OF
BLOOD ISL AN 0
ECTODERM
1 MESODERM
, , SPLANCHNIC
■^ -utrlNITIVE \ MESOTHELIUM
r ERYTHROBLASTS m F <; F N r w v u r
ARISING IN BLOOD VESSEL m t s t N u H T M t
YOLK
ENTODERM
PERIPHERAL CELL OF
a, f^^r. ISLAND
BLOOD
Fig. 333. Developing blood cells. (A) Diagram showing origin of different types of
blood cells from the primitive hemocytoblast. (Redrawn and slightly modified from
Patten. 1946. Human Embryology, Blakiston, Philadelphia.) (B) Blood-cell origin in
the yolk-sac area of human embryo. (Redrawn from Bloom and Bartelmetz, 1940. Am.
J. Anat. 67.) (C) Differentiation of blood cells and blood vessel endothelium in a
blood island of chick embryo yolk-sac area.
734
HISTOGENESIS OF CIRCULATORY SYSTEM 735
terrestrial form, Rana temporaria, the bone marrow functions in this capacity
as it does in the adults of reptiles, birds, and mammals. In the adult reptile and
bird, the bone marrow seems to function in the production of all types of blood
cells. In the mammal, the bone marrow possibly elaborates only erythrocytes
and granular, white blood cells, while the lymphocytes probably are produced
in other areas, such as the pharyngeal and palatine tonsils and lymph nodes,
etc. In all vertebrates from the teleost fishes to the mammals, it is probable
that scattered lymphoid tissue in various parts of the body functions in the
formation of lymphocytes.
During the development of the early human embryo and later fetus, the
following have been given as sites of blood-cell formation (Minot, '12;
Gilmour, '41 ) :
(a) yolk sac in embryos up to 3 mm., i.e., the end of the fourth week
of pregnancy,
(b) mitosis of previously formed erythroblasts in general circulation, yolk
sac, and chorion of embryos from 3 to 9 mm. in length,
(c) liver and yolk sac of 10- to 18-mm. embryos. In embryos of 470- to
546-mm. there is a gradual decrease in the liver,
(d) spleen, beginning in the 28-mm. embryo; thymus, and lymph glands
in the 35-mm. and larger embryos,
(e) bone marrow during the third month and later.
3) Characteristics of Development of the Erythrocyte. Most vertebrates
in the adult condition retain the nucleus in the erythrocyte or red blood cell.
To this cell is given the function of carrying oxygen from the site of external
respiration to the body tissues. It also is a factor in conveying carbon dioxide
from the tissues to the site of external respiration. The oxygen-carrying capacity
of the erythrocyte resides in the presence of the compound hemoglobin.
Hemoglobin is a complex protein molecule, containing iron atoms. The iron
atoms make it possible for the hemoglobin to convey oxygen.
In the adults of various amphibian species, there is a tendency for the
red blood cell to lose its nucleus by various means (Noble, '31, pp. 181-182).
This tendency toward loss of the nucleus reaches an extreme form in Batra-
choseps where more than 90 per cent of the red blood cells have lost their
nuclei. In adult mammals, the mature erythrocyte loses its nucleus (column
6, fig. 333 A) but it is retained in the early embryo.
4) Characteristics of Various White Blood Cells. White blood corpuscles
or leukocytes vary greatly in number and in morphological features in all
vertebrates. In general, the following two major groups of white blood cor-
puscles may be distinguished.
a) Granulocytes. Granulocytes are cells which arise from granuloblasts
(columns 3, 4, and 5, fig. 333A). These cells are characterized by the
56 THE CIRCULATORY SYSTEM
resence of an irregularly shaped nucleus and by a cytoplasm which possesses
'anules of various dimensions and staining affinities,
b) Lymphoid Forms. Lymphoid forms are of two types, namely, lympho-
f'tes and monocytes. These cells arise from lymphoblasts and monoblasts
:spectively (columns 1 and 2, fig. 333A). The lymphocytes are small,
)unded cells with a clear cytoplasm and a large nucleus. They are found
I all vertebrates and are abundant especially in fishes and amphibia. Large
jmbers are found in the lymph nodes in various parts of the body. Monocytes
:e similar to the lymphocytes but are much larger and have a tendency to
Dssess an irregularly shaped nucleus. Various hematologists hold that the
lonocyte is a special type of blood cell, distinct from other leukocytes and
" a separate developmental origin.
D. Morphogenesis of the Circulatory System
1. Introduction
The major alterations of the basic arterial and venous conditions into
le morphology present in the adult or definitive body form of the species
;cur during the larval period, or the period of transition from primitive
Tibryonic body form to the definitive or adult form. This fact is true not
[ily of the circulatory system but of all other organ systems as well (Chap.
1). The pronounced changes, therefore, which occur in the revamping of
le basic, generalized condition of the circulatory system during the larval
jriod should be regarded as transformation which adapts the basic embryonic
jndition to conditions which must be met when the developing organism
nerges into the environment of the adult.
2. Transformation of the Converging Veins of the Early
Embryonic Heart into the Major Veins which
Enter the Adult Form of the Heart
a. Alteration of the Primitive Converging Veins of the Heart in the
Shark, Squalus acanthias
An early stage of the developing venous circulation of Squalus acanthias
shown in figure 334A. Only two veins are present, the primitive vitelline
iins. They enter the sinal rudiment of the developing heart. Before the liver
ibes form, the left vitelline vein develops a new venous sprout, the intestinal
iin, which extends caudalward along the lateral aspect of the intestine to
le developing cloacal area (fig. 334B). Here it forms a collar-like venous
ructure around the cloaca and continues back below the tail gut as the
mdal vein. Meanwhile, the anterior, posterior, and common cardinal veins
sgin their development, and the liver also begins to form (fig. 334C). As
le liver develops, two prominent liver lobes are elaborated (Scammon, '13),
id the vitelline veins become surrounded by the developing liver trabeculae
MORPHOGENESIS OF CIRCULATORY SYSTEM 73
(Chap. 13). During this process, the vitelline veins are fenestrated, an(
sinusoids are produced. These sinusoids connect with the right and left vitellin
(hepatic) veins at the anterior end of the liver.
Posterior to the liver, the right and left vitelline veins form a collar aroun(
the duodenum as shown in figure 334C. The left portion of the duodena
collar then disappears, and the hepatic portal vein which receives blood fron
the developing stomach, pancreas, and intestine enters the liver as indicate!
in figure 334D.
As the above development progresses, three important changes are effectet
(fig. 334E):
( I ) The lateral veins along the lateral body wall arise and join the commoi
cardinal veins near the entrance of the right and left vitelline (hepatic
veins;
(2) the intestinal vein loses its connection with the caudal vein; and
(3) the postcardinal veins extend caudally and ct>nnect with the cauda
vein.
Meanwhile, the mesonephric kidneys begin to develop, and new veins, ii
the form of irregular venous spaces, form between the two kidneys. Thesi
new veins are the subcardinal veins. The subcardinal veins are joined by th^
internal renal veins which ramify through the kidney substance from the pos
terior cardinal veins. They course over and around the forming renal tubule
(fig. 334F, G).
Later, the two subcardinal veins extend forward and by means of an anas
tomosis on either side connect with the posterior cardinal veins anterior t(
the mesonephric kidneys. As this transformation occurs, the segment of eacl
posterior cardinal vein atrophies between the kidney and the point where th^
subcardinal venous anastomosis joins the posterior cardinal vein (fig. 334G)
While the above changes evolve, the anterior cardinal veins expand greatl;
over the dorsal pharyngeal area, where they form sinus-like spaces. Thesi
anterior cardinal venous sinuses receive the internal jugular veins from th
brain region and various pharyngeal veins. Coronary veins and externa
jugular veins also develop as shown in figure 334H.
b. Changes in the Primitive Converging Veins of the Heart in the
A nuran A mphihia
1) Vitelline Veins. As in the shark embryo and in all other vertebrates, thi
vitelline veins of the frog or toad embryo are among the first blood vessel
to be formed in the body. In frog embryos of about 3- to 4-mm. in length
the two vitelline veins begin to appear as irregular blood spaces along thi
ventro-lateral aspect of the midgut region, extending anteriad around thi
forming liver. At a point immediately anterior to the liver rudiment, thesi
vessels fuse to form the endocardinal rudiment of the heart (fig. 332I-K)
738 THE CIRCULATORY SYSTEM
Proceeding forward from the heart region, the two primitive subintestinal
blood vessels continue forward below the rudiment of the foregut where they
form the rudiments of the ventral aorta. They diverge and extend dorsad
around the foregut to the dorsal area of the foregut. These vessels which
thus pass around the foregut represent the third pair of aortal arches, i.e.,
the first pair of branchial aortal arches (fig. 335A). The first branchial aortal
arches join the forming dorsal aortae. The dorsal aortae form first as irregular
blood spaces, extending along the primitive gut from below the forming brain
posteriad to the midgut area. Here they diverge to give origin to the vitelline
arteries which ramify over the yolk substance of the midgut and there anas-
tomose with branches of the vitelline veins.
About the time of hatching, the two vitelline veins become enmeshed in
the substance of the developing liver, and the vitelline veins gradually become
divided into three groups (fig. 335B):
(a) a right and left vitelline vein between the liver and the sinus venosus
of the heart,
(b) the veins within the liver which form an irregular meshwork, and
(c) the two vitelline veins, posterior to the liver substance.
The left vitelline vein, anterior to the liver, soon atrophies and becomes
fused with the right vitelline vein as indicated in figure 335C and D. The right
vitelline vein thus receives the hepatic veins. Within the liver substance, the
two vitelline veins break up into smaller veins to form ultimately the sinusoids
of the liver (fig. 335C). Posterior to the liver, the vitelline veins form the
hepatic portal and intestinal veins (fig. 335C).
2) Lateral (Ventral Abdominal) Veins. The lateral veins form first as two
minute veins, which extend posteriad from the lateral ends of the sinus venosus
of the heart. Eventually they unite with the iliac veins as shown in figure 335D.
Fig. 334. The developing venous system in Squalus acanthias. (Modified from Hoch-
stetter, '06.) (A) An early stage in the development of the venous system. The two
primitive vitelline veins only are present. (B) Later stage in development of vitelline
veins. (C) Early stage in development of hepatic portal system. A venous ring is
formed around the duodenum. Anterior and posterior cardinal veins are evident. (D)
Later stage in the hepatic-portal system development. Left segment of duodenal collar
has disappeared. Observe that the efferent hepatic veins (V. hepaticae revehentes) repre-
sent the right and left vitelline veins between the liver lobes and the sinus venosus,
whereas the afferent hepatic veins (V. hepaticae advehentes) are the vitelline veins just
posterior to the liver. (E) Lateral veins make their appearance. Posterior cardinal veins
join veins around the cloacal area and thus assume responsibility for venous drainage
of the tail region, and the intestinal vein in consequence loses its connection with the
caudal vein of the tail. (F) Subcardinal veins appear between the kidneys. (G) Sub-
cardinal veins make connection with posterior cardinal veins. Posterior cardinal veins
regress anterior to the mesonephric kidneys where the posterior cardinal and subcardinal
veins anastomose. (H) Mature plan of the venous system showing the converging veins
of the heart. Hepatic portal vein omitted.
Fig. 334. (See facing page for legend.)
739
740 THE CIRCULATORY SYSTEM
Anteriorly, the two lateral (ventral abdominal) veins lose their connection
with the sinus venosus and merge together to form one ventral abdominal
vein; the latter acquires a connection with the hepatic portal vein near the
liver. A ventral abdominal circulation is established thus between the hepatic
portal system and the iliac veins (fig. 335E, F).
3) Formation of the Inferior Vena Cava. The inferior vena cava is a vessel
not found in the venous system of the developing shark. It is a blood vessel
associated with and characteristic of lung breathers. As such, the inferior
vena cava appears first among the vertebrates in the lungfishes (Dipnoi) and
it functions to shunt the blood from the posterior regions of the body over
to the right atrial portion of the heart. That is, the inferior vena cava is a
vessel correlated with the division of the heart into two parts. One part is
devoted to getting the non-oxygenated systemic blood into the lung region,
while the other part functions to propel the aerated blood from the lungs into
the head region and other parts of the body. This division of labor within
the heart is not necessary in strictly gill-breathing fishes, such as the sharks
and teleosts, and, in consequence, an inferior vena cava is not developed in
these vertebrates.
The formation of the inferior vena cava in the anuran amphibia is shown
in figure 335C-G and need not be explained further. It is to be observed that
it forms from four segments:
(1 ) a right vitelline vein,
(2) an hepatic segment,
(3) a segment which extends posteriad from the liver to the fused sub-
cardinal vein, and
(4) the subcardinal vein. (Consult figure 335E.)
4) Formation of the Renal Portal System. The renal portal system is inau-
gurated among the cartilaginous fishes (i.e., the shark group). It does not
exist in cyclostomes. As shown in figure 334 relative to the developing shark
embryo, it results from the formation of the subcardinal veins, accompanied
by the obliteration of the anterior portions of the posterior cardinal veins.
Fig. 335. Developing venous vessels in the anuran amphibia. (B-G, redrawn and
modified from Kampmeier, 1920, Anat. Rec. 19; H, redrawn from Kampmeier, 1925,
J. Morph. 41; I, redrawn from Goodrich after Kerr, 1930, Studies on the Structure and
Development of Vertebrates, Macmillan, Ltd., London.) (A) Primitive plan of early
circulation in frog embryo. The relationship of the primitive venous system shown in
B to the rest of the vascular system is evident. (B) Plan of venous system of 4 mm.
embryo of the toad, Bufo vulgaris. (C) Plan of venous system of 6 mm. embryo of
the toad, Bufo vulgaris. (D) Plan of venous system of 15 mm. embryo of the toad,
Bufo lentiginosus. (E) Plan of venous system of 18 mm. embryo of the toad, Bufo
lentiginosus. (F) Plan of venous system of young toad of Bufo lentiginosus, immedi-
ately after metamorphosis. (G) Plan of venous system of mature Rana pipiens. (H)
Left posterior lymph hearts of an adult Rana pipiens. (I) Internal structure of mature
frog heart.
Fig. 335. {See facing page for legend.)
741
742 THE CIRCULATORY SYSTEM
The blood from the tail and posterior trunk region of the body thus must
pass through the small blood vessels within the kidney substance. Here waste
materials and excess water are extracted before the blood is passed on to the
heart and aeration systems. The renal portal system is developed exceptionally
well in the embryos and adults of fishes and amphibia. It is inadequately de-
veloped in the adult reptile, and it is questionable whether or not the poorly
developed, renal portal system functions in the adult bird. The adult mammal
does not possess this system. However, the embryos of all reptiles, birds,
and mammals possess a renal portal system wherein blood is shunted through
the kidney substance from the posterior cardinal veins into the subcardinal
complex. It is a most transient affair in the mammalian embryo. The devel-
opment of the renal portal system in the anuran embryo is shown in figure
335C-E. Observe that pronephric and mesonephric renal portal systems are
developed.
5) Precaval Veins. The formation of the precaval veins is shown in figure
335B-G. It is to be observed that the common cardinal veins become trans-
formed into the anterior or precaval veins, while the anterior cardinals persist
as the internal jugular veins.
c. Changes in the Primitive Converging Veins of the Heart in the Chick
1) Transformation of the Vitelline and Allantoic Veins: a) Vitelline
Veins. The vitelline veins in the developing chick first make their appearance
as two delicate capillaries, one on either side of the inner wall of the anterior
intestinal portal in blastoderms of 26-28 hours of incubation. At this time
there are about four pairs of somites present. These minute blood vessels are
intimately associated with the entoderm of the anterior intestinal portal, and
eventually come to lie side by side immediately below the foregut as the anterior
intestinal portal recedes caudally. At about 27-29 hrs. of incubation, or when
the embryo has about five to six pairs of somites, the two splanchnic layers
of the hypomeric mesoderm, in the area where the heart is to form, begins
to cup around and enclose the two vitelline capillaries (fig. 332L). A little
later, at about 29-33 hrs. of incubation, these two splanchnic mesodermal
layers begin to fuse above and below the vitelline capillaries (fig. 332M).
At 33-38 hrs. of incubation, or when nine to ten pairs of somites are present,
a simple, tubular heart is present which contains the rudiment of the endo-
cardium within in the form of the two fused or fusing vitelline capillaries.
This endocardial rudiment is enclosed by the hollow, tube-like epimyocardial
rudiment derived from the fused layers of splanchnic mesoderm (fig. 336A).
At about 33-38 hrs. of incubation (fig. 336A), the primitive circulatory
system consists of the following;
( 1 ) Two vitelline veins which converge to enter the forming heart just
anterior to the intestinal portal;
(2) the primitive tubular heart;
MORPHOGENESIS OF CIRCULATORY SYSTEM 743
(3) two delicate capillaries, the future ventral aortae, course anteriad from
the heart below the foregut. As the ventral aortae approach the anterior
limits of the foregut they diverge and travel dorsad as the mandibular
aortal arches, one on either side of the gut tube, to the dorsal region.
In the dorsal area of the foregut the mandibular aortal arches become
continuous with
(4) the dorsal aortae. These two delicate vessels lie upon the foregut on
either side of the notochord, and extend caudalward into the region
of the developing midgut.
During the period of 40 to 50 hours of incubation the following changes
occur in the above system (fig. 336B and B'):
( 1 ) The rudimentary vitelline arteries extend outward over the yolk-sac
area from the dorsal aortae, forming many small capillaries.
(2) The anterior and posterior cardinal veins and connecting interseg-
mental veins are established and unite with the sinus venosus by means
of the common cardinal vein (fig. 336B').
(3) The vitelline veins extend outward over the blastoderm and continue
anteriorly around the head area as the anterior vitelline veins. The
latter veins unite with the circumferential blood sinus. A complete
circulation through the embryo and out over the yolk-sac area is thus
effected.
During the early part of the third day of incubation the right and left vitelline
veins begin to fuse in the area just posterior to the heart. This fusion forms
a single vein, the ductus venosus (fig. 337A). The latter structure joins the
sinus venosus of the heart. Posteriorly, the vitelline veins make a secondary
connection with the developing posterior vitelline or omphalomesenteric veins
which extend backward along the sides of the midgut to the area where the
vitelline arteries leave the dorsal aortae. At this point each omphalomesen-
teric vein turns sharply laterad and courses along the pathway of a vitelline
artery (fig. 336C).
At the end of the third day of incubation the ductus venosus is present as
an elongated structure lying between the anterior intestinal portal and the
heart. A posterior vitelline vein continues posteriad from the ductus venosus
around each side of the anterior intestinal portal (fig. 336D). As observed
in Chapter 13, during the third and fourth days of incubation the liver rudiment
begins to form. In doing so, the trabeculae of the liver surround the ductus
venosus. The immediate segment of the ductus venosus which becomes sur-
rounded by the forming liver substance forms the meatus venosus. As devel-
opment of the liver proceeds, two main groups of veins develop in the liver
substance (fig. 337B, D): (1) An anterior efferent group of hepatic veins
which drain blood from the liver and (2) a posterior afferent set of hepatic
CXORSAL AORT
DUCT OF C
OMPHALOMESENTERIC
IVITELLINEl
ARTERY
Fig. 336. Early development of the circulatory system in the chick. (A) Primitive
vitelline (omphalomesenteric) veins, heart, ventral aorta, and the first or mandibular
pair of aortal arches. About stage 10 of Hamburger and Hamilton, 1951, J. Morph. 88.
Approximately 33-38 hrs. of incubation. (A') Lateral view of same. (B) Lateral
view of chick circulatory system of about 45-50 hrs. of incubation. (About Hamburger
and Hamilton stage 13.) (B') Same, showing common cardinal vein (duct of Cuvier).
(C) Circulatory system of chick during early part of third day of incubation. (About
Hamburger and Hamilton stage 15.) (D) Circulatory system of chick embryo about
72 hrs. incubation.
744
MORPHOGENESIS OF CIRCULATORY SYSTEM 745
veins, representing branches of the hepatic portal vein. The latter brings blood
from the stomach and intestinal areas to the liver.
During the fifth to seventh days of incubation, the afferent and efferent
sets of hepatic veins develop profuse branchings, and venous sinusoids are
formed within the liver substance between these two sets of veins. Meanwhile,
the meatus venosus within the liver atrophies and a complete hepatic portal
system is established between afferent and efferent hepatic veins during the
seventh and eighth days of incubation as shown in figure 337E.
While the above changes in the liver are emerging, changes in the omphalo-
mesenteric veins, posterior to the liver substance, are produced as shown in
figure 337A-E. By the fifth day, a new vein, the mesenteric vein, is formed
(fig. 337D), which begins to drain blood from the developing midgut and
hindgut areas. By the eighth day, the mesenteric vein is a prominent structure
(fig. 337E). At this time, the blood from the yolk sac, via the omphalo-
mesenteric veins, and that from the mesenteric vein must pass through the
liver sinusoids en route to the efferent hepatic veins (fig. 337E).
b) Allantoic Veins. The two allantoic or lateral veins begin to develop
during the third day of incubation, and, by the end of this day, two delicate
blood vessels extend along the lateral body wall, reaching back toward the
hindgut area (figs. 336D; 337B). During the fourth day (fig. 337C), the
caudal ends of the two allantoic veins begin to ramify within the walls of
the allantois. A secondary attachment to the hepatic veins within the liver
is established also at this time (fig. 337C). During the late fourth day and
the fifth day of incubation, the right allantoic vein degenerates, and the
proximal portion of the left allantoic vein loses its connection with the common
cardinal vein (fig. 337D), During the seventh and eighth days (and until
the time of hatching), the passage of blood from the allantois through the
liver to the vena cava inferior is as indicated in figure 337E. The portion of
the allantoic vein extending anteriorly from the umbilical area to the liver
persists after hatching and drains blood from the midventral portion of the
body wall. It is called the epigastric vtMs (fig. 3371).
2) Formation of the Inferior Vena Cava. The formation of the inferior
vena cava of the chick is shown in figure 337F-I and needs no other ex-
planation. It is to be observed that, following the degeneration of the meso-
nephric kidneys and the ascendancy of the metanephric kidney, the passage
of blood by way of the renal portal system through the mesonephric kidney
is abated. In the newly hatched chick, a much-weakened, renal portal system
is established via the renal portal vein (fig. 3371: However, most of the
blood through this vein passes directly into the common iliac vein and not
through the kidney substance.
3) Development of the Precaval Veins. The precaval veins are the direct
descendants of the anterior cardinal and common cardinal veins as indicated
in figure 337F-1. In figure 3371, it is to be observed that the caudal ends of
21 DAVS
CiuDiLVElN
Fig. 337. Ventral views of developing allantoic, hepatic portal, and inferior caval veins
in chick. (Diagrams C and D are adapted from figures in Lillie, 1930, The Development
of the Chick, Henry Holt, N. Y., after Hochstetter; diagrams F-H are adapted, consider-
ably modified, from Miller, 1903, Am. J. Anat. 2.) (A) Diagram of converging veins
of heart during early third day of incubation. (B) Same at end of third and early
fourth days. (C) Middle fourth day. (D) End of fourth and early fifth days. (E)
Seventh to eighth days. (F) Development of inferior vena cava at end of fourth and
beginning of fifth day of incubation. (G) Same, 6 7 days. (H) Same, fourteenth day.
(I) Same at about hatching time, 20-21 days.
746
MORPHOGENESIS OF CIRCULATORY SYSTEM 747
the posterior cardinal system function to drain the blood from the caudal
end of the body and posterior appendages, while the anterior cardinal veins
and common cardinal veins function to drain the blood from the head, neck,
and forelimb areas.
d. The Developing Converging Veins of the Mammalian Heart
(e.g., Human)
The formation of the hepatic portal system in the human embryo is shown
in figure 338G, H, and that of the inferior and superior venae cavae is
shown in figure 338 A-F. The general principles of venous development, de-
scribed in the previous pages of this chapter, apply here, and descriptive
matter is not needed to supplement the accompanying figures. It is worthy
of mention, however, that two additional veins are introduced in the abdominal
area of the embryo, namely, the two supracardinal veins. These veins persist
as a part of the vena cava inferior and azygos veins. Anteriorly, the two
precavae, so prominent in the lower vertebrates, including the birds, are dis-
placed partially by the formation of an anastomosing vein from the left to
the right side with the dropping out, to a considerable extent, of the proximal
portion of the left precava. Thus, the common cardinal vein on the right
side comes to function as the proximal portion of the single superior or
anterior vena cava, while the common cardinal vein on the left side comes
to form the coronary sinus of the heart, and occasionally as a variant, the
oblique vein of the left atrium.
3. Development of the Heart
a. General Morphology of the Primitive Heart
In the vertebrate group, two types of hearts are present, namely, lymph
hearts (fig. 335H) and the heart of the arteriovenous system. The heart of
the arteriovenous system is a centralized, well-muscularized mechanism, placed
ventral to the esophageal segment of the gut in the anterior extremity of the
coelomic cavity. Its function is to receive blood from the veins of the body
and to propel it forward toward the anterior or head region. Fundamentally,
the embryonic heart of the arteriovenous system is a tubular affair, composed
of four segments:
( 1 ) a thin-walled sinus venosus or caudal portion of the heart, connect-
ing with a series of converging veins,
(2) the atrium, a segment lying anterior to the sinus,
(3) the ventricle, lying anterior to the atrium, and
(4) the bulbus cordis.
The ventricle and, to some extent, the bulbus cordis of the embryonic heart
later develop the structures which act as the main propulsive mechanism of
the heart, while the sinus and atrium give origin to the blood-receiving areas.
748 THE CIRCULATORY SYSTEM
b. The Basic Histological Structure of the Primitive Embryonic Heart
Structurally, the embryonic heart is composed of two parts. An inner
delicate lining, the rudiment of the endocardium, forms as a result of the
fusion of the vitelline blood capillaries in the immediate area of the forming
heart. The endocardium thus is composed of endothelium (fig. 332F-M). Sur-
rounding the endocardial rudiment, there is the epimyocardium derived from
the ventro-mesial portions of the hypomeric (splanchnopleural) mesoderm
which extends ventrally from the foregut in this area (fig. 332F-M). Basi-
cally, the mesial walls of the two hypomeric areas of the mesoderm which
lie below the foregut in this region constitute the ventral mesentery of the
primitive gut. Consequently, the epimyocardium of the primitive heart may
be regarded as modified ventral mesentery. That portion of the ventral mesen-
tery which is dorsal to the forming heart forms the dorsal mesocardium,
while that part which extends ventrally below the heart forms the ventral
mesocardium. The latter is a transient structure, no sooner formed than
obliterated in most instances. The dorsal mesocardium tends to persist for a
time, more in some species than in others. Caudally, the posterior lateral
areas of the sinus venosus project the splanchnopleural mesoderm laterally
to contact the lateral somatopleural mesoderm with which the splanchnopleural
mesoderm fuses. This outward extension of the caudo-lateral edges of the
sinus venosus produces a bridge across the coelomic space from the lateral
body wall to the sinus venosus. These bridges on either side across the
primitive coelom form the lateral mesocardia. Through these mesocardial
bridges, the common cardinal veins empty their contents into the heart.
Fig. 338. Changes in the converging veins of the heart in the mammalian embryo.
(Redrawn and modified from Patten, 1946, Human Embryology, Blakiston, Philadelphia,
after McClure and Butler.) (A-F) Developmental changes in converging veins of the
human heart. Primitive converging veins of the heart shown in black; hepatic segment
of inferior vena cava shown in white with coarse stipple; subcardinal veins shown in
light stipple; supracardinal veins in white with crossed lines. (Note: the author assumes
the responsibility for adding a vitelline venous segment to the anterior end of the devel-
oping inferior vena cava. As a result of observations on developing pig, cat, and opossum
embryos, the author is convinced that a vitelline segment is contributed to the developing
posterior vena cava in the mammal.) (A) Primitive basic condition. (B-F) Later
stages as indicated. (F) Adult condition. The following contributions appear to enter
into the formation of the inferior vena cava, viz., (I) a very short vitelline segment;
(2) an hepatic segment; (3) an anastomosis between the hepatic segment and the sub-
cardinal interrenal anastomosis; (4) a subcardinal-supracardinal anastomosis; (5) a
right supracardinal segment caudal to the kidneys; and (6) a posterior cardinal contri-
bution in the pelvic area. Note also that the uzygos vein is formed from the anterior
end of the right posterior cardinal vein plus the right supracardinal with its connections
with the hemiazygos vein. Observe further that the superior vena cava is composed of
the right common cardinal vein from the area of juncture with the azygos vein to the
point of its entrance into the right atrium. (G-J) Formation of the hepatic portal vein
in the pig. (Redrawn and slightly modified from Patten, 1948. Embryology of the Pig,
Blakiston, Philadelphia.
Fig. 338. (See facing page for legend.)
749
750 THE CIRCULATORY SYSTEM
c. Importance of the Septum Transversum to the Early Heart
There is another structure which is important to the primitive embryonic
heart and to its later development. This structure is the primary septum
transversum or the mesodermal partition which forms across the coelomic
cavity, below (ventral) to the lateral mesocardia. It forms not only a par-
tition or bulwark, separating the developing liver substance from the primitive
heart, but it is also a suspensory ligament for the caudal end of the sinus
venosus and the converging veins of the heart. (See Chap. 20.)
d. Activities of Early-Heart Development Common to All Vertebrates
The early stages of heart development, following the formation of the basic
rudiments mentioned above, are essentially the same for all vertebrates.
These changes, which result in the formation of a sigmoid or S-shaped struc-
ture, are as follows (see figs. 336, 339):
(1) The dorsal mesocardium soon disappears for most of its extent, and
the primitive heart tube begins to elongate and to change its shape
rapidly.
(2) The ventricular portion bends ventraliy and to the right and, at the
same time, grows posteriad, becoming thick-walled.
(3) The atrial area expands laterally, grows forward dorso-anteriad over
the ventricular area; and at the same time forms two lateral lobes.
(4) The sinus venosus remains thin walled and rigidly attached to the
septum transversum. The latter, in all vertebrates above the fishes,
bends forward along its upper margins during the early period of
development.
(5) The bulbus cordis extends slowly and becomes a thickened anterior
continuation of the heart from which arise the ventral aortic roots.
e. Development of the Heart in Various Vertebrates
From the generalized, S-shaped, basic condition, the hearts of the various
vertebrate groups begin to diverge in their development as follows:
1) Shark, Squalus acanthias. Starting as a straight tube when the embryo
is 5.2 mm. long (fig. 339A), the ventricular portion begins to bend toward
Fig. 339. Early stages in morphogenesis of various vertebrate hearts. (A-C) Stages
in heart development in Squalus acanthias. (Redrawn from Scammon, 1911, Chap. 12,
in Normentafeln Entwichlungsgeschichte der Wirbeltiere by F. Keibel, G. Fischer, Jena.)
(D-F') Heart development in the frog, Ratui pipiens. (D-F) Left lateral views; (F')
ventral view. (G-K) Heart development in the chick, ventral views. (H-K, redrawn
from Kerr, 1919, Text-Book of Embryology, vol. II, Macmillan and Co., Ltd., London,
after Greil.) (L-O) Heart development in the human embryo, ventral views. (Redrawn
from Kramer, 1942, Am. J. Anat. 71. L, after Davis, modified; M, after Tandler,
modified; N, after Waterston, modified.) Observe that ventricular end of the original
bulbus cordis, i.e. the conus portion, contributes to the right ventricle in diagrams N and O.
VENTHICULAR
VENTRAL AREA
AORTA
Fig. 339. {See facing page for legend.)
751
752 THE CIRCULATORY SYSTEM
the right and ventrad in the embryo of 7.5 mm. At 15 mm., the heart appears
as indicated in figure 339B, while, at 20.6 mm., it assumes the general ap-
pearance of the adult form (fig. 339C). It is to be noted that the ventricular
portion of the heart does not bend as dramatically toward the right as in
the chick or mammalian heart. In the embryo of 37 mm., the heart already
has attained the characteristics of the adult form. The following develop-
mental features are present. The bulbus cordis has transformed into the
anterior contractile chamber, the conus arteriosus; the ventricular area has
developed a pronounced musculature; the atrium is thin walled and bilobed,
while the sinus venosus is cone shaped with its base applied against the
septum transversum. Right and left valves guard the sinu-atrial entrance.
A series of semilunar or pocket valves are arranged around the atrioventricular
orifice, while, more anteriorly, cup-shaped valves are forming in transverse
rows along the inner walls of the conus arteriosus.
2) Frog, Rana pipiens. At 4Vi mm. in length, the heart is present as a
simple straight tube (fig. 339D). At 5 mm., it begins to bend, the ventricular
area moving ventrad and toward the right, and the atrial area and sinus
venosus moving anteriad over the ventricular area (fig. 339E). At 7 mm.,
the heart has assumed the typical S-shaped condition of the adult form, and
constrictions appear between the atrium and ventricle (fig. 339F). At this
time, also, a median septum begins to divide the atrial chamber. The atrial
septum begins as a fold from the antero-dorsal wall of the atrium and grows
ventrad and posteriad to divide the atrium into a larger right atrium and a
smaller left artium. Moreover, as the atrial septum is developed, it forms to
the left of the opening of the sinus venosus into the atrium. Therefore, in
the 8- to 10-mm. tadpole, the opening of sinus venosus into the atrium is
entirely restricted to the right atrium, and the flow of venous, systemic blood
is directed toward the right side of the heart. At about this time, also, the
formation of the vena cava inferior proceeds rapidly. (See fig. 335.) At 8
to 10 mm., the lung buds (Chap. 14) expand rapidly, and the pulmonary
veins begin to bring back blood from the lungs. The pulmonary veins empty
into the left atrium (fig. 257B).
During the late tadpole stages and metamorphosis, internal changes occur
which transform the heart into a complicated mechanism, designed to separate
and project the oxygenated blood anteriad toward the head and into the
systemic vessels; the non-oxygenated blood from the sinus venosus passes
into the pulmocutaneous arteries. These different blood currents within the
heart are made possible largely by the modification of the internal walls of
the primitive bulbus cordis into the highly complicated mechanism of the
contractile conus arteriosus. Aside from a series of small pocket valves, the
dorsal wall of the conus forms an elongated spiral valve which functions
to separate its channel into two parts. The non-oxygenated blood is projected
dorsally to the spiral valve and into the pulmocutaneous vessels by the spiral
MORPHOGENESIS OF CIRCULATORY SYSTEM 753
valve, while the oxygenated blood passes ventrally to the spiral valve and
into the arteries coursing toward the head and into the systems (fig. 3351).
This condition of the conus is present also in urodeles with well-developed
lungs, but, in urodeles without well-developed lungs, the spiral valve is absent
and the interatrial septum may regress (Noble, '31, pp. 187-194).
3) Amniota. The heart of reptiles, birds, and mammals differs from the
heart of the Amphibia in that a mechanism is present which separates, more
or less completely, the oxygenated blood from the non-oxygenated blood.
For example, the heart of birds and mammals is a four-chambered affair as
an interventricular septum divides the primitive ventricle into two separate
compartments while an interatrial septum separates the primitive atrium into
two atria. A double heart is produced in this manner wherein the non-
oxygenated blood returning from the organ systems passes through the right
atrium and ventricle en route to the lungs while the oxygenated blood from
the lungs journeys through the left atrium and ventricle on its way back to
the organ systems. In the heart of birds and mammals, it is to be observed
also, that only two arterial channels convey blood from the heart; namely,
a pulmonary arterial trunk and a systemic arterial trunk. Another feature is
present in the heart of the birds and mammals which serves to distinguish it
from the hearts found in all lower vertebrates, in that the sinus venosus is
absorbed almost entirely during embryonic development into the wall and
structure of the right atrium.
Turning now to a consideration of the hearts of reptiles we find that the
turtles and snakes possess a heart with two atria and a ventricular region
divided rather completely into two ventricles. However, the interventricular
septum is slightly incomplete in the region near the atria, and some leakage
of blood between the two ventricles is possible. In the crocodilians the inter-
ventricular septum is completely developed, but a small opening, the foramen
of Panizza, is present at the bases of the two systemic arterial trunks. This
foramen arises as a secondary perforation later in development and does not
represent an incompleteness of the interventricular septum. In the reptilian
heart the sinus venosus retains its identity as a separate chamber of the heart.
Furthermore, contrary to the conditions found in the avian and mammalian
heart, three arterial trunks convey blood away from the ventricles. Two of
these vascular trunks come from the right ventricle, and one from the left
ventricle, for a pulmonary trunk conveys blood from the right ventricle to
the lungs, while a systemic aortic root also carries blood from the right ventricle
to the abdominal aorta. From the left ventricle, on the other hand, blood is
propelled through a single aortic root to the head, forelimbs, and abdominal
aorta (fig. 341H).
a) Heart of the Chick. The heart arises as a simple tube during the
second day of incubation (fig. 339G). At the end of the second day and
during the third day, the primitive ventricle bends to the right, and the atrium
754 THE CIRCULATORY SYSTEM
begins to travel forward above the ventricle (figs. 336C; 339H'). At the end
of the third day, the heart attains the typical sigmoid or S-shaped condition
which arises as the first major step in heart development in all vertebrate
embryos. During the fourth day of incubation, the atrial area expands into
two main lobes, the beginnings of the right and left atria; the ventricular
area expands greatly and thickens; and the bulbus cordis lies in the median
line between the developing atria (fig. 3391). The position of the various
parts of the heart on the whole assumes more nearly the adult condition.
Internally, toward the end of the fourth day, an interatrial septum begins
to develop from the dorso-anterior area between the two atrial lobes, slightly
to the left of the opening of the sinus venosus. The septum continues to form
posteriad toward the narrowed atrio-ventricular opening between the atria
and the forming ventricles. Simultaneously in the atrioventricular opening, two
endocardial thickenings, the endocardial cushions, arise, one dorsal and one
ventral. At the apex of the ventricle, an interventricular septum appears and
grows forward toward the atrioventricular opening (fig. 340G).
During the fijth and sixth days, the two endocardial cushions grow together
and separate the atrioventricular canal into two passageways by the formation
of a cushion septum. The atrial septum grows toward the endocardial cushion
area and unites with the cushion septum. However, the atrial septum never
is completed during embryonic life, as small openings or fenestrae, appear in
the septum permitting blood to pass through the septum. During the last week
of incubation, the fenestral openings in the atrial septum become much smaller
and completely close shortly after hatching. The ventricular septum, mean-
while, grows forward to unite with the cushion septum. Up to the fifth day,
but one passageway leaves the heart via the developing bulbus cordis and
ventral aorta. However, during the fifth day, beginning at the area just anterior
Fig. 340. Early stages in morphogenesis of various vertebrate hearts (Continued).
(A-E) Internal changes in the developing heart of the pig. (A-D, redrawn from Patten,
1948. Embryology of the Pig, 3d edit., Blakiston, rn. idelphia.) (A) Diagram of 3.7
mm. pig embryo heart, ventral wall removed. (B) Similar diagram of 6 mm. pig heart.
(C) Similar diagram of 9.4 mm. pig heart. (D) S'milar diagram of dissected pig fetal
heart shortly before birth. (E) Schematic drawing of dis5 ,cted 18 mm. pig heart
viewed from right side with walls of right atrium and right v;ntricle removed. Observe
that the bulbus cordis has divided into two vascular trunks. (F) Dorsal aspect of the
heart of an 11 wk. (60 mm.) human embryo. (Redrawn and modified from Patten,
1946. Human Embryology, Blakiston, Philadelphia.) The contraction wave of the heart
beat is indicated by heavy arrows. Starting at the sinus node situated in the dorsal wall
of the right atrium, the contraction wave spreads over the atrial walls and also to the
atrioventricular node located in the atrial septum from whence it travels distally through
the ventricular tissue. (G) The developing chick heart, of about 6-7 days. Right walls
removed to show developing cardiac septa. The ventricular septum is still incomplete,
and the atrial septum is fenestrated. (This figure has been modified considerably from
Kerr. 1919. Text-Book of Vertebrate Embryology, vol. II, Macmillan, Ltd., London,
after Greil.) (H) Adult heart of the South American lung fish, Lepidosiren paradoxus,
right side removed. (Redrawn from Robertson, 1913. Quart. J. Micros. Sci., 59.)
RIGHT ATRIUM
NTERATRIAI.
OPENING
LEFT
ATRIUM
Fig. 340. (See facing page for legend.)
755
756 THE CIRCULATORY SYSTEM
to the sixth pair of aortal arches, a spiral septum begins to form within the
caudal portion of the ventral aortal sac and the bulbus cordis. This septum
grows caudalward within the bulbus in a spiral manner, separating the pul-
monary trunk ventrally and the root of the systemic aorta dorsally. It con-
tinues backward toward the interventricular septum and there unites with
a similar septum at the caudal end of the bulbus. The original bulbus cordis
thus becomes divided at about the seventh day of incubation into two separate
vessels which course spirally around each other, namely, a pulmonary trunk
which unites with the right ventricle and an aortal root which is continuous
with the left ventricle (fig. 339J).
Coincident with the above changes, the valves of the heart are developed.
As the spiral septum is developed in the region of the bulbus cordis, three
semilunar or cup-shaped valves appear at the base of each of the divisions of
the bulbus. That is, at the base of the aortic root and also at the base of the
pulmonary trunk. These valves prevent the backward flow of the blood from
the aortic root into the left ventricle and from the pulmonary trunk to the
right ventricle. When the original atrioventricular opening is divided into two
atrioventricular openings by the formation of the cushion septum, the atrio-
ventricular or cuspid valves are formed in the two atrioventricular openings.
These valves prevent the backflow of blood into the atria from the ventricles.
At the opening of the sinus into the right atrium, the right and left sides of
the opening enlarge and produce folds which project inward into the atrium.
These folds form the sinu-atrial (sinu-auricular) valves. During the last week
of incubation, a third valve, the Eustachian valve or sinus septum, arises as
a fold from the dorsal aspect of the sinus which projects into the right atrium
between the openings of the vena cava inferior and the right and left venae
cavae superior (precavae). It divides the sinu-atrial opening.
As hatching time approaches, the sinus becomes incorporated almost com-
pletely into the walls of the right atrium. A small portion of the sinus probably
is incorporated into the cardiac end of the left precaval vein. The sinu-atrial
valves also disappear and the fenestrae of the atrial septum gradually close.
b) Mammalian Heart; 1) Early Features. The early development of
the mammalian heart (fig. 339) follows the general pattern of the developing
heart of lower vertebrates. A primitive tubular heart composed of a sinus
venosus, atrium, ventricle and bulbus cordis is evolved. This simple tubular
heart is followed by a typical sigmoid-shaped structure in which the two
atrial lobes hang ventrally, one on either side of the bulbus cordis, while the
ventricular region projects caudo-ventrally (fig. 339N). The sinus venosus
is much smaller, relatively speaking, than that formed in lower vertebrates and
tends to be placed toward the right side of the heart in relation to the future
right atrium. By the fifth and sixth weeks in the human (fig. 3390), the
heart attains outwardly the general appearance of the four-chambered heart.
2) Internal Partitioning. The internal divisions of the heart begin to appear
MORPHOGENESIS OF CIRCULATORY SYSTEM 757
in the human at about the fifth week, and in the pig at about 4 mm. or 17
days. This process is similar in the human and the pig, and while the following
description pertains particularly to the pig it may be applied readily to the
developing human heart. In the pig, as in the chick, a crescentic fold or septum
of the atrial chamber begins to grow caudally toward the atrioventricular
opening from the antero-dorsal region of the atrium. This septum forms the
septum primum or interatrial septum I (fig. 340A). As this septum grows
caudad, two thickenings, the endocardial cushions, one dorsal and one ventral,
arise in the atrioventricular opening (fig. 340B). The endocardial cushions fuse
and divide the atrioventricular canal into two openings. The septum primum
ultimately joins and fuses with the endocardial cushions, but the septum as
a whole is incomplete, an interatrial opening being present (fig. 340C). Mean-
while, the sinus venosus shifts more completely toward the right atrium, and
the opening of the sinus into the right atrium also shifts dextrally. This permits
an enlarged area to appear between the interatrial septum and the valvulae
venosae, or valves of the sinus venosus guarding the sinu-atrial opening. In
this area, interatrial septum II or septum secundum, arises as a downgrowth
from the atrial roof (fig. 340C, D). This second septum eventually produces
a condition as shown in figure 340D. The arrow denotes the passageway or
foramen ovale in the septum secundum and also the outlet for the blood into
the left atrium over the dorsal part of the valve of the foramen ovale (valvula
foraminis ovalis), derived from the atrioventricular end of septum I. This
condition persists until birth. The valve of the foramen ovale derived from
septum I prevents the backflow of blood from the left atrium into the right
atrium.
The atrioventricular valves are shown also in figure 340D, together with
the fibrous attachments of these valves to the muscular columns of the left
and right ventricles. The atrioventricular or cuspid valves arise as thickened,
shelf-like growths of connective tissue, to which the tendinous cords from the
papillary muscles become attached. The left and right ventricles are produced
as in the chick by the upgrowth from the ventricular apex of the interventricular
septum. In the human, the interventricular septum fuses with the endocardial
cushions during the eighth to ninth weeks. The papillary muscles projecting
inward into the ventricular cavities (fig. 340D) represent modifications of the
trabeculae carneae (fig. 340B).
3 ) Fate of the Sinus Venosus. The developing superior and inferior venae
cavae open into the right horn of the sinus venosus. As the right atrium enlarges
it absorbs this right horn mto its walls and the venae cavae obtain separate
openings into the right atrium (fig. 340D). The body of the sinus venosus
becomes the coronary sinus which opens into the right atrium below the
opening of the inferior vena cava. The coronary veins empty into the coronary
sinus. The left horn of the sinus venosus may persist as a part of the oblique
vein of the left atrium (fig. 340F).
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MORPHOGENESIS OF CIRCULATORY SYSTEM 759
4) The Division of the Bulbus Cordis (Truncus Arteriosus and Conns).
The division of the bulbus cordis occurs synchronously with the above changes.
Two internal ridges opposite each other are formed during this process. These
ridges fuse and divide the bulbus in a spiral fashion into a dorsal aortic root
and a pulmonary trunk as indicated in figure 340E. The pulmonary trunk
opens into the right ventricle, and the aortic root opens into the left
ventricle. Three cup-shaped, semilunar (pocket) valves are developed from
internal ridges in the areas between the base of the aortic trunk and the left
ventricle and between the base of the pulmonary trunk and the conus portion
of the right ventricle (fig. 340E).
4. Modifications of the Aortal Arches
When the heart begins to form, its position is ventro-posteriorly to the
developing pharyngeal area. As the pharyngeal region enlarges, the heart
recedes, relatively speaking, and moves caudally. This caudal recession of the
primitive heart in relation to the pharyngeal area is greater in fishes than in
the amphibia and higher vertebrates. Therefore, the ventral aortae (and later
ventral aorta) are longer in fishes than in other vertebrates. Actually, in the
amphibia and particularly in the higher vertebrates, the primitive heart itself
tends to lie below the pharyngeal area. Consequently, the bulbus cordis or
anterior end of the primitive heart comes to lie below the midpharyngeal
region, and the aortal arches in amphibia and in higher vertebrates arise from
the anterior end of the primitive heart in bouquet fashion (figs. 34 IE, 342A,
E). On the other hand, in fishes, a single, elongated, ventral aorta is formed,
which extends the length of the pharyngeal area. The developing heart is
attached to its caudal end, and the aortal arches arise along its extent (fig.
341A).
The aortal arches are paired vessels which run dorsally through the sub-
stance of the visceral arches. Six pairs of these arches are formed generally
in«the gnathostomous vertebrates, although some of them are transitory struc-
tures. The first, second, and fifth pairs of aortal arches are the most transitory
in all forms above the fishes.
During development the aortal arches are modified differently in the various
vertebrate groups. In fishes, a permanent, branchial mechanism is inserted
midway along the branchial visceral arches. The aortal arch of each branchial
visceral arch is broken up into an afferent vessel, passing from the ventral
aorta to the branchial (gill) structure, and an efferent vessel, leading from
the gill mechanism to the dorsal aorta (fig. 341B). In the majority of amphibia,
the first, second, and third branchial aortal arches become involved temporarily
in the development of gill mechanisms, although some, such as Nectiirus,
retain the gills permanently. In higher vertebrates, none of the aortal arches
are concerned with gill formation, and are, in consequence, transformed
directly into the adult form.
760 THE CIRCULATORY SYSTEM
The transformation of the aortal arches in the shark, frog, chick, and
mammal is shown in figures 341 and 342. It is important to observe that,
in those vertebrates possessing lungs, the pulmonary artery grows back from
the sixth aortal arch. In a sense, however, the pulmonary arteries represent
a direct caudal growth from the posterior ventral aortae, particularly in rep-
tiles, birds, and mammals (fig. 342A, B, C, E, F, G).
5. Dorsal Aortae (Aorta) and Branches
Two dorsal aortae arise first, one on either side of the notochord and
above the primitive gut tube, and their origin is synchronous with the forma-
tion of the ventral, vitelline (subintestinal) blood vessels and the heart. Pos-
terior to the pharyngeal area, the primitive dorsal aortae soon fuse to form
a secondary vessel, the dorsal aorta, lying below the notochord. Anteriorly,
in the pharyngeal area, they remain separate, and the cephalic end of each
primitive dorsal aorta grows forward into the developing forebrain area.
These forward growths of the primitive dorsal aortae into the forebrain area
form the anterior rudiments of the internal carotid arteries. The primitive dorsal
aortae, therefore, give origin to a single secondary vessel, the dorsal aorta,
which is bifurcated at its cephalic end in the region of the pharyngeal area
of the gut.
Aside from the cephalic ends of the internal carotid arteries, three main
sets of arteries arise from the developing dorsal aorta:
1 ) Dorsal intersegmental arteries, passing between the somites and send-
ing a dorsal branch toward the neural tube and epaxial musculature
and a lateral branch into the hypaxial musculature (fig. 343A). The
lateral branches develop into intercostal and lumbar arteries of the
Fig. 341. Modifications of the aortal arches. In the following diagrams, the aortal
arches are depicted in such a way as to represent two parts, viz. an afferent system, con-
veying the blood from the heart to the branchial (gill) region, and an efferent system,
leading the blood away from the branchial area. The afferent system of vessels is finely
stippled, whereas the efi^erent system is ringed with lines. With the exception of certain
lateral views all diagrams have been made from the dorsal view. (A-D) Aortal vessel
changes in embryos of Squahis acunfhias. (A and B, adapted from actual conditions
described by Scammon, 1911. See reference under Fig. 339.) (A) Generalized, basic
condition present in embryo of 15 mm. embryo. (B) Lateral view, 20.6 mm. stage.
(C and D) The afferent and efferent systems in the adult form. D should be superim-
posed upon C. Diagrams C and D have been separated to minimize confusion. (E-G)
Modifications of the aortal arches in the frog. The modifications of the aortal arches
in the frog involve a complicated series of changes. In Fig. 335 (A) the simple tubular
aortal arches are shown during the earlier phases of development. In Fig. 257 (B) a
later stage is depicted. In the latter figure the aortal arches are separated into functional
afferent and efferent vessels supplymg the branchiae or gills. At the time of meta-
morphosis the vessels are reorganized, apparently, into tubular vessels according to the
pattern shown in Fig. 341 (E). The transformations of the basic conditions shown in
Fig. 341 (E) into the adult form are outlined in Figs. 341 (F and G). (H) The
three divisions of the bulbus cordis in the turtle.
FIRST BRANCHIAL AORTAL
ARCH (EFFERENT PARTK
LEFT VENTRICLE
Fig. 341. {See facing page for legend.)
761
762 THE CIRCULATORY SYSTEM
adult. The arteries to the bilateral appendages arise as modifications
of the lateral branches of the intersegmental arteries (fig. 343B, C).
2) Lateral arteries which are not as truly segmented as are the dorsal
intersegmental arteries. They pass laterally into the developing nephro-
tomic structures (fig. 343A). The renal and genital arteries of the adult
are derived from the lateral series of arteries.
3 ) Ventral arteries much fewer in number than the above-mentioned series
(fig. 343A). The vitelline arteries of the yolk-sac area are the first of
these ventral arteries to develop. In the Amniota, the umbilical or
allantoic arteries also belong to the ventral series of arteries arising
from the dorsal aorta. These vessels pass to the placenta or allantoic
areas. The coeliac, superior mesenteric, inferior mesenteric, and um-
bilical arteries are the adult derivatives of the ventral series of arteries
arising from the primitive dorsal aorta.
E. Development of the Lymphatic System
The lymphatic system often is called the white blood circulatory system
because red blood cells are not present normally, its blood being composed
of a lymph fluid and various types of white blood cells.
Lymph vessels are present in all gnathostomous vertebrates, particularly
in the bony fishes and in amphibia, reptiles, birds, and mammals. They appear
to be absent in cyclostomes. The lymphatic system is highly developed in the
amphibia where it possesses lymph hearts, which actively propel the lymphatic
fluid forward. Lymph hearts are found in the tail region of bird embryos,
including the chick. However, lymph flow on the whole is of a sluggish nature.
Lymph vessels never join arteries but connect in various regions with the
veins. In larval amphibia and in certain adult species of amphibia, these
connections with the venous system may be numerous.
Fig. 342. Modifications of the aortal arches (Continued). (A) Generalized, basic
condition of the aortal arches in the chick embryo developed during the first 3'/2 days
of incubation. (B) Left lateral view of condition present during latter part of the
third day. (C) Schematic representation of changes in aortal arches, dorsal aortae.
and the aortal sac of the chick embryo after the first week and a half of incubation.
Observe that each external carotid artery arises from the anterior end of a ventral
aortic root plus an anastomosis with the common carotid segment. Note further that
the right and left sixth aoral arches persist until approximately the tweaty-first day
(see diagram D). (Diagram C is based to some extent upon data supplied by Pohlman,
1920. Anat. Rec. 18.) (D) Dorsal view of adult condition of aortal-arch and bulbus-
cordis derivatives in the developing chick after hatching. (E) Generalized aortal arch
condition in mammalian embryo. (F) Dorsal view of aortal arches of about 6 mm.
human embryo. (G) Lateral view of same. (This figure redrawn and adapted from
Patten, 1946. Human Embryology, Blakiston, Philadelphia, after Congdon.) (H)
Dorsal view of aortal arches of 14 mm. embryo. (I) Left lateral view of same. (This
figure is redrawn and adapted from Patten, 1946. Human Embryology, Blakiston, Phila-
delphia.) (J) Dorsal view of conditions present after birth. (See also Fig. 379.)
Fig. 342. (See facing page for legend.)
763
764 THE CIRCULATORY SYSTEM
Two general views are held as to the origin of the lymphatic system. One
view holds that lymphatic vessels develop independently of blood vessels and
originate as small spaces in the mesenchyme, the mesenchymal cells flattening
and forming an endothelial lining for the space (Huntington, '14). Such
primitive lymph spaces fuse with nearby lymph spaces to form discrete channels
(McClure, '21). A second view maintains that the certain, small lymph sacs
arise from small endothelially lined channels which are a part of the primitive
venous plexuses in certain areas (Sabin, '12, p. 709). Both views agree, how-
ever, that once formed, the primitive lymph vessels grow and spread by sprout-
ing new channels from previously established vessels (Clark and Clark, '32).
The first lymphatic capillaries appear to develop along the main veins. In
certain regions, these capillaries give origin to the lymph sacs. Right and left
jugular lymph sacs arise in the mammal along the anterior cardinal veins at
the base of the neck (fig. 343D) . These lymph sacs grow, expand, and coalesce
with smaller adjoining lymph spaces. Various other lymph sacs arise, such as
the subclavian lymph sac which is associated with the subclavian vein in the
axillary region, the cisterna chyli which arises from the retroperitoneal, median
lymph sac in the lumbar area, and the iliac lymph sacs which arise posterior
to the retroperitoneal rudiment of the cisterna chyli. From these central lymph
sacs, the peripheral lymph channels arise and grow rapidly in a distal direc-
tion. The thoracic duct comes into existence as a longitudinal vessel along
the middorsal area of the body and together with the left jugular lymphatic
trunk opens into the venous system near the junction of the internal and ex-
ternal jugular veins. The right jugular lymphatic trunk opens into the venous
system similarly on the right side. From these main lymphatic areas, smaller
peripheral channels arise as endothelial outgrowths. Valves develop within.
Fig. 343. Branches of dorsal aorta; lymphatic structures. (A) Diagram illustrating
various branches of dorsal aorta. (B) Arteries of brain area, appendages, body wall
and umbilical cord of human embryo of seven weeks. (Redrawn from Patten, 1946.
Human Embryology, Blakiston, Philadelphia, after Mall.) (C and C) Two stages in
development of forelimb arteries of pig: C, embryo of 4.5 mm.; C, embryo of 12 mm.
(Redrawn from WooUard, 1922. Carnegie Contribution to Embryology, No. 70. Vol.
14.) (D) Formation of primitive lymph sacs in the mammal (cat). (Redrawn from
F. T. Lewis, 1906. Am. J. Anat. 5.) (E and E') Four stages in the development of
a lymph node. (Redrawn from Bremer, 1936. A Text-book of Histology, Blakiston,
Philadelphia.) Diagram E, to the left. Lymphatic vessels come to surround a mass of
primitive lymphoid tissue composed of mesenchymal tissue and lymphocytes. Primitive
connective tissue surrounds the mass. Diagram E, to the right. The ingrowing lymphatic
channels break up the lymphoidal tissue with the subsequent formation of lymph smuses.
Observe that a peripheral lymph channel is established, and also that the surrounding
connective tissue is beginning to form a surrounding capsule from which trahcculae are
growing into the lymphoidal mass. Diagram E', to the left. Further development of
growth changes shown in E, to the right. Diagram E', to the right. A loose meshwork of
lymph channels and sinuses appears in the central portion or luedulla of the lymph
node, whereas the periphery or cortex is composed of secondary nodules separated into
compartments by the ingrowth of trabecuiae from the peripheral capsule.
Fig. 343. (See facing page for legend.)
765
766 THE CIRCULATORY SYSTEM
A characteristic feature of the lymphatic system is the development of
lymph nodes (lymph glands) along the lymphatic vessels. A lymph node is
a small, rounded structure with lymph vessels entering it at various points
(fig. 343E). From these lymph vessels, a flow of lymph oozes around a
meshwork of lymphoid cords, contained within the lymph node. After pass-
ing through the meandering lymph spaces within the node, the lymph emerges
from the opposite side of the lymph node into lymphatic channels.
Lymph nodes appear to arise from lymph sacs which are invaded by in-
growing mesenchyme and connective tissue. Lymphoblasts become associated
with these connective-tissue ingrowths, and lymphocytes are differentiated in
large numbers. Eventually the developing lymph node forms two areas, an
outer cortex, containing dense masses of lymphocytes and an inner medulla,
containing a loose meshwork of lymph channels and sinuses. Connective tissue
forms a capsule around the lymph node from which partitions or trabeculae
grow inward to divide the cortex into secondary nodules. Beneath the capsule,
a peripheral lymph sinus is developed. Blood vessels enter the lymph node
at the hilus and pass along the trabeculae to the secondary nodules. The re-
turning blood vessels follow the same pathways.
The spleen is a large lymph gland attached to the omental derivative of the
dorsal mesogastrium or peritoneal support of the stomach. It arises as a
concentration of mesenchyme along the left aspect of the early mesogastrium.
This mesenchymal mass eventually increases in size and projects from the
surface of the mesogastrium from which it later becomes suspended by a
constricted peritoneal support, the gastro-splenic ligament.
The mesenchymal mass of the developing spleen is well supplied with
blood vessels, and a completely closed set of vascular channels is formed at
first. Later, however, sinus-like spaces appear which unite with the closed
vascular channels converting the closed system into one possessing open
sinuses. Lymphoid tissue forms and masses of splenic corpuscles develop
about the blood vessels. (Consult Maximow and Bloom, '42, for detailed
description of splenic structure.)
F. Modifications of the Circulatory System in the Mammalian Fetus
at Birth
Consult Chap. 22.
G. The Initiation of the Heart Beat
The first parts of the heart to be developed are the anterior regions, namely,
the bulbus cordis and the ventricle. When the ventricular region is developed
in the chick, it starts to twitch. Later when the atrial portion is formed, it
commences to contract with a rhythm different from that of the ventricular
area, and its beat supersedes that of the ventricle. Still later when the sinus
venosus is established, it emerges with its own contraction rhythm, and this
BIBLIOGRAPHY
767
rhythm then dominates the contraction wave which spreads forward over the
heart. The area of the sinus continues to be the "pacesetter" of the heart beat
throughout life, although in birds and mammals, the sinus is taken up into
the posterior wall of the right atrium. In the mammal (fig. 340F), the sinus
node, located in the right atrium, initiates, under normal conditions, each
heart beat. The contraction stimulus spreads distally to the peculiar fibrous
bundle, located in the atrial septum and the atrioventricular area. This bundle
is known as the atrioventricular node, and its fibers descend into the muscles
of the ventricular area, conveying the heart beat to the ventricles.
Though fibers from the autonomic nervous system reach the heart in the
region of the right atrium and stimuli from these nerves may greatly affect
the rhythm of the heart beat, the essential control of the beat lies within the
heart's own nodal system (fig. 340F).
Bibliography
Bloom. W. and Bartelmez, G. W. 1940.
Hematopoiesis in young human em-
bryos. Am. J. Anat. 67:21.
Clark, E. R. and Clark, E. L. 1932. Am.
J. Anat. 51:49.
Gilmour, J. R. 1941. Normal haemopoiesis
in intra-uterine and neonatal life. J. Path.
& Bact. 52:25.
Hochstetter, F. 1906. Chap. IV in Hand-
buch der vergleichenden und experimen-
tellen Entwickelungslehre der Wirbel-
tiere by O. Hertwig. Gustav Fischer,
Jena.
Huntington, G. S. 1914. Development of
lymphatic system in amniotes. Am. J.
Anat. 16:127.
Jordan, H. E. and Speidel, C. C. 1923a.
Blood cell formation and destruction in
relation to the mechanism of thyroid ac-
celerated metamorphoses in the larval
frog. J. Exper. Med. 38:529.
and . 1923b. Studies on
lymphocytes. I. Effects of splenectomy,
experimental hemorrhage and a hemo-
lytic toxin in the frog. Am. J. Anat.
32:155.
Kampmeier, O. E. 1920. The changes of
the systemic venous plan during devel-
opment and the relation of the lymph
hearts to them in Anura. Anat. Rec.
19:83.
Maximow, A. A. and Bloom, W. 1942.
A Textbook of Histology. Saunders,
Philadelphia.
McClure, C. F. W. 1921. The endothelial
problem. Anat. Rec. 22:219.
Miller, A. M. 1903. The development of
the postcaval vein in birds. Am. J. Anat.
2:283.
Minot, C. S. 1912. Chap. 18, Vol. II, p.
498, The origin of the angioblast and
the development of the blood in Human
Embryology by Keibel, F. and Mall,
F. P. J. B. Lippincott Co., Philadelphia.
Noble, G. K. 1931. The Biology of the
Amphibia. McGraw-Hill, New York and
London.
Reagan, F. P. 1917. Experimental studies
on the origin of vascular endothelium
and of erythrocytes. Am. J. Anat. 21:39.
Sabin, F. R. 1912. Chap. 18, Vol. II, p.
709, Development of the lymphatic sys-
tem in human embryology by Keibel, F.
and Mall, F. P. J. B. Lippincott Co.,
Philadelphia.
Scammon, R. E. 1913. The development
of the elasmobranch liver. Am. J. Anat.
14:333.
Stockard, C. R. 1915. The origin of blood
and vascular endothelium in embryos
without a circulation of the blood and in
normal embryos. Am. J. Anat. 18:227.
18
Tne Excretory ana Reproductive Systems
Introduction
1. Developmental relationships
2. Functions of the excretory and reproductive systems
3. Basic embryonic tissues which contribute to the urogenital structures
Development of the excretory system
1. General description
a. Types of kidneys formed during embryonic development
b. Types of nephrons or renal units produced in developing vertebrate embryos
2. Functional kidneys during embryonic development
a. Pronephros
b. Mesonephros
c. Metanephros and opisthonephros
3. Development and importance of the pronephric kidney
a. General considerations
b. Shark, Squalus acanthias
c. Frog
d. Chick
e. Mammal (human)
4. Development of the mesonephric kidney
a. Squalus acanthias
b. Frog
c. Chick
d. Mammal
5. Development of the metanephric kidney
a. Chick
1) Metanephric duct and metanephrogenous tissue
2) Formation of the metanephric renal units
b. Mammal (human)
1) Formation of the pelvis, calyces, collecting ducts, and nephric units
2) Formation of the capsule
3) Changes in position of the developing kidney
6. Urinary ducts and urinary bladders
a. Types of urinary ducts
b. Urinary bladders
c. Cloaca
768
INTRODUCTION 769
C. Development of the reproductive system
1. Early developmental features; the indifferent gonad
2. Development of the testis
a. Mammal
b. Chick
c. Frog
3. Development of the ovary
a. Mammal
b. Chick
c. Frog
4. Development of the reproductive ducts
a. Male reproductive duct
b. Female reproductive duct
5. Development of intromittent organs
6. Accessory reproductive glands in mammals
a. Prostate glands
b. Seminal vesicles
c. Bulbourethral glands
7. Peritoneal supports for the reproductive structures
a. Testis and ovary
b. Reproductive ducts
A. Introduction
1. Developmental Relationships
The excretory and reproductive systems often are grouped together as the
urogenital system. This inclusive term is appUed to these two systems because
they are associated anatomically in the adult form and, during development,
show marked interrelationships and dependencies.
An important relationship, shared by the developing reproductive and
excretory systems, involves the caudal end or cloaca of the developing digestive
tube. It is this area of the differentiating metenteron which affords an outlet
to the external environment for the urogenital ducts in the majority of the
vertebrate species. This fact will become obvious later.
2. Functions of the Excretory and Reproductive Systems
The functions of the reproductive systems of the male and female are
discussed in Chapters 1 to 4 and 22.
The excretory system is most important in the maintenance of life, and
is an important feature in the flow of fluids through the body as described
in the introduction to Chapter 17. Food substances and water pass into the
body through the walls of the digestive tract, and oxygen is admitted through
the respiratory surfaces. The veins convey these substances to the heart and
arteries (with the exception of fishes and some amphibia where oxygen passes
directly into the arterial system), and the heart and arteries propel them
to the tissues. Here the food substances and water are utilized, and excess
REGION OF PRONEPHROS
r REGION OF OPISTHONEPMROS — 1
REGION OF MESONEPHROS REGION OF MET4NEPHR0S
MESONEPHRIC RENAL UNITS
^7 ARISING BY CONDENSATION OF
GROUPS OF CELLS WITHIN
15 NEPHROGENIC CORD
- PRONEPHRIC
IMESONEPHRIC)
DUCT
VESTIGIAL
PRONEPHRIC
TUBULE
SECRETORY
PORTION
OF TUBULE
MALPIGHIAN
BODY
COELOM
NEPHROSTOMAL / SECRETING
CANAL MESONEPHRIC TUBULE
DUCT
Fig. 344. Regions of kidney origin within the vertebrate group; types of renal units
formed. (A) The regions in the body where the diflferent types of vertebrate kidneys
arise. The pronephric tubules and the pronephric duct are shown in black to emphasize
the fact that this part of the developing renal system is a fundamental and necessary
primordium without which later kidney development is distorted. (B) Differentiation
of the anterior portion of the nephrotomic plate and the common method of origin of
the pronephric duct. In the anterior region (toward the left in the figure) the nephrotomic
plate segments into individual nephrotomes from each of which a renal tubule arises
(see tubules 1 to 5). Tubules 6-9 is a vestigial area of tubule development. The anterior
mesonephric region indicated by tubules 10 to 15, etc. In the anterior mesonephric area,
e.g., tubules 10 and 11, the individual tubules show a tendency to arise segmentally,
but in more posterior mesonephric regions, e.g., tubules 12 to 15, etc., the tubules arise
through condensation of cellular masses within the nephrogenic cord. Hence, primitive
(Continued on facing page.)
77Q
INTRODUCTION 771
salts, wastes, and water are the by-products. The veins, lymphatics, and
arteries convey these substances to the areas of eUmination as follows:
( 1 ) Carbon dioxide and water are residues of carbohydrate metabolism.
The carbon dioxide and some of the excess water in the body are
discharged through the respiratory surfaces.
(2) The products of protein breakdown together with excess water and
mineral salts are conveyed mainly to the kidneys and are eliminated
there.
Exceptional areas exist for the elimination of some of the above-mentioned
materials. For example, a certain amount of salts, nitrogenous wastes, and
Fig. 344 — (Continued)
segmentation is lost. The pronephric duct is formed through coalescence of the outer
distal portions of the pronephric tubules (see tubules 3, 4, and 5). The coalesced portion
thus formed grows caudally to join the cloaca. The mesonephric tubules, however,
appropriate the pronephric duct in a secondary manner, growing outward to join this
duct (see tubules 10 to 12). The pronephric duct, after this appropriation, becomes the
mesonephric or Wolffian duct.
Figs. 344C-F are diagrams of different types of renal units (nephrons) which appear in
developing vertebrate kidneys.
(C) This diagram represents a form of renal unit which we may designate as Type I.
It is a vestigial tubule which may or may not become canalized. Its chief function is
to initiate the formation of the pronephric duct. It is found in the pronephric kidneys
of elasmobranch fishes, reptiles, birds, and mammals and, to some extent, in the anterior
portion of the mesonephric kidneys of these groups.
(D) This diagram represents a renal unit found typically in the pronephric kidneys
of larval forms such as that of the frog tadpole. It is designated as Type II. it possesses
a ciliated nephrostome connecting with the coelomic cavity and a secretory portion which
joins the pronephric duct.
(E) This diagram is given to represent the typical form of renal unit found in the
earlier phases of mesonephric kidney development of lower vertebrates. It is called Type
III. It is found also in the pronephric kidney of Hypogeophis (Gymnophiona) (see
Brauer, '02). With some modifications it may represent a type of renal unit found in
the adult kidney of the urodele, Necturus maculosus (see fig. 345D).
(F) The Type IV renal unit is similar to Type III but lacks the ciliated nephrostomal
connection with the coelomic cavity. It is the later renal unit of the mesonephric kidney
of most fishes and amphibia and the typical renal unit found in the mesonephric kidney
of reptile, bird, and mammalian embryos. With some elaboration it would represent
the nephron (renal unit) found in the metanephric kidney of reptiles, birds, and mammals.
G.I., G.2., G.3., stages in development of the mesonephric tubule in the embryo of
Squalus ucanthias. G.l. and G.2. the tubule arises from the nephrotome in a segmental
fashion and appropriates the pronephric duct. G.3. a later mesonephric tubule. In the
latter tubule the nephrostomal connection with the coelomic cavity is lost. Observe that
the tubule empties into the collecting duct, an outgrowth of the mesonephric duct. The
early primitive segmental condition is lost and many tubules are formed in each body
segment.
772 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
water pass off through the sweat glands of mammals; water and possibly
small quantities of salts and wastes find riddance through the tongue's surface
and oral cavity of dogs; and the salt-excretory glands in the gills of teleost
fishes remove excess salt materials from the blood, together with small
amounts of nitrogenous substances. On the whole, however, the kidneys
function to eliminate most of the nitrogenous residues and excess water, to-
gether with salt ions of various kinds, particularly those of chloride, sulfate,
sodium, and potassium. The dispatch of salt ions by the kidneys is all important
in maintaining the correct salt balance in the blood stream.
3. Basic Embryonic Tissues Which Contribute to the
Urogenital Structures
The basic, embryonic, cellular areas which contribute to the formation of
the excretory and reproductive structures are as follows:
(1) the nephrotomic plate (intermediate-cell-mass mesoderm) (fig. 344A).
(2) the adjacent coelomic tissue, underlying the nephrotomic plate during
its development,
(3) the entodermal lining and surrounding mesoderm at the caudal end
of the digestive tube, and
(4) the ectoderm of the integumentary areas where the urogenital openings
occur.
(5) primordial germ cells.
B. Development of the Excretory System
1. General Description
The excretory system is composed of the following:
( 1 ) a series of excretory units, known as nephric units or nephrons,
(2) the kidney, a structure in which the nephrons are grouped together,
(3) a series of collecting ducts from a particular region of the kidney,
which join the nephric units on the one hand and a main excretory
duct on the other, and
(4) the cloaca (or its derivative, the urinary bladder) and a passageway
to the external surface of the body (figs. 345A, B, D; 348G, D).
a. Types of Kidneys Formed During Embryonic Development
The kidney in Greek is called nephros and in Latin, ren. The words nephric
and renal are adjectives, pertaining to the kidney but differing etymologically.
By adding a prefix to the word nephros, various types of kidneys are denoted
as follows:
(1 ) Holonephros is a word that was introduced by Price ( 1896) and des-
ignates a kidney derived from the entire nephrotomic plate in which
a single nephron (nephric unit) arises from each nephrotome. (The
DEVELOPMENT OF EXCRETORY SYSTEM 773
word nephrotome is applied to each segmented mass or bridge of
mesoderm, developed within the nephrotomic plate, which connects
the somite to the unsegmented lateral plate mesoderm or hypomere.
See figure 344B.) The early development of the kidney tubules in
the hagfish, Polistotrema (Bdellostoma) stout i (Price, 1896), and in
the elasmobranch fish, Sqiialus acanthias (Scammon, '11), tends to
simulate holonephric conditions.
(2) Pronephros, mesonephros, metanephros, and opisthonephros are terms
for types of kidneys. Actually, during the development of all gnathos-
tomous vertebrates, the nephrotomic plate on either side produces
not one holonephros but instead three types of kidneys which are
adapted to three different developmental and functional conditions.
These kidneys develop antero-posteriorly in three general regions of
the nephrotomic plate (fig. 344A). The most anteriorly developed
kidney is called the pronephros; the kidney which develops from the
midregion of the nephrotomic plate is the mesonephros; and that which
arises from the caudal end of the nephrotomic material is the meta-
nephros. Kerr ('19) attaches the name opisthonephros to the kidney
which arises posterior to the pronephros in the late larvae of fishes and
amphibia. The opisthonephric kidney takes its origin from the entire
caudal portion of the nephrotomic plate. It therefore represents the
nephrogenic tissue of the posterior part of the embryonic mesonephric
kidney plus the nephrogenic material which enters into the formation
of the metanephric kidney of reptiles, birds and mammals.
b. Types of Nephrons or Renal Units Produced in Developing
Vertebrate Embryos
Four main types of renal units are produced during kidney development
in various vertebrate species. Consult figure 344C-F.
2. Functional Kidneys During Embryonic Development
During embryonic development, the following types of functional kidneys
occur in the gnathostomous vertebrates.
a. Pronephros
The pronephric kidney is functional in all species producing free-living
larval forms. In these larvae it operates not only to remove waste materials
but is essential also in the removal of excess water, thus preventing edema
(Howland, '16, '21; Swingle, '19). Free-living larvae are found in teleost,
ganoid and lung-fishes, and in the amphibia.
b. Mesonephros
In all free-living larvae the pronephros is succeeded by the mesonephros
during the larval period. The decline of the pronephros and the ascendancy
774 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
of the mesonephros is well illustrated in figure 335B-E relative to the devel-
oping venous system in anuran larvae. The mesonephric kidney also functions
in the embryos of elasmobranch fishes, reptiles, birds, and mammals. In the
mammals its efficiency as a renal organ appears to be correlated with the
degree of intimacy existing between the extra-embryonic and maternal tissues
in the placenta. When this relationship is intimate (fig. 373D) as in rats,
mice, humans, etc., the mesonephric kidneys are less developed, and therefore
probably less functional, than in species such as the pig. In the pig the pla-
cental relationship between embryonic and maternal tissue is not so close as
in the species mentioned above (fig. 373B), and the mesonephric kidneys
are very large and well developed.
c. Metanephros and Opisthonephros
As indicated on p. 773 the metanephros is the kidney of the adult form of
reptiles, birds, and mammals, while the opisthonephros is the mature kidney
in fishes and amphibians. As the definitive or adult form of the body is achieved
in both of these groups, the mature form of the kidney assumes the renal
responsibilities.
3. Development and Importance of the Pronephric Kidney
a. General Considerations
Observation and experimentation upon the developing urinary and genital
systems of gnathostomous vertebrates suggest that the pronephric kidney,
and particularly its duct, the pronephric duct, are most important in the later
development of the excretory and reproductive systems (Gruenwald, '37, '39,
'41). The pronephric kidney therefore may be regarded as fulfilling two im-
portant functions in the gnathostomous vertebrates, namely:
( 1 ) It operates as an early renal organ in free-living larval species, and
(2) It is a necessary precursor in the development of the reproductive
system and the later excretory system.
The pronephric kidney develops from the anterior portion of the nephro-
tomic plate at about the level of the developing heart and stomach region
(fig. 344A and B). This area of the nephrotomic plate becomes segmented
into separate nephrotomes (fig. 344 A and B). During the differentiation of
each nephrotome in the pronephric area, the connection between the nephro-
tome and the dermo-myotome disappears, and a small dorso-lateral outgrowth
from the middle portion of the nephrotome occurs (fig. 344B, 1 and 2). This
cyHndrical outgrowth proceeds dorso-laterally toward the developing skin
and then turns posteriad and grows caudally (fig. 344B, 3). In the next pos-
terior nephrotome, it meets a similar rudimentary tubule with which it unites
(fig. 344B, 3 and 4). The area of union formed by these combined tubules
DEVELOPMENT OF EXCRETORY SYSTEM 775
grows caudalward to the next nephrotome to unite with its tubule (fig.
344B, 5), etc. As a result, the fused portions of the pronephric tubules give
origin to the pronephric or segmental duct (fig. 344B).
The above method of origin of the pronephric duct has been described
for elasmobranch fishes, reptiles, birds, and mammals. A different method
of pronephric duct origin occurs in the amphibia and teleosts where the
pronephric duct apparently arises by a longitudinal splitting of the nephro-
tomic plate (Field, 1891; Goodrich, '30). The pronephric duct, once formed,
continues to grow caudalward above the nephrotomic plate until it reaches
the caudal end of the plate. In this area, the growing end of the pronephric
duct turns ventrally and joins the cloaca (figs. 344A; 346F).
The entire pronephric portion of the nephrotomic plate is never realized
in the formation of pronephric tubules. The number of tubules actually formed
varies greatly and is confined generally to a limited number of nephrotomes
in the middle or posterior pronephric area.
b. Shark, Squalus acanthias
Jn Squalus acanthias, a considerable nephrotomic area, overlying the caudal
portion of the developing heart in segments 5-11, may produce suggestive
indications of pronephric tubule formation. However, generally only three to
five pronephric tubules are definitely formed. The distal ends of these tubules
unite to form the pronephric or segmental duct and the latter grows caudal-
ward to join the cloaca. The pronephric tubules are aberrant and soon dis-
appear, but the pronephric duct remains and when joined by the mesonephric
tubules it becomes known as the Wolffian or mesonephric duct (fig. 347A).
c. Frog
In the frog, Rana sylvatica. Field (1891) describes the origin of the pro-
nephric kidney from a thickening and outgrowth of the somatopleuric layer
of the nephrotomic plate in segments 2-4. Three tubules arise from this
thickened area, one tubule in segment two, another in segment three, and
a third in segment four.
A cross section of the developing second pronephric tubule at a time when
the neural tube is wholly closed and a short while before hatching is shown
in figure 346A. At about the time of hatching the second pronephric tubule
is well advanced, as indicated in figure 346B, and the fully developed first
pronephric tubule of an embryo (larva) of about 8 mm. is shown in figure
346C. The entire pronephric kidney of one side consisting of three tubules
viewed from the ventral aspect at the 8 mm. stage is presented in figure 346E.
The general plan of the pronephric kidney at the 18 mm. stage is pictured
in figure 346F. Figure 346D lies in plane A-D of figure 346F.
Contrary to the manner of origin of the pronephric duct from the distal
ends of the pronephric tubules in the embryo of Squahis acanthias, Field de-
776 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
scribes the origin of this duct in the frog from a thickening of the somato-
pleuric layer of the nephrotomic plate in segments 4-9. This somatopleuric
thickening separates, becomes canalized, and grows caudally to join the dorsal
area of the cloaca, a union which is accomplished at about the time of hatching
(fig. 258F'). The pronephric tubules in their development unite with the
cephalic end of this duct.
As the development of the pronephric kidney advances it is to be observed
that one large glomus is formed, projecting into the restricted coelomic
chamber or nephrocoel which is shut off partly from the common peritoneal
cavity by the expanding lungs (fig. 346D). Each ciliated nephrostome opens
into this nephrocoelic chamber (fig. 346F). {Note: Reference may be made
to figure 335A-C which shows the well-developed renal portal system inserted
into postcardinal vein in relation to the pronephric kidney. The postcardinal
vein breaks up into a series of small capillaries which ramify among the coiling
pronephric tubules (see figure 346C) to be gathered up again into the posterior
cardinal vein as it opens into the common cardinal vein.)
d. Chick
The pronephric tubules of the pronephric kidney of the chick are rudi-
mentary, occupying a region of the nephrotomic plate, from the fifth to the
sixteenth somites. However, all of the tubules do not appear simultaneously.
The pronephros begins to form at about the stage of 12 to 13 pairs of
somites (stage 11, Hamburger and Hamilton, '51, or at about 40 to 45 hrs.
of incubation), and small aberrant tubules are formed (fig. 345E) which
grow caudally to give origin to the pronephric duct as indicated in figure 344A.
Fig. 345. Developing kidney tubules. (A & B) General structure of adult human
kidney. (A) This diagram represents a single renal unit in relation to blood vessels,
collecting duct and the minor calyx. Arrows denote direction of excretional flow. The
position of A in drawing B is shown by the elongated oblong in B. (A is redrawn, some-
what modified, from Glendening, 1930, The Human Body, Knopf, Inc., N. Y.) (B)
Human kidney, part of wall removed, exposing pelvis and other general structures. (Re-
drawn from Maximow and Bloom, 1942, A Textbook of Histology, Saunders, Phila-
delphia, after Brauer.) (C) Including C-1 to C-6. Stages in the development of a
mesonephric renal unit in the frog, Rana sylvatica (C to C-6 redrawn from Hall, 1904,
Bull. Mus. Comp. Zool. at Harvard College, vol. 45). C represents a section through a
developing mesonephric tubule showing cellular condensation in relation to pronephric
(mesonephric) duct. C-1 to C-6 are diagrammatic figures of a developing renal unit from
right side of body. The somatic or lateral portion of the tubule is shaded by lines, the
splanchnic portion is unshaded. (D) Diagrammatic representation of a section through
pelvic kidney of Necturus maculosus. (Redrawn and modified from Chase, 1923, J.
Morph., 37.) A tubule of the ventral series is shown with a peritoneal canal and ciliated
nephrostome which opens into the coelomic cavity. A tubule of the dorsal series also is
depicted. The latter type of tubule lacks a ciliated nephrostome opening into the coelom.
(E) Pronephric tubule in the chick. Section passes through somite 11 of embryo of 16-17
somites. (F) Section through mesonephric kidney of 96 hr. chick embryo, partly sche-
matized. (G) Schematized section through mesonephric kidney of six to seven day chick.
GLOMERULUS
COELOM
BLOOD SINUSOID
SPLANCHNOPLEURE
Fig. 345. (See facing page for legend.)
Ill
778 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
At the 16- to 21 -somite stage, the pronephric kidney is well developed, but
not all the tubules are present. At the 21 -somite stage, pronephric tubules
are present from the eleventh to fifteenth somites. Anterior to this area, they
are degenerate and rudimentary. At the 35-somite stage (65 to 70 hrs. of
incubation or stage 18, Hamburger and Hamilton, '51 ), the pronephric kidney
as a whole is undergoing degeneration, although the pronephric duct (now
the mesonephric duct) remains and, at this time, joins the dorso-lateral area
of the cloaca.
e. Mammal (Human)
In the human embryo, the pronephric rudiments extend from the seventh
to the fourteenth somites (fig. 344A), although rudimentary conditions may
extend as far forward as segment 2 (Felix, '12). The pronephric kidney
appears in embryos of about 9 to 10 pairs of somites, and begins to degenerate
at a stage of 23 to 28 segments. As in the chick and the shark, the pronephric
duct arises from the fusion of the dorso-lateral ends of the rudimentary pro-
nephric tubules and grows caudalward to open into the ventro-lateral aspect
of the cloaca in embryos of 4.2 mm., greatest length (fig. 344A). (See
Felix, '12.)
Although the human pronephros is vestigial, it is as well developed as
in any other mammalia.
4. Development of the Mesonephric Kidney
The mesonephric kidney develops in the region of the nephrotomic plate
posterior to the pronephric kidney (fig. 344A). Five features distinguish
the mesonephric kidney from the pronephric kidney:
( 1 ) The primitive segmentation manifest in the origin of the pronephric
kidney tubules is lacking generally in the mesonephric kidney, although
there is a tendency for the tubules to arise segmentally in the anterior
region. Also, a segmental origin of the tubules throughout the length
of the early mesonephros occurs in the embryo of the hagfish, Polis-
trotrema (Bdellostoma) stoiiti (Price, 1896), and a primitive seg-
mental condition is found in the early mesonephros of the shark and
frog embryos as indicated below.
(2) The mesonephric tubules join the previously formed pronephric duct
and thus appropriate this duct. The pronephric duct then becomes
the mesonephric (Wolffian) duct.
(3) The antero-posterior extent of the mesonephric kidney is much greater
than the pronephric kidney, the mesonephric kidney utilizing the
greater part of the nephrotomic plate.
(4) An innovation, the collecting duct system, is introduced in the meso-
nephric kidney as a result of outgrowths from the mesonephric duct.
STAGE m Hi,
SHORTLY BEFORE
HATCHING
ELENCEPHALON
ROOT OF VAGUS NERVE
SOMITE n
GANGLION NODOSUM
OF THE VAGUS
NERVE
PRONEPHRIC TUBULE
Fig. 346. The developing pronephric kidney in the frog, Rana sylvatica (A-C and E,
redrawn from Field, 1891, Bull. Mus. Comp. Zool. at Harvard College, vol. 21. E con-
siderably modified). (A) Transverse section through developing second pronephric
tubule of frog embryo at a time when the neural tube is completely closed, two gill
fundaments are present and the otic vesicle is a shallow depression. (B) Same tubule
at about the time of hatching. (C) Section through first pronephric tubule at 8 mm.
stage. (D) Transverse section through second pronephric tubule, see line d, fig. 346F,
of 18 mm. Rana pipiens tadpole. (E) Entire pronephric kidney of one side of 8 mm.
R. sylvatica embryo. (F) Schematic reconstruction of 18 mm. R. pipiens tadpole look-
ing down from dorsal area upon the pronephric kidneys and the developing mesonephric
kidneys,
779
780 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
The renal units empty their products into these collecting ducts in the
mature form of the kidney.
(5) Whereas the functional pronephric kidney is confined to those species
which develop free-living larvae, the mesonephric kidney is functional
in all vertebrate embryos with the possible exception of a few mam-
malian species.
a. Squalus acanthias
The mesonephric tubules in the embryo of Squalus acanthias and in other
elasmobranch fishes originate in a manner similar to the pronephric tubules.
That is, a single tubule arises from each nephrotome of the nephrotomic plate.
In doing so, the nephrotome loses its connection with the developing somite
or dermo-myotome, and its dorso-lateral aspect thickens and grows laterad
in the form of a tubule. This tubule comes in contact, and fuses, with the
pronephric or segmental duct (fig. 344B, 11; G.l, G.2). The latter then be-
comes the mesonephric or Wolffian duct. In the 20.6-mm. embryo of Squalus
acanthias according to Scammon ('11), 37 pairs of these tubules are present,
extending along the mesonephric duct to the cloaca (fig. 347A). Later, this
primitive segmentation is lost, and many tubules are developed in each seg-
ment. The anterior portion of the kidney soon degenerates; the nephrostomal
connections of the mesonephric tubules with the coelom established during the
development of the tubules are lost; and the mesonephric tubules assume the
general morphology shown in figure 344G.3). As shown in figure 3440.3, a
series of collecting ducts eventually develops to connect the mesonephric tubules
with the mesonephric duct. Renal units eventually arise in the nephrogenous
tissue overlying the cloaca. This area corresponds to the metanephric region
of higher vertebrates, and the mature kidney of Squalus acanthias thus be-
comes a combination of caudal, mesonephric, renal units, associated with
metanephric units. The mature kidney thus is an opisthonephros. (See Kerr,
'19, also p. 773). In the adult kidney, segmentally arranged nephrostomes may
be observed in a limited area along the medial side of the kidney, although
they do not connect with the renal units.
b. Frog
The mesonephric renal units in the frog begin to arise at about the 10-mm.
stage. As in the shark embryo, the early origin of the mesonephric renal units
is segmental. An intermediate zone of the nephrotomic plate between the
developing mesonephros and the pronephric kidney does not develop renal
units. Coincident with this fact those units which arise more posteriorly in
the nephrotomic plate are developed better than those which arise anteriorly.
The renal units arise as cellular condensations of mesodermal cells within
the cellular mass of the nephrotomic plate (fig. 345C-1). These cellular con-
densations elongate, become canalized, and assume a union with the meso-
DEVELOPMENT OF EXCRETORY SYSTEM 781
nephric duct as shown in figure 345C-1 to C-5. A nephrostomal connection
with the coelomic cavity also appears, but the nephrostomal segment soon
acquires a secondary connection with a renal vein (fig. 345C, 4-6). The
veins thus come to drain the coelomic cavity directly. (In the water-abiding
urodele, Necturus maciilosus, the nephrostomal connection remains in con-
tact with some of the renal units, even in the adult. See figure 345D.)
As the mesonephric kidney of the frog continues to develop, many new
mesonephric renal units are added, and several units appear in each body
segment. In consequence the primitive segmental arrangement of the renal
units is lost, particularly in the caudal region of the nephrotomic plate where
the kidney is developed most highly. Collecting ducts develop as evaginations
of the mesonephric duct and the renal units discharge their contents into these
collecting ducts.
Caudally situated nephrotomic material, comparable to the metanephric
area of the kidney of higher vertebrates, is incorporated along with the meso-
nephric kidney as in the shark embryo. The adult form of the kidney, there-
fore, may be regarded as an opisthonephros, composed of mesonephric and
metanephric renal units.
c. Chick
The mesonephros of the chick develops from the nephrotomic plate in the
region between the somites 13 and 30. The nephrotomic plate in the chick
embryo increases its substance rapidly through cell proliferation posterior to
the area of pronephric-kidney origin. The original nephrotomic plate in this
way becomes converted into an elongated mass or cord of cells called the
nephrogenic cord. The mesonephric tubules arise as condensations within
this cord of nephrogenous tissue. The renal unit emerges initially as a rounded
mass of epithelial cells as in the frog. These epithelial masses elongate. They
acquire a Malpighian body at one end, while the other end unites with the
mesonephric duct. Some of the anterior tubules may have coelomic connec-
tions, similar to the pronephric tubules, but as this portion of the meso-
nephric kidney degenerates, these nephrostomal structures have little func-
tional significance.
As development progresses, the nephrotomic substance increases greatly
through proliferation of its constituent cells, and several renal units arise in
each body segment (fig. 345F). To aid this process, the mesonephric duct
forms collecting ducts which extend outward into the region of the developing
renal units, and a group of these units joins each collecting duct (fig. 345G).
The mature form of the mesonephric tubule of the chick consists of a
glandular (secretory) segment which connects with either the mesonephric or
the collecting duct on the one hand and with a Malpighian body and its
glomerulus on the other (fig. 345G). The mesonephric kidney of the chick
is a prominent excretory organ from the fifth to the eleventh day. During
782 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
the developmental period from 8 to 10 days its tubular system is exceedingly
complex compared to that shown in figure 345G. After this period, it begins
to degenerate, and its function is taken over by the developing metanephric
kidney.
d. Mammal
As in the chick, the mesonephric kidney in many mammalian embryos is
a prominent excretory structure. However, in the rat, mouse, and certain
other mammals its function as an excretory organ is dubious, probably result-
ing from the fact that the placental connection in these forms is sufficiently
intimate to assume excretory functions. In the 10-mm. pig (figs. 261, 262),
it is a prominent structure, filling a considerable part of the coelomic cavity
on either side. In the human embryo, the condition is intermediate between
that of the pig and rat. It possibly functions as an excretory structure in the
human embryo.
The renal unit or mesonephric tubule which is evolved within the nephro-
genic cord is similar to that of the bird. It develops from a condensed mass
of epithelium within the nephrotomic plate (nephrogenic cord). This con-
densed, S-shaped mass elongates, becomes canalized, and joins the meso-
nephric duct. The mesial end of the tubule, in the meantime, develops a
Malpighian body with its glomerules and vascular connections. The glandular
tube is a highly coiled affair and is associated intimately with the veins as
indicated in figure 344F. Collecting ducts, arising as evaginations of the meso-
nephric duct similar to those in the chick mesonephros, are formed.
5. Development of the Metanephric Kidney
The metanephric kidney is the later embryonic and adult form of the renal
organ in reptiles, birds, and mammals. As observed above, the mesonephric
kidney involves three structures:
( 1 ) the urinary or Wolffian duct,
(2) a series of collecting ducts which evaginate from the mesonephric or
Wolffian duct to connect with the renal units, and
(3) the nephrons or renal units.
These same relationships are present in the developing metanephric kidney.
Fig. 347. Urogenital system relationships in various vertebrates. (A) Reconstruction
of 20.6 mm. embryo of Squalus acanthias. (Redrawn from Scammon, 1911, Ciiap. 12,
Normentafeln Entwichiungsgeschichte der Wirbeltiere, by F. Keibei, G. Fischer, Jena.)
(B) Left side view of dissection of male pickerel, Esox Indus, showing reproductive
and urinary ducts and absence of a cloaca. (Redrawn from Goodrich, 1930, Studies on
the Structure and Development of Vertebrates, Macmillan and Co., Limited, London.)
(C) Male reproductive system, ventral aspect, of the pigeon. (Redrawn from Parker,
1906, Zootomy, Macmillan and Co., Limited, London, The Macmillan Co., N. Y.)
Fig. 347. (See facing page for legend.)
783
784 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
a. Chick
1) Metanephric Duct and Metanephrogenous Tissue. The metanephric kid-
ney in the chick begins to arise at the end of the fourth day of incubation
from a diverticulum which evaginates from the caudal end of the mesonephric
duct as the latter enters the cloaca (fig. 259). The origin of the metanephric
diverticulum is similar to that of the various collecting ducts of the meso-
nephric kidney, i.e., it arises as an outpushing from the mesonephric duct.
The metanephric diverticulum enlarges as its distal end grows forward and
dorsad into the nephrogenous tissue of the caudal end of the nephrotomic
plate in trunk segments 31-33. As the metanephric diverticulum enlarges and
grows into the nephrogenous tissue in this area, the nephrogenous tissue
separates from the mesonephric tissue and, together with the metanephric
diverticulum, moves anteriad above the mesonephros to the anterior end of
the mesonephros. During this process, the distal end of the metanephric di-
verticulum enlarges into the future pelvic cavity of the kidney. Numerous
small secondary evaginations make their appearance and extend outward
from this cavity. The secondary evaginations from the primary pelvic cavity
of the kidney form the rudiments of the future collecting ducts of the kidney.
2) Formation of the Metanephric Renal Units. The formation of the meta-
nephric renal units is similar to that of the mesonephric units. At about 7 to
8 days of incubation, the nephrogenous tissue around the terminal ends of the
collecting-duct evaginations from the primary pelvic cavity of the kidney forms
dense epithelial masses. Each of these masses of condensed nephrogenous
tissue assumes an S shape. One end of the S-shaped rudiment unites with the
distal end of the developing collecting duct, while the other end forms a
Malpighian body or renal corpuscle. (Comparable stages involving the devel-
opment of the S-shaped rudiment in the mammalian metanephric kidney are
shown in figure 348 A-C.) By the eleventh day, well-formed renal units are
found in the developing kidney.
The outer capsule of the kidney arises from the peripheral portions of the
nephrogenous tissue and surrounding mesenchyme. The metanephric kidney
is retroperitoneal in position, that is, it lies outside the peritoneal cavity proper.
The posterior end of the metanephric duct or ureter acquires an independent
opening into the cloaca as the above changes occur, for the caudal end of
the mesonephric duct is drawn into, merges with, and thus contributes to the
cloacal wall as the cloaca enlarges.
b. Mammal (Human)
1) Formation of the Pelvis, Calyces, Collecting Ducts, and Nephric Units.
As in the bird, the metanephric kidney of the mammal has a dual origin.
One part, the metanephric diverticulum, arises as an evagination from the
caudal end of the mesonephric duct at the level of the twenty-eighth somite in
the 5- to 6-mm. human embryo (fig. 348H). This evagination extends dorsally
NEPHROGENOUS TISSUE
DEVELOPING
RENAL
TUBULE
ARCHED COLLECTING TUBULE
GLOMERULU
BOWMAN'S CAPSULE
LOOP OF HENLE
LUNG BUD
URACHUS
BLADDER
PHALLUS
STOMACH
UROGENITAL SINUS
aL SEPTUM Vf
CLOACA
RECTUM
SONEPHRIC KIDNEY
EPHRIC DUCT VISCERAL ROOT
PHRIC DUCT
INTESTIN
UMBILICAL
„.,,^ ARTERY
METANEPHRIC
DUCT
ARCHED COLLECTING TUBULE
IMARY OR STRAIGHT
OLLECTING TUBULE
DISTAL
ONVOLUTED TUBULE
EFFERENT
BLOOD VESSEL
AFFERENT
BLOOD VESSEL
RENAL CORPUSCLE
(MALPIGHIAN BODY)
LOOD CAPILLARIES
ASCENDING LIMB OF
HENLE'S LOOP
STRAIGHT
COLLECTING
TUBULE
MESONEPHRIC DUCT
METANEPHRIC KIDNE'
METANEPHRIC DUCT
G.
DEVELOPING
MINOR CALYCES
ALLANTOIC DUCT
MAJOR CALYX
PELVIS
POSTANAL GUT
.^'^CLOACAL SEPTUM
UROGENITAL SINUS
DEVELOPING
COLLECTING DUCTS
Fig. 348. The developing metanephric kidney. (A) Condensation of rudiment of renal
tubule in relation to rudiment of arched collecting tubule. (B) Renal tubular rudiment
has united with arched collecting tubule. (C) Later stage in differentiation of renal unit.
(D) Final stage in development of renal unit. (E) Developing mesonephric and meta-
nephric kidney of human embryo of about 5 weeks. (F & G) Mesonephric and meta-
nephric conditions in human embryo of 8 mm. or about sixth week of development. (H)
Diagram showing origin of metanephric uteric bud from caudal end of mesonephric
duct in human embryo approximating 5.3 mm. greatest length. (Redrawn from Felix,
1912, in Chap. 19, Human Embryology, by F. Keibel and F. P. Mall, Lippincott, Phila-
delphia.) (I) Differentiation of kidney pelvis in human embryo of 20 mm. length or
about seven weeks of gestation.
785
786 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
into the caudal end of the nephrotomic plate (nephrogenic cord). (See figure
348E.) The metanephric diverticulum enlarges at its distal end and thus forms
the rudiment of the pelvis of the kidney as in the chick (fig. 348F). As the
rudimentary pelvis enlarges, it sends out secondary evaginations, the rudiments
of the future collecting ducts of the kidney (fig. 3481). Surrounding these
secondary diverticula, there is the cellular substance (fig. 3481) of the
metanephrogenous tissue, derived from the nephrogenic cord posterior to the
caudal limits of the mesonephric kidney.
In human embryos of 14 to 15 mm. (about seven weeks), four definite
primordia of the metanephric urinary system are established as follows (fig.
3481):
( 1 ) Nephrogenous tissue is present which surrounds beginning diverticula
of the collecting ducts;
(2) a system of developing collecting ducts which represents evaginations
from the primitive pelvis of the kidney;
(3) from the primitive pelvis of the kidney arise the rudiments of the
anterior and posterior major calyces; and
(4) the primitive ureter (metanephric duct) of which the primitive pelvis
is the distal enlargement.
(The word calyx refers to a rounded, distal division of the pelvis of the
kidney. The plural form of calyx is calyces.)
From each major calyx, secondary or minor calyces arise (fig. 3481), and
from each minor calyx, the primary or straight collecting ducts emerge into
the surrounding, nephrogenous, cellular mass. Each primary calyx and its
straight collecting-duct rudiments, together with the surrounding nephrog-
enous cells, form the rudiment of the future renal lobe.
The straight collecting ducts continue to elongate and push out into the
surrounding nephrogenous tissue. In doing so, the distal end of each collecting
duct sends out several (usually three or four) smaller evaginations into the
surrounding nephrogenous material. These smaller terminal evaginations rep-
resent the rudiments of the arched collecting tubules of the collecting duct
system (fig. 348A). Around each of the arched-tubule rudiments, masses of
nephrogenous tissue condense into the S-shaped structure typical of the de-
veloping renal units of the mesonephric kidney of the frog, chick, and mammal
and in the metanephric kidney of the chick. A sigmoid-shaped concentration
of nephrogenous cells fuses with each arched collecting tubule and elongates
distally, differentiating into the parts of the typical, mammalian, metanephric
tubule (fig. 348A-D).
As the kidney continues to develop, the original primary or straight col-
lecting ducts branch repeatedly, forming about 12 generations by the fifth
month of human fetal existence. As these branches arise, the pelvis of the
kidney and the calyces enlarge considerably, and some of the collecting ducts
DEVELOPMENT OF EXCRETORY SYSTEM 787
are drawn into and are taken up into the walls of the expanding calyces. In
the fully formed kidney, about 20 of these large straight collecting ducts open
into the papillary ducts at the apex of the renal lobe or pyramid into a minor
calyx (fig. 345A, B) (Felix, '12). The outer peripheral portion of the kidney,
containing the glomeruli and various parts of the renal units (nephrons),
forms the cortex of the kidney, while the inner portion, in which lie the
straight collecting and papillary ducts, forms the medulla (fig. 345B).
2) Formation of the Capsule. The metanephrogenous tissue around the
developing pelvis and collecting ducts of the kidney becomes divided into
inner and outer zones. The inner zone cells differentiate into the renal units,
whereas the outer zone cells form the interstitial connective tissue and outer,
connective-tissue capsule of the kidney.
3) Changes in Position of the Developing Kidney. The early developing
kidney is located in the pelvic area at the caudal end of the mesonephric
kidney. As the mesonephric kidney declines in size and moves caudally, the
metanephric kidney pushes anteriorly and takes its final retroperitoneal posi-
tion at birth in the region of the first lumbar area. (Cf. figs. 3B-F; 348E-G.)
6. Urinary Ducts and Urinary Bladders
a. Types of Urinary Ducts
The following two types of urinary ducts were mentioned above:
( 1 ) The pronephric duct, which later becomes the mesonephric duct, is
the functional urinary duct in the larval embryonic form of fishes,
amphibia, reptiles, birds, and mammals. It continues to be the main
urinary duct in adult fishes and amphibia, particularly in the female.
(See (2) below.)
(2) A second type of urinary duct represents an outgrowth of the meso-
nephric duct. Examples of this type are: (a) the metanephric duct
and its branches in the kidneys of reptiles, birds, and mammals,
(b) the collecting ducts in the mesonephric kidney of all vertebrates,
and (c) the adult urinary ducts in the posterior kidney region of
certain male fishes, such as are present in the shark, Squalus acanthias,
and in the salamander, Triton taeniatus.
b. Urinary Bladders
During the development of the urinary system in the mammal, the ventral
portion of the cloacal area and its allantoic diverticulum become separated
from the dorsal cloacal or rectal area by the caudal growth of a fold of tissue,
known as the urorectal fold or cloacal septum. The cloacal septum even-
tually divides the cloaca into a ventral bladder and urogenital sinus region,
and a dorsal primitive rectum (fig. 348E-G). As this development proceeds,
the proximal portions of the mesonephric and metanephric ducts are taken up
788 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
into the wall of the caudal bladder region, and a considerable amount of
mesoderm is contributed to the entodermal lining of the developing bladder.
This mesodermal area presumably forms a part of the lining tissue of the
bladder (fig. 349 A, B). The metanephric duct or ureter, in the meantime,
shifts its position anteriad and becomes united with the dorso-posterior por-
tion of the bladder, while the point of entrance of the mesonephric duct mi-
grates posteriad to empty into the anterior end of the dorsal region of the
urogenital sinus (figs. 348F, G; 349A, B).
In turtles and in some lizards, the adult relationships of the urinary bladder
and rectum are established in a somewhat similar manner to that of the
mammals, although the caudal migration of the cloacal septum is not ex-
tensive. Also, the cloaca is retained.
The urinary bladder (or bladders) of some teleost and ganoid fishes arise
as swellings and evaginations of the caudal ends of the mesonephric ducts
(fig. 347B). A distinct urinary bladder is absent in elasmobranch fishes and
in birds, but is present in amphibia as a ventral diverticulum of the cloaca.
c. Cloaca
A cloaca into which open the urogenital ducts and the intestine is a common
basic condition of the vertebrate embryo. It is retained in the definitive or
adult body form of elasmobranch fishes and to a considerable extent in dipnoan
fishes. It is present also in the adults of amphibia, reptiles, birds (fig. 347C),
and prototherian mammals. A cloaca is dispensed with in the adult stage of
teleost (fig. 347B) and ganoid fishes, and also in the adult stage of higher
mammals (fig. 349A-D).
C. Development of the Reproductive System
The general features of the adult condition of the reproductive system are
described in Chapters 1 and 2. For most vertebrates, the reproductive system
consists of the reproductive glands, the ovaries or testes, and the genital ducts.
FiG. 349. Differentiation of the caudal urogenital structures in the human embryo.
(A) Later stage in differentiation of the cloaca; the rectal area is being separated from
the ventraliy placed urogenital sinus by the cloacal (urorectal) membrane. Condition
of sixth week (about 12 mm.) embryo. (B) Rectal and urogenital areas completely
separated. Miillerian and mesonephric ducts present. Metanephric duct has moved for-
ward into the posterodorsal area of the developing bladder. The Miillerian ducts have
fused at their caudal ends to form the uterovaginal rudiment. This condition is present
at about 8 weeks. (C) Male fetus of about 5 months. Testis beginning to pass into
developing scrotal sac. (See also fig. 3.) (D) Female fetus of about 5 months.
(E to K) Stages in development of external genitalia. (E) Indifferent condition (about
7 weeks). (F) Male about tenth week. (G) Male about 3 months. (H) Male close
of fetal life. (I) Female about tenth week. (J) Female about 3 months. (K) Female
close of fetal life. (L & M) Stages in development of the broad ligament and separation
of the recto-uterine pouch above from the vesico-uterine pouch below.
BROAD ' ' '
LIGAMENT
Fig. 349. (See facing page for legend.)
789
DEVELOPING
SEMINIFEROUS
TUBULES
OVARIAL SAC
Fig. 350. Sex gland differentiation. (A) Transverse section through early genital
rudiment on media! aspect of mesonephric kidney in the 10 mm. pig embryo. (B)
Transverse section through early sex gland of the chick about middle of sixth day of
incubation showing ingression of sex cord of first proliferation. Observe primordial germ
cells in germinal epithelium. Compare with fig. 345G. (Redrawn from Swift, 1915, Am.
J. Anat., 18.) (C) Transverse section through sex gland rudiment of human embryo
11 mm. greatest length. (Redrawn and slightly modified from Felix, 1912, Chap. 19, in
Human Embryology, vol. II, by F. Keibel and F. P. Mall, Lippincott, Philadelphia.)
(D) Transverse section through testis of human embryo 70 mm. head-foot length.
(Redrawn from Felix, 1912. For reference see C above.) (E) Section through human
testis of embryo 70 mm. head-foot length, showing connection between testicular cords
(developing seminiferous tubules) and developing rete tubules. (Redrawn from Felix,
1912, reference same as in C, above.) (F) Transverse section through testis of seventh
month human embryo showing developing seminiferous tubules. (Redrawn from Felix,
(Continued on facing page.)
790
DEVELOPMENT OF REPRODUCTIVE SYSTEM 791
1. Early Developmental Features; the Indifferent Gonad
The gonads or reproductive glands are associated intimately with the devel-
oping mesonephric kidneys. The typical site of origin is the area between
the dorsal mesentery and the anterior portion of the mesonephric kidney
(figs. 345F, G; 350C). As development progresses, it tends to move laterad
and in doing so becomes located along the mesial aspect of the developing
mesonephric ridge (figs. 3 A; 345G).
The reproductive gland arises as an elongated fold, the genital ridge or
genital fold. The extent of this fold, in general, is longer than the actual site
from which the rudimentary gonad or reproductive gland arises, and it may
extend for a considerable distance along the mesonephric kidney. Felix ('06)
designates three general areas of the primitive genital ridge:
( 1 ) a gonal portion, from which the sex gland arises,
(2) a progonal area in front of the gonal area, which gives origin to the
anterior suspensory ligament of the gonad, and
(3) an epigonal area behind, which continues caudally as a peritoneal sup-
port along the mesonephric kidney (fig. 3A).
The rudimentary structural parts of the early genital ridge in the gonal
area, viewed in transverse section, consist of the following (fig. 350A-C):
(1) primitive germ cells (origin of the germ cells discussed in Chapter 3,
see figure 60),
(2) the germinal (coelomic) epithelium and the primitive sex cords and
cells proliferated therefrom, and
(3) contributions from mesonephric tissue, forming in most vertebrates
the rete tissue of the urogenital union together with the primitive
mesenchyme of the gonad.
The first stages in the development of the gonad consist of a thickening
of the germinal (coelomic) epithelium and of a rapid and copious prolifera-
tion of cells from its inner surface. The primitive (primordial) germ cells
become associated with the thickened germinal epithelium and its proliferated
cells, and migrate inward into the substance of the gonad with the cells of
the germinal epithelium (fig. 350B).
As a result of the activities of the germinal epithelium, a mass of cells, the
Fig. 350 — (Continued)
1912, reference same as in C, above.) (G) Differentiating testis in the wood frog, Rana
sylvatica. (Redrawn from Witschi, 1931, Sex and Internal Secretions, edited by Allen
et al., Williams and Wili<ins, Baltimore.) (H) Ingrowth of sex cords from germinal
epithelium of ovary of 6 weeks old rabbit. (Redrawn from Brambell, 1930, The Devel-
opment of Sex in Vertebrates, Macmillan, N. Y.) (I) Section through differentiating
ovary in the opossum, 63 mm. pouch young. (J) Differentiating ovary in the wood
frog, Rana sylvatica. (Redrawn from Witschi, 1931, reference same as G, above.)
792 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
so-called epithelial nucleus (Felix, '12), is deposited in the genital ridge
between the coelomic (germinal) epithelium and the Malpighian (renal) cor-
puscles of the mesonephric kidney (fig. 350C). As the epithelial nucleus in-
creases in quantity, the genital ridge bulges outward from the general surface
of the mesonephric kidney, and, at the same time, the nuclear cells push into
the mesonephric substance against the renal corpuscles (figs. 345G; 350A-C).
During the early stages of the proliferative activities of the germinal epi-
thelium in most vertebrates, cellular cords, the sex or medullary cords, appear
to arise from the germinal epithelium (fig. 350B). These cords of cells are
composed as indicated above of epithelial and germ cells. However, in the
mouse and in the human, the proliferative activity of the germinal epithelium
is such that the cellular nucleus of the genital ridge arises without a visible,
dramatic ingrowth of cellular cords from the germinal epithehum (Brambell,
'27; Felix, '12). Still, the cellular sex cords or elongated masses of cells do
appear as secondary developments somewhat later in the genital ridges of the
mouse and human (fig. 350C).
The early gonad up to this stage of development represents an indifferent,
bipotential condition, having the structural basis for differentiation either into
the testis or ovary (see figs. 350C; 351C-3). The indifferent condition in
the human sex gland is present when the embryo is about 11 to 14 mm. long,
i.e., at about the sixth or seventh week; in the chick, it occurs during the
sixth day of incubation; and in the frog, it is present during the larval period.
2. Development of the Testis
a. Mammal (Human)
As the indifferent gonad begins to differentiate into the testis, the following
behavior is evident:
( 1 ) The germinal epithelium becomes a distinct flattened membrane, sep-
arated from the primitive tunica albuginea. Unlike the conditions in
the developing ovary, the germinal epithelium quickly loses its ger-
minative character and forms the relatively inactive, superficial mem-
brane of the sex gland (fig. 350D). (The tunica albuginea eventually
becomes a connective tissue layer below the coelomic (germinal)
epithelium of the male and female sex glands.)
(2) The primitive sex or medullary cords of the indifferent gonad grow
more pronounced, and they possibly may segregate lengthwise into
separate, elongated cellular masses (fig. 350D).
(3) These elongated cellular masses or primitive seminiferous tubules be-
come remodeled directly into the later seminiferous tubules. In doing
so, their distal ends (i.e., the ends toward the primitive tunica albu-
ginea of the sex gland) appear twisted and show anastomoses with
neighboring seminiferous tubules, while their proximal ends assume
DEVELOPMENT OF REPRODUCTIVE SYSTEM 793
a Straightened condition and project inward toward the area connect-
ing the sex gland with the mesonephric kidney (fig. 350D).
(4) In the area between the inner ends of the developing seminiferous
tubules and the Malpighian corpuscles of the mesonephric tubules, a
condensation of cellular material occurs which forms the rete primor-
dium (fig. 350D). From the rete primordium the future rete tubules
are developed.
(5) As the rete tubules form, they unite with the inner straightened por-
tions of the seminiferous tubules (the developing tubuli recti) and
distally with the renal corpuscles (Malpighian bodies) of the meso-
nephric tubules (fig. 350E). The appropriated mesonephric tubules
form to a considerable degree the efferent ductules of the epididymis.
(6) While the foregoing processes ensue, the sex gland gradually becomes
separated as a body distinct from the mesonephric kidney and appears
suspended from the kidney by a special peritoneal support, the
mesorchium. Within the mesorchium are found blood vessels, lym-
phatics, and the efferent ductules of epididymis (fig. 350D).
(7) Coincident with these changes, mesenchyme between the developing
seminiferous tubules forms a coating of connective tissue around each
tubule. This connective tissue membrane gives origin to the basement
membrane of the seminiferous tubule. Within the tubules, epithelial
elements, primitive germ cells, and sustentacular elements (Chap. 3)
or Sertoli cells appear. The Sertoli cells extend from the connective-
tissue wall of the tubule inward between the epithelial and genitaloid
cells. The genital cells lie close to the surrounding connective-tissue
or basement membrane (figs. 8; 350F).
(8) Between the developing seminiferous tubules, the various cells, blood
vessels, etc., of the interstitial tissue begin to appear (fig. 350F; see
Chap. 1).
(9) Accompanying the foregoing transformations, the primitive tunica
albuginea, which originally appeared as a narrow area, containing a
few scattered cells between the germinal epithelium and the sex cords,
becomes thickened and develops into a tough, connective-tissue layer,
surrounding the testicular structures and separating the latter from
the covering coelomic epithelium. This appearance of the tunica albu-
ginea is one of, the characteristic features of testicular development.
Extending from the tunica albuginea inward between small groups of
seminiferous tubules as far as the rete area or mediastinum, connective-
tissue partitions are formed. These partitions are the septula. Each
septulum comes to surround a small group of seminiferous tubules
and thus divides the testis into compartments or lobules (fig. 7).
Within each lobule, several seminiferous tubules are found, with the
tubuli contorti or twisted portion of the tubules lying distally within
794 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
the compartment and the tubuli recti lying proximally toward the rete
testis and mediastinum.
The formation of the rete-testis canals and of the urogenital union in general
has been the subject of much controversy. In the elasmobranch fishes, Brachet
('21) considered the rete-testis canals to be formed by the nephrostomial
canals of the anterior mesonephric tubules which unite with the developing
seminiferous tubules. *In the frog, Witschi ('21) believed a condensation of
cells in the hilus of the testis formed the rudiments of the rete tubules and
that these rudiments unite with the mediastinal ends of the seminiferous
tubules on the one hand and with the renal corpuscles of the mesonephric
tubules on the other, forming the urogenital union. In the chick, it is pos-
sible that the rete tubules arise as outgrowths from the renal corpuscles (Lillie,
'30, p. 394). In the human, Felix ('12) concluded that the rete tubules arise
from a rete rudiment in the testicular hilus, but de Winiwarter ('10) con-
sidered them as outgrowths from the renal (Malpighian) corpuscles of the
mesonephric tubules.
b. Chick
The development of the testis in the chick closely resembles that described
above for the mammal. The sex or medullary cords arise during the fifth and
sixth days of incubation from the germinal epithelium (fig. 350B). For a de-
tailed description, consult Swift, '16, and Lillie, '30.
c. Frog
The main essentials of testicular development in the frog follow the pattern
described above. However, because the gonadal rudiment of the frog differs
slightly from that described for the mammal, certain features are presented
here.
The germinal epithelium of the primitive gonad of the anuran is thin, and
the primitive germ cells lie, together with various epithelial elements, below the
germinal epithelium. In the center of this primitive gonad is the slit-like primi-
tive gonadal cavity. This cavity is surrounded by the germ cells, epithelial
cells and germinal epithelium. This condition may be regarded as the indifferent
stage of gonadal development.
In the differentiation of the testis, cellular strands, the rudiments of the
future rete tubules, grow down into the primitive gonadal cavity from the
mesonephric kidney. In the male, these mesonephric strands are thick and
grow rapidly. The primitive germ cells and epithelial cells eventually grow
inward across the primitive gonadal cavity and become clustered about the
mesonephric strands (fig. 350G).
At first the germ cells and epithelial elements form cellular nests associated
with the mesonephric strands. Later, the cellular nests and associated cells
from the mesonephric strands elongate into the primitive seminiferous tubules.
DEVELOPMENT OF REPRODUCTIVE SYSTEM 795
These seminiferous tubules develop lumina and unite directly with the rete
tubules which arise, in the meantime, from cells of the mesonephric strands.
The distal ends of the rete tubules join with the Malpighian corpuscles of
certain mesonephric tubules. The mesonephric tubules thus united to the rete
tubules are, of course, joined to the mesonephric duct. In consequence, these
mesonephric tubules become the efferent ductules or vasa efferentia of the
testis (Witschi, '21, '29).
3. Development of the Ovary
a. Mammal
1) Formation of Primary Cortex and Medulla. The early phases of differ-
entiation of the ovary varies in different mammalian species. Two features,
however, are constant — features that serve to distinguish the differentiating
ovary from the testis. One of these features consists of the fact that the ovary
is more retarded in its development than the testis; the testicular features
appear sooner in the male embryo than do ovarian features in the female
embryo. This is a negative difference, but nevertheless, it serves to distinguish
the two sexes. Another constant and positive feature, however, is that the
germinal epithelium in the ovary retains its proliferative activity, while, in
the differentiating testis, this activity is lost in the early stages of differentiation.
In the cat and rabbit (de Winiwarter, '00, '09), and in the calf and opossum,
the first stage of ovarian differentiation is indicated by a second proliferation
of sex cords (Pfliiger's cords) from the germinal epithelium (fig. 350H and I).
The earlier sex or medullary cords thus are pushed inward toward the hilus
of the ovary, and a definite compact primary cortex is established, containing
cords of epithelial and germ cells. The medullary cords become broken up
in the meantime and are pressed inward in the direction of the forming primary
medulla of the ovary. Some of the germ cells of the medullary cords undergo
the earlier stages of meiosis but soon degenerate.
Synchronized with the foregoing changes in the peripheral area of the ovary
are transformations within the hilar region, that is, the area of the ovary
nearest to the mesonephric kidney. A conspicuous feature of these changes
is the ingrowth of mesenchyme and differentiating connective tissue from
the mesonephric kidney. Three morphogenetic phenomena accompany this
ingrowth:
( 1 ) Blood vessels grow into the ovary from the mesonephric kidney to
form a primitive vascular plexus within the developing medulla.
(2) A concentration of mesenchymal cells appears in the area between
the developing ovary and the mesonephric kidney. This concentration
of mesenchyme is the rete blastema, or the rudiment of the rete ovarii.
(3 ) From the region of the rete blastema radiating columns of mesenchyme
and differentiating connective tissue fibers extend outward through
796 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
the medullary zone into the cortical zone of the ovary. These columns
establish the septa ovarii. The septa ovarii branch distally, dividing
the cortical zone into columns and compartmental areas of germ and
epithelial cells.
The proliferation of sex cords (Pfluger's cords) may continue from the
germinal epithelium for an extensive period in certain mammals, such as the
cat. De Winiwarter and Sainmont ('09) noted three successive periods, al-
though Kingsbury ('38) was unable to find a clear-cut distinction between
the first and second proliferation. In the developing opossum, active prolifera-
tion from the germinal epithelium may be observed up to a time just previous
to the fourth month, following birth (Nelsen and Swain, '42).
At an early stage of development, the primitive ovary in transverse section
presents the following features (fig. 3501):
( 1 ) an outer proliferating germinal epithelium;
(2) a primitive tunica albuginea beneath the germinal epithelium, com-
posed of epithelial and germ cells together with some connective tissue
elements contributed by the ovarian septa;
(3) the primitive cortex, a compact layer within the primitive tunica albu-
ginea, composed of masses of germ cells, egg cords, and epithelial
elements, together with strands of differentiating mesenchymal cells.
The mesenchymal strands from the ovarian septa segregate the egg
cords into separate areas of germ cells and epithelial elements;
(4) internally, near the mesovarium or the peritoneal support of the ovary,
is the primitive medulla composed of epithelial cells, mesenchyme,
blood vessels, and some oocytes and oogonia;
(5) in the region of the mesovarium is a compact cellular mass, the rudi-
ment of the rete ovarii, the homologue of the rudiment of rete testis
in the male. The fundament of the rete ovarii continues rudimentary,
but a framework of connective tissue is established in this area of
the ovary similar to that of the mediastium in the testis, and
(6) from the area of the rete ovarii, radiating strands of mesenchymal
cells, extend peripherally through the medulla and into the cortex,
and thus establish the sepia ovarii, i.e., septa of the ovary. Certain rela-
tively large "interstitial cells" appear in the septula areas.
2) Formation of the secondary cortex and medulla. During later stages in
ovarian development the following changes are effected:
(1) The primitive tunica albuginea becomes converted into a relatively
thick secondary tunica albuginea lying between the germinal epithelium
and the cells of the cortical zone. It contains connective-tissue fibrils
and fibers of larger dimension, together with mesenchyme and con-
nective tissue cells. The changes in the developing tunica albuginea
DEVELOPMENT OF REPRODUCTIVE SYSTEM 797
are associated with an ingrowth of cells from the ovarian septa into
the albuginean tunic.
(2) The primitive cortex transforms into a thick secondary cortex, con-
taining many oocytes, some of which are surrounded by epithehal
cells. The complex of an oocyte enclosed by epithelial cells forms a
primitive egg follicle, which in mammals is called a primary Graafian
follicle. The complete development of the Graafian follicle, however,
does not occur until sexual maturity, although earlier stages may be
produced previous to this period.
(3) A secondary medulla is formed containing a connective tissue net-
work, enclosing blood vessels. From these blood vessels branches extend
into the cortex. Some genitaloid cells may be found in the medulla.
(4) The rete blastema remains as a compact mass of cells, sharply de-
limited from surrounding cells. It comes to lie in the area between
the ovary and the mesovarium, and forms the rete ovarii.
The development of the human ovary differs somewhat from the account
given above in that active proliferation of cortical cords from the germinal
epithelium is problematical. The proliferation of cells in the developing human
ovary appears more gradual, and the egg cords of the primary cortex are
developed in a gradual manner from cells lying below the germinal epithelium
of the undifferentiated gonad (Felix, '12, p. 904).
b. Chick
The pattern of ovarian development in the chick follows that of the mammal,
and a cortex and a medulla are established. One clear distinction in the ovarian
development in the chick compared with that in the mammal occurs, however,
for the right sex-gland rudiment remains vestigial in the chick while the left
rudiment develops rapidly into the ovary. Thus it is, that sex differences can
be distinguished in developing chicks by macroscopic examination of the sex
glands during the latter part of the second week of incubation. The enlarged
appearance of the left ovary in the female chick becomes noticeable at
this time.
c. Frog
The developing ovary in the frog differs primarily from the developing
testis in two ways:
( 1 ) The germ cells and accompanying epithelial cells remain peripherally
near the germinal epithelium, where they multiply and increase in
number; some of them enlarge during the formative stages of the
oocyte.
(2) The mesonephric rete cords, which in the testis are much thickened,
appear slender in the developing ovary and fuse to form the lining
[
- PRONEPHRIC
DUCT GIVES
ORIGIN TO
MESONEPHRIC
DUCT
MULLERIAN
DUCT ARISES BY
LONGITUDINAL
SPLITTINGOF
PRONEPHRIC
DUCT
MULLERIAN DUCT ARISES
FROM THREE COELOMIC
INVAGINATIONS NEAR
PRONEPHRIC FUNNELS;
CAUDAL END GROWS
BACKWARD TO CLOACA
VASA
EFFERENTIA
(MESONEPHRIC
TUBULES)
RENAL
PORTION OF
OPISTHONEPHRIC
KIDNEY
SPERM SAC
POSSIBLY SOME
CONTRIBUTIONS FROM
WOLFFIAN DUCT TO
MULLERIAN DUCT IN
URODELES
* PRONEPHRIC
(MESONEPHRIC)
DUCT
DUCT OF LEYDIG =
SPERM DUCT PLUS
MFSONFPHHir / URINARY DUCT. IT
TUBULES FORM THEREFORE REPRESENTS
TESTIS- MESONEPh"r,c a specialized
DuIt CoSnIcVioS I .^.^-"-E. . QF_ WOLFFI
B-2
NDIFFERENT
CONDITION
Fig. 351. Development of the reproductive and urinary ducts in vertebrates. (A-1
to A-4) Development of the reproductive ducts in Squalus acanthias. In A-2 the origin
of the ostial funnel or coelomic opening of the oviduct is presented as a derivative of
the opening of one or more pronephric tubules into the coelomic cavity. In fig. A-3,
the urinary or opisthonephric duct is independent of the mesonephric (pronephric) duct
which now is the vas deferens. The opisthonephric duct appears to take its origin as an
evagination from the caudal end of the original pronephric duct. (B-1 to B-4) Devel-
opment of the reproductive ducts in the frog. B-1 is adapted from data given by Hall,
1904, Bull. Mus. Comp. Zool. at Harvard College, vol. 45. (C-1 to C-7) Development
of the reproductive and urinary ducts in mammals. The Miillerian duct arises as an
invagination of the coelomic epithelium at the anterior end of the mesonephric kidney.
(See fig. 35 ID.) Once its formation is initiated, it grows caudalward along the pronephric
{Continued on facing page.)
798
DEVELOPMENT OF REPRODUCTIVE SYSTEM 799
tissue of the ovarian sac or enlarged space within the ovary (fig. 350J).
The ovary of the fully developed frog (and amphibian ovaries in
general) is saccular (Chap. 2).
4. Development of the Reproductive Ducts
Most vertebrate embryos, with the exception of those of teleost and certain
other fishes, develop two sets of ducts, one set of which later functions as
reproductive ducts. These ducts are the mesonephric. Wolffian or male
ducts and the Miillerian or female ducts. In the elasmobranch fishes, the
Mijllerian duct arises by a longitudinal division of the mesonephric duct (fig.
351 A). In the Amphibia, the Miillerian duct takes its origin independently.
Anteriorly it arises as a peritoneal invagination of the coelomic epithelium,
in the region of the cephalic end of the mesonephros. Posteriorly, this peri-
toneal invagination, as it grows caudally, appears to receive, in some urodeles,
contributions from the mesonephric duct (fig. 35 IB). In the Amniota
the MUllerian duct arises independently by a tubular invagination of the
coelomic epithelium at the anterior end of the mesonephric kidney (fig.
Fig. 351 — (Continued)
(mesonephric) duct to join the cloaca (see fig. 351, C-2). The metanephric duct or
ureter arises as an evagination of the caudal end of the pronephric (mesonephric) duct
(see fig. 344A). C-2 is a drawing of the urogenital system of a 26 mm. pig embryo
viewed from the ventral aspect. Note extent of Miillerian duct growth caudalward. C-3
represents a generalized indifferent condition of the urogenital system of the mammal.
C-4 and C-5 are diagrams of later stages in the development of the female (C-4) and
the male (C-5). These conditions pertain particularly to human embryos. However, by
a division of the uterus simplex into a bicornate or duplex condition it may be applied
readily to other mammals. (C-6) Later arrangement of reproductive ducts and the
associated ovaries in the human female after the descent of the ovaries. Observe origin
of various ligaments. (In this connection see also fig. 3.) (C-7) Later development of
the reproductive duct-testis complex in the human male, during descent of the testis
into the scrotum. Observe origin of testicular ligaments. (See also fig. 3.) (D) Trans-
verse section through anterior end of the meSonephric kidney of 10 mm. pig embryo
presenting the Miillerian duct invagination of the coelomic epithelium covering the
mesonephros. E-N are diagrams showing the adult excretory and reproductive duct
relationships in various fishes. The urinary ducts are shown in black. (Redrawn and
modified from Goodrich, 1930, Studies on the Structure and Development of Vertebrates,
Macmillan and Co., Limited, London, after various authors.)
It will be observed that in the male ganoid fish. Acipenser, the vasa efferentia extend
from a longitudinal testis duct through the anterior or genital part of the kidney to the
Wolffian (mesonephric) iduct. The Wolffian duct thus becomes a duct of Leydig as in
the frog. However, in teleosts, and in Protopterus and Polypterus, a separate genital
duct which opens into the caudal end of the mesonephric duct is evolved. Hence, the
Wolffian (mesonephric) duct in these forms functions as a urinary duct only. The
separation of the genital duct from the urinary duct, with the exception of the urogenital
sinus region at the posterior end, is a fundamental characteristic of most vertebrate male
reproductive systems, including many amphibia. In female fishes, fig. 351, I-N, as in
other vertebrates, the reproductive duct is always distinct from the urinary duct. The
exact homologies of the reproductive duct in forms such as Lepisosteus (Lepidosteus)
and teleosts (fig. 351. L-N) with the Miillerian duct in other verterbates is not clear.
MULLERI4N DUCT ARISES
INDEPENDENTLY ST INV4GIN4TI0N
OF THE COELOMIC EPITHELIUM
OVERLYING MESONEPHRIC KIDNEY
AT The cephalic end of the
LATTER STRUCTURE DUCT
GROWS BACK TO JOIN THE CLOACA
COMPARE WITH C-3 C - 7.
Fig. 351 — (Continued)
See legend on pp. 798 and 799.
800
DEVELOPMENT OF REPRODUCTIVE SYSTEM
801
2 PROTOPTERUS
J W K
Q POLYPTERUS 5 AMIft
N.
J SiLMONID
Fig. 351 — (Continued)
See legend on pp. 798 and 799.
35 IC). The blind caudal end of the invagination grows posteriorly along
the side of the mesonephric duct to join the cloaca (fig. 351C-2).
a. Male Reproductive Duct
The developing gonad of the males of Amphibia, reptiles, birds, and mam-
mals, together with the elasmobranch and ganoid fishes, appropriates the
mesonephric duct for genital purposes. In this appropriation, the rete tubules
of the testis unite with certain of the mesonephric tubules. The latter form
the vasa efFerentia or efferent ductules of the epididymis (fig. 351A-C). In
teleosts, dipnoan fishes, and Polypterus, the marginal testicular duct becomes
modified into a vas deferens which conveys the genital products to the uro-
genital sinus (fig. 351F-H).
In all vertebrates and in some mammals (Chap. 1), the testis remains
within the abdominal cavity. However, in most mammals and in the flatfishes,
there is a posterior descent of the testis (figs. 3 and 5) into a compartment
posterior to the abdominal cavity proper.
802 THE EXCRETORY AND REPRODUCTIVE SYSTEMS
b. Female Reproductive Duct
In the eutherian or placental mammals, the two Mullerian ducts in most
species unite posteriorly to form a single uterovaginal complex (fig. 349B,
D). In all other vertebrates, the Mullerian ducts or oviducts remain sep-
arate (see figures 33; 351A-4, B-4). The vagina of the eutherian female
mammal probably is constructed partly of entoderm from urogenital sinus,
for entoderm from this area invades the caudal end of the uterovaginal rudi-
ment and lines the vaginal wall, at least in part (fig. 349B, D).
In the teleost fishes (fig. 35 IM, N), the origin of the MUllerian ducts is
problematical (Goodrich, '30, pp. 701-705).
5. Development of Intromittent Organs
Various types of intromittent structures are described in Chapter 4. The
development of pelvic-fin modifications under the influence of the male sex
hormone occurs in fishes. Cloacal intromittent structures are developed in cer-
tain Amphibia. A definite penis occurs in reptiles, certain birds, and in all
mammals. The transformation, occurring in the external genital structures in
male and female human embryos, is shown in figure 349E-K.
6. Accessory Reproductive Glands in Mammals
Refer to figures 2 and 349C.
a. Prostate Gland
The prostate gland arises as entodermal outgrowths from the membranous
urethra near the entrance of the genital ducts. The surrounding mesenchyme
provides the connective tissue and muscle. The paraurethral glands or ducts
of Skene in the female represent minute homologues of the prostate gland.
b. Seminal Vesicles
The seminal vesicles arise as saccular outgrowths from the mesonephric
ducts.
c. Bulbourethral Glands
The bulbourethral (Cowper's) glands in the male arise as outgrowths from
the entoderm of the cavernous urethra. The vestibular glands or glands of
Bartholin are the female homologues of the bulbourethral glands.
7. Peritoneal Supports for the Reproductive Structures
a. Testis and Ovary
The testis and ovary are pendent structures in all vertebrates and they are
supported by peritoneal extensions from the dorso-lateral region of the
BIBLIOGRAPHY 803
coelomic cavity. The support of the testis is the mesorchium and that of the
ovary is the mesovarium. However, supports other than those mentioned in
the preceding sentence are concerned with the support of the testis and ovary
during development. Figures 3A, B and 351C-3 demonstrate an anterior
Hgamentous, progonal support for the developing sex gland, whereas caudally
there is a posterior, epigonal support continuing posterially to join the inguinal
ligament of the mesonephros. In the developing mammal the progonal sup-
port merges with the diaphragmatic ligament of the mesonephros. Caudally
the inguinal ligament of the mesonephros joins a ligamentous area in the
genital swelling, known as the scrotal ligament in the male and the labial
ligament in the female. Consult fig. 351C-6 and C-7 for later history.
b. Reproductive Ducts
The male reproductive duct (vas deferens. Wolffian duct) lies close to the
kidney structures in the retroperitoneal space in most vertebrates other than
those mammals with descended testes (see Chap. 1). The male reproductive
duct, therefore, assumes a retroperitoneal position and is not suspended ex-
tensively within the coelomic cavity. On the other hand, the female repro-
ductive duct (oviduct) is a pendant, twisted structure and is supported by
a well-developed peritoneal support, the mesotubarium. In mammals, due to
the fact that the reproductive ducts tend to join posteriorly, the mesotubarial
supports, along the caudal region of the reproductive ducts, aid in dividing
the pelvic region of the coelomic cavity into two general regions, viz., a dorsal
or rectal recess, and a ventral, urinary recess (fig. 349L, M).
In the mammals, the mesotubarial support of the Fallopian tube is known
as the mesosalpinx. The mesosalpinx is continuous with the broad, shelf-like,
lateral support of the uterus, known as the broad ligament. The broad liga-
ment is developed from the mesotubarium together with the remains of the
mesonephric kidney substance (349L, M). The round ligament of the mam-
malian uterus and the ovarian ligament arise from a basic rudiment com-
parable to the gubernaculum testis in the male (see figs. 3; 351C-3, C-6, C-7).
Bibliography
Brachet, A. 1921. Traite d'Embryologie de Winiwarter. H. 1900. Recherches sur
des Vertebres. Paris. I'ovogenese et I'organogenese de Tovaire
Brambell, F. W. R. 1927. The develop- des mammiferes (lapin et homme). Arch,
ment and morphology of the gonads of biol., Paris. 17:33.
the mouse. Part I. The morphogenesis
of the indiflFerent gonad and the ovary. . 1910. Contribution a I'etude de
Proc. Roy. Soc, London, sB. 101:391. I'ovaire humain. Arch biol., Paris. 25:683.
Brauer, A. 1902. Beitrage zur Kenntniss
der Entwicklung und Anatomie der ^"^ Sammont, G. 1909. Nouvelles
Gymnophionen. III. Die Entwicklung recherches sur I'ovogenese et I'organoge-
der Excretionsorgane. Zool. Jahrbiicher, nese de I'ovaire des mammiferes (chat).
Abt. Anatomie und Ontogenie. 16:1. Arch biol., Paris. 24:1.
804
THE EXCRETORY AND REPRODUCTIVE SYSTEMS
Felix, W. 1906. Chap. 2, Part 111, in Ver-
gieichenden und Experimentellen Ent-
wickelungslehre der Wirbeitiere by O.
Hertwig. Gustav Fischer, Jena.
. 1912. Chap. 19 in Human Em-
bryology by F. Keibel and F. P. Mall.
J. B. Lippincott Co., Philadelphia.
Field, H. H. 1891. The development of
the pronephros and segmental duct in
Amphibia. Bull. Mus. Comp. Zool. at
Harvard College. 21:201.
Goodrich, E. S. 1930. Studies on the Struc-
ture and Development of Vertebrates.
Macmillan and Co., London.
Gruenwald, P. 1937. Zur Entwicklungs-
mechanik des urogenitalsystems beim
Huhn. Arch. f. Entwicklngsmech. d.
Organ. 136:786.
. 1939. The mechanism of kidney
development in human embryos as re-
vealed by an early stage in the agenesis
of the ureteric buds. Anat. Rec. 75:237.
. 1941. The relation of the grow-
ing Mullerian duct to the Wolffian duct
and its importance for the genesis of
malformations. Anat. Rec. 81:1.
Hamburger, V. and Hamilton, H. L. A se-
ries of normal stages in the development
of the chick embryo. J. Morph. 88:49.
Howland, R. B. 1916. On the effect of re-
moval of the pronephros of the amphib-
ian embryo. Proc. Nat. Acad. Sc. 2:231.
. 1921. Experiments on the effect of
removal of the pronephros of Amblys-
toma punctatum. J. Exper. Zool. 32:355.
Kerr, J. G. 1919. Textbook of Embryol-
ogy, Vol. II, Vertebrata with the Excep-
tion of Mammalia. Macmillan Co., Ltd.,
London.
Kingsbury, B. F. 1938. The postpartum
formation of egg cells in the cat. J.
Morphol. 63:397.
Lillie. F. R. 1930. The Development of
the Chick. Henry Holt & Co., New York.
Nelsen, O. E. and Swain, E. 1942. The
prepubertal origin of germ cells in the
ovary of the opossum (Didelpliys vir-
giniana). J. Morphol. 71:335.
Price, G. C. 1896. Development of the
excretory organs of a myxinoid, Bdello-
stoma stouti Lochington. Zool. Jahrb.
Anat. u. Ontogenic. 10:205.
Scammon, R. E. 1911. Normal plates of
the development of Squalus acanthias.
Chap. 12 in Normentafeln zur Entwick-
lungsgeschichte der Wirbeitiere von F.
Keibel. G. Fischer, Jena.
Swift, C. H. 1916. Origin of the sex cords
and definitive spermatogonia in the male
chick. Am. J. Anat. 20:375.
Swingle, W. W. 1919. On the experimental
production of edema by nephrectomy. J.
Gen. Physiol. 1:509.
Witschi, E. 1921. Development of gonads
and transformation of sex in the frog.
Am. Nat. 55:529.
. 1929. Studies on sex differentia-
tion and sex determination in amphib-
ians. I. Development and sexual differ-
entiation of the gonads of Rana sylvatica.
J. Exper. Zool. 52:235.
19
Tne Nervous System
A. Introduction
1. Definition
2. Structural and functional features
a. The morphological and functional unit of the nervous system
b. The reflex arc
c. Structural divisions of the vertebrate nervous system
d. The supporting tissue
B. Basic developmental features
1. The embryonic origin of nervous tissues
2. The structural fundaments of the nervous system
a. The elongated hollow tube
b. The neural crest cells
c. Special sense placodes
3. The histogenesis of nervous tissue
a. The formation of neurons
1) General cytoplasmic changes
2) Nuclear changes
3) Growth and development of nerve-cell processes
b. The development of the supporting tissue of the neural tube
c. Early histogenesis of the neural tube
d. Early histogenesis of the peripheral nervous system
C. Morphogenesis of the central nervous system
1. Development of the spinal cord
a. Internal changes in the cord
b. Enlargements of the spinal cord
c. Enveloping membranes of the cord
2. Development of the brain
a. The development of specialized areas and outgrowths of the brain
1 ) The formation of the five-part brain
2) The cavities of the primitive five-part brain and spinal cord
b. The formation of cervical and pontine flexures
c. Later development of the five-part brain
D. Development of the peripheral nervous system
1. Structural divisions of the peripheral nervous system
2. The cerebrospinal system
3. General structure and function of the spinal nerves
4. The origin, development and functions of the cranial nerves
O. Terminal
805
806 THE NERVOUS SYSTEM
I. Olfactory
II. Optic
III. Oculomotor
IV. Trochlear
V. Trigeminal
A. Ophthalmicus or deep profundus
B. Maxillaris
C. Mandibularis
VI. Abducens
VII. Facial
VIII. Acoustic
IX. Glossopharyngeal
X. Vagus
XI. The spinal accessory
XII. Hypoglossal
5. The origin and development of the autonomic system
a. Definition of the autonomic nervous system
b. Divisions of the autonomic nervous system
c. Dual innervation of thoracicolumbar and craniosacral autonomic nerves
1 ) Autonomic efferent innervation of the eye
2) Autonomic efferent innervation of the heart
d. Ganglia of the autonomic system and their origin
E. The sense or receptor organs
1. Definition
2. Somatic sense organs
3. Visceral sense organs
4. The lateral-line system
5. The taste-bud system
6. The development of the olfactory organ
a. Development of the olfactory organs in SquaUis acanthias
b. Development of the olfactory organs in the frog
c. Development of the olfactory organs in the chick
d. Developm.ent of the olfactory organs in the mammalian embryo
7. The eye
a. General structure of the eye
b. Development of the eye
c. Special aspects of eye development
1) The choroid fissure, hyaloid artery, pecten, etc.
2) The formation of the lens
3) The choroid and sclerotic coat of the eyeball; the cornea
4) Contributions of the pars caeca
5) The origin of the ciliary muscles
6) Accessory structures of the eye
8. Structure and development of the ear
a. Structure
1 ) Three semicircular canals
2) An endolymphatic duct
3) A cochlear duct or lagena
b. Development of the internal ear
c. Development of the middle ear
d. Development of the external auditory meatus and pinna
F. Nerve-fiber-effector organ relationships
INTRODUCTION 807
A. Introduction
1. Definition
The nervous system serves to integrate the various parts of the animal into
a functional whole, and also to relate the animal with its environment. It
consequently is specialized to detect changes in the environment (irritability)
and to conduct (transmit) the impulses aroused by the environmental change
to distant parts of the organism. The environmental change provides the
stimulus, the protoplasmic property of irritability detects the stimulus, and
transmission of impulses thus aroused makes it possible for the animal to
respond once the impulse reaches the responding mechanism. This series of
events is illustrated well in less complex animal forms such as an ameba. In
this organism, the stimulus aroused by an irritating environmental change is
transmitted directly to other parts of the cell, and the ameba responds by
a contraction of its protoplasm away from the source of irritation. On the
other hand, the complex structure of the vertebrate animal necessitates an
association of untold numbers of cells, some of which are specialized in the
detection of stimuli, and others transmit impulses to a coordinating center,
from whence still other cells convey the impulses to specialized effector (re-
sponding) structures (fig. 352A).
2. Structural and Functional Features
a. The Morphological and Functional Unit of the Nervous System
There are two opposing views regarding the morphological and functional
unit of the nervous system. One view, widely championed, postulates that this
unit is a specialized cell called the neuron. The neuron is a distinct cellular
entity, having a cell body containing a nucleus and a central mass of cyto-
plasm from which extend cytoplasmic processes of various lengths (fig. 352B).
The nervous system is made up of many neurons in physiological contact
with each other at specialized functional junctions known as the synapses
(fig. 352A). The synapse represents an area of functional contact specialized
in the conduction of impulses from one neuron to another. However, it is not
an area of morphological fusion between neurons. Each neuron, according
to this view, originates from a separate embryonic cell or neuroblast of ecto-
dermal origin, and each develops a definite polarity, i.e. impulses normally
pass in one direction to the cell body and from thence distad to the area of
synapse.
A contrary, older view is the reticular or nerve-net theory. This theory as-
sumes that the nerve cells and their processes are a continuous mass of proto-
plasm or syncytium in which the "cell bodies" are local aggregations of a
nucleus and a cytoplasmic mass. The entire controversy between this and the
neuron theory revolves around the "synapse area." The neuron doctrine as-
808
THE NERVOUS SYSTEM
sumes a distinct morphological separation at the synapse, but the reticular
theory postulates a direct morphological continuity. We shall assume that the
neuron doctrine is correct.
b. The Reflex Arc
While the neuron, in a strict sense, represents the functional unit of the
nervous system, in reality, chains of physiologically related neurons form the
functional reflex mechanism of the vertebrate nervous system. The functional
SPINAL CORD
TELODENDRIfl
DENDRITES
DENDRITES
SENSE ORGAN
OR
RECEPTOR ORGAN
'/id'
'0/ MUSCLE FIBER OR
^ EFFECTOR ORGAN
MYELIN SHEATH-==4^^%--4^*
NEURILEMMA CELL ^ \-A'^%'-Kml*i
PROTOPLASMIC NET
AXIS CYLINDER
NODE OF RANVIER
SHEATH CELL
Fig. 352. Neuron structure and relationships. (A) Structural components of a simple
reflex arc. (B) Diagrammatic representation of a motor neuron. (Redrawn from
Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders, after
Barker.) (C) Developing nerve fiber (process) of young neuroblast. Observe growth
or incremental cone at distal end of growing process. (Redrawn from Ranson, 1939,
The Anatomy of the Nervous System, Philadelphia, Saunders, after Cajal, Prentiss-Arey.)
(D) Neuron from spinal ganglion of a dog showing ganglion cell body with its sur-
rounding capsular cells and capsule. Observe that the capsular cells and capsule are
continuous with sheath cell and neurilemma. (Redrawn, somewhat modified, from
Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders.) (E)
Longitudinal section of myelinated nerve fiber. (Redrawn from Ranson, 1939, The
Anatomy of the Nervous System, Philadelphia, Saunders, after Nemiloff, Maximow-
Bloom.)
BASIC DEVELOPMENTAL FEATURES 809
reflex mechanism is an arrangement of neurons known as the reflex arc. Theo-
retically, a simple type of reflex arc would possess (fig. 352A):
( 1 ) a sense receiving structure, the receptor;
(2) the sensory neuron, whose long afferent or sensory fiber contacts the
sensory receptor, while its efferent fiber or axon continues from the
body of the neuron to the central nervous system. Within the central
nervous system the terminal fibers (telodendria) of the eff"erent fiber
of the sensory neuron forms a synapse with
(3) the dendrites of an efferent neuron. From the efferent or motor neuron
a motor fiber (axon) leaves the central nervous system and con-
tinues to
(4) the effector organ.
Functionally, however, even the simplest type of reflex arc may not be as
elementary as this. More probably, a system of one or more association neu-
rons placed between the sensory and motor neurons exists in most instances.
c. Structural Divisions of the Vertebrate Nervous System
The nervous system of vertebrate animals consists of
( 1 ) the central nervous system, a tubular structure composed of a coordi-
nated assembly of association neurons and their processes. The central
nervous system is integrated with
(2) the peripheral nervous system constructed of a series of sensory and
motor neurons which connect the central nervous system with distal
parts of the body. Through the medium of various types of sense re-
ceptors the central nervous system is made aware of changes in the
external and internal environment of the body.
d. The Supporting Tissue
In addition to the irritable cellular neurons, the nervous system contains
connective or supporting tissue. However, unlike most of the other organ
systems of the body, the supporting tissue of the nervous system is derived
mainly from an ectodermal source. Small amounts of connective tissue of
mesodermal origin parallel the various blood capillaries which ramify through
nervous tissue, but the chief supporting tissue of the brain and spinal cord is
the neuroglia of ectodermal origin. The neuroglia consists of two main cellular
types, the ependymal cells and the cells of the neuroglia proper.
The ependymal cells (fig. 353A) form a single layer of columnar epi-
thelium which lines the lumen of the neural tube. From the inner aspect or
base of each ependymal cell a process extends peripherad toward the external
surface of the neural tube (fig. 353F-H). Later the peripheral process may
be lost. During the earlier stages of their development the ependymal cells
are ciliated on the aspect facing the neurocoel (fig. 353A).
810 THE NERVOUS SYSTEM
The cells of the neuroglia proper lie within the substance of the nerve
tube between the neuron-cell bodies of the gray matter and also between the
nerve fibers of the white matter (fig. 353H). Conspicuous among the neuroglia
cells are the protoplasmic astrocytes (fig. 353D) which reside mainly among
the neurons of the gray matter and the fibrous astrocytes (fig. 353B) found
in the white matter. The processes of the fibrous astrocytes are longer and
finer than those of the protoplasmic astrocytes, and they may attach to blood
vessels (fig. 353B). Two other cellular types of neuroglia, the oligodendroglia
and the microglia cells, also are present (fig. 353C and E). The microglia
cells presumably are of mesodermal origin (Ranson, '39, p. 57).
B. Basic Developmental Features
1. The Embryonic Origin of Nervous Tissues
The ectoderm of the late gastrula is composed of two general organ-forming
areas, namely, neural plate and epidermal areas (fig. 192A). Both of these
primitive ectodermal areas are concerned with the development of the future
nervous system and associated sensory structures. From the neural plate region
arises the primitive neural tube (Chap. 10), the basic rudiment of the central
nervous system, whereas the line of union between the neural plate and the
epidermal areas gives origin to the ganglionic or neural crest cells which con-
tribute much to the formation of the peripheral nervous system. As observed
in Chapters 9 and 10, the determination of the neural plate material and the
formation of the neural tube are phenomena dependent upon the inductive
powers of the underlying notochord and somitic mesoderm in the Amphibia.
Presumably the same basic conditions obtain in other vertebrate embryos.
Fig. 353. Structure of the developing neural tube. (A) Ciliated ependymal cells
from ependymal layer of the fourth ventricle of a cat. (Redrawn from Maximow and
Bloom, 1942, A Textbook of Histology, Philadelphia, Saunders, after Rubaschkin.)
(B-E) Various types of neuroglia cells. (Redrawn from Ranson, 1939, The Anatomy
of the Nervous System, Philadelphia, Saunders, after Rio Hortega.) (F) Transverse
section of neural tube of three-day chick embryo. The spongioblasts are stained black
after the method of Golgi. (Redrawn from Maximow and Bloom, 1942. See reference
under A, after Cajal.) (G) Transverse section of part of spinal cord of 15 mm. pig
embryo showing structural details. This section was constructed from several sections.
The part of the section to the left reveals the neuroglial support of the developing
neuroblasts. (Redrawn from Hardesty, 1904, Am. J. Anat., 3.) (H) Transverse sec-
tion, constructed from sections, of part of the spinal cord of 55 mm. pig embryo
showing neuroglial support for developing neuron cells. (Redrawn from Hardesty, 1904.
Am. J. Anat.. 3.) (I) Transverse section of spinal cord of newborn mouse depicting
spongioblasts which are moving peripherally from the central canal. These spongioblasts
are in the process of transforming into stellate neuroglia cells or astrocytes. (J)
Transverse section of 9 mm. pig embryo portraying ependymal, mantle, and marginal
layers, external and internal limiting membranes, and blood vessels growing into the
nerve substance. (Redrawn from Hardesty, 1904, Am. J. Anat., 3.) (K) Transverse
section of spinal cord of 20 mm. opossum embryo indicating general structure of the
spinal cord. Observe dorsal root of spinal nerve growing into nerve cord at the right
of the section.
INTERNAL LIMITING MEMBRANE
EXTERNAL LIMITING MEMBRANE
!■'»( WHITE 'AM-
"■ "-TTER )lV'''^'t TJ^.'.'JZ^^^ — ^r* SPINAL GANGLION
Wr^^^^QT- MARGINAL LAVER
(»JP I ., ^f^fcliS^-l li; (WHITE MATTER]
I^W V7S»*ff/'l)W»/Vr3 OR MANTLE LAY
-V^ig;^;^ >"<^./-
-^^./■:
^^ ."^ ■ ' - ■
VENTRAL
FLOOR Plate
/ENTRAL FLOOR PLATE
NLAGE OF VENTRAL HORN
?^^^ J.
Fig. 353. (See facing page for legend.)
811
812 THE NERVOUS SYSTEM
2. The Structural Fundaments of the Nervous System
The early nervous system shortly after the neural tube is formed is com-
posed of an elongated, hollow tube, aggregations of neural crest cells, and a
series of sense placodes.
a. The Elongated Hollow Tube
The primitive neural tube, located dorsally in the median plane (fig. 217G
and H), forms the basis for the central nervous system and potentially is
composed of two major regions, namely, the future brain region at its anterior
end and posteriorly the rudiment of the spinal cord. The future brain region
quickly develops three regions, viz.:
( 1 ) the prosencephalon, or the rudiment of the forebrain;
(2) the mesencephalon, or future mid-brain region, and
(3) the rhombencephalon, or hindbrain region (fig. 354D and E).
The rhombencephalon passes imperceptibly into the developing spinal cord,
or the primitive neural tube posterior to the brain region.
The cephalic end of the primitive neural tube from the time of its formation
tends to present a primary neural flexure, the cephalic flexure (see Chap. 10).
This flexure occurs in the region of the mesencephalon. It is slight in teleost
fishes, more marked in amphibia, and pronounced in elasmobranch fishes,
reptiles, birds and mammals (fig. 354E and F).
During the early stages of neural tube development, the anterior end of the
tube tends to form primitive segments or neuromeres. These neuromeres fuse
together as they contribute to the primitive brain regions as indicated in
figure 354A-D (see Hill, 1900).
b. The Neural Crest Cells
As the neural tube is formed, the neural crest cells come to lie along the
dorso-lateral aspect of the neural tube. The crest cells soon become aggregated
together in clumps, each aggregation representing the initial stage in the for-
mation of the various cranial and spinal ganglia (see figures 347A; 357B-F).
c. Special Sense Placodes
The special sense placodes are a series of epithelial thickenings of the lateral
portions of the epidermal tube overlying the future head region. These plac-
odes, which represent contributions of the epidermal tube to the forming
nervous system, are as follows:
( 1 ) The nasal placodes, two in number, each arising on either side of the
ventro-anterior region of the primitive head.
(2) The lens placodes, two in number, each arising in relation to the
optic outpushing of the diencephalic portion of the forebrain.
BASIC DEVELOPMENTAL FEATURES
813
PROSENCEPHALON —1 \
( FOREBHilN ) L TELEN
Fig. 354. Early development of the brain in the chick and teleost fish showing the
tendency to form neural segments or neiiromercs. (All figures redrawn from Hill, 1900,
Zool. Jahrbiicher, abt. Anat. u. Ontogenie 13.) (A) Dorsal view of developing brain of
chick embryo of 4 pairs of somites. (B) Dorsal view of primitive brain or encephalon
of chick embryo of 7 pairs of somites. (C) Dorsal view of brain of chick embryo
with 11 pairs of somites. (D) Dorsal view of developing brain of chick embryo with
14 pairs of somites. (E) Lateral view of brain of chick embryo about 75 to 80 hours
of incubation. In the foregoing illustrations, observe that the neuromeres gradually fuse
to form parts of primitive five-part brain shown in E. (F) Brain, lateral view, Salmo
fario, 33 somites, 22 days old. Segments 1-3 represent the prosencephalon, 4 and 5 the
mesencephalon, 6 the anterior part of the rhombencephalon, and 7-11 to the posterior
region of the rhombencephalon. Observe that the cephalic flexure is present slightly at
this time. A little later in the 36 day embryo it is more pronounced.
(3) The acoustic placodes, two in number, taking their origin from the
dorso-lateral portion of the epidermal tube overlying the middle por-
tion of the hindbrain.
In water-dwelling vertebrates, other placodes arise in the head region as-
sociated with the lateral-line system. The lateral line placodes probably repre-
sent an extension of the acoustic placodal system in lower vertebrates. Hence,
the general term acoustico-lateral or neuromast system (see Goodrich, '30,
p. 732) may be applied to this general system of sensory structures.
(4) Taste-bud placodes. The taste buds are distributed variously in dif-
ferent vertebrate species. In man, cat and in other mammals they are
located on the tongue, particularly its posterior part (fig. 285E) on the
814 THE NERVOUS SYSTEM
soft palate, and in the pharyngeal area. In fishes, taste buds are found
generally over the buccal cavity and pharynx, and also on the outer
surface of the head and branchial region. In some teleosts they may
be distributed generally over the external surface of the body (fig.
356C). The external distribution of taste buds over the head region
occurs also in certain aquatic amphibia. Consequently, the distribution
of the epithelial thickenings which give origin to the taste buds varies
greatly in different vertebrates.
3. The Histogenesis of Nervous Tissue
a. The Formation of Neurons
The neurons of the central nerve tube arise from primitive neuroblasts.
The primitive neuroblasts in turn take their origin from the cells of the
ependymal zone of the nerve tube. The ependymal zone is the layer, two to
three cells in thickness, which lines the lumen or neurocoel of the developing
tube. Cell proliferation occurs within this zone, and the primitive neuroblasts
migrate outward into the more lateral areas. After leaving the immediate
confines of the ependymal zone, the neuroblasts presumably begin to differen-
tiate into the many peculiar forms of the neurons to be found within the
central nervous system. The neurons of the peripheral nervous system arise
from cells which migrate from the central nerve tube, and from cells of the
neural crests and certain sense placodes.
1) General Cytoplasmic Changes. The basic physiological functions of
irritability and conductivity found in living protoplasm is developed to a
high degree in the neuron or essential cellular entity of the nervous system.
In consequence, the morphological changes which the simple epithelial cell
of the forming neural tube assumes during its differentiation into a neuron
is in harmony with these basic functions. One of the morphological changes
in the developing neuroblast is the formation of coagulated threads of cyto-
plasmic material embedded in a more liquid cytoplasm. These threads are
known as neurofibrils, while the more liquid, less-differentiated parts of the
cytoplasm are called the neuroplasm. Accompanying the changes which pro-
duce the neurofibrils is the formation of another characteristic of neurons,
namely, processes or cytoplasmic extensions from the body of the cell (fig.
352B). These processes are of two general types, the dendrites and the axon
(neuraxis or axis cylinder). Several dendrites are generally present but only
one axon is developed. The exact function of the dendrites has been ques-
tioned but the possibility is conceded that they function as "the chief receptive
organelles of the neuron" (Maximow and Bloom, '42, p. 190), whereas the
axon is believed to convey the nerve impulse away from the cell body to the
terminal arborizations or teledendria (fig. 352A). The teledendria make physi-
ologic contact (i.e., they synapse) with the dendrites of other neurons or they
form a specialized relationship with effector cells such as glandular cells or
BASIC DEVELOPMENTAL FEATURES 815
muscle fibers (fig. 352A). The neurofibrils extend into the cell processes. The
precise relationship of the neurofibrils to conduction and transmission of nerv-
ous impulses is unknown. (Note: The formation of the sheaths surrounding the
nerve fiber is described on page 819.)
2) Nuclear Changes. Associated with the changes in the cytoplasm men-
tioned above are alterations of the nucleus. One of the striking features of
nuclear change is that it enlarges, and becomes vesicular, though the basichro-
matin remains small in quantity. The nucleolus experiences profound changes,
and is converted from a homogeneously staining body into a vacuolated struc-
ture in which the desoxyribose nucleic acid is irregularly localized along the
edges. Contemporaneous with the nucleolar changes there is a "marked produc-
tion of Nissl substance in the cytoplasm" (Lavelle, '51, p. 466). Accompany-
ing the changes in the nucleus is its loss of mitotic activity, although a centro-
some is present in the cytoplasm. All neuroblasts, however, do not lose their
power of division; only those which start to differentiate into neurons. During
embryonic life many potential neurons remain in the neuroblast stage and these
continue to proliferate and give origin to other neuroblasts. Shortly after birth
or hatching this proliferative activity apparently ceases, and the undifferen-
tiated neuroblasts then proceed to differentiate into neurons.
3) Growth and Development of Nerve-cell Processes. The early neuroblasts
of the central nerve tube are at first apolar, that is, that do not have distinct
processes. These apolar cells presumably transform in unipolar and bipolar
varieties of neuroblasts. The unipolar cells have one main process, the axon,
and the bipolar cells have two processes, an axon and a dendrite. From
these two primitive cell types multipolar neurons arise having several dendrites
and one axon (fig. 352B).
As the nerve-cell process begins to develop, a small cytoplasmic extension
from the cell body occurs. To quote directly from Harrison ('07), p. 118,
who was the first to study growing nerve-cell processes in the living cell:
"These observations show beyond question that the nerve fiber develops by
the outflowing of protoplasm from the central cells. This protoplasm retains
its amoeboid activity at its distal end, the result being that it is drawn out
into a long thread which becomes the axis cyhnder. No other cells or living
structures take part in the process. The development of the nerve fiber is
thus brought about by means of one of the very primitive properties of living
protoplasm, amoeboid movement, which, though probably common to some
extent to all cells of the'embryo, is especially accentuated in the nerve cells at
this period of development." The distal end of a growing nerve fiber has a
slight enlargement, the "growth cone" or "growth club" (fig. 352C). The
conclusions of Harrison on growing nerve fibers in tissue culture were sub-
stantiated by Speidel ('33) in his observations of growing nerve fibers in the
tadpole's tail.
Many different shapes of cells are produced during the histogenesis of the
816 THE NERVOUS SYSTEM
neural tube. However, two main morphological types of cells may be
considered:
( 1 ) One type of neuron possesses a short axon or axis cylinder. This type
of neuron lies entirely within the gray substance of the neural tube.
(2) In a second type of neuron a long fiber or axis cylinder is developed
and this fiber leaves the gray substance and traverses along the white
substance of the cord or within the fiber tracts of the forming brain.
In many instances, the cell body of the second type of neuron lies
within the gray matter of the spinal cord, but its axis cylinder passes
out of the nerve tube as the efferent or motor fiber of a spinal or cranial
nerve (fig. 355F and I).
b. The Development of the Supporting Tissue of the Neural Tube
The potential connective tissue cell of the neural tube is the spongioblast.
Spongioblasts are of ectodermal origin and differentiate into two main types
of cells: (1) Ependymal cells, and (2) neuroglia cells.
Spongioblasts together with primitive neuroblasts lie at first within the
ependymal zone of the neural canal particularly close to the lumen. Cilia
are developed on the free surface of each spongioblast lining the neurocoel.
From the opposite end of the cell, that is, the end facing the periphery of the
tube, an elongated process extends peripherad to the outer surface of the
neural tube. In this way a slender framework of fibers extends radially across
the neural tube, from the lumen to the periphery (fig. 353F-K). A spongio-
blast which retains a relationship with the lumen and at the same time possesses
a fiber extending peripherad is known as an ependymal cell. The ependymal
cells thus are those cells whose bodies and nuclei lie next to the lumen of
the developing spinal cord and brain but possess processes which radiate out-
ward toward the periphery of the cord (fig. 353A and F). The peripheral fiber
or extension may be lost in the later ependymal cell together with its cilia.
In fishes and amphibians the supporting elements of the central nerve tube
retain the primitive arrangement outlined above (see Ariens-Kappers, '36,
p. 46). However, in reptiles, birds and mammals, the radial pattern of many
of the primitive spongioblasts is lost, and these spongioblasts transform into
neuroglia cells, losing their connection with the lumen and with the external
limiting membrane of the tube (fig. 3531).
c. Early Histogenetic Zones of the Neural Tube
The neural plate of the late gastrula is a thickened area of cells of about
3 to 4 cells in thickness. As the neural plate is transformed into the neural
tube the majority of the neural plate cells become aggregated within the lateral
walls of the tube. The lateral walls of the developing neural tube in conse-
quence are thicker than the dorsal and ventral regions. As already observed
BASIC DEVELOPMENTAL FEATURES 817
in Chapter 10, this discrepancy in the thickness of the walls of the tube is
due (in the amphibia) to the inductive influence of the somite which comes
to lie along the lateral regions of the primitive tube. In the 9-mm. pig embryo,
the neural tube in transverse section begins to present three general zones
(fig. 353J), viz.:
(1) an ependymal layer of columnar cells lining the lumen,
(2) a relatively thick nucleated mantle layer occupying the middle zone
of the neural tube, and
(3) a marginal layer without nuclei extending along the lateral margins
of the tube.
The ependymal layer of cells lies against the internal limiting membrane
of the tube, and consists of differentiating spongioblasts as indicated above.
The mantle layer contains many neuroblasts and in consequence is referred
to as the middle nucleated zone. It forms the future gray matter of the neural
tube. The outer or marginal zone in its earlier phases of development is a
meshwork of neuroglia and ependymal cell processes. Later, however, the
processes of neurons come to lie among the fibrous processes of the neurogUa
and ependymal cells as the nerve cell fibers extend along the spinal cord. The
external limiting membrane lies around the outer edge of the marginal layer,
and thus forms the outer boundary of the tube. In figure 353H is shown the
relationships of the ependymal, mantle and marginal layers of the spinal cord
of a 55-mm. pig embryo together with the ependymal and neuroglia cells.
The arrangement of the ependymal, mantle and marginal layers in the spinal
cord of a 22-mm. opossum embryo is shown in figure 353K.
d. Early Histogenesis of the Peripheral Nervous System
The formation of the cerebrospinal series of nerves which comprise the
peripheral nervous system involves cells located within the neural crest ma-
terials and also within the mantle layer (gray matter) of the neural tube.
One feature of the development of the spinal nerves is their basic metamerism,
for a pair of spinal nerves innervates the somites of each primitive segment
or metamere.
The neuroblasts of each spinal nerve arise in two areas, viz.:
( 1 ) the neural crest material which forms segmental masses along the
lateral sides of the neural tube, and
(2) cells within the ventral portions of the gray matter of the tube.
In the development of a spinal nerve bipolar neuroblasts appear within
the neural crest material. Each bipolar neuroblast sends a process distad
toward the dorso-lateral portion of the neural tube and a second process
lateroventrad toward the body wall tissues, or toward the viscera. Later these
bipolar elements become unipolar and form the dorsal root ganglion cells.
Fig. 355. Development of general structural features of the spinal cord; the nuclei of
origin and nuclei of termination of cranial nerves associated with the myelencephalon.
(A-E) The formation of the central canal, dorsal median septum, dorsal median sulcus,
and ventral median fissure in pig embryos. Arrows in the dorsal part of the developing
nerve cord show obliteration of the dorsal part of the primary neurocoel by medial
growth of the lateral walls of the spinal cord. By this expansive, medial growth, the
dorsal median septum and the dorsal sulcus (fissure) are formed. Observe that the central
canal is developed from the ventral remains of the primary neurocoel after the obliteration
of the dorsal portion of the primary neurocoel has been effected. In diagrams C-E, the
818
MORPHOGENESIS OF CENTRAL NERVOUS SYSTEM 819
Within the ventral gray matter of the spinal cord, fusiform bipolar cells arise
which send processes at intervals out into the marginal layers and from thence
outward through the external limiting membrane of the tube at the levels
corresponding to the developing dorsal root ganglia. The groups of processes
which thus emerge from the neural tube below a single dorsal root ganglion
soon unite with the ventrolateral processes of the dorsal root ganglion cells to
form the ventral root of the spinal nerve. Within the neural tube the cell
bodies of the ventral root fibers soon form multipolar neuron cells.
As development proceeds, the cell bodies of the neurons within the dorsal
root ganglia become encased by capsular cells which develop from some of
the neural crest cells (fig. 352D). The capsular cells in consequence are of
ectodermal origin and they are continuous with the neurilemma sheath. The
cells of the neurilemma sheath also arise from certain neural crest cells and
from cells within the neural tube. These cells migrate distad as sheath cells
along with the growing nerve fiber. The neurilemma or sheath of Schwann
arises as an outward growth from the cytoplasm of the sheath cells; the
neurilemma sheath thus appears in the form of a delicate tube surrounding
the nerve fiber (axis cylinder) of the neuron (352D). Later on, a secondary
substance appears between the nerve fiber (axis cylinder) and the neurilemma
in many nerve fibers. This substance is of a fatty nature and forms the myelin
(medullary) sheath (fig. 352E). Myelin deposition by sheath cells depends
primarily upon an axis cylinder stimulus and not upon the sheath cells, for it
is only a particular type of nerve fiber, the myelin-emergent fiber, which pos-
sesses the ability to form myelin (Speidel, '33). In the peripheral nerve fibers,
the neurilemma at certain intervals dips inward toward the axis cylinder,
forming the node of Ranvier. The area between two nodes is known as an
internodal segment (fig. 352B). One sheath cell is present in each internodal
segment. The nerve fibers of the peripheral nervous system with respect to
Fig. 355 — Continued
arrows drawn in the ventral portions of the nerve tube indicate the ventro-medial ex-
pansion of lateral portions of the developing nerve tube with the subsequent formation
of the ventral median fissure. In E the dorsal, ventral, and lateral columns or funiculi
of white matter are shown. (F) Diagram depicting some of the principal fiber tracts
of the spinal cord of man. Ascending tracts on the right; descending tracts on the left.
(Redrawn from Ranson, 1939. For reference see G.) (G) Ventral view of human
spinal cord, nerves removed, showing cervical and lumbar enlargements. (Redrawn from
Ranson, 1939, The Anatomy of the Nervous System. Philadelphia, Saunders.) (H)
Diagram revealing the relation of the meninges, i.e., the protective membranes of the
central nervous system, to the spinal cord. (Redrawn from Ranson, 1939. For reference
see G.) (I) Schematic diagram of transverse section through myelencephalon (medulla),
portraying dorso-ventral position of nuclei of origin in motor plate and the nuclei of
termination in alar plate of cranial nerves associated with the myelencephalon.
820 THE NERVOUS SYSTEM
their sheath-like coverings are of two kinds, viz., myelinated fibers with
neurilemma and unmyelinated (Remak's) fibers with a thin neurilemma. The
latter are found especially among the sympathetic nerve fibers of the cerebro-
spinal series. (See Ranson, '39, p. 51.)
It may be observed here, parenthetically, that the myelinated fibers of the
brain and spinal cord differ from the myelinated fibers of the peripheral
nervous system in that the sheaths are formed by an investment of neuroglia
fibers and nuclei and not by a neurilemma sheath. Many naked axons also
are present in the central nervous system.
C. Morphogenesis of the Central Nervous System
1. Development of the Spinal Cord
a. Internal Changes in the Cord
During the early development of the spinal cord described above the fol-
lowing areas are evident:
( 1 ) the ependymal layer,
(2) the mantle layer, and
(3) the marginal layer.
The further development of these areas results in the formation of a thin
dorsal roof plate and a ventral floor plate mainly from the ependymal layer
(fig. 353 J and K). Somewhat later the neural cavity of the cord is reduced
by the apposition and fusion of the dorso-lateral walls of the lumen immedi-
ately under the dorsal plate, leaving a rounded central canal below located
near the floor plate (fig. 355A-E). Synchronized with these events the lateral
walls of the neural tube expand greatly as the mass of cells and fibers increases.
During this expansion, the two dorsal parts of the lateral walls move dorsad
and mediad and in this way come to lie apposed together in the median plane
above the central canal. This apposition forms the dorsal median septum
(fig. 355D and E). The dorsal roof plate becomes obliterated during this
process. Ventrally, also, the lateral portions of the neural tube move toward
the mid-ventral line below the central canal. However, the two sides do not
become closely apposed, and as a result the ventral median fissure is formed
(fig. 355D and E).
During the growth and expansion of the two lateral walls of the neural
tube, the neuroblasts of the nucleated mantle layer in the dorsal or alar
plate of the spinal cord increase greatly in number and form the dorsal (or
posterior) gray column (fig. 355A-E). The developing neuroblasts of the
dorsal gray column become associated with the dorsal root fibers of the spinal
nerves. Ventrally, the neuroblasts of the mantle layer increase in number in
the basal plate area of the spinal cord and form a ventral (anterior) gray
column. The ventral root fibers of the spinal nerves emerge from the ventral
MORPHOGENESIS OF CENTRAL NERVOUS SYSTEM 821
gray column. In the region of the central canal the mantle layer forms the
dorsal and ventral gray commissures which extend across the nerve cord
joining the gray columns in the lateral walls of the cord. Somewhat later, a
lateral gray column on either side may be formed between the dorsal and
ventral gray columns.
As the above growth and development of the mantle layer is achieved, the
marginal zone of the spinal cord also increases in size as nerve fibers from
the developing neurons in the gray columns and in the spinal ganglia of the
dorsal roots grow into the marginal layer between the neuroglia elements.
Moreover, nerve fibers from developing neuroblasts in the brain grow pos-
teriad in the marginal layer of the cord. As the growth and expansion of the
dorsal and ventral gray columns toward the periphery of the spinal cord
occurs, the marginal layer becomes divided into definite regions or columns
known as funiculi. The dorsal funiculus, for example, lies between the dorsal
median septum and the dorsal gray column while the ventral funiculus is
bounded by the ventral median fissure and the ventral gray column. The
lateral funiculus lies laterally between the dorsal and ventral gray columns
(fig. 355F). Below the ventral gray commissure, fibers cross from one side
of the cord to the other, forming the ventral white commissure.
Eventually the nerve fibers of each funiculus become segregated into fiber
tracts. As a result, the dorsal funiculus becomes subdivided into the two fiber-
tract bundles, the fasciculus gracilis near the dorsal medial septum and the
fasciculus cuneatus near the dorsal gray column. Other fiber tracts are shown
in figure 355F. (Consult Ranson, '39, p. 110.)
b. Enlargements of the Spinal Cord
The spinal cord in many tetrapoda tends to show two enlarged areas, viz.
(fig. 355G):
( 1 ) The brachial (cervical) enlargement in the area of origin of the brachial
nerves;
(2) The lumbar (sacral) enlargement in the area of origin of the lumbo-
sacral plexus.
Posteriorly the cord tapers toward a point, and anteriorly, in the region of
the first spinal nerve, it swells to become continuous with the myelencephalon.
c. Enveloping Membranes of the Cord
Immediately surrounding the spinal cord is a delicate membrane, the pia
mater, presumably developed from neural crest cells. More lateral is the
arachnoid layer, developed probably from neural crest cells and mesenchyme.
Between the pia mater and the arachnoid is the subarachnoid space contain-
ing blood vessels, connective tissue fibers, and a lymph-like fluid. Outside
822 THE NERVOUS SYSTEM
of the arachnoid layer is a cavity, the subdural cavity. The external boundary
of the subdural cavity is formed by the dura mater. The latter is a tough con-
nective tissue membrane of mesenchymal origin (fig. 355H).
2. Development of the Brain
a. The Development of Specialized Areas and Outgrowths of the Brain
1) The Formation of the Five-part Brain. The primitive vertebrate brain
from its earliest stages of development begins to show certain enlargements,
sacculations and outpushings. Furthermore, it possesses two main areas which
are non-nervous and membranous in character, namely, the thin roof plate
of the rhombencephalon and the thin roof plate of the posterior portion
(diencephalon) of the prosencephalon (figs. 354E; 356A). These thin roof
plates ultimately form a part of the tela chorioidea. Vascular tufts, the chorioid
plexi, also project from these roof plates into the third and fourth ventricles.
The anterior region of the primitive brain known as the prosencephalon
or forebrain soon divides into the anterior telencephalon and a more posterior
diencephalon (fig. 354C-E). The telencephalon gives origin to two lateral
outgrowths or pouches, the telencephalic vesicles (figs. 354E; 357E). The
telencephalic vesicles represent the rudiments of the cerebral lobes. From
the diencephalon, four or five evaginations occur, namely, a mid-dorsal evagi-
nation, the epiphysis or rudiment of the pineal body (fig. 356A), and in front
of the epiphysis a second mid-dorsal evagination occurs normally in most
vertebrates, namely, the paraphysis (see Chapter 21); two ventro-lateral
outgrowths, the optic vesicles (fig. 354B-D) from which later arise the optic
nerves, retina, etc., and a mid-ventral evagination, the infundibulum. The
infundibulum unites with Rathke's pouch (figs. 354E; 356A), a structure
which arises from the stomodaeum. Rathke's pouch ultimately differentiates
into the anterior lobe of the pituitary body (see Chapter 21 ).
The mesencephalon, unlike the fore- and hind-brain regions, does not di-
vide. However, from the mesencephalic roof or tectum dorsal swellings occur
which appear to be associated with visual and auditory reflexes. In fishes and
amphibia, two swellings occur, the so-called optic lobes or corpora bigemina.
In reptiles, birds and mammals four swellings arise in the tectum, the corpora
quadrigemina. (fig. 357H-0).
The rhombencephalon divides into an anterior metencephalon and posterior
medulla or myelencephalon (fig. 354E and G). Two cerebellar outpushings
arise from the roof of the metencephalon.
The primitive five-part brain forms the basic embryonic condition for later
brain development in all vertebrates.
2) The Cavities of the Primitive Five-part Brain and Spinal Cord. As pre-
viously observed, the brain and spinal cord are hollow structures, and its
generalized cavity is called the neural cavity or neurocoel (fig. 357A). From
DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM 823
the primitive neurocoel, special cavities in the brain arise, as follows (see
figure 357A):
( 1 ) The telencephalon is made up of the anterior part of the prosen-
cephalon and two telencephalic vesicles. Each vesicle ultimately gives
origin to a cerebral lobe. The cavities of the telencephalic vesicles
are known as the first and second ventricles.
(2) The cavity of the posterior, median portion of the telencephalon and
that of the diencephalon form the third ventricle.
(3) The roof of the original mesencephalon may give origin to hollow,
shallow outpushings, but the cavity of the mesencephalon itself be-
comes a narrow passageway and is known as the cerebral aqueduct
or the aqueduct of Sylvius.
(4) The cavity of the rhombencephalon is called the fourth ventricle.
b. The Formation of Cervical and Pontine Flexures
In addition to the primary or cephalic flexure previously described (p. 812)
other flexures may appear in the developing vertebrate brain, especially in
higher vertebrates. The cervical flexure develops at the anterior portion of
the spinal cord, as it joins the myelencephalon. It involves the caudal portion
of the myelencephalon, and the anterior part of the cord. It bends the entire
brain region ventrally (see figure 357D and E). The latter flexure is absent
in fishes, is present to a slight degree in the early neural tube of the amphibia,
and is pronounced in reptiles, birds and mammals. The third or pontine flexure
of the brain bends the brain dorsally. It arises in the mid-region of the
rhombencephalon, in the area between the myelencephalon and the meten-
cephalon. It appears later in development than the cephalic and cervical
flexures, and is found only in higher vertebrates.
c. Later Development of the Five-part Brain
The various fundamental regions of the five-part brain develop difi'erently
in diff'erent vertebrates. Figure 357B-G and H-O illustrates the changes
of the regions of the primitive five-part brain in the shark, frog, bird, dog,
and human. For detailed discussion of the function of the various parts of
the brain of the vertebrate, see Ranson, '39.
D. Development of the Peripheral Nervous System
1. Structural Divisions of the Peripheral Nervous System
The peripheral nervous system integrates the peripheral areas of the body
with the central nervous system. It is composed of two main parts,
( 1 ) the cerebrospinal system of nerves and
(2) the autonomic system. The latter is associated intimately with the
cerebrospinal system.
TELENCEPHALON DIENCEPHALON MESENCEPHALON METENCEPH ALON
ELENCEPHALON
PREMUSCLE
OF STERNOMASTOIO
AND TRAPEZIUS
MUSCLES OF
SHOULDER AREA
I
ABBREVIATIONS
G.SO A EX --GENERAL SOMATIC AFFERENT FIBERS
(EXTEROCEPTIVE FIBERS)
G SO A.P-- GENERAL SOMATIC AFFERENT FIBERS
(PROPRIOCEPTIVE FIBERS)
GVA = GENER4L VISCERAL AFFERENT FIBERS
SPV A = SPECIAL VISCERAL AFFERENT FIBERS
SP SO A = 5PECIA
SOMATIC AFFERENT FIBERS (EX AND P)
CUTANEOUS BRANCHES
OF THE
COMMUNIS ROOT
OF THE
RIGHT FACIAL NERVE
COMMISSURE CONNECTING
LINES OF TWO SIDES
SUPRAORBITAL LINE OF ORGANS
N SUPERFICIALIS
OPHTHALMICUS
OLFACTORY LOBE
OPERCULUM
N. HYOMANDIBULARIS
Fig. 356. The cranial nerves; nuclei of origin and termination; functional components.
(Note: The accompanying figures illustrate the nuclei of origin and nuclei of termination
of the various cranial nerves. They are generalized figures and should be regarded only
as approximate representations. This must be true, for the position of the respective
nuclei within the brain "varies greatly in different orders of vertebrates" [Ranson]. This
variation presumably is the result of a developmental principle known as neurohiota.xis.
This principle postulates that the dendrites of a neuron together with the cell body move
824
DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM 825
toward the source from whence the neuron receives its stimulation. That is, the dendrites
grow, and the neuron cell body as a whole moves, toward the particular nerve fiber
tract from which the impulses are received. As these impulses and fiber tracts vary
slightly with the particular environmental conditions under which the different animal
groups live, the location of the nuclei within the brain correspondingly will vary to a
degree within the respective vertebrate groups. It is- to be observed, also, that the nuclei
of origin of the afferent fibers of the cranial nerves, and of the cerebrospinal nerves in
general, are located outside of the central nerve tube, with the exception of the neuron
cell bodies of the second or optic nerve which are located in the retina, an extension of
the forebrain, and the mesencephalic nucleus of the fifth nerve. The nuclei of origin of
the efferent fibers are placed within the latero-basal areas of the nerve tube (fig. 3551).)
(A) The nuclei of origin of the various motor components of the cranial nerves here
are shown to be located within fairly definite regions along the antero-posterior axis of
the vertebrate brain. Reference may be made to Fig. 3551, for the dorso-ventral distri-
bution of these nuclei.
The following symbols are used:
1. Somatic motor fibers are shown in solid black.
2. Special visceral motor fibers are indicated in black with white circles.
3. General visceral motor fibers are black with white markings.
Nuclei of origin within the brain are as follows:
III — black = Edinger-Westphal nucleus, origin of general visceral efferent fibers of Oculo-
motor Nerve
III — cross lines = nucleus of origin of somatic motor fibers of Oculomotor Nerve
IV — cross lines = nucleus of origin of somatic motor fibers of Trochlear Nerve
V — cross hatched = special visceral motor nucleus, origin of special visceral motor fibers
of Mandibular division of Trigeminal Nerve
VI — cross lines = nucleus of origin of somatic motor fibers of Abducent Nerve
VII — cross hatched = special visceral motor nucleus of Facial Nerve
VII — black = superior salivatory nucleus (?), origin of general visceral motor fibers of
Facial Nerve
IX — cross hatched = origin of special visceral motor fibers of Glossopharyngeal Nerve
(this nucleus represents the anterior portion of nucleus anthiguus of Vagus Nerve)
IX — solid black = inferior salivatory nucleus (?), origin of general visceral motor fibers
of Glossopharyngeal Nerve
X — cross hatched = nucleus ambiguus or origin of special visceral motor fibers of Vagus
Nerve
X — solid black = dorsal motor nucleus, origin of general visceral motor fibers of Vagus
Nerve
XI — cross hatched = probable nucleus of origin of special visceral motor fibers of Spinal
Accessory Nerve
XII — cross lines = nucleus of origin of somatic motor fibers of Hypoglossal Nerve
(B) Sensory nuclei or nuclei of termination of fifth, seventh, ninth, and tenth cranial
nerves, shown along thC' antero-posterior axis of the vertebrate brain. (The dorso-ventral
distribution of these nuclei is presented in Fig. 3551.) The nuclei of termination of the
eighth cranial nerve has been omitted. (Figs. A and B are schematized from data supplied
by Ranson, 1939, The Anatomy of the Nervous System. Philadelphia, Saunders.)
(C) Cutaneous taste-bud branches of the right Facial Nerve in the fish, Anieiurus.
(Redrawn from Johnston, 1906, The Nervous System of Vertebrata, Philadelphia, Blakis-
ton, after Herrick.)
(D) Head of the pollack, Pollachius virens. revealing seventh and tenth cranial nerve
distribution to lateral line system of the head. (Redrawn from Kingsley, 1912, Com-
parative Anatomy of Vertebrates, Philadelphia, Blakiston, after Cole.)
826 THE NERVOUS SYSTEM
2. The Cerebrospinal System
The cerebrospinal system of nerves is composed of the cranial and spinal
nerves. Two sets of neurons enter into the composition of the cranial and
spinal nerves, viz.:
( 1 ) afferent neurons, whose fibers receive stimuli from certain receptor or-
gans and convey the impulses to the central nervous system, and
(2) efferent neurons, with fibers which convey the impulses from the cen-
tral nervous system to the peripheral areas. The central nervous system
with its multitudes of association neurons thus acts to correlate the
incoming impulses from afferent neurons and to shunt them into the
correct outgoing pathways through the fibers of the efferent neurons
(see figure 358A).
Most of the afferent or sensory neurons are located in ganglia outside of
the central nerve tube, within the dorsal root ganglia of the spinal nerves and
in the ganglia of the cranial nerves in close association with the brain (fig.
356B). On the other hand, the cell bodies of the somatic efferent or motor
fibers are found within the gray matter of the central nerve tube, and the
cell bodies of the visceral efferent or motor fibers are located within the gray
matter of the central nerve tube and also in peripheral (autonomic) ganglia.
3. General Structure and Function of the Spinal Nerves
In each of the spinal nerves the nerve fibers are of four functional varieties,
namely, visceral sensory (afferent); visceral motor (efferent); somatic sensory
(afferent); and somatic motor (efferent). The visceral components are dis-
tributed to the glands, smooth muscles, etc., of the viscera located within the
thoracic and abdominal cavities, together with the blood vessels of the general
body areas. The somatic components innervate the body wall tissues includ-
ing the skin and its appendages. A spinal nerve and its component fibers in
the trunk region is shown in figure 358A, and figure 358B shows this dis-
tribution in the region of the brachial plexus.
A typical spinal nerve is composed of the following general parts:
( 1 ) The dorsal or sensory root with its ganglion, and
(2) the ventral or motor root.
(3) Each spinal nerve divides into
(4) a dorsal ramus, and
(5) a ventral ramus. The ventral ramus may divide into
(6) a lateral branch and
(7) a ventral branch. Connecting with the spinal nerve also are
(8) the gray and white rami of the autonomic nervous system.
As the peripheral nerve fibers grow distad they become grouped together
to form peripheral nerves. Each nerve in consequence is an association of
DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM 827
bundles or fasicles of fibers surrounded and held together by connective tissue.
Most of the peripheral nerve fibers are myelinated. The connective tissue
which surrounds a nerve is called the perineurium and that which penetrates
inward between the fibers is the endoneurium (fig. 358C).
4. The Origin, Development and Functions of the
Cranial Nerves
Consult diagrams, figures 356A and B, also 3551.
0. Terminal
The nervus terminalis is a little understood nerve closely associated with
the olfactory nerve. It was discovered by F. Pinkus in 1894, in the dipnoan
fish, Protopterus, after the other cranial nerves were described. In consequence
it does not have a numerical designation. (Consult Larsell, '18, for references
and discussion.)
1. Olfactory
Arises from bipolar cells located in olfactory epithelium. These cells give
origin to fibers which grow into the olfactory bulb to synapse with olfactory-
bulb neurons (fig. 356B).
Summary of functional components: Special visceral afferent fibers.
II. Optic
The optic nerve arises from neurons located in the retina of the eye. They
grow mediad along the lumen of the optic stalk to form the optic nerve.
In mammals part of the fibers from the median half of each retina decussate,
i.e., cross over, and follow the fibers from the lateral half of the retina of
the other eye into the brain (fig. 356B). In birds, however, decussation of
the optic nerve fibers is complete, as it is in reptiles and fishes, and probably
also in amphibians.
Summary of functional components: Special somatic afferent fibers, cell
bodies in the retina. In fishes, there are efferent fibers in the optic nerve con-
trolling, possibly, movements of retinal elements (Arey, '16, and Arey and
Smith, '37).
III. Oculomotor
The third cranial nerve is composed mainly of somatic motor fibers which
originate from neuroblasts in the anterior basal area of the mesencephalon.
These fibers grow latero-ventrad from the mesencephalic wall to innervate
the premuscle masses of the inferior oblique, inferior, medial and superior
rectus muscles of the eyeball (fig. 356A).
Summary of functional components: ( 1 ) Somatic motor fibers controlling
eye muscles indicated, (2) general somatic afferent (sensory) fibers, i.e. pro-
828 THE NERVOUS SYSTEM
prioceptive fibers for eye muscle tissue, (3) general visceral efferent fibers.
The neuron bodies of the visceral efferent fibers are located in the Edinger-
Westphal nucleus of mesencephalon. The fibers from these neurons form the
preganglionic fibers which terminate in the ciliary ganglion. The postgan-
glionic fibers from cell bodies in ciliary ganglion innervate the intrinsic
(smooth) muscles of the ciliary body and iris.
IV. Trochlear
The fourth cranial nerve arises from neuroblasts in the posterior ventral
floor of the mesencephalon near the ventral commissure. The fibers grow
dorsad and somewhat posteriad within the wall of the mesencephalon to the
mid-dorsal line where they emerge to the outside and decussate (i.e. cross),
the nerve from one side passing laterad toward the eye of the opposite side
where it innervates the developing premuscle mass of the superior oblique
muscle (fig. 356A).
Summary of functional components: ( 1 ) Somatic motor fibers controlling
superior oblique muscle, (2) general somatic afferent (sensory) fibers, i.e.
proprioceptive fibers from eye muscle tissue.
V. Trigeminal
The trigeminal nerve is a complex association of sensory and motor fibers
(fig. 356A, B). It has the following divisions:
A. Ophthalmicus or Deep Profundus
Composed of somatic sensory fibers to the snout region. Fibers originate
from neuroblasts in the dorso-anterior part of the neural crest cells which give
origin to the Gasserian (semilunar) ganglion. This portion of the semilunar
ganglion probably should be regarded as a separate and distinct ganglion.
One fiber from each bipolar neuroblast grows anteriad toward the snout while
the other fiber enters the wall of the metencephalon. These neurons later
become unipolar.
Summary of functional components: General somatic afferent (sensory)
fibers.
B. M axillaris
The maxillary ramus of the fifth cranial nerve is composed of somatic
sensory fibers from the upper jaw and snout and mucous membranes in these
areas. The fibers arise from neuroblasts within the neural crest material which
forms the central mass of the semilunar ganglion. One fiber from each bipolar
neuroblast grows anteriad toward the snout while the other fiber grows mediad
to enter the wall of the metencephalon along with fibers from the ophthalmic
and mandibular divisions. These neurons later become unipolar.
Summary of functional components: General somatic afferent (sensory)
fibers.
DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM 829
C. Mandibularis
The mandibular ramus is composed of general sensory (afferent) fibers
with cell bodies lying in the mesencephalic nucleus of the fifth nerve (see
figure 356A). Associated with these sensory fibers are motor fibers (generally
spoken of as special visceral motor fibers) distributed to the muscles of mas-
tication. The latter muscles arise from mesoderm associated with the first or
mandibular visceral arch. During development the motor fibers arise from
a localized mass of neuroblasts lying in the pons of the metencephalon (see
figure 356A), and they emerge from the ventro-lateral aspect of the pons and
grow out toward the mandibular arch. Later they become associated with the
sensory fibers observed above.
Summary of functional components: ( 1 ) General somatic afferent (sensory)
fibers, of the proprioceptive variety, originating in mesencephalic nucleus of
the fifth nerve (fig. 35 6 A, B), (2) special visceral efferent (motor) fibers to
muscles of mastication from motor nucleus noted above.
VI. Abducens
The word abducens means to lead away, or draw aside. It is applied to the
sixth cranial nerve because it innervates the lateral rectus muscle of the eye-
ball whose function is to pull the eye away or outward from the median line.
It is composed almost entirely of somatic efferent (motor) fibers whose origin
is within a nucleus lying in the caudo-ventral area of the pons (fig. 356A).
In the embryo, neuroblasts in this area grow outward from the ventro-lateral
wall of the pons and forward into the developing premuscle mass of the
external (lateral) rectus muscle.
Summary of functional components: (1) Somatic efferent fibers, (2) gen-
eral somatic afferent fibers, i.e. proprioceptive fibers from the external rectus
muscle.
VII. Facial
In higher vertebrates this nerve is composed largely of motor fibers of the
special visceral variety innervating the musculature derived from the hyoid
visceral arch. As indicated previously (Chap. 16) the muscle tissue of this
arch forms the facial (mimetic) and platysma musculature of mammals and
the posterior belly of digastric and stylohyoid muscles. In fishes muscle tissue
is restricted to the region of the hyoid arch and is concerned with movements
of this arch. The motor fibers distributed to the hyoid arch of fishes are located
in the hyomandibular branch of the facial nerve (see figure 3571). Aside
from these special visceral motor fibers, sensory fibers are present whose cell
bodies lie within the geniculate ganglion of the facial nerve. The sensory fibers
which innervate some of the taste buds on the anterior two-thirds of the
tongue in mammals are special visceral afferent fibers coursing in the chorda
tympani nerve, whereas those along the pathway of the facial nerve are
830 THE NERVOUS SYSTEM
general visceral sensory fibers providing deep sensibility to the general area
of distribution of the facial nerve. The special visceral afferent fibers to the
taste bud system are prominent elements in the seventh cranial nerve of many
fishes (fig. 356C). In fishes also, the seventh cranial nerve contains lateral-
line components distributed to the lateral-line organs of the head (fig. 356D).
The special motor fibers of the facial nerve arise from neuroblasts located
in the pons as indicated in figure 356A, and the general visceral motor fibers
take origin from cell bodies in the nucleus salvatorius superior.
Summary of components: ( 1 ) Special visceral efferent (motor) fibers to
musculature arising in area of hyoid arch, (2) in mammals, preganglionic
general visceral efferent fibers by way of chorda tympani nerve to submaxil-
lary ganglion; and from thence, postganglionic fibers to submaxillary and
sublingual salivary glands. (3) Special visceral afferent fibers to taste buds
on anterior portion of tongue by way of chorda tympani nerve; in fishes, spe-
cial visceral afferent fibers are extensive. (4) General visceral afferent fibers.
(5) In fishes, lateral-line components to head region are present.
VIII. Acoustic
The acoustic nerve contains special somatic sensory components which
receive sensations from the special sense organs derived from the otic vesicle.
The otic vesicle differentiates into two major structures, viz.: (1) one related
to balance or equilibration, and (2) the other concerned with hearing or the
detection of wave motions aroused in the external medium. This differentia-
tion is obscure in fishes. However, in those vertebrates which dwell in water
other hearing devices may be used aside from those which may involve the
developing ear vesicle. One aspect of the mechanism which enables water-
dwelling vertebrates to detect pressure or wave motions of low frequency in
the surrounding watery medium is the lateral line system associated with the
fifth, seventh, ninth and tenth cranial nerves.
In accordance with the differentiation of the otic vesicle into two sense-
perceiving organs, the sensory neurons of the acoustic ganglion of the eighth
cranial nerve become segregated into two ganglia, namely, ( 1 ) the vestibular
ganglion containing bipolar neurons which transmit proprioceptive stimuli
through the vestibular nerve from the organ of equilibration composed of
the utricle, saccule and semicircular canals, and (2) the spiral ganglion con-
taining bipolar neurons which transmit somatic sensations from the spiral
or hearing organ (fig. 361H).
Summary of functional components: (1) Special somatic afferent fibers
of proprioceptive variety associated with equilibration, (2) special somatic
afferent fibers of exteroceptive variety, associated with hearing.
IX. Glossopharyngeal
The glossopharyngeal nerve is associated with the third visceral arch and
nearby areas of the pharynx. It has two major components; one of these
DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM 831
components is motor, innervating the musculature derived from the embryonic
third visceral arch, while the other component is sensory. The sensory com-
ponents are derived from neuron bodies within the superior and petrosal
ganglia (fig. 356B). Aside from receiving general sense impulses from the
pharyngeal area, many of these sensory components are associated with the
taste buds on the caudal portion of the tongue. The latter components thus
are special sensory components.
The visceral motor (efferent) components to the musculature derived from
the third visceral arch arise from neuroblasts located in the ventro-lateral
floor of the anterior part of the myelencephalon (fig. 356A). The sensory
components take origin from neural crest cells located in the region of the
third visceral arch. Fibers from these neuroblasts grow mediad into the nerve
tube, and latero-ventrad toward the third visceral arch region.
Summary of functional components: (1) General visceral afferent fibers
with cell bodies in petrosal ganglion whose peripheral fibers terminate in the
posterior tongue region and in the pharyngeal area, (2) special visceral afferent
fibers with cell bodies in petrosal ganglion whose peripheral fibers contact
the taste buds in the posterior third of the tongue, (3) special visceral efferent
fibers to musculature derived from the third visceral arch. In mammals, this
musculature is the stylopharyngeus muscle, (4) in mammals: general visceral
efferent fibers, composed of preganglionic fibers from neurons in inferior
salivatory nucleus located probably in the region between the pons and medulla
pass to the otic ganglion. Postganglionic fibers from otic ganglion innervate
the parotid gland. (5) In fishes: lateral-line components are present and dis-
tributed to posterior head region. In mammals, some general somatic afferent
fibers from cell bodies in the superior ganglion appear to innervate cutaneous
areas in the ear region.
X. Vagus
The tenth cranial or vagus nerve is composed of several functional com-
ponents. It is a prominent nerve associated with the autonomic nervous system
as indicated below. In addition to these autonomic components, the functional
components of the tenth cranial nerve are related to the visceral arches caudal
to the third visceral arch. The tenth cranial nerve thus supplies several vis-
ceral arches. In consequence, it must be regarded as a composite nerve,
arising from extensive motor nuclei, the dorsal motor nucleus and the
nucleus ambiguus in the ventro-lateral area of the myelencephalon (fig. 356A).
The tenth nerve has two main ganglia, the jugular and nodose ganglia. The
motor fibers arise from neuroblasts in the nuclei mentioned above and grow
out laterally to the visceral arch area, and the sensory components take
origin from neuroblasts of neural crest origin which become aggregated in
the jugular and nodose ganglia.
Summary of functional components: (1) Special visceral afferent fibers
HINOBRAIN
OPHTHALMIC
BRANCH OF
NERVE!
MAXILLARY
BRANCH OF
NERVE 1
Fig. 357. External morphological development of various vertebrate brains. (A)
Diagram showing the fundamental regional cavities of the primitive five-part vertebrate
brain. (B-G) External morphological changes of the developing human brain and
cranial nerves. (Redrawn, somewhat modified, from Patten, 1946, Human Embryology,
Philadelphia, Blakiston, adapted primarily from Streeter and reconstructions in Car-
negie Collection.) (B) 20 somite embryo, probably 3!/2 weeks. (C) 4 mm. embryo,
about 4 weeks. (D) 8 mm. embryo, about 5V3 weeks. (E) 17 mm. embryo, about
7 weeks. (F) 50-60 mm. embryo, about 11 weeks. The brain now begins to assume
the configuration shown by the chick at hatching (see Fig. 347L and M). Roman nu-
merals III, IV, V, VI, VII, IX, X, XI and XII indicate cranial nerves. See Fig. 356A
and B for functional components of the cranial nerves at this time. (G) Lateral
view of brain at about the ninth month. (H, I, and I') Adult form of the brain of
Squalus acanthias. It is to be observed that the brain of Squalus acanthias loses the
marked cephalic flexure (see Fig. 347A) present in the early embryo, and assumes a
straightened form during the later stages of its development. (H and I ventral and dorsal
views, respectively, drawn from dissected specimens; I' redrawn and slightly modified
from Norris and Hughes, 1919, J. Comp. Neurol., 31.) (J and K) Ventral and dorsal
832
DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM 833
whose cell bodies lie in nodose ganglion with peripheral terminations in taste
buds of pharyngeal area, (2) general visceral afferent fibers whose cell bodies
lie in nodose ganglion, with peripheral distribution to pharynx, esophagus,
trachea, thoracic and abdominal viscera, (3) general somatic afferent fibers
with cell bodies in jugular ganglion and peripheral distribution to external
ear region, (4) special visceral efferent fibers to striated musculature of
pharyngeal area; cell bodies lie in nucleus ambiguus, (5) general visceral
efferent fibers. Preganglionic cell bodies in dorsal motor nucleus; terminate
in sympathetic ganglia associated with thoracic and abdominal viscera, (6) in
fishes: a prominent lateral line component is present which is distributed
along the lateral body wall.
The special visceral motor fibers of the vagus are associated with muscula-
ture arising from the caudal visceral arches.
XI. Spinal Accessory
The spinal accessory nerve arises in close association with the vagus. It is
composed mainly of motor fibers and distributed to musculature derived from
premuscle masses in the caudal branchial area (fig. 356A). They may be
regarded as special visceral motor fibers.
Summary of functional components: ( 1 ) Special visceral efferent fibers
whose cell bodies lie in nucleus ambiguus and in anterior part of spinal cord
and distributed to trapezius, and sternocleidomastoid, muscles and striated
muscles of pharynx and larynx, (2) general visceral efferent fibers associated
with vagus nerve, with cell bodies in dorsal motor nucleus of vagus.
XII. Hypoglossal Nerve
The twelfth cranial nerve is a somatic motor nerve composed mainly of
efferent fibers distributed to the hypobranchial or tongue region. These fibers
arise from neuroblasts in an extensive nuclear region from the anterior cervical
area along the floor of the myelencephalon near the midventral line (fig.
356A). In lower vertebrates these fibers innervate certain of the anterior
trunk myotomes whose muscle fibers travel ventrad into the hypobranchial
area. In higher vertebrates the hypoglossal nerve fibers innervate the tongue
and associated muscles.
Fig. 357 — Continued
views, respectively, of the adult form of the brain in the frog, Rana cateshiana. Like the
developing brain in Sqiialus, the brain of the developing frog loses its pronounced cephalic
flexure as development proceeds. (L and M) Ventral and dorsal views, respectively, of
the adult form of brain in the chick shortly before hatching. The cervical, pontine, and
cephalic flexures are partly retained in developing brain of chick, and in this respect it
resembles the developing mammalian brain. Compare these diagrams with Figs. 354E.
259. (N and O) Ventral and dorsal views, respectively, of the adult brain of the dog.
(Redrawn from models.)
834
THE NERVOUS SYSTEM
NAS4L CiPSULE
OLFiCTORY BUI
NERVUS TERMINSLIS
OLFACTORY TR4CT
CEREBRAL HEMISPHERE
SPINAL CORD
OPTIC LOBES
TROCHLEAR NERVE (N IZ
SACCUS VASCULOSUS
ABOUCENS NEFtVE (NEE)
TRIGEMINAL L08US LINEAE LATER
NERVE (NI )
FACIAL NERVEfNYn"
ACOUSTIC NERVECN •vttt
TUBERCULUM ACUSTICUM
GLOSSOPHARYNGEAL NERVE ( '
'AGUS N£RVE(N I
MEDIAL LONGITUDINAL FASCICULU
VISCERAL LO
SPINAL CORD
OCCIPITAL NERVE Ul
NERVUS
ABOUCENS
NTZI
PRETREMATIC
RAMUS
OF N VTT
OBLONGATA
RAMUS
RAMUS SUPRATEMPORAL
'ORALIS OF
OSTTREMATIC RAMI OF
GLOSSOPHARYNGEAL i
VAGUS NERVES
/rrr-I.. VISCERAL MOTOR
^^0m SOMATIC MOTOR
Fig. 357 — Continued
For legend see p. 832.
Summary of functional components: ( 1 ) Somatic motor fibers; (2) somatic
sensory, i.e., proprioceptive fibers, from tongue musculature.
5. The Origin and Development of the Autonomic System
a. Definition of the Autonomic Nervous System
The autonomic nervous system is that part of the peripheral nervous system
which supplies the various glands of the body together with the musculature
DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM
835
HEMISPHERES
= CEREBRAL
LONGITUDINA
LOBES
L FISS
JHE
-TRANSVERSE
FISSU
E
EREBELLOM
A OBLONGATA
NAL CORD
0
Fig. 357 — Continued
For legend see p. 832.
of the heart, blood vessels, digestive, urinary and reproductive organs, and
other involuntary musculature. It differs from the cerebrospinal nerve series
in its efferent system of neurons, and not in the afferent system. The latter
is composed of ordinary afferent neurons located in the ganglia of the cerebro-
836 THE NERVOUS SYSTEM
spinal series and these differ from the somatic sensory neurons of the dorsal
root ganglia only in that they convey sensations from the viscera instead of
the body wall and cutaneous surfaces. On the other hand, the efferent system
of neurons is unlike that of the cerebrospinal series in that two neurons are
involved in conveying the efferent nerve impulse instead of one as in the
cerebrospinal series. The body of one of these two neurons, the preganglionic
neuron, lies within the brain or spinal cord, whereas the cell body of the other,
the postganglionic neuron, is associated with similar cell bodies within cer-
tain aggregations called sympathetic ganglia (fig. 358A). The axons of the
postganglionic neurons run to and end in the cardiac and blood vessel mus-
culature, gland tissue and smooth musculature in general throughout the body.
According to Ranson, '18, p. 308, "The autonomic nervous system is that
functional division of the nervous system which supplies the glands, the heart,
and all smooth muscle, with their efferent innervation and includes all general
visceral efferent neurones both pre- and postganglionic."
b. Divisions of the Autonomic Nervous System
There are two main divisions of the autonomic system, viz.:
( 1 ) The thoracicolumbar autonomic system, also called the sympathetic
division of the autonomic system, and
(2) The craniosacral autonomic system, also called the parasympathetic
division of the autonomic system (see figure 358D).
The thoracicolumbar outflow of efferent fibers has preganglionic fibers
which pass from the spinal cord along with the thoracic and upper (anterior)
lumbar spinal nerves, whereas the preganglionic fibers of the craniosacral
outflow depart from the central nervous system via cranial nerves III, VII,
IX, X and XI, and in the II, III and IV sacral nerves.
c. Dual Innervation by Thoracicolumbar and Craniosacral Autonomic
Nerves
Most structures innervated by the autonomic nervous system receive a
double innervation, one from the sympathetic and the other from the para-
sympathetic division, both, in many instances, having opposite functional
effects upon the organ tissue.
Examples of this dual innervation are:
1) Autonomic Efferent Innervation of the Eye. Preganglionic cell bodies in
oculomotor nucleus, fibers passing with nerve III to ciliary ganglion. Post-
ganglionic cell bodies in ciliary ganglion; postganglionic fibers by way of
short ciliary nerves to ciliary muscle and circular muscle fibers of iris. Func-
tion: Accommodation of eye and decrease in diameter of pupil. The foregoing
innervation is a part of the cranio-sacral autonomic outflow. A parallel inner-
DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM 837
vation to the iris of the eye occurs through the thoracicolumbar autonomic
system as follows:
Cell bodies of preganglionic neurons in intermedio-lateral column of spinal
cord, from which preganglionic fibers pass to superior cervical ganglion of
autonomic nervous system. Cell bodies of postganglionic fibers lie in the su-
perior cervical ganglion and fibers pass from this ganglion along the internal
carotid plexus to the ophthalmic division of the fifth nerve, and from thence
along the long ciliary and nasociliary nerves to iris. Function: dilation of the
pupil.
2) Autonomic Efferent Innervation of the Heart. Preganglionic cell bodies
in dorsal motor nucleus of vagus in myelencephalon. Fibers pass by way of
vagus nerve to terminal (intrinsic) ganglia of the heart. Postganglionic cell
bodies in terminal ganglia of heart; postganglionic fibers pass to heart muscle.
Function: slows the heart beat. The foregoing represents the craniosacral au-
tonomic or parasympathetic innervation. The corresponding sympathetic in-
nervation is as follows:
Preganglionic cell bodies in intermedio-lateral column of spinal cord; pre-
ganglionic fibers pass to superior, middle and inferior cervical ganglia of sym-
pathetic ganglion series. Postganglionic cell bodies in cervical ganglia from
which postganglionic fibers pass via cardiac nerves to cardiac musculature.
Function: acceleration of heart beat.
d. Ganglia of the Autonomic System and Their Origin
The ganglia of the autonomic nervous system represent aggregations of the
cell bodies of postganglionic neurons; the cell bodies of the preganglionic
neurons lie always within the central nervous system. These autonomic ganglia
arise from two sources; viz.:
1 ) The neural crest material of the dorsal root ganglion of the spinal nerves
and the neural crest material associated with certain cranial nerves, and
2) from cells of the neural tube which migrate from the tube along the
forming ventral or efferent nerve roots of the spinal nerves (Kuntz and
Batson, '20).
These migrating neural cells become aggregated to form three sets of ganglia
as follows:
1 ) The sympathetic chain ganglia lying on either side of the vertebral
column.
2) The collateral or subvertebral ganglia located between the chain ganglia
and the viscera. Examples of collateral ganglia are the coeliac, superior
mesenteric and inferior mesenteric ganglia.
3 ) The terminal or intrinsic ganglia lie near or within the organ tissue such
as the ciliary and submaxillary ganglia.
838 THE NERVOUS SYSTEM
Fig. 358. General structural features of spinal nerves, and of nerve fibers terminating
in muscle tissue. (A) Diagrammatic representation of a spinal nerve in the region of the
mammalian diaphragm showing functional components. Three facts are evident relative
to the components of a typical spinal nerve, viz., ( 1 ) The somatic efferent motor neuron
lies within the central nerve tube; its fiber extends peripherad to the effector organ. One
neuron therefore is involved in the somatic efferent system (see Fig. 352A). (2) Unlike
the somatic efferent system, the visceral efferent (motor) system is composed of a chain
of two neurons, a preganglionic neuron whose cell body lies within the central nerve
tube, and a postganglionic neuron whose cell body lies in one of the peripheral ganglia.
(3) The somatic afferent (sensory) and visceral afferent (sensory) fibers both possess
but one neuron whose cell body lies within the dorsal root ganglion. The somatic afferent
fiber connects with a sense or receptor organ lying somewhere between the viscera and
the external surface (i.e., cutaneous surface) of the body, whereas the visceral afferent
fiber contacts the structural makeup of the visceral structures. (B) A spinal nerve in
the region of the brachial plexus. The main difference between this type of nerve and
the typical spinal nerve resides in the fact that the ventral ramus proceeds into the limb
and not into the body wall. Before proceeding into the limb it inosculates with the
ventral rami of other nerves to form the brachial plexus. (C) Portion of a transverse
section of the sciatic nerve of a newborn showing groups of nerve fibers joined together
into bundles. Each nerve-fiber bundle is surrounded by connective tissue, the perineurium,
and is partly divided by septa of connective tissue, the endoneurium. External to the
perineurium is the epineurium, or the connective tissue which holds the entire nerve
together (Redrawn from Maximow and Bloom, 1942, A Textbook of Histology, W. B.
Saunders Co., Philadelphia, after Schaffer. ) (D) Diagram of the autonomic efferent
system of neurons and ganglia. The parasympathetic (craniosacral) outflow is shown in
heavy black lines with white spaces; the sympathetic (thoracicolumhar) outflow is repre-
sented by ordinary black lines. (Adapted from Ranson, 1939, The Anatomy of the
Nervous System, Philadelphia, Saunders, after Meyer and Gottlieb.)
G. cerv. sup. = superior cervical ganglion
G. stellatum = inferior cervical or stellate ganglion
G. mes. sup. = superior mesenteric ganglion
G. mes. inf. = inferior mesenteric ganglion
G. pelv. = pelvic ganglion
Neurohumoral substances are produced at the terminal (effector) tips of the various
autonomic nerve fibers. A substance similar to adrenalin appears to be produced at the
tips of the sympathetic nerves proper, whereas in the case of the parasympathetic fibers
the substance is acetylcholine. These humoral substances stimulate the effector structures.
(E, F, and G) Nerve endings associated with muscle tissue. (E) Effector (motor) nerve
endings associated with cardiac or smooth muscle. Sympathetic motor endings terminate
in small swellings. This figure portrays sympathetic motor endings on a smooth muscle
cell of an artery of the rabbit's eye. (Redrawn from Maximow and Bloom, 1942, A
Textbook of Histology, Philadelphia, Saunders, after Retzius.) (F) Another example of
the termination of sympathetic nerve fiber endings on smooth muscle fibers. In this in-
stance the bronchial musculature is the effector organ. (Redrawn from Maximow and
Bloom, 1942, A Textbook of Histology, Philadelphia, Saunders, after Larsell & Dow.)
(G and G') Nerve endings in striated muscle. (G redrawn from Ranson, 1939, The
Anatomy of the Nervous System, Philadelphia, Saunders, after Huber & De Witt; G'
redrawn from Maximow and Bloom, 1942, A Textbook of Histology, Philadelphia,
Saunders, after Boeke.) (G) Represents a neuromuscular end organ of a sensory nerve
fiber terminating within a muscle spindle in striated muscle from a dog. These muscle
spindles are in the form of a connective tissue capsule which invests spindle-shaped
bundles of muscle fibers. Within this capsule, large myelinated nerve fibers terminate
in non-myelinated branches which spiral around the muscle fibers or end in flattened
discs. (G') Represents a somatic motor (efferent) nerve fiber terminating in a motor
plate within a striated muscle fiber. The motor plate is composed of an irregular mass
of sarcoplasm below the sarcolemma of the muscle fiber. This motor plate receives the
naked terminal ramifications of the nerve fiber.
Fig. 358. (See facing page for legend.)
839
Fig. 359. Types of peripheral sense receptors (see also Fig. 358G). (A) Meissner's
tactile corpuscle. Consists of a thin connective tissue capsule. One or more myelinated
nerve fibers enter the capsule, where the myelin sheaths are lost. These terminating
non-myelinated fibers break up into branches which form a complex mass of twisting
coils. The coils show varicose enlargements. Found in the dermis of feet, hands, lips,
forearms. (B) End-bulb of Krause. Small rounded bodies somewhat resembling Meiss-
ner's corpuscles. Found in lips, conjunctiva, and edge of cornea. (C) Pacinian cor-
puscle. This type of nerve ending is in the form of a large, oval corpuscle composed
of concentric layers of connective tissue. The central axis of the corpuscle receives the
840
SENSE OR RECEPTOR ORGANS 841
The general arrangement of these ganglia and the autonomic nerve fibers
to the spinal nerve series is shown in figure 358A. It is to be observed that only
two neurons, a preganglionic and a postganglionic, are involved in the efferent
chain regardless of the number of ganglia traversed.
E. The Sense or Receptor Organs
1. Definition
The sense organs are the sentinels of the nervous system. Endowed par-
ticularly with that property of living matter known as irritability, they are
able to detect changes in the environment and to transmit the stimulus thus
aroused to afferent nerve fibers. However, the perceptive ability of all sense
organs is not the same, for specific types of sense receptors are developed spe-
cialized in the detection of particular environmental changes.
There are two general areas of sensory reception, viz.: (1) The somatic
sensory area, and (2) the visceral sensory area. The location of somatic and
visceral areas in the myelencephalon are shown in figure 3551.
The somatic sensory organs are associated with the general cutaneous sur-
face of the body and also in tissues within the body wall. Consequently, this
area may be divided for convenience into two general fields, namely, ( 1 )
Fig. 359 — Continued
terminal ends of one or more unmyelinated fibers, and also, in addition, the terminal
end of a myelinated fiber which loses its myelin as it enters the axial core of the corpuscle.
Side branches arise from the central core of nerve fibers. Found in deeper parts of dermis,
and also in association with tendons, joints, intermuscular areas as well as in the
mesenteries of the peritoneal cavity, and the Hnings of the pleural and pericardial
cavities. (D) Nerve endings in skin and hair follicles. As the myelinated fibers enter
the skin they break up into smaller myelinated fibers. After many divisions the myelin
sheaths are lost, and finally the neurilemma also disappears. The free nerve endings
enter the epidermis and after other divisions form a network of terminal fibers among
the epidermal cells. Below the stratum germinativum of the skin, some of the fibers
terminate in small, leaf-like enlargements around the hair-follicles below the level of
the sebaceous glands. (A-D, redrawn and somewhat modified from Ranson, 1939, The
Anatomy of the Nervous System, Philadelphia, Saunders.) (E) Part of longitudinal
section of the lateral line canal of a Mustelus "pup" at the level of the first dorsal fin.
Observe termination of nerve fibers among groups of sensory hair cells. The lateral line
canal communicates with the surface at intervals by means of small tubules. (Redrawn
and modified from Johnson, 1917, J. Comp. Neurol., 28.) (F) Transverse section of
lateral line canal, higher rrtagnification, showing termination of nerve endings among
the secondary sense (hair) cells. (Redrawn from Johnson, 1917, J. Comp. Neurol., 28.)
(G) The lateral line sensory cord is shown growing posteriad within the epidermal
pocket of a 21 mm. embryo of Squaliis. (Redrawn from Johnson, 1917, J. Comp. Neurol.,
28.) (H) Taste bud of human. (Redrawn from Neal and Rand, 1939, Chordate Anat-
omy, Philadelphia, Blakiston.) (I) Sagittal section through human nasal cavity de-
picting nasal conchae (turbinates) and various openings leading off from the lateral
wall of the nasal cavity. The olfactory area of the mucous membrane extends over the
superior concha and medially over the upper part of the nasal septum. Observe opening
of eustachian tube (tuba auditiva).
842 THE NERVOUS SYSTEM
The exteroceptive or general cutaneous field, having sense organs detecting
stimuli at or near the surface of the body, and (2) the proprioceptive field,
with sense organs located in the body-wall tissues, such as striated muscles,
tendons, joints and the equilibration structures of the internal ear.
The visceral sensory organs receive stimuli from the interoceptive field,
that is, the visceral structures of the body.
2. Somatic Sense Organs
a. Special Somatic Sense Organs
The visual organs, the ear, and in water-living vertebrates the lateral-line
system, are sense organs of the special variety.
b. General Somatic Sense Organs
These structures are in the form of free nerve endings, terminating among
cells and around the roots of hairs, or they are present as encapsulated nerve
endings such as the corpuscles of Meissner, end bulbs of Krause, and Pacinian
corpuscles (fig. 359A-D).
3. Visceral Sense Organs
a. Special Visceral Sense Organs
The taste buds of various sorts, located generally on the tongue, mucous
surface of the buccal cavity and pharynx and in some fishes on the external
body surface are specialized visceral sense organs (fig. 285E).
In most craniates the paired olfactory organs are exteroceptive in function,
although, possibly, olfactory organs may be regarded as primitively intero-
ceptive. The olfactory organ is regarded generally as a special visceral sense
organ.
b. General Visceral Sense Organs
General visceral sense organs are located among the viscera of the body.
They represent free-nerve endings lying in the walls of the digestive tract and
other viscera. They respond to mechanical stimuli.
4. The Lateral-line System
The lateral-line organs are a specialized series of organs located in the
cutaneous areas of the body. They are found in fishes and water-living am-
phibia. A sense organ of the lateral-line system is composed of a patch of
hair cells or neuromasts, columnar in shape, possessing cilia-like extensions
at the free end (fig. 359E). Basally the hair cells are associated with the
terminal fibrillae of sensory nerves. The hair cells are supported by elongated,
sustentacular elements. In cyclostomous fishes the neuromasts are exposed
to the surface, but in Gnathostomes they lie embedded within a canal system
SENSE OR RECEPTOR ORGANS 843
lying deep within the dermis (fig. 359F). The pit organs, ampullae, etc., lo-
cated over the head region of fishes belong to the lateral-line system. They
are highly specialized structures. A developmental stage of the lateral-line canal
in Squalus acanthias are shown in figure 359G.
5. The Taste-bud System
The taste-bud system of vertebrates is most variable in its distribution. In
mammals the taste buds are scattered over the tongue (fig. 285E), and upon
the larynx, pharynx and soft palate. The taste buds on the anterior portion
of the tongue are supplied by the chorda tympani branch of the facial nerve,
the posterior lingual taste buds by the glossopharyngeal, and those in the
region of the pharynx by the vagus. In most fishes the taste buds are spread
over the inner surfaces of the pharynx and extensively over the buccal cavity.
In some fishes and amphibia they appear also over the external surface of
the head, and in some teleosts they are found over much of the body surface
(see figure 356C).
Taste buds consist of groups of specialized columnar epithelial cells, known
as hair cells, surrounded and supported by sustentacular cells. Each hair cell
has a sensory bristle protruding to the surface, whereas basally it is in contact
with dendritic terminalizations of sensory nerves (fig. 359H).
6. The Development of the Olfactory Organ
The senses of smell and taste are much alike. Both detect chemical sub-
stances dissolved in fluid. The olfactory epithelium of the vertebrate group
is of the simple columnar variety containing neurosensory cells (fig. 356A)
supported by non-nervous epithelial elements. Each neurosensory cell at its
free surface terminates in a series of cilia-like structures, and at its basal end
is prolonged into a neurite (nerve fiber) which passes into the olfactory bulb
where it breaks up into a number of telodendria. The olfactory area of the
human nasal passageway is shown in figure 3591 (see legend).
a. Development of the Olfactory Organs in Squalus acanthias
The two olfactory sacs in Squalus develop as invaginations of a thickened
olfactory placode on either side of the antero-ventral aspect of the head near
the oral invagination. They remain as blind sacs, extensively folded internally
and closely associated with the olfactory bulbs of the brain (fig. 357H).
b. Development of the Olfactory Organs in the Frog
The olfactory organs in the frog arise from two placodes, one on either
side of the head immediately in front of the developing eyes. These placodes
invaginate, and push downward and posteriad toward the developing oral
cavity. At about the 10 to 12 mm. stage they perforate into the anterior end
of the oral cavity. The walls of the olfactory inpushing become folded to form
844 IHK NIIRVOUS SYSTLM
the complicated nasal passageway of the adult frog. The external opening of
each passageway is called an external naris while the opening into the buccal
cavity is known as the choana (fig. 257B).
c. Development oj the Olfactory Organs in the Chick
The development of the olfactory organ in the chick embryo resembles the
development of this structure in the mammal, described below.
(I. Development of the Olfactory Organs in the Mammalian Embryo
As in other vertebrates, the olfactory areas of the olfactory organs of mam-
mals develop from olfactory placodes located one on either side on the ventro-
lateral aspect of the primitive head region (fig. 256). The olfactory placodes
sink inward to form the olfactory pits, and each pit expands laterally and dis-
tally. The lateral external margin of each olfactory pit is called the lateral nasal
process, and that of the median external margin is called the median nasal
process. The median and lateral nasal processes come in contact with the
maxillary process of the upper jaw.
As the olfactory pit grows posteriad it comes to open into the roof of the
primitive oral cavity as the primitive choana (figs. 288A and 256) posterio-
medially to the junction of the maxillary and median nasal processes. Later, as
each palatal process grows mcdiad from the maxillary processes, the nasal pit
and the upper oral area become separated from the oral cavity below by the
formation of the secondary palate (fig. 289D-F). Meanwhile, the median
nasal septum (fig. 288 A) grows ventrad and posteriad from the fronto-nasal
process and unites with the secondary palate in the median line (fig. 288B).
Two nasal passageways thus are established leading posteriorly (fig. 288D)
to open into the pharyngeal area as the secondary choanac (fig. 289F). The
epithelium of the original nasal placode and pit comes to lie in the dorso-medial
and dorso-lateral areas of this nasal passageway along either side of the nasal
septum (fig. 3591). The olfactory epithelium gives origin to bipolar cells, one
pole developing cilia-like processes which lie exposed to the surface of the
epithelium while the other pole develops an elongated fiber which grows
dorsad and posteriad to enter the forming olfactory bulb of the telencephalon
(fig. 356A).
7. The Eye
a. General Structure of the Eye
The general structure of the eye is shown in figure 360A.
h. Development of the Eye
The early stages of the development of all vertebrate eyes tend to follow
certain generalized steps, and the following description of the developing eye
of the chick presents the principles involved. The eye of the chick begins to
si;nsi; ok ricij'ior orcjans 845
develop as lateral oiilgrowlhs from the caudal end of the pn)sencephaIon
(future diencephalon) (lig. 354I3-D). These outgrowths, the primary optic
cvaj;>inati(>iis, begin to appear early on the second day of incubation, even
liefore the neural tube is closed. At about the 12 somite stage, which exists
at about 38 hours of incubation, the primary optic vesicles begin to constrict
proximally in the area near the brain, and distally they come into conatct
with the overlying epidermis (fig. 36()B). At 16 somites, or about 45 to 49
hrs. of incubation, the primary optic vesicle has dilTerenliated into a proximal
constricted optic .stalk and a distal primary optic vesicle (lig. 36()C). At the
22 somite stage (about 50 hrs. of incubation), the Optic vesicle begins to
invaginate and the overlying ectoderm starts to thicken preparatory to for-
mation of the lens (fig. 360D). At 55 hrs. of incubation, invagination of the
optic vesicle is completed, and the two-layered or secondary optic vesicle is
formed. The lens rudiment at this time is an invaginated vesicle still retaining
a small, open duct to the external surface. The following features of eye
development in the 55 hr. chick are present:
( 1 ) The lens vesicle is almost completely formed.
(2) The secondary optic vesicle is in the form of a cup, who.se inner layer
forms the retinal rndinient, and its outer layer the rudiment of the
pigmented coat of the eye.
(3) The ventral or lower edge of the optic stalk also is invaginated to form
the choroid fissure, which continues the invagination of the optic cup
back into the region of the ventral area of the optic stalk (fig. 36()E).
In the 72 to 75 hr. chick (about 40 pairs of somites) the two-layered optic
cup presents an outer thinner layer, the rudiment of the pigmented coat, and
an inner, thicker retinal layer. I'he lens vesicle at this time is completely free
from the overlying ectoderm and its inner (medial) wall is thicker than the
external wall. The medial thicker wall is the rudiment of the body of the len.s
and the outer thinner wall is the anterior epithelium of the lens (fig. 360F).
At 96 to 100 hrs. of incubation the developing lens of the eye has undergone
marked changes from the condition present at 72 to 75 hrs. of incubation.
The medial wall of the lens vesicle has thickened greatly and lens fibers are
evident, while the lateral wall of the vesicle forms a relatively thin epithelial
membrane (lig. 360(i). The mesoderm below the ectoderm also forms a thin,
internal epithelial membrane which lines the developing cornea. At this time
the lips of the optic cup show the first indications of two distinct areas, viz.:
a retinal or optic part, the pars optica retinae, which forms the visual portion
of the adult retina, and a pars caeca retinae lying distally in the region of the
lens (fig. 360G). The pars caeca does not develop visual cells. At the eighth
to ninth days of incubation, the pars caeca shows the beginning stages of ciliary
body formation, and the development of the iris (fig. 360H). The mesenchyme
overlying the iris forms the condensed stromal tissue, but the sphincter and
846 THE NERVOUS SYSTEM
dilator muscles of the iris develop from the pigmented layer of the pars caeca.
Two definite layers are present in the retina, viz.: inner marginal and outer
mantle layers. The rudiment of the sclerotic coat of the eye is present, and
in front of the developing iris the mesenchyme of the sclerotic coat continues
below the external ectoderm forming the rudiment of the cornea. The massive
vitreous body is present and a delicate membrane separates the vitreous body
from the optic cup. It is probable that the vitreous body forms from contri-
butions of the optic cup and the lens vesicle. At this time, also, the rudiments
of the upper and lower eyelids are present as folds of the integument sur-
rounding the outer edges of the corneal zone of the eye (fig. 360H).
c. Special Aspects of Eye Development
The foregoing description of the developing eye of the chick presents the
common or general features of eye development. The data given below de-
scribe certain features of the later development of the vertebrate eye, particu-
larly that of the mammal and the bird.
1) The Choroid Fissure, Hyaloid Artery, Pecten, etc. The choroid fissure
is the trough-like continuation of the invaginated area of the optic cup into
the optic stalk, and it permits a ready entrance into the optic cup. Mesenchyme
extends along the fissure and invades the optic cup and its developing vitreous
body. The central artery of the retina ,in the developing eye of the pig and
human also grows inward with the mesenchyme; in the region of the optic
cup it is called the hyaloid artery (fig. 3601). The hyaloid artery gives origin
to a mass of capillaries which surround but do not enter the developing lens.
This vascularization of the peripheral lens area persists until a short while
before the time of birth but regresses rapidly as birth approaches. The hyaloid
artery also regresses completely, leaving in its previous course a lymph space
known as the hyaloid canal of the vitreous body (fig. 360A).
The choroid fissure eventually closes, including the portion which extends
into the region of the optic cup. In the region of the optic stalk it persists for
a while as a small canal containing mesenchyme and the central artery of the
retina. As the retina develops, the nerve fibers of the forming optic nerve
converge toward the optic stalk and grow inward toward the brain along the
Fig. 360. Diagrams illustrating the development of the eye. (A) General structural
features of the adult mammalian eye. (Redrawn from Morris' Human Anatomy, 1943,
Philadelphia, Blakiston.) (B-H) Development of the eye of the chick. Ages indicated
on the figures. Diagram E' represents the developing eye viewed from the ventral aspect
showing the choroid fissure into which small capillaries are beginning to course forward
into the optic cup. Mesenchyme also invades the choroid fissure. In diagram H the pecten
has been slightly schematized. (I) Sagittal section through the developing eye of an
18 mm. pig embryo. Observe the hyaloid artery coursing from the optic nerve area
across the vitreous chamber to the lens. (T) Later stage in differentiation of the retina.
The rods and cones lie in the outermost area of the retina.
Fig. 360. (See facing page for legend.)
847
848
THE NERVOUS SYSTEM
SCLEROTIC
UPPER
VITREOUS BO
TIN4L LAYER
GMENTED COiT
ANTERIOR CHAMBE
l:S CORNE
LENS EPITHELIUM
NS RUDIMENT
LENS FIBERS
ELOPING CORNEA
LOWER LIO
RS CAECA
RS OPTICA
PECT
96 00 HOURS
C INNI
MARGINAL
L MfiNT E
MARGIN OF IRIS
LENS EPITHELI
P GMENTED LAYER OF
OPT C CUP BECOMES
P GMENTED LAYER OF
RET NAL LAYER OF
OPT c CUP BECOMES
SENSORY LAYER OF
RETINA
-AR CAPSULE
LENS FORMING FRON
BRANCHES OF THE
H ALOID ARTERY
RODS AND f
CONES ^1
OUTER [U4ljllliiMlllilli,
NUCLEAR -J OO'^oOj^?*^
NUCLEAR Ho"9/«jl/Oi0(
For legend see p. 846.
lumen of the optic stalk and the central artery and vein of the retina. The
lumen of the optic stalk thus becomes converted into the optic nerve.
Turning now to the chick embryo, we observe that the choroid fissure has
an added significance. In this embryo, as in the mammal, the presence of the
choroid fissure permits mesenchyme and blood vessels to enter the vitreous
chamber (optic-cup chamber) of the eye, and the optic nerve fibers travel
toward the brain along the lumen of the optic stalk. However, as the fissure
closes in the region of the optic cup, the ectodermal edges of the cup fold
inward in the region where the optic cup joins the optic stalk and this optic-
cup fold comes to enclose the inward migrating mesenchyme and blood
SENSE OR RECEPTOR ORGANS 849
vessels. Thus it happens that in addition to the entrance of mesenchyme and
blood vessels into the vitreous chamber the pigmented and nervous layers of
the pars optica retinae fold inward around and enclose the blood vessels and
mesenchyme. This forward projection of ectodermal and mesodermal tissues
into the vitreous chamber toward the lens forms the pecten of the bird's eye
(fig. 360H). From the seventh to the eleventh days the rudiment of the pecten
increases greatly in length, and becomes very narrow, folded, and comb-
shaped. Shortly before hatching, the number of folds increases to about 18,
and the structure as a whole is highly pigmented and vascularized. The pecten
appears to increase the vascular supply to the vitreous chamber, and also, it
is possible that the pecten may act in some way to increase the visual powers
of the retina. In the reptiles a similar, but less complex projection, the vascular
papillary cone, is developed, and in the eye of teleost fishes the falciform
process may be homologous with the papillary cone of reptiles and pecten
of birds.
2) The Formation of the Lens. The early formation of the lens vesicle
from the overlying ectoderm appears to be dependent upon inductive influences
emanating from the optic vesicle in some species, e.g., Bombinator, but in
others, e.g., Rana esculenta, the lens vesicle appears to form independently.
(See Werber, '16, and Spemann, '38, Chapter 3.) The inner wall of the lens
vesicle differentiates into elongated slender cells of the lens. The nuclei remain
near the center of these slender cells, and the cells gradually transform into
the transparent lens fibers. The outer, lateral wall of the lens vesicle forms
a layer of low columnar cells, the lens epithelium (fig. 360H and I).
3) The Choroid and Sclerotic Coat of the Eyeball; the Cornea. The devel-
oping optic cup is at all times surrounded by mesenchyme. This mesenchyme
condenses around the pigmented layer of the optic cup to form two distinct
layers, namely, ( 1 ) An inner vascular coat immediately surrounding the pig-
mented layer, and (2) an outer white fibrous thick connective tissue layer.
The inner vascular coat forms the soft, vascular choroid coat of the eyeball,
whereas the fibrous layer develops the hardened sclera or sclerotic coat. The
sclerotic coat in reality is the skeletal investment of the eyeball, upon which
the extrinsic muscles of the eye insert, and from which internally the ciliary
muscles or muscles of accommodation take their origin (fig. 360A). Also,
the muscles of the iris indirectly are dependent upon the sclera for their effi-
ciency. The choroid coat is the main source of blood supply for the eyeball
as a whole. It is highly pigmented and absorbs excess light rays from the
retina. In many vertebrates, including various mammals such as the cat, dog,
cow, deer, ferret, etc., the inner layer of the choroid coat near the retina de-
velops a light reflecting surface, the tapetum lucidum. In the cat and other
carnivores, this reflecting surface appears to be due to crystals of guanine,
while in the cow it is due, probably, to connective tissue fibers which glisten
and thus reflect the light.
POSTERIOR
SEMICIRCULAR
CANAL
Jeve n? .h^ ^ /"■ J'"'''u'''u '''''°" '^^'^"^'^ developing rhombencephalon at the
lZ\ . f P'^'""^'" ^hich are beginning to sink inward below the surrounding
ectoderm epidermis). (B) The otic vesicles are forming in this 19 somite (about
thhT ;"^"^^^'°")'^h.ck embryo. Slight constriction of otic vesicle near .,s junction
with the ectoderm. (C) Left otic vesicle of 4.3 mm. human embryo. The small
850
SENSE OR RECEPTOR ORGANS 851
acoustic ganglion lies to the left in the figure. The neural tube (rhombencephalic portion)
has not closed dorsally. (D) Left otic vesicle of 9 mm. human embryo viewed from
lateral aspect. The differentiating acoustic ganglion is shovi'n to the left. It is now dividing
into vestibular and spiral ganglia. The cochlear diverticulum is shown extending ven-
trally. (E) Later differentiation (11 mm. human embryo) of left otic vesicle, lateral
view. (F) 20 mm. human embryo, left, lateral view of differentiating otic vesicle. (G)
30 mm. human embryo, left, lateral view of differentiating otic vesicle. (H) 30 mm.
human embryo, left, median view of differentiating otic vesicle. (H-1) Semischematic
plan of cochlear duct and spiral ganglion of 4 month human embryo. (H-2) High-
powered view of basilar membrane (lamina spiralis membranacea) shown in Fig. 361 H-1,
portraying the spiral organ of Corti. (LI) Three-dimensional schematic drawing of the
human ear composed of the external ear, the middle ear, and the inner ear. The external
ear is composed of the pinna and external auditory meatus. The middle ear is made up of
the middle ear cavity or cavity of the tympanum with its auditory ossicles, the malleus,
incus, and stapes. The external tympanic membrane is stretched across the entrance of the
external auditory meatus into the middle ear cavity whereas the internal tympanic mem-
brane covers the fenestra rotunda (fenestra cochlea). The internal ear located within the
petrous bone communicates with the middle ear directly by means of the fenestra oralis
(fenestra vestihuli) and indirectly by means of the fenestra rotunda. The stapes is inserted
in the fenestra ovalis and the malleus is joined to the external tympanic membrane. By
means of the stapedial articulation with the incus and the latter's association with the
malleus, the three auditory ossicles thus extend across the middle ear cavity from the ex-
ternal tympanic membrane to the fenestra ovalis.
The structural parts of the inner ear are made up of the membranous labyrinth, the
semicircular canals, utriculus, sacculus, and the cochlear duct (fig. 36 IF and G). Sur-
rounding the membranous labyrinth is the other structural part of the ear, the bony
labyrinth, which conforms to the general shape of the membranous labyrinth. A fluid,
the endolymph, is contained within the membranous labyrinth, whereas perilymph lies
in the space between the membranous labyrinth and the bony labyrinth.
The development of the membranous labyrinth is shown in Fig. 361A-H, and the
formation of the pinna and external auditory meatus is depicted in Figs. 328A and B,
and 329A. It is to be observed that swellings upon the hyoid and mandibular visceral
arches contribute to the formation of the pinna, and that the external auditory meatus
develops from the invaginating hyomandibular cleft (branchial groove) between these
two arches. The origin of the auditory ossicles is shown in Fig. 319C-1 and C-2. Fig.
3611-2 shows the early relationship of these ossicles within the mesenchymal substance
of the developing middle ear cavity. During the formation of the middle ear cavity
spaces form around the developing ossicles. These spaces then coalesce to form the
rudiments of the middle ear cavity or cavity of the tympanum. This rudimentary
tympanic cavity later unites with the distal end of the first branchial pouch. The proximal
portion of the first branchial pouch forms the eustachian tube which connects the
pharyngeal area with the middle ear cavity. The extent to which the middle ear cavity
eventually comes to be lined with entoderm from the expanded distal end of the
eustachian tube (first branchial pouch) is problematical. The external tympanic mem-
brane is developed from the ectoderm of the external auditory meatus (hyomandibular
cleft invagination) and the lining of the middle ear cavity. Between these two mem-
branes is a layer of mesenchyme which transforms into connective tissue. The malleus
remains attached to the external tympanic membrane. (1-2) Schematic diagram of an
early stage in development of the auditory ossicles and tympanic cavity in the human
embryo. Observe that the first branchial pouch is expanding into the area around the form-
ing auditory ossicles where spaces, shown in black, are beginning to appear within the
mesenchyme surrounding the developing ossicles. (J) Diagram of the ear in the frog.
Unlike the condition in the frogs a tympanic cavity is almost entirely absent in urodeles.
(K) Diagram of the ear of a reptile comparable to conditions found in the snakes. It is
to be observed that an external tympanic membrane or external ear opening is absent.
Observe that the ear ossicle is composed of stapedial and extrastapedial segments. (L)
Diagram of the ear of the chick. The ear "ossicle" is composed of two parts, viz., a
Fig. 361 — Continued
bony stapedial portion which articulates with the fenestra vestibuli and a distal carti-
laginous extrastapedial segment which connects with the external tympanic membrane.
The eustachian tube connects with the same tube on the contralateral side to form a
common opening into the dorsal pharyngeal area. The external ear opening is protected
by feathers. The ear of lizards resembles that of the bird, a short external auditory
meatus being present, protected externally in many instances by scales. In the frog, Rana
cavitympanum of Siam, the tympanic membrane similarly has moved inward and an
external auditory meatus is present. (M) The right membranous labyrinth of the shark,
Squalus acanthias. (Redrawn from Adams and Eddy, 1949, Comparative Anatomy, New
York, Wiley & Sons. (N) The right membranous labyrinth of the frog, Runa. (O)
The right membranous labyrinth of the pigeon, Columha.
852
SENSE OR RECEPTOR ORGANS 853
The cornea of the eyeball is formed mainly from the mesenchyme of the
sclerotic coat which extends forward in front of the developing anterior cham-
ber of the eye (fig. 360H). The overlying skin forms the corneal epithelium.
4) Contributions of the Pars Caeca. The pars caeca or non-nervous part
of the primitive optic cup gives origin to the smooth muscle tissues of the iris.
These muscles are derived probably from the pigmented layer of the original
optic vesicle. The zonula ciliaris or suspensory ligament of the lens also is
derived from this source.
5) The Origin of the Ciliary Muscles. The smooth muscle tissue of the
ciliary muscle together with the connective tissue of the ciliary bodies, and the
stromal tissue of the iris, are derived from the mesenchyme of the primitive
choroid coat which overlies the pars caeca of the optic cup.
6) Accessory Structures of the Eye. The upper and lower eyelids develop
as folds of the integument about the eyeball and circumscribing the corneal
area (fig. 360H). In the chick these folds are apparent on the seventh day
of incubation, and in the human embryo at about the seventh week. In mam-
mals the eyelids normally fuse after their formation, and in many they do
not reopen until some time after birth. In the dog the eyelids reopen at about
10 to 15 days after birth, while in the human and guinea pig they reopen
before birth. Complete fusion of the eyelids does not occur in the chick. A
third or rudimentary eye structure, the plica semilunaris, is present at the
inner angle of the human eye. This structure may represent the nictitating
membrane in the cat and dog, and possibly also the nictitating membrane of
the chick. The real homology of the plica semilunaris with these structures,
however, is questionable.
Accessory eye glands arise in land vertebrates. The lacrimal glands arise
as epidermal ingrowths from the inner aspect of the developing upper (man,
cat, dog) or lower (urodeles) eyelid. The lacrimal gland is developed typically
in mammals. The racemose harderian gland arises as a solid ingrowth of epi-
dermal cells at the inner ingle of the nictitating membrane. The secretion of
the harderian gland found in reptiles, birds, and also in certain mammals is
oily while that of the lacrimal gland is watery. The tarsal (Meibomian) glands
of the human eyelid arise as epithelial invaginations.
The naso-lacrimal duct in mammals arises from the naso-lacrimal groove
formed in the area of the lateral nasal and maxillary processes; it extends
from the nasal sac to the angle of the eye (fig. 256). During the formation
of the face this groove sinks inward and forms a duct which establishes a
definite connection with the inner edges of each eyelid. It opens into the nasal
chamber.
8. Structure and Development of the Ear
a. Structure
The functions of hearing and equilibration (balance) in the gnathostomous
vertebrate group involve the structure known as the membranous labyrinth
854 THE NERVOUS SYSTEM
of the inner ear. The latter structure is composed of a central saccular area
to which are attached a complex of ducts and canals (fig. 361H). It is located
within a protective encasement of cartilage or bone which conforms to the
general shape of the membranous labyrinth (fig. 3611). However, the laby-
rinth fits loosely within its protective case, and a space, filled with fluid, the
perilymph, intervenes between the walls of the membranous labyrinth and
the walls of the cartilaginous or bony labyrinth which surrounds the mem-
branous labyrinth. Within the membranous labyrinth is a fluid, the endolymph.
The function of equilibration is concerned mainly with movements or lack
of movement, i.e., inertia, of the endolymph, while the function of hearing
entails wave movements in the perilymph which in turn are transferred to
a portion of the endolymphatic fluid.
The membranous labyrinth is composed of a saccular region divided into
two compartments, the utriculus and sacculus connected by a narrow passage-
way. To the utriculus and sacculus the following ducts and canals are attached
(see figure 361H, M, N, and O).
1) Three semicircular canals, which, throughout the jawed vertebrate
group, adhere to the following pattern: (a) a horizontal canal, (b) a pos-
terior vertical canal, and (c) an anterior vertical canal. Each of these canals
is expanded at one end to form an enlargement known as the ampulla.
2) An endolymphatic duct, generally connected to the sacculus near the
connecting passageway between the sacculus and utriculus. The distal end of
the endolymphatic duct is enlarged to form the endolymphatic sac.
3) A Cochlear Duct or Lagena. The lagena is an evagination of the sac-
culus. It is abortive in lower vertebrates but greatly extended in mammals.
All of the semicircular canals are attached to the utriculus. The anterior
and posterior vertical canals generally attach at one end of the utriculus to a
common chamber, the crus commune, before joining the utriculus.
The internal lining of the membranous labyrinth possesses, in restricted
areas, specialized sensory epithelial cells, known as neuromast cells, associ-
ated with branches of the acoustic cranial nerve. In the utriculus and sacculus
these areas of sensory epithelium are called maculae. A single macula is found
in the utriculus and another in the sacculus. A gelatinous membrane is asso-
ciated with each macula and concretions or otoliths may be present in the
jelly of this membrane. Within each ampulla of the semicircular canals a
sensory area of epithelium is present known as a crista, with the cilia-like
projections from the ends of the cells embedded in a gelatinous mass. The
functions of the maculae presumably present sensations which tell the animal
how much the body is tilted up and down in one plane, i.e., static equilibrium,
whereas the semicircular canals off'er sensations which enable the animal to
detect its position when it is moving up and down or around in a series of
different planes. That is, the semicircular canals probably are concerned with
dynamic equilibrium.
SENSE OR RECEPTOR ORGANS 855
The endolymphatic duct appears to lack specialized sensory areas. In the
elasmobranch fishes, the endolymphatic ducts open by means of small pores
at the top of the head. The endolymphatic-sac area of the duct may be absent
in some fishes, but in many teleosts, reptiles, and amphibia the endolymphatic
sac is greatly enlarged. In the frog group, the endolymphatic sac is most ex-
tensive, protruding itself into the brain and spinal cord areas.
Sensory patches of epithelium are present in the lagena. In reptiles, birds
and mammals, the lagena is extended considerably. In birds and mammals
the lagena is called the cochlear duct, and it contains an extensive area of
sensory epithelium known as the organ of Corti.
In tetrapod vertebrates a middle ear containing a specialized ossicle or
ossicles, is added to the hearing mechanism (fig. 3611, J, K and L), and in
reptiles, birds and mammals an external meatus or specialized structure for
receiving sound waves is found. The external auditory meatus in mammals is
supplemented by the addition of an external ear or pinna, a funnel-shaped
structure for collecting sound waves (fig. 3611).
b. Development of the Internal Ear
The internal ear arises from the otic placode which sinks inward to form
the otic vesicle. The otic vesicle gradually transforms into the shape and
structure of the internal ear peculiar to the species. The transformation of
the otic vesicle in the human embryo is shown in figure 361C-H.
c. Development of the Middle Ear
The development of the middle ear results from an evagination of the
pharyngeal wall which primarily involves the region of the first branchial
pouch. This evagination unites distally with spaces forming around the ossicles.
The opening into the pharynx is retained, and the narrow passageway between
the pharynx and the middle ear cavity containing the ossicle or ossicles of
the ear is called the eustachian duct or tube.
d. Development of the External Auditory Meatus and Pinna
The external auditory meatus forms from an epidermal invagination in the
area of the first visceral groove, that is the region between the mandibular and
hyoid visceral arches. The pinna of the external ear in mammals arises from
swellings on the mandibular and hyoid arches. These swellings enlarge and
fuse to form the complicated form of the pinna (figs. 328 and 329).
F. Nerve fiber-effector organ relationships
(Consult figure 358F-G.)
Bibliography
Arey, L. B. 1916. The function of the ef-
ferent fibers of the optic nerve of fishes.
Jour. Comp. Neurol. 26:213.
and Smith, H. V. 1937. Anat. Rec.
67 (suppl. 4).
Ariens-Kappers, C. U., Huber G. C. and
Crosby, E. C. 1936. The Comparative
Anatomy of the Nervous System of Ver-
tebrates, including Man. Vols. I and II.
Macmillan, New York.
Goodrich, E. S. 1930. Studies on the struc-
ture and development of vertebrates.
Macmillan and Co., Ltd., London.
Harrison, R. G. 1907. Observations on
the living developing nerve fiber. Anat.
Rec. 1:116.
Hill, C. 1900. Developmental history of
the primary segments of the vertebrate
head. Zool. Jahrb. Anat. 13:393.
Kuntz, A. and Batson, O. V. 1920. Experi-
mental observations on the histogenesis
of the sympathetic trunks in the chick.
J. Comp. Neurol. 32:335.
Larsell, Olof. 1918. Studies on the nervus
terminalis: Mammals. Jour. Comp.
Neurol. 30:3.
Lavelle, A. 1951. Nucleolar changes and
development of Nissl substance in the
cerebral cortex of fetal guinea pigs. J.
Comp. Neurol. 94:453.
Maximow, A. A. and Bloom, W. 1942.
A Textbook of Histology, 4th Edition.
Saunders, Philadelphia.
Ranson, S. W. 1918. An introduction to
a series of studies on the sympathetic
nervous system. Jour. Comp. Neurol.
29:305.
. 1939. The Anatomy of the Nerv-
ous System. Saunders, Philadelphia.
Speidel, C. C. 1933. Studies of living
nerves: II. Activities of ameboid growth
cones, sheath cells, and myelin segments,
as revealed by prolonged observation of
individual nerve fibers in frog tadpoles.
Am. J. Anat. 52:1.
Spemann, H. 1938. Embryonic Develop-
ment and Induction. Yale University
Press, New Haven, Conn.
Werber, E. I. 1916. On the blastolytic ori-
gin of the 'independent' lenses of some
teratophthalmic embryos and its signifi-
cance for the normal development of the
lens in vertebrates. J. Exp. Zool. 21:347.
856
20
Tne Development or tne Coelomic Cavities
A. Introduction
1. Definitions
2. Origin of the primitive splanchnocoelic coelom
B. Early divisions of the primitive splanchnocoelic coelom
1. Formation of primitive suspensory structures
2. Formation of the primitive transverse division of the body and the primary peri-
cardial and peritoneal divisions of the coelom
a. Lateral mesocardia
b. Formation of the liver-septum transversum complex
1) Formation of the liver-septum complex through modification of the ventral
mesentery by liver outgrowth
2) Formation of the liver-septum complex in the human embryo
c. Formation of the primary septum transversum
C. Coelomic changes in fishes, amphibians, reptiles, and birds
1. In fishes
2. In amphibians, reptiles, and birds
D. Formation of the coelomic cavities in mammals
1. Formation of the pleuropericardial membrane
2. Development of the pleuroperitoneal membrane
E. Development of independent pericardial walls
1. The arrangement of the parietal pericardial wall in fishes
2. Formation of an independent parietal pericardial wall in the chick
3. Formation of the independent parietal pericardial wall in amphibians and reptiles
4. Separation of the parietal pericardial wall in mammals
F. The mammalian diaphragm
G. The pulmonary diaphragm or aponeurosis of the chick
H. The omental bursa
I. The formation of various ligaments in the stomach-liver region
1. The gastro-hepatic .and hepato-duodenal ligaments
2. The coronary ligament of the liver
3. The falciform ligament of the liver
4. The gastro-splenic ligament
A. Introduction
1. Definitions
The coelomic cavities are the spaces which come to surround the various
viscera of the body such as the pericardial cavity around the heart, the pleural
857
858 THE DEVELOPMENT OF THE COELOMIC CAVITIES
cavities surrounding the lungs, and the peritoneal cavity in which He the
stomach, intestines, reproductive organs, etc. These coelomic spaces and
recesses arise from a generaUzed basic condition known as the primitive
splanchnocoelic coelom. The primitive splanchnocoeUc coelom is the elon-
gated cavity which extends throughout the trunk region beginning just anterior
to the heart and continuing posteriorly to the base of the tail. It encloses the
developing heart and the developing mesenteron (gut) from the esophageal
region posteriorly to the anal region.
2. Origin of the Primitive Splanchnocoelic Coelom
As observed previously (Chapter 10) the elongated mesodermal masses
lying along either side of the developing neural tube, notochord, and enteric
tube have a tendency to hollow out to form a cavity within. That is, like the
neural, gut, and epidermal areas of the late gastrula, the two mesodermal
masses tend to assume the form of tubes.
In the case of Amphioxus, each individual somite forms a cavity, the
myocoel. These myocoels merge on either side in their ventral halves to form
an elongated splanchnocoel below the horizontal septum (see page 506). Later
the two splanchnocoels fuse below the developing gut to form the single
splanchnocoelic coelom which comes to surround the gut. In the vertebrate
group, however, the two elongated splanchnocoels on either side of the de-
veloping gut tube and heart form directly in the hypomeric (lateral plate)
area of the mesodermal masses without a process of secondary fusion as in
Amphioxus. In the upper part of each mesodermal mass, that is in the
epimere, and to some extent also in the mesomere (nephrotomic plate) in the
vertebrate group as in Amphioxus, there is a tendency for the coelomic spaces
to appear in segmental fashion within the primitive somites and within the
anterior portion of the mesomere. These individual spaces within the somites
are called myocoels, and the spaces which arise in the segmented portion of
the nephrotome are called the nephrocoels.
In young shark embryos, such as the 3-4 mm. embryo of Squalus acan-
thias, and in amphibian embryos of the early post-gastrular period, the myo-
coelic and nephrocoelic portions of the coelom are continuous dorso-ventrally
with the splanchnocoelic coelom (fig. 217G and H). (Actually, during the
early stages of coelomic development within the mesodermal masses, in the
shark and amphibian embryos, the coelom within the epimere and nephro-
tomic portions of the mesoderm is continuous antero-posteriorly and it is
only after the appearance of the primitive somites and segmentation within
the nephrotome that they become discontinuous.) On the other hand, in the
embryos of higher vertebrates, the respective myocoels within the somites
appear later in development, and in consequence they are always separated
from the splanchnocoel. Similarly, the nephrocoelic coelom also arises later
and only the separate nephrocoels which develop within the pronephric tubules
EARLY DIVISION OF SPLANCHNOCOELIC COELOM 859
and certain types of mesonephric tubules make contact with the splanchnocoelic
portion of the coelom.
In all vertebrates (see figures 254, 332F-M) the formation of the primitive,
generalized coelomic cavity proper or generalized splanchnocoelic portion of
the coelom is formed by the fusion around the developing heart and gut struc-
tures of the two elongated splanchnocoels present in the hypomeric portions
of the mesodermal masses as described below.
B. Early Divisions of the Primitive Splanchnocoelic Coelom
1. Formation of Primitive Suspensory Structures
The splanchnic walls of the early coelomic cavities (splanchnocoels) within
the two hypomeres become apposed around the structures, lying in the median
plane (fig. 254). In the region of the heart, this apposition gives rise to the
dorsal and ventral mesocardia and to the epimyocardium of the heart itself
(fig. 254A, B) and, in the region of the stomach and intestine, it produces
the dorsal and ventral mesenteries of the gut tube and various ligaments,
connecting one organ with another. The mesenchyme which arises from the
two splanchnic layers also gives origin to the muscles and connective tissues
of the gut and its evaginated structures (fig. 311 A, B). The ventral meso-
cardium disappears in all vertebrates (Chap. 17). The dorsal mesocardium
may persist for a while but eventually disappears entirely or almost entirely
(Chap. 17). The dorsal mesentery is present constantly in reptiles and mam-
mals but may be perforated and reduced in the intestinal area in other verte-
brate classes, so that little of the dorsal mesentery remains to suspend the
intestine in certain cases as, for example, in the shark. The dorsal mesentery
above the stomach, the mesogastrium, and also the ventral mesentery in the
immediate region between the stomach and liver and between the liver and
the ventral body wall persist in all vertebrates. As a rule, however, the ventral
mesentery disappears caudal to the liver with the exception of dipnoan and
anguilliform fishes and the ganoid fish, Lepisosteus. In these forms the ventral
mesentery tends to persist throughout the peritoneal cavity. It follows, there-
fore, that the two bilaterally developed, splanchnocoelic cavities tend to merge
into one cavity or generalized splanchnocoel with a partial retention in certain
areas of the splanchnic layers of the two hypomeres which act as suspensory
ligamentous structures for the viscera.
2. Formation of the Primitive Transverse Division of the
Body and the Primary Pericardial and Peritoneal
Divisions of the Coelom
The primitive splanchnocoelic coelom soon becomes divided into the peri-
cardial coelom, surrounding the heart, and the peritoneal or abdominal coelom,
surrounding the digestive viscera, by the formation of the lateral mesocardia
PLEURO-
ERICfiRDIAL
.■•:'■ CANAL
NEURAL TUBE
DORSAL AO
DORSAL CLOSING
CONTRIBUTION
SPLANCHNOPLE
MESODERN
DUCT OF CUV
DORSAL CLOSIN
CONTRIBUTION
SOMATOPLEU
MESODER
LATERAL
MESOCARDIU
SINUS VENO:
HEART (ATRIU
LIVER
STOMACH DORSAL
MESOGASTRIUM
ORSAL AORTA
NEURAL TUBE
Fig. 362. The lateral mesocardia form the initial division of the embryonic coelom.
(A-1 and A-2) represent idealized sections through the vertebrate embryonic body in a
plane between the caudal limits of the sinus venosus and the anterior extremity of the
potential liver region of the embryo. (A-1) Diagram of the initial stage of separation
of the pericardial and peritoneal coelomic cavities in many vertebrates. Two dorsal and
two ventral recesses or passageways above and below the lateral mesocardia and lateral
horns of the sinus venosus are evident. These passageways communicate with the peri-
cardial and peritoneal divisions of the primitive coelom. (A-2) Separation of primitive
860
EARLY DIVISION OF SPLANCHNOCOELIC COELOM 861
and the primitive septum transversum which develop in relation to the con-
verging veins of the sinus venosus and the ventro-cephalic growth of the liver
rudiment. In other words, a ventral partition is established across the primi-
tive splanchnocoelic coelom in a plane which separates the caudal end of
the heart (i.e., sinus venosus) from the anterior limits of the liver. This
primitive transverse partition partially separates the primitive splanchnocoelic
coelom into two main divisions:
( 1 ) a cephalic compartment, the pericardial cavity, around the heart and
Fig. 362 — Continued
coelom into anterior pericardial and posterior peritoneal areas in early human embryo.
The precocious development of the caudal wall of the parietal pericardium obliterates
the ventral recesses shown in A-1 previous to septum transversum formation and the
outgrowth of the liver rudiment. Communication between pericardial and peritoneal
coelomic divisions is possible only through the dorsal parietal recesses (dorsal pericardio-
peritoneal canals). (B) Schematic diagram representing the initial division by the
lateral mesocardia of the primitive coelomic cavity into anterior pericardial and posterior
peritoneal divisions in an embryo of Squalus acanthias 10 mm. long. The liver outgrowth
has been extended forward slightly for diagrammatic purposes. (C) Initial division, by
the lateral mesocardia. of the primitive coelom in the 72 hr. chick embryo. Due to the
depressed condition of the anterior end of the body much of the heart appears in the
section below the sinus venosus and lateral mesocardia. However, if the embryo were
straightened and the atrium, etc., of the heart pushed forward, the structural conditions
would appear much the same as in B. The dorsal parietal recesses appear on either
side of the esophagus. (D) Semidiagrammatic section through caudal end of sinus
venosus of 22 mm. shark embryo. The dorsal closing folds are developing on either side
of the esophagus, thus closing the dorsal recesses. The liver rudiment is expanding within
the substance of the ventral mesentery caudal to the heart to form the liver-septum
transversum complex. The latter structure obliterates the ventral recesses below the
lateral mesocardia. (E) Diagrammatic representation of the forward and ventral growth
of the developing liver within the substance of the ventral mesentery to form the liver-
septum transversum complex. (See fig. 363D.) Observe: ventral parietal recesses are
obliterated by the forward growth of this complex of tissues. The arrow denotes the
passageway from the pericardial coelom into the peritoneal coelom through the dorsal
parietal recesses (dorsal pericardioperitoneal canals). (F) Early stage in development
of human heart and septum transversum showing ingrowth of somatopleural mesoderm
between the previously formed caudal wall of the parietal pericardial membrane (see
A-2) and the entoderm of the anterior intestinal portal. (Redrawn from Davis, 1927,
Carnegie Inst. Public. 380, Cont. to Embryology, 107.) (G) Later stage of human
heart development. Mesodermal partition (septum transversum) is present as a thickened
mass of tissue below the developing sinus venosus and between the caudal wall of the
parietal pericardium and the gut entoderm. (Redrawn from Davis, see fig. 362F, for
reference.) (H) Lateral dissection of fifth week human embryo to show ingrowth of
liver tissue into thickened' septum transversum. (Redrawn from Patten, 1946, Human
Embryology, Blakiston, Philadelphia.) Arrow denotes passageway (dorsal parietal recess;
pericardioperitoneal canal; pleural canal) between pericardial and peritoneal coelomic
cavities. (I-l) Sagittal section through 15 mm. pig embryo showing thickened anterior
face of liver. This thickened anterior face of the liver later separates from the liver
as the primary septum transversum (peritoneo-pericardial membrane). (1-2) Higher
powered drawing to show condition of anterior face of liver shown in fig. 362, I-l.
(J) Transverse section through thorax and pulmonary area of the body of a bird to
show position of dorsal pulmonary diaphragm. (Redrawn from Goodrich, 1930, Studies
on the Structure and Development of Vertebrates, Macmilian Co., Limited, London.)
Observe position of liver lobes in relation to the heart. Compare with fig. 294, G-4 & G-5.
LEFT
PULMONARY-
RECESS
LIVER LOBE
STERNAL R
See legend on p. 860.
Fig. 362— (Continued)
862
EARLY DIVISION OF SPLANCHNOCOELIC COELOM 863
(2) a larger caudal compartment, the peritoneal cavity, around the diges-
tive viscera and urogenital structures.
This primary division of the early coelomic cavity is accomplished by the
formation of:
1 ) The lateral mesocardia, and
2) the primary (primitive) septum transversum.
The two lateral mesocardia are formed previous to the development of the
primitive septum transversum. Eventually the lateral mesocardia fuse in part
to the dorsal edge of the transverse septum and become a part of it. The lateral
mesocardia thus, in reality, represent the initial stage in the division of the
general coelomic cavity. In consequence we shall consider the lateral meso-
cardia as important structures which enter into the formation of the primary
transverse division of the embryonic body, but they should not be confused
with the primitive septum transversum in a strict sense.
a. Lateral Mesocardia
The lateral mesocardia (fig. 362A-1, A-2) are formed as follows:
A lateral bulging or growth from the splanchnopleure at the caudal limits
of the developing sinus venosus extends dorso-laterad on each side to meet
a somewhat similar though smaller growth mediad of the somatopleural meso-
derm. These growths form a bridge on each side across the coelomic cavity,
extending dorso-laterad from the posterior lateral edges of the ventrally situ-
ated sinus venosus to the somatic wall. The area of union of this bridge on
either side with the lateral body wall is the lateral mesocardium. The lateral
mesocardia, in other words, represent the areas of juncture between the lateral
body walls and the lateral extensions of the sinus venosus. The common
cardinal veins or ducts of Cuvier join these right and left lateral extensions
or horns of the sinus venosus in the substance of the lateral mesocardia. An-
terior to the lateral mesocardia is the pericardial coelom, while posterior to
them is the peritoneal coelom. The two passageways dorsal to the lateral
mesocardia, on either side, are called the dorsal parietal recesses of His,
while those ventral to the lateral mesocardia and on either side of the ventral
mesentery and developing liver constitute the ventral parietal recesses of His
(fig. 362A).
b. Formation of the Liver-Septum Transversum Complex
1) Formation of Liver-Septum Complex through Modification of the Ven-
tral Mesentery by Liver Outgrowth. As the liver rudiment in the shark,
chick, pig, etc., grows ventrally and forward between the two splanchnopleural
layers of the ventral mesentery, it expands the ventral mesentery laterally as
the liver substance forms within the mesenchyme between the two splanchnic
layers. The expanding liver substance eventually reaches the ventral and lateral
G4STR0MEPiTIC
LIGAMENT
UNG \ PLEURO
Fig. 363 (A-1, 2, 3). Diagrams showing the invasion of the peritoneal coelom around
the liver and relations of septum transversum and diaphragm to the liver. (A-1) The
peritoneal invasion separates the liver substance away from the lateral body wall and
also from the anterior face of the liver itself. The separated, thickened, anterior face
of the liver (see fig. 362, I-l and 1-2) forms the primary septum transversum (peritoneo-
pericardial membrane). (A-2) The relation of the liver and other viscera to the
secondary septum transversum formed by the addition of the dorsal closing folds (see
fig. 362D) to the primary septum transversum. (A-3) This is a diagrammatic repre-
sentation of conditions shown in B. Observe position of various ligaments associated
with the liver. (B) Sagittal section through opossum embryo presenting relation of the
liver to diaphragm. The ventral part of the diaphragm is the remodeled primary septum
transversum. Observe that the inferior vena cava perforates the diaphragm. The area
of attachment of the liver to the diaphragm is the coronary ligament. (The preparation
from which this drawing was made was loaned to the author by Dr. J. A. McClain.)
(C) Pericardioperitoneal opening below the esophagus in the shark, Squalus acanthias.
(See also fig. 362D.) (D) Schematic diagram, dorsal view, of initial stage of devel-
oping pleural cavities in the mammal showing the anterior and posterior lateral body
folds. The anterior lateral body fold gives origin to the pulmonary ridge or rudiment
of the pleuropericardial membrane and the posterior lateral body fold forms most of
the pleuroperitoneal membrane. Cf. fig. 362E. (E-H) Schematic diagrams showjng later
stages in separation of pleural cavities in the mammal, viewed from the dorsal aspect.
Observe that the pleuroperitoneal membrane is formed from two rudiments, viz., the
posterior lateral body fold and a very small splanchnopleuric contribution (fig. 363F).
864
EARLY DIVISION OF SPLANCHNOCOELIC COELOM
865
Fig. 363 — (Continued)
See legend on p. 864.
body wall, where it fuses with the somatopleure from the body wall. Since
the lateral expansion of the developing liver is more rapid than its forward
growth, the anterior face of the liver gradually becomes flattened in the area
just below (ventral to) the lateral mesocardia and immediately posterior to
the sinus venosus of the heart. The mesenteric tissue, covering the anterior
face of the liver, then fuses with the more dorsally located, lateral mesocardia.
A transverse division across the body is completed in this manner below the
lateral mesocardia, and the ventral parietal recesses in consequence are closed.
Passage from the pericardial cavity to the peritoneal (abdominal) cavity is
now possible only by way of the pericardioperitoneal canals (dorsal parietal
recesses) (fig. 362E).
Although liver-rudiment development in the embryo of the frog and in the
embryos of other amphibians is precocious the essential procedure in the
866 THE DEVELOPMENT OF THE COELOMIC CAVITIES
formation of the primitive liver-septum transversum complex is similar to that
described above.
2) Formation of the Liver-Septum Complex in the Human Embryo. In the
developing human embryo, medial growths on either side from the somato-
pleural mesoderm occur in the region caudoventral to the forming sinus ven-
osus, and below the developing gut tube. In this way ,a primitive transverse sep-
tum is formed below the lateral mesocardia and between the entoderm of the gut
and the caudal wall of the parietal pericardium (fig. 362F, G). This septum
fuses with the lateral mesocardia and caudal wall of the parietal pericardium.
However, when the evaginating liver rudiment grows ventrad and forward into
the splanchnopleural tissue below the gut, it ultimately appropriates the previ-
ously formed transverse septum as its anterior aspect. Consequently, the general
result of the two methods is the same, namely, the transverse septum in its
earlier stages of development appears as the thickened anterior face of the
liver associated with the lateral mesocardia (figs. 261 A; 362H, I).
c. Formation of the Primary Septum Transversum
After the liver-septum transversum complex has been established and the
potential ventral parietal recesses are closed by either of the two methods
described above, the next stage in the development of the primitive septum
transversum is correlated with the forward expansion of the peritoneal coelom
around the sides and anterior face of the liver. In doing so, the peritoneal
coelom on either side of the liver extends anteriad and mesiad and thus be-
comes involved in a secondary separation of the liver from the lateral and
ventral body wall and also from the anterior face of the liver itself which be-
comes the primary septum transversum (fig. 363 A, B). A separation does not
occur in the area traversed by the veins passing from the liver to the sinus
venosus or slightly dorsal to this area. Here the liver remains attached directly
to the septum transversum and is suspended literally from it. This attaching
tissue forms the coronary ligament of the liver. The ingrowth of the two
coelomic areas on either side of and ventral to the liver, by apposition of
the coelomic epithelium in the median plane, forms a secondary ventral mesen-
tery of the liver. This secondary ventral mesentery or falciform ligament ties the
liver to the mid-ventral area of the body wall and to the septum transversum.
(Note: The terms primary septum transversum and peritoneopericardial mem-
brane are synonymous.)
C. Coelomic Changes in Fishes, Amphibians, Reptiles, and Birds
1. In Fishes
In the adult shark, and fishes in general, the fully developed adult form
of the septum transversum forms a complete partition between the pericardial
cavity and the peritoneal cavity. In fishes the pericardial cavity in the adult
fish, as in the embryo, extends laterally and ventrally to the body wall in a
COELOMIC CHANGES IN FISHES. AMPHIBIANS, REPTILES, AND BIRDS 867
fashion similar to that of the peritoneal cavity. Also, the heart continues to
lie posterioventrally to the pharyngeal region in a manner very similar to that
of the basic, embryonic body plan (fig. 294G-I).
In the formation of the adult, piscine, septum transversum from the primary
transverse septum two membranous partitions are developed which close the
dorsal parietal recesses or the openings above the lateral mesocardia. These
partitions are called the dorsal closing folds and they arise as follows:
The splanchnopleural tissue on either side of the foregut, just anterior to
the stomach rudiment and above the primitive septum transversum, forms a
thin fold of tissue. This fold grows laterad and ventrad and fuses ultimately
with the lateral mesocardium and the somatopleuric tissue, which overlies the
common cardinal vein, as this vein travels caudo-ventrally along the body wall
to reach the lateral mesocardium and the sinus venosus. As a result of this
splanchnopleuric and somatopleuric fusion of tissues with the dorsal edge of
the primary septum transversum a dorsal closing fold is formed on either side
of the esophagus, and the two dorsal parietal recesses are obliterated, separat-
ing completely the pericardial cavity from the peritoneal cavity (fig. 362D).
However, a small pericardioperitoneal opening may be left below the esophagus
in the shark.
The secondary septum transversum thus formed is a thickened transverse
partition, composed of two walls, an anterior pericardial wall and a posterior
peritoneal wall, with a loose tissue layer between these two coelomic mem-
branes. The liver is suspended from the peritoneal or caudal aspect of the
septum transversum in the region of the coronary ligament, while the posterior
end of the sinus venosus is apposed against the anterior or pericardial face of
the transverse septum. The common cardinal and other converging veins of
the heart utilize the substance of the septum transversum as a support on their
way to the sinus venosus. The hepatic veins (the right and left, embryonic
vitelline veins) pass through the coronary ligament on their journey to the
sinus venosus.
2. In Amphibians, Reptiles, and Birds
The conversion of the primary septum transversum in amphibians, reptiles,
and birds into the secondary or adult septum transversum occurs essentially
as described above. A dorsal closing fold, obliterating the dorsal parietal
recess on either side of the gut, is developed, although, in reptiles and birds,
the inward growth and contribution of somatopleuric tissue overlying the
common cardinal ridge is more important than in fishes in effecting this closure.
However, one must keep in mind an important fact, namely, that, in am-
phibia, reptiles and birds, there is an extensive caudal migration of the heart,
septum transversum, and liver complex from their original cephalic position
just posterior to the pharyngeal area. This caudal migration produces a con-
dition in which the primary septum transversum and the dorsal membranes,
868 THE DEVELOPMENT OF THE COELOMIC CAVITIES
formed by the dorsal closing folds, are inclined to a great degree, with the
ventral end of the primary septum transversum considerably more posterior
in position than the dorsal edge of the dorsal membranes. Consequently, a
secondary recess or pocket is formed on either side anterior and dorsal to
the septum transversum. This secondary recess occurs on either side of the
gut, and, into each of these recesses, a lung extends in many reptiles and in
those amphibia which possess lungs. In this pocket also lie certain of the air
sacs of birds. Thus, the general cavity back of the pericardioperitoneal mem-
brane or secondary septum transversum (i.e., the primary septum transversum
plus the two dorsal membranes, formed by the dorsal closing folds) is known
as the pleuroperitoneal cavity in amphibia and many reptiles. In birds (see
below), the respiratory part of the lung becomes enclosed dorsally near the
vertebrae within a separate pleural cavity, separated from the peritoneal cavity
by the dorsal diaphragm (fig. 362J). The thin air sacs of the bird's lung
(Chap. 14) project from the lung through the dorsal diaphragm into the
peritoneal cavity and also into certain of the bones. In the turtle group, among
the reptiles, a dorsal diaphragm is developed below each lung, segregating
the lungs partly within dorsal cavities, thus simulating the bird condition.
D. Formation of the Coelomic Cavities in Mammals
In the mammalia, a pronounced caudal migration of the heart, liver, and
developing diaphragm occurs. Also, as in birds, a further morphogenetic fea-
ture is present which results in the development of a pleural cavity for each
lung in addition to the peritoneal and pericardial cavities present in fishes,
amphibians, and reptiles. Thus it is that the development of two partitioning
membranes on either side of the gut tube, the pleuropericardial membranes,
which correspond to the dorsal closing membranes mentioned above, together
with two additional membranes, the pleuroperitoneal membranes, are neces-
sary to effect the division of the primitive splanchnocoelic coelom into the
four main coelomic cavities in the Mammalia.
1. Formation of the Pleuropericardial Membrane
It so happens that the anterior cardinal vein develops slightly in advance
of the posterior cardinal vein. As a result the common cardinal vein, which
develops from the caudal end of the primitive anterior cardinal vein, travels
along the lateral body wall in an inclined plane to reach the area of the lateral
mesocardium and sinus venosus of the heart. This inclined pathway of the
common cardinal vein is characteristic of the vertebrate embryo. As the
common cardinal vein increases in size, a lateral ridge or elongated bulge
is formed along the lateral body wall. This ridge projects inward into the
coelomic cavity and inclines caudo-ventrally to reach the dorsal edge of the
area of the primitive septum transversum (fig. 363D).
In the mammals, the mesonephric folds (ridges), in which the mesonephric
FORMATION OF COELOMIC CAVITIES IN MAMMALS 869
kidneys develop, are large and project downward into the coelomic cavity.
The anterior ends of the mesonephric ridges continue along the lateral body
wall on either side and follow an inclined plane antero-ventrally to the dorsal
edge of the primitive septum transversum (fig. 363D). Two lateral body folds
or ridges, which incline toward and fuse with the dorsal edge of the primitive
septum transversum, are produced in this manner on either side. These folds
are an anterior lateral body fold or ridge, overlying the common cardinal
vein, and a posterior lateral body fold, which represents the antero-ventral
continuation of the mesonephric ridge as it inclines ventrally to join the lateral
edge of the primitive septum transversum (fig. 363D). A V-shaped pocket is
formed between these two ridges. This pocket represents the primitive pleural
cavity or pocket. The apex of this V-shaped pocket unites with the primitive
septum transversum. As the lung buds grow out posteriorly below the foregut,
each projects into a pleural pocket (fig. 363F).
The formation of the pleuropericardial membrane is effected by an ingrowth
of tissue along the edge of the anterior, lateral body fold, the fold that overlies
the common cardinal vein. This ingrowing tissue forms a secondary ridge,
known as the pulmonary ridge, which continues to grow mesad below the
developing lung until it reaches the splanchnopleure of the esophagus with
which it fuses. A pleuropericardial membrane, in this way, is established which
separates the pericardial cavity below from the pleural cavity above (fig.
363E-G). The pleuropericardial membranes probably are homologous with
the dorsal closing folds of the secondary septum transversum of the vertebrates
below the mammals.
2. Development of the Pleuroperitoneal Membrane
As mentioned previously, the cephalic end of the mesonephric ridge projects
forward and ventrad along the lateral body wall to unite with the primitive
septum transversum to form the posterior, lateral body fold. The medial
growth of this posterior, lateral body fold and ultimate fusion with a small
splanchnopleural outgrowth, the splanchnopleural fold, forms a second par-
titioning membrane, the pleuroperitoneal membrane, which separates the
pleural cavity from the general peritoneal cavity (fig. 363E-H). Contributions
of the somatic mesoderm to the lateral body-fold tissue are significant in the
formation of the pleuroperitoneal membrane. It is to be noted that the primi-
tive pleural cavities of the mammalian embryo are small and dorsally placed,
one on either side of the gut and dorsal to the pericardial cavity. Their later
expansion is described below. To summarize the partitioning process of the
primitive coelom in mammals, we find that the following membranes are
formed:
( 1 ) the primary septum transversum,
(2) the two dorsal closing folds or pleuropericardial membranes, and
(3) two pleuroperitoneal membranes.
^os^^Wi
Fig 364 (A) Transverse section of the thoracic area of opossum embryo showmg
the separation of the parietal pericardium from the lateral body walls by expanding
pleural sacs. (The preparation from which this drawing was made was loaned to the
author by Dr. J. A. McClain.) (B-1 ) Transverse section through lung buds and pleural
870
MAMMALIAN DIAPHRAGM 871
E. Development of Independent Pericardial Walls
1. The Arrangement of the Parietal Pericardial Wall in Fishes
The parietal pericardium of the fish embryo is fused with the lateral body
wall. The caudal area of the sinus venosus is associated intimately with the
anterior wall of the septum transversum. This condition is a primary one in
all vertebrate embryos. It is retained in the adult fish.
2. Formation of an Independent Parietal Pericardial Wall
in the Chick
In the chick, two main processes occur in development which separate the
septum transversum from the liver, and also the parietal pericardial mem-
brane from the lateral body walls. These processes are:
(a) The peritoneal cavity on either side of the liver grows forward and
separates the cardiac or anterior face of the liver from the posterior
face of the septum transversum, with the exception of the area where
the veins from the hepatic region perforate the septum. This process
frees the septum transversum from the liver surface and permits it to
function as a part of the pericardial sac as indicated in figure 294G-4;
G-5.
(b) The extending peritoneal coelom not only separates the liver from the
posterior face of the septum transversum, but it continues anteriad
followed by the liver lobes along the ventral and lateral aspects of
the body wall and splits the membranous pericardium away from the
lateral body wall. Ventrally, a median septum unites the pericardium
with the body wall (fig. 362J).
3. Formation of the Independent Parietal Pericardial Wall
IN Amphibians and Reptiles
A somewhat similar process to that described for the chick obtains in rep-
tiles and, to a modified extent, in amphibia.
Fig. 364 — Continued
cavities of a 10 mm. pig embryo showing position of the primitive mediastinum. (B-2)
Later mediastinal area development portraying adult position (black area) of the medi-
astinum. (Based on the cat.) Observe that fig. 364 (A) is an intermediate condition
between figs. 364 (B-1) and 364 (B-2). (C) Probable origin of parts of the mam-
malian diaphragm. (D) The caudal migration of the septum transversum and devel-
oping diaphragm during development. 2-position = embryo of 2 mm.; 24-position =
24 mm. embryo. (Redrawn from F. P. Mall. 1910, Chap. 13, Vol. 1, Manual of Human
Embryology, Lippincott. Philadelphia.) (E-H) Development of the mesenteries and
omental bursa or lesser peritoneal cavity in the human. The cross-lined areas in H
show areas of the mesentery which fuses with the body wall. The arrows in F-H denote
development of the lesser peritoneal cavity.
872 THE DEVELOPMENT OF THE COELOMIC CAVITIES
4. Separation of the Parietal Pericardial Wall in Mammals
On the other hand, in the mammals, it is the pleural cavities, i.e., the pleural
divisions of the splanchnocoelic coelom, which extend ventrally around the
heart and thus separate the parietal pericardium from the thoracic body wall
(fig. 364A and B). Posteriorly, they separate the pericardium from the anterior
face of the developing diaphragm (fig. 363B). The secondary condition of
the mediastinum thus is established which extends dorsoventrally between the
two pleural sacs (fig. 364B-2). It is to be observed that the medial walls of
the pleural sacs fuse with the lateral walls of the pericardium by means of
the connective tissue which forms between these two layers.
F. The Mammalian Diaphragm
The mammalian diaphragm is a musculotendinous structure, innervated by
the phrenic nerve and developed from tissues around the gut, primary septum
transversum, the two pleuroperitoneal membranes, and possibly also by con-
tributions from the body wall. Study figure 364C. The exact origin of the
voluntary musculature of the diaphragm is in doubt, but it is assumed to
come from the cervical myotomes in the region of origin of the phrenic nerve,
together with some invasion of muscle substance from the lateral body wall
posterior to the cervical area. Successive caudal positions of the septum trans-
versum and developing diaphragm, assumed during its recession in the body,
are shown in figure 364D.
G. The Pulmonary Diaphragm or Aponeurosis of the Chick
The pulmonary diaphragm in the chick is a composite structure formed
of two membranes which develop in a horizontal position in the dorsal region
of the thoracic area below the lungs. Each of these two membranes fuses
with the median mesentery and the lateral body wall and thus forms a par-
tition separating the pleural cavities above from the peritoneal cavity below
(fig. 362J). The development of this partitioning membrane is as follows:
In the four- to five-day chick as the lung buds grow out dorso-posteriad
each lung bud pushes into a mass of mesenchyme which is continuous from
the splanchnopleure around the esophagus to the dorsal region of the liver.
This connecting bridge of mesenchyme is the pleuro-peritoneal membrane
and it extends from the region of the esophagus across the lower part of the
lung bud tissue to the liver lobe on each side. The mesenchymal connection
of this membrane with the liver then spreads laterally to unite with the lateral
body wall. As a result, the pleural cavity above is shut off from the peritoneal
cavity below. A continual growth dorsoposteriad of the pleuro-peritoneal
membrane, and subsequent fusion with the dorsal body wall tissues, separates
the pleural cavity completely from the peritoneal cavity. However, certain
canals remain in this membrane for the passage of the air sacs (see Chapter
14) of the lungs. Striated musculature from the lateral body wall grows into
PULMONARY DIAPHRAGM OF CHICK 873
the pleuro-peritoneal membrane on either side and converts it into a muscular
structure. These two muscular partitions thus form the pulmonary diaphragm.
H. The Omental Bursa
In all gnathostomous vertebrates, the mesogastrium is prone to form a
primitive pocket, associated with the rotation of the stomach to the right.
This pocket is quite prevalent in most gnathostomous embryos from the elas-
mobranch fishes to the mammals and is known as the primitive omental bursa.
In mammals, the omental bursa is highly developed, and it gives rise to the
lesser peritoneal cavity, retaining its connection with the greater peritoneal
cavity by means of the foramen of Winslow. The lesser peritoneal cavity in
the cat is extensive, filling the entire inside of the omental sac. In the human,
however, the distal part of the lesser peritoneal cavity is reduced by the fusion
of the omental layers. Though a rudimentary omental bursa is formed in the
early embryonic condition of elasmobranch fishes (sharks), it soon disap-
pears, so that, in the adult fish, the omental bursa is nonexistent. Figure
364E-H presents various stages in the development of the omental bursa in
the human embryo.
I. The Formation of Various Ligaments in the Stomach-Liver Region
Ligaments are those specializations of the peritoneal tissue which unite
various organs with each other or with the body wall.
1. The Gastro-hepatic and Hepato-duodenal Ligaments. These structures
are derivatives of the ventral mesentery between the stomach-duodenal area
and the liver. The gastro-hepatic ligament ties the stomach and liver together
while the hepato-duodenal ligament unites the duodenum with the liver.
2. The Coronary Ligament of the Liver. This is the tissue which unites the
liver with the caudal face of the septum transversum and in mammals with the
later developed diaphragm. Its development is described on page 866.
3. The Falciform Ligament of the Liver. This unites the liver in the median
plane to the ventral body wall and to the septum transversum or diaphragm.
4. The Gastro-splenic Ligament suspends the spleen from the stomach and
it represents a modification of the mesogastrium (see Chapter 17).
(Note: Ligamentous structures associated with the reproductive organs are
described in Chapter 18.)
Bibliography
Goodrich, E. S. 1930. Chap. XII in Studies on the Structure and Development of Verte-
brates. Macmillan and Co., London.
Mall, F. P. 1910. Chap. 13, Vol. I, Manual of Human Embryology. Lippincott,
Philadelphia.
21
Tne Developing Endocrine Glands and Tneir Possime
Relation to Definitive Body Formation and tne
Dirrerentiation or Sex
A. Introduction
B. Morphological features and embryological origin of the endocrine glands
1. Pancreas
2. Pituitary gland (hypophysis cerebri)
a. Anterior lobe
b. Posterior lobe
c. Pars intermedia
3. Thyroid gland
4. Parathyroid glands
5. Thymus gland
6. Pineal body
7. Adrenal (suprarenal) glands
8. Gonads
C. Possible influence of endocrine secretions on the development of definitive body form
1. Thyroid and pituitary glands and anuran metamorphosis
2. Thyroid and pituitary glands in relation to the development of other vertebrate
embryos
a. Chick
1) Thyroid gland
2) Pituitary gland
b. Mammal
1) Thyroid gland
2) Pituitary gland
c. Fishes
3. General conclusions relative to the influence of the thyroid and pituitary glands
in vertebrate embryology
D. Possible correlation of the endocrine glands with sex differentiation
1. Differentiation of sex
a. General sex features in the animal kingdom
b. Chromosomal, sex-determining mechanisms
c. Possible influence of the sex field in sex determination
2. Influence of hormones on the differentiation of sex
3. General summary of the factors involved in sex differentiation in the vertebrate
group
874
INTRODUCTION 875
A. Introduction
The endocrine glands are those glands which produce hormonal secretions.
The term hormone is derived from a Greek word meaning to stimulate or to
stir up. Selye in 1948 (p. 11) defined hormones as "physiologic, organic com-
pounds produced by certain cells for the sole purpose of directing the activities
of distant parts of the same organism."
The endocrine organs may be separated into two main groups:
( 1 ) purely endocrine glands, and
(2) mixed endo-exocrine glands.
Purely endocrine glands have as their sole function the production of hor-
mones. Under this heading are included the pituitary (hypophysis), thyroid,
parathyroid, pineal, adrenal (suprarenal), and thymus glands.
Mixed endo-exocrine glands are exemplified by the pancreas, liver, duo-
denum, and reproductive organs. Parts of these organs are purely exocrine,
e.g., the pancreas where pancreatic juice is produced by the acinous cells
but which elaborates, at the same time, insulin from the islets of Langerhans.
The liver elaborates the exocrine secretion, bile, which is discharged through
the bile ducts and, concurrently, manufactures the antipernicious-anemia factor
which is dispensed into the blood stream directly. The duodenum produces
digestive substances and also secretin. Secretin is elaborated by the epithelial
lining cells of this area, and it stimulates the pancreas to secrete its pancreatic
juice.
Relative to their secretory activities all endocrine glands have this physio-
morphological feature in common: They discharge the hormonal or endocrine
substance directly into the blood stream without the mediation of a duct
system. Endocrine glands, therefore, are distinguished by this process from
exocrine glands, which exude the secretory product into a duct system from
whence the secretion passes to the site of activity.
B. Morphological Features and Embryological Origin of the
Endocrine Glands
1. Pancreas
The islets of Langerhans are small masses of cells or islands scattered
among the acini (alveoli) of the general pancreatic tissue. The pancreatic
islets appear to arise as specialized buds from the same entodermal cords
which give origin to the alveoli. The islets separate early from the entodermal
cords and produce isolated cellular cords. Blood capillaries form a meshwork
within these cords of cells (figs. 295G; 365A). Their secretion, insulin, is
concerned with sugar metabolism and prevents the malfunction known as
diabetes.
Pancreatic islets are found extensively in the vertebrates and generally are
876
THE DEVELOPING ENDOCRINE GLANDS
STOMOOAEUM
MANDIBULAR PROCESS
Fig. 365. The pancreatic islets and pituitary gland. (A) Origin of islet tissue from
developing pancreatic ducts and acini. 1 = young bud; 5 = older bud. (Modified from
Arey, '46, Developmental Anatomy, Philadelphia, Saunders.) (B-E) Diagrams of pi-
tuitary gland conditions in Petromyzon (B), Runa (C), Reptile (D), and Man(E). (Modi-
fied from Neal and Rand, 1939, Chordate Anatomy, Philadelphia, Blakiston.) (F)
Origin of Rathke's pouch material from inner layer of epidermal ectoderm in early
tadpole of Rana. (G-I) Developmental stages of hypophysis in human embryo.
associated with the pancreas. In some teleost fishes, the two glands are sep-
arated although both are derived from the entoderm. The pancreatic islets are
classified as belonging to the solid, non-storage type of endocrine gland.
2. Pituitary Gland (Hypophysis Cerebri)
Previous to the latter part of the last century, the function of the pituitary
gland was presumed to be one of mucous secretion, hence the name pituitary
from the Latin, pituita, a nasal secretion. It was so regarded by Vesalius in
1543. The English anatomist, Willis, believed that the pituitary gland secreted
the cerebrospinal fluid.
The pituitary gland (fig. 365E and I) is composed of three main parts as
follows:
a. Anterior Lobe
The anterior lobe (pars anterior) is composed of two subdivisions:
( 1 ) a large anterior lobe (pars distalis), and
(2) a smaller glandular mass (pars tuberalis).
ORIGIN OF THE ENDOCRINE GLANDS 877
b. Posterior Lobe
The posterior lobe (lobus nervosus, pars neuralis) is derived from the distal
part of the infundibulum.
c. Pars Intermedia
The pars intermedia or intermediate lobe is associated closely with the
posterior lobe but has the same embryonic origin as the pars distalis and pars
tuberalis of the anterior lobe.
In Petromyzon fluviatilis, the hypophysis is a flat, tube -like organ attached
to the infundibular evagination of the floor of the diencephalon. The anterior
lobe is represented by the hypophyseal duct which ends blindly below the
infundibulum. From this duct are proliferated the cells of the intermediate
lobe (fig. 365B). The pituitary gland shows great similarity, in all higher
vertebrates, being composed of three main parts, viz., pars anterior, pars
intermedia, and pars posterior (fig. 365C-E). However, in the chicken, whale,
manatee, and armadillo, the intermediate lobe is missing (Selye, '48).
The pars anterior and the pars intermedia of the pituitary gland develop
from Rathke's pouch as evaginations of the middorsal area of the stomodaeal
pocket, although in the frog Rathke's pouch develops precociously from the
so-called neural ectoderm above the stomodaeal invagination (fig. 365F-I).
Rathke's pouch gradually comes into contact with the ventrally directed in-
fundibular evagination from the diencephalon. The distal part of the infundib-
ular evagination forms the pars neuralis, while Rathke's pouch differentiates
into the pars distalis, pars intermedia, and pars tuberalis.
3. Thyroid Gland
The thyroid gland (fig. 366B) was described first in 1656 by Thomas
Wharton, the English anatomist, who called it the thyroid gland because of
its association with the thyroid or shield-shaped cartilage of the larynx.
After about 50 years of work by many observers on the thyroid gland and
its activities, the crystalline form of the secretory principle of the thyroid
gland was isolated by Kendall in 1919, and he called it thyroxine. This com-
pound contained 65 per cent of iodine by weight and its empirical formula
was subsequently determined as Ci-,HnO,Nl4.
One of the thyroid's functions is to govern carbohydrate metabolism, and,
in general, the gland .controls the basal metabolism of the animal together
with growth processes. In man and the cat, the thyroid gland is in the form
of two lateral lobes, located on the ventro-lateral aspect of the thyroid cartilage
of the larynx, the two lobes being joined by an isthmus. In birds, there are
two glands, both being located within the thoracic cavity; in fishes, including
the Cyclostomes, the thyroid is an unpaired structure and is to be found
generally between and near the posterior ends of the lower jaws. The gland,
therefore, is a constant feature of all vertebrates.
878
THE DEVELOPING ENDOCRINE GLANDS
PARATHYROID HI
PARATHYROIO IST
PARATHYROID
nr
PARATHYROID
ECOND RIB
MUS GLAND
B.
^M^^:^^f^0f^^^
'%
SECRETORY
EPITHELIUM
0 t«..
"»"si&''.
^^mHi\Ja?S^~\!S^^ THYROID FOLLICLE ''Sff^/i^-yf''*^ . ^ fi^h ' a^Sfl' .'4^^%'
Fig. 366. Thyroid, parathyroid, and thymus glands in human embryo. (A) The
loci of origin of thyroid, parathyroid, thymus, and ultimobranchial bodies. (B) Late
stage (somewhat abnormal) of thyroid, parathyroid, and thymus gland development in
human. (C) Early stage of thyroid follicle differentiation. (D) Later stage of thyroid
follicle differentiation.
In the embryos of all vertebrates the thyroid gland appears as a pharyngeal
derivative. In the human as in fishes and amphibia (Lynn and Wachowski,
'51), it arises as a midventral outpocketing of the anterior pharyngeal floor.
In the human embryo, this outpocketing occurs between the first and second
branchial pouches at about the end of the fourth week of development (fig,
366A). Its point of origin is observable during later development as a small
indentation, the foramen caecum, in the region between the root and body
of the tongue (fig. 285). It is a bilobed evagination which soon loses its con-
nection with the pharyngeal floor and migrates caudally to the laryngeal area
where it differentiates into a double-lobed structure, connected by a narrow
bridge of thyroid tissue, the isthmus. Occasionally, a persistent thyroglossal
duct, connecting the foramen caecum with the thyroid gland, remains (fig.
366B). While the thyroid rudiment migrates posteriad, the post-branchial
(ultimobranchial) bodies, which take their origin from the caudal margin of
the fourth branchial pouch, become incorporated within the thyroid tissue.
ORIGIN OF THE ENDOCRINE GLANDS 879
The significance of this incorporation is unknown, and evidence of functional
thyroid tissue, being derived from the post-branchial body cells, is lacking.
When the cellular masses of the developing thyroid gland reach the site of
the future thyroid gland, the cells multiply and break up into cellular strands,
surrounded by mesenchyme and blood vessels (fig. 366C). These strands in
turn break up into small, rounded, bud-like masses of epithelial cells, the young
thyroid follicles (fig. 366D). During the third month of development in the
human, colloidal substance begins to appear within the young thyroid follicles.
The colloid increases during the fourth month, and the surrounding cells of
the follicle appear as a single layer of low columnar cells. Each thyroid follicle
as a whole assumes the typical appearance of a functioning structure. Blood
capillaries ramify profusely between the respective follicles.
The colloidal substance within each thyroid follicle presumably represents
stored thyroid secretion, and the thyroid gland is regarded, therefore, as a
"storage type" of endocrine gland. The theory relative to thyroid gland func-
tion is set forth that the follicle cells may secrete directly into the capillaries
and, hence, into the blood stream, or the secretion may be stored as colloid
within the follicles. Later this reserve secretion in the form of colloid may
be resorbed by the cells in times of extreme activity and passed on into the
region of the capillaries. In certain instances, e.g., dog and rat, individual
thyroid follicles may be lined with stratified squamous epithelium (Selye, '48,
p. 695).
In the larvae of the cyclostome, Petromyzon, the so-called endostyle is lined
with rows of mucus-secreting cells, alternating with ciliated cells. This endo-
stylar organ becomes transformed into the thyroid gland upon metamorphosis.
A localization of iodine in certain of the endostylar cells in the larva has been
demonstrated (Lynn and Wachowski, '51, p. 146).
4. Parathyroid Glands
The parathyroid glands in man are four, small, rounded bodies, located
along the dorsal (posterior) median edges of the two thyroid lobes of the
thyroid gland (fig. 366B). Unlike the storage type of endocrine gland, such
as the thyroid gland with its follicles, the parathyroids contain no follicles
and, therefore, represent the solid type of endocrine gland. Blood capillaries
ramify through its substance which is composed of closely packed masses of
polyhedral epithelial cells, arranged in small cords or in irregular clumps.
Two main cell types are present in mammals, the chief or principal cells with
a clear cytoplasm and the oxyphil cells whose granules stain readily with acid
stains. The chief cells are common to all vertebrate parathyroids and thus
may represent the essential cellular type of the parathyroid gland (Selye, '48,
p. 540).
The removal of the parathyroid glands results in a reduction of the calcium
content of the blood, muscular tetany, convulsions, and ultimate death. The
880 THE DEVELOPING ENDOCRINE GLANDS
parathyroid glands in some way regulate calcium metabolism to keep the
calcium content in the blood stream at its proper level.
Parathyroid structures may be present in fish (Selye, '48), but it is gen-
erally believed that true parathyroid tissue is confined to the Tetrapoda. Two
parathyroid glands on each side are found in most urodeles and other am-
phibia, and in reptiles. The birds have relatively large parathyroid glands,
attached to the two thyroid glands located in the thoracic cavity. All mammals
possess parathyroid glands which, in some instances, are located internally
within the thyroid gland as well as externally. Accessory parathyroid glands,
apart from the two parathyroids attached to the thyroid gland, are found in
rats and mice and, consequently, may not be disturbed if the thyroid gland
is removed in these rodents.
The parathyroid glands arise in the human embryo from proliferations of
the dorso-lateral walls of the third and fourth branchial pouches (fig. 366A).
The parathyroids which arise from the third pair of pouches are known as
parathyroids III, while those from the fourth pair of branchial pouches are
called parathyroids IV. Parathyroids III arise in close proximity to the thymus-
gland rudiments (fig. 366A). However, it is to be observed that the thymus
rudiments arise from the ventral aspect of the third pair of pouches. The
parathyroid-III rudiments move caudally with the thymus gland rudiments
and come to lie in relation to the lateral lobes of the thyroid, posterior to
parathyroids IV which take their origin in close relation to the post-branchial
(ultimobranchial) bodies (fig. 366A and B).
Parathyroids IV appear to be a constant feature of all Tetrapoda. In those
species having but two parathyroids, it is probable that their origin is from
the fourth branchial pouches.
5. Thymus Gland
The thymus gland or "throat sweetbread" (the pancreas is referred to
commonly as the "stomach sweetbread") lies in the anterior portion of the
thoracic cavity and posterior neck region (fig. 366B). In some cases, it may
extend well along in the neck region toward the thyroid gland. In the thoracic
area, it lies between the two pleural sacs, that is, within the mediastinum,
and reaches as far caudally as the heart. Histologically, it is composed of
two parts:
( 1 ) a cortex and
(2) a medulla.
The cortex contains masses of thymocytes or lymphocyte-like cells, while
the medulla contains thymocytes, reticular cells, and the so-called Hassall's
corpuscles, composed of stratified, squamous, epithelial cells.
In man, the thymus gland arises from the ventral portion of the third
ORIGIN OF THE ENDOCRINE GLANDS 881
branchial pouches during the sixth week. These epithelial derivatives of the
third branchial pouch become solid masses of cells which migrate posteriad
into the anterior thoracic area.
The thymus gland is found in all vertebrates, but its morphology is most
variable. In birds, it is situated in the neck region in the form of isolated,
irregular nodules. The bursa of Fabricius, previously mentioned (Chap. 13)
as an evagination in the cloacal-proctodaeal region of the chick, is a "thymus-
like organ" (Selye, '48, p. 681 ). Thymus glands in reptiles are located in the
neck region, and, in amphibians the two thymus glands lie near the angle of the
jaws. In fishes several small, thymus-gland nodules arise from the dorsal
portions of the gill pouches and come to lie dorsal to the gill slits in the adult.
The function of the thymus gland is not clear. It appears to have some
relationship to sexual maturity. (For thorough discussion, see Selye, '48,
Chap. IX.)
6. Pineal Body
The pineal gland appears to have been first described by Galen, the Greek
scientist and physician (130-ca.200 A.D.), who believed it to function in
relation to the art of thinking. Descartes (1596-1650) considered it to be
the "seat of the soul."
During development, two fingerlike outgrowths of the thin roof of the
diencephalon of the brain occur in many vertebrates, namely, an anterior
paraphysis or parietal organ, and a more posteriorly situated epiphysis. In
certain Cyclostomes (Petromyzon), the posterior pineal body or epiphysis
is associated with the formation of a dorsal or pineal eye, while the anterior
pineal organ or paraphysis forms a rudimentary eyelike structure. In Spheno-
don and in certain other lizards, the paraphysis or anterior pineal evagination
develops an eyelike organ. Also, in various Amphibia (frogs; Ambystoma)
rudimentary optic structures arise from the fused epiphyseal and paraphyseal
diverticula. In consequence, we may assume that a primary function in some
vertebrates of the dorsal, median pineal organs is to produce a dorsal, light-
perceiving organ. In certain extinct vertebrates, a fully developed median
dorsal eye appears to have been formed in this area.
On the other hand, the epiphysis (fig. 366A) in some reptiles, in birds
and in mammals has been interpreted as a glandular organ. Various investi-
gators have suggested different metabolic functions. However, an endocrine
or essential secretory function remains to be demonstrated. (Consult Selye,
'48, p. 595.)
Many types of cells enter into the structure of the pineal gland. Among
these are the chief cells, which are large and possess a clear cytoplasm. Nerve
cells and neuroghal elements also are present. Various other cell types pos-
sessing granules of various kinds in the cytoplasm are recognized.
882 THE DEVELOPING ENDOCRINE GLANDS
7. Adrenal (Suprarenal) Glands
The adrenal bodies are associated, as the name impUes, with the renal
organs or kidneys. In fishes, definite adrenal bodies are not present, but
cellular aggregates, corresponding to the adrenal cells of higher vertebrates,
are present and associated with the major blood vessels.
In man and other mammals, the adrenal body is composed of:
( 1 ) an outer, yellow-colored cortex and
(2) an inner medullary area.
The medulla contains the chroniaffin cells — cells which have a pronounced
aflfinity for chromium salts, such as potassium dichromate, which stain them
reddish brown and produce the so-called "chroniaffin reaction."
The hormone, secreted by the medulla, is adrenaline (epinephrine). It has
marked metabolic and vasoconstrictor effects. The smooth muscle tissue of
the arrecfor pili muscles associated with the hairs in mammals contract and
raise the hair as a result of adrenaline stimulation.
The morbid state, known as Addison's disease and named after the English
physician, Thomas Addison, who first described this fatal illness, arises from
decreased function of the adrenal cortex. Various types of hormones have
been discovered which arise from the cortical layer of the adrenal body, and
a large number of steroid substances have been isolated from this area of the
adrenal gland (Selye, '48, p. 89). In fishes, the cortical cell groups are isolated
from those of the medulla, and, in the elasmobranch fishes, the cortex forms
a separate organ. Its removal may be eflfected without injury to the medulla
but with resulting debility, ending in death.
Embryologically, the adrenal cortex and medulla take their origin from
two distinct sources. The cortex arises as a proliferation of the dorsal root of
the dorsal mesentery in the area near the anterior portion of the mesonephric
kidney and liver on either side (fig. 3 67 A, B). These two proliferations give
origin to two cortical masses, each lying along the anterior mesial edge of
the mesonephric kidney. Further growth of these masses produces two rounded
bodies, the adrenals (suprarenals), lying between the anterior portions of
the mesonephric kidneys (figs. 3A and B; 367B) and later in relation to the
antero-mesial portion of the metanephric kidneys (fig. 3B-E). After the cor-
tical masses are established, the chroniaffin cells invade them from the medial
side (fig. 367C), The potential chromaffin cells migrate from the sympathetic
ganglia in this area. Upon reaching the site of the developing adrenal gland
they move inward between the cortical cells to the center of the gland where
they give origin to the medulla. With the diverse embryological origins of
the cortex and the medulla, it is seen readily why two separate glandular
structures are present in lower vertebrates.
In man and other mammals, a later developed secondary cortex is laid
down around the primary cortex. The primary cortex, characteristic of fetal
DEVELOPMENT OF DEFINITIVE BODY FORM
883
CHROMAFFIN TISSUE
FORMING MEDULLA
OF ADRENAL GLAND
Fig. 367. Differentiation of the adrenal (suprarenal) body. (A) Early stage in prolif-
eration of adrenal cortical primordium from coelomic epithelium. (B) Later stage of
cortex, forming rounded masses associated with cephalic ends of mesonephros. The
anterior end of the mesonephros lies between the adrenal body and lateral wall of the
coelom. (Compare fig. 3H and B.) (C) Cells from sympathetic ganglia penetrating
medial side of primitive cortical tissue of adrenal body to form chromaffin cells of
adrenal medulla.
life, then comes to form the "inner cortical zone" or androgenic zone
(Howard, '39).
8. Gonads
The developing gonads were described in Chapter 18, and their hormonal
functions were outlined in Chapters 1 and 2.
C. Possible Influence of Endocrine Secretions on the Development of
Definitive Body Form
1. Thyroid and Pituitary Glands and Anuran Metamorphosis
One of the earlier studies in this field of development was that by Guder-
natsch ('12 and '14) which showed that mammalian thyroid gland fed to
anuran, and urodele larvae stimulated growth, differentiation, and metamor-
phosis. In a later series of studies by Allen (see Allen, '25, for references
and review) and by Hoskins ('18 and '19), it was demonstrated that the
removal of the thyroid gland in young tadpoles of Rana and Bujo prevents
metamorphosis from the larval form into that of definitive body form (i.e.,
884 THE DEVELOPING ENDOCRINE GLANDS
the adult body form). Similar results were obtained as a result of hypophy-
sectomy (i.e., removal of the hypophysis). (See Allen, '29, and Smith, '16
and '20.) The work of these observers clearly demonstrates that the thyroid
and pituitary glands are instrumental in effecting the radical transformations
necessary in the assumption of definitive body form in the Anura.
2. Thyroid and Pituitary Glands in Relation to the
Development of Other Vertebrate Embryos
a. Chick
1) Thyroid Gland. Studies relative to the possible effect of the thyroid
gland upon the developing chick embryo are complicated by the fact that
the yolk of the chick egg is composed of many other factors besides fats, pro-
teins, and carbohydrates. The yolk is a veritable storehouse for vitamins and
for thyroid, sex, and possibly other hormones. Just what effect these sub-
stances have upon development is problematical. Some experiments, however,
have been suggestive. Wheeler and Hoffman ('48, a and b), for example,
produced goitrous chicks and retarded the hatching time of chicks from eggs
laid by hens which were fed thyroprotein. Thyroprotein feeding seemingly
reduced the amount of thyroid hormone deposited in the egg with subsequent
deleterious effects upon the developing chicks. In normal development, the
thyroid gland of the chick starts to develop during the third day and produces
follicles which contain colloid by the tenth and eleventh days of incubation.
Furthermore, Hopkins ('35) showed that thyroids from chick embryos of 10
days of incubation hastened metamorphosis in frog larvae. From days 8 to
14 the chick embryo undergoes the general changes which transform it from
the larval form which is present during incubation days 6 to 8 into the
definitive body form present at the beginning of the third week of incubation.
The foregoing evidence, therefore, while it does not demonstrate that thyroid
secretion actually is being released by the developing thyroid gland into the
chick's blood stream, does suggest that the thyroid gland may be a factor
in chick development and differentiation. If the chick's thyroid gland is se-
creting the thyroid hormone into the chick's blood stream during the second
week of the incubation period, it is evident that the developing chick during
the period when it is assuming the definitive body form has two sources of
thyroid hormone to draw upon:
(1) that contained within the yolk of the egg and
(2) that produced by its own thyroid gland.
2) Pituitary Gland. Relative to the development of the pituitary gland in
the chick, Rahn ('39) showed that the anterior lobe develops both acidophilic
and basophilic cells by the tenth day of incubation. Also, Chen, Oldham, and
Ceiling ('40) demonstrated that the pituitary of chicks from eggs incubated
DEVELOPMENT OF DEFINITIVE BODY FORM 885
for five days possessed a melanophore-expanding principle when administered
to hypophysectomized frogs.
This general evidence, relative to the developing pituitary gland in the
chick, suggests that the cells of the pituitary gland may be active functionally
during the latter part of the first week and during the second week of incu-
bation. If so, the pituitary gland may be a factor in inducing the rapid growth
and changes which occur during the second week of incubation. It suggests
further, that a possible release of a thyrotrophic principle may be responsible
for the presence of colloid within the developing thyroid follicles during the
second week of incubation.
b. Mammal
As in the chick, the developing embryo of the placental mammal is in
contact with hormones from extraneous sources. Hormones are present in
the amniotic fluid, while the placenta is the seat of origin of certain sex and
gonadotrophic hormones. Also, the maternal blood stream, which comes in
contact with embryonic placental tissues, is supplied with pituitary, thyroid,
adrenal, and other hormonal substances. This general hormonal environment
of the developing mammalian embryo complicates the problem of drawing
actual conclusions relative to the effect of the embryo's developing endocrine
system upon the differentiation of its own organ systems and growth. Never-
theless, there is circumstantial evidence, relating to possible activities of the
developing, embryonic, endocrine glands upon development.
1) Thyroid Gland. Colloid storage within the follicles of the developing,
human, thyroid gland is evident at 3 to 4 months. In the pig embryo, Rankin
('41 ) detected thyroxine and other iodine-containing substances in the thyroid
at the 90-mm. stage, and Hall and Kaan ('42) were able to induce meta-
morphic effects in amphibian larvae from thyroids obtained from the fetal
rat at 18 days. The foregoing studies suggest that the thyroid gland is able
to function in the fetal mammal at an early stage of development. (For further
references, consult Moore, '50.)
2) Pituitary Gland. Similarly, in the pituitary gland, granulations within
the cells of the anterior lobe are present in the human embryo during the
third and fourth months (Cooper, '25). Comparable conditions are found in
the pituitary of the pig from 50 to 170 mm. in length (Rumph and Smith, '26).
c. Fishes
The relationship between the thyroid and pituitary glands in the develop-
ment of fishes is problematical. There is evidence in favor of a positive in-
fluence of endostylar cells and of the cells of the developing thyroid gland
in the transformation of the ammocoetes larva of the cyclostome, Petromyzon,
into the definitive or adult body form. Similar evidence suggests a tli^roid
activity relationship in the transformation of the larvae of the trout and
886 THE DEVELOPING ENDOCRINE GLANDS ,
the bony eel. However, this evidence is not indisputable, and more study
is necessary before definite conclusions are possible. (Consult Lynn and
Wachowski, '51, for discussion and references.)
3. General Conclusions Relative to the Influence of the
Thyroid and Pituitary Glands in Vertebrate Embryology
These conclusions are:
(a) Positive activities of the thyroid and pituitary glands are demonstrated
in the transformation of the larval form into the definitive or adult
form in the Anura.
(b) Suggestive evidence in favor of such an interpretation has been ac-
cumulated in fishes.
(c) Circumstantial evidence, relative to the possible activities of the thyroid
and pituitary glands during the period when the embryos of the chick
and mammal are transforming into the adult form, is present. With
the evidence at hand, however, it is impossible to conclude definitely
that these glands are a contributing factor to a change in body form
(metamorphosis) in chick and mammalian embryos (fig. 256).
D. Possible Correlation of the Endocrine Glands with Sex Differentiation
1. Differentiation of Sex
a. General Sex Features in the Animal Kingdom
Many animal groups are hermaphroditic, that is, both sexes occur in the
same individual. Flatworms, roundworms, oligochaetous annelids, leeches,
many mollusks, and certain fishes are representatives of this condition, whereas
most vertebrates, insects, and echinoderms are bisexual. If one examines the
developing gonads in insects or vertebrates, it is evident that, fundamentally,
the potentialities for both sexes exist in the same individual. As observed
previously (Chap. 18), the early gonad is bipotential in most vertebrates, and
two sets of reproductive ducts are formed. As sex is differentiated, the gonadal
cortex and the Miillerian duct assume dominance in the female, while the
gonadal medulla and Wolffian duct become functional if the animal is a male.
Generahty, therefore, gives way to specificity. Conditions thus are established
in the developing reproductive system, similar to the generalized conditions
to be found in other systems. If we take into consideration the fact that in
a large number of animals both sexes are present in a functional state in one
individual and in many bisexual species both sexes are present in a rudi-
mentary condition in the early embryo, we arrive at the conclusion that both
sexes are fundamentally present in a large majority of animal species. Sex,
therefore, tends to be an hermaphroditic matter among many species of
animals. The problem of sex differentiation, consequently, resolves itself into
this: Why do both sexes emerge in the adult condition in a large number of
4.
CORRELATION OF ENDOCRINE GLANDS WITH SEX DIFFERENTIATION 887
animals, whereas in the development of many other animal species, only one of
the two sex possibilities becomes functional?
b. Chromosomal, Sex-determining Mechanisms
A considerable body of information has been obtained which demonstrates
a fundamental relationship between certain chromosomes and sex determina-
tion. The general topography of chromosomal sex-determining mechanisms
has been established for a large number of species. A pair of homologous
chromosomes, the so-called sex chromosomes, apparently have become spe-
cialized in carrying the genie substances directly concerned with sex determina-
tion. In many species, the members of this pair of sex-determining chromo-
somes appear to be identical throughout the extent of the chromosomes in
one of the sexes. In the other sex, on the other hand, the two sex-determining
chromosomes are not identical. When two identical chromosomes are present
in a particular sex, that sex is referred to as the homogametic sex, for the
reason that all of the gametes derived from this condition will possess identical
sex chromosomes. However, that sex which possesses the two dissimilar
chromosomes is called the heterogametic sex for it produces unlike gametes,
Often the heterogametic condition is represented by one chromosome only,
the other chromosome being absent. If under the above circumstances the
normally appearing chromosome is called X, and the deleted, diminutive or
strangely appearing chromosome is called Y, while the chromosome which
is absent be designated as O, we arrive at the following formula:
XX rr the homogametic sex and either XY or XO = the heterogametic sex.
In many (probably in most) animal species the male is the heterogametic sex
(fig. 368A-C).
In some animal groups, however, such as the butterflies, the moths, possibly
the reptiles, the birds, some fishes, and probably urodele amphibia, the female
is the heterogametic sex, and the male is homogametic. In these particular
groups, many authors prefer to use the designation ZZ for the homogametic
sex (i.e., the male) and ZO or ZW for the female or heterogametic sex. The
sex-determining mechanism in these groups, according to this arrangement,
will be ZZ:ZW or ZZ:ZO (fig. 368D).
In endeavoring to explain the action of these chromosomal mechanisms,
one of the underlying assumptions is that the genie composition of the chromo-
somes actively determines the sex. For example, in cases where the female
sex is homogametic it is assumed that the X-chromosome contains genes which
are female determining; when two (or more) X's are present, the female
sex is determined automatically. When, however, one X-chromosome is pres-
ent, the determining mechanism works toward male determination. In those
species where the female sex is the heterogametic sex it may be assumed that
the Z-chromosome (or X-chromosome, depending upon one's preference)
contains genes which are male determining. When only one of these Z-chromo-
888
THE DEVELOPING ENDOCRINE GLANDS
Fig. 368. The sex chromosomes in man, opossum, chick, and Drosophila; parabiotic
experiments in Amphibia. (A) Late primary spermatocyte in human. (A') First
maturation spindle in human spermatocyte. (Redrawn from Painter, '23, J. Exper. Zool.,
37.) (B) Dividing spermatogonium in opossum testis. (B') First maturation spindle
in spermatocyte of opossum. (Redrawn from Painter, '22, J. Exper. Zool., 35.) (C)
Sex chromosomes in female Drosophila. (C) Sex chromosomes in male Drosophila.
(Redrawn from Morgan, Embryology and Genetics, 1934, Columbia University Press,
N. Y., after Dobzhansky. ) (D) Sex chromosomes in common fowl, male. (D') Sex
chromosomes in common fowl, female. (Redrawn from Bridges, 1939, Chap. 3, Sex and
Internal Secretions, edited by Allen et al., Baltimore, Williams and Wilkins, after Sokolow,
Tiniakow, and Trofimov. ) (E-G) Diagrams illustrating the spreading of gonadal sub-
stances in frogs, toads, and salamanders. In toads, E, the gonadal influences (antagonisms)
are evident only when the gonads actually are in contact. In the frogs, F, the range of
influence is wider but its effect falls off peripherally. Figure G represents the condition
in newts and salamanders. It is evident that in this group, some substance is carried in
the blood stream which suppresses the gonads in the two females as indicated in the
diagram. (Redrawn and modified slightly from Witschi, 1939, Chap. 4, Sex and Internal
Secretions, edited by Allen et al., Baltimore, Williams and Wilkins.)
somes is present the developmental forces swing in the direction of the
female sex. Sex, from this point of view, is determined by a genie balance, a
balance which in turn is governed by the quality of certain genes as well as
the quantitative presence of genes. (For detailed discussion consult Bridges,
'39, and White, '48.)
CORRELATION OF ENDOCRINE GLANDS WITH SEX DIFFERENTIATION 889
c. Possible Influence of the Sex Field in Sex Determination
Two gonadal sex fields, the cortical field and the medullary field, are
present in the early vertebrate gonad in amphibians, reptiles, birds, and
mammals. This condition is true also of many fishes. Sex differentiation pri-
marily is a question as to which one of these fields will assume dominance.
During development in various instances, sex differentiation is clearly the
result of only partial dominance on the part of one sex field, the other field
emerging partly or almost completely. As a result, various types of intersexes
may appear. For example, in the male toad. Bidder's organ at the anterior
part of the testis represents a suppressed cortical or ovarian field, held in
abeyance by the developing testis. Surgical removal of the two testes permits
the cortical field or Bidder's organ to become free from its suppressed state.
As a result, functional ovaries are developed, and the animal reverses its sex,
becoming a functional female (Witschi, '39).
One of the classical examples which demonstrates the dependence of the
developing sex field upon surrounding environmental factors is the freemartin.
The freemartin appears in cattle when twins of the opposite sex develop in
such a manner that an anastomosis or union of some of the fetal blood vessels
occurs (Lillie, '17). Under these circumstances the female twin always ex-
periences a transformation in the direction of maleness in the gonad and sex
ducts. In those instances of freemartin development where the cortical field
of the developing ovary is suppressed and the medullary area is hypertrophic,
a partial or fairly well-developed testis may be formed. Under these conditions
it is presumed that some substance is elaborated within the medullary field of
the developing gonad of the male twin which enhances the development of
the similar field in the freemartin ovary and suppresses, at the same time,
the cortical field. The development of fully differentiated gametes (i.e., sperm)
in the freemartin "testis" has not been demonstrated, but, on the whole, the
more normally developed freemartin testis shows conditions at the time of
birth which are comparable to a similar gonad of the normal male at about
the same age, with the questionable presence or absence of very young germ
cells. Gametogenesis in the developing testis of the bull occurs after birth.
Consequently, the development of gametes in the freemartin of cattle cannot
be ascertained because the freemartin gonad remains in the position of the
normal ovary and does not descend into the scrotum as it does in the male
(Willier, '21 ). A scrotal residence (Chap. 1 ) is necessary for spermatogenesis
in all males, possessing the scrotal condition.
A particularly interesting case of intersexuality, resulting from the lack of
complete supremacy on the part of one sex field, is shown in the fowl described
by Hartman and Hamilton ('22). A brief resume of its behavior and anatomy,
as described by the authors, is presented herewith.
The bird was hatched as a robust chick and developed into an apparently normal
Rhode Island Red pullet. The following spring the comb and wattles began to
890 THE DEVELOPING ENDOCRINE GLANDS
enlarge, and the bird after a few abortive attempts, learned to give the genuine
crow of a rooster. ... It was often seen scratching on the ground and calling the
flock to an alleged morsel of food, and though it was never seen to tread hens it
would strut and make advances after the manner of cocks. . . . The female be-
havior of the bird was as follows. For years it would sing like a laying hen. On
two occasions it adopted incubator chicks, caring for them day and night and
clucking like a normal hen. ... On one occasion it dropped an egg, which though
small and elongated, showed the bird to be in possession of functional ovary and
oviduct.
Its internal anatomy demonstrated the presence of a left ovotestis and a
right testis. An oviduct was present on the left side and a vas deferens on
both sides. The right testis contained tubules, and within the tubules were
ripe sperm. The ovotestis on the left side contained a cortex studded "with
oocytes of every size up to a diameter of 20 mm." and "not unlike the ovary
of a normal hen approaching the laying season" (Hartman and Hamilton, '22).
Seminiferous tubules also were present in the ovotestis which was filled with
sperm.
An interesting example of complete sex reversal was produced experimen-
tally in the axolotl, Siredon (Anibystoma) mexicanum, by Humphrey ('41).
In doing so, Humphrey orthotopically implanted an embryonic testis of
Ambystoma tigrinum into an axolotl embryo of similar age. After the ovary
on the opposite side of the host (i.e., the young axolotl) had changed to a
testis, the implanted testis was removed. Somewhat later, the sexually reversed
female axolotl was bred with other females with success. The Fj and Fj gen-
erations suggest that the female axolotl is heterogametic whereas the male
is homogametic, with a possible XY or ZW condition in the female and an XX
(or ZZ) arrangement in the male. It is interesting to observe that Humphrey
obtained YY (or WW) females which were fertile.
Many other studies have been made along the lines of experimental trans-
formation of sex. Of these, the careful studies of Witschi ('39) are illumi-
nating. The method, employed by Witschi, was to join two embryos of opposite
sex before the period of sex differentiation. In his studies, he used toad,
frog, and urodele embryos. Three different results were obtained, in which
the medulla or developing testicular rudiment tended to dominate and sup-
press the cortex or developing female sex field. For example, in toads, it was
evident that the medulla suppressed the cortex only if the two fields came into
actual contact; in frogs, the effect of suppression was inversely proportional
to the distance of the two sex fields from each other; on the other hand, in
urodeles, the substance produced by the medulla evidently circulated in the
blood stream and produced its effects at a distance (fig. 368E-G). Witschi
postulated the presence of two, not readily diffusible, "activator" substances,
cortexin, formed by the cortex, and meduUarin, elaborated by the medulla,
to account for the results in the toad and frog embryos, and, in urodeles,
he assumed a hormonal substance to be present.
CORRELATION OH ENDOCRINE GLANDS WITH SEX DIFFERENTIATION 891
The foregoing examples and many others (Witschi, '39) suggest the fol-
lowing interpretations relative to sex determination and differentiation:
( 1 ) The germ cell, regardless of its genetic constitution, develops into an
egg or a sperm, depending upon whether it lies in a developing cortex
or in a developing medulla. That is, the influence of the sex field gov-
erns the direction of germ-cell differentiation (fig. 22).
(2) The sex field is a powerful factor in determining sex. A factor (or fac-
tors) which enables an elevation to partial or complete dominance on
the part of one sex field, which under normal conditions is suppressed,
may result in the partial or complete reversal of sex.
(3) Differentiation of sex is dependent upon an interplay between the genes
of the sex chromosomes and the bio-chemical forces present in the
gonadal sex field. This interplay may be considered to work as follows:
(a) If the male-sex field or medulla in a particular species is stronger
than the female field or cortex, that is, if it is able to compete for sub-
strate substances more vigorously and successfully and to produce dif-
fusible hormonal substance more plentifully, it will suppress the female
sex field. Under these conditions, the chromosomal sex-determining
mechanism is established in such a way that the male is the hetero-
gametic sex, composed of XY or XO chromosomal combinations,
and the female is XX, the genes of the extra X chromosome being
necessary to override the male tendency present normally in the male
sex field, (b) On the other hand, if the female sex field or cortex
is stronger physiologically, then the female is the heterogametic sex
(XO or ZW), the homozygous condition of the sex chromosomes
in the male being necessary to suppress the natural tendencies toward
supremacy of the stronger female sex field, (c) It may be that the
general characteristics and strength of the sex field are controlled by
genes present in certain autosomal chromosomes, whereas the specific
role which the particular sex field takes normally in sex differentiation
is controlled by the genes in the sex chromosomes.
2. Influence of Hormones on the Differentiation of Sex
The possible effects of hormones upon sex differentiation, particularly upon
the development of the accessory duels, have been studied with great interest
since F. R. Lillie's ('17) description of freemartin development in cattle. He
tentatively made the assumption that the male fetal associate of the free-
martin produces a hormonal substance which, through the medium of vascular
anastomoses within the placentae of the two fetuses, brings about a partial
suppression of the developing ovary and effects, in part, a sex reversal in the
developing reproductive organs of the female. The female member of this
heterosexual relationship, therefore, is more or less changed in the direction
of the male; hence, the common name freemartin.
892 THE DEVELOPING ENDOCRINE GLANDS
It should be mentioned in this connection that in the marmoset, Oedipo-
midas geoffroyi, similar anastomoses between the placental blood vessels of
heterosexual twins fail to produce the freemartin condition, both twins being
normal. Species differences in the response to hormones or other sex-modifying
substances therefore occur (Wislocki, '32).
The studies made in an endeavor to ascertain the influences which sex hor-
mones play in the development of the reproductive system and in sexual dif-
ferentiation have produced the following general results.
Developing ovaries and testes and the reproductive ducts of birds, frogs,
and urodeles may show various degrees of sex reversal when the developing
young are exposed to hormones or other humeral substances of the opposite
sex. There is some evidence to the effect that sex reversal by sex hormones
is accomplished more readily and completely from the homogametic sex to
the heterogametic sex, suggesting, possibly, that the sex field of the hetero-
gametic sex is the stronger and more resistant. The reproductive ducts are
more responsive to change than are the gonads (Burns, '38, '39a; Domm, '39;
Mintz, Foote, and Witschi, '45; Puckett, '40; Willier, '39; and Witschi, '39).
In mammals, the gonads (ovary and testis) appear iquite immune to the
presence of sex hormones, whereas the reproductive ducts respond partially
to the sex hormone of the opposite sex. The caudal parts of the genital pas-
sages are more sensitive to change than are the more anterior portions (Burns,
'39b, '42; Greene, Burrili, and Ivy, '42; and Moore, '41, '50).
Castration experiments before and shortly after birth in mammals produce
the following effects:
( 1 ) Removal of the testis results in retardation and suppression of the
male duct system, while it allows the female duct system to develop.
(2) Removal of the ovary does not affect the female duct system until the
time of puberty.
(See LaVelle, '51, and Moore, '50, for extensive references and discussion.)
The general conclusions to be drawn from the above experiments, relative
to the differentiation of the reproductive ducts, are as follows:
(1) The reproductive ducts are responsive to sex hormones after they
are formed in the embryo.
(2) The male duct system normally responds to humeral substances, elabo-
rated by the developing testis soon after it is formed.
(3) The female duct system probably is not dependent upon hormonal se-
cretion for its development until about the time of sexual maturity.
(4) The developing ovary, unlike the developing testis, probably under
normal conditions does not elaborate sex hormones in large amounts
until about the time of sexual maturity.
CORRELATION OF ENDOCRINE GLANDS WITH SEX DIFFERENTIATION 893
3. General Summary of the Factors Involved in Sex
Differentiation in the Vertebrate Group
The sex glands (gonads) and the reproductive ducts appear to arise inde-
pendently of each other.
The primitive gonad is composed of two main parts:
( 1 ) the primordial germ cells and
(2) cellular structures which act as supporting and enveloping structures
for the germ cells.
The presence of the primitive germ cells probably is a primary requisite
for the development of a functional reproductive gland (see p. 121).
In the differentiation of the gonad, two basic sex fields or territories appear
to be involved in Tetrapoda and probably also in most fishes. These terri-
tories are:
( 1 ) the medulla or testis-forming territory and
(2) the cortex or ovary-forming area.
The sex fields may be controlled by the genes in the autosomal chromo-
somes, and there probably is a tendency for one or the other of these fields
to be functionally stronger than the other. The heterogametic (XY, XO, ZW
or ZO) conditions of the sex chromosomes appear to be associated with the
stronger sex field, and the homogametic (i.e., XX or ZZ) combination is
associated with the weaker sex field.
During development, presumably, there is a struggle for supremacy through
competition for substrate substances (see Dalcq, '49) by these two sex fields
and, under normal conditions, the sex chromosomal mechanism determines
which of the two sex fields shall be suppressed and which shall rise to domi-
nation. The sex chromosomes thus control the direction of sex differentiation,
whereas the field or territory elaborates the power of differentiation.
Disturbing influences may upset the sex-determining mechanism set forth
above, and various degrees of hermaphroditism may arise in the same indi-
vidual in proportion to the degree of escape permitted the normally suppressed
sex field.
The sex ducts arise in association with the pronephric kidney and its duct,
the pronephric (mesonephric) duct. The Miillerian or female duct arises by a
longitudinal splitting of the original pronephric (mesonephric) ducts (e.g., in
elasmobranchs) or by an independent caudal growth of a small invagination
of the coelomic epithelium at the anterior end of the mesonephric kidney
(e.g., reptiles, birds, and mammals). This independent caudal growth is de-
pendent, however, upon the pre-existence of the mesonephric duct (Chap. 18).
In the urodeles, the Miillerian duct appears to arise partly from an inde-
pendent origin and in part from contributions of the mesonephric duct.
894
THE DEVELOPING ENDOCRINE GLANDS
Two sets of primitive ducts thus are established in the majority of verte-
brates in each sex, the Miillerian or female duct and the mesonephric (pro-
nephric) or male duct.
During later normal development, the Miillerian duct is developed in the
female, while, in the male, the mesonephric duct is retained and elaborated
as the functional, male reproductive duct.
The male duct system is dependent upon secretions from the developing
testis for its realization during the later embryonic period and during post-
natal development, whereas the female duct develops independently of the
ovary up to the time of sexual maturity when its behavior is altered greatly
by the presence of the ovarian hormones.
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thyroid gland and hypophysis upon
growth and development of amphibian
larvae. Quart. Rev. Biol. 4:325.
. 1925. The effects of extirpation
of the thyroid and pituitary glands upon
the limb development of anurans. J.
Exper. Zool. 42:13.
Brahms, S. 1932. The development of the
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Bridges, C. B. 1939. Chap. II, Cytological
and genetic basis of sex. Sex and Internal
Secretions, 2nd Edition. Edited by Allen,
et al., Williams & Wilkins, Baltimore.
Burns, R. K., Jr. 1938. The effects of crys-
talline sex hormones on sex differentia-
tion in Amblystoma. I. Estrone. Anat.
Rec. 71:447.
. 1939a. The effects of crystalline
sex hormones on sex differentiation in
Amblystoma. II. Testosterone propio-
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. 1939b. Sex differentiation during
the early pouch stages of the opossum
(Didelphys virginiana) and a comparison
of the anatomical changes induced by
male and female sex hormones. J. Mor-
phol. 65:497.
1942. Hormones and experimental
modification of sex in the opossum. Biol.
Symp. 9:125.
Chen, G., Oldham, F. K.. and Ceiling,
E. M. K. 1940. Appearance of the mela-
nophore-expanding hormone of the pi-
tuitary gland in the developing chick
embryo. Proc. Soc. Exper. Biol. & Med.
45:810.
Cooper, E. R. A. 1925. The histology of
the more important human endocrine
organs at various ages. Oxford Univer-
sity Press, Inc., New York.
Dalcq, A. M. 1949. The concept of physio-
logical competition (Spiegelman) and
the interpretation of vertebrate morpho-
genesis. Exp. Cell Research, Supplement
1, Bonnier, Stockholm and Academic
Press, New York.
Domm, L. V. 1939. Chap. V. Modifica-
tions in sex and secondary sexual char-
acters in birds in Sex and Internal Se-
cretions by Allen, et al., 2d ed.. The
Williams & Wilkins Co.. Baltimore.
Greene, R. R., Burrill, M. W., and Ivy,
A. C. 1942. Experimental intersexuality.
The relative sensitivity of male and fe-
male rat embryos to administered estro-
gens and androgens. Physiol. Zool. 15:1.
Gudernatsch, J. F. 1912. Feeding experi-
ments on tadpoles. I. The influence of
specific organs given as food on growth
and differentiation. A contribution to the
knowledge of organs with internal secre-
tion. Arch. f. Entwicklngsmech. d. Organ.
35:457.
. 1914. Feeding experiments on tad-
poles. II. A further contribution to the
knowledge of organs with internal secre-
tion. Am. Jour. Anat. 15:431.
Hall, A. R., and Kaan, H. W. 1942. Ana-
tomical and physiological studies on the
thyroid gland of the albino rat. Anat.
Rec. 84:221.
BIBLIOGRAPHY
895
Hartman, C. G., and Hamilton, W. F.
1922. A case of true hermaphroditism
in the fowl, with remarks upon sec-
ondary sex characters. J. Exper. Zool.
36:185.
Hopkins, M. L. 1935. Development of the
thyroid gland in the chick embryo. J.
Morphol. 58:585.
Hoskins, E. R. and M. M. 1918. Further
experiments with thyroidectomy in Am-
phibia. Proc. Soc. Exper. Biol. & Med.
15:102.
1919. Growth and development of
Amphibia as affected by thyroidectomy.
J. Exper. Zool. 29:1.
Howard, E. 1939. Effects of castration on
the seminal vesicles as influenced by age,
considered in relation to the degree of
development of the adrenal X zone. Am.
J. Anat. 65:105.
LaVelle, F. W. 1951. A study of hormonal
factors in the early sex development of
the golden hamster. Contrib. to Em-
bryol. Carnegie Inst., Washington, Publ.
34:223.
Lillie, F. R. 1917. The free-martin; a
study of the action of sex hormones in
the fetal life of cattle. J. Exper. Zool.
23:371.
Lynn, W. G., and Wachowski, H. E. 1951.
The thyroid gland and its functions in
cold-blooded vertebrates. Quart. Rev.
Biol. 26:123.
Mintz, B., Foote, C. L., and Witschi, E.
1945. Quantitative studies on response
of sex characters of differentiated Rana
clamitans larvae to injected androgens
and estrogens. Endocrinology. 37:286.
Moore, C. R. 1941. On the role of sex
hormones in sex differentiation in the
opossum (Didelphys virginiana). Physiol.
Zool. 14:1.
. 1950. The role of the fetal endo-
crine glands in development. J. Clin.
Endocrinol. 10:942. .
Puckett, W. O. 1940. Some effects of crys-
talline sex hormones on the differentia-
tion of the gonads of an undifferenti-
ated race of Rana catesbiana tadpoles.
J. Exper. Zool. 84:39.
Rahn, H. 1939. The development of the
chick pituitary with special reference to
the cellular differentiation of the pars
buccalis. J. Morph. 64:483.
Rankin, R. M. 1941. Changes in the con-
tent of iodine compounds and in the his-
tological structure of the thyroid gland
of the pig during fetal life .Anat. Rec.
80:123.
Rumph, P., and Smith, P. E. 1926. The
first occurrence of secretory products
and of a specific structural differentiation
in the thyroid and anterior pituitary dur-
ing the development of the pig foetus.
Anat. Rec. 33:289.
Selye, H. 1948. Textbook of Endocrinol-
ogy. Universite de Montreal, Montreal,
Canada.
Smith, P. E. 1916. The effect of hypo-
physectomy in the early embryo upon
growth and development of the frog.
Anat. Rec. 11:57.
. 1920. The pigmentary growth and
endocrine disturbances induced in the
anuran tadpole by the early ablation of
the pars buccalis of the hypophysis. Am.
Anat. Memoirs. 11, The Wistar Institute
of Anatomy and Biology, Philadelphia.
Wheeler, R. S., and Hoffman, E. 1948a.
Goitrous chicks from thyroprotein-fed
hens. Endocrinology. 42:326.
and
-. 1948b. Influence of
quantitative thyroprotein treatment of
hens on length of incubation period and
thyroid size of chicks. Endocrinology.
43:430.
White, M. J. D. 1948. Animal Cytology
and Evolution, Chap. XI. Cambridge
University Press, London.
Willier, B. E. 1921. Structures and homol-
ogies of free-martin gonads. J. Exper.
Zool. 33:63.
1939. Chap. III. The embryonic
development of sex in Sex and Internal
Secretions by Allen, et al., 2d ed.. The
Williams & Wilkins Co., Baltimore.
Wislocki, G. B. 1932. Placentation in the
marmoset (Oedipomidas geoffroyi) with
remarks on twinning in monkeys. Anat.
Rec. 52:381.
Witschi, E. 1939. Chap. IV. Modification
of the development of sex in lower ver-
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ternal Secretions by Allen, et al., 2d
ed.. The Williams & Wilkins Co., Bal-
timore.
PART V
Tne Care or tne Developing Emtryo
The care of the developing embryo necessitates the formation of various types of
embryonic membranes, and in many species, the retention of the developing embryo within
either maternal or paternal body structures (Chap. 22).
22
Care and Nourisnment of tne Developing Young
A. Introduction
1. Care in relation to the number of young produced
2. General environmental conditions necessary for development
3. Types of enveloping or protective membranes
4. Types of food sources
5. Mechanisms for oxygen supply and carbon dioxide removal
6. Oviparity, ovoviviparity, and viviparity
B. Formation and importance of the protective embryonic membranes
1. The egg membranes
a. Primary and secondary egg membranes
b. Tertiary egg membranes
1) Mammals
2) Birds
a) Formation of the chalaziferous layer
b) Deposition of the middle dense layer of albumen
c) Formation of the inner liquid layer of albuminous material and the
chalazae
d) Deposition of the outer liquid albuminous layer
e) Formation of the egg membranes and egg shell
3) Reptiles
4) Amphibians
5) Fishes
2. The extra-embryonic membranes
a. Yolk sac
b. Amnion
c. Chorion (serosa)
d. Allantois
e. Yolk stalk, allantoic stalk, belly stalk, and umbilical cord
3. The reproductive duct as a protective embryonic membrane
4. Uncommon or specialized structures as protective mechanisms
C. Special adaptations of the extra-embryonic membranes for uterine existence
1. Implantation
a. Definition
b. Types of implantation
2. The placenta and placentation
a. Definition
b. Types of embryonic tissues involved in placentation
899
900 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
c. Types of placental relationships in the eutherian mammals
1 ) Epitheliochorial type
2) Endotheiiochorial variety
3) Endotheiiochorial plus syndesmochorial placenta
4) Hemochorial placenta
5) Hemoendothelial placenta
3. Implantation of the human embryo
a. Preparation for implantation
b. Implantation
c. Formation of the placenta
4. Implantation in the rhesus monkey, Macaca mulatta
5. Implantation of the pig embryo
6. Fate of the embryonic membranes
a. Yolk sac
b. Amnion and allantois
D. Functions of the placenta
E. Tests for pregnancy
1. Aschheim-Zondek test
2. Friedman modification of the Aschheim-Zondek test
3. Toad test
4. Frog test
F. The developing circulatory system in relation to nutrition, etc.
G. Post-hatching and post-partum care of the young
A. Introduction
1. Care in Relation to the Number of Young Produced
In this chapter, we shall consider the methods by which developing embryos
of different vertebrate species are cared for and nourished during develop-
ment. The amount of care given to the developing egg varies greatly. How-
ever, one primary rule appears to govern the reproductive habits of the species,
namely, the species must survive. This survival is accomplished by two prin-
cipal methods:
( 1 ) by the production of enormous numbers of developing young, given
no protective care, with the result that few survive to the adult or
reproductive stage, and
(2) by the formation of fewer developing individuals with greater amounts
of protective care.
Generally speaking, the fewer the individual embryos produced, the greater
the care.
Examples of the method of species survival without parental care are evi-
dent in the codfish, Gadus, which spawns about 8 to 10 millions of eggs during
a particular breeding period or in the ling, Molva, which discharges from
14 to 60 millions of eggs at one time. In these instances, the species survive
by the sheer number of developing young produced. On the other hand, the
shark, bird, and mammal substitute an extreme care of the developing egg,
INTRODUCTION 901
with the result that the number of eggs produced at each breeding period is
reduced enormously, compared with that of the cod or ling.
2. General Environmental Conditions Necessary for
Development
Regardless of whether or not there is specialized care of the developing
young, the following conditions, concerned with the nutrition and care of the
young, are necessary in the development of all vertebrate embryos:
(a) All embryos develop within a fluid or "embryonic lake" made possible
by the presence of certain, enveloping membranes;
(b) a favorable temperature is required, particularly in warm-blooded
species;
(c) food material including water must be supplied;
(d) oxygen is necessary to the developing embryo, and
(e) the removal of carbon dioxide and other wastes is imperative.
3. Types of Enveloping or Protective Membranes
Many types of protective membranes are produced in the vertebrate group
for the purpose of caring for the developing young. These membranous and
other types of protective envelopes may be classified as follows:
a. Egg membranes.
b. Extra-embryonic membranes.
c. The uterine portion of the oviduct.
d. Uncommon or specialized structures.
The egg membranes are those membranes produced around the egg during
its formation in the ovary or during the journey down the oviduct. They are
classified generally into three categories:
( 1 ) Primary egg membranes are the membranes which are produced by
the surface layer of the egg as it develops in the ovary, e.g., the vitel-
line membrane;
(2) secondary egg membranes are the membranes contributed to the egg
by the activities of the surrounding follicle cells of the ovary, e.g., the
zona pellucida of mammals, possibly also the chorion of some fish
eggs; and
(3) tertiary egg membranes are the membranes contributed to the egg as
it passes down the oviduct, such as the albuminous layers of frog and
chicken eggs.
The extra-embryonic membranes are those membranes constructed of
embryonic tissues which extend out of and beyond the strict confines of
the embryonic body. As such they represent specialized embryonic tissues
902 CARE ANn NOURISHMENT OF THE DEVELOPING YOUNG
adapted to fulfill certain definite functions necessary to the embryo. The
extra-embryonic membranes are:
( 1 ) The yolk sac, found in most species. The yolk sac is developed as an
extension of the primitive gut.
(2) The amnion, representing a sac-like structure which surrounds the
embryo. It is found only in the Amniota, that is, the reptiles, birds,
and mammals.
(3) The allantois. This structure arises as an outpushing from the mid-
ventral area of the hindgut, and is found only in reptiles, birds, and
mammals.
(4) Pharyngeal diverticula. The pharyngeal diverticula are found in cer-
tain species of fish and in amphibians. The external gill filaments of
the shark embryo mentioned in Chapter 14 are an example of this
type of extra-embryonic membrane. Also in certain species of Am-
phibia elaborate pharyngeal placentae are evolved which function in
a respiratory capacity.
The uterine portion of the oviduct functions, of course, as a capsule to
protect the developing egg in all ovoviviparous and viviparous species.
Uncommon, specialized structures for the protection of the developing
embryo are formed in many species of fishes and Amphibia. These structures
are described more explicitly on p. 915.
4. Types of Food Sources
There are two main types of food sources for vertebrate embryos, namely,
endogenous and exogenous sources. The endogenous form of food supply is
found in all amphibian species, in the lung-fishes, Amphioxus, etc., where
nourishment necessary for development is incorporated directly within the
developing embryonic cells from the beginning cleavages of the egg. On the
other hand, in the exogenous type of food supply the nourishment necessary
for development lies outside of the developing embryonic tissues. This type
of food storage is found in elasmobranch and teleost fishes, reptiles, birds,
and mammals. Two categories are to be observed, as follows:
( 1 ) In the majority of fishes, and in all reptiles, birds, and prototherian
mammals, the food is stored within the egg. The developing embryo
which lies upon this food source utilizes a specialized type of extra-
embryonic tissue to digest and assimilate the food materials.
(2) In some fishes and in the metatherian and eutherian mammals, most,
or practically all, of the food elements come directly from the maternal
(and, in some instances in fishes, from paternal) tissues as the embryo
develops. Here also, a specialization of extra-embryonic tissue is nec-
essary to tap the supply of food.
FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 903
5. Mechanisms for Oxygen Supply and Carbon Dioxide Removal
Two types of oxygen supply and carbon dioxide removal mechanisms are
encountered. In the majority of fishes and in the larger number of Amphibia,
the surface of the developing egg functions as a respiratory membrane. In
some fishes, and in rare instances in the Amphibia, special diverticula of the
pharyngeal area are developed to care for this function. On the other hand,
in all reptiles, birds, and mammals, the allantoic diverticulum from the hind-
gut assumes respiratory responsibilities.
6. Oviparity, Ovoviviparity, and Viviparity
The word oviparous is derived from two Latin words, namely, ovum, egg,
and parere, to bring forth. Oviparous animals thus produce eggs from which
the young are hatched after the egg is laid or spawned. Among the vertebrates,
oviparous species include most of the fishes, amphibia, reptiles, birds, and
prototherian mammals. Ovoviviparity is a condition in which the egg is re-
tained within the confines of the reproductive duct or other specialized areas
where it hatches, and the young are brought forth or born alive. The greater
portion of the embryo's nourishment is derived from the nutritive materials
within the egg, while oxygen uptake, together with fluid substances and the
elimination of carbon dioxide, is effected through the oviducal wall and its
blood vessels. Ovoviviparous species include certain sharks, teleosts, certain
urodele and anuran amphibia, and various reptiles. In viviparity (Latin, vivus,
alive) the new individual is brought forth alive. In viviparity the developing
embryo obtains some or all of its nourishment through the wall of the uterus
or other specialized structure. Viviparous forms are found among the sharks,
teleosts, and reptiles, together with all species of metatherian and eutherian
mammals.
B. Foimation and Importance of the Protective Embryonic Membranes
1. The Egg Membranes
a. Primary and Secondary Egg Membranes
The formation of the primary and secondary egg membranes were described
in Chapter 3. The importance of these membranes formed around the egg,
while it develops in the ovary, is considerable. The so-called fertilization mem-
brane, produced, for example, in Amphioxus, the zona radiata and chorion
of fishes, the vitelline membrane of amphibians, reptiles, and birds, or the
zona pellucida of mammals are important structures. All these membranes
form the first or primary protective coating around the embryo. Between the
embryo and this primary embryonic membrane is a fluid-filled area, the
perivitelline space. The perivitelline fluid is favorable to the embryo. Thus,
the surrounding fertilization, vitelline, or zona membranes act as an insu-
lating wall between the outside environment and this early perivitelline pond
904 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
of the embryo. All vertebrate embryos, from the fishes to the mammals, are
protected normally by the primary embryonic membrane during the period
of cleavage, and, in many fishes and amphibians this membrane functions
until the time when the embryo hatches and assumes a free-living existence.
b. Tertiary Egg Membranes
1) Mammals. The lengths of the Fallopian tubes of different mammalian
species vary considerably. In the mouse, rabbit, human, and sow, the Fal-
lopian tubes vary in length, not only from species to species but also from
individual to individual within the species. Yet, the time of passage of the
egg through this region of the reproductive duct approximately, for all four
species, is from 3 to SVi days. On the other hand, the length of the uterine
tube of the opossum may be from 5 to 10 times that of the mouse, yet the
time consumed in egg transport in the former species is about 19 to 24 hours.
Moreover, in the sow and mouse, evidence has been accumulated which tends
to show that egg transport through the middle portion of the uterine tube
is slower than that of the portion near the infundibulum or of the part near
the uterus (Anderson, '27; Lewis and Wright, '35). In the Monotremata,
Flynn and Hill ('39, p. 540) conclude that "passage through the tube must
be fairly rapid." In all these instances, the rate of egg travel through the
uterine tube appears to be dependent upon necessary developmental changes
within the cleaving egg and functional changes within the uterus and the
uterine tube. In other words, the rate of egg propulsion through the Fallopian
tube varies with the species. The time consumed in transit is not related to
the length of the tube, but is correlated with changes in the uterus, pre-
paratory to receiving the egg at a proper developmental stage.
The deposition of protective enveloping coats around the egg during egg
passage through the Fallopian tube is encountered in certain mammals. In
the monotremes, a rather dense, albuminous coat is deposited around the egg
in the upper two thirds of the Fallopian tube, and a clearer, more fluid secre-
tion is deposited around the egg by the glandular cells in the posterior third
of the tube (Flynn and Hill, '39). A leathery shell is formed around the egg
and these albuminous coats in the posterior segment or uterus. In the opossum,
a dense albuminous coating forms around the egg during its passage down
the upper part of the Fallopian tube, while a thin much tougher membrane
is added around the outside of the albuminous material in the tube's lower
part. In the rabbit, a thick albuminous coating is deposited around the egg
as it passes downward within the Fallopian (uterine) tube. Therefore, forma-
tion of protective egg envelopes may be regarded as a specific function of the
Fallopian tube during egg passage in some mammals.
The reactions of the developing egg within the uterine portion (uterus)
of the reproductive duct in the higher mammals are dramatic events in which
the embryo develops special contacts with the uterine wall. In some cases.
FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 905
the embryo becomes entirely enclosed within the tissues of the uterus. These
phenomena are considered on pages 914, 920.
2) Birds. The passage of the hen's egg down the oviduct has been studied
at various times from the time of Aristotle to the present. In its transportation,
the "naked yellow" or ovum becomes surrounded by an intricate association
of fibers, albuminous substance, membranes, and calcareous shell which form
a system of protective envelopes. As the egg of the hen passes posteriad in
the oviduct, it rotates slowly under the influence of muscular contractions and
the spiral arrangement of longitudinal folds of the mucous membrane lining
the oviduct. This rotation aids in the deposition of the membranes and albumi-
nous layers.
a) Formation of the Chalaziferous Layer. The first coating of al-
bumen is deposited around the egg as it passes through the posterior portion
of the infundibulum (fig. 157). It is in the form of a sheet of mucin-like
fibers in the meshes of which is a dense albuminous substance. This capsule
of albumen is applied closely to the vitelline membrane of the ovum, and it
represents the membrana chalazifera, or chalaziferous layer (fig. 369A). (See
Romanoff and Romanoff, '49, pp. 137, 219.)
b) Deposition of the Middle Dense Layer of Albumen. The egg
soon leaves the infundibular area of the oviduct and enters the albumen-
secreting region where a dense layer of albuminous material, the albuminous
sac, is deposited together with mucin fibers, the albumen being enmeshed in
the latter (fig. 369A).
c) Formation of the Inner Liquid Layer of Albuminous Material
AND the Chalazae. As the egg continues its journey posteriad, it is rotated
upon the spirally arranged folds of the oviduct. This rotation twists the mucin-
like fibers in the inner portion of the dense albuminous layer, and it is believed
that this twisting motion squeezes the more fluid albumen out of the mucin
meshwork where it becomes deposited immediately around the chalaziferous
layer to form the inner liquid layer of albumen. At the same time, some of
the mucin fibers become twisted in opposite directions at the upper and lower
ends of the egg as the latter is rotated along the spiral folds of the oviduct.
These twisted fibers form a bundle at the anterior and posterior ends of the
egg and become attached firmly to the chalaziferous layer, reaching outward
into the dense albumen. These two bundles of twisted mucin fibers form the
chalazae, one chalaza being tied to the chalaziferous layer at the lower end
of the egg (i.e., the end occupying the more posterior position in the oviduct)
and the other lying attached to the chalaziferous layer at the upper end of
the egg (fig. 369A).
d) Deposition of the Outer Liquid Albuminous Layer. As a result
of the resection experiments of Asmundson and Burmester ('36), one is led
to conclude that a considerable amount of the outer, watery, albuminous layer
which comes to surround the middle dense layer of albumen is deposited in
906
CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
the anterior part (i.e., the ovarian end) of the albumen-secreting portion of
the oviduct (figs. 157; 369A). Some of the watery material is added in the
isthmus and in the uterus (Romanoff and Romanoff, '49, p. 220).
e) Formation of the Egg Membranes and Egg Shell. As the egg
reaches the isthmus, the shell membranes are formed around the albuminous
material. In the upper part of the isthmus, the thin inner membrane is formed,
while the thick, coarse, outer membrane is deposited in the posterior parts
of the isthmus. These two membranes of the egg expand considerably coin-
cident with the passage of a watery albuminous material through their mesh-
work into the outer, liquid, albuminous layer while the egg passes through
the lower part of the isthmus, and also during the first part of the egg's occu-
pancy of the uterus. As a result, the volume of the albumen is increased
rapidly and considerably in this general area.
During the latter part of the period of the egg's residence within the uterus,
calcareous concretions or mammillae are deposited upon the external face
of the coarse, outer, egg membrane (fig. 369B). Each conical concretion or
mammilla is embedded in the outer egg membrane. The broader distal end
of the mammila faces outward while the pointed proximal end is attached
to the egg membrane (fig. 369B). Small pores appear between the various
mammillae. External to the mammillary layer, a spongy layer of collagenous
fibers is formed. This spongy layer gradually becomes impregnated with cal-
cium salts which lie within the spaces between the spongy fibers and between
the mammillary and spongy layers. The calcified spongy layer and associated
mammillary concretions form the egg shell. The calcium probably is secreted
in the form of bicarbonate which later changes to calcium carbonate. Some
calcium chloride and phosphate, together with a calcium-protein substance
also are formed. The colored pigments of the egg shell in colored eggs are
OF ALBUMEN :
OUTER LIQUID
LIGAMENTUM
ALBUM
HALAZA
SPONGY LAYER ■
GG MEMBRANE
AIR CELL
»««
-■i^:«ij-S
M
AMMILLARY iffmmi^-^i^^ii^-'^''"^
LAYER ^■
SHELL MEMBRANE
EGGSHELL
Fig. 369. Structure of the hen's egg. (A and B redrawn from Romanoff and Romanoff,
1949. Wiley & Sons, Inc., N. Y. ) (A) General structure of newly laid hen's egg (after
Romanoff). (B) Detailed structure of egg shell (after von Nathusius).
FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 907
ooporphyrin pigments, derived probably from the hemoglobin of worn-out
red blood cells. A thin cuticle or protective film is applied to the surface of
the calcified spongy layer just before the egg is laid (fig. 369B).
The rate of transport of the egg through the oviduct of the hen is inter-
esting. Once the egg has entered the infundlbulum, it takes but 20 minutes
to complete its passage through this area. The infundibular region constitutes
five per cent of the length of the oviduct. In the albumen-secreting region
where it accumulates most of its albumen, the egg spends about four hours.
This segment forms about 60 per cent of the total oviducal length. The passage
through the isthmus requires approximately one hour. This region forms 15
per cent of the total length of the oviduct. The last or uterine segment is
about the same length as that of the isthmus, but the egg spends about 80
per cent of its passage time or about 19 hours in this portion. The rate of
passage, therefore, in the more anterior portion of the oviduct is rapid, some-
what slower in the isthmus, and very slow in the uterus (Romanoff and
Romanoff, '49).
3) Reptiles. Egg passage through the oviduct and deposition of the tertiary
egg membranes in reptiles probably resembles very closely that of the bird with
the exception that in a considerable number of reptiles the young develop in
the uterus and are discharged in a free-living condition (see p. 83 ). Also, the
eggs of modern reptiles have a thick leathery shell instead of the brittle cal-
careous shell of bird's eggs.
4) Amphibians. In the frog, egg transport down the glandular portion of
the oviduct appears to be effected mainly by the propelling force of the
beating cilia, possessed by certain of the cells lining the oviduct. This ciliary
action possibly is aided by some peristaltic action of the oviducal musculature.
The cilia are found on the cells which line the longitudinal ridges which run
"more or less the length of the oviduct" (Noble, '31, p. 282). As the egg
moves downward (posteriad), it is covered by mucus or similar gelatinous ma-
terial. In the common frog, Rana pipiens, three gelatinous layers are deposited
around the egg during its oviducal passage.
Passage of the egg through the oviduct in other Amphibia probably resem-
bles that of the frog.
In many Amphibia (e.g., frogs), the caudal portion of the oviduct is ex-
panded to form a special compartment, called the uterus, where the eggs
remain for a period before discharge to the outside. In some urodeles, the
eggs are retained in the oviduct, and the young are born in the larval or
fully metamorphosed state (see p. 189).
5) Fishes. Internal egg transport in fishes presents a variety of conditions.
In many teleosts, the ovary, when egg formation is completed, becomes a large
egg sac, directly connected with the short oviduct. At the time of spawning,
a general contraction of the ovarian tissues occurs, and the eggs are expelled
into the oviduct and from there to the outside. The contraction of the ovarian
908 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
tissues, together with a peristaltic behavior of the oviducal musculature, affords
the mechanism necessary to transport the eggs to the external environment.
Egg membranes are not deposited around the egg as it passes through the
oviduct in teleost fishes.
In the elasmobranch fishes, however, glandular and uterine portions of the
oviduct are present, and the large egg is transported through the upper
glandular region of the oviduct in a manner similar, presumably, to that in
the hen. Surrounding membranes of albuminous materials, and an outer chiti-
noid "shell" are produced in the glandular area. These membranes vary with
the species and some are complicated as indicated in figure 380A. In many
elasmobranch fishes and also in the so-called viviparous teleost fishes, the
egg is retained in the uterine portion of the oviduct. Here the young develop
and, when discharged to the outside, are able to fend for themselves. In these
forms, the uterus is adapted to the function of providing the embryo with
an environment suitable for its development.
In the cyclostomatous fishes, an oviduct is not present, and egg transport
resolves itself into a discharge of eggs into the coelomic cavity from which
the eggs pass through openings into the cavity of the urogenital sinus. Ovarian
membranes only are present around the cyclostome egg. These membranes
may be complex as in the hagfish, Polistotrema (Bdellostoma), (fig. 162).
2. The Extra-embryonic Membranes
The extra-embryonic membranes as indicated previously are those mem-
branes produced from the embryonic tissues. These membranes are the yolk
sac, amnion, chorion (serosa) and allantois. In a strict sense, the periderm
(see Chapter 12) probably should be included as an extra-embryonic mem-
brane for it is elaborated at the surface area of the epidermis and functions
to protect and presumably to regulate the possible entrance of substances
from the surrounding environment.
a. Yolk Sac
A yolk sac is present in all reptiles, in birds and mammals, and in those
fishes which have megalecithal eggs, that is, having a large amount of yolk
substance stored within the egg. Two types of yolk sacs are found among
the vertebrates, viz.:
( 1 ) a yolk sac whose walls are composed of entoderm, mesoderm and
ectoderm in the form of closely associated layers. This type of yolk
sac is found in the embryos of the hagfishes, Polistotrema (Bdellostoma)
stouti and Myxine glutinosa, in most elasmobranch fishes, and in
teleosts (fig. 370A). Some of the amphibia with a large quantity of yolk
in the egg such as Necturus maculosus, also approach this condition.
(2) a second type of yolk sac is found in reptiles, birds and mammals. In
these instances the wall of the yolk sac is composed mainly of ento-
FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES
909
AMNION
ALLANTOIS
SPLANCHNOPLEURE OF ALLANTOIC SAC
EXTRA- EMBRYONIC MESODERM
Fig. 370. Diagrams of extra-embryonic membranes. (A) Transverse section of yolk
sac and developing body in teleost and elasmobranch fishes showing relation of body
layers to the yolk mass. (B) Transverse section of yolk sac and forming serosa (chorion)
in reptiles, birds, and prototherian mammals. (C-E) Diagrams showing extra-embryonic
membranes in the pig. (C) Conditions in 16-17 somite pig, age approximately 16 days.
The ends of the diagram have been omitted in part, because of length of embryonic vesicle.
(D) Conditions in embryo of 5 mm. or about 17-18 days of age. (E) The extra-
embryonic membranes in embryo of about 4-5 weeks of age.
derm and mesoderm, i.e., the splanchnopleure, as the extra-embryonic
coelom tends to separate the splanchnopleure from the somatopleure
in these forms (fig. 370B).
b. Amnion
The amnion is a specialized sac which comes to encompass the embryo in
reptiles, birds and mammals (fig. 370B-E). Because of its restriction to these
vertebrates, the reptiles, birds and mammals are grouped together as the
Amniota, the fishes and amphibia being designated as the Anamniota.
Eggs which are spawned into the surrounding water, as in fishes and am-
phibia, are cradled or cushioned by the surrounding fluid, and the embryo
is free to develop without undue pressure from any side. In the Amniota,
however, this watery environment must be established artificially and hence
the amnion is formed to accommodate and enclose the fluid of this individ-
ualized embryonic "swimming" pool.
910 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
The amnion arises generally in two ways, as follows, although intermediate
forms are found among certain mammals (see Mossman, '37).
( 1 ) By a dorsal folding of the somatopleure, in which anterior, lateral,
and posterior amnionic folds project dorsad and fuse (see figures 23 8B;
242C and G; 370B-E). This method is found in reptiles, birds, pro-
totherian mammals, the opossum, pig, rabbit, etc.
(2) The second main method is by cavitation, i.e., a cavity develops within
the cells forming the inner cell mass of the early embryo (fig. 372A
and B). Found in human, mouse, rat, etc. In the monkey, the forma-
tion of the amnion is somewhat intermediate between the folding and
cavitation methods.
c. Chorion (Serosa)
The formation of the amnion by the folding method also results in the de-
velopment of the chorion or serosa, in that it separates the somatopleure from
the splanchnopleure of the yolk sac (fig. 370B and C). However, in those
forms which utilize the hollowing out or cavitation method of amnion forma-
tion as in the human, the chorion forms directly by the attachment of extra-
embryonic mesoderm to the inner aspect of the trophectoderm (fig. 372 A
and B).
d. Altantois
In most fishes and amphibia, external respiration of the developing embryo
is possible by a direct interchange of oxygen and carbon dioxide across the
perivitelline fluid and primary embryonic membranes into the surrounding
watery medium. However, in eggs which are deposited on dry land, such as
those of birds, reptiles, and prototherian mammals, a specialized embryonic
structure, the allantois, is formed to permit external respiration to occur. The
allantoic diverticulum arises as a mid-ventral outpushing of the caudal end
of the hindgut (fig. 370C). The allantois is a hollow, sac-like structure com-
posed of entoderm on the inside and splanchnopleuric mesoderm externally.
As it extends outward, blood vessels develop in the mesoderm. It eventually
comes in contact with the chorion with which it fuses to form the chorio-
allantoic membrane (fig. 370D and E). The chorio-allantoic membrane in
reptiles and birds contacts the surface membranes of the shell (fig. 299E).
In the higher mammals an allantoic diverticulum also is formed. In this
group of vertebrates, the allantois not only serves the function of external
respiration but also is the main instrument in nutrition. In the human embryo,
the entodermal evagination from the hindgut forming the allantoic diverticulum
is small, and blood vessels develop precociously within the mesoderm of the
body stalk (see figure 372B). These blood vessels course distad to the de-
veloping chorion and its villi where external respiration is accomplished.
FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES
911
However, in the pig and many other mammals, the allantoic diverticulum is a
large, spacious structure (see figure 370D and E).
Respiratory devices thus arise as diverticula from two general areas of the
vertebrate body, viz.:
(1) the pharyngeal area (see Chapter 14) and
(2) the hindgut area.
e. Yolk Stalk, Allantoic Stalk, Belly Stalk, and Umbilical Cord
As the embryo increases in size (see figures 370C-E; 372B-D), the yolk-
sac connection with the mid-gut area of the embryo becomes relatively
smaller. The constricted area of entoderm and mesoderm which connects the
yolk sac with the midgut is called the vitelline duct or yolk stalk. Similarly,
the constricted area of the allantois which connects the allantoic diverticulum
with the hindgut area is called the allantoic stalk. As the embryo continues
to enlarge, the yolk stalk and allantoic stalk are brought closer together and
their mesoderms fuse. The closely associated yolk and allantoic stalks form
the belly stalk in the area where they attach to the belly (ventral) wall of the
embryo (fig. 370E). The narrowing ring-like area between the ventral body
Fig. 371. Brood compartments for care of young. (A) Pregnant female of the lizard,
Chalcides tridactylus (Seps chalcides), showing uterine compartments containing develop-
ing eggs. (Redrawn from Needham, 1942, Biochemistry and Morphogenesis, Cambridge
University Press, London.) (B) Dorsal brood pouch in the anuran, Gastrotheca
pygmaea. (C) Dorsal brood pouch in Gastrotheca marsupiata. Observe small dorsal
opening of pouch. (D) Dissection of vocal (brood) pouch in male of Rhinoderma
darwinii. (B-D, redrawn from Noble, 1931, The Biology of the Amphibia, McGraw-
Hill, N. Y.)
912
CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
SINUSOIDS LACUNAE IN TROPHOBLAST ALLANTOIC DIVERTICULUM
\yTROPHOBLAST_ \ ^^V^^rX'l^ BODY OF EMB
Fig. 372. Extra-embryonic membranes in human embryo. (A) Diagrammatic rep-
resentation of extra-embryonic membranes in embryo of about 12 days of age, shortly
after enclosure within uterine endometrium. (Redrawn and modified from Hertig and
Rock, 1941. Carnegie Contr. to Embryology, vol. 29.) (B) Extra-embryonic membranes
in embryo of about 16 days. (C) Extra-embryonic membranes in embryo of about 28
days. (D) Extra-embryonic membranes in embryo of about 12 weeks.
wall of the embryo and the yolk and allantoic stalk tissues is a passageway
for blood vessels to and from the yolk and allantoic stalk tissues. It is called
the umbilical ring, umbilicus or omphalos. As the embryo continues to enlarge,
the amnion in the mid-ventral area of the embryo is reflected downward from
the umbilical ring or umbilicus over the yolk-stalk and allantoic-stalk tissues
and thus eventually encloses the yolk and allantoic stalks (figs. 370E; 372C
and D). This entire structural complex composed of amnionic tissue, together
with enteric and allantoic diverticula and splanchnopleuric mesoderm, is called
the umbilical cord (fig. 372D).
In the human embryo, that portion of the mesoderm which connects the
VITELLINE VEIN.
YOLK sue WA
'■.\ -r — VITELLINE ARTERY
'\\\\' ,'_ DUCT FROM YOLK SAC TO
INTESTINE
UTERINE WALL
GLAND LUMEN
ALLANTOIC ARTER
6ELLY STALK
ALLANTOIC VEIN
Fio. 373. Placental relationships. (A) Placenta of Mustelus laevis. This is a yolk-
sac placenta, and the yolk sac tissues burrow into the wall of the uterus, invading the
uterine glands. It does not erode the endometrium, however, and therefore resembles the
placental conditions in the pig, shown in fig. 373B. It is essentially an epithelio-chorial
type of placenta. (Redrawn from Needham, 1942, Biochemistry and Morphogenesis,
Cambridge University Press, London.) (B) Placental relationships in the pig. The
913
914 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
forming allantoic diverticulum with the chorionic mesoderm is called the
body stalk (fig. 372B).
3. The Reproductive Duct as a Protective Embryonic
Membrane
The developing egg is retained within the oviduct in all metatherian and
eutherian mammals and in various species in the other major vertebrate
groups with the exception of the birds. Even in the birds (fig. 157), a partial
development of the egg normally occurs within the confines of the oviduct.
Oviparity thus encroaches upon ovoviviparity in birds, and ovoviviparity in-
fringes upon viviparity in certain sharks (Squalus acanthias), reptiles (vari-
ous snakes and lizards), and prototherian mammals. However, oviparity has
this feature which distinguishes it from ovoviviparity and viviparity, namely,
the new individual always hatches or leaves the confinement of the egg mem-
branes outside the protective environment of the reproductive duct (or other
protective structures). On the other hand, ovoviviparous and viviparous forms
are released from the egg membranes and thus "hatch out" within the oviduct
or other covering structure. The more viviparous the particular species, the
sooner the new individual hatches from its egg membranes. In most cases
of ovoviviparity and viviparity, the reproductive duct (specifically, the uterine
segment) acts as a protective embryonic membrane which surrounds the de-
veloping embryo or embryos. Thus, a definite area of the reproductive duct
is temporarily allotted to the embryo. If several embryos are present, a par-
ticular segment of the uterus is assigned to the care and protection of each
embryo (see TeWinkle, '41, '43, and '50) (fig. 371 A). For further description
of the uterine portion of the oviduct as a protective mechanism see p. 919.
Fig. 373 — Continued
placenta is of the epitheliochorial variety, i.e., the epithelium of the chorionic tissue comes
into contact with the epithelium of the uterus without erosion of either. (C) Placental
relationships in the dog. This figure represents a small area at the edge of the zonary pla-
centa shown in fig. 378D, as indicated. (Redrawn and modified from Mossman, 1937,
Carnegie Institute Publications, vol. 26, Contributions to Embryology, No. 158.) This
placenta is a dual type, in that the edge of the placenta resembles somewhat the henw-
chorial type, i.e., maternal blood in direct contact with the chorionic epithelium of the
villus, while the center of the placental zone is of the endotheliochorial type of placenta-
tion, i.e., the epithelium of the chorionic villus is in contact with the endothelial lining of
the maternal blood capillaries. (D) Placental relationship in human. (Redrawn and
modified from Spanner, Zeitschrift fiir Anatomic, vol. 105, Julius Springer, Berlin, Ger-
many.) The placenta is made up of many cotyledons, each cotyledon being composed of a
main stem villus, which contains the larger fetal blood vessels, and from the large stem
villus smaller branching villi extend out into the surrounding maternal blood. Imperfectly
developed septa separate the various cotyledons. This type of placentation is of the hemo-
chorial variety, i.e. the chorionic epithelium is in contact with the maternal blood. (E)
Diagram illustrating the hemoendotlielial type of placentation in the late gestation period
of the rabbit. Here the chorionic epithelium is eroded and the capillaries of the chorionic
villi lie within the maternal blood.
FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES
915
UTERINE LUMEN
MESOMETRIUM
9 9-
UTERINE GLAND
Fig. 374. Placentation in the mouse. (A) Blastocyst within fold of the uterine
mucosa. (B) Longitudinal section of uterine site of placentation showing mesometrial
and antimesometria! aspects. (C) Later stage of conditions shown in B. Observe that
placentation of the embryo is in the antimesometrial side of the uterus. The placenta
is probably of the hemochorial relationship at first becoming hemoendothelial later as
in the rabbit. (See Mossman, '37.) (A~C, redrawn from Snell. 1941, The Early Embry-
ology of the Mouse. Blakiston, Philadelphia.)
4. Uncommon or Specialized Structures as Protective
Mechanisms
Many structures other than the oviduct are used by various vertebrate
species to accommodate and protect the developing egg. In the teleost,
Heterandria formosa, the eggs are retained within the ovary (Scrimshaw, '44).
Although a typical, teleostean, oil droplet is present in the egg which measures
0.39 mm. in diameter, it is not utilized until late in development, and most
of the nourishment is afforded by a vascular sac which partly encloses the
embryo. In the teleost, Gambusia affinis, the egg also develops in the ovarian
follicle, but, in this case, most of the nourishment is derived from yolk which
is contained within the egg. In the sea horses. Hippocampus, and in the pipe-
fishes, Syngnathus, the eggs are transferred to a pouch, formed by folds of
skin located in the ventral body wall of the male. Here the embryos develop
(fig. 106). Many teleost fish are "mouth breeders," that is, they carry the eggs
for various periods in the buccal cavity.
916 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
The amphibia show an array of protective devices for young. The mar-
supial frogs are most interesting. In Gastrotheca (Nototrema) pygmaea, the
"maternal purse," formed by cutaneous folds, spreads over the dorsal area of
the trunk, and an elongated opening in the middorsal line permits passage
into the sac (fig. 371 B). In Gastrotheca marsupiata, the opening of the dorsal
brood pouch is located in the sacral area (fig. 37 IC). The brood pouch of
Gastrotheca ovijera is similar to that of G. marsupiata (Noble, '31, pp. 60,
510). In some forms, such as G. weinlandii, the skin of the back is covered
by calcareous dermal plates and in such species Noble says the young are
"enclosed within a veritable coat of mail!" Lastly, mention may be made of
the little Chilean frog, Rhinoderma darwinii. In this instance the male frog
carries the few eggs and young, through metamorphosis, in his vocal pouches
(fig. 370D). (See Noble, 31, pp. 71 and 507.)
C. Special Adaptations of the Extra-embryonic Membranes for
Uterine Existence
1. Implantation
a. Definition
Implantation is the process whereby the embryo becomes attached to a nu-
tritional substrate. The term is applied generally to those embryos which be-
come associated intimately with the uterine wall. This is the common usage of
the term. However, it is well to point out that the embryos of teleost and elas-
mobranch fishes as well as those of reptiles, birds and prototherian mammals
become attached to the yolk substrate of the egg. Moreover, this attachment
entails the elaboration of an extra-blastular or extra-embryonic tissue (i.e., the
periblast tissue) of a syncytial nature similar to that present where embryos
attach intimately to the uterine wall in the higher mammals. Most vertebrate
embryos thus rely upon a process of implantation for nutritional support.
b. Types of Implantation
When implantation occurs in such a way that the embryo remains within
the lumen of the uterus while the extra-embryonic membranes make a super-
ficial attachment to the uterine mucosa, it is called central or superficial im-
plantation. This type of implantation is found in all cases of implantation in
lower vertebrates. In the marsupial mammals it is present in Perameles and
Dasyurus, and among the eutherian mammals in the pig, cow, rabbit, sheep,
dog, cat, etc. In the mouse and rat the early blastocyst comes to lie between
the uterine epithelial folds in an antimesometrial position. These folds soon
enclose the blastocyst almost completely (fig. 374A-C). This type of im-
plantation is called ecceutric implantation and it borders upon the complete
interstitial variety. In still other mammals, such as the guinea pig, man, chim-
panzee, the embryo burrows into the uterine mucosa below the epithelium
FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 917
and in this way becomes surrounded completely by the endometrial tissue of
the uterus. This condition is known as complete interstitial implantation
(fig. 375A-C).
2. The Placenta and Placentation
a. Definition
The process of implantation implies an interaction and attachment between
the extra-embryonic membranes and the uterine wall. This area of attachment
between maternal and embryonic tissues is called the placenta, and the word
placentation denotes the general process effecting this attachment. The word
placenta is derived from the Greek and it means a flat cake. It received this
name because the human placenta is a flat, rounded mass shaped more or
less like a pancake. The placenta may be defined as the association between
embryonic and uterine tissues for the purpose of physiological exchange of
materials. It is evident that this is a restricted definition applicable only to
uterine types of implantation.
b. Types of Embryonic Tissues Involved in Placentation
In all vertebrate embryos it is the extraembryonic somatopleure (extra-
embryonic ectoderm plus extraembryonic somatopleuric mesoderm) which
contacts the uterine mucosa during placentation. In those species which pos-
sess a yolk-sac placenta, for example, in the dogfish, Mustelus laevis, the
midgut extension of the splanchnopleure which surrounds the yolk unites with
the extraembryonic somatopleure to form the embryonic contact (fig. 373A).
On the other hand, in the chorio-allantoic placenta of the lizard, Chalcides
tridactylus, and in the chorio-allantoic placenta of all eutherian mammals, //
is the allantoic evagination of the hindgut which contacts the extraembryonic
somatopleure (called the chorion in higher vertebrata) and unites with it to
form the embryonic part of the placenta (fig. 373B). However, in all of these
instances the epithelium of the extraembryonic somatopleure makes the direct
contact with the maternal tissue. Certain exceptions to this general rule appar-
ently exist, for m the rabbit during the later stages of gestation, the epitheUum
of the chorion may disappear in certain areas, permitting exposure of the fetal
blood vessels to the maternal blood (fig. 373E).
c. Types of Placental Relationships in the Eutherian Mammals
1) Epitheliochorial Type. If the epithelium of the uterus is not destroyed,
and the embryonic tissue merely forms an intimate contact with the uterine
epithelium, the placenta is called an epitheliochorial placenta, e.g., pig (fig.
373B). Under these conditions the placental area is large and diffuse (see
figure 378A). (The placenta of the dogfish, Mustelus laevis (fig. 373 A) is
essentially of this type.)
2) Endotheliochorial Variety. If the epithelium of the uterus is eroded,
918 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
and the embryonic tissue (i.e., chorionic epithehum) comes in contact with
the endothelium of the maternal blood vessels, the attachment is called an
endotheliochorial placenta (e.g., dog, cat, and other Carnivora, figure 373C).
As the placental attachment becomes more intimate the placental area be-
comes restricted. Compare figure 378A and B with C, D and E.
3) Endotheliochorial Plus Syndesmochorial Placenta. In the Ungulata
(cows, sheep, goats) the placenta is an extensive affair similar to that of the
pig. However, the attachment between embryonic and maternal tissues occurs
in certain areas known as cotyledons (fig. 378B). In parts of these cotyledons
the association of maternal and embryonic tissue is of the endotheliochorial
variety, but in other areas of the cotyledons only the epithelium of the uterus
disappears, leaving the chorionic epithelium of the extra-embryonic tissue in
contact with the connective tissue of the uterine wall. A condition where the
chorionic epithelium makes contact with the connective tissues of the uterine
wall is called a syndesmochorial relationship.
4) Hemochorial Placenta. In the rodents, primates (including man), shrews,
moles, and bats the endothelium of the maternal blood vessels is destroyed
by the erosive activity of the embryonic tissues, and the chorionic epithelium
of the embryonic portion of the placenta comes directly in contact with the
maternal blood (fig. 373D). This type of association is known as a hemo-
chorial placenta.
5) Hemoendothelial Placenta. In the rabbit, the initial contact of the fetal
tissues with the uterine epithelium forms an epitheliochorial relationship.
Still later it becomes, after erosion of maternal tissue, a hemochorial condition,
and finally, during the latter phases of pregnancy, even the chorionic epithelium
disappears, leaving the endothelium of the embryonic blood vessels in contact
with the maternal blood (fig. 373E). This type of association is the most inti-
mate placental contact known and it is called a hemoendothelial relationship.
3. Implantation of the Human Embryo
a. Preparation for Implantation
In all cases of uterine care of the developing egg, the uterus must be pre-
pared for the event. This preparation is induced by the activities of the ovarian
hormones (see Chapter 2 and figures 53 and 59). Implantation of the embryo
occurs in the early luteal phase of the reproductive cycle when the endometrial
mucosa is in an optimum condition for the reception of the developing egg.
b. Implantation
As indicated above, p. 904, the process of egg transport down the Fallopian
tube occurs at a rate which permits the developing egg (embryo) and the
uterine tissue to prepare themselves for the implantation event. About three
to three and one-half days elapse during the passage of the egg through the
000 SPACE
M^^ R-
^'MJ>^,'.^^h^.iMk.^.^ J
Fig. 375. Implantation in human and monkey. Trophoblastic ectoderm shown in com-
plete black in the following diagrams. (A) Human about 71/2 days. Blastocyst almost
completely inside of the endometrium. (B) Human about 11 days. Blastocyst within
endometrium. Trophoblast enlarging. (C) Human about 12 days. (D) Condition of
human embryonic vesicle at about 13-15 days. Observe enormous thickening of tropho-
919
920 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
Fallopian tube. As a result, when the developing human egg reaches the uterus
it is in the early blastula (blastocyst) condition (Chap. 6). The zona pellucida
or secondary egg membrane is still intact. The blastocyst remains free within
the uterus presumably for about four days. During this period, it becomes
separated from the zona pellucida (i.e., it hatches) and the blastocoelic cavity
of the blastocyst (blastula) subsequently enlarges greatly. The implantation
site for man (and also monkeys) under normal conditions is the mid-dorsal
or mid-ventral area of the uterus (Mossman, '37). The human embryo pre-
sumably begins to implant about 7 to 8 days after fertilization (Hertig and
Rock, '45). In doing so that pole of the blastocyst which contains the devel-
oping germ disc becomes attached to the uterine epithelium. As this occurs
the uterine epithelium becomes eroded in the area of immediate contact with
the blastocyst, and the epithelial cells of the trophoblast layer of the blastocyst
increase in number. As a result, the trophoblast tissue enlarges greatly in the
contact area (fig. 375A, F and G). During this process a change occurs in
the trophoblast cells for the external cells fuse together to form a syncytium,
the so-called syntrophoblast, while the inner trophoblast cells remain cellular
and form the cytotrophoblast (fig. 376A). The syntrophoblast presumably
acts as the invading tissue. {Note: the trophoblast tissue in figures 372A and
in 375 is shown in black.) As the syntrophoblast increases in quantity it comes
to enclose irregular spaces, the trophoblastic lacunae (fig. 375B-D). Simul-
taneously localized areas of the syntrophoblast extend outward to form the
primary villi (fig. 376A). These primary villi at first lack a mesenchymal core,
but soon they become invaded by the mesoderm of the somatopleure to form
the secondary villi (figs. 372B; 376B). At about 11 days, the developing
human embryo is completely inside of the uterine wall (fig. 3758). At 12 to
15 days (fig. 375C and D), the syntrophoblast has expanded considerably
and secondary villi begin to appear around the inner portions of the tropho-
blast (figs. 375D; 376B). Meanwhile (fig. 375D), some of the endometrial
tissue close to the invading chorionic vesicle, including blood vessels, is
Fig 375 — Continued
blast tissue, the presence of trophoblastic lacunae containing endometrial residues, and
the formation of the secondary chorionic villi. (A-D, redrawn from Corner, 1944, Our-
selves Unborn, Yale University Press, New Haven, Conn.) (E) Placental relationships
at about 12 weeks. (Redrawn and modified from De Lee and Greenhill, 1943, The
Principles and Practice of Obstetrics, Saunders. Philadelphia.) (F) Early stage in
implantation of the monkey. Macaca mulatta, blastocyst about 9 days of age. (G)
Monkey blastocyst about 10 days. (H) Monkey blastocyst about 10 days. (I) Monkey
blastocyst 11 days. (J) Blastocyst of 13-day monkey embryo showing primary and
secondary implantation sites. (F-J redrawn from Wislocki and Streeter, 1938, Carnegie
Instit. Contributions to Embryology. Vol. 27, Contributions to Embryology. No. 160.)
(K) Placentae of Lasiopyga callitrichus. Observe that umbilical cord and its blood vessels
attach to the primary placental disc, while blood vessels are given off from the primary
disc to the secondary disc. (Redrawn from Wislocki. 1929. Carnegie Contributions to
Embryology, Vol. 20, Contributions to Embryology, No. 111.)
FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES
921
broken down to form liquefied areas, the embryotroph. It is possible that this
liquefied material is assimilated by the syntrophoblast and passed inward to
the developing germ disc. If this histological material thus is utilized it forms
a source of nutrition, and it may be called histotrophic nutrition.
c. Formation of the Placenta
As the developing chorionic vesicle grows within the endometrium of the
uterus, the uterine mucosa expands over the growing vesicle (fig. 377A and B ) .
That part of the endometrial tissue overlying the chorionic vesicle is called
the decidua capsularis (fig. 377A), and the portion of the endometrial
lining of the uterus not concerned with the enclosure of the chorionic vesicle
is called the decidua vera or decidua parietalis. The part of the endometrium
lying between the muscle tissue of the uterine wall and the enlarging villi
(fig. 372C and D) of the chorionic vesicle is the decidua basalis (fig. 377A).
At first chorionic villi are developed over the entire chorionic vesicle (fig.
372B), but as development goes on the villi in relation to the decidua parietalis
are resorbed gradually to form a smooth area of the chorion, the chorion laeve
(fig. 372D). Finally, only those villi in relation to the decidua basalis remain
(fig. 372D). The villi within the decidua basalis enlarge and become the
SYNTROPHOBLAST
TROPHOBLASTIC
LACUNAE
CYTOTROPHOBLAST
EXTRA - EMBRYONIC
MESODERM
MESENCHYMAL CELLS
BRUSH BORDER
CYTOTROPHOBLAST
(LANGHAN'S LAYER)
SYNTROPHOBLAST --j^, 0^ / ^_,-i, <«; ^-«> ;^»
„, -BLOOD VESSEL ■* -^ \ V r^ ^^r""^ -^^ --^^'^t.
HOFBAUER CELL— S ^ ^ .^ ) >^,» 5u^« 8 -
CELLS OF
REGRESSING
CYTOTROPHOBLAST
C LAYER
CAPILLARY
Fig. 376. Structure of villi in human chorionic vesicle. (A) Primary villus. (B)
Secondary villus. (C) Villus from chorion at about 4 weeks. (D) Villus at about
14 weeks. Observe gradual disappearance of cytotrophoblast.
922 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
main villi for physiological interchange of materials between the embryo and
the maternal tissues. This portion of the chorionic vesicle with the enlarged
chorionic villi is known as the chorion frondosum (fig. 372D). The villi of
the chorion frondosum and the tissue of the decidua basalis together form
the placenta. The embryonic mesodermal tissues of the placenta are continuous
with mesoderm of the umbilical cord, and the embryonic blood vessels of
the placenta are directly continuous with the blood vessels of the umbilical
cord (fig. 372D). The placental area thus is a dual structure composed of
the decidua basalis or maternal placenta (placenta materna) and the chorion
frondosum or fetal placenta (placenta fetalis) (fig. 375E). The placental
area gradually expands during the early months of pregnancy until at about
the fifth month when it reaches its greatest relative size or about one-half the
internal aspect of the uterus.
The early chorionic villi of about the fourth week of pregnancy are com-
posed of four constituent parts, viz.:
( 1 ) blood capillaries which course within
(2) the mesenchymal cells of the mesodermal core. Surrounding the in-
ternal core of mesenchyme is the trophectodermal layer composed
of an inner
(3) cytotrophoblast, which is surrounded externally by the
(4) syntrophoblast (fig. 376C and D).
As development proceeds, the central core of mesenchyme with its blood
capillaries increases in size, and the cytotrophoblast layer of the trophectoderm
decreases in quantity, until, at about the fourth month, little remains of the
cytotrophoblast layer with the exception of a few scattered cells below the
syntrophoblast (fig. 376D).
The placental villi are grouped together into groups known as cotyledons.
Between the cotyledons are the placental septa, which incompletely separate
the various cotyledons from each other. The origin of the placental septa is
uncertain, possibly being contributed to by both embryonic and maternal
tissues. Surrounding the villi within each cotyledon is a pool of maternal blood
which bathes the surfaces of the syntrophoblast of the villi. A hemochorial
relationship is in this way established (fig. 373D).
4. Implantation in the Rhesus Monkey, Macaco mulatta
The various stages of implantation and placental formation of the rhesus
monkey are shown in figure 375F-K. It is to be observed that the monkey
develops a primary placenta (fig. 375H and I) which later is supplemented
by another placenta, the secondary placenta, attached to the opposite uterine
wall (fig. 375J and K). Also, the embryo of the rhesus monkey, unlike the
human embryo, does not bury itself within the uterine mucosa, and the cho-
FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES
923
PL4CENTA
Fig. 377. Human placentation. (A) Condition at about 4 weeiis. (B) About six
weeks. Villi disappearing on one side, while those of chorion frondosum are enlarging.
(A and B redrawn from Corner, 1944, Ourselves Unborn, Yale University Press, New
Haven, Conn.) (C) Placental relationships in dizygotic (i.e. two fertilized eggs) twins
implanted close together. Observe two chorionic vesicles, and two placentae. (D) Pla-
cental relationships in monozygotic (one fertilized egg) twins. Observe one chorionic
vesicle, two amnions, and one placenta. (C and D redrawn from Dodds, 1938, The Essen-
tials of Human Embryology, John Wiley & Sons, New York.)
rionic vesicle remains within the lumen of the uterus (see Wislocki and
Streeter, 38).
5. Implantation of the Pig Embryo
As in the human the passage of the cleaving egg of the pig through the
Fallopian tube is slow, consuming about 2V2 days. When the egg reaches the
uterus it still is surrounded by the zona pellucida and developmentally is in an
advanced state of cleavage or early blastocyst formation (fig. 145H). It re-
mains free in the uterine horn for about 6 to 7 days. During this period the
blastocyst enlarges and elongates at a rapid pace, particularly during the sixth
924
CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
and seventh days of uterine existence (i.e., 9 and 10 days after copulation)
(fig. 145I-L). The blastocyst eventually forms a much elongated attenuated
structure about 1 meter long. During the earlier portion of the free uterine
period the many blastocysts of the ordinary conceptual process in the sow
become spaced within the horn of each uterus, an intriguing process which
continues to remain baffling. From 10 to 13 days after copulation the blasto-
cysts experience the gastrulation processes (see figure 145M-R; and figures
208 and 209); from days 13 to 15 body form is developed gradually (fig.
242A-F) and the amnion and chorion are formed (fig. 242G).
From days 14 to 17 the allantoic diverticulum grows rapidly (figs. 242G;
370C-D). At this time the chorionic vesicle as a whole shortens and becomes
much larger in transverse section. The yolk sac of the embryo of 16 to 17
days is greatly enlarged in relation to the size of the embryo, and the entoderm
at its distal end lies closely apposed against the chorionic ectoderm (figs.
242F; 370C). As the allantoic cavity expands, the yolk sac, relatively speak-
ing, contracts, and a relationship is established similar to that in figure 370D.
As the allantois expands its mesoderm comes in contact with the mesoderm
of the chorionic membrane and fuses with it (fig. 370E). This new layer
forms the chorio-allantoic membrane. The chorio-allantoic membrane becomes
folded into elongated folds which fit into similar folds of the uterine mucosa.
A relationship thereby is formed as shown in figure 373B.
COTYLEDONS
DOG, CAT, SEAL, GENET
Fig. 378. External appearance of chorionic vesicles in various mammals. (A) Pig.
This placental type is called diffuse. (A') Enlarged drawing of small cotyledon or
areola. (B) Cow. Observe large cotyledons. This type of placenta is called cotyledonary.
(C) Brown bear. Special zonary placenta. (D) Dog, etc. Zonary placenta. (E) Rac-
coon, incomplete zonary placenta. (A, B-E, redrawn and modified from Hamilton,
Boyd and Mossman, 1947, Human Embryology, Williams and Wilkins, Baltimore.)
FUNCTIONS OF THE PLACENTA 925
In certain areas of the chorion, speciaUzed structures or areolae, containing
small villi, appear to slightly invade the uterine glands (fig. 378A'). How-
ever, the epithelium is not destroyed, and at all times the maternal and fetal
aspects of the greatly expanded placental area (see figure 378 A) may be
separated without injury either to the chorionic or to the uterine epithelium.
6. Fate of the Embryonic Membranes
a. Yolk Sac
The yolk sac of teleost and elasmobranch fishes is withdrawn gradually
toward the ventral body wall and intestine. The contribution of the yolk sac
differs considerably in the two groups. In the teleost fishes, the somatopleuric
portion of the yolk sac contributes much to the body wall while the splanch-
nopleuric tissues of the yolk sac form a considerable part of the latero-ventral
region of the intestine. In the elasmobranch fishes, the somatopleuric layer of
the yolk sac forms only a small area of the ventral body wall in the anterior
trunk region, and the splanchnopleuric tissue of the yolk sac is withdrawn
inward toward the duodenal area. This withdrawal of the splanchnopleuric
tissue is a complex affair, for as the external yolk sac is withdrawn an internal
yolk sac is developed as an evagination from the yolk stalk (vitelline duct)
near the duodenum (fig. 296A). While the external yolk sac gets smaller
the internal yolk sac increases in size, and after the external yolk sac has
been entirely withdrawn a considerable part of the internal yolk sac remains.
Ultimately the splanchnopleure of the internal yolk sac forms a small area
of the duodenal wall.
In the chick the yolk sac is still large as hatching approaches. During the
eighteenth and nineteenth days the yolk sac containing a considerable amount
of yolk is withdrawn into the body cavity through the umbilicus. Here the
yolk is absorbed rapidly and the yolk sac tissues are taken up into the wall
of the intestine about 5 or 6 days after hatching.
The yolk sac of the higher mammals does not contain yolk substance. One
of its main functions is the formation of the first blood cells (see Chapter 17).
The yolk stalk and yolk sac increase somewhat in size during the early phases
of development. Ultimately the yolk stalk becomes greatly elongated and
separates from the yolk sac. The proximal portion of the yolk stalk is taken
up into the wall of the intestine. In the human embryo, the area of yolk stalk
inclusion into the intestinal wall is about 18 to 24 inches proximal to the
ilio-caecal area.
b. Amnion and Allantois
The amnion and allantois of the Amniota function until birth. During par-
turition the amnion generally ruptures, but may remain intact around the
offspring. For example, in a litter of six puppies, half of the amnions may
be ruptured and half may be intact. The intact amnion must then be ruptured
926 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
or the puppy will suffocate. In the human the after-birth consists of the
following:
(a) the maternal membranes — decidua vera, decidua basalis, and decidua
capsularis (vera), and
(b) the fetal membranes — chorion frondosum, chorion laeve, amnion, yolk
sac, allantois, and umbilical cord.
D. Functions of the Placenta
The functions of the placenta are many, and the more intimate the contact
with the maternal tissue the functions appear to increase. The various func-
tions of the placenta may be listed as follows:
( 1 ) Food materials pass from the maternal blood stream to the blood
stream of the embryo.
(2) Waste materials pass from the embryo's circulatory system to the
blood stream of the mother.
(3) Serves as the external respiratory mechanism for the embryo.
(4) It functions to elaborate two ovarian hormones, estrogen and proges-
terone (see Chapter 2) together with chorionic follicle-stimulating and
luteinizing hormones. The production of estrogen and progesterone
helps maintain pregnancy (see Chapter 2) and at the same time
brings about the development of the mammary glands.
(5) The placenta and after-birth tissues form a source of nourishment to
the female of many mammals, for it is generally eaten by the mother.
E. Tests for Pregnancy
The elaboration of chorionic follicle-stimulating and luteinizing hormones
by the placenta in increasing amounts during the first part of pregnancy and
their excretion by the kidneys makes possible certain tests for the detection
of pregnancy (see Engle, '39).
1. ASCHHEIM-ZONDEK TeST
Aschheim and Zondek were the first investigators to detect gonad-stimu-
lating principles in the urine of pregnant women. The excretion of these sub-
stances in pregnancy urine begins during the second week, about the fifteenth
day, rises sharply to the thirtieth day and then gradually falls to the ninetieth
day (Siegler and Fein, '39). This secretion probably is elaborated by the
trophoblast of the developing chorion during the second week of pregnancy
and later by the epithelium of the chorionic portion of the placenta. The pres-
ence of these gonad-stimulating substances in the urine provokes reproductive
changes in the ovaries of common laboratory animals when injected with the
urine. Aschheim and Zondek were the first to use this method for detecting
pregnancy. The method consists of the injection of small amounts of preg-
TESTS FOR PREGNANCY 927
nancy urine into mice and rats and, later, observing the appearance of hemor-
rhagic conditions of the foHicles within the ovaries. A modification of the
Aschheim-Zondek or A-Z test used by Kupperman, Greenblatt, and Noback,
'43, consists of the injection of 1.5 cc. of a morning sample of urine into the
lower portion of the abdomen of immature rats. The animal is killed with
ether after two hours and pronounced hyperemic conditions of the ovary are
observed as a positive test.
2. Friedman Modification of the Aschheim-Zondek Test
In this test 10 cc. of the suspected urine is injected into the marginal vein
of the rabbit's ear. In about 12 to 24 hours a positive test is denoted by ovula-
tion points (blood points) on the ovarian surface and by hemorrhagic con-
ditions within the follicles. This test is as accurate as the original A-Z test
and works in almost 98 to 99 per cent of the cases.
3. Toad Test
When the "clawed toad" of South Africa, Xenopus laevis, is injected with
pregnancy urine, the animal ovulates within a few hours and the eggs are
easily detected.
4. Frog Test
Wiltberger and Miller, '48, advocate the following test. Five cc. of a first
morning (overnight) sample of urine is carefully injected subcutaneously into
the dorsal or lateral lymph sacs of a male frog. Two or more frogs are used.
Each frog is then placed in a clean, dry, glass jar with perforated lid. After
2 to 4 hours at ordinary room temperature, any urine that is voided by the
frogs is examined microscopically. If urine is not present, the frog is seized
by the hand while still in the jar. This treatment usually results in urination.
Sperm in the urine denotes a positive test.
F. The Developing Circulatory System in Relation to Nutrition, etc.
All of the developing systems undergo gradual alterations which are inte-
grated with, and contribute to, the ever-changing demands involved in the
welfare of the embryo. However, the circulatory system is the one system
which must assume the burden of transport of food materials, oxygen, and
water to the developing systems. Synchronously it transports deleterious sub-
stances to the areas of elimination. While assuming this burden it also must
evolve its own development to bring about the structure of the adult form of
the circulatory system.
A striking example of the dual burden carried by the developing circulatory
system is presented in the changes which go on a short time before and after
birth (mammals) or hatching (reptiles and birds). The placental area in mam-
mals and the chorio-allantoic structures in reptiles and birds act as respiratory
928
CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
SUPERIOR VENA CAVA
RIGHT LUNG
LEFT LUNG
LIGAMENTUM
ARTERl
DUCTUS
ARTERl
RIGHT AT
RIGHT VENTRICLE
AORTA
LIGAMENTUM TERES
PORTAL VEIN
URACHUS
INFERIOR VENA CAVA
UMBILICAL A
Fig. 379. Diagrams of probable fetal and postpartum circulations through the heart
in the mammal. (A) Fetal circulation. Oxygenated blood passes through umbilical vein,
to liver. Passing through the liver by means of the ductus venosus it gathers blood from
the liver veins and empties into the inferior vena cava through the hepatic vein. Within
the inferior vena cava it mixes with non-oxygenated blood from the posterior part of
the body. Reaching the right atrium it passes across the atrium through the foramen
ovale and into the left atrium and from thence into left ventricle. The blood from the
superior vena cava crosses to one side of the blood current from the inferior vena cava
in the right atrium on its way to right ventricle. Most of the blood from the right ventricle
courses through the ductus arteriosus into the descending aorta. A small amount goes to
the lungs via the pulmonary arteries. (B) Circulation after birth. Observe there is no
passage of blood from the right atrium into the left atrium. The blood in the left atrium
is returning oxygenated blood from the lungs. The ductus arteriosus has atrophied. (See
text.) (A redrawn and modified from Windle, 1940, Physiology of the Fetus, Saunders,
Philadelphia. B adapted from A.)
and excretory regions before birth and hatching. The circulatory system
therefore must accommodate these areas in the fulfillment of the respiratory
and excretory functions. However, at the same time the developing heart and
immediate blood vessels in relation to the heart also must look forward, as
it were, to the requirements of the period after birth (mammals) or after
hatching (reptiles and birds). A diagram of the circulation of the blood
through the heart previous to birth in the mammalian heart is shown in figure
379A, and figure 379B delineates the pathway of the blood after birth.
Before birth the valve-like arrangement of the interatrial septa, I and II, per-
mits the oxygenated blood from the placenta to flow from the right atrium
TESTS FOR PREGNANCY
929
into the left atrium. From the left atrium the blood passes into the left ven-
tricle and from thence out through the aortic root to supply heart tissues,
head region and systemic structures in general. On the other hand, the blood
from the superior vena cava flows through the right atrium to the right ven-
tricle, and from there it is propelled out into the proximal portion of the
pulmonary artery, and through the ductus arteriosus (Botalli) (left sixth aortal
arch) to the systemic aorta. The unaerated blood from the right ventricle
Fig. 380. Care of young. (A) Egg capsule of Scyllium canicula (dogfish) contain-
ing developing egg fastened on seaweed by means of terminal tendrils. (Redrawn from
Kyle. 1926, The Biology of Fishes, Sidgwick and Jackson, Ltd.. London, after Varges.
(B) Male bowfin (Amia) guarding young. (Redrawn from Dean, 1896, Quart. J. Micros.
Sci. 38.) (C) Scarlet tanager feeding young. (Redrawn from photo by A. A. Allen, in
Pennsylvania Bird-Life, pub. by Pa. Game Commission, Harrisburg, Pa.) (D) Opos-
sum, suckling young. (E) Female hedgehog (Erinaceus eiiropetis) guarding young. (Re-
drawn from Figuier. 1870, Mammalia. D. Appleton and Co., New York.)
930 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG
in this way is mixed with aerated blood within the descending aorta. Some
circulation to and from the capillary bed within the lungs also occurs at
this period.
At birth and after, the change in the place of oxygenation of the blood
from the placental area to the lungs with the stoppage of the blood flow
through the umbilical vessels, necessitates the changes shown in figure 379B.
The closure, normally, of the foramen ovale in interatrial septum II, together
with the shrinkage of the ductus arteriosus to form the ligamentum arteriosum
accommodates this change in direction of blood flow. The alterations which
effect the stoppage of blood flow through the foramen ovale and ductus arte-
riosus are functional and they actually precede the morphological closure
changes. The foramen ovale is functionally closed by the apposition of Septum
I and Septum II. This apposition is effected by the equalization of the blood
pressures in the right and left atria. However, the structural closure of the
foramen ovale is produced by the growing together and gradual fusion of
the two interatrial septa. The process is variable in different human individuals,
and failure to attain complete structural closure of the foramen ovale occurs
in about 20 to 25 per cent of the cases. Functionally, this failure to close
may not be noticeable. On the other hand, in the heart of the kitten, failure
to develop a complete morphological closure by 6 to 8 weeks after birth
is rare.
The morphological closure of the ductus arteriosus also is gradual. This
does not interfere with the relative normal functioning of the lungs for the
opening up of the capillary bed within the lungs together with the concomitant
voluminous flow of blood through the pulmonary arteries to the lungs, asso-
ciated with the pressure exerted at the distal end of the ductus arteriosus by
the blood within the descending aorta, aids the functional closure of the
ductus arteriosus. In some individuals, the ductus arteriosus may remain
open, to some degree, even in the adult.
G. Post-hatching and Post-partum Care of the Young (fig. 380)
Although care of the young after hatching or after birth is beyond the
province of this work, it should be observed that such care is characteristic
of birds and mammals, and is present in certain instances in fishes and am-
phibia (fig. 380B). In the marsupial mammals, the early post-partum care
of the young in the marsupial pouch of the mother is closely related to the
pre-hatching or pre-partum care of the young in other animal groups. In the
opossum, for example (fig. 380D), the utterly helpless young are firmly at-
tached to the nipples of the mother for about 50 days (McCrady, '38). This
attachment in reality constitutes a kind of "oral placenta." From this view-
point, the care of the developing embryo in marsupial mammals may be
divided into two phases, namely, a uterine phase and an early post-partum
POST HATCHING AND POST-PARTUM CARE OF YOUNG
931
phase. The first phase in the North American opossum consumes about 13
days, and the latter about 50 days. After the young become free from their
nipple attachment they spend about 40 days in and out of the marsupium.
Bibliography
Anderson. D. H. 1927. The rate of pas-
sage of the mammalian ovum through
various portions of the Fallopian tube.
Am. J. Physiol. 82:557.
Asmundson, V. S. and Burmester, B. R.
1936. The secretory activity of the parts
of the hen's oviduct. J. Exper. Zool.
72:225.
Eigenmann, C. H. 1892. On the viviparous
fishes of the Pacific coast of North
America. Bull. U. S. Fish Commission,
Vol. 12:381.
Engle, E. T. 1939. Gonadotropic sub-
stances of blood, urine, and other body
fluids. Chapter 18, Sex and Internal Se-
cretions. Ed. by Allen, et al., Williams
& Wilkins, Baltimore.
Flynn, T. T. and Hill, J. P. 1939. Part
IV: Growth of the ovarian ovum, mat-
uration, fertilization, and early cleavage.
Trans. Zool. Soc. London. 24 Part 6:445.
Hertig, A. T. and Rock, J. 1945. Two
human ova of the pre-villous stage, hav-
ing a developmental age of about seven
and nine days respectively. Carnegie Inst.
Publ. No. 557. Contrib. to Embryol.
31:65.
Hisaw, F. L. and Albert, A. 1947. Obser-
vations on reproduction of the spiny
dogfish, Squaliis accmthias. Biol. Bull.
92:187.
Kerr, J. G. 1919. Textbook of Embryol-
ogy, Vol. II, The Macmillan Co., London.
Kupperman, H. S., Greenblatt, R. B., and
Noback, C. R. 1943. A two- and six-
hour pregnancy test. J. Clinical Endo-
crinol. 3:548.
Lewis, W. H. and Wright. E. S. 1935. On
the early development of the mouse egg.
Carnegie Inst. Publ. Contrib. to Embryol.
25:113.
McCrady, E., Jr. 1938. The embryology of
the opossum. The American Anatomical
Memoirs, No. 16. The Wistar Institute
of Anatomy and Biology. Philadelphia.
Mossman, H. W. 1937. Comparative mor-
phogenesis of the fetal membranes and
accessory uterine structures. Carnegie
Inst. Publ. No. 479. Contrib. to Embryol.
26:129.
Noble, G. K. 1931. The Biology of the
Amphibia. McGraw-Hill Book Co., New
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Romanofi', A. L. and Romanoff", A. J.
1949. The Avian Egg. John Wiley and
Sons, New York.
Scrimshaw, N. S. 1944. Embryonic growth
in the viviparous poeliciid, Heterandria
fonnosa. Biol. Bull. 87:37.
Siegler, S. L. and Fein, M. J. 1939. Studies
in artificial ovulation with the hormone
of pregnant mares' serum. Am. J. Obst.
& Gynec. 38:1021.
TeWinkle, L. E. 1941. Structures con-
cerned with yolk absorption in Squalus
acanthias. Biol. Bull. 81:292.
. 1943. Observations on later phases
of embryonic nutrition in Squalus acan-
thias. J. Morph. 73:177.
1950. Notes on ovulation, ova, and
early development in the smooth dog-
fish, Mustelus canis. Biol. Bull. 99:474.
Wiltberger. P. B. and Miller, D. F. 1948.
The male frog, Rana pipiens, as a new
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107:198.
InJ
ex
Page references in italic type refer to illustrations in text; those followed by t refer
to tables.
A-Z test, for pregnancy, 927
Abomasum, of cow or sheep, 623
Acetylocholine, 838
Acids, role of in egg activation, 218
Acipenser, reproductive and urinary ducts
of, 799
A. julvescens, xvii
A. sturio, cleavage in, 308, 309
Acoustico-lateral system, 813
Acraniata, xv
Acroblast, 126, 150
in guinea pig sperm, 150
in human sperm, 150
multiple, in grasshopper, 750
Acrodont tooth, 607
Acrosome, 148
formation of, 150
role of in egg-sperm contact, 232
shapes and positions of, 143
Activation, of egg {see also Egg, activa-
tion of)
acids in, 217
artificial, 217
in Arbacia, 218
role of temperature in, 219
by strichnine, 218
complete, 212
fertilizin in, 225
in hypertonic sea water, 217, 218
membrane formation during, 218
partial. 212, 217
of sperm, during fertilization, 212
Addison, Thomas, 882
Addison's disease, 882
Adeimann, H. B., 716
Adipose glands, 665
Adipose tissue, 658
formation of, 664
Adrenal glands, 882
differentiation of, 883
role of secretions of in color change,
594
structure of, 882
Adrenalin (Adrenaline), 838
Adrenaline (Adrenalin), 882
Aerial adaptations, of muscles, 708
After-birth, in human, 926
Aftershaft, of feather, 571, 578
Agglutination, of egg during fertilization,
and sperm secretions, 230
of sperm, 225
Agglutinin factor, 225
Air-bladder evagination, of pharynx, dor-
sal, 601
dorso-lateral, 601
Air sacs, cellular composition of, 649
formation of in chick embryo, 646
in bird group, 644
interclavicular, 647
intermediate, anterior, 647
posterior, 647
Alar plate, 820
Albumen, deposition of in birds, middle
dense layer of, 905
outer liquid layer of, 905
Albumen-secreting region, in oviduct of
bird, 905
Albuminous layer, of rabbit and opossum
eggs. 303
Albuminous sac. of bird's egg, 905
Allanson, M., 46
Allantoic stalk, 911
Allantoic veins {see Veins, allantoic)
Allantois, 902, 908, 910
fate of, 925
Allen, B. M., 883, 884
Alligator, ventricles of heart of. 704
Alligator mississippiensis, xviii
egg. characteristics of, 204t
fertilization, site of, 204t
sperm entrance into egg, place of. 204t
Allophores, 591
Alveolar ducts, 650
Alveolar sacs, 650
Alveolus (Alveoli), of lung, structure of,
650
of jaw, 6 1 1
Ambystoma, effect of primordial germ cells
on gonad maturation in, 121
fate of pre-chordal plate mesoderm in,
525
pineal organ of, 881
spermatheca of, 190
Ambystoma maculatum, equatorial plane
of cleavage in, 283
933
934
Amby stoma maculatum (punctalum), xvii
cleavage in, 305
early, 306
Ambystoma mexicanum, xvii
gills of, 639
Ambystoma punctatiim, limb-bud field in,
509
neural crest cells in, 464
Ambystoma tigrinum, sex reversal in, 890
Ameiiirus, facial nerve in, 825
Ameloblasts, 563, 611
Amia (bowfin), egg of, 309
egg membranes of, 310
male guarding young, 929
male reproductive duct in, 18
vertebrae formation in, 685
Amia calva, bowfin, xvii
air bladder of, 643
cleavage in, 309, 310
egg of, characteristics of, 203t
fertilization, site of, 203t
gastrulation in, 444
neurocranium, developmental stages of,
670
sperm entrance into egg, place of, 203t
Amitosis, 281
Amnion, 902, 908, 909
fate of, 925
formation of, 910
Amniota, 902
explanation of term, 909
heart, development of, 753
paired appendage rudiments in, 508
Amphiarthrosis, 695
Amphiaster, of first cleavage, 247, 259
influence on polyspermy, 286
Amphibia, xvii
anuran, definitive body form assumption
in, 884
gastrula of, beginning, 355
metamorphosis of, role of thyroid and
pituitary glands in, 883
organ-forming area relationships at
end of gastrulation in, 460
coelomic changes in, 867
epidermal tube development in, 474
gastrula of, ectodermal potencies of, 414
gastrulation in, 406
movements of parts of blastula during,
411
gill resorption in, 642
gymnophiona, blastulae of, 370
cleavage in, 312
gastrulation in, 444
intromittent organ of, 192
independent pericardial wall develop-
ment in, 871
myotome differentiation in, 714
A mphibia — (Continued)
skin of, characteristics of, 565
vertebrae development in, 685
vitelline membrane in, 167
zona radiata in, 167
Amphioxus, xv
anus formation in, 502
blastopore closure in, 501
blastula of, 356
organization of, 342
cleavage in, 288, 290, 342
egg of, bilateral symmetry of, 162
egg membranes of, 165
emboly in, 402
epiboly in, 403
epidermal overgrowth of neural plate in,
502
fertilization of, 247
fertilization of egg in, 227
gastrula, antero-posterior extension of, 403
gastrulation in, 402, 404
resume of, 405
germinal vesicle in, 136
importance of end-bud growth in, 494
maturation of egg in, 227
mesoderm position in embryo of, 494
mouth formation in, 502
notochord position in embryo of, 494
organ-forming areas in egg of, 161
presumptive organ-forming areas of, 353
production of twin embryos through iso-
lation of first two blastomeres in,
335
skin of, origin of dermal component of,
559
somite differentiation in, 504
stages of development of, 503
stomach area of, 621
tubulation in, 494
of entodermal area, 500
of mesoderm, 503
Amphioxus (Branchiostoma) lanceolatum,
egg, characteristics of, 203t
fertilization, site of, 203t
sperm entrance into egg, place of, 2031
Amphiuma, external gills of, 639
Amphiuma means, xvii
Amplexus, in frog, 201
in toad, 185
Ampulla (Ampullae), 843, 854
of vas deferens, 20
Anal membrane, 600
Anal opening, of mammals, 715
Analogy, 551
Anamniota, explanation of term, 909
rudiments of paired appendages in. 508
Ancel, P.. 249
Anderson, D. H., 904
INDEX
935
Andrenosterone, 27
Andrews, F. N., 195
Androgamic substances (androgamones),
228
Androgen(s), 21
site of, 27
synthesis of, 27
Androgenesis, 263
Androgenic zone, of adrenal gland, 883
Andromerogony, 265
Androsterone, 21-22
Anestrus, definition of, 94
Angioblast theory of primitive blood vessel
development, 730, 731
Animal breeding, and sperm survival, 198
Animal pole of egg of higher mammals,
determination of, 299
Animalculists, 56
Anisotropic substance, definition of, 703
Ankylosis, 695
during development of skull bones, 695
of frontal bones, 695
Anolis carolinensis, xviii
Anseriformes, xix
Anticoagulating substance of blood in cy-
clostomatous fish, 617
Antifertilizin, 225, 227
sperm, 230
Antilocapra americana, pronghorn ante-
lope, horns of, 586
Antipernicious-anemia factor, 875
Antlers, loss of "velvet," 28
shedding of, 28
Antrum, of mature Graafian follicle, 71,
74
Anura, xviii {see also Amphibia, anuran)
Anus, formation of, in Amphioxus embryo,
502
Aorta(ae), dorsal, 727
branches of, 764
development of, 760
lateral arterial, 762
ventral arterial, 762
Aortal arch (arches), afferent system of,
760
efferent system of, 760
mandibular, of chick embryo, 743
modifications of, during embryonic de-
velopment, 759
in various vertebrates, 760, 762
within visceral arches, 619
Apical body {see Acrosome)
Apis mellifica, natural parthenogenesis in,
216
Aponeurosis, pulmonary, of chick, 872
Appendages, bilateral, of long bones, de-
velopment of, 692
paired, 690
Appendages — (Continued)
bi\a\QT?L\—{C ontinued)
paired — (Continued)
appendicular skeleton, development
of, 690
development of, early, 508
development of musculature of, 718
types of, 691
median unpaired, 690
Apteria, 573
Apteryx, kiwi, xix
Aqueduct of Sylvius, of brain, 823
Arachnoid layer, 821
Arbacia, sea urchin, artificial activation of
egg in, 218
egg water of, 225
fertilization membrane separation in,
237
Arbacia pustulosa, fertilizin of, 225
Archaeornithes, xix
Arcualia, 682
Area opaca, of chick blastoderm, 357
Area pellucida, of chick blastoderm, 357
Areolae, 925
Aristotle, 905
Armadillo, dermal bony armor of, 697
polyembryony in, 383, 384
Aronson, L. R., 59
Arteriovenous system, 726
basic plan of, 726
development of, 726
Artery(ies), carotid, internal, 727, 760
central, of retina, 846
coeliac, 762
development of, 732
genital, 762
intersegmental, dorsal, 760
lateral, 762
lumbar, 760
Arthrosis, 695
Artificial parthenogenesis {see Partheno-
genesis, artificial)
Artiodactyla, xx
Ascaphus, 4
Ascaphus truei, xviii
intromittent organ in, 790
sperm transport by means of cloacal tail,
189
Ascaris, cell lineage in, 331
chromatin diminution of, 116
fertilization, stages of, 265
separation of fertilization membrane in,
237
Ascaris megalocephala, continuity of Keim-
bahn in, 1 17
egg maturation in, 214
fertilization in, 214
Aschheim, S., 40, 926
936
Aschheim-Zondek test, for pregnancy, 926
Asdell, S. A., 19, 103, 197
Asmundson, V. S., 905
Asterias forbesi, artificial mechanical acti.-
vation of, 219
fertilization cone in, 243
Astylosternits robust us, respiratory villosi-
ties of, 636
Asymmetron macricaudatum, xv
Asynchronism, in cleavage of frog's egg,
295
Atrioventricular node, 754, 161
Atrioventricular valves, of chick heart, 756
Atrium, of developing heart, 747, 750
fate of in various vertebrates, 758
Auditory meatus, external, 851
Autonomic nervous system, 823
definition of, 834
development of, 834
divisions of, 836
ganglia of, 837
origin of, 837
gray and white rami of, 826
origin of, 834
Autonomous potency {see Potency, au-
tonomous)
Autonomous theory of gastrulative move-
ments, 447
Auxiliary cells, of blastula, 341
Auxiliary glands, 4
Auxocyte, definition of, 125
Aves, xix (see also Bird)
Axial filament, formation of, 152
Axiation, of specific organ-forming terri-
tories, a main function of gastru-
lation, 447
Axis cylinder, 814
Axon, 814
B
Baitsell, G. A., 664
Baker, J. R., 47
Balfour, F. M., 536, 716
Barb(s), of feather, 571
manner of formation of, 576
Barbicels, of feather, 573, 576
Barbule(s), of feather, 573, 576
formation of, 576
Bartelmez, G. W., 733
Bartholin, glands of, 802
Basal plate area of primitive skull, 671
Basalia, 685
Basement membrane, of mature Graafian
follicle, 71, 74
Basidorsals, of arcualia, 682, 685
Basihyal element, 673
Basioccipital center of ossification, 695
Basisphenoid portion of sphenoid bone, 696
Basiventrals, of arcualia, 682, 685
Bat, Myotis, descent of testes in, 7
sperm production in, 30
spermatogenesis in, 23
Bataillon, E., theory of fertilization of, 268
Bataillon, method for producing artificial
parthenogenesis, 219
Bathygobius soporator (gobiid fish), de-
velopment of, 250
egg membrane expansion at fertilization,
235
micropyle in egg of, 249
Balrachoseps, red blood cell nuclei of, 735
Batson, O. V., 837
Beams, H. W., 150
Belly stalk, 911
Bending, of body of vertebrate embryos,
511
Benoit, J., 120
Bhattachorya, 167
Bidder's organ, 889
Bilateral symmetry, of egg, 161, 162
Bile, 875
Bile duct, common, 627
Biogenetic principle of recapitulation, 347
Biophor, 334
Bipotential condition, of early gonad, 792
Bird(s), blastoderm of, hypoblast (ento-
derm) origin in, 358
blastula in, mature, 357
coelomic changes in, 867
copulatory organ of, 191
developmental stages of neurocranium
in, 673
egg of, albuminous sac of, 905
chalaziferous layer, formation of, 905
egg membranes of, tertiary, 905
female reproduction of, 192
follicle and periphery of oocyte of, 156
lung, position of in body, 649
oviparity and ovoviviparity in, 914
pectoral girdle of, 689
skin, characteristics of, 571
development of, 573
tongue of, copula of, 604
Bischoff, 213
Bisexual species, 886
Bishop, D. W., 27
Bissonnette, T. H., 44, 46
Bitch, condition of egg at fertilization, 222
Bladder, air, development of, 651
types of, 643
Blastaea. definition of, 347
Blastaea-gastraea theory, 347
Blastocoel, definition of, 280
development of in frog's egg, 296, 297
formation of in Rana pipiens, 295
importance of, 391
INDEX
937
Blastocoelic cavity, primary, of chick blas-
toderm, 3 19
Blastocoelic fluid, alkalinity of, 297
Blastocyst, 300
early, of rabbit, 303
mammalian, types of, 303
primary condition of, 305
secondary, 305
Blastoderm, 280
avian, origin of hypoblast (entoderm)
in, 358
central, of reptilian blastula, 360
of chick, delamination of hypoblast (en-
toderm) cells from, 359
pre-primitive streak, 35S
presumptive organ-forming areas of,
360
opaque, of reptilian blastula, 360
Blastomere(s), definition of, 280
equal, 288
totipotent, 336
Blastoporal canal, 417, 469
Blastopore, closure of, 404
in Amphioxus, 501
in frog embryo, 469
comparison with primitive streak, 450
definition of, 390
dorsal lip of, of teleost blastoderm, 438
history of in developing embryo of Rana
pipiens, 416
Blastosphere, definition of, 341
Blastula, of Amphibia, 370
auxiliary cells of, 341
chordate, as a bilaminar structure, 352
descriptions of various types, 352
relation of to future three germ-
layered condition, 352
significance of, 340
formative cells of, 341
importance of in embryonic develop-
ment, 351
importance of in Henkel's theory of, 347
in relation to twinning, 380
late, organization center in, 283
importance of, 386
morphological relationships of, 282
movements of parts of, during gastrula-
tion in amphibia, 411
of opossum, 366
organization of, in Amphioxus, "iAl
physiological relationships of, 282
primary, of chick egg, 319
of Echidna, 363
protochordate, 352
relationship of to gastrula and primitive
body form, 393t
reptilian, 360
Blastula — {Continued)
secondary, formation of in Echidna, 363
solid, 312
Blastulation, a main purpose of, 351
Blood, composition of, 726
relation of to connective tissues, 654
Blood capillaries, subintestinal, 726
Blood-cell formation, sites of, 733
in human embryo, 735
Blood-cell origin, theories of, 732
Blood cells, developing, 734
early embryonic origin of, 733
white, 726
characteristics of. 735
Blood corpuscles, red, 726
{see also Erythrocytes)
white, 726
Blood islands, of yolk sac, 733
Blood platelets, 726
Blood sinus, circumferential, of chick
embryo, 743
Blood vessel(s), later development of, 731
primitive, formation of, 727
supraintestinal, 727
theories of primitive development of, 730
Bloom. W., 582, 733
Blount, M., 320
Body fold(s), lateral, 869
anterior, 869
posterior, 869
Body form, definition of. 518
definitive, definition of, 519
possible influence of endocrine secre-
tions on development of, 883
development of, 462
in vertebrate group, basic similarity of,
514
larval, definition of, 518
organization of, influences which play a
part in, 512
primitive embryonic, xiv
definition of, 518
relationship of to blastula and gas-
trula, 393
types of, during embryonic development,
531
Body plan, primitive, in relation to gas-
trulation, 391
Body ridges, lateral, 869
Body stalk, 914
Bomhinator embryo, organ-forming areas
of blastula in. 474
Bone(s). 656
cancellous, 660 {see also Bone, spongy)
conversion of into compact bone, 668
characteristics of, 659
compact, 660
conversion of cancellous bone to, 668
938
INDEX
Bone(s) — (Continued)
dermal, in reptiles, 570
development of, 666
development of types of, 660, 661
formation of, endochondral, 666, 667
intracartilaginous, 666
intramembranous, 666
membranous, 661, 666
perichondria! (periosteal), 667
parts of, 662
spongy, 660 (see also Bone, cancellous)
characteristics of, 662
types of, 662
Bone cells, 659 {see also Osteocytes)
Bonnet, 55, 57, 216
Bony labyrinth, 851, 854
Bony layers, 659, 662
Bony platelets, of teleost fish scale, 565
Border fibrils, of smooth muscle, 702
Bouyancy structures, development of, 642
Boveri, T., 116, 215
theory of fertilization, 267
Bowen, R. H., 126
Bowfin (see Amia)
Brachet, A., 794
Brachet, J., 159, 240, 268, 270. 271
Brachial enlargement, of spinal cord, 821
Brain, development of, 822
external morphological, 832
in chick, 813
in teleost fish, 813
five-part primitive, formation of, 822
later development of, 823, 832
primitive cavities of, 822
Brain region, of central nervous system, 812
Brambeil, F. W. R., 140, 792
Branchial arch(es), 527, 618, 619
first, development of musculature of, 718
Branchial (gill) arches, 527
Branchial grooves (furrows), 527, 619
Branchial organs, 636 (see also Gills)
Branchial pouches, 527, 601, 619
Branchial rays, 673
Branchiomerism, 540
Branchiostoma, xv
Branchiostoma lanceolatum
(see Amphioxus)
Branchynema, sperm development in, 151
Brauer, A., 369, 370. 444, 446
Broad ligament, 803
Bronchus (Bronchi), apical, of mammalian
lung, 649
formation of, in chick's lung, 647
in mammalian lung, 649
primary, in mammalian lung, 649
recurrent, 647
stem, of mammalian lung, 649
Brood compartments, for care of young,
911
Brood pouch (es), in pipefish, 188
of marsupial frogs, 916
Brook lamprey, shrinkage of egg at fertili-
zation, 235
Biifo, role of thyroid gland in metamorpho-
sis of, 883
Bufo ainericanus, xviii
Bufo lentiginosus, venous system of
embryos of, 740
Bufo vulgaris, venous system of embryos
of, 740
Bulbourethral glands, 802
Bulbus cordis, 750
division of in developing mammalian
heart, 759
fate of in various vertebrates, 758
of developing heart, 747
Bunodont teeth, 607
Burmester, B. R., 905
Burns, R. K. Jr.. 892
Burrill, M. W., 892
Bursa, inguinal, 6, 13
Bursa ovarica, 66, 200
Byrnes, E. F., 720
Caecilia tentaculata, xviii
Calamus, of feather, 571
Calcareous platelets, of teleost fish scale,
565
Calyx (calyces), definition of, 786
formation of in mammal (human), 784
minor, 786
Canal, birth, pseudo-vaginal, of the opos-
sum, 63
inguinal, 6
vaginal, 62
Canaliculi, 563
absence of in cartilage, 659
of bone, 662
Canine teeth, 607
Canis familiaris (see Dog)
Capillary(ies), air, of chick's lung, 647
development of, 732
effect upon skin color, 592
Capsular cells, of nervous system, 819
Capsule(s), of cartilage cells, 659
of kidney, formation of, 787
Carapace, xix
formation of in turtle, 569
Carbon dioxide, elimination of, 771
mechanisms for, in developing em-
bryo, 903
Cardiac region, of stomach, 621
Cardinal system, 727
Cardinal vein (see Vein, cardinal)
INDEX
939
Carinatae, xix
Carnivora, xx
Carotenoids, effect upon skin color in the
fowl, 592
Cartilage(s), 656
articular, 697
definition of, 658
development of, 665
elastic, 659, 665
epiphyseal, 691
formation of, in human embryo, 666
pre-cartilage stage of, 665
hyaline, 658, 665
interstitial growth of, 665
palatoquadrate, 672
parachordal, of primitive skull, 671
peripheral growth of, 665
pterygoquadrate, 672
types of, 658
Cartilaginous labyrinth, 851, 854
Cartilaginous tissue, types of, 658
Casuarius, xix
Cat, Australian native, Dasyurus viverrinus,
sperm survival in female genital
tract, 197
chromatin and nuclear changes in oocyte,
136
Felis, egg, characteristics of, 207t
facial and cervical muscles derived
from hyoid arch mesoderm in,
719
fertilization, site of, 207t
sperm entrance into egg, place of, 207t
female reproductive organs of, 65
Caudal bud, 471
Caudal fin, in fishes, types of, 715
Caudal gut, 471
Caudata, xvii
Cavia (see Guinea pig)
Cavia porcellus (guinea pig), characteris-
tics of egg, site of fertilization,
and place of sperm entrance into
egg, 206t
Cavitation method of amnion formation,
910
Cell(s), epithelial, human, 180
general tissue, of ovary, 60
germ, of ovary, 60
interstitial, 17
movement of, importance of during de-
velopment and in gastrulation,
394
morphogenetic, 394
Sertoli, 16
spermatogenic, 16, 17
Cell center, 259, 261
Cell division, mechanisms associated with,
284
Cell lineage, 343
definition of, 292
in Ascaris, 331
in Styela (Cynthia) partita, 344
of isolated blastomeres of cleaving sea-
urchin egg, 329
Cell principle, as enunciated by Schleiden
and Schwann, 281
Cell proliferation, in gastrulation, defini-
tion of, 397
Cellular differentiation, definition of, 517
Cellular injury, and egg activation, 219
Cementoblasts, 613
Cementum, 613
Central body, 259, 261
Central canal, of spinal cord, 820
Central cells, in chick blastoderm, 316
Central nervous system, basis of, 812
morphogenesis of, 820
primitive, areas of, 812
Centriole, of sperm, anterior, 149
distal, 149
formation of, 152
proximal, formation of, 152
ring, 149
Centromere, 135
Centrosome, 259, 261
Centrosphere, 158
Cephalic flexure, 511, 812. 833
in vertebrate embryos, 511
Cephalic outgrowth, 461
Cephalochordata (Lancets), xv
Ceratohyal part of second visceral arch, 673
Cerebral aqueduct, of brain, 823
Cerebral lobe(s), 823
rudiments of, 822
Cerebrospinal system, 823, 826
Cerfontaine, P., 288, 404
Cervical enlargement, of spinal cord, 821
Cervical flexure, 833
formation of, 823
Cervical ganglion, superior, 837
Cetacea, xxi
Chaetopterus, annelid worm, artificial par-
thenogenesis in, 217
egg of, viscosity changes during cleav-
age in, 286
Chalaziferous layer, of bird's egg, forma-
tion of, 905
Chalcides tridactylus (Seps chalcides), liz-
ard, chorio-allantoic placenta of,
917
uterine compartments of, 911
Cheiropterygium, 691
Chelydra serpentina, xix
Chemodifferentiation, 375
and gastrulative process, 402
Chemotaxis, 225
940
Chen. G., 884
Chevron bones, 686
Chiasma (chiasmata), 134
definition of, 137
Chick, blastoderm of, area opaca of, 357
area pellucida of, 357
cleavage in, 313, 315
delamination of hypoblast (entoderm)
cells from, 359
presumptive organ-forming areas of,
360
brain development in, 813
circulatory system in, early development
of, 744
dorsal aorta of, 743
egg of, bilateral symmetry of, 162
feather development in, 572
feather pattern experiments with, 592
gastrula of, mosaic distribution of de-
velopmental tendencies, 429
gastrulation of, 420
resume of morphogenetic cell move-
ments during, 426
gut structures of, morphogenesis of, 604
heart of, 753
converging veins of, 746
early development of, 725
independent pericardial wall develop-
ment in, 871
inferior vena cava, development of, 746
liver rudiment, development of. 623
lung development in, 645, 645, 646
mesonephros, development of, 781
metanephric kidney, development of,
784
migration of cells during gastrulation in,
423
movements in epiblast layer of during
gastrulation, 426
musculature associated with mandibular
visceral arch of, 717
of 100 hours of incubation, special refer-
ence to nervous and urinary sys-
tems, 341
of seventeen to nineteen pairs of som-
ites, sections of, 487
of seventy-two to seventy-five hours of
incubation, 490, 542-544
of twelve to thirteen pairs of somites, 486
of twenty-seven to twenty-eight pairs of
somites, 488
olfactory organ development of, 844
ovarian development of, 797
palatal conditions in, 615
pancreas rudiments development of, 628
pelvic girdle of, 689
Chick — (Continued)
pituitary and thyroid glands, effect of
on development of, 884
post-gastrular development of, early, 480
pre-primitive streak, early blastoderm of,
358
pronephros, origin of, 776
pulmonary diaphragm of, 872
relation of embryonic chondrocranium
to adult skull, 678-679t
skin, development of, 570
testis, development of, 794
tongue, origin of, 603, 608
transverse sections of five pairs of som-
ites, 483
tubulation of epidermal area in, 476
twelve-day embryo, air sacs of, 645
vertebrae, development of, 685
vitelline and allantoic veins of, 742
Chief cells, of parathyroid glands, 879
of pineal gland, 881
of stomach mucosa, 621
Chief piece, of sperm flagellum, 149
Chiroptera, xx
Choana(ae), 844
primitive, 844
secondary, 844
Chondrichthyes, xvi
Chondrin, 659
Chondriosomes, 126, 157 {see also Mito-
chondria)
Chondroblasts, 665
Chondroclasts, 667
Chondrocranium, developmental stages of,
in Squalus acanthias, 669
relation of to adult skull, of chick, 678-
679t
of frog, 616-6111
of human, 680-681t
Chondrocytes, 659
Chondrostei, xvii
Chordamesodermal canal, 417
Chordata, characteristics of phylum, xiv
major divisions of phylum, xv
Chorio-allantoic membrane, 910, 924
Chorio-allantoic placenta, 917
Chorioid plexi, 822
Chorion, 162, 903, 908, 910
in Styela, 162, 163
true, 165
Chorion frondosum, 922
Chorion laeve, 921
Chorionic vesicles, in various mammals,
924
Choroid coat, of eyeball, formation of, 849
Choroid fissure, 846
of developing eye, 845, 846
Chromaffin cells, 882
INDEX
941
Chromaffin reaction, 882
Chromatid, 132
Chromatin, diminution of, in Ascaris, 116
in Miastor, 118
Chromatophores, 591
origin of, 557, 560
role of in producing skin-color effects,
591
Chromatophoric activity, environmental
control of, 594
genetic control of, 592
hormonal control of, 593
Chromidia, during fat formation, 664
Chromosomal mechanisms, for sex deter-
mination, 887
Chromosome(s), accessory, 215
bivalent, 132
homologous, definition of, 130
"lamp-brush," 136, 142
reduction of, 145
sex, 887
tetrad, 132
twin, 132
Chrysemys picta, turtle, blastoderm of dur-
ing gastrulation, 418
female, reproductive organs of, 192
sperm transport in female, 191
zona radiata of, 166
Cilia, distribution of in peritoneal cavity of
adult female frog, 201
peritoneal, of Runa pipiens, 195
pro-ovarian, 191
Ciliary action, and external migration of
the egg, in frog, 201
Ciliary body formation, of developing eye,
845
Ciliary ganglion, of oculomotor nerve, 828
Ciona, early cleavage planes in, 287
Ciona intestinalis, xv
Circulation, fetal and postpartum, 928
Circulatory system, 724
definition of, 725
histogenesis of, 730
in mammalian fetus, modifications of at
birth, 766
in relation to nutrition, 927
major subdivisions of, 726
morphogenesis of, 73,6
Circulatory tubfcs, primary, 463
Citellus tridecemlineatus
{see Ground squirrel)
Claude, A., 159
Clavelina. early cleavage planes in, 287
Clavicle, 693
of intramembranous origin, 693
Claws, development of. 584
diagrams of. 583
in reptiles, 570
Cleavage, cytoplasmic substances, influence
of, 284
definition of, 280
determinate definition of, 287
egg organization, influence of, 284
equatorial plane of, 283
factors involved in, 284
functions of, 333
geometrical relations of, 283
history of, early, 281
holoblastic, 288
transitional or intermediate type, 305
in Ambystoma maculatum (punctatum),
305, 306
in Amia calva, 309, 310
in Amphioxus, 288, 290, 342
latitudinal plane of, 284
meridional plane of, 283
meroblastic, 288
definition of, 312
qualitative, 336
influence upon later development, 334
quantitative, 336
influence upon later development, 334
superficial, 288
suppression of, by heparin, 219
transitional or intermediate, definition,
305
types of, 288
vertical plane of. 283
viscosity changes during, 286
Cleavage-blastuiar period, importance of,
281
Cleavage laws, 286
Cleavage path, 247
Cleavage planes, relation of to antero-pos-
terior axis of embryo, 286
Clemmys guttata, xix
Clemmys leprosa, formation of hypoblast
in, 361
Cloaca, 18, 772
area of, diverticula of, 602
diff'erentiation of, 787
glands of, 189
presence of in vertebrate group, 788
primitive, divisions of in mammals, 715
Cloacal septum, 787
Closing folds, dorsal, 861, 867
Coccyx, 686
Cochlear duct, 557, 854, 855
Coelenterates, gemmation in, 55
Coeloblastula, definition of, 341
typical, basic structure of, 342
Coelom. definition of, 857
extra-embryonic, 909
Coelom formation, 490, 492
Coelomic cavities, 857
formation of, in mammals, 868
942
INDEX
Coelomic changes, in amphibians, 867
in birds, 867
in fishes, 866
in reptiles, 867
Coelomic covering, of the heart, 703
Coelomic tissue, contribution of to urogen-
ital system, 772
Cole, F. J., 213
Collagenous fibers, characteristics of, 657
Collar cells, of developing contour feather,
575
Collateral ganglia, of autonomic nervous
system, 837
Collecting duct(s), 781
of kidney, 772
formation of in chick embryo, 784
mesonephric, 778
metanephric, formation of in mammal
(human), 784
primary, 786
straight, 786
Collecting tubules, arched, of developing
mammalian kidney, 786
Colon, 630
Color-pattern production, manner of, 591
Coluinba, membranous labyrinth of, 852
Coluinbiformes, xix
Competence, definition of, 379
as a function of developmental time se-
quence, 380
Concrescence, in gastrulation, 397
Conductivity, 814
Conklin, E. G., 224. 287, 288, 353. 404.
502
Connective tissues, characteristics of, 656
fibrous, formation of, 663
types of, 657
kinds of, 656
loose, 663
mucoid, 663
proper, 656
formation of, 663
soft, types of, 657
Contractility, a generalized property of
living matter, 700
Contraction wave, of heart beat, 754
Conus, 759
Conus arteriosus, of developing Squalus
heart, 752
of frog heart, 752
Convergence, 551
definition of, 396
during gastrulation, 396
Cooper, E. R. A.. 885
Coordinating center, of nervous system,
807
Copula, of mammalian tongue, 605
Copula protuberance, of developing bird
tongue, 604
Copulation path, female, 239
of sperm and egg in Styela. 247
Copulatory organs, extensible, in reptiles,
mammals, and some birds, 191
Coracoid, 693
Cord, spermatic, 19
Corium, 556
Cornea, 846
development of, 849
Corner, G. W., 5, 499
Corona radiata, dispersal by hyaluronidase,
229
Coronary ligament, 864. 866
of liver, 873
Coronary sinus, of mammalian embryo,
747
Corpora bigemina, 822
Corpora quadrigemina, 822
Corpus albicans, 85
Corpus hemorrhagicum, 84
Corpus luteum, differentiation of, in the
snake, 84
effect on growth of egg follicle, 81
formation of, 83
hormone of, 89
of opossum, 84
of ovulation, 85
of platypus, Ornithorhynchus, 84
of pregnancy, 85
Corpus sterni, 687
Cortex, of hair shaft, 582
of ovary, 58
position occupied by primitive germ
cells, 68
primary, formation of, 795
secondary, formation of, 796
Cortexin, 890
Corti, organ of, 855
Cortical changes, importance of during fer-
tilization, 239
Cortical field, a gonadal sex field, 889
Cortical granules, action of in formation of
fertilization membrane, 237
Cortical zone, of adrenal gland, 883
Cotyledons, 918, 922
Cow, horn of, 586
Cowper's glands, 802
Craig-Bennett, A., 46
Cranial ganglia, 812
Cranial nerves (see Nerves, cranial)
Craniosacral autonomic system, 836
Cranium, proper, 668
Crista, of semicircular canals, 854
Crocodila. xviii
Crocodilia, gastralia of, 697
Crocodylits acutus, xviii
INDEX
943
Crop, of chick, 623
Crossing over, phenomenon of, 133, 135
Crotalus horridus, rattlesnake, xix
oral glands of, 616
Crown, of tooth, 605
Cryptohranchus, external gills of, 639
Cryptobranchus alleganiensis, xvii
respiratory surfaces of, 636
Cumulus oophorus, of mature Graafian
follicle, 71, 74
Cuneus, of hoof of horse, 585
Cup-shaped valves, of chick heart, 756
Cushion septum, of chick heart, 754
Cuspid valves, development of in mam-
malian heart, 757
of chick heart, 756
Cutaneous field, general, 842
Cuticle, of hair shaft, 582
Cytochemistry, xii
Cytogenesis, definition of, 517
Cytolysis, 221
Cytomorphosis, definition of, 517
Cytoplasm, germinal, 116, 117
Cytoplasmic changes, of developing neu-
rons, 814
Cytoplasmic extensions, from nerve cell
body, 814
Cytoplasmic inequality, of cleavage blasto-
meres, 328
Cytoplasmic segregation, during fertiliza-
tion, in Styela, 245
Cytotrophoblast, 920
D
Dalcq, A. M., 270, 893
Dasyurus, superficial implantation in, 916
Dasyurus novemcinctus, egg, characteris-
tics of, 206t
fertilization, site of, 206t
sperm entrance into egg, place of. 206t
Dasyurus viverrinus (Australian marsupial
cat), early blastular conditions
of, 304
egg, characteristics of, 205t
fertilization, site of, 205t
inner cell mass of blastocyst of, 304
sperm entrance into egg, place of, 205t
sperm survival in female genital tract,
197
Dean, B., 309, 310, 311, 444, 458, 639
Deansley, R., 46
De Beer, G. R., 682. Ill
Decidua basalis, 921
Decidua capsularis, 921
Decidua parietalis, 921
Decidua vera, 921
Deciduous teeth, 613
Deep profundus, division of trigeminal
nerve, 828
Deer, Odocoileus virginianus borealis, ef-
fect of testosterone on, 27
spermatogenesis in, 23
testicular activity in relation to sea-
sons, 24
white-tailed, antler of, 586
Definitive body form, xii {see also Body
form, definitive)
Definitive state, of germ cell, 113
de Graaf, Reinier, 5, 5
early description of egg follicles, 71
Dehydroisoandrosterone, 22
Delamination, in gastrulation, 390
definition of, 398
of three germ layers in teleost fishes, 438
Demersal eggs, 251
De Meyer, J., 223
Demibranch, definition of, 638
Dendrites, 814
Dental lamina, 609
Dental papilla, 61 1
Dentinal fibers, of odontoblasts, 611
Dentine, of teeth, 605, 611
substance similar to, in shark embryo,
563
Dentition, types of, 607
Dependent differentiation, definition of, 375
Dermatome, 507, 711
Dermatomic fold, 507
Dermis, 556
development of in chick, 573
embryonic rudiment of, 530
structures of in Necturus, 567
Dermo-myotome, 526
De Robertis, E., 43
Deroceras laeve, formation of polar bodies
in, 214
Descartes, 881
Desmocranium, 671
Desmognathus, muscles of, 707
spermatheca of, 790
Desmognathus fuscus, xvii
Desoxyribose nucleic acid, 815
Determination, in differentiation, definition
of, 378
positional or presumptive, 378
rigid, in early Styela blastomeres, 330
rigid or irrevocable, definition of, 378
Deutoplasm, formation of, 157
Developing body, mesodermal contribu-
tions to, 524
Development, definition of, xi
environmental conditions necessary for,
901
944
Development — (Continued)
later, influence of quantitative and qual-
itative cleavages on, 334
periods of, xii
Developmental potencies, differences in
amphibian egg materials and
early blastomeres, 330
of isolated blastomeres of cleaving sea-
urchin egg, 329
de Winiwarter, H., 794, 795. 796
Diabetes, 875
Diakinesis, 128, 137
of hen's egg, 253
terminalization in, 139
Diaphragm, dorsal, 868
mammalian, 872
origin of musculature of, 718
pulmonary, of chick, 872
Diaphysis, of bone, 691
Diarthrosis, 695
definition, 696
Dictyosomes, 126
Didelphys, facial and cervical muscles de-
rived from hyoid arch mesoderm
in, 719 (see also Opossum)
pelvic girdle in, 689
Didelphys aurita, inner cell mass of blasto-
cyst of, 304
Didelphys virginiana, xx
blastula, formation of, 364
egg, characteristics of, 206t
fertilization, site of, 206t
inner cell mass of, 304
sperm entrance into egg, site of, 206t
uterine horn, anterior end of, 194
Diencephalon, 822
thin roof plate of, 822
Diestrus, definition of, 93
lactational, definition of, 94
Differentiation, biochemical, 375
definition of, 374
dependent, definition of, 375
implications of, 375
morphological, 375
physiological, 374
problem of, 374
Digestive tract, human, characteristics of
mucous membrane in, 620
structural composition of walls of, 629
Digestive tube, development of, 602
diagrams showing basic features of,
598
general morphogenesis of, 602
Diphyodont dentition, 615
Diplonema stage, 137 (see also Diplotene
stage)
Diplospondyly, 685, 688
Diplotene stage, 128, 137
of hen's egg, 253
Dipnoi, xvi
Discoblastula, definition of, 341
Discoglossus, primitive frog, 187
Discoglossus pictus, xviii
secretions causing lysis in, 229
Divergence, in gastrulation, definition of,
398
Diverticulum, dorsal, left, 502
right, 502
Division, meiotic (see Meiotic division)
Dizygomatic twins, definition of, 380
Dog (Canis familiaris), egg, characteristics
of, 207t
fertilization, site of, 207t
sperm entrance into egg, place of, 207t
Dog shark, development of intestine in,
626 (see also Sqiialus)
Dogfish, Scyllium canicula, germinal vesi-
cle in, 136
Domm, L. V., 594, 892
Dornfeld, E. J., 119
Dorsal aorta (see Aorta, dorsal)
Dorsal arching movement, 461
Dorsal lip, of blastopore, of teleost blasto-
derm, 438
Dorsal median septum, 818
Dorsal root ganglion cells, 817
Dorsal septum, 706
Dorsal sulcus (fissure), 818
Dorsal upgrowth movement, 461, 476
Driesch, H., equipotential state of, 377
Dry fertilization, 232
Duct, ejaculatory, 20
epididymal, 15, 18
convoluted, absence of, 28
function of, 29
presence of, 29
reproductive, 15
and external fertilization, 17
and internal fertilization, 18
in male vertebrates, 757
sperm-conveying, 4
Ductuli efferentes, 15
Ductus choledochus, 627
Ductus deferens, 19
cross section of ampullary region of, 183
muscular character of, 182
Ductus venosus, of chick embryo, 743
Dukes, H. H., 103
Dumas, 281
Dunn, L. C, 197
Dura mater, 822
Durken, B., 331
Dushane, G. P., 592, 593
Dyad, 132
INDEX
945
Ear, development of, 850, 853
external, 851
structure of, 853
Echidna, blastoderms of, early, 362
blastula of, primary, 363
blastula of, secondary, 363
cleavage in, 324, 325
egg of, 76
characteristics of, 205t
fertilization in, 254, 255
site of, 205t
maturation in, 254
sperm entrance into egg, place of, 205t
Echidna aculeata, xix
cleavage in. 326
egg of, telolecithal, 255
Echinarachinus, separation of fertilization
membrane in, 237
Echinochrome A, 228
Ectobronchi. of chick, 645, 647
Ectoderm, introduction of word, 347
Ectodermal expansion, 395
{see also Epiboly)
Edentata, xx
bony armor of dermal. 697
Edinger-Westphal nucleus, of oculomotor
nerve, 828
Effector structures, 807
Efferent ductules, activities of in sperm
transport, 180
of epididymis, 793
Efferent fiber, 816
Efferent neurons, 826
Efferent system, of aortal arches, 760
Egg {see also Oocyte)
Egg(s), activation of {see also Activation,
of egg)
artificial, 217
mechanical, of Asterias forbesi, 219
cellular injury in, 219
complete, 212
natural, 216
partial, 212, 217
ribonuclease in, 219
types of, 216
and developing yolk body, of fowl, 755
bilateral symmetry of, 161, 162
chart of passage from ovary down ovi-
duct. 317
chordate. characteristics of, summary,
203t
fertilization, site of. 203t
cytoplasmic differentiation of, 154
definition of, 125, 154
Harvey's conception of. 55
demersal. 251
development of, summary of, 170t
Egg(s) — {Continued)
exudation of substances from at fertili-
zation. 240
fertilization of, in Amphioxus, 227
summary of activities of in initial
stages, 233
homolecithal, 156
inhibited or blocked condition of, at
fertilization, 221
invisible morphogenetic organization of,
160
isolecithal, 156
maturation of. in Amphioxus, 227
in Styela (Cynthia) partita, 224
migration of. to brood pouch of male
pipefish, 199
external (peritoneal), 199
and ciliary action in frog. 201
internal (oviducal), 199
in frog, 201
monospermic, 259
numbers produced by different verte-
brate ovaries, 81
of bitch, formation of first polar body,
222
of frog, cleavage plane of, first, 292
second, 292
third or latitudinal, 293
fertilization of, 247
of hen, fertilization processes in, 251,
252
maturation in, 252
first division, 253
vitelline membrane (zona radiata) of,
167
of rabbit, development of, early, 300
fertilization stages in, 251
of teleost fish, fertilization of, 249
organization of, dependence of first
cleavage spindle upon, 285
parthenogenesis from non-sexual, 216
pelagic. 251
pigeon, fertilization processes in, 251
polarity of, 161
polyspermic, 259
secretion of, in egg water of /I r6ac/a, 225
sperm entrance into, 203t
surface of, contraction of during fertili-
zation, 240
telolecithal, 156
telolecithality, 156
transport of, from ovary to site of fer-
tilization, 199
from ovary to uterine tube, and estro-
genic hormone, 200
in amphibia other than frog. 202
in birds, 199
in ectopic pregnancy, 201
946
INDEX
Egg( s ) — (Continued)
transport of — (Continued)
in fishes, 202, 907
in frog, 201
oviduct, 907
in mammals, 200
in Salamandra atra, 202
through oviduct of hen, 907
types of, 156
viability of after discharge from ovary,
82
Egg discharge, factors effecting, in the ver-
tebrate group, 80
Egg follicle {see Follicle, egg)
Egg membrane(s), 901
formation of, 903
importance of, 903
in bird's egg, 906
oi Amphioxus, 165
of Fundulus heteroclitus, 165
of perch, 165
origin of, 162
primary, 901
formation and importance of, 903
secondary, 901
formation and importance of, 903
tertiary, 108, 901
of birds, 905
of mammals, 804
Egg passage, through oviduct, in reptiles,
907
Egg permeability, loss of before fertiliza-
tion, 222
Egg pronucleus, influence of in stimulating
enlargement of male pronucleus,
262
Egg protoplasm, influence of in formation
of first cleavage amphiaster, 261
Egg respiration, low level of at fertiliza-
tion, 222
Egg secretions, formation of, 223
Egg shell, formation of, in bird's egg. 906
Egg streak, 239
Egg transport {see Egg, transport of)
Egg water, of Arbacia, 225
Ejaculate, single, volume of, 32t
Elasmobranch fish(es), blastula of, 320,
368
cleavage in. 320
early, 318
gastrulation in, 441
periblast tissue of, 321
sagittal sections of. embryos of, 477
seminal vesicles of, 186
tubulation of epidermal area in, 476
vitelline membrane of, 165
zona radiata of. 165
Elastic fibers, characteristics of, 657
Elastic tissue, 657
El-Toubi, M. R., 669
Emboitement, 57
Emboly, 395, 449
definition of, 394, 396
in gastrulation, of amphibian, 408
of Amphioxus, 402
processes involved in, 396
Embryo, definition of, xi
secondary, induction of, 411
Embryology, biochemical, xii
causal, xi
chemical xii
definitions relative to, xi
descriptive, xi
experimental, xi
Embryonic knob, 303
Embryonic membrane(s), fate of, 925
primary, 162, 904
Embryonic mesenchyme, 532
Embryonic period, xii
Embryonic shield, of reptilian blastoderm,
360
of teleost blastoderm, 437
Embryonic tissue, of chick blastoderm, 319
Embryotroph, 921
Enamel, of teeth, 605, 611
Enamel organ (s), differentiation of, 611
rudiments of, 609
End bud, 471
growth of, importance of in Amphioxus
embryo, 494
in vertebrate embryo, 494
of chick embryo, definition of, 431
End piece of sperm flagellum, 149
Endocardial cushions, of developing chick
heart, 754
of developing mammalian heart, 757
Endocardial primordia, 702
Endocardium, 748
of chick heart, 742
Endochondral bone formation, 667
Endocrine glands, definition, 875
embryonic origin of, 875
influence of on sex differentiation, 886
morphological features of, 875
solid, non-storage type of, 876
Endocrine secretions, possible influence of
on development of definitive body
form, 883
Endoderm, introduction of word, 347 {see
also Entoderm)
Endo-exocrine glands, 875
Endogenous sources, of food for embryo,
902
Endolymph. 851
of ear. 854
Endolymphatic duct, 854
INDEX
947
Endolymphatic sac, 854
Endoneurium, 827
Endoskeleton, 668
development (morphogenesis) of. 668
Endosteum, 693
Endostylar cells, 885
Endostyle, of Petromyzon larva, 879
Endotheliochorial placenta, 918
Endotheliochorial plus syndesmochorial
placenta, 918
Endothelium, 520, 748
of blood capillaries, development of, 731
Enterocoel, within somite of Amphioxus
embryo, 505
Entoblast, 347 {see also Hypoblast)
Entobronchus (Entobronchi), 647
anterior, 646
Entoderm, 347
formation of in reptiles, 361
Entoderm cells, mother, in opossum blas-
tula, 365
Entodermal area, developmental tenden-
cies of during gastrulation, 412
of late blastula and gastrula, later deriva-
tives of, 533
tubulation of in Amphioxus, 500
Enveloping membranes, types of, 901
Environmental conditions, necessary for
development, 901
Environmental factors, external, and testis
function, 43
of sexual cycles, in field mouse, 47
Enzyme, starch-splitting, 617
Epaxial (epiaxial) region, of vertebrate
body, 494
Epaxial muscle groups, 706
Ependymal cells, 809. 816
Ependymal layer, of developing neural
tube, 817
Epiblast, 347, 352
Epiblast layer, of chick blastula, 343
Epiboly, 395, 449
definition of, 394, 395
description of processes concerned with,
395
during gastrulation in Amphioxus, 403
in amphibian gastrulation. 411
Epicardial rudiment, 703 '
Epicardium, 703
Epidermal area, of late blastula and gas-
trula, later derivatives of, 533
Epidermal cells, ciliated. 810
Epidermal tube, development of, in am-
phibia, 474
Epidermis, 556
development of in chick. 573
two fundamental parts of, 559
Epididymis, as sperm-ripening structure, 29
as storage organ, 30
definition of, 18
efferent ductules of, 15, 793
Epigastric vein, of chick embryo, 745
Epigenesis, theory of, 345
Epiglottis, 650
development of, 650, 650
Epigonal area, of genital ridge, 791
Epigonal support, for developing sex gland,
803
Epimere, definition of, 490
Epimyocardial rudiment, of chick heart,
742
Epimyocardium, 748. 859
Epinephrine, 882 {see also Adrenalin)
Epiphysis (Epiphyses). 822, 881
of bone, 691
Epipubic midpiece, of pelvic girdle, 693
Epithelial bed, of hair follicle, 581
Epithelial matrix, of hair bulb, 581
Epithelial nucleus, of primitive gonad, 792
Epithelial sheath, of tooth, 613
Epitheliochorial placenta, 917
Epithelium, germinal, 17
(coelomic), 791
effect of X-rays on, 120
of ovary, 58
origin of germ cells from, 68
primitive, 5 19
Epitrichium, 559
Equatorial plane, of cleavage. 283
Equilibrium, dynamic, 854
static. 854
Equipotential state, of Driesch. 377
Equus cahallus (horse), egg, characteristics
of, 207t
fertilization, site of. 207t
sperm entrance into egg, place of, 207t
Ericulus, eutherian mammal, ovarian fer-
tilization in, 197
Erinaceus europeus {see Hedgehog)
Erythroblasts, 733
Erythrocyte(s), 726, 733
characteristics of development of, 735
Esophageal glands, of esophagus, 620
Esophagus, histology of, 620
Estradiol. 85
structural formula of, 86
Estrogen (s), 21, 85 {see also Hormone,
estrogenic; Hormone, sex, fe-
male)
efi"ect of upon female mammal. 88
efi'ects of upon vertebrates other than
mammals. 89
Estrogen formation, primary control of, 87
Estrone, 85
structural formula of, 86
948
Estrous cycle, factors controlling, 96
follicular phase of, 92
in mammals, 92
luteal phase of, 92
Estrus, definition of, 93
relation of to activity of oviduct during
egg transport, 200
relation of to ovulation, 95
Eustachian duct (see Eustachian tube)
Eustachian tube, 841, 851. 855
Eustachian valve, of chick heart, 756
Eutheria, xx
Evagination, scrotal, 1 1
Evans, H. M., 41
"Ex ovo omnia," 53, 54
Excretory duct, main, of kidney, 772
Excretory system, 768
development of, 772
functions of, 769
Exoccipital center, of ossification, 695
Exocrine glands, 875
Exogastrulation, 449
Exogenous sources, of food for embryo,
902
Exoskeleton, 668
Extension, in gastrulation, definition of,
398
External auditory meatus, 855
External naris, 844
Exteroceptive field, 842
Extodermal expansion, definition of, 395
{see also Epiboly)
Extra-embryonic coelom, 909
Extra-embryonic membranes, 901
diagrams of. 909
in human embryo, 972
Eycleshymer, A. C, 306, 308, 473
Eye, accessory structures of, 853
development of, 844
special aspects of, 846
extrinsic muscles of, development of,
716
general structure of, 844
Eyelids, 853
Fabricius, bursa of, 602, 881
Falciform ligament, 866
of liver, 873
Falciform process, of teleost fish eye, 849
Fallopian tube, in rabbit, behavior in sperm
transport, 193
Fankhauser, G., 160, 270
Fascia deep, 556
spermatic, external, 13
internal, 13
middle, 13
Fascia deep — (Continued)
superficial, 556
perineal, 12
Fasciculus (Fasciculi), definition of, 701
or muscle fiber bundles, 701
Fasciculus cuneatus, 821
Fasciculus gracilis, 821
Fat, brown, 664, 665
types of, 664
white, 664
Fat droplets, origin of, 157
Feather(s), after shaft, formation of, 571,
578
contour, 571
development of, 575
early phase of, 575
secondary phase of, 575
development of, 577
in chick, 572
down, 573
later, development of, 578
filoplumous, development of, 578
general structure of, 571
nestling down, development of, 573
Feather follicle, 571
Feather vane, formation of, 576
Felis (see Cat, Felis)
Felix, W., 778, 785, 787, 791, 792, 794, 797
Fenestra cochlea, 851
Fenestra ovalis, 851
Fenestra rotunda, 851
Fenestra vestibuli, 557
Fenestrae, of atrial septum of chick heart,
754
Ferret, pituitary ablation of, 39
Ferret, Putorius vulgaris, effect of light
upon reproductive activities, 44,
45
Fertilization, 21 1
areas of, 189
behavior of gametes during, 221
changes in physiological activities at, 243
definition, 113, 211
dry, 232
external, and reproductive duct, 17
fusion of gametes at, 234
in Echidna, 254, 255
in Gambusia affinis, 199
in hen's egg, 252
in Heterandria formosa, 199
in ovary, 197
in Styela (Cynthia) partita, 224, 245
internal, general features of, 189
reproductive duct in, 18
sperm transport in, 189
metabolic change at, 243
movements of ooplasmic substances dur-
ing Styela, 264
INDEX
949
Fertilization — (Continued)
of Amphioxus, 247
of teleost fish egg, 249
second stage of, 234
shrinkage of egg at, 235
sites of, in vertebrate group, 779
theories of, Bataillon, E., 268
Boveri, T., 267
Heilbrunn, L. V., 271
Lillie, F. R., 269
Lillie, R. S., 271
Loeb, J.. 268
Runnstrom, J., 271
two stages of, 243
Fertilization complex, 223
Fertilization cone, 241
in Asterias forbesi, 243
in Nereis virens, 241
in Toxopneustes variegatus, 241
Fertilization membrane, 235, 903
action of cortical granules in formation
of, 237
formation of in Arbacia, 237
in Nereis, 239
in egg of sea urchin, Toxopneustes livi-
dus, 214
separation of in Ascaris, 237
in Echinarachinus, 237
Fertilizin, 225, 269
action of, 225
and sperm antifertilizin in the fertiliza-
tion process, 230
distribution of in animal eggs, 227
in egg activation to develop, 225
inactive, 269
presence in egg of sea urchin, 227
Fetal placenta, 922
Fetus, mammary, xi
Fevold, H. L., 40, 41
Fiber, of skeletal muscle, dark (red), 700
pale (white), 700
Fiber tracts, of spinal cord, 821
Fibrillary plate, of teleost fish scale, 565
Fibrils, of muscle, 700
Fibrinogenase, 33
Fibrinolysin, 33
Fibroblasts, 663
Fibrocartilage, 659, 665
Fibrois astrocytes, 810
Fibrous tissue, white, 657
development of, 663
Field, early, limitations imposed upon, 510
Field, H. H., 720, 775, 779
Field concept, of development, 509
Filament, axial of sperm, 149
undulating (vibratile), 149
Fin(s), anal, 690
median dorsal, 690
tail, 690
Fin-ray cavities, 507
Fish(es), bony, skin of, 559
caudal fin of, types of, 715
coelomic changes in, 866
cyclostomatous, glands of, 617
egg transport in, 907
gills of, arrangements of, 639
development of, 638
influence of pituitary and thyroid glands
on development of, 885
integument of, anatomical characteristics
of, 561
intestinal tracts of, diagrams of, 617
male, intromittent organ in, 185
myotomes, differentiation of, 714
parietal pericardial wall in, 871
respiratory surface relationships in, 638
skin, development of, 561
vertebrae, formation of, 685
Flagellum, of sperm, 149
role of in fertilization, 232
Flemming, W., 664
Flexure, cervical, 511
cranial, 51 1
nuchal, 51 1
Flint, J. M., 648
Flounder, Limanda ferruginea, female re-
productive system of, 58
testes, position of, 7, 10
testis in relation to reproductive ducts,
19
Fluid, seminal {see Seminal fluid)
Fluid vehicle, of lymphatic blood, 726
Flynn, T. T., 200, 324, 325, 362, 363, 904
Fold, inguinal, 1 1
Fol, H., 214, 235
Folley, S. J., 108
Follicle(s), egg, definition of, 70
effects on development by gonado-
trophic hormones, 72
history of after ovulation, 83
hormonal factors concerned with de-
velopment of, 72
mature, structure of, 74
structure of in metatherian and
eutherian mammals, 74
of lower vertebrates, 75, 75, 76
prototherian, structure of, 75
Graafian, 70
development of, in opossum ovary, 70
mature, 71, 74, 74
in opossum, 72
primary, 70
secondary, 70, 72
tertiary, 70, 72
950
INDEX
Follicle ( s ) — (Continued)
Graafian — (Continued)
tertiary conditions in opossum ovary, 71
Follicle stimulating hormone (FSH), folli-
cle development, effects on, 72
response to, of seminiferous tubules, 24-
25
role of in spermatogenesis, 24-25, 40-41
testis activity, effect on, 40
Follicular phase, of female reproductive
cycle, 93
Fontanel (s), anterior, 696
definition of, 696
lateral, 696
posterior, 696
Food sources, for embryo, types of, 902
Foote, C. L., 892
Foramen caecum, 878
Foramen of Panizza, of crocodilian heart,
753
Foramen ovale, of developing mammalian
heart, 757
Forebrain, 812, 822
Foregut. 478, 600
Formative cells, of blastula, 341
of opossum, 366
Formative tissue, of chick blastoderm, 319
Formic acid, in egg activation, 218
Fovea, of Ambystoma egg, 305
Fowl, common, spermatogenesis in, 153
egg and developing yolk body of, 755
Sertoli-cell conditions in, 139
young oogonia of, 154
zona radiata of egg of, 166
Fox, S. W., 225
Fox, silver, seminiferous tubules in rela-
tion to seasons, 25
Fraser, E. A., 716
Fraternal twins, definition of, 380
Freemartin, 889
lack of in marmoset, 892
Friedman modification, of Aschheim-
Zondek pregnancy test, 927
Frog (see also Rana)
amplexus in, 201
blastopore closure in, 469
chondrocranium of, relation of, to adult
skull, 616-(ilH
cleavage in, latitudinal plane of, 284
egg of, bilateral symmetry of, 162
blastocoel development of, 296, 297
ciliary action and external migration
of, 201
egg transport in, 201
through oviduct, 907
gastrulation in, 406
heart of, early development of, 728
Frog — (Continued)
lateral vein development in, 738
lungs, development of, 644
marsupial, 916
mesonephric kidney, development of,
780
migration of egg in internal, 201
muscles of, 707
musculature associated with mandibular
visceral arch of, 717
neural fold stage of, early, 462, 463
neurenteric canal formation in, 469
neurocranium, developmental stages of,
672
of hoof of horse, 585
olfactory organ development of, 843
palatal conditions in, 615
pancreas rudiments, development of, 628
pectoral girdle of, 689
pineal organ of, 881
renal portal system of, 776
reproductive structures of female, 66
tadpole, development of teeth in, 607
testis, development of, 794
tongue of, 603
urostyle of, 685
vertebrae, development of, 685
vitelline membrane of, 166
vitelline vein development in, 737
Frog test, for pregnancy, 927
Frontal bone, development of in human,
695
Fronto-nasal process, 844
Fructose, 34
possible elaboration in seminal vesicle
and prostate, 34
sperm utilization of, 34
FSH {see Follicle stimulating hormone)
Fundic region, of stomach, 621
Fundulus, red blood cell origin of, 733
Fundulus heteroclitus, killifish blastoderm
of, late, 367
cleavage planes in, first, 287
egg, characteristics of, 204t
egg membranes of, 165
fertilization, changes during, in egg of,
244
site of, 204t
oxygen consumption in at fertilization,
244
sperm entrance into egg, place of, 204t
Funiculus (Funiculi), 821
dorsal, 821
lateral, 821
ventral, 821
Fiirbringer, M., 708
INDEX
951
Gadus, codfish, lack of parental care in,
900
Galen, 881
Galliformes, xix
Callus, bird, pectoral girdle of, 689
Callus (dotnesticus) gallus, characteristics
of egg, site of fertilization, and
place of sperm entrance into egg,
205t
gut structures of, morphogenesis of, 604
Galtsoff, P. S., 228
Gambusia affinis, mosquito fish, sperm sur-
vival within female genital tract
of, 197
egg development in ovarian follicle in,
915
fertilization in, 199
intromittent organ of, 755
ovarian fertilization in, 197
Gamete(s), condition of at fertilization,
221
cytosomal (cytoplasmic) maturation of,
124, 145
definition, 1 13
differentiation of, 124
female, activities of in aiding sperm and
egg contact, 223
characteristics of at fertilization, 221
fusion of at fertilization, 234
male, activities of in aiding contact of
two gametes at fertilization, 228
characteristics of, 223
nuclear maturation of, 124
physiological maturation of, 124, 169
specific activities of in fertilization, 223
Gametic fusion, important studies of, 214
in Toxopneustes lividus, 214
Gamones, 228
Ganglia, 812
of autonomic nervous system, 837
Ganglionic crest (neural crest), 469
Ganoidei, xvii
Ganoin, 564
Gasserian (Semilunar) ganglion, 828
Casterosteus, male sex hormone produc-
tion in, 25
Gastraea, theory of, definition of, 347
Gastralia, 697
Gastro-hepatic ligament, 873
Gastro-splenic ligament, 766, 873
Gastrotheca marsupiata, dorsal brood
pouch in, 911
Gastrotheca pygmaea, dorsal brood pouch
in, 911, 916
Gastrula, 347
an embryonic form common to all meta-
zoan animals, Haeckel's theory
of, 347
antero-posterior extension of, in Amphi-
oxus, 403
definition of, 390, 391
late, a mosaic of specific organ-forming
territories, 446
relationship of to blastula and primitive
body form, 393
Gastrulation, 281
definition of, 390, 391
extension in, 398
in Amia calva, the bony ganoid, 309,
310, 444
in Amphibia, 355, 406. 411, 444
in Amphioxus, resume of, 405
in chick, resume of morphogenetic move-
ments of cells during, 426
in elasmobranch fishes, 441
in gymnophionan amphibia, 444
in mammals, 431, 435
in relation to primitive vertebrate body
plan, 391
in reptiles, 417
in teleost fishes, 436, 440
two of the main functions of, 447
Gastrulative movements, autonomous the-
ory of, 447
Gastrulative streaming, of entoderm in
chick embryo, 426
Gatenby, J. B., 150
Gecko (Platydactylus), formation of hypo-
blast in, 361
Geiling, E. M. K., 884
Gemmation, 55
Gemmules, 114, 348
Generation, spontaneous, 57
Geniculate ganglion, of facial nerve, 829
Genital ducts, 788
Genital ridge (fold), 791
Germ cell(s), definitive state of, 113, 117
factors determining fate of, 43
migration of, 115
by active amoeboid movement, 120
by shifting of tissues, 120
in blood stream, 120
origin of, 68, 114, 118, 119, 121t
primitive (primordial), 117, 772, 791,
793
effect on developing gonad, 121
of Amby stoma, 121
position occupied in ovarian cortex, 68
primary, 117
secondary, 1 17
Germ-cell areas, of late blastula and gas-
trula, later derivatives of, 534
952
Germ gland, primitive gonia in, 115
Germ-layers, concept of, during develop-
ment, 345
Germ plasm, 1 14
an immortal substance, 114
continuity of, 1 14
Germ ring, developmental potencies of in
teleost fishes, 441
embryonic portion of, 438
extra-embryonic portion of, 438-439
of teleost blastoderm, definition of, 436
Germ-track, theory of, 114
Germinal disc, of hen's egg, 313
Germinal plasm (germ plasm), 117
Germinal vesicle, 143
function of in yolk synthesis, 159
in egg of Amphioxus, 136
in egg of dogfish, Scyllium canicula, 136
in hen's egg, 251
of pigeon's egg, 267
Gestation, length of in common mammals,
100
Gila Monster (Heloderma suspectum), poi-
son glands of, 617
Gill (Branchial) arches, 527
Gill filaments, external, in Squalus acan-
thias, 638
Gill pouches, perforation of, 619
Gill septum, in Squalus acanthias, 638
Gill slit, first, formation of in Amphioxus,
502
Gills (Branchial organs), 636
development of in fishes, 638
external, 636, 639, 640
internal, 636
of teleost fishes, 639
resorption of in Amphibia, 642
Gingiva, 609
Gizzard (grinding organ) of chick, 623
Gladiolus (Corpus sterni), 657
Glands, alveolar, in skin of Necturus, 567
apocrine, definition of, 587
auxiliary, 20
bulbourethral (Cowper's), 20, 31
cardiac, of esophagus, 620
coagulating, 33
granular (poison gland), in skin of Nec-
turus, 567
hibernating of woodchuck, 665
holocrine, definition of, 587
intermaxillary, 617
labial, of rattlesnake, Crotalus horridus,
616
lingual, 617
mammary, changes in relation to repro-
duction, 105
mucous, in skin of Necturus, 567
of Littre, 20
Glands — (Continued)
oral, 617
parotid, 617
embryonic origin of, 618
poison, in skin of Necturus, 567
of Gila Monster, 617
of snakes, 617
preening (Uropygial) in bird, 571
prostate, 20
salivary, 617
skin, development of, 687
subaceous, rudiment of, 581
sublingual, origin of in mammals, 617
submaxillary, origin of in mammals, 617
sweat (Sudoriferous), 587
unicellular, in skin of Necturus, 567
vesicular {see Seminal vesicles)
Glass, F. M., 23
Glomus, 776
Glossopharyngeal nerve (Cranial N. IX),
718, 830
Glottis, 650
Glutathione, as a spawning-inducing agent,
228
Gnathostomata, xvi
Gobiid fish (Bathygobius soporator), de-
velopment of, 250
Goedart, 216
Goethe, 536
Goldsmith, J. B., 120
Golgi substance, 126
passing of from the follicle cells into
ooplasm of developing oocyte,
158
Gomphosis, definition of, 615
Gonadal cavity, 794
Gonadotrophins, mammalian pituitary, ef-
fect of on frog ovary, 74
Gonal portion, of genital ridge, 791
Gonia, definitive, 117
primitive, in germ gland, 115
Goodrich, E. S., 528
Gorilla sp., characteristics of egg, site of
fertilization, and place of sperm
entrance into egg, 207t
Graafian follicle, primary, 797
primary, secondary and tertiary, 70
Grafts, testis, 37
Granule, neck of sperm, 149
Granuloblasts, 733, 735
Granulocyte, 733
Grasshopper, conjugate sperm of, 141
multiple acroblast in, 150
"sperm boat" of, 141, 147
Gray column, dorsal (posterior), of spinal
cord, 820
lateral, 821
953
Gray commissure, dorsal, 821
ventral, 821
Gray crescent, appearance of at fertiliza-
tion in egg of Styela, 246, 286
formation of in frog's egg, 249
formation of in Rana sylvatica, 293
Gray matter, 810
Greenblatt, R. B., 927
Greene, R. R., 892
Gregory, P. W., 305
Grinding organ (Gizzard), of chick, 623
Groove, labial (Labiogingival), 609
Ground hog, Marmota monax, seasonal
descent of testes in, 6
Ground squirrel, Citellus tridecemlineatus,
accessory reproductive gland de-
velopment of, 22
descent of testes in, 6
effect of temperature on males, 46
seasonal spermatogenesis in, 22
spermatogenesis in, 23
Ground substance, of bone, 662
Growth (incremental) cone, of developing
neuroblast fiber, 808, 815
Gruenwald, P., 774
Guanine, 849
Guanophores, 591
Gubernaculum of testis, 9, 11, 803
Gudernatsch, J. F., 883
Guinea pig, fertilization in, 236
sperm, acroblast in, 150
sperm morphogenesis of, 146
spermatogenesis in, 148
Gum elevation, 609
Gut tube, primitive, formation and regions
of, 478
Guyer, M. F., 218
Gytnnophiona, xviii {See also Amphibia
gymnophionian)
blastoderms of late, 369
branchial pouch perforation of, 640
Gynogamone I, 228
Gynogamone II, 228
Gynogamones (gynogamic substances),
223
Gynogenesis, 262
Gynomerogony, 267
H
Haeckel, recapitulation theory of, 347
Hagfish, Myxine, gill arrangements in, 639
Hair, bulb, 581
canal, 581
color of, factors concerning, 582
development of, 579
diagrams of, 582
diagrams of development of, 580
down, fine, lanugo, 581
Hair — (Continued)
follicle of, structure of, 581
mature, structure of, 581
Hair cells (Neuromasts), 842
of taste buds, 843
Hair cone, 581
Hair follicle, diagrams of, 582
Hair rudiment, 581
Hair shaft, 581
Half embryo, development of in isolated
blastomeres of Styela partita, 332
Haliotis cracherodii, secretions producing
lysis in, 229
Hall, A. R., 885
Hall, R. W., 798
Haller, 56
Ham, 213
Hamburger, V., 488, 744, 776. 778
Hamilton, H. L., 488, 744, 776, 778
Hamilton, W. F., 889, 890
Hammond, J., 197
Hanson, F. B., 687. 693
Hapiosis, 124
Harderian gland, 853
Harmonious totipotency, definition of, 376
Harmonious totipotential system, 377
Harrison, R. G., 815
Hartman, C. G., 195, 197, 305, 363, 364,
889, 890
Hartmann, M.. 225
Hartsoeker, N. (1656-1725), 56
Harvey, William, 53
Hassall's corpuscles, 880
Hatschek, B., 288
Haustra, 628
Havers, Clopton, 662
Haversian canal, of Haversian system, 662
Haversian system, 662
Head area, innervation of premuscle masses
of, 720
Head cavities, 502
as origin of eye muscles in shark group,
716
Head (cephalic) outgrowth, 461
Head fold, 476
Head gut (Seessel's pocket), 599
Head mesoderm, contributed by neural
crest material, 525
derived from pre-chordal plate, 523
originating from post-otic somites, 525
Head organizer, ability of, 401
transplantation of, 512
Head (pre-oral) gut, 482, 484
Head-process stage, of chick blastoderm,
431
Head region, mesoderm of, origin of, 522
Heart, atrial area of, 750
atrium of, 747
954
INDEX
Heart — (Continued)
converging veins of, 736
development of, 726, 728, 747, 750
early embryonic, fate of divisions of in
various vertebrates, 758t
embryonic, divisions of, 747
mammalian, converging veins of, 747,
748
primitive embryonic, histological struc-
ture of, 748
valves of, in check, 756
ventricles of alligator, 704
Heart beat, contraction wave of, 754
initiation of, 766
Heat, effect upon sperm formation, 36
Hedgehog (Erinaceus europeus), descent of
testes in, 7
female guarding young, 929
Heilbrunn, L. V.. 217, 237, 271, 286
Heloderma suspectum (Gila Monster),
xviii
Hemal arches, 688
Hematopoiesis (Hemopoiesis), 732
Hemicentetes semispinosus, Madagascan in-
sectivore, blastula of, 366
Hemichordata, subphylum, xv
Hemochorial placenta, 918
Hemocytoblast, 733
Hemoendothelial placenta, 918
Hemoglobin, 735
Hemopoiesis (Hematopoiesis), 732
Hen, effect of progesterone on ovulation,
80
egg, germinal disc of, 313
first cleavage plane in egg of, 287
maturation and fertilization in egg of,
252
sperm transport in, 191
Henle, layer of in hair, 583
Hensen's membrane, 702
Hensen's node, formation of in pig gas-
trula, 433
Heparin, suppressed cleavage by, 219
Hepatic portal system, of chick embryo,
745, 746
Hepatic portal vein, 737
Hepatic veins, 738, 743
Hepaticae advehentes, 738
Hepaticae revehentes,'7i5
Hepato-duodenal ligament, 873
Hermaphroditic species, 886
Hershkowitz, S. G., 563
Hertig, A. T., 920
Hertwig, O., 214. 239
Hertwig, R., 218
Hertwig's laws of cleavage, 286
Heterandria formosa, characteristics of egg,
site of fertilization, and place of
sperm entrance into egg, 204t
egg retention of, 915
fertilization in, 199
ovarian fertilization in, 197
Heterogametic sex, 887
Heuser, C. H., 305, 432
Hibbard, H., 187
Hildebrand, S. F., 197
Hill, C, 812, 57i
Hill, E. C, 11
Hill, J. P., 197, 200, 305, 362, 363, 904
Hill, M., 44
Hilus, of ovary, 58
Hindbrain, 812
Hindgut, 481, 600
junction with midgut, diverticula of, 601
Hippocampus, male egg pouch of, 915
His, Wilhelm. 731
enunciation of principle of organ-form-
ing germ-regions, 343
recesses of, 863
Hisaw, F. L., 40, 103
Histochemistry, xii
Histogenesis, definition of, 517
Histology, definition of, 517
Histotrophic nutrition, 921
Hoagland, H., 198
Hochstetter, F., 738, 746
Holoblastic cleavage, 288
Holobranch, definition of, 638
Holonephros, definition of, 772
Holostei, xvii
Holz-Tucker, M., 59
Homo sapiens (man), characteristics of
egg, site of fertilization, and place
of sperm entrance into egg, 207t
Homogametic sex, 887
Homologous chromosomes, definition of,
130
Homology, basic, of vertebrate organ sys-
tems, 545
definition of, 545
Homonculus, 54, 56
Homoplasy, 551
Honeybee, natural parthenogenesis in, 216
Hoofs, development of, 584
diagrams of, 583
Hopkins, M. L., 884
Hormonal control, of ovulation, in lower
vertebrates, 80
of ovulatory process, 78
Hormones, estrogenic, estradiol, 85
estrone, 85
in ovary of hen, 73
influence upon sperm transport in ovi-
duct, 197
INDEX
955
Hormones — (Continued)
gestational (progesterone), 85
gonadotrophic, effects on development of
mammalian egg follicle, 72
influence of on mammalian development,
885
influence of on sex differentiation, 891
lactogenic, luteotrophin, 103
male, in mating urge, 22
Horns, of various mammals, 586
Hoskins, E. R., 883
Hoskins, M. M., 883
Howard, E., 883
Howland, R. B., 773
Huber, E., 708, 718
Huber, G. C, 305
Human, after-birth in, 926
chorionic vesicle of, structure of villi in,
921
development of frontal bone in, 695
development of occipital bone in, 695
development of ovary of, 797
development of temporal bone in, 695
development of testis in, 792
development of tongue of, 609
developmental features of face, 535
digestive tract of, characteristics of mu-
cous membrane of, 620
structural composition of walls of, 629
facial and cervical muscles derived from
hyoid arch mesoderm in, 719
female, artificial insemination of, 199
female, reproductive cycle in, 107
formation of metanephric kidney in, 784
heart of, converging veins of, 747
implantation of embryo in, 919
kidney of, 776
male, acroblast in forming sperm of, 150
later stages of spermatogenesis in, 148
morphogenesis of sperm in, 146
morphogenesis of digestive tract of, 606
olfactory area of nasal passage in, 843
pectoral girdle in, 689
pelvic girdle in, 689
placentation in, 918, 923
relation of embryonic chondrocranium
of to adult skull, 680-68 It
Human ear, three-dimensional schematic
drawing of, 851
Human embryo, development of liver in,
626
differentiation of truncal myotomes in,
714
extra-embryonic membranes in, 972
formation of placenta of, 921
late gastrula of, 432
liver-septum complex formation in, 865
Human embryo — (Continued)
muscle development of, 710, 711, 712,
713
of ten somites, 499
parathyroid gland of, 878
sites of blood-cell formation in, 735
thymus gland of, 878
thyroid gland of, 878
Humphrey, R. R., 120, 890
Huxley, layer of in hair, 583
Huxley, T. H., 536
Hyaline cartilage, 658, 665
Hyaloid artery, 846
canal, 846
Hyaluronidase, 33, 229
Hydrochloric acid, secretion of, 621
Hydromantes genei, sperm transport in fe-
male, 191
Hydromantes italicus, sperm transport in
female, 191
Hyla crucifer, xviii
Hyoid arch, facial and cervical muscles in
mammals derived from meso-
derm of, 719
Hyoid somite, in sharks, 716
Hyomandibula portion of hyoid arch, 673
Hyomandibular cleft, 851
Hypaxial (hypoaxial) region, of vertebrate
body, 494
Hypaxial muscle groups, 706
Hypaxial musculature, of Necturus, Idl
Hypermastia, 590
Hyperthelia, 590
Hypertonic sea water, and egg activation,
217, 218
Hypoblast, 347, 352
formation of in reptiles, 361
primary, conversion into secondary hy-
poblast in teleost fishes, 438
Hypoblast layer, of chick blastula, 343
Hypochord (subnotochordal rod), 655
Hypodermis, 556
Hypogeophis alternans, xviii
blastulation and gastrulation in, 446
late blastoderm of, 369
Hypogeophis rostratus, beginning gastrula
of, 369
Hypoglossal nerve (cranial n. xii), 717, 833
Hypoischial midpiece, of pelvic girdle, 693
Hypomere, contributions of, to developing
heart and gut structures in rep-
tiles, birds, and mammals, 529
to formation of gut tube, 528
to formation of heart structures, 528
definition of, 492
derivatives of, 527
early differentiation of, 527
Hypophysectomy, and testis, 39
956
Hypophysis, and relation to testicular func-
tion, 39
anterior lobe, results of removal, 39
structural composition of, 39
structure of in various vertebrates, 877
Hypophysis cerebri, 39, 876 {see also Pi-
tuitary gland)
Hyporachis (aftershaft), 571, 578
I
Ichthyopterygium (a type of bilateral ap-
pendage), 691
Identical (monogametic) twins, definition
of, 380
Idioplasm theory, of Nageli, 215
Idiosome (idiozome), description of, 126
nature of, 126
Ilium, 693
Implantation, definition of, 916
in Macaca rnulatta (rhesus monkey),
922
in monkey, 919
of human embryo, 918, 919
of pig embryo, 923
types of, 916, 917
Inactive fertilizin, 269
Incisor teeth, 607
Incremental cone, of developing neuroblast
fiber, 808
Incus, 674, 851
Individuation, definition of, 378
of specific organ-forming territories, a
main function of the gastrulative
process, 447
processes involved in, 379
Induction, 400
of a secondary embryo, 411
Inductors, xii
Inferior caval veins, of chick embryo, 746
Inferior vena cava, 740
Infundibula (vestibules) of chick's lung,
647
Infundibulum, 822
relation of to ovary, in opossum, 194
Ingalls. N. W., 721
Ingression, during gastrulation, definition
of, 397
Inguinal fold, 1 1
Inguinal ligament, of mesonephros, 803
Inner cell mass embryonic knob, 300, 303
Inner ear, 851. 854
Innervation, dual, of autonomous nervous
system, 836
of premuscle masses in head and pha-
ryngeal areas, 720
Insectivora, xx
Insemination, artificial, of human female
and domestic animals, 199, 229
Insulin, 875
Integument, vertebrate, definition and gen-
eral structure of, 556
origin of component parts of, 557
Interatrial opening, of developing mam-
malian heart, 757
Interatrial septum, of developing heart,
753, 754, 757
Interbasalia, 685
Intercalated discs, of cardiac muscle, 701,
704
Intercoelomic membrane, 506
Intercostal arteries, 760
Interdorsals, of arcualia, 682, 685
Interkinesis, 131, 132
Intermediate cell mass, synonymous with
mesomere and intermediate meso-
derm, 492
Internal ear, development of, 855
Internal migration, of egg, in frog, 201
Internal transverse muscle, of frog, 707
Internodel segment, of peripheral nerve
fibers, 819
Interoceptive field, 842
Interparietal bone, 695
Intersegmental veins, of chick embryo, 743
Intersexes, 889
Interstitial tissue, of testis, 793
Interventrals, of arcualia, 682
Interventricular septum, of developing
heart, 753. 754, 757
Intervertebral disc, 682
Intestinal folds, 632
Intestinal vein, 736
Intestine, anterior area, diverticula of, 601
histogenesis of, 631
morphogenesis of, in various vertebrates,
630
small, 630
torsion and rotation of. 630
Intrinsic ganglia, of autonomic nervous
system, 837
Intromittent organ, development of, 802
in Ascaphus truei, 190
in male fishes, 755
of gymnophionan amphibia, 792
Invagination, definition of, in gastrulation,
390, 397
Involution, definition of, during gastrula-
tion, 396
Iris, development of, 845
Irritability, 807, 814
Ischium, 693
Isotropic substance, 703
Isthmus, of bird oviduct, albumen addition
to egg in. 906 {see also Fig. 157)
Isthmus, of thyroid gland, 877
Ivy, A. C, 892
INDEX
957
Jaws, rudiments of, 603
Johnson, C. E., 716
Joints, formation of, 695
Jones, L., 574, 575
Jordan, H. E., 667, 664, 733
Jugular ganglia, of vagus nerve, 831
Jugular veins, external, 737
internal, 737
Juhn, M., 577
Jupiter, 53
Just, E. E., 237
K
Kaan, H. W.. 885
Keel, solid, of neural ectoderm, in teleost
and bony ganoid fishes, 439, 45S
Keibel. P., 498
Keimbahn, 114, 117
Keimplasma, 114
Kendall, E. C, 877
Kenneth. J. H., 103
Keratin, in stratum corneum of skin, 568
Kerkring, valves of, 632
Kerr, J. G., i07 , 324, 773, 780
Kidney, 772
collecting ducts of, formation of in chick
embryo, 784
functional, during embryonic develop-
ment, 777, 773
mesonephric, 9 {see also Mesonephric
kidney)
metanephric, 9 {see also Metanephric
kidney)
pronephric, development of in frog, 779
importance of, 774
regions of origin of within vertebrate
group, 770
retroperitoneal position of, 787
types of, 772
Kidney tubules, developing, 776
Kinetochore, 134, 135
Kingsbury. B. F., 717. 796
Kinoplasmic bead (droplet), in relation to
sperm maturation, 169
of mammalian sperm, 148, 168
Koch. F. C. 22
Kolleker. 213
Kopsch, F.. 324
Kowalewski. A.. 288
Krause, end-bulb of, 840, 842
Krause's membrane, 702
Kuntz, A.. 837
Kupffer's vesicle. 439
Kupperman, H. S.. 927
Labial groove, 616
Labial ligament, 803
Lacerta viridis, zona radiata of, 167
Lacertilia, xviii
Lacrimal glands, 853
Lacunae, 563
of bone, 659, 662
trophoblastic, 920
Lagena, 557, 854, 855
Lallemand, 213
Lamella, of bone, 659, 662
Larnpetra ayresii, sea lamprey, xvi
Langerhans, islet of, 625, 630, 875
Langworthy, O. R., 722
Lankester, R., 390
Lanugo. 581
Larsell. O., 645, 645. 646, 648
Larsell, Olof, 827
Larval forms, free-living, xiii
non-free-living, xiii
Larval period, xiv
of development, xiii
Laryngotracheal groove, 645, 649
Larynx, 718
Lasiopyga callitrichus, placenta of, 920
Lateral (allantoic) veins, of chick embryo,
745
Lateral body folds, 461
Lateral-line organs, of head, 830
Lateral-line system, 813, 842
Lateral rectus muscle, innervation of, 829
Lateral veins, development of, of frog em-
bryo, 738
Latitudinal plane, of cleavage, 284
Lavelle, A., 815
LaVelle. F. W.. 892
Law. Van Beneden's. 215
Lebistes (guppy). sperm survival within fe-
male genital tract of, 197
Leblond. C. P., 160
Leeuwenhoek. and the concept of an in-
tangible preformationism, 213
and the concept of gametic union, 213
belief in preformationism, 56
Lens, anterior epithelium of, 845
body, rudiment of, 845
formation of, 849
Leonard, S. L., 40
Lepidosiren paradoxa, lungfish, xvi
cleavage in, 307
egg, size of, 307
external gills of larval form, 640
heart of, 754
Lepisosteus osseus, xvii
air bladder of, 643
cleavage in, 312
development of skin in, 563
958
Lepisosteus osseus — (Continued)
early development of, 311
formation of scale in, 564
reproductive and urinary ducts of, 799
ventral mesentery of, 859
Leplonema condition (of meiosis), 135
Leptotene stage of meiosis, 128, 135
Leuchtenberger, C, 126, 233
Lewis, F. T., 629, 764
Lewis, M. R., 701
Lewis, W. H., 305, 702, 717, 718, 720,
721, 904
Leydig, cells of, 17
and male sex hormone production, 25
LH (luteinizing) factor, and physiological
maturing of sperm, 43
as interstitial-cell-stimulating hormone,
ICSH, 40
effects on follicle development, 72
LieberkiJhn, crypts of, 620
Ligament, suspensory, anterior, 9
Ligamentum arteriosum, 930
Light, as a factor in reproduction, in Eu-
ropean starling, Sternus vulgaris,
44, 45
in ferret, 44
reflection of, and skin color, 592
Light spot, of Amhy stoma egg, 305
Lillie, F. R., theory of fertilization, 269
{see also 320, 577, 794, 891)
Lillie, R. F., 286
Lillie, R. S., theory of fertilization, 271
Limax cainpestris, formation of polar bod-
ies in, 214
Limb, extrinsic mass of premuscle tissue of,
721
intrinsic mass of muscle-forming mesen-
chyme of, 721
Limb-bud f^eld, in urodele, Ainby stoma
unctatum, 509
Limiting membrane, external, of neural
tube, 817
internal, of neural tube, 817
Lineback, P. E., 628. 631
Lipoblasts, 664
Lipochromes, and affect upon skin color in
the fowl, 592
Lipocytes, 664
Lipogenesis, 664
Lipophores, 591
Liposome, 664
Lips, formation of, 616
Liquor folliculi, of mature Graafian folli-
cle, 71, 74
Liver, development of, 736
in human embryo, 626
histogenesis of, 626
Liver cords, 626, 627
Liver rudiment, development of, 623, 625
Liver-septum transversum complex, 861,
863
Liver trabeculae, 736
Lobules, of testis, 793
Local origin theory, of primitive blood ves-
sel development, 730, 731
Locy, W. A., 536, 645, 645. 646, 648
Loeb, J., theory of fertilization, 268
Long bones, of appendages, development
of, 692
Lophodont teeth, 607
LTH, leuteotrophin, 79
Lumbar (sacral) enlargement, of spinal
cord, 821
Lumen, ovulatory, in Tilapia macrocephala,
59
Lungs, 636
cellular composition of, 649
development of, 642, 644
mammalian, alveoli of, 650
formation of respiratory area of, 649
relationships of, 643
Lung pipes (parabronchi), 647
Lunglessness, 651
Luteal phase, of female reproductive cycle,
93
Lutein cells, 85
Luteinization factor (hormone) (LH;
ICSH), and release of sperm
from Sertoli cells, 43
and sperm development, 41
and spermatogenesis, 40-41
effect on testis activity, 40
response to, of seminiferous tubules,
24-25
Luteotrophin. LTH, 79, 103
as involved in functional behavior of cor-
pus luteum in progesterone secre-
tion, 91
Luther, W., 441
Lymph, relation of to connective tissues,
654
Lymph fluid, 726
Lymph hearts, 726, 747, 762
Lymph nodes, 726, 766
formation of, 766
Lymph sacs, 764
Lymphatic structures, development of, 764
Lymphatic system, 726
development of, 762
Lymphatic vessels, 726
Lymphoblasts, 733, 736, 766
Lymphocytes, 736, 766
Lymphoid forms, of blood cells, 736
Lynn, W. G., 878, 879, 886
Lysis, secretions which cause, during fer-
tilization, 229
INDEX
959
M
Macaca mulatta (rhesus monkey), charac-
teristics of egg, site of fertihza-
tion, and place of sperm entrance
into egg, 207t
implantation of, 920, 922
MacBride, E. W., 55
McClain, J. A., 864, 879
McClung, C. E., sex chromosome hypothe-
sis of, 215
McClure, C. F. VV., 731
McCrady, E., Jr.. 364, 930
Macromastia, 590
Maculae of ear, 854
Malaclemys centrata, sperm survival within
female genital tract of, 197
Male, amplectant, 248
Mall, F. P., 664, 721
Malleus, 674, 851
Malpighian body, 781, 782
Malpress, F. H., 108
Mammae, xix, 587
Mammalia, xix
Mammalian embryo, olfactory organ de-
velopment of, 844
Mammals, changes in converging veins of
heart in, 748
chorionic vesicles in, 924
copulatory organ of, 19
development of heart in, 757
development of lungs in, 648, 649
development of sternum in, 687
development of vertebrae in, 685
developmental stages of neurocranium
and splanchnocranium in, 675
divisions of primitive cloaca in, 715
effect of pituitary and thyroid glands on
development of, 885
eutherian, cleavage in egg of, 297
facial and cervical muscles derived from
hyoid arch mesoderm in, 779
formation of coelomic cavities in. 868
formation of sternum in. 688
gastrulation in. 431
heart of. converging veins of, 747
metatherian, cleavage in egg of, 297
modifications of circulatory system of at
birth, 766
musculature associated with mandibular
visceral arch of, 717
origin of definitive germ cells in. 118
origin of musculature of diaphragm of,
718
other than pig, gastrulation in. 435
palatal conditions in, 615
parietal pericardial wall development in,
872
Mammals — (Continued)
prototherian, egg, bilateral symmetry of,
162
tertiary egg membranes of. 904
tubulation of epidermal area in, 476
Mammary cycle, 92
Mammary glands, 587
changes in during reproductive cycle, in
bitch, 102
development of, 588, 589
Mammary (milk) ridges, 587
Mammilla (teat; nipple), 589
Mammillae, depositions of in bird's egg-
shell, 906
Mammogen, 107
Mandibular somite, in shark, 716
Mandibularis, division of trigeminal nerve,
829
Manta birostus, ray, xvi
Mantle (nucleated) layer, of developing
neural tube, 817
Marcus, H., 716
Marginal cells, of chick blastoderm, 316
of Echidna blastoderm, 326 {see also
Vitellocytes)
Marginal layer, of developing neural tube,
817
Marmoset, Oedipomidas geofjroyi, lack of
freemartin condition in. 892
Marrow, red, 694
yellow, 694
Marrow areas, secondary, 668
Marrow cavities, 662
primary, 667. 668
Marshall, F. H., 7
Marsupial frogs, 916
Maternal placenta (pi. materna), 922
Mathews, A. P., 219
Mating urge, and spermatogenesis, 22
Matrix, of nail, 584
Matthews, S. A., 23
Maturation of egg, in Echidna, 254
in egg of pigeon. 267
in hen's egg. 252
Maxillaris, division of trigeminal nerve, 828
Maximow, A. A., 582, 664
Mead, A. D., 217
Meatus venosus, of chick embryo, 743
Mechanism, for controlling time of sperm
entrance into egg, 257
Meckel's cartilage, 672
Median fissure, ventral, 820
Median septum, dorsal, 820
Mediastinum, 793, 871, 880
Medulla, of hair shaft, 582
of ovary, 58, 795, 796, 797
Medullarin, 890
Medullary cords, 792
960
INDEX
Medullary field, a gonadal sex field, 889
Medullary sheath of nerve fiber, 819
Megakaryocytes, 726
Megathura crenulata, secretions producing
lysis in, 229
Meibomian glands, 853
Meiocyte, definition of, 124, 125
female, dependent nature of maturation
divisions in, 144
Meiosis, 124
and nuclear growth, 142, 143
crossing-over phenomena associated with,
134
general description of, 130
in amphibian egg, 138
nuclear changes during, 128
peculiarities of nuclear behavior in oo-
cyte during, 141
Meitoic division, 128, 132
disjunctional, 133
equational, 133
in spermatocyte and oocyte, 144
reductional, 133
Meiotic mitosis, 130
Meiotic phenomena, resume of, 145
Meissner, corpuscles of, 840, 842
Melanin, 577, 592
granules of, in skin of mammals, 579
Melanophores, 591, 592
Membrana chalazifera, of bird's egg, 905
Membrana granulosa, of mature Graafian
follicle, 71, 74
Membrane, embryonic, {see Embryonic
membrane)
extra-embryonic, 908
post-nuclear, 152
vibratile, description of in sperm tail, 153
Membrane formation, and activation of
egg, 218
Membranous labyrinth, of inner ear, 851,
853
Menstruation, definition of, 93
possible factors effecting, 93, 107
Meridional cleavage furrows, in chick
blastoderm, 315
Meridional plane, of cleavage, 283
Meroblastic cleavage, 288
Merogony, 265
Mesencephalon, 812, 822
Mesenchyme, 520, 522, 656
embryonic, contribution of to adult
skeletal tissue, 655
derivatives of, 532
primitive skeletogenous, 526
Mesenchymal packing tissue, of early em-
bryo, importance of, 656
Mesentery, dorsal, 859
ventral, 859
Mesoblast, 347
Mesobronchus of chick lung, 646
Mesocardium, dorsal, 748, 750, 859
lateral, 728. 748, 859, 863
formation of, 863
role of in initial division of embry-
onic coelom, 860
ventral, 748, 859
Mesoderm, extension (migration) of in the
urodele, Pleurodeles, 417
introduction of word, 347
of head region, origin of, 522
of tail, origin of, 525
position of in embryo of Amphioxus,
494
in vertebrate embryo, 494
pre-chordal plate, definition of, 523
precocious, elaborated in human embryo,
436
tubulation of, in Amphioxus, 503
Mesodermal areas, of late blastula and
gastrula, later derivatives of, 534
Mesodermal bands, in Amphioxus post
gastrula, 505
Mesodermal cells, types of, 522
Mesodermal grooves, in Amphioxus post
gastrula, 505
Mesogastrium, 859
Mesomere, definition of, 492
Mesorchium, 793, 803
Mesosalpinx, 803
Mesosternum, 687
Mesothelium, 522, 530
Mesovarium, 58, 803
Metabolic change, at fertilization, 243
Mesonephric kidney (Mesonephros), de-
velopment of 773, 778, 780, 781,
782
Mesonephric (Wolffian), duct, 771. 778,
787
origin of in Squcdus, 775
Metamerism, 534
basic metamerism, 534
of spinal nerves, 817
Metamorphosis, of sperm, 17
role of thyroid and pituitary glands in,
883
Metanephric diverticulum, origin of in
chick, 784
Metanephric duct, 784, 787
Metanephric kidney, development of, 782,
784, 785
retroperitoneal position of, 784
Metanephric renal units, formation of in
chick embryo, 784
Metanephrogenous tissue, 786
Metanephros, 773
Metatheria, xx
961
Metencephalon, 822
Metenteron, definition of, 597
development of, 602
early, main types of, 597
esophagus and stomach region of, mor-
phogenesis and histogenesis of,
621
hepato-pancreatic area, morphogenesis
and histogenesis of, 623
primitive, basic cellular units of, 600
tubular, formation of in flat blastoderms,
482
Metestrus, definition of, 94
Miastor, chromatin diminution in, 118
early development of, 118
Microglia, 810
Micromastia, 590
Micropyle, 167, 257
in egg of Bathygohius soporator, 249
Midbrain, 812
Middle ear, 557
development of, 855
Middle piece, of sperm, 149
Midgut. 478, 600
junction with hindgut, diverticula of, 601
Milk (deciduous) teeth, 613
Milk ridges, 587
Miller, A. M., 746
Miller, D. F., 927
Minot, C. S., 498
Mintz, B.. 892
Mitchell, G. A. G., 6
Mitochondria, 126, 157
role of in fat formation, 664
transformation into yolk spheres, 156
Mitochondrial body, or nebenkern, 152
Mitochondrial cloud, 159
Mitochondrial material, role in formation
of middle piece of sperm, 152
Mitochondrial yolk body, 159
Mitosis. 281
mechanisms associated with, 284
Molgula manhattensis, xv
Molva (ling), lack of parental care in, 900
Monad condition, of developing gamete,
133
Monestrus, definition of, 94
Monkey, implantation of embryo, 919, 922
Monoblasts, 733, 736
Monocyte, 733, 736
Monoploid condition, of developing
gamete, 133
Monotremata, egg transport through Fal-
lopian tube in, 904
Monozygotic twins, definition of. 380
Moore, Carl R., 35, 119, 885, 892
Moore, J. A., 297
Morgan, T. H., 216, 297, 382
Morphogenesis, definition of, 517
Morphogenetic movements, summary of,
during gastrulation, in frog and
other amphibia, 415
Mossman, H. W., 910, 915, 920
Motor fiber, 816
Motor nucleus, dorsal, of vagus nerve, 831
Motor plate, 838
Mouse, placentation in, 915
Mouth, formation of, in Amphio.xus em-
bryo, 502
Mucosal walls, characteristics of, 620
Mucous layer, of stomach, characteristics
of, 620
Miillerian duct, 799
Muscle(s). 701
abductor caudae externus. 716
abductor caudae internus, 716
adaptations of, aerial, 708
natatorial, 706
terrestrial, 706
adductor mandibulae, 717
arrector pili, attached to hair follicle, 581
result of adrenaline stimulation on,
882
associated with hyoid visceral arch, de-
velopment of. 717
associated with mandibular visceral arch,
development of, 717
associated with spinal accessory nerve,
development of, 718
branchial, development of in various
vertebrates, 708, 718
cardiac, characteristics of, 700, 701
histogenesis of, 702
ciliary, origin of, 853
coccygeo-iliacus, of frog, 707
coccygeo-sacralis, of frog, 707
depressor mandibulae. of birds, 718
of frog, 717
derived from posterior visceral arches,
development of, 718
digastric, 717
dorsalis trunci, of Nectiirus, 101
extrinsic, of eye, 716
innervation of, 716
facial and cervical, derived from meso-
derm of hyoid arch, 779
first ventral constrictor muscle shark,
717
iliocostalis, of human embryo, 714
iliopsoas in human, 715
intercostales externi, of human embryo,
715
intercostales interni, of human embryo,
715
interneurales, of frog, 707
interspinales, in human embryo, 714
962
INDEX
M uscle (s ) — (Continued)
intertransversarii, in human embryo, 714
of frog, 707
levatores costarum, of human embryo,
714
longissimus dorsi, in human embryo, 714
of frog, 707
longus capitis in human, 715
longus colli in human, 715
masseter, 708, 111
multifidus, of human embryo, 715
oblique, external of scrotum, 12
internal of scrotum, 12
obliquus abdominis externus in human,
715
obliquus abdominis internus in human,
715
obliquus externus, of Necturus, 701
obliquus externus superficialis, of frog,
707
obliquus inferior of eye, 716
obliquus internus, of Necturus, 707
obliquus superior of eye, 716
of cloacal and perineal area, 715
of head-pharyngeal area, development
of, 716
of post-branchial area, development of,
717
of tail region, development of, 715
of tongue, 717
of trunk and tail, characteristics and de-
velopment of, 705
of visceral skeleton, development of, 717
pyramidalis, 716
quadratus, 716
quadratus femoris, 701
rectus abdominis, 715
of frog, 707
of Necturus, 707
rectus externus (posterius or lateralis)
of eye, 716
rectus inferior of eye, 716
rectus internus (anterius) of eye, 716
rectus superior of eye, 716
retractor oculi of eye, 716
rotatores, in human embryo, 714
semitendinosus, 701
serratus posterior inferior in human, 715
serratus posterior superior in human, 715
skeletal (striated), 700
histogenesis of, 702
types of muscle fibers in, 700
smooth. 700
characteristics of, 701
histogenesis of, 704
somitic, development of in various ver-
tebrates, 708
sphincter colli, of birds, 717
Muscle (s) — (Continued)
spinalis dorsi, of human embryo, 714
transversus, 12
transversus abdominis in human, 715
transversus, of frog, 707
Muscle column, of visceral arch, 619
Muscle fibers, 700
relation of to tendinous attachment, 701
Muscle septa (Myosepta; Myocommata),
706
Muscle tissues, arrangement of, 704
general structure of, 700
histogenesis of, 702
structure of, 703
Muscular contraction, as a means of sperm
transport, 193
Muscular system, definition of, 699, 700
morphogenesis of, 705
Musculature, adaptations of, 706
of mammalian diaphragm, origin of, 718
of paired appendages, origin of, 718
Mus musculus (mouse), characteristics of
egg, site of fertilization, and place
of sperm entrance into egg, 206t
Muskox, effect of testosterone on, 27
Mustelus laevis, dogfish, placenta of , 9 1 3, 9 1 7
Myelencephalon, 822
Myelin-emergent fiber, 819
Myelin (Medullary) sheath, 819
Myelinated fibers, 820
Mylohyoid muscle, 717
Myoblast, 702
Myocardial primordium, 703
Myocardium, 703
Myocoels. 526. 858
Myocommata (Myosepta), 706
Myofibrils, 700
arrangement of in skeletal muscle, 700
fine, of smooth muscle. 702
Myoglial fibers, 704
Myoglial fibrils, of smooth muscle, 702
Myosepta (Myocommata), 706
Myotome, 506, 711
differentiation of, in fishes and
amphibia. 714
truncal, differentiation of in higher ver-
tebrates, 714
Mystacoceti, xxi
Myxine glutinosa, hagfish, formation of
adipose tissue in, 664
yolk sac of, 908
Myzostoma glahrum, fertilization in, 261
origin of centrioles in first cleavage in,
261
N
Niigeli, "idioplasm theory" of, 215
Nail field, of developing finger nail, 584
INDEX
963
Nail fold (groove), of developing finger
nail, 584
Nail matrix, 584
Nail plate (Unguis), 584, 585
ventral (subunguis), 585
Nails, development of, 584
diagrams of, 583
Nasal processes, 844
median, 844
Nasal septum, 844
Naso-lacrimal duct, 853
Naso-lacrimal groove, 853
Natatorial adaptations, of muscles, 706
Natrix sipedon, xix
Neal, H. V., 716
Nebenkern, mitochondrial, 151
or mitochondrial body, 152
Neck, of sperm, 149
of tooth, 605
Nectophrynoides vivipara, xviii, 189
Necturus, development of vertebrae in,
685
differentiation of myotomes in, 714
muscles of, 707
skin of, 566
spermatheca of, 190
Necturus maculosus (Mud puppy), xvii
branchial-pouch-groove perforation of,
640
characteristics of egg, site of fertiliza-
tion, and place of sperm entrance
into egg, 204t
cleavage of egg in, 307, 308
development of gills in, 641
development of skin in, 567
egg, size of, 307
external gills of early stages, 639, 640
first cleavage plane of dgg in, 287
lung of, 644
skeletal muscle development in, 708
stages of normal development of, 473
yolk sac of, 908
Nelsen, O. E., 796
Neoceratodus forsteri, lungfish, xvi
Neornithes, xix
Nephric, use of term, 772
Nephric (Renal) units (Nephrons), 772
development of in mammalian kidney,
786
formation of in mammal (human), 784
Nephrocoel, of frog pronephric kidney, 776
Nephrogenic cord, 781, 782, 786
Nephrons (Nephric units), 772
types of, in developing vertebrate em-
bryos, 773
Nephros, 772
Nephrostomes, 780
Nephrotomes, or segments of nephrotomic
plate, 773, 774
Nephrotomic mesoderm (urogenital meso-
derm), synonymous with meso-
mere, 492
Nephrotomic plate, 772, 786
Nereis, fertilization in, 239, 241, 260
sperrn entrance into egg of, 212
Nerve(s), abducens (cranial n. VI), 829
acoustic (cranial n. VIII), 830
chorda tympani, 829, 843
cranial, abducens, (cranial n. VI), 829
facial (cranial n. VII), 829
functional components of, 824
glossopharyngeal (cranial n. IX), 718,
830
hypoglossal (cranial n. XII), 717, 833
nervus terminalis (cranial n. O), 827
nuclei of origin and termination of
cranial nerves, 824
oculomotor (cranial n. HI). 827
olfactory (cranial n. I), 827
optic (cranial n. II), 827
origin, development, and function of,
827
spinal accessory (cranial n. XI), 718,
833
trigeminal (cranial n. V), 717, 828
trochlear (cranial n. IV), 828
vagus (cranial n. X), 831
Nerve fibers, termination of, 838
Nerve-net theory (doctrine), of nervous
structure, 807
Nervous layer, of primitive epidermis of
frog embryo, 568
Nervous system, 805-856
capsular cells of, 819
central. 809
definition of, 807
functional unit of, 807
peripheral, 809
structural fundamentals of, 812
supporting tissue of, 809
vertebrate, structural divisions of, 809
Nervous tissues, embryonic origin of, 810
histogenesis of, 814
Neural ectoderm, solid keel of, in teleost
and bony ganoid fishes, 458
Neural fold method, of neuralization, 466
Neural (ganglionic) crest, 469, 810, 812
Neural groove, 466
Neural plate, in Amphioxus gastrula, 495
Neural plate area, of late blastula and gas-
trula, later derivatives of, 533,
810
Neural tissue, 520
964
Neural tube, bilateral symmetry of, 513
dependency of upon surrounding tissues,
513
developing, structure of, 8J0
histogenetic zones of, 816
supporting tissue of, development of, 816
Neuralization, definition of, 465
neural fold method of, 466
thickened keel method of, 465
Neuraxis, 814
Neurenteric canal, 471
formation of in frog embryo, 469
in Amphioxus, 497
Neurilemma (sheath of Schwann), 819
Neurobiotaxis, definition of, 824
Neuroblasts, apolar, 807, 815
bipolar, 815
primitive, 814
unipolar, 815
Neurocoels, 822, 858
Neurocranium, 668
basic cartilaginous foundation of, 652
definition of, 669
development of, 669
development of in Amia calva, bowfin,
670
developmental stages in frog, 672
developmental stages of in bird, 673
mammalian, developmental stages of,
675
of chicks, 678
of frog. 676
of human, 680
types of in vertebrates, 669
Neurofibrils, 814
Neuroglia, 809
Neuroglia cells, 810, 816
Neurohumoral substances, 594, 838
Neuromast system, 813
Neuromasts, 842, 854
Neuromeres. 812, 813
Neuron structure and relationships, 808
Neuron theory (doctrine), of nervous
structure, 807
Neurons, afferent, 826
association, 809
cytoplasmic changes of, 814
formation of, 814
multipolar, 815
nuclear changes of, 815
Neuroplasm, 814
Neuropore, anterior, 471, 497
posterior, 471
Neurosensory cells, of olfactory epithe-
lium, 843
Neurula, 468
Newport, 213, 281
Nickerson, W. S., 564
Nictitating membrane, 853
Nieuwkoop, P. D.. 120
Nipple (teat), 589
eversion, 589
inversion, 589
Nissl substance, 815
Noback, C. R., 927
Noble, G. K., 189, 591
Nodose ganglia, of vagus nerve, 831
Normality, male, effects of FSH and LH
upon, 41
Notochord, a specialized, median portion
of middle germ layer of meso-
dermal tissue, 493
position of in embryo of Amphioxus,
494
position of in vertebrate embryo, 494
Notochordal area, as center of organization
of the late blastula, 350
of late blastula and gastrula, later de-
rivatives of, 533
Notochordal canal, 417
in human gastrula, 432
Notochordal-neural canal, 474
Nourishment, in relation to testicular
function, 38
of young, 899
Nuclear changes, of developing neurons,
815
Nuclear equality, of cleavage blastomeres,
328
Nuclear pole, of egg and sperm entrance,
259
Nucleated (mantle) zone, middle, of neu-
ral tube, 817
Nucleation, delayed, in developing egg of
Tritiirus viridescens, 332
Nucleolus, of developing neuroblast, 815
Nucleus, metamorphosis of, during
spermiogenesis, 153
Nucleus ambiguus, of vagus nerve, 831
Nucleus pulposus (pulpy nucleus), 682
"Nurse cells," 147
Nussbaum, M., 114
Nutrition, circulatory system in relation to,
927
histotrophic, 921
Occipital bone, development of in human,
695
Oculomotor nerve (cranial n. Ill), 827
O'Donoghue, C. H., 197
Odontoblasts, 61 1
dentinal fibers of, 611
in skin of shark embryo, 562
Odontoceti, xxi
INDEX
965
Oken, 536
Okkelbergia lamotteni, sea lamprey, xvi
Okkelburg, P., 235
Oldham, F. K., 884
Olfactory nerve (cranial n. I), 827
Olfactory organ, development of, 843
Olfactory pits, 844
Olfactory placode, 843, 844
Oligodendroglia, 810
Olsen, M. W., 287, 313, 320
Omasum (Psalterium; Stomach manyplies),
of cow's stomach, 623
Omental bursa, 873
"Omnis Cellula e Cellula," dictum of R.
Virchow, 281
Omohyoid muscle, innervation of, 717
Omphalomesenteric veins, of chick em-
bryo, 743
Omphalos (Umbilical ring; Umbilicus), 912
Oncorhynchus tschawytscha, king salmon,
xvii
Ontogeny, recapitulates phylogenetic pro-
cedures and not adult morpho-
logical stages, 351
Oocyte, (see also Egg)
c>toplasm of, and formation of a second
kind of yolk, 158
definition of, 68, 125, 154
degree of fusion with sperm at fertiliza-
tion, 212
inequality of cytoplasmic division in, 144
mammalian, membranes of, 168
maturation divisions in, as compared
with spermatocyte, 144
in relation to sperm entrance and egg
activation, 256, 269
membranes in relation to, 162
nucleus during meiotic prophase, 136
polarization of, 160
primary, 145
primitive, characteristics of, 68
reptile, zona radiata (zona pellucida) of,
167
secondary, 145
Oogonia, 1 14
young, of fowl, 154
Ooplasmic movements, and sperm entry,
243
Ooporphyrin pigments, 907
Ootid, 145
Opercular opening, of gill chamber in frog
tadpole, 641
Operculum, formation of in frog tadpole,
461
of gill chamber, in teleost fishes, 639
Ophthalmicus (deep profundus), division
of trigeminal nerve, 828
Opisthonephros, 773, 780
Opossum, bifid penis in male, 196
blastula of, formative cells of, 365, 366
corpus luteum of, 84
early development of blastoderm, 363
female reproductive tract in, 196
female suckling young, 929
fertilization in, 252
Graffian follicle in, mature, 72
tertiary conditions of, 71
Graffian follicle in developing ovary, 70
pseudo-vaginal birth canal of, 63
relation of ovary to infundibulum in,
194
reproductive system, female, 63, 64
young female, primitive germ cells near
germinal epithelium, 119
Oppenheimer, J. M., 287, 441
Opsanus (Batrachus) tan, origin of oral
cavity in, 603
Optic evaginations, primary, 845
Optic lobes, 822
Optic nerve (cranial n. II), 827
Optic nerve fibers, decussation of, 827
Optic stalk, 845
Optic vesicles, 822
primary and secondary, 845
Oral cavity, origin from stomodaeal invagi-
nation, 603
Oral evagination, of foregut, 482
Oral glands, 676
Oral membrane, 599
Organ, intromittent, 4, 18
Organ-forming areas, major, antero-pos-
terior extension of, 457
starting point for tubulation of, 459
major presumptive, of late blastula and
gastrula, summary of later deriv-
atives of, 533
relationships of at end of gastrulation
in anuran amphibia, 460
migration of in the amphibia, 409
of the chordate blastula, 352
presumptive, in amphibian late blastula,
355
of chick blastoderm, 360
of chordate blastula, 344
of late amphibian blastula, destiny of,
354
of Salmo irideus (trout) blastoderm,
369
of the blastula, principle of, 344
Organ-forming germ-regions, principle of,
enunciated by Wilhelm His, 343
Organ of body, definition of, 517
Organ system, definition of, 518
early, fundamental similarity of, 520
vertebrate, basic homology of, 545
Organism, definition of, 518
966
Organization center, 398
characteristics of, 399
different action of compared to ordinary
induction of neural tube, 400
dual appearance of in trout blastoderm,
382
importance of, 395
in late blastula, 283
isolation of in early duck embryo, 385
necessity for, 381
of late blastula, importance of, 386
relation of to gastrulative process, 398
Organizers, xii
primary, importance of, 395
Organogenesis, definition of, 518
Origin of germ cells, 12 It
Ornithorynchus paradoxus, xx
Oryctolagus cuniculus (rabbit), character-
istics of egg, site of fertilization,
and place of sperm entrance into
egg, 206t
Ossification, centers of, 695
of bony skulls, 674
Osteoblasts, 666, 693
in developing scale of Lepisosteus, 563
Osteoclast, 661, 693
action of in bone formation, 667
Osteocollagenous fibers, 659
Osteocytes, 659, 662, 666
Ostium tubae abdominale, 63
Ostrea virginica, spawning reaction in, 230
Otic capsules, 671
Otic ganglion, association with parotid
gland, 831
Otic placodes, 850
Otic vesicle, differentiation of, 830
Otoliths, of ear, 854
Ova, degeneration of, 67
Ovarian capsule, around ovary of rat, 194
Ovarian cycle, 92
Ovarian fertilization, in eutherian mammal,
Ericulus, 197
in Gambusia affinis, 197
in Heterandria fonnosa, 197
Ovarian ligament, 803
Ovarian sac, 799
Ovariectomy, during pregnancy, 99
Ovaries, 788
activities of, 67
as dynamic center of reproduction for
most animal species, 56
as source of estrogen, 86
as "storehouse" of oogonia, 67
bird ovary, 61
cortex of, 58, 67, 68
cyclic changes in, 60
development of, in chick, 797
in mammal, 795
Ovaries — (Continued)
effects of vitamin deficiency upon, 66
fertilization in, 197
general cell structure of, 60
germinal epithelium of, 58
hilus of, 58
human, development of, 797
importance of, 53
importance of in mammary gland devel-
opment and lactation, 103
internal conditions as an ovulatory
factor, 81
medulla of, 58
of the hen, estrogenic hormone in, 73
other possible functions of, 108
relation of to infundibulum, in opossum,
194
role of in gestation, 98
role of in parturition, 101
saccular, 60, 62
structure in teleosts, 59
structure of, 57
tunica albuginea of, 58
use of words, 5
Oviduct, glandular portion of, 62
of birds, albumen-secreting region of,
905
ostium of, in Rana pipiens, 195
uterine portion of, 62
protective function of, 901
uterine segments of, degrees of fusion of,
64
Oviparity, 903
Oviparous, definition of, 903
Ovis aries (sheep), characteristics of egg,
site of fertilization, and place of
sperm entrance into egg, 207t
Ovists, 56
Ovotestis, 890
Ovoviviparity, 903
Ovoviviparous species, 902
Ovulation, dependent, 82
factors controlling, 75
hormonal control of, in lower verte-
brates, 80
in lower vertebrate groups, 79
in the hen, role of progestrone, 80
in the rabbit, 77
internal conditions of the ovary as a
factor, 81
process of in higher mammals, 76
spontaneous, 82
Ovulatory process, hormonal control of, 78
Ovum, definition, Harvey's conception of,
55
Owen, R., 536
INDEX
967
Oxygen consumption, at fertilization, in
Fundulus heteroclitus, 244
effects of fertilization on, 240
Oxygen supply, mechanisms for, in devel-
oping embryo, 903
Oxyphil cells, of parathyroid glands, 879
Oxytocin, 101
P
Pachynema chromatin conditions, 137
Pachytene stage of meiosis, 128, 132, 137
Pacinian corpuscle, 840, 842
Palade, G. E., 159
Palatal conditions, in chick, 615
in frog, 615
in mammal, 615
Palate, primary, 615
secondary, 844
formation of, 615, 615
Pancreas, a compound alveolar gland, 630
development of rudiments of, in chick
embryo, 628
in frog embryo, 628
in shark embryo, 628
endocrine aspect of, 630
exocrine aspect of, 630
histogenesis of, 630
Pancreatic acini, 625
Pancreatic rudiment, development of, 623,
625
Pangenesis, theory of, 114, 348
Panniculus carnosus, 705, 722
Papilla, dermal, in developing feather germ
in chick, 574
of hair, 581
Papillary cone, vascular, of reptilian eye,
849
Parabronchi, of bird lung, 647
of chick, 645
Parachordal cartilages, of primitive em-
bryonic skull, 671
Parallelism, 551
Paralutein cells, 85
Paraphysis, 822, 881
Parascaphirhynchus albus, xvii
Parasympathetic division, of autonomic
nervous system, 836
Parasynapsis, 137
Parathyroid glands, 619, 879, 880
development of, 880
distribution of in vertebrate groups, 880
of human embryo, 878
role of in calcium metabolism, 879
Parietal cells, of stomach mucosa, 621
Parietal mesoderm, 506
Parietal organ, 881
Parietal pericardial wall, formation of, 871
Parkes, A. S., 44
Parmenter, C. L., 219
Parotid gland, innervation of, 831
Pars caeca retinae, of developing retina,
845, 853
Pars intermedia, of pituitary gland, 877
Pars optica retinae, of developing retina,
845
Parthenogenesis, artificial, Bataillon's
method for producing in
frog, 218
in annelid worm, Chaetopterus, 111
results obtained by work on, 220
from non-sexual egg, 216
natural, 216
Parthenogenetic merogony, 266
Passerifonnes, xix
Pasteels, J., 354, 355
Pasteur, Louis (1822-1895), 57
Path, cocopulation, of sperm during ferti-
lization, 248
initial entrance of sperm during fertili-
zation, 248 .
Patten, B. M., 305, 364
Patterson, J. T., 313, 315, 320, 383, 384
Payne, L. F., 193
Pecten, of bird's eye, 846, 849
Pectoral girdle, 689
Pelagic eggs, 251
Peltier, 213
Pelvic girdle, 689, 693
Pelvis, formation of in mammal (human),
784
Penetration path of sperm, 248
Penis, bifid, in male opossum, 196
Pepsin, secretion of, 621
Perameles, superficial implantation in, 916
Perch, Perca flavescens, egg membranes of,
165
sperm production in, 30
Perforatorium of sperm, 149
Periblast, central, 319
Periblast, peripheral, 319
Periblast tissue, of elasmobranch fishes,
321
origin of in teleost fishes, 324
Pericardial cavity, 857, 861
Pericardial division, primary, of coelom,
formation of, 859
Pericardial walls, independent, develop-
ment of, in amphibians, 871
in chick, 871
in mammals, 872
in reptiles, 871
Pericardioperitoneal canals, 861
Perichondria! bone formation, 667
Perichondrium, definition of, 665
Peridental membrane, definition of, 614
968
Periderm, 558, 559, 579, 908
of primitive epidermis of frog embryo,
568
Perilymph, of ear, 854
Perimysium, external, 701
internal, 701
Perineurium, 827
Periosteal bone formation, 667
Periosteal tissue, 693
Periosteum, 666, 667, 693
Peripheral nervous system, development
of, 823
early histogenesis of, 817
structural divisions of, 823
Perissodactyla, xx
Peritoneal cavity, 858, 863, 873
Peritoneal division, of coelom, primary,
formation of, 859
Peritoneal support of the ovary, 58
Peritoneopericardial membrane, 864, 866
Perivitelline fluid, 235
Perivitelline space, 167, 235, 903
Petroniyzon, ammocoetes larva of, 885
embryo of, partial eversion of oral cav-
ity in, 607
pineal body of, 881
stomach area of, 621
Petroniyzon ftuviatilis, hypophysis of, 877
Petroniyzon marinas, sea lamprey, xvi
Pflijger's cords, proliferation of, 796
Pharyngeal area, role of in respiration, 619
Pharyngeal (branchial) pouches, 527, 618,
619
Pharyngeal diverticula, 902
Pharyngeal glands, of internal secretion.
619
Pharyngeal grooves, 618
Pharyngeal membrane, 599, 632
Pharyngeal (oral) evagination, of foregut,
482
Pharyngeal placentae, 902
Pharyngeal veins, 737
Pharynx, 618, 718
Pharynx, diverticula of, 601
functions of, 601
Phillips, R. W., 195
Phrenic nerve, origin of, 718
Phyrynosoma cornuturn, xviii
Physiological maturation, of sperm, in re-
productive ducts, 169
in testes, 169
Physoclistous type, of air bladder, 643
Physostomous type, of air bladder, 643
Pia mater, 821
Pig embryo, developing coils in digestive
tract of, 628
development of body form in, 498
early development of, 298, 364, 496
Pig embryo — (Continued)
gastrulation in, 433
implantation of, 923
of about 9.5 to 12 mm., 546-550
primitive streak, notochord, and meso-
dermal migration in, 434
sections of, 497
sections of embryonic (germ) disc of,
435
tooth development in, 614
Pigeon, female, reproductive organs of,
192
maturation phenomena in egg of, 267
Pigment, in hair, 592
Pigment cells, origin of from neural crest
ceils, 469
Pigmented coat, of eye, rudiment of, 845
Pike, walleyed, fertilization of, 232
Pincus, G., 12, 119, 198, 219, 305
Pineal body, 822, 881
Pinkus, F., 583, 827
Pinna (External ear), 851, 855
Pinnipedia, xx
Pipa pipa, xviii
Pipefish, brood pouch in, 188
Pipefishes, egg migration of to brood pouch
of male, 199
Pit organs, 843
Pituitary gland (Hypophysis cerebri), 39,
876
anterior lobe of, 876
efi"ect of on development of chick, 884
effect of on development of mammal,
885
in vertebrate embryology, general con-
clusions, 886
influence of in anuran metamorphosis,
883
influence of on development of fishes,
885
pars intermedia of, 877
posterior lobe of, 877
role of secretions of in color change, 594
Pituitary gonadotrophins, effects on ovaries
of vertebrates other than mam-
mals, 73
Placenta, 917, 922
as source of estrogen, 87
formation of, in human embryo, 921
functions of, 926
of mouse, 915
oral, 930
primary, of monkey, 922
secondary, of monkey, 922
types of, 917, 918
Placenta fetalis, 922
Placenta materna, 922
Placental relationships, 913
INDEX
969
Placental septa, 922
Placentation, definition of, 917
human, 923
types of embryonic tissues involved in,
917
Placodes, acoustic, 813
lateral line, 813
lens, 812
nasal, 812
taste bud, 813
Plasis, 547
Plasm, polar, 1 17
Plasma, 726
seminal, 21, 32
Plastron, xix
Piatt, J. B., 603, 716
Platydactylus, formation of hypoblast in,
361
Platypus (Ornithorhynchus), xx, character-
istics of egg, site of fertilization,
and place of sperm entrance into
egg, 205t
cleaving eggs of, 324, 326
corpus luteum of, 84
Plethodon cinereus, xvii
Pleural cavities, 857. 869
Pleurodeles, mesoderm extension (migra-
tion) in, 417
Pleurodont tooth, 607
Pleuropericardial membrane, formation of,
868
Pleuroperitoneal membrane, 864, 868
development of, 869
Plica semilunaris, 853
Plug, vaginal, 31, 33
Pluma, 571
Plumules. 573
Pluripotent state, definition of, 377
Pneumatic duct, of air bladder, 643
Pocket (semilunar) valves, in aortic and
pulmonary trunks, 759
Pohlman. A. G.. 762
Poison glands, in skin of fishes. 561
of rattlesnake, Crotahis horridus, 616
Polar bodies, formation of in Deroceras
laeve (Limax campestris), 214
in Haemopis, 214
in Nephelis, 214
Polar plasm, 1 17
Polarity of cytoplasm, theories of, 158
PoUstotreina (Bdellostoma) stouti, xvi
gill arrangement in, 639
kidney tubules of, 773
mesonephric kidney of, 778
yolk sac of, 908
Pollachius virens, pollack, cranial nerve
distribution of, 825
PoUister, A. W., 297
Polocyte, 145
Polyembryony, definition of, 380
in armadillo, Tatusia novemcincta, 383,
384
Polyestrous conditions, 94
Polyinvagination, during formation of sec-
ondary blastula in Echidna, 364
in gastrulation, definition of, 397
Polyphyodont condition, of tooth develop-
ment, 607
Polypteriis, male reproductive duct in, 18,
799
Polyspermy, in Bryozoa, 259
in hen's egg, 253
in insects, 259
in pigeon's egg, 253
in Triton, European newt, 270
influence of first cleavage amphiaster on,
286
Pontine flexure, formation of, 823, 833
Porichthys notatus, development of phos-
phorescent organ in, 561
Post-anal gut, 471, 600
Post-branchial bodies, 878, 880
Post-embryonic period, xii
Postganglionic neuron, 836
Post-nuclear cap of sperm head, formation
of, 149, 150
Potency, autonomous, 379
versatility of, definition of, 379
definition of, 376
in relation to differentiation, 376
prospective, definition of, 379
Potency expression, definition of, 379
Potency release, definition of, 379
Potency restriction, as a characteristic of
cleavage and the blastulative
process, 377
Pre-cartilage stage, of cartilage formation,
665
Precaval veins, development of in chick
embryo, 745
Pre-chordal plate, and cephalic projection
in various chordates, 449
Pre-chordal plate material, 512
importance of in late gastrula, 414
Pre-chordal plate mesoderm, definition of,
523
Pre-chordal (trabecular) plate area, of
primitive skull, 671
Preformationism, emboitement, theory of,
57
in modern embryology, 57
intangible, Leeuwenhoek's concept of,
213
past and present, 56
Preganglionic neuron, 836
970
Pregnancy, determinative tests for, 108,
926
ectopic, and egg transport, 201
Pregnancy cycle, in cow, 104
in sow, 102
Pre-mandibular somite, in sharks, 716
Pre-oral (head) gut, 482, 484
Pre-oral pit, 502
Pre-otic somites, as origin of eye muscles
in shark group, 716
Prepuberal period, of post-embryonic de-
velopment, xiv
Presphenoid portion, of sphenoid bone,
696
Prevost, 281
Price, G. C, 773
Primary blastular condition, of fish blasto-
derm, 368
Primary organizer, characteristics of, 399,
400, 401 (.see also Organization
center)
Primary primordial germ cells, 117
Primary tubes (tubulations), of primitive
vertebrate body, 456
Primates, xxi
Primitive knot (Hensen's node), formation
of in pig gastrula, 433
Primitive plate, of reptilian blastoderm, 360
Primitive streak, body of. formation of in
pig gastrula, 433
comparison with blastopore, 450
developing, 425
development of in chick blastoderm, 420
formation of. cell movements in epiblast
involved in, 421
Principal (chief) cells, of parathyroid
glands, 879
Principal piece, of sperm flagellum, 149
Principle, of presumptive organ-forming
areas of the blastula, 344
Proboscidea, xx
Process, ovulatory, 75
Processus vaginalis, 6
Procoracoid, 693
Proctodaeal invagination, 485
Proctodaeal membrane, 632
Proctodaeum, 471. 482, 600
area of diverticulae of, 602
Proestrus, definition of, 93
Progestational phase, of female reproduc-
tive cycle, 93
Progesterone, effect on ovulation, in the
hen, 80
effects of, 81, 91, 98
hormone of the corpus luteum, 85, 89
structural formula of, 91
Progonal area, of genital ridge, 791
Progonal support, for developing sex
gland, 803
Projection, cephalic, and pre-chordal plate,
in various chordates, 449
Prolactin, 73
Pronephric duct, 777, 774, 775, 787
Pronephric kidney (pronephros), develop-
ment of, 774
origin of in chick, 776
Proprioceptive field, 842
Prosencephalon, 812
Prospective fate, definition of, 379
Prospective potency, definition of, 379
Prostate gland, development of, 802
function of, 31
possible elaboration of fructose by, 34
Protective membranes, reproductive duct
as, 914
types of, 901
Protein breakdown, products of, elimina-
tion of, 771
Protein synthesis, problematical, 159
Protoplasmic astrocytes, 810
Protopterus annectens, lungfish, xvi
reproductive and urinary ducts of, 799
skin of, 559
stomach area of, 621
terminal nerve (cranial n. o.) of, 827
Prototheria, xix
Proventriculus (glandular stomach) of
chick. 623
Psalterium, of cow's stomach, 623
Pseudobranchus striatus, external gills of
larval form, 640
Pseudopregnancy. 93
Pseudo-vaginal birth canal, of the opos-
sum, 63
Pterygoid muscle, 717
Pterygotemporal muscle, 717
Pterylae, 573
Ptyalin, 617
Puberal period, of post-embryonic devel-
opment, xiv
Pubis, 693
Puckett, W. O., 892
Pulmonary organs. 636
Pulmonary ridge. 869
Pulpy nucleus (Nucleus pulposus), 682
Purser, G. L., 197
Pyloric area, diverticula of, 601
Pyloric ceca, 601
Pyloric end. of stomach. 621
Quill (calamus), of feather, 571
INDEX
971
R
Rabbit, artificial insemination in, 229
early development of egg of, 296, 300
Fallopian (uterine) tube of, behavior in
sperm transport, 193
fertilization in, 253
Rachis (shaft), of feather, 571
Rahn, H., 197. 884
Rami, of autonomic nervous system, 826
Rana, role of thyroid gland in metamor-
phosis of, 883
Rana catesbiana, xviii, brain of, 833
pelvic girdle of, 689
testis in relation to reproductive ducts,
19
Rana cavitympanuin, tympanic membrane
of, 852
Rana fusca, first cleavage plane in, 287
Rana pipiens (leopard frog), xviii
branchial-pouch-groove perforation, 640
characteristics of egg, site of fertilization,
and place of sperm entrance into
egg, 204t
cleavage in, 292, 294
deposition of gelatinous layers of egg of,
907
development of gills in larva of, 641
development of heart of, 752
development of intestine in, 626
development of liver rudiment of, 623
development of skin of, 568
early development of, 294
early neural tube stage of, 466
effect of mammalian pituitary gonado-
trophins on ovary of, 74
external gill filaments of, 636
external views of embryo, 470
fertilization phenomena in, 239
' first cleavage plane in, 287
formation of blastocoel of, 295
gill development in tadpole of, 642
history of blastopore in embryo of, 416
peritoneal cilia and ostium of oviduct of,
195
pronephric and mesonephric kidneys of,
779
skin of, 566
sperm survival of, 198
spermatogenesis in, 23
10 mm. tadpole of, 538-539
venous system of, 740
Rana sylvatica, xviii
cleavage in egg of, 292, 293
external views of embryos, 470
formation of gray crescent in, 293
normal development of, 293
pronephric kidney of, 775
development of, 779
Rana temporaria, association of sperm and
egg pronuclei in, 214
origin of blood cells in, 735
Rankin, R. M., 885
Ransom, R. M., 47
Ranson, S. W., 810
Ranvier, node of, 819
Rasmussen, A. T., 6, 665
Rat, female reproductive organs of, 66
origin of germ cells in, 119
ovarian capsule around ovary of, 194
Rathke's pouch, 601, 877
Ratitae, xix
Rattus rattus (rat), characteristics of egg,
site of fertilization, and place of
sperm entrance into egg, 206t
Rawles, M. E., 592
Reagan, F. P.. 731
Receptor organs, 841
Rectal area, 630
Rectal recess, 803
Rectum, 787
Redi (1626-1697), 57
Reflex, suckling, 107
Reflex arc, 808
component structures of, 809
Relaxin, 103
Remak's fibers, 820
Ren, 772
Renal, use of term, 772
Renal arteries, 762
Renal lobe, of developing kidney in mam-
mal, 786
of developing mammalian kidney, 787
Renal portal system, of frog tadpole, 776
Renal pyramid, 787
Renal units, development of in mammalian
kidney, 786
metanephric, formation of in chick em-
bryo, 784
types of, 770
Reproduction, asexual, 53, 55
sexual, 55
Reproductive climax, 92
Reproductive cycle, 92
female, changes in reproductive organs
and mammary glands in bitch,
102
in cow, 104
in lower vertebrates, 96
in sow, 702
follicular phase of, 91, 93
luteal phase of, 91, 93, 98
male, relation to reproductive conditions
in the female, 47
of human female, 707
972
Reproductive duct, as a protective embry-
onic structure, 914
development of, 798, 799, 801, 802
Reproductive organs, changes in during re-
productive cycle, in bitch, 102
female, of the cat, 65
of the rat, 66
of the human female, 59
Reproductive state, in relation to reproduc-
tive cycle, 92
Reproductive structures, accessory, male
sex hormone and development of
in male, 29
of female, 61
of male, 17
female, of the frog, 66
of the opossum, 64
of the urodele, Necturus maculosus,
63
Reproductive system, 768-804
development of, 788
female, dependency on general body con-
ditions, 65
of the common flounder, Limanda
ferruginea, 58
of the opossum, 63, 196
functions of, 769
male, 4
anatomical features of, 5
of the vertebrate female, 57
Reptile, blastoderm, embryonic shield of,
360
sagittal section showing notochordal
inpushing (notochordal canal or
pouch), 419
claws of, 570
coelomic changes in, 867
copulatory organ of, 191
dermal bones in, 570
egg passage through oviduct in, 907
entoderm formation in, 361
heart of, 753
independent pericardial wall develop-
ment in, 871
skin of, characteristics of, 568
tubulation of epidermal area in, 476
Reptilia, xviii
Respiration, buccopharyngeal, 651
external, 635
internal, 635
Respiratory areas, of chick's lung, 647
Respiratory bronchioles, 650
Respiratory surfaces, structural relation-
ships of, 636
Respiratory system, cellular composition
of, 650
Rete blastema, 795
Rete ovarii, rudiment of, 795
Rete primordium, 793
Rete-testis canals, 794
Rete tubules, 793
Reticular theory (doctrine) of nervous
structure, 807
Reticular tissue, 657, 663
Reticulum, of cow's stomach, 623
Retina, 845
central artery of, 846
Retinal rudiment, of eye, 845
Retzius, G., 140
Rhahdites aberrans, and gynogenesis, 262
Rhabdites pellio, and gynogenesis, 262
Rhineodon typiis, whale shark, xvi
Rhinoderma darwinii, vocal brood pouch
in male, 911, 916
Rhombencephalon, 812, 822
Rhomboidal sinus, 471
Ribonuclease, and egg activation, 219
Ribs, 686
dorsal, 686
ventral or pleural, 686
Ridge, of presumptive neural plate ma-
terial, 439
Robson, J. M., 12
Rock, J., 920
Rocket sperm, of certain decapod Crus-
tacea, 232
Rodentia, xx
Romanoff, A. J., 905, 906, 907
Roof plate, dorsal, 820
ventral, 820
Rooster, Callus (domesticus) gallus, testis
in relation to reproductive ducts,
19
Root, of hair, 582
of tooth, 605
Root sheath, external, of hair root, 583
internal, structure of, 583
Rotation, of vertebrate embryo body, 511
Round ligament, 803
Roux, Wilhelm, 246
hypothesis of, 215
Rowlands, I. W., 229
Rugh, R., 23, 201, 297
Rumen, of cow's stomach, 623
Rumph, P., 885
Runnstrom, J., 229
theory of fertilization, 271
Sacculus, 851, 854
Sachs' rules of cleavage, 286
Sacral (lumbar) enlargement, of spinal
cord, 821
Sacrum, formation of, 690
Sainmont, G., 796
INDEX
973
Salamandra atra, egg transport in, 202
sperm transport in female, 191
Salamandra salamandra, sperm transport
in female, 191
Salienta, xviii
Salmo fario, brain of, 813
early development of, 323
Salmo irideus, presumptive organ-forming
areas of blastoderm of, 369
Salmo salar, xvii
Salvelinus (trout) beginning gastrula of,
437
characteristics of egg, site of fertilization,
and place of sperm entrance into
egg, 204t
Salvelinus fontinalis, xvii
Santorini, duct of, of pancreas, 629, 630
Sarcolemma, of muscle cell, 700
Sarcoplasm, of muscle cell, 700
Sayles, L. P., 563
Scales, development of, in shark skin, 562
formation of, in Lepisosteus
(Lepidosteus) osseus, 564
in reptiles, 570
Scalopiis aquatic us (mole), characteristics
of egg, site of fertilization, and
place of sperm entrance into egg,
206t
teeth of, 615
Scammon, R. E., 760, 773
Scape, of feather, 571
Scaphiopus holbrookii, xviii
Scaphirhynchus platorlTychus, xvii
Scapula, 693
Schleiden, 281
Schrader, F., 126, 233
Schiickling, A., 223
Schwann, 281
sheath of, 819
Schwenk, E., 27
Sclera, of eye, 849
Scleroblasts, in developing scale of Lepi-
sosteus, 563
Sclerotic coat, 846, 849
Sclerotome, 526, 711
Scolecomorphus uluguruensis, xviii
Scrimshaw, N. S., 197, 915
Scrotal ligament, 803
Scrotum, 6
as regulator of testicular temperature, 36-
37
median septum of, 12
structure of, 7, 12
Scylliuni canicula, dogfish, basic plan of
head, 528
blastoderm of, 318
egg capsule of, 929
Scy Ilium canicula — (Continued)
surface views of developing blastoderms
of, 442
vitelline membranes of, 163
Sea bass, early development of, 323
Sea urchin, egg of, developmental potencies
(cell lineage) of isolated blasto-
meres of, 329
nuclear equality in, 335
presence of fertilizin in egg of, 227
Sebaceous glands, 587
Secondary hypoblast, formation of in tele-
ost fish blastoderm, 370
Secondary primordial germ cells, 117
Secretin, 875
Seessel's pocket. 599
Segmental (pronephric) duct, origin of,
775
Segregation of different substances, one of
functions of cleavage, 333
Self-differentiation, definition of, 375
Semen, 15, 21
coagulation of, 33
function of, 32
sperm density in, 32t
Semicircular canals, 851, 854
Semilunar (gasserian) ganglion, 828
Semilunar valves, in aortic and pulmonary
trunks, 759
of chick heart, 756
Seminal fluid, 21
amount of, 32
factors involved in passage of from testis
to main reproductive duct, 179
factors which propel, 182
functions of, 32
Seminal plasma, influences of in effecting
sperm contact with egg, 231
Seminal vesicles, 802
function of, 31
in certain birds, 186
in ovenbird, 186
in robin, 186
in towhee, 186
in wood thrush, 186
of caudal end of reproductive duct, in
elasmobranch fishes, 186
possible elaboration of fructose in, 34
Seminiferous tubules, primitive, 792
Semispinalis muscle, of human embryo, 715
Senescence, period of, xiv
Sense placodes, 812
special, 812
Sense (receptor) organs, 841
somatic, general, 842
special, 842
visceral, general, 842
special, 842
974
Sense receptors, types of, 840
Septa ovarii, 796
Septula, of testis, 793
Septula compartments, 13
Septulum, 793
Septum primum, of mammalian heart, 757
Septum transversum, primary, 627, 864,
866
secondary, 867
formation of in amphibians, reptiles,
and birds, 867
Serial homology, 534
Serosa (chorion), 908, 910
Serous cavity, 6, 13
Serpentes, xviii
Serranus atrarius (sea bass), early develop-
ment of, 323
ectoderm of, 559
Sertoli cell, 793
as nurse cell for developing sperm, 147
conditions of in fowl, 139
Sex cells, definitive, structure of, 126
Sex characteristics, secondary, effects of
male sex hormone on, 27
Sex chromosomes, 887
Sex cords, 792
Sex-determining mechanisms, chromoso-
mal, 887
possible influence of sex field on, 889
Sex differentiation, factors involved in,
summary of, 893
influence of endocrine (hormone) sub-
stances on. 886, 891
Sex features, general, in animal kingdom,
886
Sex field, cortical, 889
medullary, 889
possible influence of on sex determina-
tion, 889
Sex gland differentiation, 790
Sex hormone, androgenic, 20, 21
gynogenic, 85
male, biological effects of, 27, 28, 29
production of, and cells of Leydig, 23,
24, 25
sources of, 26
Sex reversal, in axolotl, 890
Sexual cycle, 92
female, length of, 92, 94
non-ovulatory (anovulatory), 96
male, as influenced by external environ-
mental factors, 43, 47
Seyle, H., 108, 879
Shaft, of bone, 691
of hair, 582
Shaft (rachis; scape), of feather, 571, 576
Shapiro, H., 219
Shark, origin of eye muscles in, 716
skeletal muscular development in, 708
"tongue" of, 603
Shaver, J. R., 219, 704
Sheath, inner hair (epithelial), of hair fol-
licle, 581
outer, of hair follicle, 581
Sheath cuticle, inner, of hair root, 583
Sheldon, E. F., 665
Shell membranes, 906
Shettles, L. B., 198
Shreiner, K. E., 664
Shumway, W., 297
Simpson, M. E., 41
Sinu-atrial (auricular) valves, of chick
heart, 756
Sinus, urogenital, 18
Sinus node, 754, 167
Sinus septum (Eustachian valve), of chick
heart, 756
Sinus venosus, 728, 747, 750, 757, 758
Sinusoids, of liver, 627
Siredon {Amby stoma) mexicanum, sex re-
versal in, 890
Siren lacertina, xvii
Sirenia, xx
Skeletal portion, of visceral arch, 619
Skeletal system, definition of, 653, 654
Skeletal tissue, adult, contribution of em-
bryonic mesenchyme to, 655
development of, 663
Skeletogenous septum, horizontal, 706
Skeleton, appendicular, 668
axial, 668
development of, 674
of tail, 688
basic embryonic, origin and significance
of, 655
dermal, 568
external (exoskeleton), 668
primitive or "ghost," 655
visceral, 668, 669
muscles of, 717
Skin, accessory structures associated with,
development of, 579
color of, factors concerned with, 590
component parts of, origin of, 557
development of, in fishes, 561, 565
general functions of, 557
mammalian, characteristics of, 578
development of, 579
origin of dermal component of, 559
vertebrate, basic structures of, 557
coloration and pigmentation of, 590
definition of, 556
INDEX
975
Skull, 668
adult, relation of to embryonic chondro-
cranium, in chick, 678-679t
in frog, 676-677t
in human, 680-681t
development of, 669
Smith, P. E., 884, 885
Snake, differentiation of corpus luteum in,
84
garter, Thainnophis radix, sperm produc-
tion in, 23, 30
poison glands of, 617
Snell, G. D., 305
Sole, position of testes in, 7
Somatic mesodermal layer, 506
Somatoplasm, 1 14
Somatopleure, definition of, 493, 530
Somites, in head region, 492
relation to, primitive segmentation of de-
veloping body, 491
Somitic mesoderm, synonymous with epi-
mere, 490
Spallanzani, 56, 281
Spalteholz, 704
Spawning-inducing, agent, 228
Specific organ-forming areas, history of
concept, 343
Speidel, C. C, 733, 815, 819
Spemann, H., 381
Sperm, 16, 21
accessory, 32, 33
activation of during fertilization, 212
agglutination of, 225
antifertilizin formation by, 230
chordate, types of, 140
conjugate, 147
of grasshopper, 141
copulation path of, in frog's egg, 248
development, and luteinizing hormone,
41
summary, 170t
entrance into egg, 203, 243, 253
enzyme-protecting substances of, 34
flagellate, structure of, 147
head of, 142, 148
mammalian, kinoplasmic bead or droplet
of, 765
metamorphosis of, 17, 147
morphogenesis of, in guinea-pig, 146
in human, 146
non-flagellate, 141, 147
of frog, Rana pipiens, survival of, 198
of urodele amphibia, 143
older concept as parasites in seminal
fluid, 213
penetration path of, 246
physiological differentiation of, 169
Sperm — (Continued)
physiological maturing, dependence upon
LH, 43
pigmented trail of during fertilization,
248
point of entrance of into egg of Styela,
245
probable immotility of in epididymal
duct, 180
production of, in bat, Myotis, 30
in garter snake, Thainnophis radix, 30
in perch, Perca flavescens, 30
release from Sertoli cells by luteinizing
hormone, LH, 43
secretions, 228
spatulate, structure of, 142, 148
spermiogenesis of, 147, 151
spermioteleosis of, 147
storage of, 30
structural parts of, and fertilization, 232
summary of activities of in initial stages
of fertilization, 233
survival of, in female, genital tract, 197
survival outside male and female tracts,
198
survival under artificial conditions, 198
transfer of from male to female, 189
transport of, estrogenic hormone influ-
ence on, 197
in mammal, 178
in vertebrates other than mammals,
183
within female reproductive tract, 191
within male accessory reproductive
structures, 178
types of, 147
Sperm antifertilizin, and fertilizin of the egg
in the fertilization process, 230
Sperm aster, division of, 247, 259
Sperm bead, 148
"Sperm boat," of grasshopper, 141, 147
Sperm density, in semen, 32t
Sperm entrance into egg, time of in relation
to maturation divisions, 144
Sperm extracts, effecting liquefaction of egg
cortex, 229
Sperm number, and fertilization, 230
Sperm pronucleus, movements of during
fertilization in Styela, 246
Sperm "ripeness," and ability to fertilize, 30
Sperm secretions, and agglutination of eggs
during fertilization, 230
Sperm survival, and animal breeding, 198
Sperm transport, activities of efferent duc-
tules in, 180
by means of cloacal tail in Ascaphus
triiei, 189
importance of muscle contraction in, 181
976
Sperm transport — (Continued)
in Chry semes picta female, 191
in external watery medium, 186
in forms where fertilization is internal,
189
in hen, 191
in Hydromantes genei female, 191
in Hydromantes italicus female, 191
in neotropical urodele, Oedipus female,
191
in oviduct, influence of estrogenic hor-
mone on, 197
in rabbit, peculiar behavior of Fallopian
(uterine) tube in, 193
in Salamandra atra female, 191
in Salamandra salamandra female, 191
muscular contractions as a means of, 193
outside of genital tract of male, 186
vas deferens as an organ of, 181
Spermatheca, 4, 189, 190
Spermatid, 125
Spermatocyte, definition of, 125
maturation divisions in, as compared
with oocyte, 144
Spermatogenesis, and its stimulation by
FSHandLH (ICSH), 40-41
and mating urge, 22
in bat, Myotis, 23
in common fowl, 153
in common frog, Rana pipiens, 23
in deer. Odocoileus virginianus borealis,
23
in garter snake, Thamnophis radix, 23
in ground squirrel. Citellus tridecemline-
atus, 23
in the grasshopper, 131
later stages, of human, 148
seasonal, and accessory gland develop-
ment, 22
stages of. in guinea-pig, 148
Spermatogonia, 16, 17, 114
Spermatophore, 4, 18, 21, 189
Spermatozoa, 16, 21
origin of term, 213
Spermia. 16. 21
Spermiogenesis, comparison of in mammals
and insects, 154
definition of, 149
description of, 149
Sphaerodactylus notatus, xviii
Sphenodon, Tuatera, gastralia of. 697
pineal organ of. 881
Spinal accessory nerve, 833
muscles associated with, development of,
718
Spinal cord, as area of central nervous sys-
tem, 812
brachial enlargement of, 821
Spinal cord — (Continued)
cavity of, 822
development of, 820
enveloping membranes of, 821
general structural features of, develop-
ment of, 818
Spinal ganglia, 812
Spinal nerve, component parts of, 826, 838
formation of ventral root of, 819
Spiral folds (valves), of intestine, 632
Spiral ganglion, of acoustic nerve, 830
Spiral septum, 756
Spiral valve (septum), of frog heart, 752
Splanchnic, definition of, 530
Splanchnic mesoderm, 506
Splanchnocoelic coelom, primitive, 858,
659
Splanchnocranium, 668, 669, 679t
development of, 672
mammalian, developmental stages of,
675
of frog, 677t
of human, 68 It
Splanchnopleure, definition of, 493, 530
in yolk sac wall, 909
Spleen, 726, 766
Splenic corpuscles, 766
Sponges, gemmation in, 55
Spongioblasts, 810, 816
Spongy layer, of bird's egg, 906
Squalus, lateral line sensory cord of, 841
Squalus acanthias, dog fish, xvi
adult brain of. 832
alteration of primitive converging veins
of heart of, 736
aortal vessel changes in embryo of, 760
characteristics of egg. site of fertilization,
and place of sperm entrance into
egg, 203t
coelom of, 858
developing skin of, 558
development of heart of, 728, 750
development of intestine in, 626
development of mesonephric duct in, 777
development of mesonephric kidney in,
780
development of musculature of head of,
717
development of olfactory organs of, 843
development of pancreas of, 628
development of reproductive ducts in,
798
developmental stages of chondrocranium
of, 669
early stages of tubulation of neural and
epidermal organ-forming areas,
475
external gill filaments of, 636, 638
INDEX
977
Squalus acanthias — (Continued)
gill septum in, 638
initial division of coelom of, 861
kidney of, 773
lateral-line canal of, 843
liver rudiment of, 623, 625
morphogenesis of digestive structures of,
599
ovoviviparity and viviparity in, 914
pectoral girdle of, 689
pelvic girdle of, 689
pericardioperitoneal opening of, 864
pronephric kidney in, 775
skin, development of, 562
sperm survival within female genital
tract of, 197
tail musculature of, 704, 706
testis in relation to reproductive ducts, 19
venous system development in, 738
Squamous inferior, center of ossification,
695
Squamous superior, center of ossification,
695
Stapes, 674, 851
Starling, European, Sturnus vulgaris, effect
of added electric lighting upon
reproductive activities, 44, 45
Stearns, M. L., 664
Stem villus, 914
Steno, 5
Stereoblastula, 312, 341
Stereocilia, of epithelial cells. 180
Sterna hirundo, tern, feather rudiment of,
577
Sternebrae, 687
Sternocleidomastoid musculature, 718
Sternohyoid muscle, innervation of, 717
Sternum, 688
Stickleback, Gasterosteus aculeatus, repro-
ductive activity and temperature
in, 26, 46
interstitial tissue, 25
Stimulus, 807
Stockard, C. R., 384, 731, 733
Stomach, development of, 618, 623
regions of, 62 1
true, definition of, 621
Stomach sweetbread, 880
Stomodaeal invagination, 484-485, 603
Stomodaeum, 482. 598, 601
Stratum corneum, of epidermis. 568, 579
Stratum germinativum. 558. 559, 568, 579
Streeter. G. L., 305, 434, 923
Strichnine, activation of egg by, 218
Strong. R. M.. 577
Strongylocentrotus purpuratus, fertilizer of.
225
Struthio, xix
Styela, chorion in egg of. 162. 163
early cleavage planes in. 286
egg of. bilateral symmetry of, 162
organ-forming areas in egg of, 161
"test" cells of egg of, 164
Styela partita, xv
cell lineage in. 344
characteristics of egg. site of fertilization,
and place of sperm entrance into
egg, 203t
development of half embryos in isolated
blastomeres of, 332
differences in early blastomeres in, 328
fertilization in, 224, 245
movements of ooplasmic substances in
egg of at fertilization, 264
Subarachnoid space, 821
Subcardinal veins, 737
Subdural cavity, 822
Subhyoideus muscle, 717
Subnotochordal rod, 655
Substances, spawning-inducing, 228
Subunguis, 584
Subvertebral ganglia, of autonomic nervous
system, 837
Sudoriferous glands, 587
Supporting tissue, of nervous system, 809
Supracardinal veins, of mammalian em-
bryo, 747
Suprarenal body, differentiation of. 882, 883
Suprascapula, 693
Sus scrofa (pig), characteristics of egg.
site of fertilization, and place of
sperm entrance into egg, 207t
Suspensory structures, primitive, formation
of, 859
Sustentacular elements, 793
Suture, of frontal bony areas of skull, 695
Swain, E. R., 796
Swammerdam, 56, 281
Sweetbread, stomach, 880
throat, 880
Swift, C. H.. 790, 794
Swim-bladder, relationships of. 643
Swingle, W. W., 773
Sylvius, aqueduct of, of brain, 823
Sympathetic chain ganglia, of autonomic
nervous system, 837
Sympathetic division, of autonomic nerv-
ous system, 836
Sympathetic ganglia, 836
Synapsis, 132
Synaptene stage, 135
Synarthrosis, 695. 696
Syncytium, 360, 703
Synergism. 41
Syngamy. 113, 211
978
INDEX
Syngnathus (pipefish), male egg pouch of,
915
Synizesis, definition of, 137
Synosteosis, 695
Synovial membrane, 696
Syntrophoblast, 920
Syrinx, of bird, 649
System development, definition of, 518, 520
Tadpoles, early, development of early sys-
tems in, 537
Tail axial skeleton of, 688
muscles of, development of, 705, 710,
715
Tail bud, 47 1
Tail gut, 471, 482, 600
Tail outgrowth, 476
Tapetum lucidum, 849
Tarsal glands, 853
Taste-bud system. 843
Taste buds, caudal, innervation of, 831
description of, 843
innervation of, 843
Tatusia, armadillo, pelvic girdle and sa-
crum in, 689
polyembryony in, 383, 384
Tavolga, W. N., 235
Teat (nipple), 589
Tectum, of mesencephalon, 822
Teeth, absence of in vertebrates, 605
bony (true), 605
development and arrangement of in vari-
ous vertebrates, 672
general characteristics of, 605
horny, 605
types of, 605, 607
Tela choriodea, 822
Tela subcutanea, 556
Teledendria, 814
Telencephalic vesicles, 822, 823
Telencephalon, 822
Teleost fish, blastoderm of, presumptive
organ-forming areas of, 367, Alil
blastula of, 368
cleavage and early blastula formation in,
322
development of brain in, 813
fertilization of egg of, 249
gastrulation in, 436, 437, 440
germ ring of, developmental potencies
of, 441
gills of, 639
origin of periblast tissue in, 324
spermatogenesis in, 23
structure of ovary in, 59
twinning in, 382
zona radiata of, 165
Teleostel, xvii
Teleostomi, xvi
Telosynapsis, 137
Temperature, and artificial egg activation,
219
as a factor in reproduction, 46
Temporal bone, development of in human,
695
Temporal muscle, 717
Tensor tympani muscle, 717
Tensor veli palatini muscle, 717
Terminal arborizations (teledendria), of
nerve fibers, 814
Terminal ganglia, of autonomic nervous
system, 837
Terminal nerve (cranial n. o.), 827
Terminalization, in diakinesis, 139
Terrapene Carolina, xix
Terrestrial adaptations, of muscles, 706
"Test," cells, of Styela egg, 164
Testes, 788
Testicle, use of word, 4
Testis, activities of, 22, 34, 47
and hypophysectomy, 39
anterior suspensory ligament of, 9
cryptorchid, 35
descent of, 6, 7, 9. 10, 11, 12
in bat, Myotis, 7
in European hedgehog, 7
influence of testosterone in, 12
development of, in chick, 794
in frog, 794
in human, 792
in mammal, 792
during and after abdominal confinement,
37
factors influencing, 34
function and anatomical position, 34
and external environmental factors, 43
and hypophysis, 39
and nourishment, 38
and temperature, 34
in relation to reproductive ducts, 19, 19
lobuli of, 13
location of, 6
mediastinum of, 13
of stickleback, activities in relation to
seasons, 26
of vertebrates in general, 17
position of, in flounder, Limanda ferru-
ginea, 10
position of, in sole, 7
rete portion of, 16
retroperitoneal, 10
septula of, 13
size of, 17
types of, relative to sperm formation, 31
INDEX
979
Testosterone, 22, 23, 26, 41
examples of stimulation, 27
in testicular descent, 12
Testudinata, xix
Testudo graeca, developing egg of, 167
Tetrad, 137
Tetrapoda, pectoral girdle of, 689
pelvic girdle of, diagrammatic, 689
TeWinkle, L. E., 914
Thamnophis radix, garter snake, xix
Theca externa, of mature Graafian follicle,
71, 74
Theca interna, of mature Graafian follicle,
71, lA
Thecodont teeth, 605
development of, 610
Theelin, 85
Theria, xx
Thickened keel method, of neuralization,
465
Thigmotaxis, during cleavage, 328
Thimann, K. V., 12
Thin roof plate, of developing brain, 822
Thoracic duct, 764
Thoracicolumbar autonomic system, 836
Throat sweetbread (thymus), 880
Thymocytes, 880
Thymus gland, 619, 726, 878, 880
Thyng, F. W., 625
Thyroglossal duct, 878
Thyroid gland, 619, 877
effect of on development of chick, 884
effect of on development of mammal,
885
embryonic origin of, 878
histogenesis of, 879
in vertebrate embryology, general con-
clusions, 886
influence of on development of fishes,
885
influence on anuran metamorphosis, 883
isthmus of, 877
of human embryo, 878
Thyroid-gland diverticulum, 601
Thyroxine, 877
Tichomiroff, 217
Tissue, definition of, 517
four fundamental, of the embryo, 519,
520
interstitial, 15, 17
and male sex hormone production, 23,
24
Toad, amplexus in, 185
Toad test, for pregnancy, 927
Tongue, bird, copula of. 604
development of, 603
innervation of, 717
mammalian copula of, 605
Tongue — (Continued)
of frog, 603
of shark, 603
origin of musculature of, 717
protrusile, 603
true, flexible, 603
Tonsils, 726
Tooth, development of in mammals, 608
egg, in chick, 608
epithelial sheath of, 613
Torpedo ocellafa, germ disc of, 318
Tortoise shell, of commerce, 569
Totipotency, definition of, 376
in teleost blastoderm, 385
Totipotential condition, of Roux, 377
Townsend, G., 228
Toxopneiistes lividus, fertilization mem-
brane in, 214
gametic fusion in, 214
Toxopneiistes variegatus, fertilization cone
in, 241, 263
Trabeculae, of spongy bone, 662
Trabeculae carnae of heart, 757
Trabecular area, of primitive skull, 671
Trachea, cellular composition of, 649
Transplantation experiments, in Triton, 381
Transverse division, primitive, formation
of, 859
Trapezius musculature, in human, 718
Trichosurus, derivation of eye muscles in,
716
Trigeminal nerve (cranial n. v.), 717, 828
Triton, European newt, gastrula, 355
polyspermy in, 270
Triton cristatus, xvii
transplantation experiments in, 381
Triton taeniatus, transplantation experi-
ments in, 381
Triturus viridescens, xvii
cleavage of partially constricted egg of,
332
first cleavage plane in, 287
reproductive activity and temperature in,
46
Truncus arteriosus, 759
Trunk, muscles of, development of, 705,
710
Trunk or tail organizer, ability of, 401, 512
Trutta irideus, xvii
Tuatera, xviii
Tube, uterine (Fallopian), 64
Tuberculum impar, of developing bird
tongue, 603
of mammalian tongue, 605
Tubulated areas, regional modifications of,
457
980
Tubulation, auxiliary processes of, 461
epidermal, 474, 476
immediate processes of, 460
in Amphioxus, of neural, epidermal, en-
todermal and mesodermal organ-
forming areas, 494
influences which play a part in, 512
of entodermal area, in Amphioxus, 500
of major organ-forming areas, starting
point for, 459
of mesoderm, in Amphioxus, 503
of the mesodermal areas, 490, 492
primary, of primitive vertebrate body,
456
processes concerned with, 460
Tubules, rete, 15, 16
seminiferous, 15
and response to follicle stimulating
hormone (FSH), 24-25
straight (tubuli recti), 15, 793, 794
Tubuli contorti, 793
Tubuli seminiferi, 15
Tunica albuginea, 13, 58, 792
Tunica dartos, 12
Tunica vaginalis communis, 13
Tunica vaginalis internus, 13
Tunica vaginalis propria, 13
Tunicata, xv
Turner, C. D., 37
Turner, C. L., 27, 197
Turtle, carapace formation in, 569
development of skin of, 569
female reproduction of, 192
gastrula of, transverse sections, 420
stages in development of, 479
surface views of blastoderm of during
gastrulation, 418
Twin embryos, production of in Amphi-
oxus, 335
Twinning, in teleost fishes, 382, 386
true, in teleost fishes, 386
requisite conditions for, 380
Twinning conditions, experimentally pro-
duced, 381
Twins, dizygomatic, definition of, 380
fraternal, 380
Tyler, A., 225, 228
Tympanic membrane, internal, 851
Tympanum, 851
U
Ultimobranchial bodies, 619, 878, 880
Umbilical arteries, 762
Umbilical cord, 912, 922
Umbilical ring (Umbilicus), 912
Umbilical veins, 730
Umbilicus, 462, 912
inferior, of feather shaft, 571
superior, of feather shaft, 571, 578
Unfertilized egg, oxygen consumption of,
222
LJnguiculata, xx
Unguis, 584
Vngulata, xx
Unipotency, definition of, 377
Unmyelinated (Remak's) fibers, 820
Ureter, 788
Urethra, cavernous, 20, 802
Urinary bladders, 787
Urinary ducts, 787
development of in vertebrates, 798
types of, 787
Urinary recess, 803
Urochordata (Tunicata), subphylum, xv
asexual reproduction in, 53
Urodela, xvii
Cryptobranchus alleganiensis, testis in
relation to reproductive ducts, 19
Necturus maculosus, reproductive struc-
tures of, female, 63
saccular ovary of, 62
testis in relation to reproductive ducts,
19
neotropical, Oedipus, sperm transpc?rt in
female, 191
sperm of, 143
spermatophores in, 21
Urogenital mesoderm, synonymous with
mesomere, 492
Urogenital opening, of mammals, 715
Urogenital sinus region, 787
Urogenital structures, differentiation of in
human embryo, 788
embryonic tissues which contribute to,
772
Urogenital system, definition of, 769
relationships in various vertebrates, 782
Urogenital union, 794
Uromastix hardwicki, developing egg of,
167
Uropygial glands, in bird, 571
Urorectal folds, 787
Urostyle, 686
Uterine cycle, 92
Uterine horn, anterior end of, in opossum,
194
Uterine segments, of oviduct, degrees of
fusion of, 64
Uterine tube, in rabbit, peculiar behavior
in sperm transport, 193
Uterus, 64
a preformationist conception of, 56
development of, 802
INDEX
981
Uterus — (Continued)
of amphibians, 907
of bird, albumen addition to egg in, 906
Utriculus, 851, 854
V
Vacuole, inside head of sperm, 142
Vagina, 62
Vaginal cycle, 92
Vaginal plug, 31, 33
Vagus nerve (n. x), 831
Valerianic acid, in egg activation, 218
Valves, of veins, 732
Valvula foraminis ovalis, of developing
mammalian heart, 757
Valvulae venosae, of developing mam-
malian heart, 757
Van Beneden, E., 214
Van Beneden's law, 215
van der Stricht, 222
van Leeuwenhoek, A. (1632-1723), 56
Vane, of feather, 571
Vas deferens, 19
ampulla of, 20
as an organ of sperm transport, 181
as sperm storage organ, 30
Vascular buds, activities of in bone forma-
tion, 667
Vascular system, primitive, early develop-
ment of, 728
regions of, 730
Vegetal pole, of developing egg of higher
mammals, determination of, 299
Veins, abdominal, ventral, 740
allantoic, 730
of chick embryo, 745, 746
azygos, of mammalian embryo, 747
cardinal, anterior, 727, 736, 743
common, 728, 736, 743
posterior, 728, 736, 743
coronary. 737
development of, 732
lateral, 730
oblique, of mammalian heart, 747, 757
Vena cava, anterior (superior), of mam-
malian embryo, '747
inferior (posterior), formation of in
anuran amphibia, 740
formation of in anuran embryo, 740
formation of in chick embryo, 745
of mammalian embryo, 747
superior, of mammalian embryo, 747
Ventral constriction, 478
Ventral median fissure of spinal cord, 819
Ventral mesentery, 748
Ventricle, first, of brain, 823
second, of brain, 823
third, of brain, 823
fourth, of brain, 823
fate of in various vertebrates, 758
of developing heart, 747
Ventricular portion of heart, 750
Vertebrae, acoelous (amphiplatyan), 686
amphicoelous, 685
development of, 682, 685
formation of, in fishes, 685
heterocoelous, 655
procoelous, 685
Vertebral column, divisions of, 683
Vertebral theory of skull, 536
Vertebrata, xv
Vertebrate head, basic plan of, 528
Vertebrate hearts, early stages in morpho-
genesis of, 754
Vertebrate morphogenesis, basic features,
chapt. 1 1
Vertebrates, classification of, xiv
numbers of practicing internal fertiliza-
tion, 189
Vertical plane, of cleavage, 283
Vesalius, 876
Vesicle, seminal, 18, 20
Vesiculase, 33
Vestibular ganglion, of acoustic nerve, 830
Vestibular glands, 802
Vestibules, of chick's lung, 647
Villi, 632, 914, 921
primary, of trophoblast, 920
secondary, of trophoblast, 920
Villus, stem, 914
Vincent, W. S.. 119
Vintemberger. P., 249
Virchow. R.. "Omnis Cellula e Cellula,"
281
Visceral arches, 527. 618. 619
hyoid, 527, 619, 717
mandibular, 527, 619
development of muscles associated
with, 717
posterior, muscles derived from. 718
Visceral (branchial) pouches, 601, 619
Visceral furrows, 527, 619
Visceral mesoderm, 506
Vital-staining technic. 120
Vitamin A, 66
Vitamin B, 66
Vitamin C, 66
Vitamin E, 67
role in chick development. 67
Vitamins, 66
Vitelline arteries, 727, 743
Vitelline duct, 911
982
INDEX
Vitelline membrane, 903
as osmotic membrane during early chick
and frog development, 241
in egg of amphibia, 167
in egg of elasmobranch fishes, 165
of egg of Scy Ilium canicula, 163
of frog's egg, 166
of hen's egg, 167
Vitelline veins, 727, 737, 743
primitive, 736
transformation of in chick, 742
Vitellocytes, 326
Vitreous body, of eye, 846
Viviparity, 903
Viviparous species, 902
Vogt, W., 344, 355
Voice box, of bird, 648
von Baer, Karl Ernst (1792-1876), 71,
213, 521
laws of, 521, 522
von Dungern, E., 223
W
Wachowski, H. E., 878, 879, 886
Wagner, 213
Waldeyer, W., 114
Wang, H., 119
Water, elimination of, 771
Weber, M., 6
Weismann, A., 114, 334
Wells, L. J., 6, 12, 46
Whale, characteristics of egg, site of ferti-
lization, and place of sperm en-
trance into egg, 206t
Wharton, Thomas, 877
Wharton's jelly, 663
White commissure, ventral, 821
White matter, 810, 819
Whitman, C. O., 473
Williams, L. W., 570
Williams, R. G., 41, 43
Willier, B. H., 120, 592, 593, 892
Willis, T., 876
Wilson, E. B., 286, 287
Wilson. H. v., 324, 437, 458, 559
Wilson, J. M., 308, 473
Wilson, W. L., 286
Wiltberger, P. B., 927
Wimsatt, W. A., 197
Winslow, foramen of, 873
Wirsung, duct of, of pancreas, 629
Wirsung, J. G., 629
Wislocki, G. B., 6, 23, 27. 892, 923
"Witch's" milk, in newborn human male
and female, 588
Witschi, E., 791, 794, 795. 891, 892
Wolffian (mesonephric) duct, 777, 799
origin of in Squalus, 775
Woodchuck, hibernating gland of. 665
Woollard, H. H., 764
Wright, E. S., 305, 904
X
Xenopus laevis, xviii
use of in pregnancy test, 927
X-rays, effect on germinal epithelium, 120
Yellow crescent, appearance of at fertili-
zation in egg of Styela, 246. 286
Yochem, D. E.. 196
Yolk, localization of at egg pole, 160
"Yolk-attraction sphere," 157
Yolk bodies, 157
Yolk body, in egg of fowl. 755
Yolk nucleus, 157
Yolk sac. 902
blood islands of. 733
external, in shark embryo. 925
fate of. 925
internal, in shark embryo, 925
types of. 908
Yolk-sac placenta, 917
Yolk spherules, origin of, 157
Yolk stalk, 911
Yolk synctium, in elasmobranch fishes,
322
Young, care of, 899, 929
number produced and care. 900
Young, W. C, 36
Zona pellucida, 162. 168. 901, 903
of reptile oocyte, 167
Zona radiata. 162. 903
in amphibia. 167
of Chrysemys picta, 166
of egg of fowl, 766, 167
of egg of teleost fishes, 165
of elasmobranch fishes, 165
of reptile oocyte, 167
Zonary placenta, 914
Zondek, B.. 40. 926
Zonula ciliaris, 853
Zygonema meiotic threads. 135
Zygote. 113. 211
Zygotene stage of meiosis. 132, 135
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