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Cat. No. 23-221

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THE BIOLOGY OF THE FROG

THE MACMILLAN COMPANY

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Rana pipiens. Upper figure, ordinary resting attitude ; lower
figure, crouching oosition. (From photographs by Mr. F.
M. Abbott.)

THE

BIOLOGY OF THE FROG/

BY

SAMUEL J. HOLMES, Ph.D.

PROFESSOR OF ZOOLOGY IN THE UNIVERSITY
OF CALIFORNIA

Fourth Revised Edition

THE MACMILLAN COMPANY

By THE MACMILLAN COMPANY.

Copyright, 1906, 1927,

5Wl

By SAMUEL J. HOLMES.

Copyright, 1934,

All rights reserved — no part of this book may be re-
produced in any form without permission in writing
from the publisher, except by a reviewer who wishes
to quote brief passages in connection with a review
written for inclusion in magazine or newspaper.

Sixteenth Printing, 1964

• PRINTED IN THE UNITED STATES OF AMERICA •

PREFACE TO FIRST EDITION

The present book is the outgrowth of a course of lectures
delivered during the past six years at the University of
Michigan. This course with the accompanying laboratory
work was based on the frog which was chosen as a con-
venient form with which to introduce students to a knowl-
edge of the morphology, physiology, and life history of ver-
tebrate animals. In writing this book I have had in mind
the needs of students, such as most of those taking this
course, who have had some preliminary work in general
biology, but who have forgotten most of what little of the
elements of physiology they may have learned in the schools.
A certain amount of physiology of a more or less general
nature has accordingly been introduced in addition to the
descriptions of the special functions of the various organs
of the body. The book is more suitable for use as a text
in college or university classes than in high schools, although
it is hoped that it will prove of service to teachers in high
schools where the frog is studied in the course in zoology.

Considerable space has been devoted to describing the
habits and natural history of the frog, and the endeavor
has been made throughout the work to correlate the study
of structure with that of the physiological functions of the
body and the activities of the organism as a whole in rela-
tion to the environment. There is, I believe, a value in
getting a fairly adequate idea of the whole life of any one

vi

PREFACE

organism which is not attained by the usual course of a
study of types.

In preparing this work it has cost much deliberation in
many cases to decide what material to include and what
to reject, and probably several things have been omitted
which it might have been desirable to have retained. The
literature dealing with the frog is almost appalling in its
extent. Perhaps no animal, except man, has been the sub-
ject of so many scientific investigations. One seldom picks
up a volume of a physiological journal without finding that
the frog comes in for a share of attention in one or more
articles. It indeed seems, as is often remarked, that the
frog is especially designed as a subject for biological re-
search. In fact, most of what is known in certain depart-
ments of physiology is derived from a study of this animal.

The anatomy of the frog has been most exhaustively
treated in Gaupp's excellent revision of Ecker and Wieder-
scheim's "Anatomie des Frosches," and I am naturally
under great obligations to this work. Most of the state-
ments made in the present book regarding the anatomy
of the frog, however, I have verified by personal observa-
tion. Much of the material dealing with the physiology
and natural history of the frog is here brought together
for the first time. Where not otherwise mentioned, the
statements in this book are made with reference to the
common leopard frog of North America, Rana pipiens,
except in cases where they may be understood to apply
equally well to any species of the genus.

I am indebted to Professor Jacob Reighard for several
suggestions and criticisms in connection with the prepara-
tion of this work; to Dr. W. P. Lombard for reading and
criticising several of the chapters dealing with the physi-
ology of the frog; and to Dr. J. E. Duerden for generously
undertaking the labor of critically reading the whole of

PREFACE

vii

the manuscript before it went to press. To Dr. Ernst
Gaupp and his publisher, Friedrich Vieweg, I wish to ex-
press my thanks for their permission to reproduce several
of the figures in Gaupp's revision of Ecker and Wieder-
sheim's "Anatomie des Frosches."

PREFACE TO FOURTH REVISED EDITION

It is now twenty-one years since the issue of the first
edition of The Biology of the Frog. After this lapse of
time a book dealing with any of the rapidly advancing
fields of biology begins to exhibit the infirmities of age.
Since 1906 a good deal has been learned in several of the
i provinces of biological science to which the frog is accus-
tomed to make contributions. Besides, a few of the things
which it was thought were known when the first edition
of this work was written are now found to be not quite
as stated, if not positively erroneous. There have been, of
course, changes in the scientific names of a number of the
species. With the aid of my colleague, Dr. C. L. Camp,
an attempt has been made to bring the generic and specific
names into harmony with present usage among specialists
in the systematic zoology of the Amphibia.

The continued use of The Biology of the Frog, despite the
fact that parts of it were getting out of date, prompted me
to undertake a thorough revision of the book. Although
relatively little modification was necessitated in the sec-
tions on morphology, many changes have been made in the
discussion of other topics, especially those dealing with
physiology. The chapter on internal secretion has been
made over twice its previous length on account of the re-
markable discoveries of the past two decades in the field
of endocrinology. Several new figures have been added

viii PREFACE

and some of the old ones have been replaced by better
substitutes. It is hoped that after these efforts to adjust the
volume to its changing environment it will prove more use-
ful than its predecessor to teachers and students of biology.

University of California,
Berkeley, California, 1927,

S. J. H.

CONTENTS

JHAPTER PAGE

I. The Amphibia in General and Frogs in Par-
ticular 1

II. The Habits and Natural History of the Frog 27

III. External Characters of the Frog .... 65

IV. Preliminary Account of the Internal Struc-

ture 72

V. The Development of the Frog. . 85

VI. Histology of the Frog 125

VII. The Digestive System and its Functions . . 138

VIII. The Vocal and Respiratory Organs . . . . 168

IX. The Skin L 182

X. The Excretory System

XL The Reproductive Organs and the Fat Bodies. 218

XII. Internal Secretion and the Endocrine Glands 225

XIII. The Skeleton 245

fXIV. The Muscular System ^261

XV. The Circulatory System 274

XVI. The Nervous System 299

XVII. The Sense Organs 337

XVIII. Instincts and Tropisms as related to Reflex

Action 357

XIX. The Intelligence of the Frog 368

Index . 375

ix

THE BIOLOGY OF THE FROG

THE

BIOLOGY OF THE FROG

CHAPTER I

THE AMPHIBIA IN GENERAL AND FROGS IN
PARTICULAR

For a long time the frog has been a favorite object for
the study of animal structure and function. Probably no
species except man has been the subject of so many inves-
tigations. Whenever a physiologist plans to undertake a
piece of research his first impulse is to pounce upon the
poor frog. A mere list of the articles written about this
well known animal would fill a volume larger than the pres-
ent one. Doubtless the fact that frogs are usually easy to
obtain is one reason why they have been so extensively
studied. Then they are nice clean animals, easy to dissect
and admirably adapted for physiological experimentation.
The biological sciences owe much to the peculiar virtues
of these humble creatures.

To the student of fundamental life processes it is a
matter of secondary importance whether he devotes his at-
tention to an Amoeba, a bean plant, a frog or a man. If
we had a profound and exhaustive knowledge of any one
organism we should know most of the fundamental secrets
of life in every form. Different species have their peculiar
advantages for the investigation of this or that problem.
But for a large part of the problems with which a biologist

J

THE BIOLOGY OF THE FROG

deals, the choice of a form for study is determined by such
considerations as convenience and availability.

The circumstances which have made the frog a popular
object of investigation have also caused it to be widely
used in instruction. For anyone entering upon the study
of biology there are several advantages in selecting an ani-
mal whose structure is more or less similar to that of the
human body. No student can get very far in becoming ac-
quainted with the anatomy of the frog without coming to
realize that frogs and human beings exhibit numerous simi-
larities of organization which the uninitiated observer
would never have suspected.

THE FROG'S PLACE IN NATURE

Both frogs and human beings belong to that large sub-
division of the animal kingdom known as the Vertebrata,
a group characterized, as the name implies, by the posses-
sion of a vertebral column, or backbone. The term back-
bone, however, is sometimes a misnomer, since in the primi-
tive fishes and their still simpler relatives the vertebral
column is not bony, but cartilaginous. The vertebrae in all
vertebrates are formed around a rod-like structure called
the notochord. This organ is no longer found in the adult
stage of the higher vertebrates, but it always appears in
the embryo and forms the first rudiment of the spinal
column.

In all vertebrate animals the brain and spinal cord lie
dorsal to the digestive canal, whereas in such typical in-
vertebrates as the insects and crustaceans the nerve cord
extends along the ventral side of the body. In the verte-
brates the heart is ventral in position while in the inverte-
brates we have mentioned it is dorsal. All vertebrates
also have red blood which circulates in well defined blood

AMPHIBIA IN GENERAL

vessels, and there is a well developed coelom, or body cav-
ity between the digestive canal and the body wall. The
orientation of several of the chief organ systems of the
vertebrate body is just the reverse of that found in typical
invertebrates. This is why the French naturalist Saint
Hilaire was led to compare a vertebrate with an inverte-
brate walking on its back.

The lowest classes of vertebrates, the cyclostomes (hag
fishes, lampreys) and the fishes, are aquatic animals and
breathe by means of gills. The three highest classes, the
reptiles, birds and mammals, are usually terrestrial in habi-
tat and breathe by means of lungs. The frog belongs to a
class called the Batrachia, or Amphibia which is partly
aquatic and partly terrestrial, and which contains both
gill breathers and lung breathers. In many respects the
Amphibia are intermediate between aquatic and terres-
trial vertebrates. Some of the more primitive species live
permanently in the water, and even those forms which are
best adapted to live on dry land usually repair to the water
to lay their eggs which develop into gill-breathing tad-
poles.

The Amphibia as a class are typically furnished with
four legs fitted for locomotion on dry land or on the bot-
tom of ponds and streams. Unlike the fins of fishes which
are provided with many rays, the legs of the Amphibia are
of the five-fingered, or pentadactyl type characteristic of
the higher classes of vertebrates. One or more of the digits
may be absent in some species, but the general plan of the
feet and limb bones is essentially like that of the leg of a
reptile or mammal.

The skin in the Amphibia lacks the scales characteristic
of the fishes and the reptiles, and is usually smooth and
moist in adaptation to its important function as an organ
of respiration. The heart has three chambers i.e., a ven~

4

THE BIOLOGY OF THE FROG

tricle and two auricles, and there are paired aortic arches.
One of the peculiarities which distinguish the Amphibia
from the reptiles is the characteristic covering of the eggs.
Among the Amphibia the eggs are surrounded by a jelly-
like substance secreted by the walls of the oviduct. The
eggs of reptiles are enclosed by a calcareous shell, and the
embryo during development is provided with an amnion
and an allantois, while in the Amphibia such embryonic
membranes are not formed.

Most amphibians differ from reptiles in undergoing a
metamorphosis, the eggs hatching into aquatic gill-breathing
tadpoles. There are a few^ amphibians which dispense with
a metamorphosis, but in most cases gills are developed in
the embryo, although they may not be retained after the
young emerge from the coverings of the egg.

Amphibians are confined to the torrid and temperate
zones. In the temperate zone they hibernate during the
winter, crawling into the mud or other sheltered localities
out of the reach of frost. In higher latitudes, where on
account of the extreme cold the ground becomes frozen in
the winter to a depth of several feet, the Amphibia are
absent, since they could not easily get into situations where
they could escape being frozen.

The Amphibia may be divided primarily into the Stego-
cephali, or Labyrinthodonts, and the Lissamphibia. The
former group consists exclusively of fossil forms which first
appear in the Carboniferous era and extend into the Upper
Triassic. The bodies of the Stegocephali were usually
covered with scales and bony plates, and the skull was
provided with numerous dermal bones. Some of these
extinct animals were of very large size, and in many species
the dentine of the teeth was folded in a very complicated
manner, — a feature wrhich caused the name Labyrinthodont
to be given to the group,

AMPHIBIA IN GENERAL

The Lissamphibia lack the extensive dermal skeleton of
their extinct predecessors. Their tooth structure is simple
and they never attain large proportions. They are com-
monly divided into three orders which may be determined
by the following key: —

Legs absent Apoda.

Legs present.

Tailed amphibians Urodela.

Tailless amphibians Anura.

THE APODA

The Apoda, or Coecilians, are

creatures of wormlike form, en-
tirely devoid of limbs and limb
girdles. The skin is smooth and
thrown into transverse rings. In
some forms small scales are
found embedded in the integu-
ment. The eyes are small func-
tionless rudiments buried be-
neath the skin. The Apoda are
generally found in moist ground,
in which they burrow, and they
are confined to tropical or subtropical regions. No species
occur in North America north of Mexico.

THE URODELA, OR TAILED AMPHIBIANS

The tailed amphibians occupy a more primitive position
than the tailless forms. A large proportion of them live in
the water, and some members of the group retain their gills
in the adult state. The Urodela are divided by Gadow into
four families as follows: —

Fig. 1. — A legless amphi-
bian, Ichthyophis glutinosa,
female guarding her eggs.
(From the Cambridge Nat-
ural History, after Sarasin.)

6

THE BIOLOGY OF THE FROG

Jaws without teeth. No hind limbs . . . Sirenidce.
Both jaws with teeth. Fore and hind limbs
present.

Gills persistent. No eyelids or maxillary bones Proteidw.
Gills usually absent in the adult. Maxillary
bones present.

Eyes with lids Salamandridce*

Eyes devoid of lids Amphiumidce.

The Proteidse constitute the most primitive of the Uro-
deles. At the sides of the neck there are three pairs of
external gills. The species are aquatic in habit. There are
only three genera, two of which, Necturus and Typhlo-
molge, are confined to North America. The remaining
genus, Proteus, represented by a single species, P. anguinus,
is found only in the caves of Austria. This species occurs
in deep cool water in regions of complete darkness. Its eyes,
like those of many cave animals, are rudimentary. Its
color is nearly white, but if exposed to light its skin
gradually turns dark and eventually may become nearly
black.

Fig. 2. — Proteus anguinus. Front view of the mouth in the upper left
corner. (After Gadow, Cambridge Natural History.)

A cave salamander, Typhlomolge rathbuni, closely allied
to Proteus, was found only a few years ago in Texas, where
it was discovered in water thrown up from an artesian well.

AMPHIBIA IN GENERAL 7

The body of this species is slender and provided with a long,
flattened tail. The legs are long and slender. The eyes,
like those of Proteus, are rudimentary and buried beneath
the skin. The most common representative of the Proteidae
are the "mud puppies" or "water dogs," which belong to

Fig. 3. — Necturus maculosus. (From Hegner.)

the genus Necturus. Necturus maculosus is the most
abundant species. It occurs in the northern and eastern
part of the United States, west of the Alleghenies, and is
especially abundant in the region of the Great Lakes. Its
general color is brown above, marked with darker spots,

Fig. 4. — Siren lacertina. (From the Cambridge Natural History.)

and a dirty white or dusky color below. It has bushy red
gills, which are kept moving back and forth at frequent
intervals. Like most amphibians, it is most active at night ;
during the day it lies concealed out of the reach of light.

The family Sirenidae is represented by two genera, Siren
and Pseudobranchus, both of which are confined to North
America. Each genus contains but a single species. The

s

THE BIOLOGY OF THE FROG

larger of these, Siren laeertina, is found in the rivers and
ponds of the Southern States, from Texas to North Carolina.
The body is long and snakelike in appearance. The fore
legs are very short and situated close behind the external
gills; the feet are four-toed. There are three pairs of gill
slits. The genus Pseudobranchus has only one pair of gill
slits instead of three, and the feet possess but three toes.
The single species, P. striatus, occurs in Georgia and Florida.

The Amphiumidae include forms of quite diverse appear-
ance, which are sometimes placed in distinct families. The
genus Amphiuma is represented by a single species, A.
means (Fig. 3), found in the Southern States of North
America. The body is eel-like, with the very small legs
situated far apart, near the two extremities. There is a
single pair of gill slits behind the head, near the fore legs.
The length of this species is often over two feet. The female
lays her eggs in the latter part of the summer, and lies
coiled about them in some protected spot, until they hatch.

The genera Cryptobranchus and Megalobatrachus are
sometimes placed in a distinct family, the Cryptobranchidse.
The former is represented by the large "hellbender, " C.
alleghaniensis, of the eastern United States. This species
may reach a length of twenty inches. Its body and head
are much flattened, and the sides are bordered with curious
fluted folds of skin. The eyes are relatively very small
The hellbender is very sluggish in its habits, but it is,
nevertheless, a very voracious eater. Its vitality, judging
from an account by Mr. Frear,1 is certainly remarkable.
Mr. Frear tells of one specimen which had been picked up
after it "had lain exposed to a summer sun for forty-eight
hours." It was then brought into the museum and left a
day before it was placed in alcohol. After it had been left
in the alcohol "for at least twenty hours" it was taken out,

Mm. Nat,, Vol. 16, 1882.

AMPHIBIA IN GENERAL

9

"when it began to open its big mouth, vigorously sway its
tail to and fro, and give other undoubted signs of vitality."
The giant salamander of Japan, Megalobatrachus maximus,
is closely related to the preceding species. The largest
specimens known exceed five feet in length.

Fig. 5. — Amphiuma means. (From the Cambridge Natural History.)

The Salamandridae form a large family, which is fre-
quently divided into several different families by many
writers. Only a few of the more noteworthy forms, there-
fore, will be described.

The group is divided by Gadow into four subfamilies as
follows: —

A. Series of palatal teeth transverse or posteriorly
converging.

B. Parasphenoid without teeth. Vertebrae am-

phiccelous. Toes 4-5 Amblystomatincv.

10

THE BIOLOGY OF THE FROG

BB. Parasphenoid with teeth.
C. Vertebrae opisthoccelous. Toes 5. Tongue

largely free . Desmognathince.

CC. Vertebrae amphiccelous. Tongue small

and largely free Plethodontirue.

A A. Series of palatal teeth in two longitudinal
series diverging behind. Parasphenoid
toothless . . . . . . . . Salamandrime.

The subfamily Amblystomatinae is represented in this
country mainly by the two genera Amblystoma and Chon-
drotus. Amblystoma contains quite a large number of
species. They are mostly of considerable size and fre-
quently spotted in color. They are very retiring in their
habits, and are not often seen except in the spring, when
they go to the water to breed. Their eggs, which are sur-
rounded by a very thick coat of jelly, are found in rounded
or irregular masses attached to twigs or stems of grass.

The larvae of A. tigrinum were formerly considered a
separate species, the axolotl, which was placed in a distinct
genus, Siredon, among the urodeles with permanent gills.
Under certain conditions the external gills of this larva
may be retained until after the breeding season, and this
peculiarity led to its being mistaken for a normal adult
form. It has been contended that the metamorphosis of
the axolotls could be accelerated if they were forced to
breathe air, but Professor Powers has shown that the factor
of nutrition is probably the most important one, although
others are influential, in producing this change, since it
usually follows in sufficiently mature larvae upon a sudden
diminution of the food supply.

The Desmognathinae include three genera, of which
Desmognathus is the most common. It contains only three
species, all of which are confined to the eastern part of the
United States. The species live concealed in the daytime
under stones or in sheltered nooks where the Air is moist.

AMPHIBIA IN GENERAL

11

The female of D. fuscus lays her eggs in two long strings
which she wraps around her body after having resorted to a
suitable hiding place. ^
Another represent a- , '-^^P^iJ^^Ste-

The species of Pletho-

don, Eurycea and Batrachoseps, the more common genera,
are mostly of small size. They are usually found in damp
situations under rocks or decaying masses of wood. A Cali-
fornia species, Aneides lugubris, has been found by Ritter 1
to have the peculiar habit of laying its eggs in holes high
up in the branches of live-oak trees.

The Salamandrinae are mainly found in the Old World.
The well known fire salamander of Europe, Salamandra
maculosa, reaches a length of from six to eight inches. The
skin is smooth and shiny, and colored black except where
marked with large irregular yellow spots. The conspicuous
color of this species is frequently cited as an example of
"warning coloration/' since the glands of the skin secrete a
substance which is very poisonous. By advertising its dis-
agreeable qualities in this way the salamander is rendered
free from the attacks of many animals which would other-
wise unwittingly destroy it. Gadow tells of the dearly
bought experience of two American bullfrogs that were
kept in an inclosure with several salamanders. The next

1 Univ. of Calif. Publications. Zoology, Vol. 2, 1904.

The Plethodontinae

form a large group,
which is mainly con-
fined to America.

tive of this sub-
family is Typhlotri-
ton spelceus, a blind
species found in a
cave in Missouri.

Fig. 6. — Desmognathus fuscus. Female
with egg-mass. (After Wilder.)

12

THE BIOLOGY OF THE FROG

morning after they were put in "the huge frogs were found
dead, each having swallowed a salamander, which they were
not acquainted with and had taken without suspicion. "
The poison of the salamander, which has been analyzed by
Faust, is very different from that of the toad and has an
especial affinity for the nervous system.

Salamandra atra is a shiny black species which lives high
up in the Alps. The young are retained in the uterus until
they attain an advanced stage of development. When they

Fig. 7. — Triton cristatus. 1, female ; 2, male as he appears during
the breeding season. (After Gadow. )

are born they have no external gills, as the young of the
preceding species do, but these organs are nevertheless
fully developed in the unborn larva, in which they attain
a remarkable degree of development. The large size of the
gills is doubtless dependent on the fact that they are
worked in for the purpose of absorbing food.

The genus Triton is remarkable on account of the marked
sexual dimorphism which occurs in several of the species,
especially during the breeding season. The male of T. cris-
tatus at this time possesses a high serrated crest above the

AMPHIBIA IN GENERAL

IS

head and body, and is marked with conspicuous colors.
After the breeding season the dorsal crest becomes greatly
reduced and the coloration becomes duller. The female
has no crest and is not so conspicuously colored as the male,
although she also becomes duller in color after the breeding
period is past.

A close relative of the Tritons is the common newt
(Triturus viridescens) of the northern and eastern parts
of this country. It is a pretty species, being colored an
olive-green, reddish or reddish brown above, orange or
lemon-yellow below, and having a lateral row of scarlet
spots, each surrounded by a black ring. A variety, miniatus,
which has been described and which is characterized by
possessing a vermilion red color, is said by Gage to be
only an immature form of this species. Egg laying was
found by Jordan to take place near Worcester, Massachu-
setts, from about April 10 to June. The eggs are laid in
small nests attached to masses of vegetation, or wrapped
within leaves of aquatic plants.

THE ANURA

The Anura, or tailless Amphibia, have a short, broad
body, with well-developed hind legs fitted for jumping.
They are divided by Gadow as follows: —

A. Tongue absent Aglossa.

AA. Tongue present (Phaneroglossa) .

B. Halves of the shoulder girdle overlapping
in the middle line (Arcijera).
C. Sacral diapophyses dilated.
D. Terminal phalanges not claw shaped.

Ribs present. Upper jaw with teeth . Discoglossidce.
No ribs. Upper jaw with teeth . . Pelobatidce.
No ribs. Both jaws without teeth . Bufonidce.
DD. Terminal phalanges claw shaped, usu-
ally supporting adhesive disks . . Hylidce.

14

THE BIOLOGY OF THE FROG

CC. Sacral diapophyses cylindrical . . . Cystignathidce.
BB. Halves of the shoulder girdle meeting in
the middle line and forming a me-
dian bar (Firmisternia) .
C. Sacral diapophyses dilated .... Engystomatidce,
CC. Sacral diapophyses cylindrical . . . Ranidce.

The Aglossa include only a few aberrant forms charac-
terized by the absence of a tongue and the fact that the
Eustachian tubes open by a single median aperture in the

Fig. 8. — The Surinam toad, Pipa americana. (From Mivart.)

posterior side of the palate. The most noteworthy member
of this group is the peculiar Surinam toad, Pipa americana,
from the northern part of South America. This creature
has a most grotesque appearance. The back is broad and
flattened, the head small, triangular, depressed, and fur-
nished with irregular flaps near the lips; the eyes are small
and have a round pupil. The most remarkable feature of
the species is the mode in which the female carries the eggs
and young. Aftei the eggs are laid and fertilized, they be-

AMPHIBIA IN GENERAL

15

come pushed upon the back of the female, to which they
adhere. The skin then grows up around the eggs; inclosing
them in separate cavities which become entirely covered
over. The tadpole stage is passed within these cavities.
When the young Pipa is quite fully formed, it breaks out
and makes its escape.

The Discoglossidae are not represented by any American
species. One of the most noteworthy of the European
species of this family is the so-called
obstetrical toad, Alytes obstetricans.
In the breeding season the male clasps
the female in the usual way, and when
the egg strings are extruded, he tan-
gles them around his hind legs and
carries them about with him. When
the young larvae are about ready to
escape, the male takes to the water
and frees himself of the mass.

The Pelobatidae include but one
American genus, Scaphiopus. These
forms are commonly known as the
spade-foot frogs, on account of the
peculiar horny appendage which oc-
curs on the inner side of the hind foot.
This structure is employed in digging in the ground, where
the animal is concealed during the day. Scaphiopus hot-
brooki, which is found in the southern and eastern parts of
the United States, is very capricious in making its ap-
pearance. After rains in the spring or summer the spade-
ioot frogs come out in great numbers and lay their eggs,
making a great clamor with their song. Then they dis-
appear, and may not again show themselves for several
years.1

1 See Abbott, Am. Nat., Vol. 16, and Hargitt, Am. Nat., Vol. 22.

Fig. 9— The obstet-
rical frog, Alytes ob-
stetricans; male, with
string of eggs. (From
Sedgwick's Zoology,
after Claus.)

16

THE BIOLOGY OF THE FROG

The Bufonidae, or toads, comprise a large family which
is found on all the continents of the globe. The principal
genus is Bufo, which includes the best-known representa-
tives of the family. The toads of this genus possess a very
rough, warty skin, whose irregularities are caused by the
large number of poison glands contained in it. These
glands secrete a whitish, milky fluid of a very poisonous

nature. Even a very small quantity of this substance
when injected into the blood of a small animal soon pro-
duces fatal effects. The abundance of this secretion affords
the toad very efficient protection, and not many animals
have the hardihood to attack the creature. Faust 1 has iso-
lated and analyzed two similar compounds from the poison-
ous secretion, bufonin and bufotalin. These are allied to
digitalin and they produce similar effects. In weak doses
they accelerate the action of the heart, but in larger quan-
tities they act as a virulent heart poison. In addition to the
poison, the skin secretes mucus, as in other amphibians, al-
though not in great quantity.

1 Arch. exp. Pathol. Bd. 47, 49, 1902-3.

Fig. 10. — Bufo horeas. (After Storer.)

AMPHIBIA IN GENERAL

1?

The color of toads, like that of frogs, may change under
the influence of different external conditions. When ex-
posed in a light-colored environment, the skin usually be-
comes lighter in color. In a dark environment it becomes
darker, thus bringing about a certain adaptation of the color
of the animal to that of its surroundings. This change is
effected by means of changes in the pigment cells of the skin -
in the same manner as in the frog, which will be more fully
described later.

Toads are nocturnal in habit. During the day they lie
concealed under stones or in other damp, shady localities,
venturing out only toward evening. They hop about like
frogs, although with much less agility. On the other hand,
they climb with considerable readiness. They feed upon
earthworms, snails, and all sorts of insects. The latter are
generally caught by suddenly throwing out the tongue and
then withdrawing it along with the insect to which it ad-
heres. Toads are very useful in destroying large numbers
of injurious insects, and hence deserve all possible encour-
agement and protection. Kirkland 1 and Garman,2 who
have carefully examined the contents of the stomachs of a
large number of toads, find that the variety of insects de-
voured is very great. Ants were the forms most commonly
met with in the stomachs, and beetles, bugs, moths, and
caterpillars were found by Garman to follow successively
in order of frequency.

Toads keep within a certain locality for a long period.
They have their particular holes or nooks, where they rest
during the day and to which they return after their night's
journey in search of food. Their sense of locality is appar-
ently quite good, as is shown not only by the fact that they
find their way home, but by their habitually visiting certain

1 Kirkland, Bull. No. 46, Mass. Agric. Exp. Sta.
* Garman, Bull. No. 91. Ky. Exp. Sta.

IS

THE BIOLOGY OF THE FROG

spots in the course of their nocturnal wanderings. The
longevity of toads is somewhat uncertain. Boulenger kept
one specimen for twelve years. There is a record of a
specimen which lived to be thirty-six years old, and was
then accidentally killed. Cases are recorded in which a
toad has occupied a certain retreat for a longer period; but
the identity of the individual is not assured. There are
numerous stories of live toads found embedded in rocks or
sealed up in trees ; but usually they do not give evidence of
sufficiently careful investigation to compel belief. Buck-
land 1 has shown that toads may live without food, when
sealed up in blocks of limestone, for over a year; toads im-
prisoned in the limestone for two years were invariably
found to be dead. We should be skeptical, therefore, about
accepting stories about toads having been found alive in
situations where they must have remained for a much
longer time.

Toads hibernate under rocks, or in cavities in the ground,
where they are protected from extreme cold. Often several
toads may be found huddled together in one hiding place.
Here they lie benumbed and almost stiff, although not
actually frozen, until spring. Soon after their emergence
from their winter sleep they usually betake themselves to
water to deposit their eggs. The breeding period of Bufo
lentiginosus in Massachusetts, according to Kirkland, is in
April; in Ithaca, New York (Gage),2 from the middle of
April to May; in Ann Arbor, Michigan, I have found this
species breeding in the latter part of April. The eggs are
embedded in long strings of jelly which are usually found
among vegetation near the shore. The males of B. lenti-
ginosus are much smaller than the females. During the
breeding season they frequently utter a peculiar shrill

1 Buckland, "Curiosities of Natural History."
aGage, Proc. Am. Ass. Adv. Set., Vol. 47, 1898.

AMPHIBIA IN GENERAL

19

sound. After this period, according to Allen, the song
changes to "a shorter, lower-toned note that, at night, has
a peculiar weirdness, and reaches almost a wail. This note
is heard mostly at evening and during the night, though I
have occasionally heard it early in the morning and late
in the afternoon. " According to Courtis,1 Miller,2 and other
observers the females are attracted by the sounds produced
by the males.

When toads are handled, and often even when ap-
proached, they swell their bodies with air. Slonaker 3
tells of a toad which when approached by a snake would
swell up and orient itself with its back toward the enemy.
The inflation of the body makes it more difficult to
retain hold of the creature, as any one may readily
determine.

There are several species of toads in North America, but
most of them are confined to the Western States.

The Hylidse, or tree frogs, form an extensive and widely
distributed family. The tips of the toes are furnished with
small adhesive disks which enable the animal to climb up
the trunks of trees. Many species are able to climb up a
vertical surface of smooth glass. This is rendered possible
not so much through the suction of the disks as by a stickj
secretion which is produced by the glands of the skin at
these points.

Male tree frogs are usually able to make a noise which is
astonishingly loud for creatures of so small a size. In Hyla
and its allies the vocal sac of the male is capable of great
distension, and when fully inflated, becomes much larger
than the head. The voice is heard most often in the breed-
ing season, but it may also be heard during most of the
summer, especially about dusk. The note of the tree frog

-Am. Nat., Vol. 41, 677, 1907. 2 Am. Nat., Vol. 43, 641, 1909.
3 Proc. Indiana Ac. Sci., 1900.

20

THE BIOLOGY OF THE FROG

is often regarded as indicative of approaching rain. It
is frequently heard immediately before a shower. The
integument of the creature is easily affected by changes in
moisture. Situations where the air is damp are always
preferred, and it is not unnatural that their song should
be heard when the atmosphere approaches the point of
saturation.

A great many species of tree frogs have a remarkable
power of changing their color under different external con-
ditions. When among green leaves their color is usually
green, but when on the bark of trees or on the ground their
color may change to a brown or gray.

The North American species of this family north of
Mexico and Texas fall into three genera. These are sepa-
rated by Jordan by the following key: —

A. Disks small. Fingers not webbed. Palustrine.

B.. Toes broadly webbed. Tympanum indistinct . Acris.

BB. Toes scarcely webbed. Tympanum distinct . Pseudacris.
AA. Disks round, conspicuous. Fingers somewhat

webbed. Skin roughened. Arboreal . . Hyla.

Acris is represented by a single species, A. gryllus, the
common "cricket frog/' which is distributed over the greater
part of the United States. Its typical color is brown or gray
above. There is a dark triangular patch between the eyes,
and the middle of back and head is bright green or reddish
brown. Specimens from different regions show variations
in color and there is a considerable power of color change
in the individuals themselves. This species is usually
found along the banks of ponds and swamps. Its note re-
sembles that of a cricket.

Hyla is represented by over a hundred species, about
twenty of which occur in North America. The species are
mainly found in trees. H. versicolor, so-called on account

AMPHIBIA IN GENERAL

21

of its remarkable change in color, is one of the most com-
mon and largest of the North American species, reaching
a length of two inches. The eggs of this species are de-
posited singly or in small clusters on grass growing near
the water's edge.

Many of the Hylidae possess singular devices for carrying
the eggs. In Hyla goeldii of Brazil the eggs are carried on
the back of the fe-
male, the skin being
produced into a fold
which borders the
egg mass. Noto-
trema, the "marsu-
pial frog" of South
America, has a large
pouch opening near
the posterior end of
the back, in which
the eggs are re- . Y / Kf,

Fig. 11. — Hyla aremcolor. (After Storer.)

ceived, and where

they undergo development as far as the tadpole stage.

The Engystomatidae contain but one North American
species, Gastrophryne carolinense, which is found in the
Southern States, from South Carolina to Texas. The large
family Cystignathidae is represented on this continent by
only three species, which are confined to Mexico and
Florida.

The Ranidae, or true frogs, comprise numerous genera,
only one of which, the typical genus Rana, is found in North
America. This genus contains over one hundred and fifty
species, which occur in all of the continents of the globe,
although they inhabit only the extreme northern parts of
South America and Australia. There are seventeen North
American species. Only a few of the better-known forms

22

THE BIOLOGY OF THE FROG

can be mentioned here. Full descriptions of the species may
be found in Cope's "Batrachia of North America/' and in
Miss Dickerson's "Frog Book."

Rana catesbiana, the bullfrog. — This is by far the largest
cf North American species of Rana, and one of the largest
of the genus. It attains a length of five to eight inches.
It is widely distributed throughout the United States, east
of the Rocky Mountains, from Mexico to Canada. The

Fig. 12. — Rana catesbiana, the bullfrog.

color of the upper surface varies from green to olive-brown,
marked with small darker spots. The head is usually
bright green, and the legs are marked with blotches of
darker color. The tympanum or eardrum is very large,
especially in the male. The toes of the hind feet are broadly
webbed, the web extending to the tip of the fourth toe.

This species rarely goes far from the water. It is usually
found either partly immersed in the water or sitting on the
bank of some pond or stream. It makes for the water very

AMPHIBIA IN GENERAL

23

quickly when alarmed, and usually skims along the surface
t for several yards before diving below. According to Kalm,
it may leap to a distance of three yards, but Abbot, who
experimented with several specimens, found none that could
jump quite seven feet. The males have a very loud, hoarse
bass voice, which has been compared to the bellowing of a
bull. When a number of them are croaking near by, the
noise, as Kalm observes, is "so loud that two people talking
by the side of a pond cannot understand each other. They
croak all together; then stop a little, and begin again. It
seems as if they had a captain among them; for when he
begins to croak, all the others follow; and when he stops,
the others are all silent. When this captain gives the signal
for stopping, you hear a note like 'po-opF coming from
him. In daytime they seldom make any great noise, unless
the sky is covered. But the night is their croaking time;
and when all is calm, you may hear them, though you are
near a mile and a half off."

Bullfrogs feed not only upon the creatures devoured by
other species of frogs, but they frequently capture other
animals which their smaller relatives are unable to swallow.
They often devour full-grown specimens of other species of
Rana, the young of ducks, and other water fowl, and even
small chickens which venture too near their haunts.

The bullfrog requires two years to complete its metamor-
phosis. I have often captured its large tadpoles beneath the
ice in midwinter.

A very closely allied species of bullfrog, R. grylio, has
recently been described from Florida by Stejneger. It has
somewhat longer toes and a darker color than catesbiana,
and is said to have a quite different voice.

Rana clamituns. — This species has a nearly uniform green
or brownish color above, marked only with small irregular
black spots. The dermal plicae are conspicuous. The hind

24

THE BIOLOGY OF THE FROG

legs are short and the web extends well out on the toes.
The most conspicuous feature of this species is the very
large tympanum, which in the male considerably exceeds the
diameter of the eye. In the female the tympanum is con-
siderably smaller, being about three fourths the diameter of
the eye and "distant from the latter by nearly half its own
diameter." This species is widely distributed from the
Eastern States to Missouri and Minnesota and from Can-
ada to Florida and Mississippi. It is closely confined to
water like the bullfrog. It may reach a length of three
inches.

Rana sylvatica, the wood frog.— Unlike the two preced-
ing species,^, sylvatica is usually found in damp woods
often far from water. It occasionally occurs at a consider-
able elevation, one specimen having been taken by Mr.
Allen near the top of Mount Bartlett, New Hampshire, at
an altitude of twenty-five hundred feet. This frog, says
Mr. Allen, "is commonest in the beech woods and so closely
resembles in color the dead beech leaves, that not infre-
quently, even after having seen one jump, it is with diffi-
culty distinguished from the background. When frightened
it takes prodigious leaps in an erratic course, and usually
escapes into some hole or under a log. At night, while
walking in a damp spot in the woods, I found numbers of
them congregated in the path, where they had probably
come to feed. . . . Rarely have I heard them utter a
sound in the summer, though occasionally, when in the
woods at night, I have detected their faint, rasping 'craw-
aw-auk.'^}

Rana pipiens, the leopard frog. — This is perhaps the most
common of all the North American species of Rana. Its
ground color is green marked with rather large black
blotches edged with whitish. The legs are crossed above
with black bars which may or may not be interrupted in the

AMPHIBIA IN GENERAL

25

middle. There are usually two irregular rows of black
spots on the back, between the prominent dermal plicae; the
| lower side of the body is pale. The tympani are smaller
than the eyes and there is no black ear patch. The vomer-
ine teeth lie between the posterior nares. The legs are
long, so that when the heel is brought forward it extends in
front of the tip of the snout.

Cope distinguishes four varieties of this species, for a
description of which the reader may be referred to this
author's "Batrachia of North America."

Rana palustris, the pickerel frog. — This species resembles
the preceding one. It is usually brownish in color, with
two rows of large rectangular dark brown blotches between
the dermal plicae. There is a brown spot above each eye
and a dark line between the eye and the nostril. The body
is whitish below, but the lower side of the hind legs is
yellow. External vocal sacs are absent.

This species is quite common in the eastern part of the
United States. It is said by Cope to prefer "cold springs
and streamlets, but is of all our frogs the most frequently
seen in the grass."

REFERENCES

Abbott, C. C. A Naturalist's Rambles about Home, 2d ed., 1894.
Allen, G-. M. Notes on the Reptiles and Amphibians of Intervale,
New Hampshire. Proc. Bos. Soc. Nat. Hist., Vol. 29, 1901.
Boulenger, G. A. The Tailless Batrachians of Europe, 1897.
Brehm, A. C. Thierleben, Bd. 7.

Cope, E. D. Batrachia of North America. Art. "Amphibia" in the
Riverside Natural History.
Dickerson, M. The Frog Book, N. Y., 1906.

Dumeril et Bibron. Erpetologie Generale ou Histoire complete des
Reptiles.

Durigen, B. Deutschlands Amphibien und Reptilien, 1897.

Fischer-Sigwart, H. Biologische Beobachtungen an unseren Am-
phibien. Vierteljahrsch. d. Naturf. Gesell. Zurich, LXII, Jahrg.
1897.

26

THE BIOLOGY OF THE FROG

Gadow, H. Amphibia and Reptiles. Vol. 8 of the Cambridge
Natural History.

Hay, O. P. The Batrachians and Reptiles of the State of Indiana,
1892, 17th Ann. Rep. Dept. Geol. and Natural Resources.

Hoffmann, C. K. "Amphibien," in Bronn's Classen und Ordnungen
des Thierreichs, Bd. VI, 2.

Holbrook, J. E. North American Herpetology.

Jordan, D. S. A Manual of the Vertebrate Animals of the North-
ern United States, 9th ed., 1904.

Leydig, F. Die anuren Batrachier des deutschen Fauna, 1877.

Miller, N. The American Toad. Am. Nat. Vol. 43, 641, 730, 1909.

Rosel von Rosenhof. Historia naturalis ranarum nostratium, 1758.

Spallanzani, L. Experiences pour servir a l'Histoire de la genera-
tion, 1787.

Storer, T. M. A Synopsis of the Amphibia of California. Univ.
Calif. Pub!. Zool. Vol. 27, 1925.

Wright, A. H. North American Anura. Carnegie Inst. Publ. No.
197, 1914. Frogs. Rep. U. S. Bur. Fish., 1919, 1921.

Accounts of the general anatomy of the frog are contained in the
following works:—-

Bourne, G. C. An Introduction to the Study of the Comparative
Anatomy of Animals, 2 vols., 1900.

Ecker, A. Anatomy of the Frog, translated by George Haslam,
Oxford, 1889.

Ecker und Wiedersheim. Anatomie des Frosches, auf Grund
eigener Untersuchungen durchaus neu bearbeitet von Dr. Ernst
Gaupp, 1896-1904.

Hempelmann, F. Der Frosch. Leipzig, 1908.

Howes, G. B. Atlas of Practical Elementary Zootomy, 1902.

Huxley and Martin. General Biology, 1889.

Marshall, A. M. The Frog: an Introduction to Anatomy, 6 ed.,
1896.

Mivart, St. George. The Common Frog, 1874.
Parker and Parker. An Elementary Course in Practical Zoology,
1900.

Vogt und Yung. Lehrbuch der praktischen vergleichenden Anato^
mie, 2 Bd.

CHAPTER II

THE HABITS AND NATURAL HISTORY OF THE FROG

Habitat. — The habitat of Rana pipiens, like that of most
species of frogs, is usually in or near the water. In damp or
wet weather, however, this species frequently wanders for a
considerable distance from its aquatic home. It is liable to
be found almost anywhere near the shores of lakes, ponds, or
streams in the wide territory over which it is distributed.
Its range as given by Cope is from "Athabasca Lake, in the
north, to Guatemala inclusive, in the south," and from the
Atlantic coast to the Sierra Nevada Mountains. It has,
therefore, the widest distribution of any of the North Amer-
ican species of Amphibia, although it is not known to occur
on the Pacific slope.

That Rana pipiens is confined to the neighborhood of
water depends in great measure on the fact that the skin
must be kept moist in order that cutaneous respiration may
take place. As soon as the integument becomes dry, as it
quickly does if the frog is exposed to warm dry atmosphere,
it is no longer capable of serving as an organ of respiration,
and the animal soon perishes. The frog, unless it is among
wet grass or weeds, or in a moist atmosphere, must remain
where it can moisten the skin by an occasional plunge into
the water. Another circumstance which serves to keep the
frog in close proximity to water is the means thus afforded
of escaping from enemies. Anyone who has walked along
the margin of a pond or stream must have observed that
when a frog is started up it almost invariably makes a jump

27

28

THE BIOLOGY OF THE FROG

for the water. In this way the creature has a ready mode of
escaping, not only from man, but from a number of other
enemies which might easily overtake it in a fair field. After
its first plunge the frog usually swims some distance under
the surface and then comes up, exposing only the tip of
its snout above the water to get air. Frequently, if there
is grass or weeds near the water's edge, the frog will swim
a few strokes away from the shore and then turn back
and quietly come to the surface among the vegetation, where
its advent would usually not be suspected by the observer.

During the breeding season in the spring, frogs are more
closely confined to the water than at other times of the year.
In the summer they wander farther from the water in search
of food. Different species vary greatly, however, in this
respect. \^The wood frog, Rana sylvatica, is commonly
found in woods miles away from any pond or stream^ Most
of the other North American species of Rana are more
closely confined to an aquatic habitat. <5n Europe the water
frog, R. esculenla, is decidedly aquatic on its habits, whereas
other species, commonly spoken of as the grass frogs, scatter
through the meadows and woodlands after the breeding
season.

Food. — The food of frogs consists of earthworms, in-
sects, spiders; in fact, of almost any kind of animal small
enough to be seized and swallowed.1 Large frogs have no
sentimental scruples against devouring their smaller rela-
tives. The large bullfrog is an especially dangerous enemy
to other members of its genus. I have often found the
stomach of this animal greatly distended from its having
swallowed nearly full-grown specimens of Rana pipiens.
Earthworms are a favorite article of diet; a hungry frog
will devour several large worms one after the other, often

1 Drake, C. J., The Food of Rana pipiens. Ohio Nat., Vol. 41, 257,
1914.

HABITS AND NATURAL HISTORY OF THE FROG 29

seizing a new worm before having finished the one it is
attempting to swallow. According to Fischer-Sigwart, Rana
\fusca will devour large May beetles, employing both fore
limbs to push the rough legs of the insect into such a posi-
tion that the prey can be forced down the throat, an opera-
tion which is accomplished only after considerable difficulty.
The same observer found that R. fusca would devour large
snails (Helix hortensis and H. nemoralis) after it had been
accustomed to that form of diet by being fed with speci-
mens from which the shell had been removed. After a short
preliminary education the frog would catch the snails, of
its own accord, and swallow them shells and all, one frog
devouring six large specimens in succession. According
to Fischer-Sigwart, this frog does not ordinarily devour
snails although Durigen reports Rana muta as swallowing
specimens of Helix as well as species of mollusks devoid
of shells. Bees and wasps are eaten with avidity notwith-
standing their stings, which apparently affect the frog but
little.

While the frog is a gourmand, he is nothing of an epicure.
Almost any sort of living creature is acceptable to him, and
even decayed meat when once seized is readily swallowed.
Both taste and smell are apparently obtuse; if anything is
taken into the mouth, it usually continues its course down
the alimentary canal. Objects of a too objectionable na-
ture may, however, be ejected.

In seizing food, the frog usually makes use of its extensile
tongue, which can be thrown out of the mouth with surpris-
ing rapidity. The tongue is attached by its anterior end to
the tip of the lower jaw, while the forked posterior end lies
free. In the capture of prey the posterior end of the tongue
is thrown forward until it comes in contact with the object,
when it is quickly withdrawn. The sticky secretion wTith
which the tongue is covered enables it to adhere to the

30

THE BIOLOGY OF THE FROG

objects it strikes against, so that they may be conveyed to
the mouth.

The frog has an instinct to snap at small moving objects
that come sufficiently near. This action is determined more
by the motion and size of the objects than their form. Un-
less a thing is moving, the frog pays little attention to it.

Frogs may often be
caught by dangling small
bits of red yarn before
them on a hook. When
the yarn is seized, the ani-
mal may be jerked out of
the water. According to
Knauer, frogs and toads
have the power of eject-
ing indigestible bodies
from the stomach by way
of the mouth. Bits of grass or moss accidentally swallowed
with the food are gotten rid of in this way.

Protrusion of the Tongue. — The frog is able to throw
out its tongue with remarkable rapidity, but the method by
which this feat is accomplished was, until recently, but in-
adequately understood. Hartog 1 and Gaupp 2 have found
that the protrusion is brought about by the pressure of the
lymph in the large sublingual lymph sac. This may be
readily shown if we cut off the upper jaw of the frog and
inject air or liquid through the mylohyoid muscle, which
extends beneath the tongue. The lymph spaces become
filled, and this causes the tongue to be raised up and thrown
forward. "If," says Hartog, "we inject with melted cocoa
butter colored with carmine or alkanet, and keep up the
pressure until the mass sets, we find that it fills an enormous

hartog, Ann. Nat. Hist., May, (7), 7, 1901.
3 Gaupp, Anat. Anz., 19, 1901.

Fig. 13. — Figure showing the tongue
of the frog in three different posi-
tions. (After Wiedersheim. )

HABITS AND NATURAL HISTORY OF THE FROG 31

lymph sac between the muscle and the body of the hyoid,
extending through the median intermuscular fissure into the
tongue itself, sending branches between the fan-shaped ram-
ification of the intrinsic muscles at the edges of the tongue
and into its terminal dilatations. " According to Hartog,
the contraction of the mylohyoid muscle expels the lymph
from the subhyoid space into the tongue and thus effects the
protrusion of this organ.

Locomotion. — The locomotion of the frog is effected by
leaping and swimming, and in both of these operations the
long hind legs play the chief part. In the ordinary resting
position the body is inclined upward in front, being sup-
ported on the fore legs, which are held in a peculiar twist so
that the large thumb points nearly backward; the posterior
part of the body rests upon the ground, and the hind limbs
are folded up ready for a spring. No preliminary move-
ments are required in order to get the animal in readiness
for escape. By a sudden extension of the hind legs the
body is propelled through the air. In leaping, the fore
limbs are used more to hold up the anterior part of the
body and to point the animal in the desired direction of
movement than as actual organs for propulsion. If one
causes a frog to leap in various directions, it will be observed
that the body is adjusted before each leap in a new direc-
tion by the movements of the fore limbs. An ordinary
specimen of Rana pipiens may leap from two to three feet.

The movements of the hind legs in swimming are very
much like those performed in jumping. In both operations
the hind legs are alternately drawn up in the form of a Z
and quickly extended. As they are pushed back, the toes
are spread apart, and as the web between them affords a
considerable resistance to passing through the water, this
motion gives the body a forward impulse. The fore limbs
are held back against the body, after the stroke, and if the

32

THE BIOLOGY OF THE FROG

frog does not make several strokes in quick succession, the
hind limbs are held extended behind the body, so that the
animal affords as little resistance as possible to gliding
through the water. The fore limbs are also used in swim-
ming, taking strokes sometimes together and sometimes al-
ternately. To a certain extent they aid in propelling the
animal forward, but they are also employed, as in locomo-
tion on land, to guide the direction of movement. ) When
the animal starts to swim downward, the fore legs beat
backward and upward, the hand being twisted so as to
press its broad surface against the water. This naturally
pushes the anterior part of the body down. In starting to
swim upward, the fore legs beat downward, elevating the
anterior part of the body, which is then pushed upward by
the strokes of the hind legs. The fore legs are also used
in causing the body to move from side to side, and unequal
movements of the hind legs are employed for the same
purpose. Bendings of the body are also used to help steer
the course of the animal. The hind legs usually make a
stroke at the same instant, but the frog not infrequently
uses them alternately, especially when struggling near an
obstacle.

Attitude when Floating on the Surface. — When frogs
are kept in water beyond their depth, they spend a consid-
erable portion of their time at the surface with just the tip
of the nose exposed, for the purpose of breathing air. The
distance which the head projects from the water may be
varied at will, as it depends upon the amount of air taken
into the lungs. The more the lungs are inflated, the less the
specific gravity of the animal becomes, and the higher,
therefore, it rises in the water. When at the surface the frog
usually lies quiet, hanging obliquely with the hind legs in a
state of moderate extension. The fore legs generally are
held out from the body. In such a position the frog may

HABITS AND NATURAL HISTORY OF THE FROG 33

rest for a long time without performing any other move-
ments besides those involved in respiration. The extended,
sprawled-out attitude of the frog when resting at the sur-
face contrasts markedly with its resting position on land,
when its hind legs are closely doubled up and already set for
a spring. One probable reason for the extension of the hind
legs is that there is nothing to support them from below,
and they would naturally hang down, when relaxed, from
their own weight. However this may be, the extended con-
dition of the hind limbs is of service in enabling the animal
to suddenly draw itself downward whenever danger threat-
ens from above.

Diving. — If a frog is approached when it is resting at the
surface of the water, it will dive downward with great
celerity and make several strokes, carrying it some distance
away from its resting place. The action is performed so
quickly that it is not easy at first to see how it is accom-
plished. At one moment the frog is resting in perfect quiet
and at the next instant we perceive him making vigorous
kicks and rapidly swimming away. By experimenting with
frogs kept in a glass dish and concentrating our attention
on one feature of their behavior at a time, we may gain an
idea of the way this feat is accomplished. To swim down-
ward through the water the animal has to reverse its posi-
tion, as an extension of the hind legs in its normal resting
attitude would tend to throw it out of the water. The
first movement is that of withdrawal from the surface,
which is accomplished by suddenly bringing the hind legs
forward, thus giving the body a backward impulse. This
brings the hind limbs up into a position for making the
ordinary swimming stroke. Along with the withdrawal of
the body from the surface the fore legs make a sudden
stroke backward and upward, thus throwing the anterior
end of the body down. Then the hind legs extend and shoo?"

34

THE BIOLOGY OF THE FROG

the animal farther downward through the water. The
attitude of the body, as the frog rests at the surface, is one
of preparation for the act of diving, just as its attitude on
the ground is one of readiness for a spring. At the moment
the frog leaves the surface, bubbles of air may generally be
seen to escape from the nostrils.

Righting Movements. — Like most animals, the frog when
placed upon its back will regain its normal position. It
does so, too, with remarkable quickness, certainly in less
than half a second. A frog will right itself a great many
times in quick succession, and in course of time will become
so fatigued that it will act slowly enough to give the ob-
server a chance of following its movements. These move-
ments, which involve the coordinated action of several
muscles, vary a good deal in different acts, but they com-
monly occur in about the following way: If the frog rolls
over toward its left side, the right hind leg is brought dor-
sally by a contraction of the muscles of the dorsal side of
the thigh; the muscles of the ventral side of the left thigh
also contract; both these movements tend to roll the body
over to the left. The right hind leg is often brought for-
ward so that the thigh lies at a considerable angle from
the body, and this gives the limb a greater purchase in
rolling the body over. The left fore leg is brought down
alongside of the body, and the opposite member is thrown
over to the left side, thus assisting the hind legs in the act
of rotation.

The Voice. — The croaking of Rana pipiens may be rep-
resented, although rather inadequately, by the syllables
"au-au-au-au-auk." The voice of the male is louder and
deeper than that of the female and is more often beard.
In large frogs the notes are deeper than in small ones. The
notes of frogs are more often heard during the breeding sea-
son, when they are supposed to serve the purpose of a sex

HABITS AND NATURAL HISTORY OF THE FROG 35

call. It has been denied that the female frogs are attracted
by the sounds of the male, but it seems fairly well estab-
lished that the females of tree frogs and toads are guided
to the males by this means. In the summer, however, it is
not unusual to hear the croaking of frogs, especially in
the evening. A damp atmosphere is conducive to their
song, and for this reason the voices of these animals are
often heard upon the approach of a shower. The tree
frogs seem to be especially sensitive to atmospheric
changes, and the popular reputation which these creatures
enjoy as prognosticators of the weather is not entirely
unmerited.

The croaking of frogs is readily evoked by rubbing the
back or side of the body. After each stroke the frog usually
responds by a croak and then lapses into silence. Croaking
is often caused through accidental contact with other indi-
viduals. Two frogs which were kept in a dish on my table
were in the habit of croaking at frequent intervals, and I
observed that each time the back or side of one frog was
touched by the other, the individual would respond by a
croak. If not disturbed, the frogs would remain silent
indefinitely.

Frogs croak as well under water as on land. As the air
is forced out of the lungs, past the vocal cords, into the
mouth, the external nares are closed so as to prevent its
escape. Then the buccal cavity contracts, forcing the air
back into the lungs again ; and the same process is repeated.
If the head of the frog is held under water while the animal
is croaking, it may be seen that the air is forced back and
forth between the mouth and the lungs, while only a little,
if any, is allowed to escape through the nares.

Under conditions which are particularly agreeable, frogs
often give out a low grunting sound as if of contentment.
On the other hand, when frogs are seized by a snake or other

36

THE BIOLOGY OF THE FROG

enemy or are severely injured, they sometimes utter a sort
of cry whicli is called the pain scream.

Instincts for Protection. — When a frog is seized in the
hands, it usually makes violent efforts to escape. If it is
held by the anterior part of the body, the hind legs are used
to push against one's hand with considerable force. At the
same time the body is generally inflated with air, which
enables it to slip away more readily from one's grasp. The
sudden ejection of fluid from the bladder, which takes place
when the frog is caught, may also be of occasional service
in its attempts to get free.

Frogs sometimes swell the body before being seized as if
in anticipation of their capture, and they are especially apt
to do this after being lightly touched. Touch a frog that is
resting quietly, and if the creature does not hop away, one
may see the body puff up; and if the body is touched two
or three times, the swelling will continue until the lungs con-
tain their maximum amount of air. An animal such as a
snake which was attempting to swallow a frog wrould find
the operation somewhat more difficult if the body of its
victim were strongly inflated. Frogs often avoid capture
better by remaining perfectly quiet than by attempting to
get away by jumping. Fear prompts the creatures now to
the one and now to the other method of escape. Safety is
also sought occasionally by crouching close to the ground,
and more often by crawling under some object that promises
to afford shelter.

Stimuli that irritate the surface of the body are gotten
rid of in different ways. If the eye is touched, it is quickly
drawn into the head and covered by the lower eyelid. Curi-
ously enough, the same action is performed if the nose is
touched or any part of the head near the eyes. Stimuli on
the right side cause the right eye to wink, or if the stimulus
is on the left side, the left eye responds. The fore foot is

HABITS AND NATURAL HISTORY OF THE FROG 37

often brought forward to remove the stimulus, the foot on
the side stimulated being always employed. Stimuli applied
to the side of the body often cause the hind foot to be
brought forward to the stimulated spot. There is also a
twitching of the muscles of the side of the body near a
stimulated spot which reminds one of the twitchings pro-
duced by the skin muscles of a horse. If the tip of the
urostyle is irritated, the heels of both hind legs are brought
up to that point. A frog may be caused to repeat these
reactions many times, as a rule, but after a while it at-
tempts to avoid further persecution by hopping away.

Seasonal Changes. — The frog undergoes an unusual
amount of change in relation to the different periods of the
year. One reason for this is the fact that it is a cold-
blooded creature and cannot maintain itself in the same
condition winter and summer, as is done, for instance, by
man. A frog's condition changes markedly with reference
to differences in temperature to which it adapts itself.
Another reason for periodic changes is the ripening of the
reproductive cells, which, especially in the female, makes
extensive draughts upon the stored-up nutriment of the
body. Then there are the changes correlated with the re-
currence of the period of sexual activity, such as the de-
velopment of the nuptial excrescences on the thumb of the
male, the occurrence, in some species, of papillae on the
back and sides of the female, and the breeding instincts,
which appear only at this time.

In the fall of the year the body is richly stored with
nutriment accumulated during the summer while food is
abundant. During the winter this material is employed not
only in maintaining the temperature of the body and fur-
nishing the energy necessary to carry on the various activi-
ties of the organs, but it is drawn upon to contribute to the
growth of the reproductive cells. A part of this material

38

THE BIOLOGY OF THE FROG

is stored in the muscles, which during the winter decrease in
weight in relation to the rest of the body. Gaule 1 found
that in female frogs killed in July the gastrocnemius muscle
weighed on the average 32.6 mg. for every gram of body
weight. In August the ratio rose to 34.8. In December it
sank to 26.1. In January it was 26.4, and in June, the
laying period, 27.1. In the male the decrease in relative
weight of the muscles is not nearly so great, as there is
much less material to be employed in the development of
the sexual products.

The liver undergoes marked seasonal changes which will
be more fully described in connection with the account of
that organ. In the winter it contains a large amount of
glycogen, which almost entirely disappears by the end of the
breeding season. Until early spring, however, the glycogen
suffers comparatively little loss. The color of the liver also
varies between winter and summer, owing probably to differ-
ences of nutrition. In winter there is an accumulation of
pigment which gives the liver a dark appearance. In sum-
mer this pigment in most frogs largely disappears and the
liver becomes lighter in color. The size of the cells varies,
increasing through the summer, reaching its maximum in
Rana temporaria in November, then decreasing through the
winter and early spring, reaching the minimum in April
(Leonard) 2 or May (Funke).3 The size of the liver in
relation to the rest of the body, according to Langendorff,
Ploetz, and Funke, even increases during the winter months.
At the close of the breeding season the minimum size is
reached, after which there is a gradual increase during the
summer. Apparently, therefore, there is either a growth
of the liver during the winter at the expense of the rest

1 Gaule, Arch. ges. Phys. Bd. 81, 1900.

2 Leonard, Arch. Anat. u. Phys., phys. Abth. Suppl., 1881.

* Funke, Denkschr., Wien Akad. math- nat. CI. Bd. 68, 1900.

HABITS AND NATURAL HISTORY OF THE FROG 39

of the body, or the various other organs decrease more
rapidly than the liver in size.

The blood of the frog undergoes in the spring, after the
animal has begun to take food, a rapid regeneration, a pro-
cess which in higher animals takes place at all times of the
year. There is a great increase in the number of blood
corpuscles, both red and white. The marrow of the bones,
where the new blood cells are mainly produced, shows in the
spring a lymphoid structure, becoming more and more fatty
toward fall, after the production of new blood cells has
mainly ceased.

The changes in the fat body at different times of year
have been studied by Ploetz and Funke, both of whom
found in the two species studied (Rana temvoraria and
Rana esculenta) that this organ changed but little during
the winter months, but suffered a marked diminution in
size just before and during the breeding period in the late
spring. After this there is a gradual increase in the size
of the fat body until fall, when it reaches its maximum.

The advent of the breeding season is marked by great
changes in the reproductive system, both in the gonads, or
organs which produce the sex cells, and the various acces-
sory organs. The variation in the size of the ovary before
and after the discharge of the ripe ova is enormous. After
the eggs are laid in the spring, the ovary shrivels to a small
fraction of its previous dimensions. During the summer it
increases in size, and in the fall it may fill most of the body
cavity. The oviduct is also enlarged before and during the
breeding season. The glands in its wall reach a high degree
of development and secrete an enormous amount of a mucus-
like substance around the eggs as they pass down the lumen.
After the eggs are discharged, the glands diminish in size
and activity , and the size of the whole duct is much reduced.
There is a diminution in the size of the testes after the

40

THE BIOLOGY OF THE FROG

escape of the spermatozoa and then a gradual increase in
size until mid-summer or fall.

Correlated with the ripening of the spermatozoa and the
appearance of sexual instincts of the male frog there is an
increased development of the base of the inner digit of the
fore arm and an enlargement of certain muscles which are
concerned in the clasping reflex. Both the inner digit and
clasping muscles are larger in the breeding period than at
other times, and it is probable that their increased develop-
ment is dependent upon changes taking place in the sexual
glands. Sometimes there are certain external characters
developed in the female also during the breeding season. In
the females of Rana temporaria Huber has described der-
mal papillae which occur especially upon the back and sides
of the body and the upper surface of the legs. On the back
they are usually confined to the posterior half of the body,
but on the sides they extend forward nearly to the tip of the
nose. In the male the skin is entirely smooth or possesses
in a few cases only very small papillae. The color of these
papillae is a whitish or light rose, and they are rounded
or cone-shaped in outline, and four to five millimeters in
diameter. They are richly supplied with blood, but are
entirely devoid of dark pigment. When sectioned they
are shown to be due mainly to a thickening of the outer
portion of the cutis and to be made up largely of con-
nective tissue. The overlying epidermis is not noticeably
thicker than it is elsewhere. Since these organs appear
during the breeding season, it is probable that they have
some function in relation to reproduction. If they do not
directly serve to enable the male to retain his hold of the
female, they may act as stimuli, causing him to clasp more
tightly when he feels the female slipping from his grasp.

Color Changes. — One of the most remarkable adaptations
of many kinds of frogs for concealment from their enemies,

HABITS AND NATURAL HISTORY OF THE FROG 41

is the power of changing their color in harmony with their
surroundings. The tree frogs possess this property in the
highest degree. When these animals are among the green
leaves of a tree, they assume a bright green color. When
on the bark, their skin turns to a gray or brown. In both
cases the color of the frog closely resembles that of the
surroundings and serves to make its possessor difficult to
distinguish. The value of such a power as a means of
protection from enemies is obvious. No frog, however
remarkable may be the changes in color it may undergo,
is able to assume all shades and hues. Frogs possess
the property of adapting themselves only to the pre-
dominant colors of their environment, which are green,
the color of vegetation, and some shades of gray or
browrn, the usual color of the soil and the bark of trees.
They cannot turn red or blue or violet, and, in fact, the
power to do so would be of little value to them if they
possessed it.

Rana pipiens, like most of the members of its genus,
possesses a much lesser range of color variations than the
tree frogs; nevertheless it can change its color to quite a
marked degree. If in a dark environment, its skin becomes
much darker; the black spots contain so much pigment
that they remain unchanged under all conditions, but the
lighter regions between them are subject to marked changes.
Exposure to bright light gives the skin a much lighter color,
the green and golden colors come out to a much greater
extent, and the black pigment cells become less conspicuous.
There is little doubt that power of color change in Rana
pipiens is of service to the animal as a means of conceal-
ment. The frog is less conspicuous in a dark environment,
when its skin assumes a darker hue, and when in the grass
or weeds its green coloration serves the same purpose. The
mechanism of color changes, and the various stimuli by

42

THE BIOLOGY OF THE FROG

means of which they are set up, will be treated of in the
description of the skin.

Enemies. — As frogs are among the most defenseless of
animals, they fall an easy prey to a variety of carnivorous
creatures, who devour them in great numbers. First of
these enemies in order of destructiveness is doubtless to be
counted man, who, on account of his fondness for frogs'
legs, to say nothing of his scientific curiosity, has almost
exterminated some species in many localities. It is in the
breeding period in the early spring that the destruction of
frogs is greatest, since the animals then appear most abun-
dantly and are most easily caught. They are devoured by
water rats, mink, weasels and skunks. There are many
birds which prey upon frogs, such as cranes, herons, hawks
and crows; but their greatest enemies, next to man, are
the various species of snakes, of which, according to Fischer-
Sigwrart, they have an intense instinctive fear. When in
the water they may also fall a prey to the larger species of
turtles. In Europe several fishes, such as the larger herring
and trout, prey upon frogs; and smaller fishes are very de-
structive to the tadpoles.

To a certain extent frogs are preyed upon by other mem-
bers of their own class. The large Cryptobranchus devours
frogs, and even toads. I have several times found large
bullfrogs with Rana pipiens in their stomachs, and it fre-
quently happens that small individuals fall victims to larger
members of their own species.

Among the invertebrates there are few species that
actively prey upon the frog if we exclude those forms which
are parasitic. Many aquatic bugs, such as Belostoma,
Benacus, Zaitha, Ranatra, and even the small back-swim-
mers, Notonecta, catch the young tadpoles and suck out
their blood. Water beetles, such as Dytiscus, and the
stealthy larvae of the dragon flies make use of the same

HABITS AND NATURAL HISTORY OF THE FROG 43

source of food. Mortality among the tadpoles is natu-
rally high, as they are preyed upon by many forms which
are unable to cope with the adult frog. Water fowl,
fishes, and aquatic insects prevent the great majority
from reaching maturity; and the young frog is ex-
posed to many dangers from which older and larger
individuals are exempt. Probably only a few even of
the adults are allowed to die of old age. The stomach
of some larger animal forms the inevitable destina-
tion of all but a small per cent of the product of any
brood.

The crayfish is often found devouring the dead bodies of
frogs, and it is not improbable that occasionally it may cap-
ture an unwary specimen alive; but, for the most part, it
probably makes use of frogs killed by some other means.
Certain species of Glossiphonia (Clepsine), among the
leeches, live upon frogs and turtles; but they do not
require a very large quantity of food, since one meal
may suffice to keep them alive for over a year. Like
higher animals, frogs are attacked by mosquitoes, but it
is uncertain how much inconvenience arises from this
source.

Parasites. — The frog, like most of the higher animals,
is afflicted by a large number of parasitic forms, belonging
both to the animal and the vegetable kingdoms. The leeches
mentioned in the previous section might almost be said to
be parasitic, since they remain attached to the frog for a
long period. The larvae of blowflies (Calliphora, Lucilia)
sometimes infest the intestine of frogs; but they usually
prove a greater pest to toads. The female lays its eggs in
the nostrils of the toad, and the larvse feed upon the mem-
branes of the nasal cavity, and may work their way into
the brain and sometimes the eyes of their host. I have
found no record of their occurrence in the nasal cavities of

44

THE BIOLOGY OF THE FROG

frogs, although it is not improbable that they are occasion-
ally found there.

Of the several species of Nematodes (round worms) found
in the frog, Rhabdonema nigrovenosa, which occurs in sev-
eral European species, is, perhaps, the best known, since
its life history presents several exceptional and interesting
features. A kind of alternation of generations occurs in this
species, there being a free form living outside the body,
and a parasitic form which is usually found in the lungs.
The latter is hermaphroditic, and produces eggs which give
rise to rhabditiform embryos which pass into the alimentary
canal and thence outside the body. These embryos develop
into the free form, which consists of both males and females.
The eggs produced by the female are fertilized and develop
within her body. Here the embryos live and grow by
devouring the internal organs of their mother, after which
the young matricides make their escape into the water.
When opportunity offers, they crawl into the lungs of a
frog, and there develop into the parasitic hermaphroditic
form. An allied species, according to Leidy, occurs in this
country in the lungs of Rana pipiens.

Other nematodes (Strongylus, Nematoxys, and Oxysoma)
are occasionally found in the intestine of frogs, and the
ventricle of the heart and the blood vessels may be in-
fested by species of Filaria. The hair worm, Gordius, oc-
curs in the frog among other forms during a part of its
larval life.

The Acanthocephali are represented by one species,
(Acanthocephalus ranee = Echinorhynchus hoeruca Rud.)
parasitic in the intestine of the frog. Another species, A.
lesiniformis (Malin), has been described in an encysted
state in the peritoneum.

Tapeworms (Cestodes) are rarely found in frogs. Tcenia
dispar Goeze, which occurs in the intestine of several Euro-

HABITS AND NATURAL HISTORY OF THE FROG 45

pean species of Rana, has been found by Leidy 1 in this
country in the intestine of Rana pipiens and Bufo lenti-
ginosus.

Frogs harbor several species of Trematodes, or flukes,
which occur chiefly in the intestine and bladder, but they
are sometimes found also in the lungs and other parts of
the body. Loos 2 describes eight species of Distomum from
frogs found in Europe and Stossich 3 mentions ten species
of Distomum occurring in Rana esculenta and nine in R.
temporaria, but some of these forms have been united by
subsequent writers. Several species of
Distomum are common to the frogs of
both Europe and North America. The
peculiar genus Polystomum, character-
ized by having a circle of several distinct
suckers at the posterior end of the body,
is often found in the urinary bladder of
frogs.

Frogs harbor several species of proto-
zoan parasites most of which are de-
scribed or referred to in Wenyon's monu-
mental work on Protozoology. Among
the Infusoria one of the commonest spe-
cies is the wrell known Opalina ranarum which is frequently
found in the rectum. This species has a flattened, ciliated
body and many small nuclei. Like the other members of
the Opalinidae, it has no mouth. At times it becomes en-
closed in a cyst and it may be conveyed in this form from
one host to another. Several genera and species of Opa-
linidae occur in frogs and other amphibians in various parts
of the world. The Opalinas differ so much from other

1 Leidy, "Researches in Helminthology and Parasitology," Smith-
sonian Inst., 1904.

a Loos, "Die Distomen unserer Fische und Frosche, 1894.
4 Stossich. "I Distomi degli Anfibi," 1889.

Fig. 14. — Opalina
ranarum; a, a, nu-
clei. (From Heg«
ner.^

46

THE BIOLOGY OF THE FROG

members of the Infusoria that some writers remove them
from that class entirely. So far as known they do little
damage to their hosts and should probably be regarded as
commensals intsead of true parasites.

Of the parasites belonging unquestionably to the In-
fusoria there are several species of Balantidium. In the
frog they appear to live upon the intestinal contents, al-
though a species in man, B. coli, actively invades the tis-
sues and becomes pathogenic. Nyctotheres cordiformis, a
form related to the species of Balantidium, is a common
parasite of the frog's rectum.

Of the flagellate Protozoa there are several species which
infest the intestine. Copromonas subtilis, whose life his-
tory has been worked out by Dobell,1 is frequently found
in the rectum of frogs and in the faeces outside of the body.
Hexamita intestinalis, which commonly occurs in the in-
testine, may invade the tissues and is sometimes found
in the blood. Several species of trypanosomea occur in
the blood of various kinds of frogs in which they
may become strongly pathogenic. A common species,
T. rotatorum (Mayer), is transmitted from one animal
to another by a leech which preys upon tadpoles. The
parasites multiply in the digestive canal of the leech and
are conveyed to other tadpoles during the act of sucking
blood.

A few parasitic rhizopods have been reported from the
intestine of the frog; one of these, Entamoeba ranarum, is
allied to one of the amoeboid organisms which attacks the
human intestine. Another rhizopod, Chlamydophrys ran-
arum, which is often enclosed in a thin shell, occurs in the
frog's intestine. Whether either of these parasites causes
much damage to its host is not known. Other rhizopods

1 Dobell, "Researches on the Intestinal Protozoa of Frogs and
Toads." Quart. Jour. Mic Sci., Vol. 53, 201. 1909,

HABITS AND NATURAL HISTORY OF THE FROG 47

have been observed in the frog, but little is known of their
life history.

The largest number of protozoan parasites of the frog
fall within the exclusively parasitic class of Sporozoa. The
order Coccidiida is represented by two species of Eimeria,
E. ranarum (Labbe) and E. ranee Dobell, which infest the
cells of the intestinal epithelium. Isospora lieberkilhni is
found chiefly in the cells of the kidney.

Of the blood infesting Sporozoa the curious parasite
Lankestrella minima (= Drepanidium ranarum) occurs at
first in the endothelial cells of the blood vessels and finally
comes to live in the red blood corpuscles. These parasites
may be conveyed from one frog to another by the bite
of a leech. Cytamoeba bacterifera and Dactylosoma ran-
arum also occur in the red corpuscles.

Leptotheca ohlmacheri. one of the Myxosporidia, has been
reported in the tubules of the kidney both in European
toads and frogs, and in Rana clamitans and R. pipiens in
North America (Kudo).

Of plant parasites, there is a species of Saprolegnia which
sometimes attacks the skin of Rana pipiens, and probably
of other species of frogs. It forms large, light-colored
blotches which may spread over a considerable part of the
body. Necturus and other amphibians are liable to attacks
from the same fungus. Eidam 1 has described a species of
fungus, Basidiobolus ranarum, which inhabits the alimen-
tary canal of Rana esculenta and R. oxyrhina.

Frogs have so many other parasites that they deserve to
be relatively free from bacterial diseases, but they are at
least not completely exempt. Lichtenstein 2 has reported
a case of a frog whose liver was badly infected with tu-

1 Eidam, "Cohn's Beitrage zur Biolo&ie dor Pflanzen," Bd. 4, p. 181.

2 Lichtenstein, Zeit. f. Bakt. Parasit. Infektionskr., Oris. Bd. 85,
249, 1920.

48

THE BIOLOGY OF THE FROG

bercle bacilli, and he succeeded in making a culture of
these organisms which caused the development of tubercu-
losis when injected into other frogs. A fairly common bac-
terial infection which is often a source of trouble to raisers
of frogs, is the disease known as red leg. This fatal mal-
ady causes a reddish color of the legs and under side of
the body due to the congestion and hemorrhage of the cu-
taneous blood vessels. The animals often become bloated
from the accumulation of fluid in the lymph spaces. Em-
erson and Norris 1 have made cultures of a bacterium
{Bacillus hydrophilus fuscus) derived from infected ani-
mals, and have found that it produced red leg when injected
into healthy frogs.

Times and Places of Breeding. — The breeding period of
frogs is in the early spring, soon after the animals have
emerged from their winter quarters. As in most amphibians,
the eggs are laid in the water, usually among the vege-
tation near the shore, and receive no attention from the
parents after they have been deposited. When the breeding
season is over, the frogs scatter and resume an active preda-
tory life. In exceptional cases, however, they have been
known to resume for a time their winter sleep after the
breeding season is over. At Dorpat, Marquis observed that
in one year Rana fusca, immediately after the breeding sea-
son, crawled back into the mud and remained two weeks
before again emerging.

The commencement of the breeding depends in a great
measure upon the temperature. In general, a late spring
delays the time of breeding, and warm weather, on the
other hand, hastens it. The breeding period depends to a
certain extent upon local conditions. In shallow ponds
which are exposed to the sun, and where consequently the
water becomes warmed early in the season, frogs breed much

lJour. Exp. Med., Vol. 7, 32, 1905.

HABITS AND NATURAL HISTORY OF THE FROG 49

earlier than in water which on account of its depth or the
lack of sunshine is heated only very slowly. In masses of
water fed by cool springs the breeding of frogs and of other
forms of life is much delayed.

(_Th&_rlifferent specie^jaLirogS- breed, ai_ different times, and
the breeding period of a species naturally varies with the
latitude, coming on later as we pass northward .and earlier
as we pass south. Thus Rana temporaria, whose breeding
season in England and middle Europe is in March, does
not breed until May in Norway, but in southern countries
it-may breed even as early as January.
\ Dr. Morgan 1 has recorded the results of four years' ob-
servations of several American species of frogs in the vicin-
ity of Baltimore, Maryland. "The first frogs to lay, and
among the very first (Acris gryllus excepted) to appear, are
the wood frogs (Rana sylvatica). A few warm days in
early spring suffice to bring them out. The following rec-
ords give a general idea of the time. February 23, 1891, and
March 8th, 9th, and 10th, 1880. The eggs of these had
been laid several days. The egg bunches are found in small
pools on the edges of weeds, generally among the low hills,
and are often stuck to twigs of bushes. TKe bunches are
generally large, four to six inches in diameter, and contain
very many good-sized eggs.i In the same pools it is quite
usual to find the firmer eg~gounches of Amblystoma, for this
Urodele also lays its eggs very early.

"Somewhat later two species of tree frogs appear in the
small pools in the woods, generally in quite small, and,
therefore, during the day often quite warm, puddles; some-
times in the same pools as the wood frogs, oftener in the
ditches by the side of the road. These two frogs are Hyla
pickeringii and Chorophilus triseriatus.2 The eggs of these

1 Morgan, Am. Nat., Vol. 25.

2 Now Pseudacris triseriata.

50

THE BIOLOGY OF THE FROG

species are very similar, and I know of no certain method
of distinguishing the one from the other. The bunches
are small, attached to bits of grass, or lie simply on the
bottom, and each bunch contains from five or six to fifteen
or twenty eggs. I have the. following record of the times at
which the eggs were found: Hyla, March 9, 10, 13; April
5, 1890. Chorophilus, February 23, 1891; March 13 and
24, 1890.

"The eggs of Rana halecina are found still later, some-
times in the same localities as the wood frogs, oftener in
pools in the open ground quite away from the woods. The
following are the records: March 25, April 15, 1890. Eggs
of Bufo lentiginosus were reported from the same region on
April 14, 1890, and April 5 and 6, 1891."

Duration of the Breeding Period. — The duration of the
breeding period, like the time of its first appearance, is
very dependent upon external conditions, especially tem-
perature. The period of copulation, or the time during
which males may be found clasping the females, often con-
siderably exceeds the period between the first and last
deposition of eggs. This is due to the fact that the males
seize the females often several days before the first eggs
are laid. According to Fischer-Sigwart's observations of
Rana fusca, the females may lay eggs, if the weather is
warm, three days after the beginning of copulation; but in
cold weather the laying may be deferred for twenty or
thirty days. The duration of copulation in frogs kept in
the terrarium was found to vary between six and twenty-
three days. The less variability of the breeding period in
this case is doubtless due to the more uniform conditions
under which the animals are kept. The period between the
appearance of the first egg mass and the time when all the
females have extruded their eggs was found to vary in
different localities, between four and twenty-seven days, ac-

HABITS AND NATURAL HISTORY OF THE FROG 51

cording to the temperature; in the terrarium a variation
between ten and twenty-eight days was observed. Accord-
ing to Pfliiger, the laying period in Rana esculenta lasts only
two days. It is quite probable that a single copulation
extends through the whole breeding season. In Rana fusca
Pfliiger has observed that a male may clasp a female for
several weeks, and in Rana esculenta the same observer
has recorded an embrace which lasted a month. Steinach
records a still longer period for Rana jusca, the pairs re-
maining united for as much as seven weeks. In these cases
the weather was cool, otherwise the discharge of the sexual
products would have occurred much more quickly.

Copulation. — In copulation the male clasps the female
just behind her fore legs, where he hangs tightly and main-
tains his hold persistently against all efforts to dislodge
him. Often the thumbs are interlocked to increase the firm-
ness of his hold. The body of the female is very much
compressed in consequence of this, but it apparently causes
her little inconvenience. The labor of locomotion of the
pair falls mainly upon the female, the male exerting him-
self only occasionally for the maintenance of equilibrium.
With the exception of the effort necessary to enable him to
retain his hold, the male during copulation is singularly in-
active, and will endure very unfavorable conditions rather
than make any effort to seek a better situation. Some-
times several males will be found clasping different parts of
the body of one female, forming masses, "Begattungsklum-
pen," in the midst of which the female can scarcely be seen.
Males will often clasp females which are dead, or females
of another species, or even toads. The males of Rana jusca
have been observed clasping carp and being carried around
by the fish, which were unable to divest themselves of their
burden. The eyes of many of these fishes are destroyed by
the thumbs of the clasping frogs. Males will usually clasp

52

THE BIOLOGY OF THE FROG

one's linger, or almost any object they can seize, although
they will not as a rule retain hold of objects other than
female frogs for a very long time. The clasping instinct
to a large extent overcomes fear. A frog which will make
violent efforts to escape from the hands will often hold to
one's finger and entirely desist from its efforts to escape
during the period of copulation. The clasping instinct is so
strong that the animal may be severely injured without
showing any diminution of its ardor. The body of a male
frog may even be cut in two in the middle and the fore
part will still cling tenaciously to the female for hours,
only desisting when the creature becomes so weak from
loss of blood that it can no longer retain its position.
Further data on the clasping reaction and the recognition of
sex are given in the chapter on the nervous systems (p. 320) .

Egg Laying and Fertilization. — The extrusion of eggs
usually occurs only after the male has clasped the female
for several days. The eggs, which are discharged from the
ovaries rather slowly, are conveyed from the body cavity
into the oviduct, through which they pass into the uteri,
where they finally all collect, distending the thin walls of
these organs to an enormous degree. As Spallanzani dis-
covered long ago, females killed in the first part of the
breeding season have most of the eggs still in the ovary.
Later many eggs are found in their passage down the ovi-
duct, although they are not often met with in the body
cavity. Females killed near the end of their breeding
period have nearly all the eggs in the uterus. At the be-
ginning of the breeding period the seminal vesicles of the
males are empty, or contain very little seminal fluid. Stein-
ach found on examining a large number of copulating
specimens of Rana temporaria, which he received March 5,
1893, that the seminal vesicles contained no traces of
spermatozoa. Only on March 8 were the seminal vesicles

HABITS AND NATURAL HISTORY OF THE FROG 53

in some specimens partly filled, and there were yet several
males in which the vesicles were empty on March 9. Cop-
ulation may begin, therefore, several days before the sexual
products of either sex are ready for discharge.

When the eggs are extruded through the cloaca of the
female, the male discharges his spermatic fluid over them,
and the spermatozoa penetrate the jelly around the eggs
and complete the act of fertilization. The operation is
somewhat similar to the fertilization of the eggs of most
fishes, where the male who accompanies the female during
the breeding period discharges his milt or sperm over the
eggs at the moment when they are extruded. The clasping
instinct of the male frog serves to insure the proximity of
the two sexes when the proper moment for fertilizing the
eggs arrives. When this time approaches the pair sink to
the bottom of the water, where they remain quiet until the
sexual products are discharged, when they separate. The
male then loses his clasping instinct and is totally indifferent
xo the other sex. As a rule he fertilizes but one batch of
eggs a year.

Fischer-Sigwart is of the opinion that all of the eggs are
not generally fertilized at the time of their extrusion, but
that many are fertilized later by other males, which are
usually found among the masses of eggs in the breeding
places. This observer has often seen males discharging
their fluid over masses of eggs. The supernumerary males,
therefore, play a part in fertilizing the eggs, as well as those
which have succeeded in obtaining a mate.

What the stimulus is that prompts the male to discharge
his sperm at the same time the eggs are extruded from the
female is not altogether evident; the same problem presents
itself in the case of fishes.

Congregating at Breeding Grounds. — During the breed-
ing season frogs usually congregate at certain points in

54

THE BIOLOGY OF THE FROG

shallow water in considerable numbers. At this time there
seems to be manifested a gregariousness which does not
appear under ordinary circumstances, and which is not en-
tirely accounted for by the tendency of the animals to seek
a similar habitat for breeding. According to Fischer-Sig-
wart's observations on Bana jusca, if only a few pairs
occur in any locality, they get as closely together as pos-
sible, and their egg masses form almost a continuous sheet.
The laying grounds are the scenes of lively activity, "Alles
hastet und drangt." The supernumerary males crawl over
and work through the masses of eggs and, according to
Fischer-Sigwart, effect the fertilization of the ova which
may not have been reached by spermatozoa at the time of
their discharge.

When the sexual products are discharged, the frogs go
back upon the land and scatter in all directions. Most
frogs leave the breeding grounds at nearly the same time.
At one day a place may be teeming with these creatures,
while on the following day not a single individual can be
found. Henceforth the frog is a solitary animal, having
lost all its sexual instincts and social proclivities. It leads
its life as if no other member of its species were in existence.

Egg Laying without the Presence of the Male. — The
question whether or not the female frog will lay eggs with-
out the embrace of the male has been investigated by Nuss-
baum 1 in Rana jusca. Females were isolated soon after
they came out from their winter quarters, when the eggs
were still in the ovary. The eggs were found to leave the
ovary, pass down the oviduct, and collect in the uterus as
they do under normal conditions. They were not extruded
from the uterus all at once, but were passed out slowly, a
few at a time, some eggs often being retained until consid-
erably after the breeding season. In a female killed late

1 Arch. mik. Anat., Vol. 46, 1895.

HABITS AND NATURAL HISTORY OF THE FROG 55

in the summer one uterus was found to contain a large
amount of jelly, but the eggs themselves had broken down
and disappeared.

Egg laying in Rana esculenta is apparently more depen-
dent upon external conditions. While R. fusca readily lays
its eggs when in captivity, esculenta does so only if the
eggs have accumulated in the uteri at the time of capture
When the females are taken earlier, the eggs that are in
the ovary are not extruded, but are retained there until they
are finally resorbed.

Proportions of the Sexes. — A male frog, as a rule, pairs
with only one female during the breeding season, and it is
advantageous to the species that the sexes should be nearly
equal in numbers. This relation is, in fact, usually found
to obtain among adult frogs. Born,1 in studying the pro-
portion of the sexes in young frogs, came to the conclusion
that the females often greatly preponderate over the males.
In one brood the females were found to constitute ninety-
five per cent of the total number. Pfliiger2 and Griesheim,3
who investigated the subject with considerable thorough-
ness, found that in young frogs it is often very difficult to
distinguish the sexes. The gonads of the males frequently
contain numerous egg follicles and have the general ap-
pearance of ovaries, and many frogs which would naturally
be taken for females show only male characters at a later
period of development. Estimates based on the appearance
of the sex organs of frogs during the first year of their life
gave a very large percentage of females. This percentage
was found to vary greatly in lots of frogs taken at different
localities. The degree of development of the sex organs
does not closely correspond with the development of the

1 Born, Breslauer drtzl. Zeitsch., 1881.

2 Pfliiger, Arch. yes. Phys., Bd. 29, 1882.
2 Griesheim, Ibid., Bd. 26, 1881.

56

THE BIOLOGY OF THE FROG

other parts of the body, and there is consequently a great
variation in the appearance of these organs in specimens of
the same age. As there is no evidence that the mortality
of the young females greatly exceeds that of the young
males, and as the proportion of the adults of the two sexes
is nearly the same, it is probable that many of the young
frogs which would ordinarily be diagnosed as females are
"juvenile hermaphrodites" in which the female characters
predominate for a time but which later develop into males.
This conclusion is supported by the fact that there are
many cases in which the sex glands are intermediate in ap-
pearance between ovaries and testes. Egg follicles are
often found in the testes of frogs at a later period of
development.

Witschi and Swingle have found that different local races
have quite different sex ratios. In some races the differenti-
ation of sex is distinct, and males and females occur in ap-
proximately equal numbers. In other races there is a
longer indifferent period and the eventual determination of
sex may depend upon external conditions. Frogs are crea-
tures in which the distinctions of sex are apparently not so
sharp as in most other organisms.

Copulation in Late Summer. — Fischer-Sigwart has ob-
served that in July and August the sexual instinct of the
males kept in terraria often asserts itself a second time,
especially if they have been well fed and are in good condi-
tion. No sexual products are extruded at this time from
either sex, although the males show a strong proclivity to
clasp the female and will even clasp other males. A case is
related of a male who observed a female which had seized
an earthworm, and in attempting to share the morsel clam-
bered over her back, when the clasping instinct suddenly
took possession of him, and he remained clasping her body.
Later in the season this clasping instinct disappears.

HUBITS AND NATURAL HISTORY OF THE FROG 57

Hibernation. — In the late fall frogs betake themselves
to water and bury themselves in the mud out of reach of
frost. Here they lie in a dormant condition until the next
spring. The general vital activities of the animal run down
so low that little expenditure of energy is required to main-
tain life. There is need, therefore, for only a small amount
of oxygen, and skin respiration then suffices. During the
whole winter the frog does not breathe air with the lungs.
The temperature of the body sinks until it is only a few
degrees above that of the surrounding medium. As the
frog takes no food during this time, it must keep up its vital
activity at the expense of material stored in its tissues.
Its temperature, even if only a little above that of its sur-
roundings, requires the use of a certain amount of combus-
tible material for its support. During the summer the frog
feeds voraciously, and when cold weather ensues, its sys-
tem is stored with a rich supply of food material which is
gradually expended through the winter months. This food
must keep the heart beating and support the various activ-
ities of the physiological machinery of the body. And in
addition to supplying energy for this purpose, it must af-
ford the substance for the growth of the sexual products,
which increase during the winter at the expense of the other
parts of the organism.

Life in Summer. — After the eggs are laid in the early
spring, the frog leads an active predatory and solitary life.
After its long winter sleep and the expenditure of substance
and energy during the breeding season which closely fol-
lows the awakening in the spring, the frog is naturally in
great need of food, and it becomes a very voracious feeder.
During the early part of the summer it is busily engaged
in the attempt to satisfy its hunger. In midsummer, when
the body has compensated for its losses, the frog often be-
takes itself to a place of concealment, coming out only at

THE BIOLOGY OF THE FROG

intervals to obtain food. This period of comparative inac-
tivity has been spoken of as a summer sleep, but according
to Fischer-Sigwart a true summer sleep does not occur either
in frogs or toads. There is only a period of comparative
rest after the need for a large amount of food has ceased.

Injuries; Power of Regeneration. — As the frog is preyed
upon by several enemies, specimens are often found in
which fingers or toes, or sometimes the entire hand or foot,
are missing. In most cases they are doubtless individuals
which have had these parts bitten off and were fortunate
that only a portion of their body was left in the possession
of the enemy. Even severe wounds in the frog heal very
readily. Fischer-Sigwart has observed that if a frog is
wounded it betakes itself to the water, and if an individual
keeps in the water during the summer, one may be pretty
sure that it has received some injury.

The power of regeneration possessed by some of the
lower amphibia is very marked. Triton is able to regen-
erate its tail or limbs, or even its eye, if a small portion
of that organ is left. But in the frog, and, so far as is
known, in the other Anura, the power of regeneration is
almost entirely lost. Even in the tadpole stage it is much
reduced. The tadpoles of the higher amphibia are able to
regenerate the tail if it is cut off, but their power of regen-
erating the limbs is very limited. The regeneration of the
limbs of tadpoles was first recorded by Spallanzani. Fraisse,
however, who cut off the limbs from both young and old
tadpoles, arrived at entirely negative results. Later Bar-
furth carried on several experiments on both old and young
tadpoles, and found that if the legs were cut off from
young specimens these organs would be frequently regen-
erated, although slowly. The powTer of the tadpole to re-
generate missing limbs was found to decrease rapidly with
age.

HABITS AND NATURAL HISTORY OF THE FROG 59

Effects of Heat and Cold. — The frog belongs among
those animals which are commonly spoken of as cold-
blooded. This expression doubtless takes its origin from
the fact that the temperature of such forms is usually low.
The higher vertebrates, such as the mammals and birds,
have a high bodily temperature, and, what is most remarka-
ble, the temperature in most of these keeps nearly constant
under most diverse conditions. A bird or a mammal may
live in extremes of climate below 40° and 50° below zero
F. and considerably over 100° F., and yet the temperature
of the blood will not vary more than a very small amount.
These animals have an almost perfect mechanism for the
regulation of the bodily heat. With a rise of temperature
there is, in the mammals, an increase of perspiration and an
increased evaporation from the surface of the body which
tends to cool the blood. When the temperature sinks, there
is less evaporation from the surface, and the cold acts
indirectly as a stimulus to increase metabolism and causes
the combustion of the bodily fuel to proceed at a more
rapid rate and thereby to compensate for the loss of heat
by radiation. By virtue of this mechanism the higher
animals are able to keep in an active condition in both
winter and summer. With the cold-blooded animals it is
different. Their temperature rises and falls in correspond-
ence with the temperature of their environment. In the
cold their metabolism is slow, their temperature runs down,
and consequently they become sluggish and inactive. As
it becomes warmer, their temperature rises, their metabolism
increases, and they become more active and alert. A lizard
which is made almost stiff when the weather approaches
the freezing point becomes the most agile of creatures in
the sunshine of a hot day.

The temperature of the cold-blooded animals is not,
however, entirely at the mercy of the environment. Evapo-

60

THE BIOLOGY OF THE FROG

ration from the surface of the body tends to keep the
temperature of the animal in warm weather below that of
the surrounding atmosphere. And as the weather ap-
proaches the freezing point, the small amount of meta-
bolism in the animal serves to keep its temperature some-
what above that of its environment.

The effect of high temperature on the frog has^Jbeen
studied by Maurel and Lagriffe.1 At 26° to 30° cTthe
frogs become active and restless. At 31° to 33° C. they
show evident signs of discomfort. From 34° to 36° C. they
jump about wildly without any apparent sense of direction.
At a temperature of 37° to 39° C. they lose their sense of
equilibrium, and if exposed to a temperature of 39° to 40° C.
they die. If, however,- they are exposed only for a short
time at the latter temperature, they may subsequently
recover, although they may at first appear as if dead. The
cause of death is probably the coagulation of certain protein
compounds in the blood and tissues.

Frogs have little power of withstanding extreme cold for
the reason that they have no means of keeping their tem-
perature very much above that of their surroundings, and
their tissues consequently become frozen. On the other
hand, they can withstand a reduction of their own bodily
temperature far below the point which would be quickly
fatal to any warm-blooded animal. They may be even
frozen in ice for a short time and subsequently recover
if gradually thawed out. Knauthe 2 found that frogs which
were exposed to a temperature of from — 1° to — 5° C. for
twelve hours became stiff and the limbs lost their pliability.
The animals were then laid in wet moss and kept for
several days slightly above the freezing point (.2° to .5°
C), under which condition they gradually came back to

1 Maurel and Lagriffe, Oomp. rend. Soc. Biol. Paris, torn. 52, 1900.

2 Knauthe, Zool. Anz.. Bd 74.

HABITS AND NATURAL HISTORY OF THE FROG 61

activity. The bodily temperature of the frogs in the ex-
periment sank to from — .2° to — .8° C. Examination of
the web of the foot and the tongue revealed no signs of
circulation of the blood, which seemed to be no longer in a
fluid state.

The bodies of some specimens were cut open and the
heart was found to have entirely stopped beating. Accord-
ing to Knauthe, — and other observers have obtained the
same results, — if the tissues of the frog become entirely
frozen, the animal will not recover. The bodily temperature
cannot be lowered much below — 1° C. without producing
a fatal result. The animal may, perhaps, be frozen and then
recover, but it cannot be frozen hard.

Miiller-Erzbach 1 performed the experiment of placing
the frog in a dish of water, which was gradually cooled off
until it froze. As a film of ice began to form, the frog
attempted to keep up at the surface of the water, but it was
pushed down below and forced to remain there until frozen
in a solid cake of ice. Here it was kept for five hours, while
the surrounding temperature ranged from — 6° to — 8.7° C.
When the ice was then thawed, the frog was stiff and
showed no signs of life, but after an hour and a half it
had revived.

Frogs from warm countries cannot endure so low a tem-
perature as those from higher latitudes. But the frogs in
northern regions are often killed by cold, especially during
severe winters. If they are prevented from burying deep
enough in the mud, the frost may overtake them. In some
localities the frogs may be largely exterminated during a
period of severe cold, so that few are found the next spring.
In regions of high latitude, where the ground is permanently
frozen below the surface, thawing out only for a few feet
during the summer, frogs do not occur, since no means are

1 Muller-Erzbach, Zool. Anz.. Bd. 14.

62

THE BIOLOGY OF THE FROG

afforded to escape from being solidly frozen during the
winter.

It was observed by Knauthe that the color of frog?
exposed to the cold became very dark, even when they
were placed in sunlight, which under normal conditions
causes the skin to assume a lighter hue.

Absorption of Water. Frogs do not drink like the
higher animals, but absorb the water they require through
the skin. The meager and shriveled condition of a frog
which has been kept some time in dry air contrasts markedly
with its plump appearance after it has just been taken from
the water. The skin is loosely attached to the body, and
a considerable quantity of water may collect in the large
subcutaneous lymph spaces. Donaldson found that a group
of frogs after being kept in dry air for several hours lost
14 per cent of their weight. When placed back in the
water again they regained very nearly their previous weight
in twenty-four hours. Both the loss and absorption of
water were found to take place more rapidly in the summer
than in the winter. F. G. Hall placed Rana pipiens in very
dry air and found that they lost 41 per cent of their total
weight, but although they were apparently dead they re-
vived upon being placed again in water. An experiment by
Townson on a species of tree frog showed that a specimen
weighing ninety-five grains increased in weight by seventy-
six grains after being kept in the water for an hour.

Shedding of the Skin. — At certain periods the frog casts
off its cuticle or outer layer of skin. The part shed consists
merely of a very thin transparent membrane only one or
two cells thick. This comes off in large patches which may
be seen adhering to the animal here and there, the skin
covering the toes usually coming off last. The first molt
takes place in the spring at about the time of the breeding
season. In Rana fusca Fischer-Sigwart found that after the

HABITS AND NATURAL HISTORY OF THE FROG 63

first molt, which occurs from* late in February to early in
April, a second molt follows in the latter part of May or the
first part of June, a third in July, and usually a fourth in
August. In colder seasons the period of the molt comes
later, and the fourth molt may then not occur. Five molts
were not observed even during the warmest summers. Frogs
often eat their shed skin
after they have rubbed
it off with the aid of
their feet. The same
habit has been observed
in toads and in the
large salamander, Cryp-
tobranchus.

Hypnotism. — A frog
may be thrown into the
so-called hypnotic state
in several ways. If it is
seized in the hands, laid
upon its back, and held
a few moments until it
has ceased its struggles,
it will usually remain
motionless for a consid-
erable time, sometimes
for hours. The position
taken is a variable one. There is a tendency to assume an
attitude such as would be produced if the movements of the
frog were checked sometime during its efforts to regain an
upright position. According to Verworn,1 the muscles in-
volved in the righting movements are in a state of tonic con-
traction as if these movements were suddenly inhibited. The
breathing movements and the heart beats are at first accel-

1 Verworn, "Die sogenannte Hypnose der Thiere," 1898.

Fig. 15. — Rana temporaria in the so-
called hypnotic state. The upper
figure shows the position assumed
when the back is rubbed with the
finger. The same attitude is main-
tained when the frog is placed on
its back, as is shown in the lower
figure. (Modified from Verworn.)

64

THE BIOLOGY OF THE FROG

erated, but at a later stage their rate falls below the normal
(Heubel), and there is a decreased responsiveness to ex-
ternal stimuli. Different frogs vary greatly as regards
both the ease with which they may be hypnotized, and the
duration of the hypnotic state. In some cases if a frog is
simply placed on its back without being held, it may become
hypnotized after it has righted itself a few times, and lie
for a long time in some phase of the process of turning over.
Specimens of Rana esculenta, according to Verworn, when
laid on their backs, sometimes quickly draw the hind legs
close to the body, close their eyes, and lie with their muscles
in a state of tonic contraction, — a condition which suggests
the death feigning of certain insects.

Tonic contractions of different parts of the body may
often be induced by rubbing the back and sides. If the
frogs are in a normal resting position, they frequently raise
themselves up on their legs and remain motionless and rigid
for some time. If when in this state they are laid on their
backs, the legs still retain the same attitude as before.

Frogs may be awakened from their state of hypnosis by
any sudden stimulus, and their recovery is often immediate.
The duration of this state may be much prolonged if all
sensory impressions are so far as possible removed.

REFERENCES

See especially the works of Abbott, Allen, Boulenger, Brehm,
Dumeril et Bibron, Dickerson, Durigen, Fischer-Sigwart, Hay, Gadow.
Leydig, Rosel von Rosenhof, Spallanzani and Wright cited in the
references to the preceding chapter.

CHAPTER III

EXTERNAL CHARACTERS OF THE FROG

It will be convenient to begin our study of the frog by a
description of the principal external features of its structure.
The flattened more or less triangular head is broadly united
to the trunk, there being no region that can be properly
called a neck. The large eyes commonly protrude consid-
erably, but can be withdrawn into the orbits. Press upon
one of the eyes with the fingers, and it will be found that it
can be forced inward even beyond the general surface of the
head. If now the mouth of the frog be opened, it will be
seen that there is a marked prominence in the roof due to
the fact that the eye is pressed against the membrane lining
that portion of the cavity. The orbit or eye socket of the
frog, therefore, is not separated from the mouth by any of
the bones of the skull, which is a very different condition
from what we find, for instance, in ourselves. In the center
of the eye is a dark oval opening, the pupil, which is sur-
rounded by a brightly colored ring, or iris. The eye, as in
ourselves, can be covered by a pair of eyelids. The upper
eyelid, however, is capable of but little movement, but the
lower lid, which is thin and more or less transparent, can
be drawn up so as to cover nearly the whole eye. It will be
noticed that each time the frog closes its eyelids the eye is
pressed into the head, a fact which gives the winking of the
frog so peculiar an appearance. The lower lid of the frog
is not quite the same organ as the lower eyelid of most
animals. It corresponds rather to the lower eyelid proper,

65

66

THE BIOLOGY OF THE FROG

plus a nictitating membrane. The latter structure in most
animals in which it occurs is very distinct from both the
other eyelids. In a bird, for instance, it appears as a thin
membrane which can be drawn over the eye from the inner
angle of the orbit. In the frog, however, it is situated
just above the lower lid, of which it appears to form a

Pig. 16. — Outline of frog, a.n., anterior nares ; b.s., brow spot ;
d.p., dorsal plica? ; /.a., fore arm ; fem., femur ; t., tympanum ; til).,
tibia.

continuation. It is thinner and more transparent than the
lower lid and separated from it by a shallow groove.

Behind the eye is a nearly circular area covered by a
tense membrane, known as the tympanic membrane, which
forms the covering of the drum of the ear. Near the center
of this membrane may be seen a small prominence caused
by the end of the columella, or bone which connects at its
inner end with a small opening in the skull which communi-
cates with the inner ear. When the tympanic membrane is

EXTERNAL CHARACTERS OF THE FROG 6?

set in motion by the waves of sound which strike it, the
vibrations thus caused are communicated to the internal
ear, and thus give rise to the sensation of hearing, as will
be treated more in detail in a later chapter.. On the inner
side of the tympanic membrane lies a cavity, the Eustachian
tube, which opens internally into the mouth. If a bristle be
passed through this membrane, it will be seen to emerge
through a rather large rounded opening near the angle of
the jaw. The external features of the auditory organ of the
frog differ markedly from those of man in that all traces of
an external ear are absent, and the tympanic membrane lies
exposed at the surface of the body instead of lying at the
inner end of a long passage.

Above and behind the blunt tip of the snout lie the nos-
trils or external nares. These openings are guarded by
valves which open and close in connection with the move-
ments concerned in respiration.* The tip of the upper jaw
is slightly movable, and if it ISe pressed upward, the valves
of the nostrils become closed, and prevent the passage of
air through the nares. Pass a bristle through the nostril and
it will be found to emerge into the mouth through one of a
pair of rounded openings, the internal nares, situated some-
what behind the corresponding external openings. The
anus, or opening of the cloaca, lies somewhat dorsal in posi-
tion at the posterior endof~the body. .

On the upper side of the head, in front of the eyes, there
usually occurs a small, light-colored mark, the brow spot.
In some specimens this spot may be entirely concealed by
pigment; but in most cases it may be detected, although it
is often quite inconspicuous. The brow spot is a feature of
considerable interest, from the fact that in the embryonic
development of the frog it is connected with a peculiar
outgrowth of the brain known as the epiphysis or pineal
gland. When the outer or distal portion of this structure

68

THE BIOLOGY OF THE FROG

becomes constricted off, after the bones of the skull have
developed, it is quite widely separated from the basal
portion, which persists in connection with the brain even in
the adult frog. The pineal gland is found in almost all
vertebrates, including man, in whom it was given by the
philosopher Descartes the important function of being the
seat of the soul. It has been ascertained that this structure
is a rudiment of a stalk which formerly connected with a
median eye; in fact, there are certain reptiles (Hatteria) in
which this eye is fairly well developed, containing a cornea,
lens, retina, coats of pigment, and other structures charac-
teristic of a well-developed visual organ; but in most verte-
brates the eye no longer appears, or is represented by the
merest rudiment. The connection of the pineal gland with
the surface in the development of the frog, and the persis-
tence of the brow spot marking the point of this former

Fig. 17.— Hand and forearm of rudiment of an additional
frog. A, male ; B, female. (After digit on the inner side

skin. This rudiment corresponds to the thumb of our
hands. The two inner digits contain three joints each, the
two outer ones, four. There are no claws or nails on the

connection, are facts of
considerable significance in
relation to the evolution of
this form

The twro pairs of legs are
very different in form and
in function. The fore limbs
are short, and consist of
three divisions, the upper
arm, the forearm, and the
manus, or hand. The hand
has four fingers, and the

Storer. )

which can be felt under the

EXTERNAL CHARACTERS OF THE FROG 69

digits of the frog in either pair of limbs. The fore limbs,
when the animal is in a resting position, are held in a
peculiar twist; the forearm, and to a greater extent the
hand, are turned inward, so that the large inner finger often
points backward. The fore limbs of the male frog differ
in a peculiar manner from those of the female, being
modified in relation to the clasping instinct of the male
which appears at the breeding season. The forearm is
relatively thicker than that of the female, owing to the
greater muscular development of that portion of the limb.
The inner finger of the hand also becomes much larger in
the male and swollen at the base. The swelling is due
mainly to a thickening of the glandular portion of the skin
in this region, and becomes reduced in size when the breed-
ing season is past.

The hind limbs are longer and admirably adapted for
jumping and swimming, but are of little service in walking,
as the frog can scarcely be said to employ this method of
locomotion. Like the fore limbs, the hind limbs are divided
into three parts: an upper portion or thigh, a middle part,
crus or shank, and the foot, or pes. The latter is very well
developed and has the ankle remarkably elongated; there
are five toes and the rudiment of a sixth toe, termed the pre-
hallux, which is situated on the inner side of the foot. The
toes increase successively in length from the first, or inner
one, to the fourth, the fifth toe being commonly a little
shorter than the third. The first two toes contain three
joints each, the third and fifth, four each, and the elongated
fourth toe, five joints. On the under side of the articula-
tions between the bones of the toes are small cushions
termed subarticular pads. The toes are connected together
by web, which serves to make the foot an efficient paddle
as the animal swims through the water. The amount of
web between the toes varies greatly in different species of

70

THE BIOLOGY OF THE FROG

frogs, and is a character which is therefore made use of for
purposes of classification.

The ordinary resting position of the frog is a squatting
posture, with the anterior part of the body elevated on the
fore limbs, which are bent at the elbows and turned inward.
Near the middle of the back is a sort of hump due to a
bend at this place in the vertebral column. The hind limbs
are folded together, the knees pointing outward and forward,
and the ends of the ankles lying near each other at the hind
end of body. In this position the frog is in readiness to
leap, when alarmed, by the sudden extension of the hind
legs.

The skin of the frog is almost everywhere smooth, with
the exception of small scattered prominences occurring
mainly on the back and on the dorsal side of the hind legs.
Nothing corresponding to true hair or scales is to be found
in the frog; in fact, with the rare exception of rudimentary
scales in some forms, such structures are entirely absent
from all of the recent Amphibia. The general looseness of
the attachment of the skin is a feature which cannot fail to
be noticed.

Behind the eyes there extend two, usually light-colored;
ridges formed by a thickening of the skin, and known as
the dorso-lateral dermal plicce or folds. There are usually
several smaller and somewhat irregular longitudinal folds
of skin between these. The color of the skin is much darker
on the upper or dorsal side than below, where it is usually
white. In Rana pipiens the large black pigment spots which
occur on the dorsal side of the body and legs are subject
to much variation in size and shape. There is usually a
pair of large spots between the eyes and a single median
spot in front of these. The spots between the dorso-lateral
folds show a tendency to arrange themselves in two rows.
The spots on the hind legs are frequently elongated so as

EXTERNAL CHARACTERS OF THE FROG

71

to form transverse bands. In addition to the black pigment
there are green and golden colors, which are present in vary-
ing proportions. The changes in color which the skin may
undergo under certain conditions will be discussed in a later
chapter.

CHAPTER IV

PRELIMINARY ACCOUNT OF THE INTERNAL
STRUCTURE

Mouth Cavity. — If the mouth of the frog is held widely
open, the following parts will appear. In the roof of the
mouth there is a pair of rounded prominences caused by the
eyes, as has already been mentioned. Around the margin
of the upper jaw is a row of fine, sharp, closely set teeth
which are conical in shape and curved inward more or less
at the tip. External to the teeth is a fleshy fold, or upper
lip, and on the inner side is a groove, the sulcus marginalis,
which receives the lower jaw when it is closed. Anteriorly
this groove is crossed on each side by a low elevation, the
pulvinar rostrale; immediately behind the tip of the jaw
the sulcus is deepened again, forming the median subrostral
fossa; on each side of the pulvinars are the lateral subrostral
fossce, which are mere deepenings of the sulcus marginalis.
The lower jaw is entirely devoid of teeth and is held tightly
pressed against the upper jaw, the mandibular muscles
being normally in a state of tonic contraction; the tip of
the lower jaw is flexible and is capable of being elevated or
depressed independently of the rest of that structure, the
joints of the movable part, pars mentalis, lying under the
pulvinars of the upper jaw. The elevation at the extreme
tip of the lower jaw (tuber culum prelinguale) fits into the
median subrostral fossa, and there is a slight depression on
either side of this tubercle corresponding to the pulvinars.
The two jaws, therefore, fit together, part for part, with
great nicety. In fact, they form an air-tight joint which,

72

INTERNAL STRUCTURE

73

as we shall see later, is a necessary feature in relation to
the peculiar mode of breathing which the frog is forced to
employ. The tip of the upper jaw is likewise movable, its
free portion corresponding in
extent to that of the lower
jaw, so that the two parts can
be raised and lowered to-
gether. It may be noted that
the elevation of the tip of the
jaw effects the closure of the
nares, a point of considerable
importance in relation to the
process of respiration.

If a bristle is passed into
one of the anterior nares, it
may be seen to emerge into
the anterior portion of the
mouth cavity by one of a pair
of rounded or oval openings,
the choance, or posterior nares.
Between the choanae is a pair
of prominences which bear
the vomerine teeth. At the
posterior end of the buccal
cavity near the angle of the
jaw are the large openings,
Eustachian tubes, which lead
outward to the tympanic

membrane, as may easily be demonstrated by means of
a bristle. In the male frog another and a smaller pair
of openings may be seen on the lower side of the buccal
cavity, a little in front of the Eustachian tubes; these
are the openings of the vocal sacs, and their continuity
with these organs may be shown by passing a bristle into

Fig. 18— Mouth of the frog
widely opened. E, Eusta-
chian tubes ; G, glottis ; J9
lower jaw ; L, lateral sub-
rostral fossa ; M, median
subrostral fossa ; N, posteri-
or nares ; O, oesophagus ; P,
pulvinar rostrale ; S, open-
ing of the vocal sac ; T,
tongue ; tp. tuberculum pre-
linguale ; V, vomerine teeth.

74

THE BIOLOGY OF THE FROG

them or by inflating them by means of a blowpipe. Pos-
teriorly the buccal cavity presents two openings in the
middle line. The ventral opening, or glottis, is a narrow
longitudinal slit in the middle of a prominence caused by
the cartilages and muscles of the larynx; it is kept closed
except during the passage of air into or out of the lungs.
The dorsal opening marks the beginning of the esophagus,
which leads to the stomach. Although capable of great
distention, the esophagus is kept closed except when food
is being swallowed.

The floor of the mouth is very distensible and undergoes
continuous movement in respiration. In the middle part
may be seen the hyoid cartilage, which gives attachment for
the tongue and several muscles that move the floor of the
mouth. The tongue of the frog is attached in front to the
lower jaw and below to the hyoid cartilage. Its shape is
subject to great variation according to the degree of con-
traction of its various muscles, but in its normal relaxed
condition it is oblong, flattened, somewhat narrowed in
front, and produced at its posterior angles into two lobes
which extend backward on either side of the glottis; the
posterior margin is concave, and the sides, which project
over the attachment at the base, leave a considerable space
of the floor of the mouth uncovered. In the mucous mem-
brane covering the tongue there are numerous glands which
secrete the mucus by which this organ is always covered.
There is also a large number of papillce, of which there are
two kinds: the filiform, which are conical or threadlike in
shape, and the fungiform, which are larger and less numer-
ous than the former. The latter are narrow at the base and
expanded at the distal end. In an average specimen of R.
fusca Fixsen found two hundred and thirty-eight of these
papillae. In a specimen of R. pipiens I have found as many
as six hundred and forty.

INTERNAL STRUCTURE

75

The tongue of the frog can be readily thrown out of the
mouth, as it is in capturing an insect, and withdrawn again
with great quickness. The sticky secretion with which it
is covered causes it to adhere to insects or other prey with
which it comes in contact. The victims are then drawn
back into the mouth, where the tongue may assist in pushing
them back into the throat, where they can be swallowed.
The sticky substance on the frog's tongue is not produced
by the mucous glands, but is derived, in part at least, if not
entirely, from the intermaxillary gland, which lies above the
anterior part of the roof of the mouth. This gland is partly
inclosed by the premaxillary bones just in front of the nasal
cavities. It really consists of an aggregate of several small
glands (twenty to twenty-five in Rana esculenta) , with as
many independent ducts leading into the cavity of the
mouth. Wiedersheim has shown that the secretion of these
glands is remarkably adhesive.

The mouth cavity in general is lined by a mucous mem-
brane which varies considerably in structure in different
regions. Posteriorly it is thrown into folds which converge
toward the esophagus. The epithelium which forms the
superficial portion of this membrane is ciliated over a large
part of the mouth, and there are numerous goblet cells
scattered about among the others. The action of the cilia
may be demonstrated in a live or recently killed frog by
scattering powdered carmine over the roof of the mouth.
The carmine grains will be seen to be carried very slowly
backward, and eventually they will be drawn into the
esophagus. The whole membrane of the mouth takes part
to a greater or less extent in the production of mucus.

The Teeth. — The teeth of the frog are very numerous,
but of small size and uniform structure. With the excep-
tion of the two patches of vomerine teeth, they are confined
to the upper jaw. The jaw teeth rest against the dental

THE BIOLOGY OF THE FROG

processes of the maxillary and premaxillary bones, to which
they are attached by cement substance. They are em-
bedded in the mucous membrane of the mouth, beyond
which they project only for a short distance. Each
tooth is approximately cylindrical in form, tapering slightly
toward the upper end, which is somewhat incurved. The
of the tooth which is fastened to the
jaw is called the root. Upon this rests the
crown, which is separated from the root by
a transverse furrow. In the center is a
cavity filled with the pulp, which is a very
vascular tissue in which the cells (odonto-
blasts) are situated that produce new mate-
rial for the growth of the tooth. The
greater portion of the crown is composed of
substance called dentine which forms a hard
calcareous wall, traversed by numerous fine
branching canals which lead from the pulp
cavity. The upper half of the crown is
coated with a very hard, resistant layer
of enamel which is considerably thickened
over the tip. The enamel shows a strati-
fied structure, but it does not contain the
vertical prisms found in higher forms. Out-
side of the enamel there is a thin, resistant membrane, the
cuticula dentis. The root of the tooth is composed of a
substance resembling bone.

' The teeth of the frog are not used for mastication, but
only for holding prey, which is the primitive function of
teeth among vertebrate animals. There is a continual re-
placement of old teeth by new throughout most of the life
of the animal, the process ceasing only in old individuals.
The walls of the old teeth become partly absorbed by means
of large multinucleate cells, the osteoclasts; in this way they

basal portion

A B

Fig. 19 —Teeth
of the bull frog.
A, view of in-
ner face ; B,
lateral face ; c,
crown ; r, root ;
/, section of
lower jaw ; p,
pulp cavity.

INTERNAL STRUCTURE

77

become freed from their attachment to the jaw bone, and
are then cast out. New teeth are producd below the old
ones, whose place they finally take.

Organs in the Body Cavity. — If the ventral body wall of
the frog be cut through and the cut edges be spread apart
and pinned down, there will be opened up a large cavity
containing the principal internal organs of the body. This
space is called the body cavity, or ccelom. It lies ventral to
the vertebral column, or backbone, which may be seen when
the internal organs are pushed aside. If the middle part
of the pectoral girdle, or bony support of the fore limbs, is
cut away, the exposure of the parts will be made more
complete. Near the anterior end of the body cavity lies
the heart, not on the left side of the body, as in ourselves^
but very nearly in the middle line. It is inclosed in a
transparent sac, the pericardium, through which one may
see the two auricles, which are thin-walled and appear dark
red from the blood they contain, and a posterior cone-
shaped division, the ventridle, which has a very thick mus-
cular wall and is of a pink or light reddish color. The
pericardium is united to the ventral body wall by a thin
sheet of membrane, the posterior edge of which is free and
incloses the anterior abdominal vein, which runs along the
mid-ventral line and finally empties into the liver.

The liver is a large, reddish brown organ lying above and
partially surrounding the pericardium; it consists of three
lobes, — a median, a right, and a left. On the ventral side
of the median lobe lies the gall bladder, which is connected
with the intestine by means of the g^ll duct. Projecting
into the body cavity from in front ar^ two thin, very dis-
tensible sacs, with a reticulated appearance, the lungs.
With a blowpipe they may be inflated from the glottis and
swell enormously. When the air is expelled from them,
they contract to a very small size.

78

THE BIOLOGY OF THE FROG

nght aurscle

riqht oviducts

12 j

ventricle

left auricle
left oviduct1

fight lung

Systemic trunKs-
coel.mes.artery-
pt cv. vein
r Kidney
r oviduct

ureter
dorsal aorta -
right oviduct"

postcaval vein
liver j
gullet
-left lung

"left oviduct
-left ovary

-left oviduct"
-left oviduct

urinary bladder

rectum

abd.vem

Fig. 20. — Organs of a female frog. The alimentary canal has been
cut off at the gullet and rectum and most of the liver has been
removed and the ventricle of the heart turned forwards. The
abdominal vein has been cut and turned back. (From Newman,
redrawn from Parker and Parker.)

The different parts of the alimentary canal vary consider-
ably in size and texture. Above and projecting behind the
liver is the stomach, a thick-walled, muscular organ which

INTERNAL STRUCTURE

79

tapers toward the posterior end where the pyloric constric-
tion marks its point of separation from the small intestine.
Anteriorly the stomach is connected with the short esopha-
gus leading from the posterior end of the buccal cavity.
The small intestine, which proceeds from the pyloric end of
the stomach, at first bends forward and runs nearly parallel
with the stomach; this portion is called the duodenum; the
portion behind this, or the ileum, curves abruptly backward,
and, after forming several coils, suddenly widens out into
the large intestine, jjhe anterior portion of the large intes-
tine is called the rectum; posteriorly it narrows into the
cloaca^ which passes above the ventral part of the pelvic
girdle and terminates in the anus, or v ent[ Along its whole
extent the alimentary canal is suspended from the mid-
dorsal portion of the body cavity by a thin, transparent
sheet of membrane, the mesentery.

In the U-shaped loop made by the stomach and intestine
lies an elongated, light-colored organ of irregular shape, the
pancreas^ There is a dark red, rounded body, the spleen,
attached to the mesentery near the anterior end of the
large intestine.

The reproductive organs, or gonads, lie on either side of
the alimentary canal and are supported from the dorsal body
wall by special sheets of membrane like the mesentery. The
gonads of the female, or ovaries, during the breeding season
are greatly enlarged, and may be recognized by the small,
globular, dark-colored eggs which form the greater part of
their mass. External to the ovaries are the large, white
convoluted tubes, the oviducts. These are also suspended
to the dorsal body wall by thin sheets of membrane; they
have no connection with the ovaries; anteriorly they open
into the body cavity near the base of the lung by a wide
funnel-shaped mouth into which the eggs find their way
after they have been discharged into the coelom. The walls

THE BIOLOGY OF THE FROG

of the oviducts are thick and glandular except toward the
posterior end, where they become expanded into thin, very
distensible sacs, the uteri, in which the eggs collect before
being laid. The uteri open separately into the dorsal side of
the cloaca.

The gonads of the male, or testes, are very different in
size and appearance from the ovaries. Each testis is an
ovoid, whitish organ, occupying a similar position to the

Fig. 21. — Rana temporaries. Dissection from the left side ; the viscera
somewhat displaced, an, anus ; b. d, bile duct ; b. hy, body of hyoid ;
bl, urinary bladder ; bl', its opening into cloaca ; c. art, conus arteri-
osus ; cblm, cerebellum ; cl, cloaca ; cn. 3, centrum of third vertebra ;
cp. ad, corpus adiposum ; crb. h, cerebral hemisphere ; d. ly. s, dorsal
lymph sinus ; du, duodenum ; ep. cor, epicoracoid ; eus. t, Eusta-
chian tube ; FR. PA, fronto-parietal ; gl, glottis ; gul, gullet ; il,
ilium ; is, ischium ; kd, kidney ; I. au, left auricle ; I. Ing, left lung ;
Ir, liver ; m. mck, mentomeckelian ; n. a. 1, neural arch of first ver-
tebra ; olf. I, olfactory lobe ; opt. I, optic lobe ; o. ST, omo- and epi-
sternum ; ped, pericardium ; PMX, premaxilla ; pn, pancreas ; p. na,
posterior naris ; pu, pubis ; ret, rectum ; r. Ing, right lung ; s. int,
small intestine ; sp. cd, spinal cord ; sph. eth, sphenethmoid ; spl,
spleen ; st, stomach ; s. v, sinus venosus ; tng, tongue ; ts, testis ; ur,
ureter; ur', its aperture into the cloaca; tjst, urostyle ; v, ventricle;
v. ly. s, ventral lymph sinus ; vo. t, vomerine teeth ; vs. sent* vesicula
seminalis.

INTERNAL STRUCTURE

81

ovary in the body cavity and suspended in a similar way
by a membrane, the mesorchium, to the dorsal body wall.
At the anterior end of both the ovaries and testes are at-
tached the fat bodies, corpora adiposa, which are easily
recognizable on account of their yellow color and their
division into a number of finger-like lobes.

The kidneys are reddish, flattened, oblong organs lying
against the dorsal body wall on either side of the vertebral
column. Their position is somewhat behind the middle
of the body cavity, and they are brought into view when
the other abdominal viscera are removed or pushed aside.
Each kidney is connected with a tube, the ureter, which
runs along its outer edge and empties, near the opening of
its fellow, into the dorsal side of the cloaca. In the male
the ureters are expanded distally to form seminal vesicles,
which in some species of frogs reach a considerable size.
The urinary bladder is a large, bilobed sac lying in the
posterior end of the body cavity, and opening into the
ventral side of the cloaca just below the openings of the
ureters. If the bladder is inflated by means of a blowpipe
inserted into the cloaca, it will swell to a large size, and an
accurate idea may be gained of its form and connections.

The walls of the body cavity and the various organs
contained in it are covered by a thin, moist, glistening
membrane, the peritoneum. This membrane is perfectly
continuous throughout, and is simply reflected over the
various organs. If we imagine that the body cavity were
originally empty, and that the organs it contains were
pushed into it from the outside, carrying the peritoneum in
front of them, it will help us to understand the relation of
this membrane to the structures it surrounds. It is not to
be inferred, however, that these relations were brought
about in just this way, but the device will be useful in
enabling us to get the conditions clearly in mind. The

82

THE BIOLOGY OF THE FROG

alimentary canal, the liver, the lungs, the gonads, oviducts,
bladder, fat bodies, and other organs are covered with peri-
toneum, which usually adheres closely to the surface. The
mesenteries and the similar sheets of membrane supporting
the ovaries, oviducts, and testes are double, as would
naturally be the case if these organs were pushed in in the
way mentioned. The peritoneum passes down from the

urostyle
dorsal aorta
ilium
postcaval ve;n
Kidney

spermary

jntestine-

muscle

ubcutaneous lymph sac

SKin

parietal layer of
peritoneum

subcutaneous lymph

muscle

abdominal vein

Fig. 22. — Diagram of a cross section of the body of a frog showing
the course of the peritoneum by a dotted line. (After Parker and
Parker.)

body wall, covers these organs, then passes back to the
body wall again, the two sheets of membrane coming close
together except where they are separated by the organ they
surround. The arteries and veins supplying the organs
generally run between the two layers of the supporting
membranes. The portion of the peritoneum surrounding the
alimentary canal and its appendages is called the visceral
layer; the part applied to the body wall, the parietal layer.
For the most part the parietal layer is grown fast to the

INTERNAL STRUCTURE

S3

body muscles, but on the dorsal side of the body it is
separated from the wall, forming a large lymph space, the
cisterna magna, or suhvertebral lymph sinus. The kidneys
lie in this space; hence they are covered with peritoneum
only on the ventral side, and
not completely invested by it
like the other viscera. The
membrane previously men-
tioned, which extends be-
tween the ventral body wall
and the pericardium and
liver, is a portion of the
peritoneum, forming a sort
of ventral mesentery; it is
reflected upon the ventral
body wall on the one hand
and spread out over the peri-
cardium and liver on the
other.

The coelom is filled with a
transparent fluid, the cielo-
mic or peritoneal fluid,
which is essentially the same
as the lymph found in other
portions of the body. Owing
to the fluid in which they
lie and the smoothness of
their peritoneal coating, the
organs in the body cavity are enabled to glide over each
other with little friction.

Organs outside of the Body Cavity. — Above the ccelom
there is a second cavity surrounded by the bones of the
vertebral column and skull and containing the central
nervous system. Tbi« neural cavity, as we may weii call

Fig. 23. — Organs in the neural

cavity. The anterior part of
this cavity contains the brain,
which is composed of the olfac-
tory lobes, olf. I; the cerebral
hemispheres, crb. h; the dien-
cephalon, dien; the optic lobes,
opt. I; cerebellum, cblm; and
medulla oblongata, med. obi.
n. c, neural canal ; sp. cd, spi-
nal cord ending in the filum
terminale, /. t ; e, eye. (After
Parker and Parker.)

M

THE BIOLOGY OF THE FROG

it, exte nds farther forward than the ecelom, and is separated
from the latter by the bases, or centra of the vertebra. The
anterior portion of the central nervous system, or brain,
lies in the skull, and is continued posteriorly as the spinal
cord, which is inclosed within the vertebral column. If
we make a cross section through the frog somewhere near
the middle, we shall find that the body contains two longi-
tudinal cavities separated by the centra of the vertebrse, —
the ecelom below, and the neural tube above. Around both
of these is a layer of muscles which is much thicker dorsally
although it completely surrounds the ecelom below. And
outside of the muscles, from which it is separated by large
lymph spaces crossed by a few bands of connective tissue,
is the skin.

With the exception of the loose attachment of the skin,
all the features of structure mentioned in the last paragraph
belong to the vertebrate animals in general. We have next
to see how these fundamental features of structure came to
be established.

CHAPTER V

THE DEVELOPMENT OF THE FROG

The frog, like all higher animals which are developed
through sexual reproduction, begins its existence as a single
cell, the ovum or egg. The eggs or ova arise in the ovary,
and, when full grown, break out into the body cavity, where
they float around freely until they are carried by the action
of cilir into the funnel-shaped mouths of the oviducts.
They are then carried down the oviducts by the action of
cilia lining the walls and, during their passage, they become
surrounded by a thick coating of gelatinous substance.
From the oviduct they pass into the large, thin-walled uteri,
where they remain until they are discharged into the water,
where they are fertilized and undergo their development.

There is perhaps no phenomenon in nature more marvel-
ous than the formation of a complex organism from a single
and apparently simple cell. At the one end of the process
we have a small mass of seemingly lifeless material with
only the simplest visible features of structure, and at the
other a creature with a complex organization composed of
parts beautifully coadapted and working in harmony, with
numerous instincts by which it adjusts itself to the various
objects in its environment, and most remarkable of all, with
volition and intelligence which, in the highest forms, may
attain a high degree of development. The production of
?ven the simplest animal seems almost a miracle, and our
wonder is only increased the more we attempt to understand
bow and why the process takes place.

85

86

THE BIOLOGY OF THE FROG

Many of the older writers who speculated on the problem
of development were of the opinion that the egg contains in
miniature all of the organs of the body in the same form
and relation in which they occur in the adult, and that the
parts simply expand as nutriment is absorbed until the
embryo attains its maximum size. The process of de-
velopment was compared to the unfolding of a flower from
a bud in which the parts occur in the same relative position
as in the expanded blossom and simply unfold through the
absorption of sap until the flower reaches its final form.
This view, which is known as evolution, or preformation,
was championed by such men as Haller, Leibnitz, Bonnet,
and Spallanzani ; it really amounted to a denial of develop-
ment. What appears to be such, according to this interpre-
tation, is simply growth, the expansion of something pre-
formed. The Abbe Spallanzani, who made extensive and
most excellent studies on the breeding habits of Amphibians,
considered the eggs of these animals to be small embryos
of tadpoles whose parts through the influence of the sper-
matic fluid were stimulated to growth and activity. Other
writers, among whom C. F. Wolff occupies the most promi-
nent place, regarded the egg as a mass of simple unorganized
material which becomes more and more complex as de-
velopment proceeds. This doctrine, which is generally
known as the theory of epigenesis, has become more widely
accepted in recent times, although no one at present espouses
either epigenesis or preformation in the forms advocated in
the eighteenth century.

The cell theory, or the doctrine that animals and plants are
composed of living units, or cells, which was promulgated
by Schleiden and Schwann in 1839, did much to correct
the extravagant forms of the older theories of preforma-
tion, and started investigators on the road to a truer con-
ception of development. A farther step in advance was

THE DEVELOPMENT OF THE FROG

87

made when it was ascertained that the egg is a single cell.
Not many years afterward it was discovered that the sper-
matozoon is likewise a single cell, and that in fertilization
there is a union of the nuclei of the ovum and spermatozoon,
each contributing an equal share of chromatin to the nucleus
of the fertilized egg, and thence to all the cells of the body
of the embryo. The whole aspect of the problem of develop-
ment is very different from what it appeared to the older
naturalists. However we may regard the modern forms of
the doctrines of preformation and epigenesis, — for both
points of view are still held, — it is certain that development
actually proceeds from a single cell to a body consisting of
a multitude of cells of great variety of form and function.
This cell may be enormously complex, containing somehow
features which represent all the different organs of the
body, or it may be comparatively simple in structure, and
the differentiations appearing in the embryo may be the
results of the interactions of its parts and the influence of
external conditions. Preformation and epigenesis both
have their advocates, but their differences have become less
wide as knowledge of embryology has advanced.

The Jelly and its Uses. — The egg of the frog when it
is laid in the water is surrounded by a spherical mass of
transparent jelly. At first the coat of jelly is less than the
diameter of the egg in thickness, but through the absorption
of water it gradually swells until it becomes two or three
times this diameter. The jelly consists of three layers, a
thin inner layer closely applied to the egg, a thick middle
layer of more fluid consistency, and a thick outer layer.
The coats under the microscope show a concentric series of
fine lines, indicating the stratification of the material. The
function of this jelly is primarily the protection of the eggs.
It keeps them free from dirt, bacteria, and the spores of
fungi, and also from the attacks of aquatic insects and

88

THE BIOLOGY OF THE FROG

snails, and various mechanical injuries which would other-
wise affect them. Bernard and Bratuschek 1 consider that
the jelly also serves to keep the eggs warmer than the
surrounding water, which in the spring, when the eggs are
laid, is often very cold, and frequently covered with ice.
The jelly is supposed to act upon the principle of a hotbed,

allowing free en-
trance to the sun's
rays, but checking
radiation from the
egg. The heat waves
that radiate from
the eggs are longer,
less refrangible, and
pass through the
jelly less readily
than the shorter
waves, which are
abundant in the di-
rect heat of the sun.
Bernard and Bra
tuschek 2 found in
experimenting upon the jelly that the greater the wav
length, the less heat passed through in comparison with an
equal amount of water under the same conditions. The
jelly was thus shown to have the property required to keep
the eggs warmer than the surrounding medium. The black
pigment of the eggs which readily absorbs the heat rays
also functions in the same manner.

Resistance of the Sexual Products to Cold. — The egg
masses of the frog are laid so early in the spring that the
water containing them is frequently frozen. Fischer-Sigwar

1 Bernard und Bratuschek, "Der Nutzen der Schleimhiillen fur di
Froscheier," Biol. Centrlb., Bd. 11, 1891.
3 Ibid.

Fig. 24.— Egg in jelly. (After Schultze.)

THE DEVELOPMENT OF THE FROG

89

records finding egg masses which were frozen solid for two
days, during which the temperature sank to — 8° C. When
the eggs were gradually thawed out, they underwent a
normal process of development, although somewhat slower
than usual, and gave rise to larvae which left the jelly two
days afterward. How long eggs may be frozen and how low
a temperature they can endure and still retain their power
of development is not determined.

The spermatozoa of the frog may also be frozen without
fatal results. The Abbe Spallanzani showed that sperma-
tozoa frozen in ice for half an hour are able to cause eggs to
develop, but if kept in ice for several hours, this power is
lost. The spermatozoa apparently have less power of re-
sistance to cold than the eggs; careful comparisons, however,
have not yet been made.

Structure of the Undivided Egg. — The eggs of the frog
as they occur in the body of the female after their dis-
charge from the ovary are surrounded by a very thin v itel-
line membrane, which represents its cell wall. On one side,
representing the animal pole, the egg is colored by black
pigment, which, as is shown in sections, is mainly confined
to the periphery immediately under the vitelline membrane.
This pigment is in the form of minute granules embedded in
the protoplasm. The nucleus of the egg lies excentrically
near the middle of the dark pole. When the egg is in the
ovary, the nucleus is large and contains a large amount of
fluid known as nuclear sap, but just preceding the escape of
the egg into the body cavity and during its passage down
the oviduct the nucleus becomes shrunken through the exu-
dation of its fluid, and undergoes a process of division in-
volved in the formation of the first polar body. As Schultze
has shown, along with the shrinkage of the nucleus there
appears a mass of fluid beneath the animal pole which he
considers to be the nuclear sap. The place is marked by a

90

THE BIOLOGY OF THE FROG

light-colored spot near the center of the dark cap. The
greal mass of the frog's egg is made up of yolk, which
occurs in the form of granules embedded in the cytoplasm.
The yolk is a semi-fluid nitrogenous substance which is em-
ployed for the nutrition of the developing embryo. It is
more abundant toward the light-colored or vegetal pole of
the egg, the region around the animal pole containing rela-
tively more cytoplasm. The yolk granules are of various
sizes, and are usually spherical or oval in form. By the
action of certain reagents they may be broken up into flat-
tened plates which, according to Schultze, do not exist as
such in the living egg.

Maturation. — The process of maturation consists in two
successive divisions of the ovum, resulting in the formation
of the two polar bodies. These bodies are minute globules
extruded at the animal pole of the egg. Morphologically
they are cells, produced by very unequal divisions of the
egg, which is only a cell of very large size. The first polar
globule is given off while the egg is within the body,
the preliminary steps of the process occurring just be-
fore the egg leaves the ovary. The large watery nucleus
shrinks, the chromatin becomes aggregated into definite
bodies, or chromosomes, a spindle forms at right angles
to the surface of the egg, and half the chromatin, with a
small amount of cytoplasm, is extruded as the first polar
body.

In Rana fusca the second polar body is given off after
the egg has been laid, and within half an hour after fertiliza-
tion; no resting stage of the nucleus intervenes between the
two maturation divisions. The two polar bodies in Rana
fusca are of about equal size, and are either attached to
the animal pole or float in the fluid which accumulates be-
tween the egg and the vitelline membrane. If the jelly is
removed from a freshly laid egg, and the light spot, or fovea,

THE DEVELOPMENT OF THE FROG 91

at the animal pole observed with a hand lens, the extrusion
of the second polar body may be witnessed.

When the maturation divisions are completed, the amount
of chromatin in the egg has become much reduced and the
number of chromosomes diminished by one half. That the
number is not reduced to one fourth is due to the fact that
only one of the two maturation divisions is a true reducing
division. Just previous to maturation a pairing of the
chromosomes occurs which is known as synapsis. The

Fig. 26. — Division of the spermatocytes of Rana temporaria. The
figure at the right shows the chromosomes of two daughter cells.
(After Witschi.)

chromosomes come to lie side by side in pairs, and one of
the maturation divisions consists in separating the chro-
mosomes which had previously been joined. The other
maturation division involves a longitudinal splitting of each
chromosome, and is hence of the non-reducing type which
is found in the ordinary process of mitosis.

A corresponding pairing and reduction of chromosomes
takes place during the development of the sperm cells,
so that these finally contain half the usual number of
chromosomes. When fertilization occurs there is restored
the full number of chromosomes characteristic of the species.
In Rana pipiens, according to Swingle, and in Rana tem-

92

THE BIOLOGY OF THE FROG

poraria, according to Witschi, the number of chromosomes
is 26 and therefore 13 in the mature sex cells. In the bull-
frog Swingle finds 28 chromosomes.

The sperm cells of the frog fall into two classes, one
containing a large X chromosome and the other a smaller
Y chromosome. There is probably an X
chromosome in every egg, and, in accordance
with the usual scheme for the determination
of sex, if an egg is fertilized with a sperm cell
containing an X chromosome it produces a
female; if it is fertilized by a sperm cell con-
taining a Y chromosome it produces a male.

Fertilization. — The act of fertilization
consists in the union of two cells, the ovum
from the female and the spermatozoon from
the male. As bearers of hereditary qualities
these two cells are equal, but in size and
form they are as dissimilar as they can well
be. The sperm cell is a minute, elongated
body consisting of a narrow, pointed head
formed mainly of the nucleus, a short middle
piece just behind the head, and a long, very

pIG 26. Sper- slender tail. As before described, the seminal

matozoon of fluid of the frog is shed over the egg masses
lent! 6(Af~ as they are extruded into the water from the
ter La Va- body of the female. The spermatozoa which
George.) * swarm in the seminal fluid are active, and
swim about by the movements of the
tail, working their way through the jelly of the egg mass
until one comes in contact with an egg. Then the sper-
matozoon slowly penetrates the egg substance. The en-
trance of one spermatozoon seems to cause some change in
the substance of the egg whereby other spermatozoa are
prevented from entering it, as normally an egg is fertilized

/

THE DEVELOPMENT OF THE FROG 93

by only one sperm cell, although there may be thousands
of others in the immediate vicinity. When the head of the
spermatozoon has entered the egg, it begins to enlarge, and
its nucleus, which is now known as the male pronucleus,
assumes a spherical form. It migrates slowly toward the
central part of the egg, dragging in behind it a mass of
pigment granules from the periphery, so that its course
comes to be marked by a dark streak. Before the male
pronucleus has penetrated very far the process of matura-
tion is brought to completion by the formation of the second
polar body. The nuclear material remaining in the egg
after this second maturation division goes into a resting
stage, forming the female pronucleus, or the nucleus of the
matured ovum. The male and female pronuclei approach
and finally fuse into one, which is called the copulation
nucleus. The number of chromosomes contributed by both
parents to the nucleus of the fertilized egg is the same, and
this has doubtless a fundamental relation to the fact that,
on the average, offspring inherit qualities from both their
parent forms in an equal degree. This correlation of
equality of inheritance from the two parents with the
equality of their contributions of chromatin material to
the fertilized eggs lends strong support to the view that it is
to the chromatin of the nucleus that we must look for the
bearer of hereditary qualities; and especially since the
cytoplasm in the two germ cells differs so enormously in
amount.

Very soon after the spermatozoon has entered the egg a
mass of fluid collects between the egg and vitelline mem-
brane. This is called the peri-vitelline fluid, and is doubt-
less derived from the egg itself. Owing to the accumulation
of this fluid the egg becomes free to rotate, and the dark
pole, which has a less specific gravity than the light-colored
yolk-laden region, soon comes to lie uppermost. Before

94

THE BIOLOGY OF THE FROG

fertilization the eggs lie in all possible planes, but soon after
fertilization (hey all assume the same position, with the
black pole above.

Cleavage. — Usually between two and a half and three
hours after fertilization the egg begins to undergo its first
division. The process begins as a small depression at the
animal pole, which gradually extends in the form of a
groove until it finally surrounds the egg. This groove marks
the outer boundary of a cleavage plane which extends
through the egg, dividing it into approximately equal cells.
The cleavage of the mass of the egg is preceded and ac-
companied by a karyokinetic division of the nucleus, so
that each daughter cell contains a nucleus derived from the
copulation nucleus of the fertilized egg; and in all subse-
quent cleavages of the ovum there is a separation of nuclear
as well as cytoplasmic material, and chromatin derived from
both parents comes to lie in all the cells of the body of the
embryo.

The egg before cleavage is not entirely a radially sym-
metrical structure, but the dark pole inclines somewhat to
one side. There is consequently only one plane which can
divide the egg into two symmetrical halves. As a rule the
first cleavage furrow lies in this plane of symmetry; more
i arely it lies at right angles to it, although it may occur in
almost any intermediate position.

The second cleavage appears about three quarters of
an hour after the first; the furrow extends gradually
from the animal to the vegetal pole, at right angles to
the first furrow, and divides the egg into four cells. The
first and second cleavage planes stand in a tolerably
constant relation to the axes of the body of the embryo,
the first cleavage plane marking the median, or sagittal
plane of the future animal; but this is a rule not without
exceptions.

THE DEVELOPMENT OF THE FROG

95

ABC

D E F

G H

Fig. 27. — Segmentation of egg. (After Schuitze.) A, two-cell stage,
with the beginning of the second furrow ; B, eight-cell stage, show-
ing the cross furrow at the animal pole ; E, eight-cell stage ; O, D,
F, Cr, sixteen-cell stages, showing variations in the plan of cleavage ;
H, thirty-two-cell stage. (From Morgan's "Development of the
Frog's Egg.")

96

THE BIOLOGY OF THE FROG

G H ,

Wig. 28. — Segmentation of the egg and formation of the blastopore.
A, eight-cell stage seen from one side ; B, beginning of sixteen-cell
stage ; C, thirty-two-cell stage ; D, forty-eight-cell stage ; E, F, and
G, successive later stages of cleavage ; H, beginning of the blasto-
pore in the form of a small crescent ; I, circular blastopore on the
vegetative side of the egg. (After Morgan.)

THE DEVELOPMENT OF THE FROG

97

The third cleavage furrow comes in a little above the
equator of the egg and at right angles to the other two;
the four upper cells cut off by this division are a little
smaller than the lower four. At the next cleavage the fur-
rows run nearly vertically and hence at right angles to the
third cleavage plane. Sometimes the furrows meet at the
animal pole, but more frequently they cut through the first
or second cleavage furrows, producing thus a bilateral ar-
rangement of the cells. The fourth cleavage furrows are
subject to more variation at the vegetal pole of the egg, and

Fig. 29. — Vertical section through the blastula of a frog in different
stages. B, segmentation cavity or blastocoel. (After Marshall.)

the subsequent divisions soon become so irregular that it
is impossible to trace out any plan of procedure. A fifth
cleavage occurs typically parallel to the third, appearing
first in the upper hemisphere and then in the lower. The
cleavages thus far usually follow the rule that each cleavage
plane comes in at right angles to the previous cleavage
plane. Deviations from the typical method of cleavage
are apparently of little moment, even in the first few divi-
sions, as such abnormally dividing eggs may nevertheless
produce perfect embryos.

98

THE BIOLOGY OF THE FROG

Cleavage takes place more rapidly in the dark or animal
pole, since at that place the protoplasm is most dense. Yolk,
which is most abundant in the white or vegetative side of
the egg, delays cell division, and we find in the later stages
of segmentation the cells are much larger at the vegetative
pole and gradually become smaller toward the opposite
side of the egg. In the first few cleavages the planes of
division lie at right angles to the surface of the egg, but
subsequently planes of division occur parallel to the surface,
so that the egg comes to consist of more than one layer of
cells in thickness. A cavity makes its appearance near the
center of the egg and gradually increases in size as cleavage
proceeds. This is the blastoccel, or segmentation cavity,
and the egg at this time is called the blastula. It is es-
sentially a hollow sphere of which the wall on the vegetative
side is very much thicker than it is above and composed of
large yolk-laden cells.

Gastrulation. — In the embryonic development of most of
the many-celled animals a stage is passed through which is
known as the gastrula. In its typical form a gastrula is a
sort of double-walled sac such as may be produced, accord-
ing to a well-worn illustration, by pushing in one side of a
hollow rubber ball with the finger. The mouth of the gas-
trula is called the blastopore, and this opening naturally be-
comes smaller as the process of inpushing is completed.
The process of gastrulation, which is exemplified in its
typical form in the development of a starfish or sea-urchin,
becomes very much modified in different animals. Such
is the case in the development of the frog. The large ac-
cumulation of yolk at the vegetal side of the blastula pre-
vents the invagination of this region from taking place in
the typical way. The same end is reached partly by a
process of in-pushing and partly by the overgrowth of the
white pole by the dark. The inpushing and overgrowth take

THE DEVELOPMENT OF THE FROG 99

place more on one side of the egg than the other, and these
processes are first indicated by the appearance of a
crescentic groove a little below the equator of the egg. The
crescent represents the beginning of the blastopore. The
groove is deepest at the center and thins out towards the
edges, which gradually extend around the lower pole of the
egg. In this way the crescent becomes converted into a

Pig. 30. — Sagittal section through a frog embryo. B, blastocoel or
segmentation cavity ; BP, lip of blastopore ; EE, outer or epidermic
layer of ectoderm ; EN, inner or nervous layer of ectoderm ; Y, yolk
cells. (After Marshall.)

circle, and the circle gradually becomes smaller and smaller
until only a small part of the light-colored yolk, known as
the yolk plug, appears in the midst of the dark area. The
white pole is thus overgrown by the dark, but not with
equal rapidity from all sides, the closing-in taking place
much more rapidly on the side where the crescentic fold
originally appeared, and which subsequent events prove
to be the anterior end of the embryo.

100

THE BIOLOGY OF THE FROG

If we make a vertical section through the embryo at right
angles to the crescentic blastopore, we shall find the latter
is the mouth of a cavity which extends some distance into
the egg. Above this cavity, which is called the archenteron,
is a comparatively thin roof, closely applied to the upper
wall of the embryo, and at the floor of the cavity is a large
mass of yolk cells. The archenteron represents the cavity
produced by the process of gastrulation. It is due, in great
measure at least, to the overgrowth of the dorsal lips of the
blastopore, the cells forming the floor being formerly at the
surface of the egg. According to Marshall, the cavity
arises in great part through the splitting apart of the yolk
cells, but while this may be a factor in the case, it certainly
cannot be the predominant one. (See Robinson and Asshe-
ton '91, 1 Assheton '94,2 Morgan '97.3) As the archenteron
increases in size, the blastoccel or segmentation cavity
necessarily becomes smaller. According to Marshall the
former breaks through into the latter, and the two form
one cavity. This, however, does not occur in all cases.

The Germ Layers. — The formation of the gastrula pro-
duces a two-layered embryo, each layer being several cells
thick. The outer of these layers is the ectoderm; the inner,
the entoderm. The cells of the former are small and pig-
mented; those of the latter for the most part are compara-
tively large, lighter in color, and contain a large amount of
yolk. The two layers are continuous with each other at the
lips of the blastopore. Before the process of invagination
is completed there appears a third germ layer, the mesoderm,
or mesoblast, between the other two. The mesoderm ap-
pears all around the blastopore, and as this opening closes
mainly from in front backward, the two masses of meso-

1 Robinson and Assheton, Quart. Jour. Mic. Sci., Vol. 32, 1891.

2 Assheton, Ibid., Vol. 37, 1894.

3 Morgan, "The Dpvelopment of the Frog's Egg," 1897.

THE DEVELOPMENT OF THE FROG 101

derm on either side are brought near each other in the mid-
dorsal line. The free ventral edges of the masses or sheets
of mesoderm extend ventrally until they meet below and
come to surround the archenteron, except for a short space
along the dorsal side. The sheets of mesoderm soon be-
come split into an inner or splanchnic layer, which lies next

Fig. 31. — Sagittal section through a frog embryo. B, blastocoel ; BP,
dorsal lip of blastopore ; BP', ventral lip of blastopore ; EE, epi-
dermic layer of ectoderm ; EN, inner or nervous layer of ectoderm ;
H, hypoblast or entoderm ; T, mesenteron or gastrula cavity ; Y,
yolk plug. (After Marshall.)

to the archenteron, and an outer, parietal, or somatic layer,
which lies next to the ectoderm. The space between these
two layers of mesoderm is the beginning of the ccelom, or
body cavity. It is at first small, but as development pro-
ceeds, it widens out more and more (Fig. 33, C).

The cells just above the mid-dorsal wall of the archenteron
form a thickening which soon becomes marked off sharply

102

THE BIOLOGY OF THE FROG

from the mesodermic layers on either side and the wall of
the archenteron below. This thickening is the beginning
of the notochord, a structure forming the beginning of the
vertebral column, and occurring in the embryo, when not
also present in the adults, of all vertebrate animals. It
is always the first part of the skeleton to make its appear-
ance in the embryo, as it was the first part to appear in the

M

Fig. 32. — Transverse section through the middle of a frog's embryo.
GH, notochord ; E, ectoderm ; M, mesoderm ; NG, neural groove ;
NP, neural plate; T, mesenteron ; Y, yolk cells. (After Marshall.)

evolution of the race. Whether in the frog it is entodermic
in origin, as it certainly is in some of the Amphibia and in
many other vertebrates, or whether, as maintained by Mor-
gan, it is developed from the mesoderm, is a matter about
which there is a difference of opinion. Miss H. D. King 1
has studied the formation of the notochord in Bufo lenti-
ginosus and Rana palustris, and has come to the conclusion

1 King, Biol Bull, Vol. 4, 1903.

THE DEVELOPMENT OF THE FROG 103

that the notochord in the anterior end of the embryo arises
from the mesoderm, whereas in the posterior part of the
embryo it is developed from both mesoderm and entoderm.

Fig. 33. — Transverse section through a frog embryo before the closure
of the medullary or neural folds. C, coelom or body cavity ; OH,
notochord ; EE, epidermic layer of ectoderm ; EN, nervous layer of.
ectoderm ; M, mesoderm ; ME, outer or somatic mesoderm ; MH, in-
ner or splanchnic mesoderm ; NC, neural groove ; ND, dorsal root of
spinal nerve ; NS, spinal cord ; T, archenteron ; W, liver diverticu
lum; Y, yolk. (After Marshall.)

External Changes. — At the time when the blastopore is
nearly closed the egg is still in a spherical form, except
that along what is to be the dorsal side of the body of the
embryo there is the beginning of a broad depression known

104

THE BIOLOGY OF THE FROG

as the primitive groove. On either side of this are two
folds, the inner and the outer medullary folds, which are
continued as an elevation around the anterior end of the
primitive groove and are produced backward on either side

Fig. 34. — Development of the embryo. A, yolk-plug stage ; B, showing
the medullary folds, the blastopore nearly closed, and below the lat-
ter the invagination which is to form the anus ; C, D, later stages ;
E, the medullary folds have grown together and covered the blasto-
pore. Above the anus is the rudiment of the tail. (From Morgan,
after Ziegler.)

of the blastopore. The outer medullary folds gradually
fade away, but the inner ones become elevated and arch
over the groove between them. Finally the two inner fold?
meet and fuse along the median line, converting the groove
into a tube. The point where they first fuse corresponds
to the neck region of the embryo; and the closure of the

THE DEVELOPMENT OF THE FROG 105

tube proceeds both forward and backward from this point.
The fusion extends backward so that folds on either side of
the blastopore close in above that opening in such a way
that it becomes no longer visible from the outside. As the
medullary tube is completed it is constricted off from the
ectoderm above, and the latter becomes continuous over the

Fig. 35. — Embryos. Gp, gill plate ; Gs, Gs', two gill slits ; $, suckers ;
Sp, serse plate; An, anus. (From Morgan, after Schultze.)

mid-dorsal line. Subsequently it develops into the brain
and spinal cord of the embryo.

As the above changes are taking place the embryo
elongates in the direction of the neural tube, which marks
the longitudinal axis of the future animal. On either side
of the anterior end of the neural tube there appears a pair
of thickenings of the ectoderm. The anterior members of

106

THE BIOLOGY OF THE FROG

each pair, the sense plates, grow forward and meet in front
of the end of the neural tube; a depression appears in each
plate and marks the beginning of the ventral sucker of the
tadpole. Subsequently these depressions meet in front and
become converted into a U-shaped groove. In the posterior
pair of plates, the gill plates, there appears tow vertical
grooves, which later become converted into the gill slits;
later two additional slits appear, one before and one behind
the other two, but none of them breaks through until after
the tadpole leaves the jelly. In the middle line, just above
the ventral sucker, the beginning of the mouth appears as
a hollow depression of the ectoderm, but it does not com-
municate with the archenteron until a much later period.
The anus begins as an invagination of the ectoderm a short
distance behind the point where the blastopore was closed
over. Later this invagination meets and fuses with a
diverticulum from the posterior part of the archenteron,
thus establishing an opening between the latter and the
exterior. The tail arises as an elevation of the region in
front of the blastopore, which grows backward and pushes
the anus to a more ventral position. Later it becomes
flattened from side to side, and its upper and lower edges
become produced into a thin expansion, or tail tin.

The nostrils appear as a pair of external depressions or
pits a little above the rudiment of the mouth. These pits
deepen, and finally communicate with the buccal cavity.
Above and to the sides of the nasal pits the beginning of
the eyes is indicated as a pair of thickenings of the ecto-
derm. The outline of the enlarged anterior portion of the
medullary tube may be observed from the surface. It is
bent downward in front, and shows a division into three
regions, which become the three primary vesicles of the
brain. Near the posterior of these vesicles there is developed
on either side an invagination or pit of the ectoderm, which

THE DEVELOPMENT OF THE FROG

107

finally sinks in and becomes cut off from the surface and
forms the vesicle of the inner ear.

At the time the neural tube is formed, the superficial cells
of the ectoderm become furnished in many places with cilia
by means of which the embryo slowly rotates within the
jelly. The general direction of the stroke of the cilia is
from before backward. The movement is strongest at the
anterior end of the body, and is weaker on the ventral than
on the dorsal side. "A tadpole of 6 or 7 mm. will progress,
if placed upon its side in water, along the bottom of a flat
glass vessel, at the rate of one millimeter in from four to
seven seconds, " (Assheton '96.) After the tadpole is
hatched from the jelly the cilia gradually disappear.

Organs from the Ectoderm. — In addition to forming
the outer layer of the skin over the entire surface of the
embryo the ectoderm gives rise to certain other structures
which come to lie within the body. Chief among these is
the central nervous system whose beginning in the medullary
groove has already been described. The neural tube into
which the medullary groove develops loses its original con-
nection with the surface; anteriorly it becomes enlarged
and forms the brain, the remaining portions developing into
the spinal cord. The thickening of the walls of the portion
of the tube which forms the cord diminishes the central
cavity until it becomes reduced to a fine canal, known in
the adult as the canalis centralis. The anterior portion of
the tube becomes divided by slight constrictions into three
vesicles, which form, designating them from before back-
ward, the fore, mid and hind brain. The hindbrain becomes
widened from side to side, especially in front; its floor and
sides thicken, but the roof, except for a small fold at the
end which develops into the cerebellum, remains thin and
membranous, and becomes thrown into a series of folds
which support a mass of blood vessels known as the choroid

108 THE BIOLOGY OF THE FROG

plexus. The portion of the hindbrain which does not form
the cerebellum is converted into the medulla. The central
cavity becomes widened out, forming the fourth ventricle;
which communicates posteriorly with the canalis centralis
of the cord and anteriorly with the ventricle of the mid-
brain.

The midbrain grows out dorsally and laterally into a pair
of hollow processes, the optic lobes, whose cavities or ventri-
cles communicate with the median canal, which becomes
narrowed by the thickening of its walls, and forms the aque-
duct of Sylvius, or iter a tertio ad quartum ventriculum.
The floor of the midbrain forms the crura cerebri.

The forebrain soon becomes separated into two parts,
the thalamencephalon behind, and the cerebral hemispheres,
which grow out from the latter in front. The floor and walls
of the former become thickened to form the optic thalami,
the roof remains thin and membranous, and the cavity
becomes the third ventricle. From the roof of the thala-
mencephalon there arises a median hollow outgrowth, the
pineal gland, which extends dorsally, reaching the surface
ectoderm, where it becomes expanded into a small knob.
The knob becomes constricted off when the bones of the
skull develop and forms the brow spot, previously described.
The floor of the thalamencephalon gives rise to a hollow
outgrowth, the infundibulum, which extends downward.
The extremity of this outgrowth comes into contact with an
invagination from the roof of the mouth cavity, the two
parts combining to form the pituitary body which is there-
fore an organ of dual origin. The sides of the thalamen-
cephalon give rise to a pair of lateral diverticula, the optic
vesicles, which grow out until they come in close contact
with the surface ectoderm. The distal end of the vesicles
widens out to form the retina of the eyes, the stalk giving
rise to the optic nerve.

THE DEVELOPMENT OF THE FROG iug

Fig. 36. — Sagittal sections through two embryos. In A the blastopore
is overarched and there is the beginning of the proctodeum or anal
invagination. In B the proctodeum has met and fused with an
evagination of the archenteron. A, anus ; FB, forebrain ; HB,
hindbrain ; LV, liver diverticulum ; MB, midbrain ; N, notochord ;
NT, neurenteric canal ; PD, proctodeum ; PR, pharynx ; PN, pineal
body; PT, pituitary body, (From Morgan, after Marshall.)

110

THE BIOLOGY OF THE FROG

The anterior wall of the forebrain produces a pair of
pouches, the cerebral hemispheres, which finally become the
largest part of the brain. Their cavities, the lateral ven-
tricles, communicate with the third ventricle by an opening,
the foramen of Monro,

so
Sp

Fig. 37. — Cross section of a frog embryo. AR, arehenteron ; MS,
mesoblastic somites ; N, notochord ; JSS, neural crest ; M, medullary
tube ; PR, pronephros ; SN, subnotochordal rod ; SO, SP, somatic
and splanchnic mesoderm. (From Morgan, after Marshall.)

The nerves arise as paired outgrowths both from the brain
and cord, pushing their way between the cells of the other
organs, dividing and ramifying, as they push outward
toward the various parts they supply. The spinal nerves
begin as two independent outgrowths, representing the
dorsal and ventral roots; these soon unite into a single
nerve.

THE DEVELOPMENT OF THE FROG

111

The lining of the mouth cavity is formed from an invagin-
ation of ectoderm, the stomodeum, which pushes in until it
breaks through into the archenteron. A similar ectodermal
invagination, the proctodeum, forms the lining of a small
part of the posterior end of the alimentary canal. The lens
and cornea as well as the retina of the eye, and the vesicle
of the inner ear, also take their origin from this layer.

Organs from the Entoderm. — The entoderm, or the germ
layer which is invaginated within the egg, gives rise to the
lining of the alimentary canal and of all organs which arise
as outgrowths from it. The first of these to be formed is the
liver, which at the beginning appears as an outpocketing
of the ventral side near the anterior end. The outpocketing
becomes folded and branched, being converted finally into
a number of clusters of tubules, all emptying into the com-
mon canal, the bile duct, which is produced by a lengthening
of the neck of the original outgrowth. A lateral outgrowth
of the bile duct forms the gall bladder. The cells lining the
terminal branches of the hepatic diverticula become the
secreting cells of the liver. The connective tissue, blood
vessels, and outer coating of the liver are derived from the
mesoblast.

The pancreas arises much in the same way as the liver,
but as a pair of outgrowths instead of a single one. They
form, however, a single organ, and their ducts later become
connected with the bile duct. Only the secreting portion of
the pancreas and the lining of its ducts are of entodermic
origin, the connective tissue, blood vessels, etc., arising, as
in the liver, from the middle germ layer.

The bladder arises as an outgrowth of the ventral side
of the alimentary canal, near the posterior end; its lining,
therefore, is of entodermic origin.

The lungs appear as a pair of pouches from the sides of
the esophagus. They make little growth until quite late in

112

THE DEVELOPMENT OF THE FROG 113

the life of the tadpole. The region of the esophagus from
which the lungs arise becomes depressed and partly sepa-
rated off from the part above to form the larynx, the mouth
of the depressed portion going to form the glottis of the
adult.

The gill slits in the frog appear in the form of five solid
outgrowths on each side of the anterior portion of the
archenteron. In section they are shown to be in the form
of a double fold such as would be produced if the walls of
a pouch-like diverticulum were brought into contact. At
their outer ends the slits come into contact with the ecto-
derm, with which they fuse. The first two slits are the first
to form; the others appear in order from before backward.
When the tadpole escapes from the jelly into the water, the
walls of the solid gill slits separate, the ectoderm breaks
through at the outer end, and a free communication is estab-
lished between the throat and the outside. The first slit,
the hyomandibular, does not break through to the outside;
its entodermic lamellae separate and form a pouch which
communicates with the pharynx. In most forms the hyo-
tnandibular cleft forms the Eustachian tube and its covering
over its outer end, the tympanic membrane ; but in the frog,
according to Marshall, the Eustachian tube has a different
method of origin. (See " Vertebrate Embryology, " p. 143.)
The four following slits are known as the branchial clefts;
of these the second and third open first, then the first, and
finally the fourth.

The thyroid gland begins as a longitudinal groove along
the floor of the pharynx. It gradually sinks below the sur-
face and becomes converted into a solid, elongated mass of
cells. Later it divides into right and left portions, which
are completely separated.

The thymus, according to Maurer, arises by a sort of
budding process from the epithelium of the dorsal end of

114

THE BIOLOGY OF THE FROG

the first branchial cleft. The end then separates from its
point of origin and becomes carried backward, finally lying
behind the tympanic membrane. The thymus is relatively
larger in young frogs than in older ones. Other bodies of
similar epithelial origin from the gill clefts are, according to
Maurer, the post-branchial bodies, the epithelial bodies, and
the pseudothyroid ("ventraler Kiemenrest").

The entoderm forms only the inner portion of the alimen-
tary canal and its diverticula. The connective tissue, mus-
cular, and peritoneal layers are derived from the mesoblast.
For the most part it is composed of but a single layer of
cells. The epithelium of the mouth and a small portion of
the cloaca are produced by the ectoderm, these being the
only portions of the lining of the alimentary canal not of
entodermic origin.

Organs from the Mesoderm. — The development of the
mesoderm has been traced to the stage in which it consists
of two double-layered sheets of tissue extending from the
notochord above to the ventral side of the body. The two
sheets of mesoderm are separated by the notochord except
for a short distance in front of and behind this structure,
where they become continuous across the middle line. A
division soon occurs in the mesoderm, separating a dorsal
portion, known as the vertebral plate, from a ventral part,
called the lateral plate. The former becomes divided trans-
versely into a number of blocks called myotomes, or muscle
segments. Each of these becomes thickened so that the cen-
tral cavity becomes reduced in size and finally disappears.
The division of the vertebral plate into segments begins in
the neck region of the embryo and proceeds backward.
The segments soon become separated from each other by
septa of connective tissue which assume the form of a V
with its apex pointing toward the anterior end of the body.
The myotomes are easily seen at the sides of the body of a

THE DEVELOPMENT OF THE FROG

115

young tadpole, especially in the region of the tail. The cells
of the myotomes elongate in a direction parallel with the
long axis of the animal and become converted into muscle
fibers.

The two layers of the lateral plates become widely sepa-
rated by the enlargement of the intervening body cavity
or coelom. The inner or splanchnic layer becomes closely
applied to the entoderm of the archenteron and forms the
supporting tissue and musculature of the alimentary canal
and its diverticula. The outer or somatic layer comes to lie
against the outer ectoderm and forms the inner portion
(connective tissue, muscle, and peritoneum) of the body
wall. The innermost portion of both the somatic and
splanchnic layers of mesoblast become differentiated as a
separate layer, the peritoneum, which is continuous all
around the body cavity. As the right and left halves of the
coelom arise independently and gradually extend toward the
mid-ventral line, they are separated for a time by a median
ventral partition. This subsequently breaks down along
most of the length of the alimentary canal, putting the two
sides of the body cavity in connection with each other. The
median partition persists, however, for a short distance
anteriorly, forming the vertical membrane which extends
from the liver and pericardium to the ventral body wall. A
still smaller portion occurs between the body wall and the
ventral side of the cloaca.

The heart and pericardium take their origin from the
mesoderm near the anterior end of the ventral side 0/ the
body. A pair of fissures appears in the sheet of mesoderm
in this region; these gradually enlarge and extend toward
the middle line. The layer roofing over these fissures be-
comes raised up on either side, and the two folds thus formed
meet each other above, forming a sort of tube. Within this
tube are inclosed some scattered cells which arrange them-

116

THE BIOLOGY OF THE FROG

selves into a layer that becomes the endothelial lining of the
heart. The cavity outside the tube becomes the cavity of
the pericardium, and the tube itself thickens and becomes
transformed mainly into the heart, but its outer layer gives
rise to a thin sheet of tissue, the visceral portion of the peri-
cardium. The tissue which at first connects the heart with
the ventral side of the pericardium becomes broken through,
and the two sides of the pericardial cavity become continu-

PE

Fig. 39. — A, B, C, three stages in the development of the heart. E,
endothelium ; PE, pericardium ; PH , pharynx ; TP, wall of heart.
(After Morgan.)

ous; the dorsal connection of the heart disappears at a later
period. The visceral layer of pericardium which closely
invests the heart becomes reflected upon the sides of the
surrounding cavity, where it becomes continuous with the
parietal layer, the relations of the two parts being essentially
the same as that of the portion of peritoneum surrounding
the alimentary canal and that lining the ccelom. Owing to
its increase in length the heart becomes bent in the form of
an S; anteriorly it becomes continued into the truncus

THE DEVELOPMENT OF THE FROG 117

arteriosus, which divides into two branches which proceed
toward the gills, where they break up into the aortic arches,
which distribute branches to the gill filaments. The blood
vessels first appear as lacunae or spaces between the cells of
the mesoderm; the spaces enlarge, become continuous, and
the cells surrounding them take on a definite arrangement
and form the walls. The blood corpuscles arise either from
cells originally inclosed in the vessels or from cells budded
off from the lining membranes.

In the development of the renal organs there first appears
on each half of the body a temporary organ known as the
pronephros, which later disappears without contributing to
the formation of the permanent kidney. The duct of the
pronephros, or segmental duct, arises, according to Field,1 as
a thickening of the mesodermic wall of the body cavity. It
becomes hollowed out secondarily, and at its anterior end
it divides into three tubules which open into the coelom.
Posteriorly the duct joins the cloaca. The tubules increase
in length and become more convoluted, and the duct itself
in the region of its tubules becomes bent and twisted owing
to its increase in length, but its hinder portion remains
straight. The mouths (nephrostomes) of the tubules become
lined with cilia which carry material into the canals.

The pronephros, which is the functional kidney of early
larval life, is replaced by the mesonephros, or Wolffian body,
which is the renal organ of the adult. The mesonephros
makes its first appearance as a series of small tubules on
either side of the body, between the aorta and the segmental
duct. The tubules are at first solid, but they soon acquire a
lumen which communicates with that of the segmental duct,
with which they fuse. Their distal ends become swollen out
into a sort of sac, one wall of which becomes pushed in by a
knot of blood vessels or glomerulus derived from the renal

1 Field, Bull Mus. Comp. Zodl. Harvard, Vol. 21. 1891.

118

THE BIOLOGY OF THE FROG

Fig. 40. — Horizontal section through an advanced embryo. AR,
archenteron; BR1, BR2, BR3, branchial arches; H\ H\ H3, gill
slits ; HB, hyoid slit ; HM, hyomandibular cleft ; HY, hyoid arch ;
IN, infundibulum ; OF, olfactory pit ; OS, optic stalk ; P, proneph-
ros ; 8, segmental duct. (From Morgan, after Marshall.)

THE DEVELOPMENT OF THE FROG 119

arteries, thus forming the Malpighian bodies found in the
adult kidney. Owing to their growth in length the tubules
become contorted; branches are given off which later open
into the ccelom by funnel-shaped ciliated mouths, or nephro-
stomes, but the latter soon lose their connection with the
tubules and acquire secondarily an opening into the branches
of the renal veins in the ventral part of the kidney. The
tubules increase to a very large number and become richly
supplied with blood vessels; they form with the connective
tissue which binds them together a compact mass which
assumes the form of the kidney of the adult. The segmental
or pronephric duct which served as the outlet of the pro-
nephros is worked in to form the Wolffian duct or ureter of
the adult. The Miillerian duct was formerly supposed to
arise by a splitting of the segmental duct, but according to
MacBride,1 Marshall,2 Gemmill,3 and more recently Hall,4 it
develops quite independently of that structure.

The reproductive organs first appear as ridges of the
peritoneum near the base of the mesentery (Marshall) . As
the genital ridges increase in size they become constricted
at their points of attachment, and finally hang supported by
a peritoneal membrane. In the male the testis becomes con-
nected with tubes which grow out of the renal tubules and
form the vasa efferentia. The genital ridges in the two sexes
have a similar appearance until near the close of larval life,
when those of the female undergo a much more rapid growth.

The beginning of the vertebral column is represented by
the notochord, but this structure forms but a relatively small
portion of the backbone of the adult frog. Loose meso-
dermic cells, or mesenchyme, produced from the periphery of
the somite, collect around the notochord, forming a tubular

1 MacBride, Quart. Jour. Mic. Sci., Vol. 33, 1892.

2 Marshall, "Vertebrate Embryology."

8 Gemmill, Arch. f. Anat. u. Phys., Phys. Abth., 1897.
*Hall, Bull Mus. Comp. Zobl. Harvard, Vol. 45. 1904.

120 THE BIOLOGY OF THE FROG

investment. From the dorsal side of this mass ridges or
folds grow up and surround the spinal cord. The mesoderm
covering the notochord then becomes divided by transverse
septa which alternate with those between the somites, but
these do not cut across the notochord itself. The segments
they cut off represent the vertebrae; they soon become car-
tilaginous, and finally ossify. The cartilaginous sheath
grows inward at the ends of the vertebrae, constricting and
finally cutting through the notochord, so that in the adult
all that remains of this structure are small portions inclosed
wTithin the centra of the vertebrae.

Metamorphosis. — At the time of hatching the tadpole is
a fish-like creature, having a long, vertically flattened tail,
by means of which it swims through the water. The sides
of the tail show the markings of the muscle segments
through the skin. The flattened expansions of the integu-
ment on the upper and lower sides of the tail are thin and
nearly transparent, so that one may easily observe with a
microscope the blood flowing in the capillaries.

The mouth breaks through into the archenteron a few
days after hatching, the larva, previous to this time, living
at the expense of the food yolk in the alimentary canal.
The intestine increases very rapidly in length, and becomes
coiled in the form of a spiral, which may often be seen
through the ventral body wall. The external gills grow
rapidly after the tadpole is hatched, and soon are converted
into long, branching tufts. Three pairs of external gills are
developed, the posterior pair making its appearance after the
first two. The gill slits grow about the time the mouth is
fully formed, and the water which is taken in at the mouth
is passed through the gill slits to the exterior. In addition
to the external gills there are developed somewhat later four
pairs of internal gills, which are produced by foldings of the
membrane lining the gill slits. Both external and interna)

8

[G. 41. — Metamorphosis of Rana temporaria. 1, tadpole just hatched,
dorsal aspect ; 2, 3, older tadpoles, side view ; 4, 5, later stages, dorsal
views showing external gills and development of operculum ; 6, older
j tadpole, left side, showing single opening of operculum; 7, older stage,
right side, showing hind leg and anus; 8 and 10, lateral view of two later
stages showing development of hind legs ; 9, dissection of tadpole to
show internal gills, spiral intestine, and anterior legs developed within
operculum ; 11, advanced tadpole just before metamorphosis ; 12, 13, 14,
stages in metamorphosis, showing gradual resorption of tail; 15, juvenile
frog after metamorphosis. (From Newman, redrawn after Leuckart
Nitsche wall chart.) *

121

122

THE BIOLOGY OF THE FROG

gills receive an abundant blood supply from the vessels that
form the aortic or branchial arches. The disappearance of
the external gills is associated with the growth of a fold,
the operculum, which arises on either side of the head and
gradually extends backward. The free posterior edge of the
fold fuses with the body behind and below the gill region,
leaving only an open space on the left side of the body,
which is known as the spiracle. The water which passes
out of the gill slits comes into a chamber bounded exter-
nally by the opercular wall, and thence passes through the
spiracle to the outside. Soon after the completion of this
chamber the external gills disappear and the internal gills
function in their stead.

The jaws of the tadpole are furnished with horny coatings
which function as teeth, but these are shed in later larval
life. In addition both upper and lower lips also contain
transverse rows of fine teeth, which vary in number and
arrangement in the different species. Around the outside
of the lip there are numerous small papillae, which also vary
considerably in tadpoles of different species of frogs. The
nasal pits do not break through into the mouth until some
time after hatching. The eyes are situated on the dorsal
side of the head, and look obliquely upward. There are
several rows of sense organs on the skin of the tadpole,
but these disappear when the animal assumes a terrestrial
mode of life. The ventral sucker in the recently hatched
larva is in the form of a horseshoe. The ectodermic cells
covering it are partly glandular, and they form a mucous
secretion by means of which the larvae adhere to various
objects. Later in larval life the sucker becomes divided in
two in the middle. The two parts become carried farther
back on the ventral side of the head, and gradually decrease
in size, and finally disappear.

The hind limbs, which are the first ones to appear, bud

THE DEVELOPMENT OF THE FROG

123

out as small papillae on either side of the base of the tail.
They gradually increase in size, become jointed in structure,
and later bud out the toes at the distal end. The fore
limbs develop in much the same manner; the left limb
passes through the spiracle, the right one pushing through
the wall of the operculum.

Towaid the end of the larval period the tail begins to dis-
appear; its tissues break down and are resorbed, serving,
doubtless, as food material for building up the other organs
of the body. During the transformation of the tadpole into
the young frog, the intestine shortens, the mouth becomes
much wider, and the horny jaws are shed, the tongue in-
creases greatly in size, the legs grow rapidly, the rounded
body changes in form, and the gills become resorbed; the
lungs then develop rapidly, and the tadpole frequently comes
to the surface for air.

The food of the tadpole is mainly vegetable matter.
Spirogyra and other algae are common articles of diet; ani-
mal food, however, is greatly relished. Tadpoles will feed
eagerly on decaying insects, earthworms, or almost any kind
of meat. They will also eat bread or fruits; there are few
things, apparently, in the way of food, which they disdain.

REFERENCES

The most complete accounts of the development of the frog are
contained in Morgan's book, "The Development of the Frog's Egg,"
and Marshall's "Vertebrate Embryology." A more condensed account
is to be found in the small work on "The Frog," by the latter author.

More recent accounts of the development of the frog may be
found in the following works: —

Jenkinson, J. W. Vertebrate Embryology. Oxford and London,
1913.

Kellicott, W. Chordate Development, N. Y., 1913.
McEwen, R. Vertebrate Embryology, N. Y., 1923.
Reese, A. M. Introduction to Vertebrate Embryology, N. Y.;
1904,

124

THE BIOLOGY OF THE FROG

The following papers deal with the characteristics and metamor-
phoses of tadpoles: —

Barfurth, D. Versuche iiber die Verwandlung der Froschlarven.
Anat. Anz., Bd. 1.

Boulenger, G. A. A Synopsis of the Tadpoles of European Batra-
chians. Proc. Zool. Soc. London, 1891.

Camerano, L. Observationi sui girini degli Anfibi anuri. Boll.
Mus. Torino, 8, 1893.

Copeland, E. B. Heterogeneous Induction in Tadpoles. Science,
n. s.. Vol. 13, 1900.

Hinkley, M. H. On Some Differences in the Mouth Structure of
Tadpoles of the Anurous Batrachians found at Milton, Mass. Proc.
Bos. Soc. Nat. Hist., Vol. 21.

Ryder, J. A. The "Ventral Sucker" or "Sucking Disks" of the
Tadpoles of Different Genera of Frogs and Toads. Am. Nat., Vol. 22.

For descriptions of tadpoles of different species of Anura see the
references to Dickerson, Storer, and Wright in Chapter II.

CHAPTER VI

HISTOLOGY OF THE FROG

Since Schleiden and Schwann promulgated the cell theory
in 1838-1839 we have been accustomed to regard organisms
as composed of little units or cells. Most cells of the body
of the higher organisms are united to form tissues which
are aggregations of cells of similar character bound together
by means of an intercellular substance. In the bodies of
animals the classes of tissues commonly distinguished are
the following: —

1. Epithelial.

2. Connective.

3. Muscular.

4. Nervous.

These broad divisions include nearly all the manifold
variety of cells occurring in the body. The blood and lymph
are sometimes added as forming a distinct class of tissues,
sometimes classed as a form of connective tissue with fluid
intercellular substance, and sometimes treated of as if they
were not tissues at all. They will be described in a later
chapter.

In the epithelial tissues the cells lie in layers with only a
small amount of intercellular substance. We meet with this
class of tissue on the surfaces of organs, or lining the cavi-
ties of organs, and forming the lining of glands, ^blood^
vessels, and ducts of all kinds. The various kijads of epithe-
15urn~are distinguished according to the shapes of the cells.
An excellent example of flattened or squamous epithelium

125

126

THE BIOLOGY OF THE FROG

may be obtained in the outermost skin which is cast off
during the molt. The cells of this layer are broad and ex-
ceedingly thin, and show a rounded nucleus near the center.
The cells of the peritoneum are mostly of the same flattened
type. In columnar epithelium the cells are elongated per-
pendicularly to the surface and are usually prismatic in
outline, owing to mutual pressure; such epithelium is com-
mon in the mucous layer of the in-
testine. In many places, as in the
outer skin, there may be all transi-
tional stages between columnar epi-
thelium and squamous epithelium.
Layers such as this which are sev-
eral cells deep are called stratified
epithelium.

In some parts of the body there
Flfu 42'TA Por^10o of occurs a peculiar variety called cili-

the epidermis of Rana ^ . *

pipiens. s, stoma cell, ated epithelium in which the cells are
furnished with cilia at their outer
ends. Usually such cells are columnar, but they may
be cuboid or even somewhat flattened. Ciliated epithe-
lium occurs in the mouth and throat of the frog, in cer-
tain parts of the peritoneal lining of the body cavity,
on the inner lining of the oviducts, in the mouths of the
ciliated funnels of the kidney, in the ventricles of the
brain, and, in early larval life, on the outer surface of the
body. If the roof of the mouth of a frog be scraped with
a knife and the cells removed and examined under a micro-
scope, a shimmering movement may be seen on one side of
each cell. This is due to the rapid movement of the cilia or
fine hairlike processes on the surface. The cilia of all the
cells of a particular area beat most strongly in one direction
and the effect of this common movement is to create a cur-
rent which carries small objects in the direction of the

HISTOLOGY OF THE FROG

127

ciliary beat. The action of cilia may easily be demonstrated
by sprinkling some powdered carmine on the roof of a frog's
mouth. Soon one may observe that the substance is slowly
carried backward down the esophagus into the stomach.

The connective tissues embrace a large number of tissues
whose general function it is to support and hold together
the various other parts of the body. While in the other
kinds of tissue the intercellular substance is relatively very
small in amount, in the connective tissues it is usually very
abundant. Nearly all of the connective tissue is derived
from the middle germ layer, or mesoderm. It arises chiefly
from scattered cells, or mesenchyme, and in the early stages
of its differentiation the amount of intercellular substance is
very small, and of a jelly-like consistency. The intercellular
substance becomes modified in various ways in the different
varieties of connective tissue. In some cases it remains
soft, in others it becomes fibrous, in bone it becomes hard-
ened through deposits of carbonate and phosphate of lime.
The principal kinds of connective tissue found in the frog
are the following: —

White fibrous connective tissue is the variety which has
the widest distribution. A good example of this may be
obtained from the membranes which connect the skin with
the body wall. If a portion is spread out on the slide and
examined with the microscope, it will be seen to be made up
of a clear homogeneous portion, or matrix, of a gelatinous
substance in which are imbedded numerous fibers ; the fibers
are usually unbranched and have a characteristic wavy
appearance. They are frequently united in bundles which
run in all directions. When treated with acetic acid, they
swell up and disappear, and when boiled, become converted
into gelatin. Scattered among the white fibers there are
generally a few yellow elastic fibers; these are straight and
not wavy; they are not affected by acetic acid and do not

128 THE BIOLOGY OF THE FROG

yield gelatin when boiled; they frequently branch, and
when cut across, the ends do not curl like those of the white
fibers. Imbedded in spaces of the matrix here and there
are the connective tissue corpuscles or cells. These cells
vary considerably in their form and in the appearance of
their cytoplasm; usually they are branched, and the
branches of neighboring cells often unite or anastomose,

the frog, c, connective tissue corpus- ing the muscles to

posed of sheets and strands intersecting each other in al
planes. It forms a coating or fascia for each muscle, an
toward the ends of the muscles it is frequently modified int
tendon which is very dense and inelastic, and mainly com
posed of fibers, all of which lie in one direction. The loos
tissue of lymphatic glands belongs to a variety called ade
noid, which is composed of an irregular network of sheet
-and strands forming a fine meshwork which supports th

Fig. 43. — Fibrous connective tissue from

forming an irregular
network, the meshes
of which are filled
with the intercellular
substances. These
processes of the cells
run in canals which
allow a circulation of
the fluid among the
spaces or lacuna? in
which the cells lie
White fibrous tissue
varies greatly in con
sistency and textur
in different parts
The loose tissue bind

Cies ; e, elastic fibers ; w, white fibers.
(After Parker and Parker.)

gether is called areo
lar tissue, and is com

HISTOLOGY OF THE FROG

129

cells. The ligaments uniting the bones together are formed
of a very dense and inelastic variety of white fibrous tissue.
Modifications of the same kind of tissue occur in the cutis
of the skin, in the-submucosa of the alimentary canal, in the
substance of glands and the capsules surrounding various
organs.

Adipose tissue may be regarded as a form of connective
tissue in which many of the cells have become enlarged
through being gorged with fat; the nucleus with a small
amount of protoplasm lies to one side of the cell, and the
cell wall and a thin pel-
licle of protoplasm sur-
round the globule of fat.
In its early stages the fat
cell may contain several
isolated droplets of oily
substance which as they
grow coalesce into a single
large mass.

Cartilage is a dense
massive variety of con-
nective tissue. In the
clear hyaline cartilage
which is the predominant
variety in the frog, the matrix appears transparent and
homogeneous, although under proper treatment it may be
shown to contain numerous fibers which ordinarily are not
evident. The cells are contained in rounded spaces or
lacunae, scattered irregularly through the matrix; in some
cases minute channels have been observed connecting the
neighboring lacunae together. Two or more cells are often
found in one lacuna, a fact which indicates that they have
recently arisen by the division of the parent cell. Each cell
causes the deposit around it of intercellular substance; and

Fig. 44. — Cartilage from the head of
the femur, c, cells ; c', cells in
process of division ; c. s, empty
cell space ; m, matrix. (After Par-
ker and Parker.)

THE BIOLOGY OF THE FROG

the cells separated by cleavage soon form a partition be-
tween each other which gradually increases in thickness and
presses the cells farther and farther apart. The outer sur-
faces of cartilages are covered by a layer, or perichondrium,
which consists of an outer fibrous membrane, below which
are connective tissue corpuscles, which, as the cartilage
grows, sink into the matrix and become transformed into
ordinary cartilage cells. Hyaline cartilage occurs at the ends
of the bones of the limbs, between the vertebrae and at the
ends of their transverse processes, at the tip of the urostyle,
in the pubis of the pelvic girdle, in the hyoid and the carti-
lages of the larynx, and at both ends of the sternum; it forms
the basis of the cranium and the central axis of the lower jaw.

Calcified cartilage, which contains a deposit of lime salts
in the matrix, occurs in the suprascapula, the pelvis of old
frogs, and at the ends of some of the larger bones of the
limbs; viz. the heads of the humerus and the femur.

The structure of bone is similar to that of cartilage in that
it contains cells imbedded in a solid matrix. In bone the
matrix is rendered firm by the deposit of carbonate and
phosphate of lime. By immersion in acid the lime salts may
be removed and a cartilaginous body having essentially the
same histological structure as bone remains. Bone, how-
3ver, is not merely calcified cartilage; it differs from it both
histologically and chemically. Cartilage is often the precur-
sor of bone, but in such cases the former is broken down
and bony tissue built up in its place.

Two principal varieties of bone are usually distinguished,
— compact bone, which is very firm and dense, and spongy
or cancellous bone, which is made up of plates and bars
forming a structure which is comparatively loose and lack-
ing in strength. The latter is found within the center of
the vertebrae and to a small extent within some of the long
bones. A good example of compact bone may be obtained.

HISTOLOGY OF THE FROG

131

by making a cross section of the femur. The central part
of the bone is hollow and filled with marrow, and the outer
surface is covered by a layer of periosteum, which is similar
in structure to the perichondrium surrounding the cartilage.
The bony substance is arranged in concentric layers, or
lamellce, which contain numerous lacunce, in which lie the
bone cells. From the lacunae fine branching tubes, or
canaliculi, containing processes from the bone cells, are
given off which ex-
tend in all directions
and anastomose
with the canaliculi
of neighboring
spaces.

Bones increase in
thickness by the ad-
dition of successive
layers to the outside.
The osteoblasts, or
cells forming the in-
ner layer of the pe-
riosteum, give rise
continually to new
bone c e 1 1 s which
cause the deposition
of new layers of bony substances between the periosteum
and the old bone. New layers may also be added from with-
in by a layer of cells lining the inner surface of the walls
of the marrow cavity.

Muscle is composed of elongated cells or muscle fibers
united by connective tissue. Two varieties of muscle are
commonly distinguished, the striated, or so-called voluntary,
and the unstriated, or involuntary. In the latter the cell
structure is relatively simple; the fibers are commonly

Fig. 45. — A part of a cross section of the
femur of the frog, c, canaliculi ; Ic, la-
cunae ; Im, lamellse; m, marrow cavity.
(After Parker and Parker.)

132

THE BIOLOGY OF THE FROG

7LU

spindle-shaped, with a single nucleus near the center, which
is usually elongated in the direction of the fiber. The ends
of the libers are sometimes branched, but they are more
commonly entire. The length of the unstriated muscle
libers varies greatly; they may be very narrow and attenu-
ated, as in the walls of the bladder, or short and com-
paratively thick, as in the walls of
th^ smaller blood vessels. While
the fibers usually show no cross
striation, the cytoplasm shows deli-
cate longitudinal strands, or fibril-
Ice, which are considered by most
investigators to be the contractile
elements of the cell. The cell wall
is very thin and transparent. In its
action unstriated muscle is slow; a
considerable time elapses before it
responds to a stimulus, and it is
also slow to relax. It is found in
those parts of the body where there
is little occasion for sudden move-
ment. It occurs in the muscular
coats of the alimentary canal, in
the walls of the blood vessels and of
many ducts, in the lungs, urinary
and gall bladders, around many of
the glands of the skin, and in the
iris and ciliary muscle of the eye. It
is concerned in the production of slow movements, like the
contractions of the intestine, the expansion and contraction
of blood vessels, the change in shape of the pupil of the eye.

The fibers of striated muscle are more complicated in
structure. They possess several spindle-shaped nuclei,
scattered about through the cell, each of which is surrounded

Fig. 46. — Unstriated mus-
cle fibers from the in-
testine of the frog. nuy
nucleus. (After
Howes.)

HISTOLOGY OF THE FROG

133

by a small amount of unmodified cytoplasm. There is a
thin, but well-defined, cell wall, or sarcolemma, which is
best seen in places where the contents of the fiber are crushed
or broken apart. Each fiber of voluntary muscle is to be
regarded as a single cell, with numerous nuclei scattered
about through its cytoplasm. In its early stages of develop-
ment a voluntary muscle cell possesses but one nucleus. As
the fiber grows, the nucleus divides repeatedly, but as the
cytoplasm does .not
divide at the same
time, there come
filially to be numer-
ous nuclei within the
limits of a single cell
wall. The cytoplasm
shows both a longi-
tudinal striation,
and a cross striation
consisting of alter-
nate lights and dark
bands. The longi-
tudinal striation is
due to the existence
of minute strands,
the sarcostyles or
fibrillce, which ex-
tend the length of
the cell. The fibrillse, which are supposed to represent the
contractile elements of the fiber, are separated by a semi-
fluid substance, the sarcoplasm. There is an arrangement
of the fibrillae into bundles, the muscle columns, which are
separated from each other by a thicker layer of sarcoplasm
than that between the fibrillae.

The appearance of cross striation is brought about by the

Fig. 47.— A, part of a fresh muscle fiber of
a frog ; B, the same after treatment with
distilled water followed by methyl green.
b, light bands ; d, dark bands ; n, nuclei ;
s, sarcolemma showing more clearly
where the fiber is broken. (After Par-
ker and Parker.)

134

THE BIOLOGY OF THE FROG

division of the fibrillae into segments, or sarcomeres. The
sarcomeres are separated from each other by a very fine
dark line known as KrauseJs membrane, which extends not
only across the individual fibrillae, but across the sarcoplasm
between the fibrillae of the fiber. Krause's membrane lies in
the center of a comparatively clear and lightly staining
band formed by the opposed ends of the two contiguous
segments. The middle portion of each sarcomere forms the
so-called dark band. Across the center of this band there
extends a second very delicate membrane, known as the
line of Hensen. When the fiber is relaxed, this line may be
seen to lie in the center of a comparatively light band, which
is usually not evident when the muscle is in a contracted
state. The dark bands of the muscle fiber are composed
of material which is anisotropic, or doubly refracting, while
the lighter areas on either side of Krause's membrane are
isotropic, or singly refracting, like the sarcoplasm. When
viewed with polarized light the differences between these
two substances are clearly brought out.

A transverse section of a muscle fiber presents the appear-
ance of a number of polygonal areas called Cohnheim's
fields, which represent the cut ends of the muscle columns,
the spaces between the fields being filled with sarcoplasm.
Each of the fields shows a dotted appearance, due to the
cut ends of the individual fibrillae.

The muscle fibers of the heart differ from both of the
above classes. They are cross-striated, but each fiber con-
tains but a single nucleus. Each muscle cell is furnished
with branches which connect with the branches of contigu-
ous muscle cells, so that the whole mass forms a sort of
network.

The tissue of the nervous system consists of nerve fibers,
and nerve, or ganglion cells. Each nerve is composed of
usually a large number of nerve fibers, held together by con-

HISTOLOGY OF THE FROG

135

nective tissue and surrounded by a common sheath. A
typical nerve tiber presents the following parts: a central
strand, or axis cylinder; a sheath of fatty substance around
this called the medullary sheath, or white substance of
Schwann; and a delicate external membrane, neurilemma,
or sheath of Schwann. At intervals constrictions occur,
called the nodes of Ranvier, where the white substance is
interrupted, although the axis cylinder and neurilemma are
continuous. Immediately beneath the neurilemma occur
the nuclei, each surrounded by a small amount of proto-
plasm. Each internodal segment, or space between twc
nodes of Ranvier, contains several oblique markings across
the medullary sheath, which are known as the incisures of
Schmidt.

The axis cylinder of a nerve is simply the elongated proc-
ess of a ganglion cell, and under high magnification is
found to be made up, much like a muscle cell, of very fine
fibrillse, with an intervening substance of more fluid con-
sistency. The white or medullary substance contains a large
amount of fatty material called myelin; if a fresh nerve
is placed in water, this substance will swell up and collect
in drops, giving the nerve a very irregular outline. The
medullary sheath is supposed to act as a sort of insulator,
like the coatings that are wound around an electric wire.

The nerve fiber, unlike that of muscle, is a composite
structure, being formed of cellular elements of diverse origin.
The sheaths of the nerve represent a series of cells which
have become applied to but have an entirely different origin
from the axis cylinder. The latter is always an outgrowth
of a nerve or ganglion cell and is always of ectodermic
origin. In the development of a nerve the axis cylinder is
always the first part to make its appearance; as it grows
out, pushing its way through the other tissues, it becomes
surrounded with nucleated cells which flatten out and form

136

THE BIOLOGY OF THE FROG

the neurilemma; the white substance appears at a compara-
tively late period. The cells forming the sheath of a nerve
arc of mesodermic origin; the nerve fiber being therefore a
structure derived from two germ layers.

The regeneration of nerve fibers which have been cut in
two shows an intimate dependence of the axis cylinder upon

ABC

Fig. 48. — Nerve cells and fibers of the frog. A, fresh nerve fiber. B.
nerve fiber with the myelin swollen through the absorption of water.
C, cross section of nerve fibers. D, ganglion cells, ax, axis cylin-
der ; ax.p, axis cylinder process of ganglion cell ; I.S, incisure of
Schmidt ; m.s, medullary sheath ; n, nucleus ; nl, neurilemma ; N.R>
node of Ranvier ; p.p, protoplasmic process of ganglion cell.

the ganglion cell from which it arises. The portion of the
axis cylinder distal to the cut, and consequently.no longer
connected with the nerve cell, degenerates, and becomes
replaced by an outgrowth from the proximal part which fol-
lows the track of the degenerating fiber until the structure
of the whole nerve is restored. This phenomenon is but a
special case of the general principle that a portion of a cell

HISTOLOGY OF THE FROG

137

cut away from the part containing the nucleus invariably
dies.

The nerve or ganglion cells are found in those parts
which are spoken of as the nerve centers; viz. the brain,
spinal cord, spinal ganglia, and the various ganglionic
masses of the sympathetic system. These centers are made
up of ganglion cells and their fibers, together with the con-
nective tissue which binds them together and the vessels
which supply them with nutriment and carry away their
waste products. Ganglion cells are generally irregular in
outline, with a nucleus near the center. Their cytoplasm is
granular and under proper treatment shows a network, the
strands of which are connected with the fibrillse of the nerve
fiber and other processes of the cell. Two kinds of proc-
esses are commonly distinguished: the axis cylinder proc-
ess, which acquires a sheath and forms a part of a nerve
fiber; and the protoplasmic processes, often several in num-
ber, which are shorter than the former and generally
branched. Nerve cells are designated as unipolar, bipolar,
or multipolar, according as they possess one, two, or three or
more processes. Unipolar ganglion cells are found in the
sympathetic ganglia.

CHAPTER VII

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS

One of the characteristics of all forms of life is the need
of food. The matter which composes the bodies of living
organisms is being continually broken down and eliminated
as waste products. New matter is consequently required
to make good the loss if the vital process be kept going.
In the frog a part of the material is taken from the oxygen
of the air and from the water absorbed through the skin;
but neither of these sources supplies the carbon, nitrogen,
and other elements which form essential parts of all living
substance. Life phenomena are associated especially with
certain compounds called proteins. These are complex
substances containing carbon, oxygen, hydrogen, and nitro-
gen, and, in many cases, also sulphur, phosphorus, calcium,
potassium, sodium, magnesium, iron, chlorine, iodine, and
occasionally other elements. Living substance, or proto-
plasm, is of protein nature, but it is probable that it is a
group of compounds rather than a particular compound
which we might express by a definite chemical formula.
This living matter is the subject of chemical changes which
are spoken of under the general term metabolism. The
synthetic or building-up processes by which this substance
is formed from simpler compounds are called anabolism;
the opposite, or tearing-down processes by which it is re-
solved into simpler substances are known as katabolism.
If an organism grows, it is evident that the anabolic side of
the process must predominate over the katabolic. If kata-

138

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS J 39

bolism predominates, or, in other words, if waste exceeds
repair, the organism must diminish in size.

Now the function of food is not merely to compensate for
the material which is broken down and eliminated, but to
afford the energy necessary to carry on the various activi-
ties of the organism. Food is to the body what fuel is to
a steam engine. The body is continually expending energy
in the form of heat. The amount of energy lost in this way
depends upon circumstances, and it may be comparatively
small when the temperature of the animal is only slightly
above freezing. But so long as life lasts there is some heat
produced, and this heat results from the breaking down of
some of the constituents of the body. Every movement
which the frog performs involves the expenditure of energy,
which must come ultimately from its food supply. An
organism has often been compared to a vortex which main-
tains its form, while the material of which it is composed js
subject to continual change. The matter composing the
tissues of an animal is not the same during successive years,
nor quite the same during successive days. It is being con-
tinually drawn through the vortex, where it gives up a part
of its energy for the maintenance of the vital processes.
The substances eliminated by an animal possess, therefore,
less energy than the food material taken in. The amount
of energy obtainable from a gram of any particular com-
pound, such as cane sugar, when it undergoes decomposi-
tion, may be measured with considerable accuracy. If we
measure the energy resulting from the splitting up of a cer-
tain amount of food substance and compare it with the
energy obtainable from an equal amount of material after
it has been eliminated from the organism, we should find
the energy of the latter to be much less in amount. If now
we could measure the energy expended by the organism in
radiating heat and performing work during the time this

140

THE BIOLOGY OF THE FROG

material is consumed, we should probably find it to be equal
to the difference between the potential energy of the food
and that of the eliminated products. All of our experience
goes to prove that the great law of conservation of energy
applies as strictly to organisms as to the phenomena of the
inorganic world. Living beings are not sources of energy
in themselves, but are dependent upon their environment for
energy as much as they are for the material composing their
bodies.

^n order that food material may be assimilated or built
up into the tissues of the bodies, it must be rendered soluble,
so that it can pass through the lining of the alimentary
canal into the blood and lymph, and from these fluids
through the walls of the cells in the different parts of the
body. This process of converting food into a soluble state
ready for absorption is called digestion./ There are certain
mechanical processes involved in digestion, such as (in
higher animals) chewing the food, moving it about by the
contractions of the walls of the stomach, and passing it along
the intestine by the peristaltic contraction of the walls. The
frog, however, like most lower vertebrates, does not chew the
food taken into the mouth, but swallows it whole down the
very distensible esophagus into the stomach, where it is
acted upon by the gastric juice/ The principal part of the
process of digestion consists in the chemical changes pro-
duced in food by the action of the various digestive fluids.
These changes are mainly of the nature of fermentations
caused by substances called enzymes, or ferments. What
the chemical nature of enzymes is still remains very much
in the dark, since they cannot be completely freed from their
association with other substances, but it has often been held
that they are some form of protein. They have the prop-
erty of causing chemical changes in other bodies without
suffering any, or at least but very little, destruction of their

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 141

own substance. A very minute amount of enzyme will
cause the fermentation of a very large amount of other
material. Near the freezing point the action of enzymes
is almost nil ; but with increase of temperature their action
goes on much more rapidly until a maximum is reached
beyond which further increase of temperature checks the
process. A temperature of 100° C. destroys the action of
most ferment entirely.

The substances which may serve as food are the proteins,
fats, carbohydrates, water, vitamines, salts of various kinds,
and a few other substances not falling into any of these
categories.

The proteins are the most essential of the food materials,
since they contain in addition to the carbon, oxygen and
hydrogen found in carbohydrates and fats, the element nitro-
gen, and in many cases a certain number of other elements
besides. The white of egg, muscle, in fact most animal foods
with the exception of fat, consist largely of different forms
of protein. In fats only carbon, oxygen, and hydrogen are
present, and the proportion of oxygen is small. Chemically,
fats are compounds of glycerin with some fatty acid.

The carbohydrates are compounds of carbon, oxygen, and
hydrogen, the two latter elements being in the proportion in
which they occur in water (H20) ; in other words, there are
twice as many atoms of hydrogen as of oxygen in each car-
bohydrate molecule. Sugar and starch are examples of this
class of food.

All of these classes of food are acted upon by specific fer-
ments, which render them soluble and capable of diffusing
through the walls of the alimentary canal. The action of
the different digestive fluids will be described in connection
with the organs by which they are produced.

The Esophagus and Stomach. — The esophagus is very
short and remarkably distensible, as is proven by the rela-

142

THE BIOLOGY OF THE FROG

lively large animals the frog
is capable of swallowing.
The inner surface is thrown
into longitudinal folds which
extend also into the stomach.
There is no sharp line of de-
marcation separating the
esophagus from the pharynx
on the one hand and from
the stomach on the other.
The anterior end of the
stomach is considerably
wider than the esophagus,
and the organ1 tapers gradu-
ally to the posterior or py-
loric end, where it is sepa-
rated by a constriction, the
pylorus, from the small in-
testine. The stomach lies
mainly in the left half of the
body, and is curved so that
the convex side is toward
the left. It is suspended
dorsally by a fold of peri-
toneum, the mesogaster, and
from the ventral side arises
a second sheet of peritoneum
(the gastro-hepato-duodenal
ligament) , which extends to
the duodenum and liver. The
wall of the stomach is much thicker than that of the esopha-
gus or the intestine. The inner surface is thrown into
several longitudinal folds, which become less prominent
posteriorly, and near the pyloric end entirely disappear.

Fig. 49. — Alimentary canal of
Rana esculenta. A, opening of
the rectum into the cloaca, CI;
Duy duodenum ; D, ileum ; t>
boundary between the latter
and the large intestine, R; HB,
urinary bladder ; M, stomach ;
Mz, spleen ; Oe, esophagus ; Pyy
pylorus. (After Wiedersheim. )

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 143

In a cross section of the stomach one may observe a very
thin outer layer composed of much flattened cells; this is
the serous coat or serosa, and it is formed by the peritoneum.
Within the serosa is a thicker layer, the subserosa, consist-
ing mainly of connective tissue. This layer has been fre-
quently described as a layer of longitudinal muscles, and it
has the appearance of such; but if treated with the proper
stains, it can readily be shown to be mainly connective
tissue. Some writers (Valatour, P. Schultze), have been
disposed to deny the existence of longitudinal muscles in
the frog's stomach. In sections across the cardiac end of
the stomach, however, one may detect a few muscle fibers
among the connective tissue, and in the pyloric end, accord-
ing to Gaupp, there are a few longitudinal fibers which are
continuous with those of the intestine.

Within the subserosa is a thick layer of circular muscles
which becomes thicker toward the pylorus. Internal to the
circular muscles is a layer of connective tissue, the sub-
mucosa, in which there are numerous blood vessels. The
tissue of the submucosa extends into the folds of the inner
coat. Between the mucosa and submucosa there is a thin
muscular layer, the muscularis mucosas, composed of an
inner layer of circular fibers and an outer stratum of longi-
tudinal ones.

The mucosa of the stomach is a thick layer composed of
glands embedded in a supporting matrix of connective tissue.
These glands represent invaginations of the epithelium
lining the inner surface of the stomach. They are elongated
tubular structures set very closely together, and frequently
more or less branched. The glands differ in structure at
the two ends of the stomach. In the cardiac region the
glands are very long, the mouth of the gland is quite deep,
and lined with elongated cells whose clear inner ends are
filled with a substance which probably forms mucus. Near

144

THE BIOLOGY OF THE FROG

the outer end of the gland the cells are more elongated,
like those of the surface epithelium; behind the clear sub-
tance the cytoplasm of the cells is granular, the nucleus is
elongated, and the outer ends are drawn out into a long
narrow process. Passing down the mouth of the gland the
cells become shorter, the nuclei more rounded, and the tail-
like processes finally almost disappear. In the neck regions
of the gland there are usually a few rather large cells con-

Fig. 50. — Glands of the stomach. A, from cardiac end ; B, from py-
loric end ; m, mouth ; n, neck ; b, body of gland.

taining a large clear vacuole which pushes the nucleus and
most of the cytoplasm to one side. It is usually in the
region of these clear cells that the glands branch. The cells
composing the body of the gland lie just below the clear
cells and present a very different appearance from the cells
lining the mouth and neck. They are polygonal in outline,
with large round or oval nuclei and granular cytoplasm ; the
lumen, or central cavity of the gland in this region, is very
small and at times almost obliterated. The lower ends of
the glands extend as far as the muscularis mucosae.

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 145

In the pyloric end of the stomach the glands are less deep.
The mouth of the gland, however, is relatively deeper than
in the cardiac end, but is lined by much the same kind of
cells. At the bottom of the gland there are several large
polygonal cells with very large clear vacuoles much like
the cells in the necks of the cardiac glands. Occasionally
there may be a few polygonal granular cells below these.
In general, however, the pyloric glands may be said to corre-
spond to the mouth and neck of the glands of the cardiac
end of the stomach. Like the latter, these glands frequently
branch, but the branching commonly takes place above the
body of the gland.

The histological structure of the esophagus resembles in
a general way that of the stomach. There is an external
layer of longitudinal muscles and an inner layer of circular
fibers, but both are comparatively thin. A muscularis
mucosae is lacking except close to the stomach, where it is
represented by a few scattered fibers. The mucosa is well
developed; the surface epithelium consists of cylindrical
mucous cells with ciliated cells scattered among them.

The glands of the mucous layer are comparatively large
and much branched; and in many cases the branches, which
may be as many as fifteen in number, redivide. Near the
mouth the glands are small in size, and toward the stomach
they become smaller again and more simple in structure.
The cells of the body of the esophageal glands have a granu-
lar appearance much like the corresponding cells of the
glands of the cardiac end of the stomach. The mouths of
the glands are lined with a short cylindrical epithelium
with occasional ciliated cells.

Gastric Digestion. — In the stomach the food is subjected
to the action of the gastric juice, which is secreted by the
glands of the mucosa. Gastric juice is acid in reaction from
the presence of a small quantity of free hydrochloric acid,

140

THE BIOLOGY OF THE FROG

and it contains also a ferment, pepsin, which acts upon the
proteins, converting them into soluble peptones. Neither the
fats nor the carbohydrates undergo digestion in the stomach.
By digesting out the protein portion of foods in which fats
and carbohydrates are contained the gastric juice helps to
render these substances more readily digestible by other
fluids.

The action of the gastric juice of the frog may be readily
demonstrated by siphoning off some of this fluid from the
stomach by means of a bent glass tube and placing in it a
small bit of the white of a hard-boiled egg. The piece of
egg after a time will be seen to be corroded, and finally it
will become entirely dissolved.

The secretion of the esophagus has a strong digestive
power, but its reaction is alkaline instead of acid, and it is
capable of acting only after it has been rendered acid
through mixture with the fluid of the stomach (Nussbaum).

When gastric digestion is completed, the food passes
through the pylorus into the small intestine.

Changes in the Glands during Digestion. — The changes
undergone by the glands of the esophagus and stomach have
been studied by Partsch, Swiecicki, Nussbaum, Griitzner,
Langley, and Sewall. In frogs which have been kept for
several days without food Langley found the cells of the
body of the gland to be enlarged so as practically to obliter-
ate the central canal. The contents of the cell are uniformly
granular and the cell outlines are very indistinct. "In one
to two hours after feeding the lumina begin to be obvious,
and the granules to disappear from the inner border of the
cells. . . . Up to the fifth hour these changes become more
and more marked, and at the same time the cells and the
remaining granules they contain become distinctly smaller,
and the cell substance stains more deeply. ... At the
period of maximum change the nucleus is much larger

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 147

compared with the cell substance than it is during rest; it
is still surrounded by finely granular protoplasm, and it is
sometimes placed near the outer border of the cells. The
return to the normal appearance begins about the fifth hour,
so that during the
greater part of the di-
gestive period the for-
mative processes go
on whilst the secre-
tory are still active.
In twenty-four hours
the glands have
nearly or altogether
returned to the hun-
gry condition." The
time and extent of the
changes produced in
the glands were found
by Langley to vary
enormously with the
amount of food given
and the state of the
frog. "If a frog is fed
with several worms
so that the stomach
is much distended
with digestible food,

Fig. 51. — Showing changes in the gastric
glands of the frog. A, gland from a
hungry frog which had not been fed for
five days. The cell outlines are indis-
tinct and the granules are scattered
throughout the cells. B, gland three
hours after a meal ; the granules have
disappeared along the inner border of
the cells ; lumen of the gland visible.
C, gland twenty-five hours after a
heavy meal ; the cells are shrunken and
not so full of granules. (After Lang-
ley.)

the changes are
greater and persist for a much longer time. ... In twenty-
four hours the glands, instead of having returned to the
hungry state, are still small and consist of somewhat small
cells with a more or less distinct inner non-granular border;
the lumina are frequently large. " The increase in the size of
the lumen is accompanied and probably caused by the

148

THE BIOLOGY OF THE FROG

decrease in the size of the cells. "In frogs to which an
excess of food has been given, the non-granular inner zone
is usually most obvious about the eighteenth or twentieth
hour after feeding. The cells there have increased and
are still increasing in size; the greater clearness with which
the non-granular zone can be seen is then probably due
to the net increase in the cell granules taking place more
slowly than the increase in the cell protoplasm."

Langley found that the effect of fasting in winter is not
very great ; the cells of the gland become somewhat smaller,
but they are fairly well filled with granules. If, however,
the winter frogs are kept warm, or if frogs at other times of
the year are kept for a long time in a fasting condition, the
cells shrink in size and become clear along the inner border
as they do after secretion.

The mucigen content of the cells lining the mouth of the
gastric glands is large in amount before and for some time
after a meal, but during the height of the digestive process it
becomes much diminished. The changes in the pyloric
glands are much like those in the mouth and neck of the
cardiac glands. "The maximum amount of mucigen is
contained by the pyloric and similar gland cells after a mod-
erately prolonged fast. The minimum amount of mucigen
is contained by these cells twelve to eighteen hours after a
heavy meal; it is then only with difficulty that the mucous
can be distinguished from the subcubical cells."

The changes undergone by the esophageal glands differ
somewhat from those of the glands of the stomach. Langley
and Sewall, and also Griitzner, found that in normal hungry
frogs the cells were granular throughout. Some time after
food is taken the granules begin to disappear near the outer
end of the cell; i.e. the end away from the lumen of the
gland instead of in the opposite end as in the gastric glands.
The outer clear zone thus produced increases in size as

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 149

digestion proceeds, and the whole cell grows smaller. "Aa
the outer zone increases, the granules in the inner end be-
come smaller. The diminution in the size of the granules is
very marked in cells in which the outer zone takes up the
larger part of the cell. . . .

"During the first hour and a half after feeding no distinct
change is to be seen. After this period a diminution in the
number of granules in the outer half of the cell becomes
obvious. Usually this is first seen in the glands close to
the stomach. The disappearance of granules in the outer
portion of the cell goes on so that a clear zone is formed.
The clear zone steadily increases until the sixth to twelfth
hour, or even later, the time varying with the state of the
animal and the amount of food given. The glands then
begin to become more granular, the time of complete re-
covery varies enormously: in some cases the glands are
throughout granular in twenty-four hours from the time
of feeding the animal, in others they do not become so for
several days."

If the frog is fed with pieces of sponge instead of food, a
secretion is set up both in the stomach and the esophagus,
the change being as a rule the greater, the larger the sponge.
Changes take place in the cells similar to those produced by
digestible food, but they occur much more slowly, beginning
generally only three or four hours after the sponge is placed
in the stomach; the granules begin to increase again in the
esophagus only after some days.

Of what significance are these changes in the granular
contents of the gland cells? It is evident that they have
something to do with the formation of digestive fluids of
the esophagus and stomach, and it is probable that the
granules are composed of a substance which is transformed
into pepsin. That they are not composed of pepsin itself,
but of some substance which has been called pepsinogen, is

THE BIOLOGY OF THE FROG

indicated by the following experiments. "If the esophagus
or stomach of a frog be placed in glycerin as rapidly as
possible after removal from the body, the glycerin extract
lias only a weak peptic power. If the esophagus or stomach
of a frog be kept moist for twenty hours before it is placed
in glycerin, the glycerin extract has a very much greater
peptic power. If the esophagus and stomach which has
been extracted with, say, 5 cu. cm. of glycerin for a week
be washed free of glycerin and treated with 5 cu. cm. of
dilute hydrochloric acid, then an enormously greater amount
of pepsin is found in the acid than is found in the glycerin
extract."

The amount of pepsin content is greatest in those glands
in which there is the greatest number of granular cells.
The pepsin content of the esophagus was found by Swiecicki,
Langley, and Sewall to be greater than that of an equal area
of the stomach. In the pyloric region, where the granular
cells are few in number, the pepsin content of the glands is
much less than in the cardiac end. Langley found that if
pieces of equal size were cut out of the esophagus, cardiac
end, middle, and pyloric end of the stomach, and the pepsin
content of each estimated, the power of converting protein
was much the greatest from the piece from the esophagus,
and became less respectively in the pieces from the other
regions named. Partsch, Nussbaum, Swiecicki, Langley,
and Sewall have all investigated the relative digestive power
of the glands well filled with granules and glands from which
the granules have mainly disappeared, and all agree that
the pepsin content of the former is much the greater.

Rapidity of Digestion. — The digestive processes of the
frog compared with those of the higher vertebrates proceed
slowly, due probably to the fact that the frog is a cold-
blooded animal. The length of time taken to digest a meal
varies with the amount of food. Langley found that a small

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 151

earthworm was digested in somewhat less than twenty-four
hours, but if several worms were given, they do not disap-
pear from the stomach until a longer period. The rate of
digestion, as Riddle has found,, is rapidly increased by a
rise of temperature from 25° C. to 30° C.

Structure of the Intestine. — The *small intestine begins
just behind the pyloric constriction, and runs forward as the
duodenum for some distance, when it turns abruptly back-
ward as the ileum, which after coiling about in an irregular
manner, widens out abruptly into the large intestine near the
posterior end of the body. The diameter of the small intes-
tine, which is nearly uniform throughout its course, is much
less than that of the stomach, and its walls are much
thinner. The intestine is fastened by a mesentery to the
mid-dorsal portion of the body cavity, and its duodena]
portion is connected to the liver and stomach by the previ-
ously mentioned remains of a ventral mesentery, the gastro-
hepato-duodenal ligament.

A cross section of the small intestine shows the following
layers: At the outside is a very thin coat of peritoneum
similar to that coating the stomach. Within this is a well-
marked layer of longitudinal muscle fibers; then comes a
thicker layer of circular muscle fibers, and within this the
submucosa; the latter is connective tissue layer containing
numerous blood vessels. There is no sharp line of division
between the submucosa and the connective tissue portion of
the mucosa; the latter is more dense, and contains more
cellular elements; between the mucosa and submucosa are
large, irregular lymph spaces which frequently extend into
the folds. The existence of a muscularis mucosae has been
affirmed by some investigators (Howes, Grimm, Langer,
Ecker), but others (Valatour, Heidenhain, Gaupp) were
unable to verify the observation. At most this layer can
consist of but a few scattered cells. In sections which I

152

THE BIOLOGY OF THE FROG

have studied there are connective tissue fibers just
below the epithelium which give an appearance very much
like that of a thin muscle layer, but I have been unable
to convince myself of the existence of muscle cells in that
region.

The epithelium of the mucosa consists of a layer of
cylindrical cells among which two varieties may be distin-
guished, the goblet, or beaker cells, and the ordinary type of
absorptive cells. The goblet cells may be distinguished by
the large, oval vacuole, the inner end of which is filled with
a transparent, more or less granular substance, which prob-
ably gives rise to mucus. The nucleus is situated near the
base of the cell, and the part between the nucleus and the
inner globule is constricted, and contains several small vacu-
oles (Bizzozero). The absorbing cells are narrow, with an
oval nucleus near the base; the outer free border is thick-
ened, and shows a cross striation. According to Bizzozero
the mucous cells do not arise from the transformation of
cells of the ordinary type, as Paneth maintains, but are a
distinct kind of cell. The young stages of the goblet cells
may be seen wedged in between the bases of the other cells,
and all intermediate stages between these and the mature
type may be traced.

Leucocytes are often found between the epithelial cells,
and also wandering cells of larger size with bodies of vari-
ous kinds in their protoplasm (Heidenhain, De Bruyne).
Wandering cells containing pigment have been found to
occur in the lower end of the small intestine (Oppel).

The mucosa of the small intestine is thrown into numer-
ous folds, but there are no true villi nor definite glands nor
crypts such as occur in the higher vertebrates. Just behind
the pylorus the folds take the form of an irregular network,
but a short distance farther back they become arranged in
two series of transverse semilunar plications the free edges

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 153

of which are produced backward, forming a double series
of pockets which tend to check the flow of food in the
direction of the stomach. The pockets are connected by
smaller folds which run mainly in a longitudinal direction.
Farther back, a little beyond the middle of the intestine,
the folds lose their regular arrangement, and in the posterior
third they assume a longitudinal direction.

The large intestine is composed of the same layers as the

Fig. 52. — Part of a cross section of the small intestine of the frog.
bl, blood vessels ; eg, goblet cells ; ep, ordinary epithelial cell ; e.s,
submucosa ; m.c, circular muscles ; m.l, longitudinal muscles ; pe,
peritoneum. (After Howes.)

small. The inner surface is thrown into folds, which at the
proximal end form an irregular network, but in the rectum
they become longitudinal. The epithelium of the mucosa
consists of cylindrical cells, among which numerous goblet
cells are to be found.

The Pancreas. — The pancreas is an elongated gland of
irregular shape situated between the stomach and the duo-
denum, and extending from the liver to within a short dis-
tance of the pylorus. It is traversed by the common bile

L54

THE BIOLOGY OF THE FROG

dud into which its ducts enter. Of these there is a principal
duct, and several smaller ducts from the portion of the
gland near the liver.

The pancreas is a much-branched tubular gland, the ter-
minal branches of the glands being often curved and twisted
in an irregular manner. The tubules are coated exter-
nally with a basement membrane, and held together by a
delicate connective tissue in which lie the blood vessels and
nerves.

The secretory cells of the tubules contain numerous
zymogen granules, which, when the frog is in a hungry state,
are found in great abundance, especially at the inner or free
end of the cell. These disappear after the animal is fed,
like the granules in the glands of the stomach. A peculiar
darkly staining body (paranucleus, nebenkern) is usually
found near the nucleus toward the outer, or basal end of
the cell.

The fluid secreted by the pancreas is alkaline, mainly
from the presence of sodium carbonate (Na2C03), and it
contains three ferments: steapsin, or lipase which causes a
splitting of fats into fatty acid and glycerin; amylopsin,
which converts starch into sugar; and trypsin, which con-
verts proteins into various simpler compounds and finally
into amino acids. Trypsin carries protein digestion farther
than it is carried by the pepsin of the gastric juice. Trypsin
differs also from pepsin in that it acts in an alkaline or
neutral medium; in a strongly acid medium its action is en-
tirely stopped.

The Liver. — The liver is a massive gland whose secre-
tion, the bile, is conveyed to the intestine through the bile
duct along with the fluid secreted by the pancreas. The
organ is of a dark reddish color, and is divided into a right,
a left, and a middle lobe. The middle lobe is small and
concealed from view by the heart. The left lobe is divided

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 155

by an oblique incision into an anterior and a posterior
portion, the latter occupying the middle of the posterior
part of the liver.

The greater portion
of the liver is covered
by a closely adherent
layer of peritoneum,
which is continued to
form attachments with
the pericardium, ven-
tral body wall, dorsal
body wall, and the
stomach and intestine.

The bile duct is
formed by the conflu-
ence of the hepatic
ducts leading from the
lobes of the liver. The
gall bladder lies on the
dorsal side of the liver,
between the right and
left lobes. It is
rounded or oval in out-
line, and usually ap-
pears green from the
color of the bile seen
through its thin walls.
The gall bladder is
connected with the cys-
tic ducts, the one leading to one of the hepatic ducts, the
other joining the common duct farther down, usually within
the substance of the pancreas.

The histological structure of the liver differs considerably
from that of the pancreas, although both organs are to be

Fig. 53. — Liver and pancreas of frog.
Dc, common bile duct ; Dcy, cystic
ducts ; Dh, Dh1, hepatic ducts, which
with the cystic ducts combine to form
the common bile duct ; G, gall blad-
der ; L, L1, L2, L3, lobes of the liver
turned forwards ; Lhp, hepatoduo-
denal ligament ; M, stomach ; P, pan-
creas ; P\ pancreatic ducts entering
the common bile duct ; Py, pylorus.
(After Wiedersheim.)

150

THE BIOLOGY OF THE FROG

regarded as much-branched, tubular glands. The terminal
branches inclose the ultimate ramifications of the hepatic
ducts, or bile capillaries. These capillaries come to branch
and anastomose in an irregular manner so as greatly to ob-
scure the original tubular structure of the organ.

The bile capillaries may be surrounded by five or six cells
in cross section, or they may run between but two cells; they
also give off lateral branches which penetrate the cell bodies.
The secretory cells of the liver are cubical or polyhedral in
form, with large nuclei; the cytoplasm contains protein
granules, small drops of fat, lumps of glycogen, and often
pigment.

The liver receives blood from two sources: (1) the hepatic
artery, which conveys arterial blood, and (2) the portal sys-
tem, which includes the anterior abdominal vein from the
ventral body wall, and the portjgllvein, which receives blood
from the stomach, intestine, pancreas, and spleen. The
materials absorbed by the blood from the organs of diges-
tion pass, therefore, through the liver before entering the
general circulation. All of the blood leaves the liver by
the hepatic veins, which lead, from the dorsal side of that
organ to the posterior vena cava.

The liver is well supplied with lymph vessels which form
perivascular lymph spaces around the capillaries.

The liver of the frog generally contains a considerable
amount of pigment. Two forms of pigment occur, accord-
ing to Leonard, the black or dark brown, and the golden.
A certain amount of pigment granules occurs in the ordinary
cells of the liver parenchyma, but most of this substance is
found in pigment cells which are scattered about through
the whole organ.

Eberth held that the pigment cells lie within the blood
vessels, and that they resulted in large part at least, from
the transformation of leucocytes. Ponflck and Leonard

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 15?

regard them as lying outside the blood vessels in the peri-
vascular lymph sinuses. Braus, however, finds pigmented
cells both in the blood and in the lymph vessels.

Fig. 54. — Three phases of the hepatic cells of the frog. A, cells rich
in glycogen taken from a frog during winter. There are numerous
proteid granules around the lumen, and several larger fat globules
toward the outer ends of the cells. B, cells poor in glycogen
taken from a winter frog that had been kept for ten days at a tem-
perature of 22° C. The proteid granules are scattered uniformly
throughout the cell. Much the same appearance is presented by the
hepatic cells of a frog in summer. C, cells taken from a frog starved
for a long time in summer. The cells are shrunken and the glyco-
gen has almost disappeared. (From Foster's Physiology, after
Langley.)

There is no evidence that the pigment cells are derived
from the ordinary secreting cells of the liver (Oppel).
Colorless amoeboid cells have been observed in the lymph
spaces of the liver, and it is not improbable that a large part
of the pigment cells may result from the accumulation of

15S

THE BIOLOGY OF THE FROG

pigment by such cells which have wandered into the liver
from other sources.

The secreting cells of the liver present different appear-
ances in relation to changes in their activity. The granules
of the cells were found by Langley to increase in number
after a meal. "The changes are much more marked when
the cells have, to start with, a small outer non-granular
zone; in such cases in the 6th to 8th hour of digestion, the
outer zone is large, and in the 24th to 30th, the cells become
granular throughout." The decrease of granules was found,
as a rule, to be accompanied by an increase in the glycogen
in the cells, and vice versa. From analogy with the be-
havior of similar granules in other gland cells, Langley con-
siders the granules in the liver to be concerned in the
secretion of bile. Lahousse finds that granules disappear
from the cell almost entirely eleven or twelve hours after
feeding. Five or six hours after food is given the liver cells
are considerably enlarged, and the capillaries congested.
By the eleventh hour after feeding the congestion has dis-
appeared, and the cells diminish somewhat in size.

Functions of the Bile. — The bile, which is secreted by
the cells of the liver, makes its way by means of the gall
capillaries to the hepatic ducts, and thence into the gall
bladder, where it is stored until food passes out of the
stomach, when it is discharged through the common bile
duct into the intestine. Bile is an alkaline fluid of complex
composition. Some of its constituents, such as the fatty
substance, cholesterin, and the bile pigments, are simply
waste products, but others play a certain part in the diges-
tion. In higher vertebrates it has been shown that the
bile helps to emulsify fats and facilitates their absorption
from the intestine.

Intestinal Digestion and Absorption. — The food, when
it is passed from the stomach into the duodenum, possesses

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 159

an acid reaction due to the acidity of the gastric juice with
which it is mixed. In the duodenum it becomes mixed with
the pancreatic juice and bile, both of which are alkaline, and
its acidity is neutralized. The proteins which may have
escaped the fermentative action of the pepsin in the
stomach, together with the peptones which result from
peptic digestion, are acted upon by the trypsin of the pan-
creatic juice and converted into simpler compounds, chief
among which are the amino acids. The starchy constituents
of the food are converted into sugar by the secretion of
the pancreas, and the fats are emulsified and split into fatty
acids and glycerin by the pancreatic ferment lipase.

The role of the intestinal juice in the frog is little known,
but in the mammals it has several important digestive func-
tions. It contains a ferment, erepsin, which converts pep-
tones and other products of protein digestion into amino
acids; it contains a ferment which splits the more complex
sugars, the disaccharids, into the simpler monosaccharids,
dextrose, lsevulose and galactose; it secretes a substance
which activates the protein-splitting enzyme of the pan-
creas; and it produces still other substances which play an
important part, directly or indirectly, in digestive processes.

When the various constituents of the food are digested,
or rendered soluble by the action of the digestive juices,
they are absorbed through the walls of the intestine into the
blood and lymph. In the higher vertebrates most of the
fat is taken up by the lymph vessels of the intestine, and it
is generally held that a large part of the sugar and products
of protein digestion is absorbed by the capillaries of the
blood vessels. Whether the absorbed materials follow the
same course in the frog is not certainly known.

Probably but d small fraction of food is absorbed by the
stomach; most of the cells of the lining of that organ are of
the secretory type. The inner surface of the intestine is

100

THE BIOLOGY OF THE FROG

especially adapted for absorption on account of the large
number of folds it contains which give an extensive surface
for contact with the food. The numerous blood and lymph
vessels near the epithelium of the mucous layer afford ready
means of transport of substances which diffuse into them
through their walls.

The Glycogenic Function of the Liver. — One of the
principal functions of the liver is the formation of glycogen,
a carbohydrate, having the same empirical formula as
starch, C6H10O5. This substance is, in fact, often referred
to as "animal starch," and it possesses several points of
resemblance to the starch found in plants. It is soluble in
water, forming a milky white solution. When treated with
iodine its solution gives a reddish, port-wine color. In its
dry state it forms a white powder.

Glycogen occurs in the cells in the form of granules or
even lumps of considerable size. Its presence may be
detected by staining with iodine sections of liver prepared
by hardening the organ in absolute alcohol and then embed-
ding it and cutting without allowing the tissue to pass
through water. In this way the glycogen may be prevented
from dissolving out. Glycogen may be prepared by throw-
ing the liver of a recently killed frog into boiling water,
then grinding it up with sand in a mortar, extracting with
water and filtering. A milky fluid will thus be produced
which can then be evaporated until the residue is obtained,
which is largely glycogen.

If the liver of a frog be left for some hours before boiling
and then tested for glycogen, it will be found that the
amount of this substance obtained is comparatively small,
and if appropriate tests be applied, it may be shown that a
certain amount of dextrose has appeared in its stead. The
liver contains a ferment which has the power of converting
glycogen into dextrose; as the ferment is destroyed by

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 161

boiling, a greater amount of glycogen can be obtained from
the liver if it is boiled soon aftei it is removed from the
body.

The glycogen content of the liver not only increases in
the fall and decreases in the spring and summer, but it
undergoes changes in relation to variation in the amount of
food, and to changes of temperature of short duration.
After feeding there is a slight increase in the amount of
glycogen in the liver; this slowly disappears if the frog is
kept several days without food. In winter, if frogs in which
the liver is well filled with glycogen be kept for a few days
in a warm room, the glycogen content of the liver rapidly
decreases. On the other hand, if summer frogs, which gen-
erally contain little glycogen, be kept at a low temperature
for several days, the amount of glycogen in the liver be-
comes markedly increased.

The glycogen stored in the liver may be given out slowly
into the blood in the form of dextrose, into which it is
changed by an enzyme in the hepatic cells. The liver acta
as a sort of reservoir of food, storing it up in a compara-
tively insoluble form when it is in excess, and expending it
gradually to tide over periods of fasting. The frog begins
its long* period of hibernation with a large reserve supply of
this material, which is slowly used up during the winter
and more rapidly consumed in the early spring.

While glycogen occurs in greatest abundance in the liver,
forming at times over 8 per cent of the weight of that organ,
it is found also in many other organs of the body,
The muscles contain a considerably less percent of gly-
cogen than the liver; but owing to their much greater
bulk their total glycogen content may exceed that of the
Jiver. although it is usually less. Smaller quantities of
glycogen are found in the ovaries, central nervous system,
and skeleton.

162

THE BIOLOGY OF THE FROG

Periodic Changes in the Liver. — The liver of the frog
undergoes important changes in relation to food and tem-
perature. There is a regular seasonal change which affects
not only the size and general appearance of the organ, but
also the amount of pigment contained in it, and the contents
of the secreting cells. In the summer the liver is usually
large, comparatively light in color, and furnished with little
pigment (Weber, Eberth, Leonard). In the winter and
early spring, before the feeding period, the liver becomes
relatively small in size and dark in color, the number of
pigment cells increases, and there are more pigment granules
contained in the secreting cells. Miss Leonard, who has
made a study of the percentage of pigment in relation to the
whole mass of the organ in different times of the year,
arrives at the following result: —

November. .7 per cent June, 2.77 per cent

December, 4.13 per cent July, .68 per cent

April, 11.12 per cent

It may thus be seen that the relative amount of pigment
contained in the liver increases through the winter, then
diminishes in the spring after the period of feeding.

The same observer found that in winter and early spring
the average size of the secreting cells and also their nuclei
was smallest in early spring, and increased during the
summer as is shown in the following table: —

November

December

April

June

July

Average diameter

of cells
Average diameter

of nuclei

.0292 mm.
.006 mm.

.0162 mm.
.0044 mm.

.012 mm.
.0076 mm.

.0172 mm.
.0065 mm.

.0274 mm.
.0065 mm.

Similar measurements by Funke gave results approximately
the same as those obtained by Miss Leonard. The mini-

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 163

mum size of the cells in R. temporaria according to Funke
occurs in May, the maximum in July and August. In
R. esculenta the minimum falls in June and the normal
size is reached two or three months later, but there is no
well-defined period of maximum growth. In both species
the minimum size of the liver cells as well as the liver as a
whole occurs at the time of breeding.

The fat content of the liver was found by Funke to vary
in an irregular manner both in R. esculenta and R. tem-
poraria. In the first species the fat content of the liver in
many instances almost entirely disappeared in June. Dur-
ing the summer fat is stored in the liver, and in the winter
it suffers very little diminution if it does not actually in-
crease in amount. In R. temporaria the amount of fat in
the liver is very small compared with that in R. esculenta,
and no definite conclusion could be drawn regarding the
general course of its seasonal changes. According to Lang-
ley's observations upon frogs in England, "the fat in the
liver cells reaches its maximum amount in February and
March. In January it is as a rule somewhat less. In April
it rapidly decreases, from May to December it is present in
comparatively small though varying amounts. It is usually
present in minimum amount in September and October."

Miss Leonard found that the relative proportion of blood
vessels to the whole mass of the liver varies in different sea-
sons. The following table represents the percentage of
area of cross sections of blood vessels in relation to the
whole areas of the sections studied, during different seasons
of the year: —

November, 17.23 per cent June, 9.82 per cent

December, 10.105 per cent July, 6.58 per cent

April, 7.47 per cent

Comparing this with the previous tables, it will be seen
^hat as the size of the cells of the liver increases, the relative

164

THE BIOLOGY OF THE FROG

proportion of blood vessels and pigment to the whole mass
of the liver decreases.

Variations in glycogen contents of the liver at different
times of year have been studied by several investigators
(Langley, Luchsinger, Von Wittich, Barfurth, Langendorff
and Mozeik, Athanasiu). In the spring during the breeding
season the amount of glycogen is at its minimum, there be-
ing often scarcely a trace of this substance in the liver cells.
After the frog begins to take food glycogen slowly accumu-
lates, but during the active life of the animal in summer it
is not stored in the liver in any great quantity. In the fall,
when the weather becomes cooler and the frogs less active,
the glycogen becomes much increased in amount. During
the winter sleep glycogen is used up only to a slight extent,
but as the temperature rises on the approach of spring, and
the sexual products are maturing, the store of glycogen is
rapidly diminished. Athanasiu, who has investigated the
amount of glycogen in the whole body of the frog (R.
esculenta) at different seasons, finds that the minimum
quantity (slightly over one tenth per cent of the body
weight) occurs in June, then there is a slow accumulation
until September, when there is a rapid increase to the maxi-
mum (1.43 per cent of the body weight) followed by a
slow diminution during the winter and then a rapid falling
off in the spring. The amount of glycogen in the liver alone
was found to increase and decrease along with that of the
body as a whole. The amount of glycogen in the liver was
found to be more variable than the glycogen content of
the rest of the body, exceeding the latter in the fall and
early winter, while in the spring the reverse relation obtains.

The variations in the weight of the liver as a whole have
been studied in detail by Gaule in Rana esculenta. The
weight of the liver was found to be relatively greater in
males than in females and to possess a somewhat greater

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 165

range of seasonal variations. The following table taken
from Gaule's estimates shows the weight of the liver per
gram of body weight in the two sexes during the different
months of the year: —

Male

Female

05775

.0430

0436

.0382

0502

.0348

0370

.0323

.0232

0244

.0214

July

0317

.0311

0360

.0360

0710

.0509

0571

.0574

0723

.05882

06001

.0567

The numbers in the table represent the average weights
of the livers and bodies of a number of individuals (usually
15 to 25) sacrificed for each determination. The variations
in the size of the livers are thus shown to correspond in
general to the variations in the glycogen content. In
November the liver may become between two and three
times as large as it is in June.

Fate of the Different Kinds of Food. — The functions of
food, as we have seen, are to build up tissue and to supply
the organism with the energy for carrying on its vital pro-
cesses. Only the proteins are capable by themselves of
forming living tissue, as they alone possess all the necessary
elements. The fats and carbohydrates, however, are also to
a certain extent tissue builders, but they can supply only
three of the elements of living matter; namely, carbon5
oxygen, and hydrogen.

166

THE BIOLOGY OF THE FROG

The fat stored in the cells of adipose tissue may be ob-
tained from fat contained in the food, but it may also be
derived from carbohydrates and even from proteins.

The principal functions of both fats and carbohydrates is
the production of energy. These compounds are split up
and oxidized to carbon dioxide and water, yielding energy
in this way for the performance of bodily movements and
the maintenance of the temperature of the animal. Energy
is also derived from the breaking down of proteins, so that
it may be said that all of the principal classes of foods are
tissue builders and also energy producers. After the food
stuffs have played their part and become broken down into
simpler compounds, they are eliminated from the body
through the organs of respiration and excretion.

REFERENCES

Contejean, C. Sur la digestion stomachale de la grenouille. C. R.
Ac. Sci., Paris, T. 112, 1891.

Dewevre. Note sur la fonction glycogenique chez la grenouille
d'hiver. C. R. hebd. Soc. Biol., Paris, 1892.

Eberth, C. I. Die Pigmentleber der Frosche und die Melamie.
Virchow's Archiv, Bd. 40, 1867.

Grutzner, P. Ueber Bildung und Ausscheidung von Ferment en.
Arch. ges. Phys., Bd. 20, 1879.

Grutzner und Swiecicki. Bemerkungen iiber die Physiologie der
Verdauung bei den Batrachiern. Arch. ges. Phys., Bd. 49, 1891.

Heidenhain, M. Ueber die Structur der Darmepithelzellen. Arch,
mik. Anat., Bd. 54, 1899.

Heidenhain, R. Untersuchungen iiber den Bau der Labdrlisen.
Arch. mik. Anat., Bd. 6, 1870.

Langley, J. N. On the Histology and Physiology of the Pepsin-
forming Glands. Phil. Trans. Roy. Soc, Vol. 172, part 3, 1881.

Langley and Sewall. On the Changes in Pepsin-forming Glands
during Secretion. Jour. Phys., Vol. 2, 1880.

Leonard, A. Der Einfluss der Jahreszeit auf der Leberzellen von
Rana temporaria. Arch. Anat. u. Phys., phys. Abth. Suppl. Bd., 1887.

Moraczewski, W. von. Die Zuzammensetzung des Leibes von

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 167

hungernden und blutarmen Froschen. Arch. Anat. u. Phys., Suppl.
Bd. 1900.

Nussbaum, M. Ueber den Bau und die Thatigkeit der Driisen.
Arch. mik. Anat., Bd. 13, 15, 16, and 21.

Oppel, A. Lehrbuch der vergleichenden mikroskopischen Anatomie.

Partsch, C. Beitrage zur Kentniss des Vorderdarmes einiger Am-
phibien und Reptilien. Arch. mik. Anat., Bd. 14, 1877.

Riddle, O. The Rate of Digestion in Cold-Blooded Vertebrates.—
The Influence of Season and Temperature. Am. Jour. Physiol., Vol.
24, 447, 1909.

Stolkinow. Vorgange in den Leberzellen, insbesondere bei den
Phosphorvergiftung. Arch. Anat. u. Phys., Suppl. Bd., 1887.

Swiecicki, H. Untersuchungen iiber die Bildung und Ausscheidung
des Pepsins bei den Batrachiern. Arch. ges. Phys., Bd. 13, 1876.

Weber, C. H. Ueber die periodische Farbenveranderungen welche
die Leber der Huhner und der Frosche erleijdet. Bericht. Verh-
Konigi. sachs, Ges. Wiss. Leipzig, Math .-phys. CI.. 1850.

CHAPTER VIII

THE VOCAL AND RESPIRATORY ORGANS

In the vertebrate animals the vocal and respiratory
organs are intimately associated owing to the fact that the
production of sound is caused by the expulsion of air from
the lungs. With the exception of the sounds made by a few
fishes the voice makes its first appearance in the vertebrate
series among the Amphibia. In the Urodeles, or lowest
division of the group, the voice is, as a rule, feebly de-
veloped or entirely absent. It attains its maximum develop-
ment among certain of the Anura, but in not a few members
of this order it is small and weak.

The Vocal Apparatus: — The sound-producing organs of
the frog are located in a sort of box called the larynx, situ-
ated just below the pharyngeal cavity at the beginning
of the entrance into the lungs. The larynx opens into the
pharynx through the slit-like glottis above, and by a pair
of openings behind, into the lungs. It is held between the
stout, bony thyroid processes of the hyoid apparatus, to
which it is attached by muscles as well as connective tissue.
The skeleton of the larynx is composed mainly of the cricoid
and arytenoid cartilages. The former consists of a slender
ring surrounding the larynx and lying in nearly the same
plane as the thyroid processes of the hyoid, to which it is
closely attached; at its posterior end it is produced into a
spine which extends backward between the lungs. From
near the middle of. its ventral surface it gives rise to a sort
of loop, the tracheal process, which is bent backward and

Jfi8

THE VOCAL AND RESPIRATORY ORGANS 169

serves as a means of attachment for the necks or roots of
the lungs. The arytenoid cartilages are a pair of semilunar
valves, which rest upon the cricoid cartilage; their uppei
edges form the lateral margins of the glottis; they afford
attachment to muscles by which the glottis may be opened
or closed. The true sound-producing organs consist of a

Fig. 55. — Lungs of the frog. A. ventral vein, the right lung cut open
to show the inner surface ; h. hyoid. B, section through lung ;
a. artery; g. glottis; v. vein. (B, modified from Renaut.)

pair of elastic bands, the vocal cords, extending longitudi-
nally across the larynx. They may easily be seen from
above by spreading apart the two sides of the glottis, or
from below by removing the membranous floor of the laryn-
geal cavity. Their median edges are thickened and lie near
each other in the middle line. Sound is produced by
the expulsion of air from the lungs which sets the free
edges of the vocal cords in vibration. Variations in the

170 THE BIOLOGY OF THE FROG

sound are caused by altering the tension on the cords
through the action of the laryngeal muscles. The vocal
apparatus of the male frog is much larger than that of the
female.

The males of many species of Rana possess a pair of
rocal sacs situated at the sides of the pharynx. These sacs
are out-pocketings of the pharyngeal wall which extend
backward between the skin and the body. They communi-
cate with the mouth by small openings in the floor a short

Fig. 56. — Cartilages of the larynx of the frog. A, from above ; B,
from the side ; Ca, arytenoid cartilage ; (7. V- to C. I4, cricoid carti-
lage ; P, expansion of the cricoid ; Sp, spinous process of the cricoid ;
* * *, prominences of the arytenoids. (After Wiedersheim.)

distance in front of the angle of the lower jaw. Besides a
lining of mucous membrane they possess a muscular coat
which consists of fibers drawn out from the subhyoideus
muscle. The vocal sacs are distended during the croaking
of the frog through the pressure of the air in the buccal
cavity. They serve as resonators to reenforce the sound
produced by the vocal cords. They are absent in the
female. Their size in the males of Rana pipiens is very
variable; in some of the varieties of this species they are
absent entirely.

THE VOCAL AND RESPIRATORY ORGANS 171

The Lungs. — The lungs are ovoid, thin-walled sacs of
comparatively simple structure. They are capable of great
distension and may be readily inflated through the glottis;
they do not collapse when the body is cut open, owing to
the fact that the glottis under ordinary circumstances re-
mains closed. When air is let out of the lungs, they shrivel
to an inconspicuous size. The inner surface of the lungs is
divided by a network of septa into a series of small cham-
bers or alveoli, by means of which the amount of surface
exposed to the air is very greatly increased. The walls of
the alveoli are richly supplied with blood vessels which
break up to form a fine capillary network. The inner sur-
face of the alveoli is covered with a single layer of epithelial
cells which are very thin and flattened except on the edges
of the septa, where they become cylindrical and ciliated.
Outside the epithelium is a connective tissue layer which
contains the blood and lymph vessels, and numerous un-
striated muscle cells which give the lungs their great power
of contraction. The outer surface of the lungs is coated
with peritoneum.

The area of the inner surface of the lungs of Rana escu-
Icnta has been carefully calculated by Krogh. In a speci-
men weighing 40 g. it was found to be 98 sq. cm. The
total surface of the skin was estimated to be 154 sq. cm. in
the same specimen.

The Respiratory Movements. — Since the frog has no
ribs, it is unable to draw in air by enlarging the cavity con-
taining the lungs as the higher animals do, and it has
recourse to a more indirect method of inspiration. If one
watches the respiratory movements of a frog, it will be
seen that the floor of the mouth rises and falls at quite
regular intervals. Usually at somewhat greater intervals
there may be seen a contraction followed by a sudden ex-
pansion of the body wall; and accompanying the latter

172

THE BIOLOGY OF THE FROG

movement there is a brief closure of the nares. The respir-
atory movements of the frog fall into two classes: (1) the
oscillatory throat movements, and (2) the movements
directly concerned in filling and emptying the lungs. The
throat movements may continue for quite a long period,

Fig. 57. — Diagrams to illustrate the respiratory movements of the
frog. In A the floor of the mouth is depressed, the nares are open,
and air rushes through them into the buccal cavity. In B the floor
of the mouth is raised, the nares are closed, and air is forced from
the buccal cavity into the lungs, e.n, external nares ; gl, glottis ;
gul, gullet ; i.n, internal nares ; Ing, lung ; olf.s, olfactory chamber ;
pmx, premaxillary bone; trig, tongue. (After Parker and Parker.)

especially if the frog is kept quiet and where it is cool,
without any movements of the body or nares. During this
time the glottis remains closed and no air passes into or out
of the lungs. The nares are kept open, and air is drawn
through them into the buccal cavity as the floor of the
mouth is lowered, and forced out through them as the floor
nf the mouth is raised. These oscillating movements per-

THE VOCAL AND RESPIRATORY ORGANS 173

form two functions: (1) they are subservient to the respira-
tion which takes place in the mucous walls of the mouth and
pharynx, and (2) by renovating the air discharged into the
buccal cavity after each expiration from the lungs they en-
able comparatively pure air to be forced into the lungs again
at the next inspiration. The breathing in which the lungs are
involved is indicated by movements of the flanks, or regions
above and behind the fore legs. Certain small movements,
however, occur in these regions which appear to be inci-
dentally associated with the oscillatory movements of the
floor of the mouth and play no part in lung respiration; the
true flank movements are quite well marked. After each
drawing in of the flank or expiration there follows im-
mediately a swelling of the flank due to inspiration, but
there may elapse a considerable interval before the next
expiration occurs, so that the lungs are always filled with
air during the pause between successive respiratory acts.
Expiration is effected by the contraction of the muscles of
the body wall aided by the elasticity of the walls of the
lungs. During the act of expiration the glottis opens and
almost immediately afterwards closes. If the sides of the
body are cut open so that the lungs cannot be compressed
by the muscles of the body wall, air will be expelled, though
more slowly, every time the glottis opens. A frog thus
operated on is still capable of both inspiring and expiring
air, the mere elasticity of the walls of the lungs being
sufficient for the latter function.

In filling the lungs the buccal cavity acts as a sort of
force pump. As the floor of the mouth rises, the nares are
closed, the glottis opens, and the air in the buccal cavity
thus subjected to pressure and having no other avenue of
escape is forced through the glottis into the lungs. The
glottis then closes, and the movements of the floor of the
mouth may continue for some time before the next in-

174

THE BIOLOGY OF THE FROG

spiration takes place. The rising of the floor of the throat
and the closure of the nares take place almost at the same
time that air is expelled from the lungs; and the expansion
of the lungs follows almost immediately afterward. Much
of the air expelled from the lungs in expiration does not
escape from the buccal cavity, but is forced back into the
lungs again at the next inspiration. It is mixed, however,
with the comparatively pure air previously in the mouth
cavity. The valvular arrangement for closing the nares is
an essential part of the mechanism for filling the lungs.
It was formerly thought that the nares were closed by
special muscles attached to the valves, but it was shown by
Gaupp that this function is performed through raising the
tip of the lower jaw, thus elevating the premaxillaries and
thereby closing these openings. It may be readily shown
that the closure of the nares can be brought about in this
way by pressing upward against the premaxillaries with the
finger. (So long as its mouth is kept open the frog is unable
to close its nares; being unable to force air into the lungs,
such a frog will sooner or later die of asphyxiation^ During
all of the respiratory movements the mouth of the frog
is held tightly closed through the tonic contraction of the
muscles of the lower jaw. As has been explained in a
previous chapter, the tip of the lower jaw is independently
movable, owing to the existence of the small mento-
meckelian bones, which are opposed to the premaxillaries.
The contraction of the small submentalis muscle, which runs
transversely across the tip of the jaw, causes this part to be
raised above the general level and by pressing upward
against the premaxillaries closes the nares.

As air is forced into the lungs, the pressure in the buccal
cavity is indicated by the slight protrusion of the eyes and
tympanic membranes. Sometimes, however, when the frog
is making strong inspiratory efforts, the eyes are drawn

THE VOCAL AND RESPIRATORY ORGANS 175

inward during each gulp of air, thus aiding the process by
diminishing the size of the buccal cavity.

According to Baglioni the external aperture of the nares
does not remain closed during the last phases of the eleva-
tion of the floor of the mouth; nevertheless, air does not
escape from the buccal cavity, as may be shown by placing
the nose of the frog beneath water, when no bubbles arise
from the nostrils. The muscles which draw the hyoid ap-
paratus and tongue forward and upward cause the tip of
the jaw to be depressed when a certain position of these
organs has been reached and the nares open. Why, then,
does not air pass out of the nares as the floor of the mouth
continues to be raised? As Baglioni maintains, this is be-
cause the nasal passages are closed from behind by means
of the anterior processes of the hyoid cartilage, which are
so formed and situated that they fit neatly into the posterior
nares as the hyoid apparatus is drawn upward and forward
in the act of inspiration.

Changes in the Blood in Respiration. — The respiratory
movements that have been described are subsidiary to keep-
ing fresh air in close relation with the blood. On the one
hand we have the organs of respiration and the distribution
within them of the blood vessels, which are so arranged that
the blood is brought very close to the surface over a large
area. And on the other hand we have a complicated and
beautifully adaptive mechanism for keeping the large por-
tion of the respiratory surface included in the lungs in
contact with pure air. These devices facilitate the exchange
of gases which takes place between the air and the blood by
means of diffusion across the intervening membranes. The
blood receives oxygen from the air and gives off carbon
dioxide, so that the air which has been expired from the
lungs or buccal cavity always contains less of the former
&nd more of the latter gas. The process of respiration falls

176

THE BIOLOGY OF THE FROG

into two phases: (1) external respiration, or the exchange
of gases between the blood and the surrounding medium,
and (2) internal respiration, or the exchange of gases be-
tween the blood and the tissues. The metabolism of every
cell involves the consumption of oxygen which is received
from the blood, and as the result of the oxidation of com-
pounds of carbon which occurs throughout the body every
cell produces carbon dioxide, which is given off into the
blood. The blood, therefore, acts as a means of transporting
oxygen from the organs of respiration to the tissues and of
carbon dioxide from the tissues to the organs of respiration.
It thus serves as the medium between internal and external
respiration. The greater portion of oxygen in the blood is
carried by the red corpuscles in combination with hemo-
globin. This peculiar substance has the power of forming
a wreak and unstable chemical union with oxygen. As the
blood passes through the capillaries of the respiratory
organs, oxygen diffuses into it and combines with the
hemoglobin; when the blood reaches the tissues where the
partial pressure of the oxygen is diminished, the hemoglobin
parts with its oxygen to the surrounding cells. Hemoglobin
is a protein compound containing iron; it is readily soluble
in water and may be obtained by evaporation from its solu-
tion in the form of crystals. When combined with oxygen,
it assumes a bright red color, but when it loses its oxygen,
it becomes a much darker and more bluish tinge. It is to
this change in the hemoglobin that the difference in color
between arterial and venous blood is due. Blood that has
been oxygenated is bright red, while blood that has not been
purified has a much darker color.

The Respiratory Function of the Skin. — The skin of the
frog is an organ of respiration of the utmost importance.
During the winter when the frog lies buried in the mud it |
becomes practically the only respiratory organ. Frogs may

THE VOCAL AND RESPIRATORY ORGANS 177

be kept alive when submerged in water at 0° to 13° C. for
several days. At a higher temperature frogs tend to come
to the surface oftener for air, and if prevented from doing
so, they may die of asphyxiation. The skin functions as a
respiratory organ both in water and in air. If the nostrils
of a frog be plugged with wax, the animal may be kept alive
in cool air for several days.

The experiments of several investigators have shown that
more carbon dioxide is given off through the skin than
through the lungs. Klug found that the ratio of C02 given
off by the lungs to that given off through the skin varied
in the different specimens investigated from 1:2.5 to 1:4.46.
The frogs which Klug experimented upon wTere put in a
chamber divided by a partition which contained an aperture
surrounded by rubber. The frog was placed so that its
head projected through the partition, and was held tightly
by the rubber so that one chamber was completely shut off
from the other. Air was passed through both chambers,
and the amount of carbon dioxide given off into each
measured and compared. The one chamber received the
output from the skin only, while the other received that of
the lungs together with the small amount exhaled from the
skin of the head. The method of Klug was an improvement
over those of his predecessors, although not entirely free
from objections, the principal one being that the pressure
of the rubber necessary to produce an air-tight fit would
impede the normal movements of respiration. Experiments
of ligating or extirpating the lungs, removing the skin, tying
the cutaneous blood vessels, plunging the frog in oil nearly
up to its nostrils, etc., in order to eliminate one or the other
modes of respiration are all open to the same criticism
that they do not tell us anything of the relation of skin and
pulmonary respiration under ordinary conditions. If carbon
dioxide is prevented from escaping through the skin, more

ITS

THE BIOLOGY OF THE FROG

of it will be exhaled through the lungs, or if the lungs are
tied, more carbon dioxide will be eliminated through the
skin.

The relation between the cutaneous and pulmonary respi-
ration of the frog has recently been quite exhaustively
studied by Krogh. The lungs were supplied with air by
means of artificial respiration, and the income of oxygen
and the output of C02 from both the lungs and skin com-
pared under various conditions. In Rana fusca at a tem-
perature of 20° C. the average ratio of oxygen income to
C02 output in several experiments on frogs taken at dif-
ferent times of year was, in pulmonary respiration, 02 105 :
C0245; in cutaneous respiration 02 52 : C02 129. It is thus
evident that in the lungs the oxygen taken in is greatly in
excess of the C02 given out; while in the skin the reverse
relation obtains. In Rana esculenta relatively more oxygen
is taken in thiough the skin and relatively less C02 elimi-
nated through the lungs. The respiratory quotients (i.e.
ratio of 02 to C02) in the two species at 20° C. are as
follows: — -

Cutaneous

Pulmonary

Respiration

Respiration

R.

fusca

2.48 r.q.

.32 r.q.

R.

esculenta

1.92 r.q.

.32 r.q.

Influence of External Conditions upon Respiration. —

The respiratory functions uf both the lungs and the skin
vary in different periods of the year even when the animals
are placed under the same degree of temperature. The
amount of oxygen taken in by the lungs is greatest during
the breeding season; then it rapidly decreases during the
summer, and reaches its minimum in the winter, the ratios
of oxygen absorption at a temperature of 20° C. being as

THE VOCAL AND RESPIRATORY ORGANS 179

follows: spring, 134.5; summer, 82; winter, 54. The output
of C02 by the lungs varies in a similar manner (spring, 62;
summer, 42; winter, 16.5). The cutaneous respiration is
subject to much less seasonal variation; the absorption
of oxygen is practically constant; the elimination of C02
is considerably increased during the breeding period, but
for the rest of the year it varies but little. While the
amount of oxygen taken in by the lungs during the spring
and summer considerably exceeds that absorbed by the
skin, the cutaneous absorption of oxygen becomes much
greater than the pulmonary in the winter. In winter, there-
fore, the skin becomes relatively more important in respira-
tion than during the rest of the year.

Whether the skin functions more efficiently as a respira-
tory organ in air or in water the few and contradictory
results of Bohr and Krogh do not enable one to determine.
Few experiments have been made upon the relation between
temperature and the rapidity of respiration, although it is
known that respiration takes place much more rapidly when
the temperature is increased. At low temperatures respi-
ratory changes are slight.

Moleschott and Fubini have shown that light has a
marked effect upon respiration of the skin, the amount of
C02 produced at a given temperature being much greater
in the light than in the dark. This was held to be due in
part to a direct action of light upon the skin, because the
increase occurs in frogs whose eyes have been removed,
although to a less extent than in normal specimens. The
more refrangible rays have the greatest effect upon skin
respiration, as was shown by measuring the C02 output in
frogs exposed to differently colored lights. The ratios of
C02 production under violet, yellow, and red light were
found to be as 114, 103, and 100 respectively. In red light
there is but little more C02 output than in the dark. The

ISO

THE BIOLOGY OF THE FROG

influence of heat was excluded in the experiments by pass-
ing the light through a vessel of water. As frogs which are
placed in the light Lccume restless and excited and fre-
quently make efforts to go toward the source of illumination,
it is probable that these differences in respiration result
from variations in the animal's activity. The fact that the
phototactic activities of the frog become greater under the
more refrangible rays would naturally lead to a parallel
increase in respiration under the same conditions. That
differences in respiration occur in blinded frogs under dif-
ferently colored lights is not inconsistent with this interpre-
tation, since phototaxis still occurs in frogs from which the
eyes have been removed.

REFERENCES

JSabak, E. Die Mechanik und Innervation der Athmung. Winter-
stein, Handb. vergl. Physiol., Bd. 1, 2nd H, 706, 1913-1914.

Baglioni, S. Zum Athmungsmechanismus des Frosches. Arch. Anat.
u. Phys., phys. Abth., Suppl. Bd., 1900.

Berg, W. Untersuchungen iiber die Hautathmung des Frosches.
Inaug. Diss. Dorpat, 1868.

Bert, P. Des mouvements respiratoires chez les Batrachiens et les
Reptiles. Jour. Anat. et Phys., T. 6, 1869. Legons sur la physiologie
comparee de la respiration, Paris, 1870.

Bohr. Ueber die Haut- und Lungenathmung der Frosche. Skan-
dinav. Arch. f. Phys., Bd. 10, 1899.

Dissard, A. Influence du milieu sur la respiration chez la gre-
nouille. C. R. Ac. Sci., Paris, T. 116, 1893.

Gaupp, E. Zur Lehre von dem Athmungsmechanismus beim
Frosch. Arch. Anat. u. Phys., Anat. Abth., 1896.

Klug. Ueber die Hautathmung des Frosches. Arch. Anat. u. Phys..
phys. Abth, 1884.

Krogh, A. On the Cutaneous and Pulmonary Respiration of the
Frog. Skandinav. Arch. f. Phys., Bd. 15, 1904.

Martin, H. N. The Normal Respiratory Movements of the Frog
Jour. Phys, Vol. 1, 1878-1879.

THE VOCAL AND RESPIRATORY ORGANS 181

Milne-Edwards, H. De Pinfluence des agens physiques sur la vie,
Paris, 1824. Legons sur la physiologie et Panatomie comparee de
Phomme et des animaux, 1857-1865.

Moleschott and Fubini. Suir influenza della luce mista e
chromatica nelP esalazione di acido carbonico per P organismo
animale. Atti delP Acad. Torino, 15, 1879.

Regnault et Reiset. Recherches chimiques sur la respiration.
Ann. chem. et phys., Ser. 3, T. 26.

Schafer, G. D. Respiratory Exchange of the Frog. Science, n.s.,
Vol. 60, 179, 1920.

Wedenski, N. Ueber die Athmung des Frosches. Arch. ges. Phys.,
Bd. 25, 1881.

Willem, V. Les mouvements respiratoires chez la Grenouille.
Arch. Neer. Physiol. T. 3, 315, 1919. Observations sur la respiration
des Amphibiens. Bull. Ac. Roy. Belg. 1920, 298.

CHAPTER IX

THE SKIN

External Characters. — The skin is an organ of unusual
importance in the life of the frog, because, in addition to the
functions which it commonly performs among other animals,
it has a number of special functions which are peculiar to
the Amphibia, and which, in most cases, reach their fullest
development among the Anura^j)As in^most of Jjie Am-
phibia^the gkin^of the fr^g is smoofji^and moist; it is very
loosely att^che^l^
bands of connectiveTtiis^^

cutllieousnym^ very tough^but

it is considerably thickeFtin'the dorsal side^oFthe^odv than
it is below. In certain regions it presents special thicFen-
ings; such as the dermal plicae, which extend backward
from near the posterior angles of the eyes, the subarticular
pads beneath the joints of the digits of the feet, the swell-
ing at the base of the first finger of the arm, the protuber-
ance over the sixth toe or prehallux, and the upper eyelids
and lips. Small papillae often occur, especially on the dorsal
side of the body, some of which, the tactile papillae, are
permanent; others, the sexual papillae of the female, occur
only during the breeding period.

Histological Structure. — The skin is composed of two
principal layers, the epidermis, and the corium, or cutis. A
third layer of subcutaneous connective tissue, not belonging
to the skin proper, lies underneath the corium and forms
the septa uniting the skin to the body wall.

182

THE SKIN

183

The epidermis, or outer portion of the skin, is composed
of several layers of cells. The cells of the innermost layer
are columnar; but in passing toward the outer surface the
cells become more and more flattened, until those of the
outermost or horny layer (stratum corneum) become very
broad and thin. It is the stratum corneum that is shed
during the molting process. The gradual change in shape
between the cells of the inner and outer surfaces of the
epidermis is due to the fact that there is a continual pro-
duction of new cells in the inner layer which are gradually
pushed outward, becoming more and more flattened the
farther they are pressed away from their point of origin.

The epidermis, especially on the dorsal side of the body,
usually contains more or less dark brown or black pigment.
This pigment is partly within special cells, the chromato-
phores, and partly in and between the typical cells of the
epidermis. In certain regionsjillj^

mis may corill^ Ehrmann found*

that in the same region of epidermis pigment would appear
and disappear in the course of a few months. The chro-
matophores of the epidermis resemble the dark pigment
cells of the corium. Whether they are derived from cells
of the corium which have wandered into the epidermis, or
whether they arise through the transformation of cells of
the epidermis itself, is a matter of controversy. Loeb and
Strong 1 have come to the conclusion that the chromato-
phores that appear in the regenerated epithelium of the frog
are derived from epithelial cells, and not from cells that
have wandered in from the cutis. Chromatophores in the
epidermis are not usually abundant. The main source of
the color of the skin is in the pigment cells of the corium.

The inner layer of the epidermis contains several stellate
cells, which, according to Mayer, arise from the modifica-

1 Loeb and Strong, Am. Jour. Anat., Vol. 3, p. 275, 1904.

184

THE BIOLOGY OF THE FROG

tion of cells of the typical form, and, by acquiring pigment,
become later transformed into chromatophores. In the
outer portion of the epidermis occur scattered oval or flask-
shaped cells, the upper portion or neck of which lies just
beneath the stratum corneum. According to F. E. Schultze
they produce a secretion which passes between the stratum
corneum and the subsequent layer of cells and aids in
shedding the skin. Pfitzner, on the other hand, regards
them as degenerate epithelial cells which retain the me-
chanical function of holding the stratum corneum in contact
with the underlying layer. Modifications of the outer layer
or stratum corneum are found in the small stoma cells,
which are situated over the necks of the cutaneous glands.
The necks of these glands open to the surface through a
small triradiate aperture which is raised slightly above the
general level. This aperture has generally been regarded
as passing through a single cell (Harless, Ciaccio, Eberth,
Engelmann, Heidenhain, Nicoglu) , but, according to Junius,
what has been heretofore considered as one cell is really
made up of several, the boundaries between which have
disappeared.

The corium is separable into two layers, an outer com-
paratively loose layer (stratum spongiosum), which con-
tains most of the glands, and an inner layer (stratum com-
pactum), which is formed of very dense connective tissue.
The stratum spongiosum consists of a loose network of
fibrous connective tissue, richly supplied with lymph spaces
and blood vessels. Just beneath the epidermis it forms a
thin layer which contains numerous pigment cells. In the
deeper portion are embedded the glands. Thickenings of
the stratum spongiosum form the basis of the dermal papillae
mentioned above.

The stratum compactum is mainly composed of a cense
layer of connective tissue, whose fibers run in a wavy course

THE SKIN

185

parallel to the surface of the skin. At intervals this layer
is crossed by vertical strands, which often extend through
the stratum spongiosum into the epidermis. In addition to
fibrous connective tissue, these strands frequently contain
smooth muscle fibers, elastic fibers, nerves, and blood ves-
sels. It is probably due to the contraction of these muscle
fibers that the papillation of the skin is produced after cer-
tain conditions of stimulation. The fibers may also aid in
squeezing out the secretion of the cutaneous glands.

The subcutaneous connective tissue forms a loose layer
beneath the stratum compactum and a second very thin
layer next to the muscles, the two layers being separated by
large lymph spaces except in the septa, where they become
continuous. The outer of the two layers is very vascular
and contains numerous stellate cells, within which are nu-
merous grayish white pigment granules. These cells are
especially abundant on the ventral side of the body, where
they produce the white coloration characteristic of that
region.

Glands of the Skin. — The skin of the frog, like that
of most of the Amphibia, is richly furnished with glands.
These glands are of the simple alveolar type, and lie mainly
in the stratum spongiosum of the corium. Only rarely, as
in the large glands of the inner finger, do they extend into
the deeper portions of the skin. Typically the glands are
spherical or oval in form, and open to the surface through
a narrow neck which extends through the epidermis and
terminates in the triradiate opening of a so-called stoma
cell at its outer end.

The skin glands of the frog have been studied by numer-
ous investigators, but there remain the widest differences
of opinion regarding many of the most important features
of their structure and functions. Two varieties of gland are
commonly distinguished which may be designated as the

186

THE BIOLOGY OF THE FROG

mucus glands and the poison glands. While Heidenhain,
Nicoglu, and others regard these two types of gland as
specifically distinct, other investigators (Calmels, Ley dig,
Sczcesny, Junius) consider them as different phases in the
development of one and the same gland. However this
may be, the glands of the frog's skin may be grouped into
two classes which are structurally and functionally different,
and we shall describe them separately without regard to the
question as to whether they are genetically connected.

The mucus glands are smaller and much more abundant
than the poison glands, and are found over practically the
entire surface of the body. In some places they are so thick
that they nearly touch. In Rana fusca, according to Engel-
mann, they average about sixty to each square millimeter of
surface. Their ducts are narrow, and lined with a layer of
small flattened epithelial cells. The body of the gland is
lined with epithelial cells which form a single layer except
near the opening of the neck, where there are two layers.
It is this epithelium which forms the mucus which is dis-
charged into the lumen of the gland, and poured out through
the neck over the surface of the skin. The appearance of
the secreting epithelium varies greatly in different glands.
In some cases, more often in the smaller glands, the epi-
thelial cells are low, clearly marked off from each other, and
from the large lumen of the gland, and contain nuclei which
take up a large part of the cell. In other glands the cells
are elongated so that they fill a large part of the lumen; the
nucleus is relatively small, and situated near the base of the
cell, and numerous granules occur toward the free ends.
During secretion these granules swell up, and become con-
verted into a transparent substance which is discharged into
the central cavity (Biedermann) , and it is probable that
they represent a stage in the formation of mucus. Numerous
transitional stages between these two varieties of epithelium

THE SKIN

13?

occur, and it is quite certain that the differences are due to
the age of the glands, and their different states of secretion.
Changes in the form of the cells, however, are produced to
a certain extent by the con-
traction of the gland.

Outside of the epithelium is
a muscular coat composed of
smooth muscle cells which lie
in a meridional direction. The
outermost coat of the gland is
formed by a layer of fibrous
connective tissue. The func-
tion of the muscle cells is the
expulsion of the secretion of
the gland. The glands of the
skin are in constant motion
(Ascherson, Engelmann) , as
may be seen by an examina-
tion of the glands in the web
of the foot. They change not
only in size, but also in form,
being now rounded and now
wrinkled and angular. Con-
traction may be caused by
stimulation of the skin with
irritant solutions or by the
electric current.

The poison glands are
larger and less abundant than
the mucus glands, and less
uniformly distributed over the surface of the body. They
are more numerous on the dorsal side of the body and
hind legs, and they are especially abundant, and unusu-
ally large, in the lateral dermal plicae. According to

PS

Fig. 58. — Cross section of the
skin of the frog. D, dermis
or cutis ; E, epidermis ; b.v,
blood vessel ; c. gl, cutaneous
gland cut through the center ;

c. gV , the same from one side ;

d, duct of gland ; h.f, h.f.
h.f, horizontal fibers of con-
nective tissue : h.l, outer or
horny layer of the epidermis ;
m.l, Malpighian layer of the
epidermis ; pg, pigment cells.
(After Howes.)

1SS

THE BIOLOGY OF THE FROG

Junius, they occur on all parts of the skin, although
they may be comparatively scarce in certain situations.
Like the mucus glands they possess a muscular and a
connective tissue coat outside the layer of epithelium.
The chief differences in the two types of glands, with the
exception of size and the thickness of the tunics, lies in the
secreting cells. Engelmann described the epithelium as

M. G

M. G

•P. G

P. G

Fig. 50. — Section across a dermal plica of Rana esculenta. M. G,
mucus glands ; P. G, poison glands ; the granular epithelium has an
indefinite outline and shows no cell walls. (After Gaupp.)

consisting of cylindrical cells nearly filled with granules.
The boundaries of the cells apparently disappear under cer-
tain conditions of secretion, the epithelial lining forming a
continuous irregular layer of protoplasm (Gaupp).

The secretion of the poison glands is a whitish fluid with
a burning taste. It may be caused to exude from the skin,
especially of the bullfrog, by placing the animal under
chloroform. Of its properties in the frog comparatively
little is known. Paul Bert found that a goldfinch which was
inoculated with the dermal secretion of Rana esculenta died

THE SKIN 189

within one minute; a frog inoculated with the poison of
another frog of the same species died within an hour and a
quarter.

In many other Amphibia, especially the toads and sala-
manders, poison glands are very extensively developed, and
yield an abundant secretion.

Sex Differences. — The skin of the frog presents certain
differences characteristic of sex, some of which are perma-
nent, while others occur only during the breeding period.
In Rana jusca, according to Leydig, and in R. arvalis,
according to Steenstrup, the web on the hind feet of the
males is more fully developed than in the females. The
swelling on the inner side of the first finger of the male,
which has been mentioned in a previous chapter, is caused
by modifications both of the corium and the epidermis,
This swelling is much larger in the breeding period than at
other times, and it doubtless subserves the functions ot aid-
ing the male to retain hold of the female. The cutaneous
glands in this region are much enlarged, and become elon-
gated into a tubular form, and extend through the entire
thickness of the skin. The epidermis in the breeding period
is proliferated to form small papillae with a thick, rough,
horny layer. After the breeding period the epidermis
becomes smooth again, and there is also a partial disappear-
ance of the pigment of the corium, so that the swelling loses
its dark color.

The occurrence of dermal papillae in the female of Rana
fusca during the breeding period has already been suffi-
ciently described (see Chapter II). The males of certain
species assume at this time a blue coloration which appears
mainly on the ventral side of the body. In Rana arvalis
(R. oxyrrhinus) it has been described by Steenstrup and by
Siebold. In Ra?ia fusca Falio described a blue coloration
appearing on the throat during the breeding period. Leydig

190

THE BIOLOGY OF THE FROG

found that this color disappeared soon after the animal was
take n from the water. Both Leydig and Haller, who studied
the same phenomenon in Rana temporaria} regard the blue
as an interference color produced by minute granules in the
skin. It is probable that the appearance of the blue color
is associated with the absorption of water. Frogs which
have lost the blue color when kept in the air soon regain it
when placed in the water again. After the breeding period
is over, the blue color quickly disappears. A reddish brown
color during the breeding season has been described by
Leydig in the female of Rana fusca, and Smith has observed
a blue coloration of the throat which he regards as charac-
teristic of the female of that species at this time.

The skin of the male of Rana fusca in the breeding sea-
son becomes swollen and may hang down at the sides.
The stratum compactum of the corium becomes more or
less gelatinous and the subcutaneous lymph spaces become
filled with a material resembling the vitreous humor of
the eye,

With the exception of the swollen first finger of the male
and the dermal papillae of the female there is no evidence
as to what functional significance'the above characters pos-
sess, if they possess any. They may be the incidental prod-
ucts of the important constitutional changes which take
place during the breeding period, without being of any
direct value to the organism.

Seasonal Changes. — Most of the seasonal changes in the
skin are correlated with the sexual differences that occur
during the breeding season, and have been treated under
that head. There are some other seasonal changes, how-
ever, which occur apparently without regard to the develop-
ment of the sexual products. In the winter and early spring
frogs are darker in color than in summer, owing probably
in large part to differences of temperature. According to

THE SKIN

191

Donaldson the power of the skin to absorb water is greater
in summer than in winter.

Color Changes. — The power of the skin to change its
color in relation to surrounding conditions depends upon
changes which occur in the pigment cells, or chromato-
phores. Of these there may be distinguished the following
varieties: black pigment cells ( melanophores ) , interference
cells (leucophores), golden pigment cells (xanthophores,
xantholeucophores), and in some species of frogs red pig-
ment cells.

The black chromatophores are stellate cells with irregu-
larly branching processes. There is a single nucleus near
the center of the cell. The dark pigment is in the form
of numerous small brown or black granules of a substance
called melanin, which is a very resistant compound remain-
ing unaffected by most reagents. The black chromato-
phores are most abundant on the dorsal side of the body,
especially in the black spots where they are massed together
very thickly. On the ventral side they are almost entirely
absent over a considerable area. They are found mostly in
the superficial layer of the corium just below the epidermis.
Scattered chromatophores occur in the epidermis and the
deeper layers of the corium. They tend to aggregate in
regions which are most abundantly supplied with blood
vessels. The pigment of the chromatophores undergoes
remarkable changes in form under certain conditions. When
the pigment is most expanded, it is widely spread out into
numerous branching processes, giving the whole skin a
much darker color; at other times it may be contracted
into a small rounded mass. Some investigators (Pouchet,
Leydig) have attributed the change in the form of the pig-
ment to changes in the shape of chromatophores, which
were supposed to send out processes and draw them in
again like an Amoeba. Such movements undoubtedly occur

192

THE BIOLOGY OF THE FROG

in the pigment cells of many of the lower animals, but many
investigators consider that the movement of the pigment
m the chromatophores of the frog takes place along pre-
formed paths, the outline of the cell remaining approxi-
mately constant while the pigment granules flow back and
forth within the processes, which are transparent, and hence
invisible except when containing pigment. Virchow, Von

j M IT

Fig. 60. — Pigment cells from the frog, in different states of extension.
(From Verworn's "General Physiology.")

Wittich, and Biedermann think that the changes in the
chromatophores may involve both a change in the shape of
the cell and a flow of the pigment within the cell.

By teasing up bits of frog's skin in a suitable medium I
have been able to isolate living pigment cells and to observe
that they actually change in shape and creep about much
like an Amoeba. Some of the processes were often devoid
of pigment, but the granules could be seen to flow in and
out of them as the cell changed in form. It is probable,

THE SKIN

193

therefore, that pigment cells behave in much the same man-
ner in their normal environment in the skin, and that both
changes in outline and the flow of granules within the proc-
esses are involved in changes in the distribution of the black
pigment.

The cells which give the skin its golden and green colors
form a layer immediately beneath the epidermis. Unlike the

Fig. 61. — Isolated pigment cells of the skin of the frog cultivated in
lymph. Figures 3-8 indicate successive changes in one cell during
a half hour. All figures drawn to the same scale.

black chromatophores they are usually rounded or polygo-
nal in form, and they lie a little above the black cells, which
constitute a sort of dark background. Their golden color is
due to a fatty pigment or lipochrome, which is sometimes
diffused throughout the cell and at other times aggregated
into large drops (Biedermann) . This pigment is soluble in
alcohol, chloroform, and ether, giving a golden yellow solu-
tion which turns to a yellowish green when very dilute.
The same substance, according to Kiihne, produces the

194

THE BIOLOGY OF THE FROG

yellow color of the fat body. In frogs which have been pre-
served for some time in alcohol this pigment disappears,
and consequently the specimens lose their golden and green
coloration.

An important additional source of color in the frog is
furnished by the so-called interference granules. These
granules have a crystalline structure, and they show a cross
st nation due to the fact that they consist of a series of
lamellae. According to Ewald and Krukenberg they are
composed of guanin. By transmitted light they are brown
or gray, but in reflected light they are usually blue. These
granules give the skin a whitish or sometimes a bluish color.
Cells having these or similar granules are called leucophores.
It has been held that many cells (xantholeucophores) con-
tain both yellow pigment and guanin granules, but according
to Ficalbi, von Wittich, and more recently W. J. Schmidt,
the so-called xantholeucophores are really double cells, an
outer one containing yellow pigment, and a deeper one con-
taining the granules of guanin.

Red stellate pigment cells have been described in Rana
fusca by Von Wittich and Schmidt. They occur in the
corium, and were observed to undergo changes in the dis-
tribution of their pigment like those of the black chromato-
phores.

Nearly all of the color changes which the skin of the frog
undergoes depend upon the differences in the distribution
of two elements, the black and the yellow pigment. When
the pigment of the black chromatophores is expanded, the
skin becomes dark in color, owing to the fact that the black
pigment is spread over a greater amount of surface. When
the skin is light in color, the black pigment becomes con-
tracted into small masses, thus allowing the light to be
reflected from the other pigment cells. These facts may
easily be demonstrated by comparing the skin of a dark

195

196

THE BIOLOGY OF THE FROG

frog with that of a light one, when great differences in the
chromatophores will almost certainly be observed. Although
the black chromatophores lie mainly below the golden cells,
their branches cover the latter to a greater or less extent,
and when the black pigment is fully expanded, it cuts off
much of the light which would otherwise be reflected from
them.

The golden color that appears in the frog's skin is due
directly to the pigment in the golden cells, but the green is
not produced in so simple a manner. There is no green
pigment in the frog's skin, and various explanations have
been offered as to how this color comes to appear. The
subject has been investigated by Briicke, Harless, Von Wit-
tich, Eberth, Biedermann, and Ehrmann, each of whom
disagrees in certain particulars with the others. Briicke
regarded the green color as a simple interference phenome-
non caused by the granules of guanin; but that the golden
pigment is necessary to the production of green was subse-
quently shown by the fact that when the golden pigment is
dissolved out of the cells the green color disappears although
the granules may remain unchanged. It is quite well estab-
lished that the green is a combination effect of light reflected
from the guanin granules, and the golden pigment through
which the light passes. As the light reflected from the
granules contains a large proportion of blue rays, we have
what is practically equivalent to a blue background seen
through a yellow medium, the result of which is to produce
green. The yellow medium absorbs most of the colors of
the spectrum, allowing yellow and a certain amount of green
light to pass through. The blue background reflects only
blue and green light. Since green rays are the only ones
which are capable both of reflection from the blue back-
ground and of passing through the yellow medium, the back-
ground appears of a green color.

THE SKIN

197

The green is produced, according to Biedermann, when
the black chromatophores are expanded beneath the yellow.
Then most of the light is reflected from the granules. When
the black chromatophores are contracted so that the yellow
cells have a lighter background, light may be reflected from
other elements than the blue granules, and a yellow or

Fig. 63. — Sections through the skin of the tree frog. In A the skin
appears yellow ; the black pigment is concentrated, and considerable
light is reflected through the yellow chromatophores from the deeper
tissues. In B the color of the skin is green ; the black chromato-
phores are in a state of moderate extension, forming a dark layer
beneath the yellow cells, so that most of the light passing through
the yellow cells is reflected from the bluish granules. In C the pig-
ment from the black chromatophores has surrounded the yellow
cells, giving the skin a very dark color. (From Gaupp, after Ehr-
mann.)

golden color may predominate. The role of the contraction
and expansion of the golden pigments is not accurately de-
termined. It is probable that the gray or grayish blue color
which is sometimes assumed may be produced by the simul-
taneous contraction of both the black and the golden pig-
ments, since frogs with the black pigment spots contracted
often exhibit these hues when the golden pigment has been
dissolved out in alcohol. Von Wittich found in the tree

B

c

198

THE BIOLOGY OF THE FROG

frog that a gray color was associated with the contraction
of both kinds of pigment. In the ordinary color changes
variations in the concentration of the golden pigment are
much less important, however, than the changes in the black
cells.

The color changes in the skin are produced by numerous
agencies which act upon the pigment cells either directly or
through the central nervous system. The chromatophores
of the frog form a very delicate and responsive system which
is constantly undergoing changes in response both to stimuli
from the environment and the varying internal states of the
animal. One of the most important of the external stimuli
affecting the skin is light. It is a well-known fact that frogs
exposed to a bright light become light in color, while if
they are kept some time in the dark, the skin turns much
darker. These changes are much more pronounced in tree
frogs (Hyla) than in the species of Rana, and they bring
about an adaptation of the color of the animal to that of
its environment which is often very close. The question
whether light affects the chromatophores directly or through
the central nervous system has received considerable atten-
tion. The latter alternative was espoused by Lister, who
found that a blinded frog no longer changes its color in
response to changes in the intensity of light. Lister's con-
clusion has been only partially confirmed by subsequent
investigators. Steinach found that if both the nerves and
blood vessels supplying any portion of the skin were cut in
two, there still remained in that part a certain capacity for
color change in response to light of different intensities.
When pieces of dark paper were laid over portions of the
skin thus treated, or even upon portions of skin entirely re-
moved from the body, the areas covered were found to be
considerably darker than those exposed to the light. Speci-
mens of Hyla in which certain parts were shaded while other

THE SKIN

199

parts were exposed to light became light colored in all except
the shaded areas. This was found to occur both in normal
frogs and in frogs whose spinal cord was destroyed.
Color changes were found by Dutartre to take place more
rapidly in normal frogs than in specimens which had been
blinded, but the same reactions occurred in both cases.
There is no doubt, therefore, that light brings about
color changes both directly and through the central nervous
system.

How the nervous system functions in the control of color
changes is uncertain. There are no connections known to
exist between nerve fibers and the chromatophores such as
have been observed in some fishes; and the chromatophores
of frogs do not show the quick response to nervous stimula-
tion or to the cutting of nerves which the pigment cells of
certain fishes exhibit. Hogben, who has recently reviewed
the question, is of the opinion that "the regulation of colour
response by fluctuating pituitary secretion is adequate to
interpret all the accredited phenomena in adult Amphibia,
without invoking a direct innervation of melanophores." As
will be seen in Chapter XII, the pituitary secretion exercises
a profound effect upon the pigment cells in both larval and
adult frogs. The amount of this secretion in the blood would
naturally vary according to different conditions of nervous
stimulation, so that the nervous system might exercise an
indirect control of pigmentation through the intermediary
of the pituitary hormones.

Destruction of the optic thalamus of the brain has been
claimed to produce a darkening of the skin (Steiner, Bieder-
mann) , but Hogben suggests that this may be due to injury
to the pituitary resulting from operating on a near-by part
of the brain. He finds that operations on the brain in which
injury to the pituitary is carefully avoided produce no dark-
ening of the skin color. The light color following stimula-

200

THE BIOLOGY OF THE FROG

tion of the medulla might be interpreted as due to an in-
creased stimulation of the pituitary. The experiments of
cutting and stimulating the nerves of the hind leg have
yielded contradictory results. Hogben cut and stimulated
various nerves without obtaining any consistent effect upon
pigmentation.

The condition of the pigment cells is profoundly influ-
enced by changes in the circulation. An arrest of the blood
flow causes a paling of the skin. If the leg of a dark-
colored frog be tightly ligatured around the knee, the part
below the ligature will soon assume a much lighter color.
The same result follows if the blood vessels alone are tied,
and is effected more quickly if the ligature is made around
an artery instead of a vein. These results are not inconsis-
tent with the theory of control by means of internal secre-
tions, since the arrest of the blood flow would naturally re-
duce the amount of the available secretion.

Raising the temperature causes the pigment of the skin
to contract. Cold, on the other hand, causes the pigment
to expand and the skin to assume a dark color. The dark
color of winter frogs is in part at least the effect of cold,
and the lighter color of summer frogs in part the result of a
higher temperature. A dark-colored frog may readily be
made to turn much lighter if it is placed for several minutes
in water of a temperature 27° C. Changes of temperature
affect the concentration of pigment even in isolated pieces
of skin.

Various chemical substances affect the chromatophores,
some causing a contraction, others an expansion of the pig-
ment. Adrenalin causes the melanophores to strongly con-
tract and the extract of the pituitary will cause them to
expand even in excised pieces of the skin. Carbon dioxide
produces a darkening of the skin; carbon monoxide, on the
other hand, causes the skin to turn pale. Chloroform and

THE SKIN

20i

some other anaesthetics as well as certain irritants, such as
croton oil and cantharides, cause an expansion of the pig-
ment on the parts of the skin to which they are applied.
Dryness tends to cause the skin to turn pale, while immer-
sion in water produces the reverse effect. This has been
observed especially in Rana fusca by Biedermann and in
R. agilis and Hyla by Werner.

Biedermann has shown that color changes are influenced
in a remarkable way by contact stimuli. Specimens of
Hyla placed where the skin comes in contact with rough
substances become very dark in color even when surrounded
with bright-colored materials. Hylas which were placed
upon smooth green leaves became light colored even in the
dark. While the influence of light is admitted to be an
important factor, the color changes of Hyla are regarded by
Biedermann as determined to a great extent by the nature
of the material with which the skin comes in contact. Since
in the life of the tree frog rough surfaces are generally asso-
ciated with a dark environment, while smooth surfaces are
usually afforded by green leaves, this method of reaction to
contact stimuli conspires to bring about, in most cases, an
adaptation of the color of the animal to that of its sur-
roundings. In the species of Rana studied this mode of
reaction to contact was not observed. Finally it may be
observed that color changes are associated with the psychic
states of the animal. Frogs, like men, may turn pale
through fear, but the mechanism of the process is very dif-
ferent in the two cases. If frogs are held in the hand for
some time, the skin turns paler; this may in part be a re-
action to temperature, but the same effect is produced if the
animal is pursued and caused to jump about vigorously in
its attempts to escape.

Absorption and Excretion. — The power of the frog's skin
to absorb water has already been described. Several in-

202

THE BIOLOGY OF THE FROG

vestigators have endeavored to ascertain if fluids pass
through the skin with equal facility in both directions. Ac-
cording to Reid a five per cent solution of sugar in distilled
water passes through the skin more rapidly from within
outward than from without inward; but if the same per-
centage of sugar is dissolved in a normal salt solution the
fluid will pass more rapidly from without inward. Overton
maintained that water Would pass from without inward
through the frog's skin when the osmotic pressure is the
same on the two sides, but his conclusions were not sup-
ported by the experiments of Maxwell. Przylecki has
shown, however, that a frog's skin containing a one per cent
solution of sugar will not allow any sugar to pass through
even in eight to twelve hours, whereas if the sugar solution
is on the outside it will pass through with a fair degree
of rapidity. After twelve hours this selective action of the
skin disappears and it behaves more like a non-living mem-
brane. Chloroform and other depressants decrease the rate
of passage of fluid from without and increase its rate of
passage from within.

The amount of fluid that can be forced through the skin
under pressure depends also upon the direction of flow.
Cima found that as much water under a pressure of 10 cm.
of mercury would pass through the skin of the frog from
within outward in five minutes as would pass through in the
reverse direction in thirty-seven minutes.

Of the excretory function of the skin of the frog practi-
cally nothing is known.

Another function which has been attributed to the skin is,
curiously enough, nutrition. According to Przylecki the
skin secretes diastase and has a power of digesting carbo-
hydrates, but the nutriment which a frog can secure in this
way must, under ordinary circumstances, be quite infinitesi-
mal. Experiments to test the ability of tadpoles to utilize

THE SKIN

203

nutriment absorbed through the skin have led to opposed
conclusions.

REFERENCES

Ascherson. Ueber die Hautdriisen der Frosche. Arch. Anat. u.
Phys., 1840.

Babak, E. Zur chromatischen Hautfunktion der Amphibien.
Arch. ges. Phys., Bd. 131, 87, 1910.

Bert, P. Venin cutane de la grenouille. C. R. Soc. Biol. (8), T. 2,
1885.

Biedermann, W. Zur Histologic und Physiologie der Schleimse-
cretion. Sitzb. d. k. Ak. Wiss. Math.-nat. CL, Bd. 94, Abth. 3, 1886;
Vienna, 1887; Ueber den Farbenwechsel der Frosche. Arch. ges.
Phys., Bd. 51, 1892.

Boulenger, G. A. The Poisonous Secretion of Batrachians. Nat.
ScL, Vol. 1, 1892.

Donaldson, H. H. On the Absorption of Water by Frogs. Science,
n.s., Vol. 13, 1901.

Drascli, O. Beobachtungen an lebenden Driisen, etc. Arch. Anat.
u. Phys., phys. Abth., 1889.

Dutartre, A. Sur les changements de couleur chez la grenouille
commune (Rana esculenta). C. R. Hebdom. Ac. Sci., T. 3, 1890.

Ehrmann, S. Zur Physiologie der Pigmentzellen. Cent. f. Phys.,
Bd. 5, 1891.

Engelmann, T. W. Die Hautdriisen des Frosches. Arch. ges.
Phys., Bd. 5, 1872.

Fuchs, R. F. Die Farbenwechsel und chromatische Hautfunk-
tionen der Tiere. Winterstein, Handb. vergl. Physiol., Bd. 3, H. 1, 2nd
Th. 1189, 1914. (Full bibliography.)

Gadow, H. Color in Amphibia. Proc. Roy. Inst. Great Britain,
1902.

Harless, E. Ueber die Chromatophoren des Frosches. Zeit wiss.
Zool., Bd. 5, 1854.

Heidenhain, M. Die Hautdriisen der "Amphibien." Sitzb. Wiirzb.
phys.-med. Ges., 1893.

Hogben, L. T. The Pigmentary Effector System. Edinburgh and
London, 1924.

Holmes, S. J. The Movements and Reactions of the Isolated
Melanophores of the Frog. Univ. Calif. Publ. Zool., Vol. 13, 167.,
1914.

204

THE BIOLOGY OF THE FROG

Hooker, D. The Reactions of the Melanophores of Rana in the
Absence of Sensory Control. Zeit. allg. Physiol., Bd. 14, 93, 1912.

Huber, O. Ueber Brunstwarzen bei Rana temporaria. Zeit. wiss.
Zool., Bd. 45, 1S87.

Junius, P. Ueber die Hautdriisen des Frosches. Arch. mik. Anat.,
Bd. 47, 1896.

Leydig, F. Ueber die allgemeinen Bedeckungen der "Amphibien."
Arch. mik. Anat., Bd. 12, 1876; Die anuren Batrachier der deutschen
Fauna, 1877; Integument briinstiger Fische und Amphibien. Biol.
Cent., Bd. 12, 1892.

Ueber das Blau in der Farbe der Thiere, Zool. Anz., Bd. 8, 1885;
Blaufarbiger Wasserfrosch. Zool. Garten, Bd. 33, 1892.

Maxwell, S. S. On the Absorption of Water by the Skin of the
Frog. Am. Jour. Physiol., Vol. 32, 287, 1913.

Overton. 39 Thesen liber die Wasserokonomie der Amphibien, etc.
Verh. phys.-med. Ges. Wurzburg, Bd. 36.

Pfitzner, W. Die Epidermis der Amphibien. Morph. Jahrb., Bd.
6, 1880.

Przylecki, S. J. Proprietes digestives de la peau des grenouilles.
Arch. Internat. Physiol., T. 22, 208, 1923. L'absorption cutanee chez
les grenouilles, I.e., T. 20, 144, 1922.

Reid, W. Osmosis Experiments with Living and Dead Membranes.
Jour. Phys., Vol. 11, 1890.

Reid and Hambly. On the Transpiration of Carbon Dioxide
through the Skin of the Frog. Jour. Phys., Vol. 18, 1895.

Rynberk, G-. van. Ueber die durch Chromatophoren bedingten
Farbenwechsel der Tiere. Ergeb. der Physiol., Bd. 5, 1906.

Schmidt, W. J. Ueber pigmentfreie Auslaufer Kerne und Zen-
tren der Melanophoren bei den Froschen. Arch. f. Zellforsch.,
Bd. 15, 269, 1920. Beobachtungen iiber den roten Chromato-
phoren in der Haut von Rana jusca. Anat. Hefte, I Abt. 58, H. 3,
643, 1920.

Ueber die Xantholeukosomen von Rana esculenta. Jen. Zeit., Bd.
57 (n.s. Bd. 50), 219, 1921.

Seeck, O. Ueber die Hautdriisen einiger Amphibien. Inaug. Diss.
Dorpat, 1891.

Steinach, E. Ueber Farbenwechsel bei niederen Wirbelthieren
bedingt durch directe Wirkung des Lichtes auf die Pigmentzellen.
Cent. f. Phys., Bd. 5, 1891.

Stieda, L. Ueber den Bau der Haut des Frosches. Arch. Anat. a
Phys., 1865.

THE SKIN

205

Stirling, W. On the Extent to which Absorption can take Place
through the Skin of the Frog. Jour. Anat. and Phys., Vol. 11, 1877.

Strieker und Spina. Untersuchungen liber die mechanischen Leis-
tungen der acinosen Driisen. Sitzb. Ak. Wiss. Math.-nat. CI., Bd. 80.
Abth. 3, 1879, Vienna, 1880.

Townson, R. Observationes physiologicse de Amphibiis, Gottingae,
1795.

Werner, F. Ueber die Veranderung der Hautfarbe bei europai-
schen Batrachiern. Verh. d. k. k. zool.-bot. Ges. Wien, Bd. 40, 1890,
Albinismus und Melanismus bei Reptilien und Amphibien. Ibid., Bd.
43, 1893.

Wittich, W. von. Die griine Farbe der Haut unserer Frosche, ihre^
physiologische und pathologische Veranderungen. Arch. Anat.- u.
Phys., 1854; Entgegnung auf Herr Harless': "Ueber die Chromato-
phoren des Frosches." Ibid., 1854.

CHAPTER X

THE EXCRETORY SYSTEM

The process of excretion is an essential part of the activ-
ity of all living substance. The substances resulting from
the breaking down of living matter and the various mate-
rials taken into the organism which are never built up into
living substance give rise to many compounds no longer
useful which must be gotten rid of if the life of the organism
be maintained. Every cell of the body excretes as well as
assimilates and respires. A part of the waste is eliminated
in the form of carbon dioxide, which is thrown off from the
body through the organs of respiration. The solid products
of metabolism, however, cannot be disposed of in this way,
and specialized organs are developed for their removal. In
excretion, as in respiration, we must distinguish between the
discharge of substances into the blood which takes place
throughout all parts of the organism, and the elimination of
these substances from the blood to the outside of the body.
The latter function is carried on by several organs. The
skin is to a certain extent an organ of excretion, although
little is known of its function in this respect among the
Amphibia. In higher forms in which sweat glands occur a
certain amount of salts and other substances is gotten rid of
by cutaneous excretion. The liver is an important excre-
tory organ, and the walls of the intestine also subserve the
same function. The most important organs of excretion,
however, are the kidneys, of whose structure and function
we shall give a short account.

206

THE EXCRETORY SYSTEM

207

Structure and Function of the Kidneys. — The kidneys
of the frog are oval, flattened, dark red bodies lying dorsal
to the peritoneum of the posterior portion of the body
cavity. The duct of the kidney, or ureter, is joined at about
the posterior third or fourth
of the outer margin; it then w4o
runs for a short distance
along the dorsal surface and
finally becomes embedded in
the substance of the kidney,
running near the margin to
the anterior end of that
organ. The ventral surface
of the kidney is flatter than
the dorsal and is traversed
longitudinally by the yel-
lowish adrenal body (Fig.
77). The kidneys are cov-
ered by peritoneum only on
the ventral surface with the
exception of a very short
space where this membrane
is folded in over the edges.

The kidney may be re-
garded as a compound, tub-
ular gland, made up of a
large number of coiied uri-
niferous tubules. Each uri-
niferous tubule begins in a

Malpighian body near the ventral surface. A Malpighian
body consists of two parts, a knot of blood vessels, the
glomerulus, and a surrounding membrane, or Bowman's
capsule. The artery, vas afjerens, entering the capsule
breaks up into several capillaries which, after forming a

Fig. 64. — Male urinogenital or-
gans. Ao, Aorta ; CI, cloaca ;
Cv, postcaval vein ; FK, fat
bodies ; Ho, testes ; C7r, ureters
opening into the cloaca at $,
8'; Vr, renal veins. (After
Wiedersheim. )

208

THE BIOLOGY OF THE FROG

few coils, emerge as the efferent blood vessel from the same
opening. Bowman's capsule is an exceedingly thin mem-
brane; there is an inner fold closely ap-
plied to the glomerulus which is con-
tinuous with the outer wall at the point
where the blood vessels enter the cap-
sule. Bowman's capsule is simply the
thinned out and expanded end of a
uriniferous tubule which has become
pushed by the glomerulus as one might
push in the end of a finger of a glove.
The capsule, however, has grown
around the glomerulus and closely
surrounds the afferent and efferent ves-
sels. At the dorsal side of the capsule,
and usually opposite the point where
the blood vessels
enter, the outer
wall passes into
the neck of the uriniferous tubule. The
very thin cells of this wall shade off
gradually into cells of columnar epi-
thelium which for a short distance
carry very large cilia. Beyond the
neck, which is somewhat narrower than
the rest of the tubule, the cells are
lined with much shorter cilia. Each
tubule is lined with a single layer of
cells which varies in character in the
different parts. The course of each
tubule is quite complicated. At first it
runs dorsally, where it forms a more or less complicated coil,
then it proceeds to the ventral side of the kidney, forms a
second coil, and finally runs dorsally again, emptying into

Fig. 65. — A urinifer-
ous tubule, c, col-
lecting tubule, m,
Malpighian body ;
t, uriniferous tu-
bule leading from
the latter to the
collecting tubule.
(After Nussbaum.)

Fig. 66. — Malpighian
body. b.v., blood
vessel ; g.l., glom-
erulus ; u.t., urinif-
erous tubule.

THE EXCRETORY SYSTEM

209

one of the collecting canals which extend transversely across
the dorsal surface of the kidney from the inner margin to
the ureter. The tubules are held together by connective
tissue which forms a support also for the numerous blood
vessels with which the kidney is supplied.

The ventral surface of the kidney is furnished with nu-
merous ciliated funnels, the nephrostomies, whose expanded
ends open into the coelom. At
their other end the nephro-
stomes empty into branches
of the renal veins, and the
cilia with which they are lined
beat toward the upper end of
these organs and thus create
a current of lymph from the
body cavity into the blood.
This relation of the nephro-
stomes is a peculiar one and
occurs only in the Anura.
The lower Amphibia preserve
the typical arrangement of
these organs, as the nephro-
stomes are connected with the
renal tubules. This condition,
as Marshall has found, occurs
also in the early stages of the
life of the frog, but later the
nephrostomes lose their orig-
inal connection with the tu-
bules and become united sec-
ondarily with the renal veins.

The kidney of the male frog stands in an intimate relation
to the sexual organs. The vasa efferentia, or ducts which
convey the spermatozoa from the testis, pass into the sub-

Fig. 67. — Diagram of a kidney
showing the ureter and col-
lecting tubules. (7, collect-
ing tubules ; L, longitudi-
nal canal of Bidder ; S, semi-
nal vesicle ; T, testis ; U, ure-
ter ; VE, vasa eff erentia.

210

THE BIOLOGY OF THE FROG

stance of the kidney, and the spermatozoa are carried
through this organ to the ureter, which thus serves also as a
vas deferens. The vasa efferentia are originally outgrowths
of the walls of the Malpighian corpuscles which become con-
nected with the testis. In some species (R. esculenta) the
Malpighian bodies, which give rise to these outgrowths, still
preserve their original function, and during the period of
sexual activity spermatozoa may be seen in them as well as
along the whole length of the renal tubules which arise
from them. The vasa efferentia lead into a longitudinal

Urinif erous Dorsal surface

Fig. 68. — Diagram of a cross-section of the kidney of the frog.

canal (Bidder's canal) which runs near the median edge
of the kidney.

In Rana jusca, according to Beissner, this canal is con-
nected with the collecting tubules which extend across the
dorsal side of the kidney to the ureter. The short tubes
which connect the longitudinal canal with the collecting
tubules widen out near the latter to form an ampulla. This
enlargement is formed by a Malpighian body which has lost
its glomerulus and consequently its original function. In
Rana jusca there is a comparatively direct connection estab-
lished between the vasa efferentia and the collecting tubules,
and the spermatozoa, therefore, are not found in the
Malpighian bodies and functional renal tubules. Bidder's

THE EXCRETORY SYSTEM

211

canal occurs in the kidneys of both sexes, but its function in
the female is not known.

In many of the lower vertebrates (Elasmobranchs,
Amphibia) the kidney is divided into an anterior, or
sexual portion, and a posterior, or excretory portion.
The frog presents only the beginning of such a differ-
entiation. The vasa efferentia are connected with the
anterior part of the kidney, but the excretory function
of this region is still retained. The course of the sper-
matozoa through the kidney varies considerably in dif-
ferent species of frogs, as is evinced by the fact that it
is much more direct in Rana fusca than in R. esculenta.
The latter presents, doubtless, the more primitive con-
dition.

The kidney is supplied with blood from two different
sources: (1) the renal arteries, which rise from the urino-
genital arteries, or direct from the aorta, and (2) the renal
portal veins, which convey venous blood from the posterior
portion of the body. The renal arteries, of which there are
usually from four to six, enter the kidney at the median
edge or near the latter on the ventral surface. The divisions
of the renal arteries are distributed to the renal tubules
(arterise rectse), and also to the glomeruli (vasa efferentia).
The renal portal vein runs along the dorsal surface of the
kidney very near the outer margin. From the transverse
branches of this vein, which extend across the dorsal surface,
small vessels are given off which penetrate the substance of
the kidney and form capillary networks around the renal
tubules. The vasa efferentia, which emerge from the glo-
meruli, together with the efferent veins arising from this
capillary network, go to form the beginnings of the renal
veins which convey the blood from the kidney to the
posterior vena cava. The glomeruli are supplied only with
arterial blood, while the renal tubules receive blood from the

212

THE BIOLOGY OF THE FROG

renal portal veins, and also, although to a less extent, from
the renal arteries.

The function of the kidney is the elimination of waste
matters from the blood. The renal excretion, or urine, is a
fluid containing a large number of compounds in solution.
Most of the nitrogen leaves the body in the form of urea,
(NH2)2CO, which is a white crystalline compound, very
soluble in water. Urea represents the final product of the
breaking down of the nitrogenous substances of the body,
and it has been shown that the formation of this substance
takes place to a large extent in the liver, from which it
passes into the blood. The kidney also excretes several salts
such as the chlorides, sulphates, and phosphates of sodium,
potassium, calcium, and magnesium, and numerous other
substances in smaller proportions.

The specific roles of the glomeruli and tubules in renal
excretion have long been a matter of dispute. It is certain
that water and other substances diffuse from the blood
through the walls of the capillaries of the glomeruli into
the renal tubules. It has been held, especially by Ludwig
and his followers, that practically all of the substances
excreted by the kidney pass through the glomeruli, and that
the function of the tubules is to absorb the excess of water
and certain other materials which pass down the lumen. By
other physiologists it ha& been maintained that both the
glomeruli and the renal tubules are secretory, but that they
eliminate different products. Nussbaum's ingenious experi-
ments on the frog seemed to offer a solution of this problem.
As the glomeruli are supplied by branches of the renal
arteries, Nussbaum concluded that the blood supply of these
organs would be cut off if the renal arteries were tied. The
opportunity was thus presented of comparing the excretion
of the kidney in which the glomeruli are rendered function-
less with that of the normal organ. It was found that irj

THE EXCRETORY SYSTEM

213

frogs with the renal arteries tied the secretion of urine was
much diminished in amount. Solutions of sugar, peptones,
and egg albumen, which when injected into the blood of
normal frogs soon make their appearance in the urine, could
not be detected, after injection into the blood, in the urine
of frogs whose renal arteries were ligatured, even after the
flow of urine was increased by the simultaneous injection of
urea. Nussbaum came to the conclusion that albumen,
sugar, and most salts are excreted by the glomeruli, while
urea is eliminated l?y the cells of the uriniferous tubules.
There is a source of error in such experiments, since ligat-
ing the renal arteries alone does not entirely cut off the
blood supply of the glomeruli; there are anastomoses with
the genital arteries by means of which these organs may
receive blood in a somewhat roundabout way. Adami
found that some of the glomeruli became filled by injecting
the aorta of a frog in which the renal arteries were tied.
This observer, in repeating Nussbaun's experiments, failed
to confirm some of the latter's results and considered that
the conclusions that were founded upon them were not
established on a firm basis.

It seems evident that the renal tubules absorb several
substances that pass through the glomeruli. Peter found
that trypan blue injected into the blood was taken up first
in the inner ends of the cells of the tubules. Later it passed
to the outer ends and finally was absorbed into the blood.
It is certain that sugar and various salts pass through the
capillaries of the glomeruli. According to Clark the renal
tubules absorb sugar from the urine, even when the urine
contains much less sugar than the blood.

The Bladder. — The bladder is a thin-walled, bilobed sac
attached to the ventral side of the cloaca, just below the
openings of the ureters. It arises as an outpushing of the
ventral wall of the cloaca like the allantois of the embryos

214

THE BIOLOGY OF THE FROG

of the higher vertebrates, with which it is regarded as homol-
ogous. It is surrounded by peritoneum which is continued
as a median dorsal sheet attaching it to the rectum; a ven-
tral sheet of peritoneum connects it with the ventral body
wall, and a lateral peritoneal extension on either side joins
the sides of the bladder to the dorso-lateral regions of the
body wall.

The inner surface of the bladder is lined with a layer of
epithelium about three cells thick (List), the inner layer

Fig. 69. — Diagram of the bladder and rectum of the frog ; A, from the
side ; B, from below ; Bl, bladder ; (7, cloaca ; R, rectum ; S, sphinc-
ter muscle; U, ureter; Ut, uterus. (Modified from Gaupp.)

resting upon a membrane of connective tissue. Numerous
goblet cells occur among the other epithelial cells. The
middle layer of the bladder consists of a network of smooth
muscle fibers. The fibers are sometimes single, and some-
times united into bundles, and they extend in all directions.
Outside of the muscle layer is a thin sheet of connective
tissue which is covered externally by the peritoneum.

The bladder is very distensible, as may readily be shown
in a recently killed frog, by inflating it by means of a blow-
pipe introduced into the cloaca. When entirely empty, the
bladder shrinks to an inconspicuous size. It was formerly

THE EXCRETORY SYSTEM

215

doubted whether the bladder of the frog serves as a recep-
tacle for urine, as it has no direct connection with the ducts
from the kidneys. Townson, whose conclusions were fol-
lowed by Dumeril in his great work on reptiles and am-
phibia, regarded the bladder as a sort of reservoir for water
absorbed through the skin. The contents of the bladder
were stated to be nearly pure water, and the urine proper
was supposed to pass out and through the cloaca without
entering the bladder at all. According to Dumeril,1 "the
so-called urinary bladder of frogs, tree frogs and toads, as
well as that of the salamanders, is a sort of reservoir to
which an almost pure aqueous humor, destined to be
exhaled through the skin, appears to be carried either by
the veins or by the lymph vessels. " The subsequent investi-
gations of Davy, Nussbaum, Adami and others have shown
conclusively that the fluid contained in the bladder is de-
rived from the kidneys, and that it contains urea and other
substances characteristic of renal secretion.

The urine of the frog is much less concentrated than that
of a human being, but it is subject to much variation ac-
cording to whether the frog is kept in water or dry air.
Water is absorbed by the skin and passed out through the
kidneys. If urine is prevented from escaping by closing the
cloaca the frog continues to absorb water only for a time
(Parnas). The skin ceases to take in water after a certain
concentration of the blood has been reached.

The end of the cloaca is commonly held closed by the
contraction of its circular muscles, and the urine which is
thus prevented from passing out collects in the bladder.
The contents of the bladder are expelled suddenly by the
contraction of the muscles of the body wall, which naturally
subjects the bladder to a considerable pressure. The ex-
pulsion of urine often takes place when the frog leaps, and

1 Dumeril, "Erpetologie generate."

216

THE BIOLOGY OF THE FROG

it is very apt to occur as a consequence of the struggles of
the animal if the frog is taken in the hands, as every one
who has handled frogs has doubtless discovered. The belief
that the content of the bladder of the toad is poisonous is
entirely without foundation.

REFERENCES

Adami, J. G. On the Nature of the Glomerulus Activity in the
Kidney, Jour. Phys., Vol. 6, 1885.

Atkinson, M., Clark, G-. A., and Menzies, J. A. The Function of
the Urinary Tubules in the Frog. Jour. Physiol., Vol. 55, 253, 1921.

Bainbridge, F. A., and Beddard, A. P. Secretion by the Renal
Tubules of the Frog. Biochem. Jour., Vol. 1, 255, 1906.

Beissner, H. Der Bau der samenaBleitenden Wege bei Rana jusca
und Rana esculenta. Arch. mik. Anat, Bd. 53, 1898.

Bieter, R. N., and Hirschf elder, A. D. The Excretion of Dyes
and Other Substances in the Frog's Kidney and its Bearing upon
Theories of Renal Secretion. Am. Jour. Physiol., Vol. 68, 326, 1924.

Braemser, P., and Hahn, A. Ueber die Ausscheidung von Glucose
durch die Glomeruli der uberlebenen Froschniere. Zeit. f. Biol., Bd.
74, 37, 1921.

Clark, G-. A. Glucose Absorption in the Renal Tubules of the
Frog. Jour. Physiol, Vol. 56, 201, 1922.

Farrington, O. C. The Nephrostomes of Rana. Trans. Conn. Ac.
Sci, Vol. 8, 1892.

Frankl, O. Die Ausfuhrwege der Harnsamenniere des Frosches.
Zeit. wiss. Zool, Bd. 63, 1897. See also Arch. Mik. Anat, Bd. 51, 1898.

Heyde, H. C. van der. The Composition of the Urine and Blood
of the Hibernating Frog. Jour. Biol. Chem, Vol. 46, 421, 1921.

Nussbaum, M. Ueber die Secretion der Niere. Arch. ges. Phys,
Bd. 16, 1878. Fortgesetzte Untersuchungen, etc. I.e., Bd. 17, 1878.
Ueber die Entwickelung der samenableitenden Wege bei den Anuren.
Zool. Anz, Bd. 3, 1880. Ueber die Endigung der Wimpertrichter in
der Niere der Anuren, I.e., Bd. 3, 1880. Ueber den Bau und die
Thatigkeit der Driisen. Arch. mik. Anat, Bd. 27, 1886. See also Anat.
Anz., Bd. 1; Zool. Anz, Bd. 20; and Arch. mik. Anat, Bd. 51.

Parnas, J. K. Neue Untersuchungen liber den Wasserhaushalt der
Frosche. Biochem. Zeit, Bd. 114, 1, 1921.

THE EXCRETORY SYSTEM

217

Peter, K. Zur Histophysiologie der Amphibienniere. Zeit. ges.
Anat., Bd. 73, 145, 1924.

Richards, A. N., and Schmidt, C. F. The Glomerulus Circula-
tion in the Frog's Kidney. Am. Jour. Physiol., Vol. 59, 489, 1922.

Scott, E. L., and Kleitman, N. Sugar in the Blood of the Common
Frog. Am. Jour. Physiol., Vol. 55, 355, 1921.

Yoshida, H. Ueber die Harnbildung in der Froschniere. Arch,
ges. Physiol., Bd. 206, 274, 1924.

CHAPTER XI

THE REPRODUCTIVE ORGANS AND THE FAT BODIES

The reproductive system has the functions of producing
the sex cells and transporting them outside of the body.
The first function is discharged by the gonads, which are
known in the female as ovaries, and in the male as testes,
or spermaries. While the ovaries and testes are homologous
organs, the sexual products are carried to the outside in the
two sexes by very different methods.

Organs of the Female. — Each ovary of the frog is in the
form of a sac which is more or less lobulated. Its internal
cavity is divided by several partitions into chambers which
are filled by fluid. Externally, the ovary is covered by
peritoneum, which is continued on the dorsal side to form
a double membrane, the mesovarium, which suspends the
ovary from the dorsal body wall. The blood vessels and
nerves which supply the ovary run between the two mem-
branes of this supporting structure. The inner surface of
the ovary is lined by a single layer of flattened epithelial
cells, the origin of which may be traced to outgrowths from
the kidney in early development. The stratum medium,
or middle portion of the wall of the ovary, varies greatly in
thickness in different parts and at different times. It is
composed mainly of ova and follicle cells in various stages
of development. The eggs lie within small chambers or
follicles ; these consist of a layer of cells (membrana granu-
losa) lying next to the vitelline membrane, and outside of
this a very vascular network, the theca folliculi. After the

218

REPRODUCTIVE ORGANS AND FAT BODIES 219

eggs reach their full development they break through the
follicle and the outer wall of the ovary, and are discharged
into the body cavity.
When the eggs are
all extruded, the ova-
ries, which before
had filled the greater
part of the body cav-
ity, become reduced
to small wrinkled or-
gans, containing the
minute ova for the
following year.

The oviducts are a
pair of convoluted
tubes extending the
length of the body
cavity on either side
of the middle line.
They are surrounded
by peritoneum which
is continued dorsally
to form a supporting
membrane which ex-
tends to the dorsal
body wall outside of
the mesovaria. An-
teriorly, each ovi-
duct opens by a wide
mouth, or ostium,
into the body cavity
near the base of the lung. At the posterior end it en-
larges to form the thin-walled, very distensible uterus;
the openings of the two uteri lie close together on the

Fig. 70. — Urinogenital organs of a female
frog. N. kidneys ; Od, oviduct ; Ot, its
opening into the coelom ; Ov, ovary ; P,
opening of the oviduct into the cloaca :
S, S', openings of the ureters ; Ut, uter-
ine dilatation of the oviduct. (After
Wiedersheim.)

220

THE BIOLOGY OF THE FROG

dorsal wall of the cloaca. With the exception of the
uterus, and a short space at the anterior end, the ovi-
ducts possess a thick glandular wall. The inner surface of
the oviduct is thrown into longitudinal ridges, which are
covered with ciliated epithelium. The grooves between the
ridges receive the openings of the numerous glands which
secrete the gelatinous coats of the eggs. These glands are
mostly of the simple tubular type; they are lined by a single
layer of cylindrical secreting cells which become very much
enlarged during the breeding season. When the secretion is
discharged, the outer membrane of the cells is burst
(Lebrun) , and Lhe contents, which formed a greater part of
the bulk of trie cell, flow into the lumen of the gland. After
the discharge of the secretion the glands become very much
reduced in size, and the whole oviduct much thinner, and of
a yellowish color from the accumulation of fat. As Boett-
cher has shown, the oviducts in the breeding period possess
a remarkable capacity for the absorption of water. A pair
of oviducts, which when just taken out of the body weighed
9.6 g., were found to weigh 1084 g. after they had lain some
time in water; i.e. they had increased in weight 113 times.
After the breeding season this power of absorbing water is
very much reduced. The eggs, as they are discharged from
the ovaries, are taken into the mouths of the oviducts by
means of ciliary action. They_are then carried down the
oviducts_by means of the cilia on the ridges of the inner
walls. Dirrlngjhis passage jAey~receive their coats of jelTy,
after which they _ciille£i-4^t^

greatly distend TTprp. thpy may remamfor sev^raT^ays, the
l^ngjjnjyfJwLP d^nd^g upon the presence orabsence of
the male_jlsee p. 54).

The males of several species of Rana possess a curious
homologue of the oviduct of the female. In Rana pipiens
it is very well developed, and contains an enlargement at its

REPRODUCTIVE ORGANS AND FAT BODIES 221

posterior end representing a uterus. It lies just external to
the ureter, and extends as a fine tube some distance in front
of the kidney. Its function, if it has any, is unknown.
Cases are not uncommon in which organs characteristic of
one sex are found in a rudimentary form in the other, and
it is not improbable that the oviduct of the male frog is
simply a useless, although rather large, rudiment of this
kind. In the bullfrog (Rana catesbiana) this duct is absent.

Organs of the Male. — The testes are rounded or ovoid
organs lying ventral to the kidneys. Like the ovaries, they
are surrounded by peritoneum, which is extended dorsally
as a double membrane, the mesorchium, to the dorsal side
of the body cavity, where it becomes continuous with the
general ccelomic lining. The vasa efferentia, or ducts of
the testes, consist of a variable number of slender tubes,
which extend within the mesorchium to the inner margin
of the kidney, where they connect with Bidder's canal. The
vasa efferentia often branch and anastomose more or less,
in a way which varies greatly in different individuals.

The testis is made up essentially of a mass of tubules,
together with blood vessels and nerves, and a small amount
of connective tissue binding the tubules together. The whole
is surrounded by a connective tissue membrane, the tunica
albuginea, outside of which is the peritoneum. Toward the
outer portion of the testis the tubules extend radially, and
end blindly next to the tunica albuginea. Near the point
where the vasa efferentia enter they become coiled irregu-
larly. The vasa efferentia form a network within the testis,
into which the tubules open at their inner ends. Each
tubule possesses an outer membrana 'propria and an inner
lining of cells, some of which (spermatogonia, spermato-r
cytes, and spermatids) represent stages in the formation of
spermatozoa; others form the so-called "follicle-cells," and
the flattened cells described by Bertacchini, which lie next to

THE BIOLOGY OF THE FROG

the outer membrane. The follicle cells form a sort of wall
around groups of cells from which the spermatozoa take
their origin.

The spermatozoa of the frog pass through the substance
of the kidney into the ureter. In many species of frogs the
free portion of the ureter is dilated to form a seminal recep-
tacle in which the spermatozoa are stored against the time

Fig. 71. — A, cross section of one of the tubules of the testis ; sp,
bundles of spermatozoa ; t.e, epithelial lining of the tubule. B,
stages in the development of spermatozoa. (After Parker and
Parker.)

of their discharge from the body. The seminal receptacle
is poorly developed in Rana pipiens and R. catesbiana. In
the European species R. jusca it becomes very large and
divided into a number of compartments.

Corresponding to the various stages in the development
of the spermatozoa the testes of the frog assume a different
appearance in different times of the year. In Rana fusca,
according to Nussbaum and Ploetz, the testes are smallest
in May, after they have discharged their spermatozoa.\
Then they gradually increase in size until August, when they

REPRODUCTIVE ORGANS AND FAT BODIES 223

attain their maximum, after which they decrease in size dur-
ing the fall and less rapidly during the winter. In Rana
esculenta, according to Ploetz, the testes vary little in size
in different months. This is, perhaps, due to the fact that
during most of the year all stages of spermatogenesis may be
met with in some of the tubules. The interstitial substance
between the tubules increases in Rana fusca from March to
September. There is a storage of fat and pigment during
this period which later disappears (Ploetz, Friedmann). In
Rana esculenta there is most interstitial substance around

o 0

April June July Sept. March

May August

Fig. 72. — Seasonal changes in the size of the testes of Rana
temporaria. Natural size. (After Witschi.)

those tubules in which the process of sperm production is
most rapidly going on.

The Fat Bodies (Corpora Adiposa). — The fat body is a
yellowish organ lying just in front of the gonads. It is fur-
nished with a number of finger-like processes whose number
varies not only in different individuals but also in the same
individual at different times. In the male the fat body is
broadly and closely attached to the anterior end of the
testis. In the female it is less closely attached to the gonad
than in the male.

The fat bodies serve as a sort of storehouse of nutriment.
They undergo great changes in size during different seasons
of the year, as has been described in a previous chapter.
The histological phenomena wThich accompany these changes

224

THE BIOLOGY OF THE FROG

have been studied by Toldt, Neumann, and Giglio-Tos. In
the spring nearly all of the fat disappears from the cells
(Toldt) , and as there are usually two or more nuclei in each
cell at this time, it is probable that cell division takes place.
After the feeding period begins there is a rapid storage of a
yellowish fat in the cells, which become greatly increased in
size.

The development of the fat body is closely connected with
that of the gonads. Both, in fact, arise from the differen-
tiation of the genital ridge, the anterior portion of which
forms the fat body, the posterior portion the ovary or testis.

REFERENCES

Boettcher, A. Ueber den Bau imd die Quellungsfahigkeit der
Froscheileiter. .Virchow's Archiv, Bd. 37, 1866.

Bouin, M. Histogenese de la Glande Genitale Femelle chez Rana
temporaria. Arch, de Biol., T. 17, 1901.

Crew, F. A. E. Sex Reversal in Frogs and Toads. Jour. Genetics,
Vol. il, 141, 1921.

Funke, R. Ueber die Schwankimgen des Fettgehaltes der Fettfuh-
renden Organe im Kreislauf des Jahres. Denkschr Ac. Wiss. Math.-
nat. CI., Bd. 68, 1900, Wien.

Giglio-Tos, E. Sur Jes corps gras des Amphibies. Arch. Ital. d.
Biol., T. 25, 1896.

. Ploetz, A. J. Die Vorgange in den Froschhoden unter dem Einfluss
der Jahreszeit. Arch. Anat. u. Phys., phys. Abth. Suppl. Bd., 1890.

Tarchanoff, J. R. Zur Physiologie des Geschlechtapparatus des
Frosches. Arch. ges. Phys., Bd. 40, 1887.

CHAPTER XII

INTERNAL SECRETION AND THE ENDOCRINE GLANDS

Most secreted substances with which we are familiar are
formed in glands and discharged through a duct. There are
several organs, however, whose products instead of being
carried off by a duct are diffused into the blood or lymph.
The latter process is known as internal secretion. The term
internal secretion was first employed by Claude Bernard in
connection with his classical investigation on the glycogenic
function of the liver. While the liver secretes bile which is
carried to the intestine through the bile duct, the dextrose
which is derived from the stored glycogen passes into the
blood. Urea is also formed in the liver and is likewise
passed into the blood stream. But quite regardless of the
original use of the term, the production of sugar and urea
are no longer considered as typical examples of internal
secretion. Whether warranted or not, the term internal
secretion is commonly employed in a more restricted sense
to designate the production of a class of compounds called
hormones which have an important influence in stimulating
the activity of other parts of the body.

Organs which produce hormones are spoken of as en-
docrine glands. Several so-called ductless glands belong in
this category, but the presence or absence of a duct has no
necessary relation to the production of hormones. The
pancreas and the testes have ducts for the discharge of
some of their products, nevertheless they elaborate im-
portant hormones which pass into the blood.

225

226

THE BIOLOGY OF THE FROG

In recent years the study of internal secretions has be-
come one of the most important and fruitful fields of
physiological research. It has not only thrown much light
on the coordination of physiological functions, but it has
yielded results of great value in the practice of medicine. A
few of these endocrine glands were considered by some in-
vestigators as mere rudiments of organs useful once, but now
lunctionless. It is now known that some of these organs,
far from being useless rudiments, are absolutely essential for
the maintenance of life. Just how they act is still an un-
settled problem, but their secretions appear to afford stimuli
which are necessary to evoke the normal functioning of
other organs of the body.

The Pancreas. — In addition to the pancreatic juice which
performs so many important digestive functions, the
pancreas secretes a hormone which is of even greater impor-
tance in the vital economy of the body. Removal of the
entire pancreas from one of the higher vertebrates causes
the onset of diabetes which soon leads to death. In diabetes
there is an abnormal amount of sugar in the blood. From
the blood it is eliminated by the kidneys and hence appears
in the urine. Under ordinary circumstances the undue pro-
duction of sugar is prevented through the agency of the
internal secretion which is given off from the pancreas into
the general circulation. If the duct of the pancreas is tied
so as to prevent the discharge of the external secretion of
this organ, there is no abnormal production of sugar and
the animal may live for a long time. A large part of the
pancreas may be removed, or the whole organ may be re-
moved and grafted into some other part of the body without
producing fatal results. So long as the body receives the
sugar-regulating hormone secreted by the pancreas it may
be kept alive, but without this precious substance, fata?
effects soon follow.

INTERNAL SECRETION AND ENDOCRINE GLANDS 227

The part of the pancreas concerned in the elaboration of
this hormone is considered to be the groups of cells known
as the islands of Langerhans which lie between the tubules
which secrete the pancreatic juice. This conclusion is
strengthened by the fact that if the pancreatic duct is tied
the cells of the tubules atrophy and only the islands of Lan-
gerhans persist. It is also supported by the fact that in
many cases of diabetes the cells of the islands of Langer-
hans show a pathological condition.

Efforts to extract the sugar-regulating hormone of the
pancreas proved unavailing until a few years ago when Dr.
Banting succeeded in obtaining a substance called insulin
which when injected into the blood of animals suffering from
diabetes had a most remarkable effect in checking the ab-
normal production of sugar. This striking triumph of
patient and ingenious experimentation has proven a means
of saving many human lives from a malady with which the
medical profession had long struggled in vain.

Most of our knowledge of the functions of the pancreas
has resulted from experiments on mammals. Islands of
Langerhans occur in the pancreas of the frog and are formed
quite early in development (Aron and Alfonsi). Experi-
ments on injecting insulin into the blood of the frog were
found by Schwartz and Bricka to reduce the sugar content
of the blood, although the glycogen reserve of the liver was
little affected. Gayda has found that the removal of the
pancreas from the frog resulted in the appearance of ab-
normal quantities of sugar in the urine. It is probable,
therefore, that the pancreas of the frog functions in essen-
tially the same manner as it does in higher animals.

The Spleen. — The spleen of the frog is a rounded, reddish
body lying dorsal to the anterior end of the cloaca, where
it is attached to the supporting mesentery. It receives blood
from a branch of the anterior mesenteric artery, and gives

228

THE BIOLOGY OF THE FROG

off the splenic vein, which forms a branch of the hepatic
portal system; both blood vessels enter at a common point
called the hilus. The spleen is surrounded by a fibrous
membrane outside of which the greater part of the surface
is coated with peritoneum. The inner framework of the
spleen consists of a network of areolar tissue which contains
the essential part of the organ, the spleen pulp. The latter
is composed of several kinds of cells, many of which repre-
sent stages in the development of leucocytes, of which the
spleen contains a large number. There are numbers of
large cells containing an abundance of pigment, both yellow
and black. The pigmented cells have the property of ab-
sorbing pigment granules with which they come in contact;
if coloring matters are injected into the blood, they are
taken up by these cells in large quantities (Ponfick,
Siebel). The spleen also contains large cells in which red
blood corpuscles are frequently found in all stages of degen-
eration.

The spleen is an organ having various functions. It is a
place where red blood corpuscles are destroyed, probably
when they have reached a moribund condition. Pigment
and other foreign matters in the blood are taken up by
certain cells of the pulp. Leucocytes are in all probability
formed in the spleen, as various stages in their production
have been observed, and it has been found that there is a
greater number of these cells in the blood of the splenic vein
than in that of the splenic artery. Red blood corpuscles and
spindle cells are produced in the spleen according to H. E.
Jordan who regards the spleen as the chief organ for renew-
ing all kinds of blood corpuscles.

The Thyroid Glands. — The thyroid glands of the frog are
completely separated from each other, being situated on
either side of the hyoid apparatus in a small space between
its posterior lateral and thyro-hyoid processes. Gaupp has

INTERNAL SECRETION AND ENDOCRINE GLANDS 229

Fig. 73. — D iagram

showing the position
of the thyroid glands,
/; I, lateral process of
hyoid cartilage ; t.p,
thyro-hyoid process of
hyoid.

described some thyroid tissue (accessory thyroid) on the
ventral side of the hyoglossus muscle, and I have been able
to confirm this observation in Rana
pipiens. The tissue of the thyroid
shows a unique structure, being com-
posed of a mass of rounded follicles
united by a small amount of con-
nective tissue in which there is a rich
supply of blood vessels. Each fol-
licle is a perfectly closed sac lined
by a single layer of cubical epithelial
cells. In the center of each follicle
is a colloidal mass of transparent
substance which probably represents
the secretion of the epithelial lining.
What knowledge we have of the functions of the thyroid

in the frog indicates
that they are much the
same as in man and
other mammals. The
removal of the thyroid
in the higher verte-
brates commonly pro-
duces fatal results un-
less there are accessory
organs, such as the
parathyroids, which in
some of the mammals
are able to maintain
life in the absence of
the thyroid. Removal
of a part of the gland creates, as a rule, but little disturb-
ance. Life may be maintained for a considerable period
after complete removal of the thyroid by giving injections

e •>

Fig. 74. — Part of a cross section of the
thyroid of Rana pipiens. e, epithelial
layer of vesicles ; m, colloidal sub-
stance in vesicle.

230

THE BIOLOGY OF THE FROG

of extracts of the gland into the blood. In man the disease
called myxoedema, or, cretinism, caused by atrophy or func-
tional insufficiency of the thyroid, is often much helped, or
even cured by the administration of thyroid extract. The
substance to which the thyroid owes its important function
is a crystalline compound known as thyroxin which contains
a comparatively large amount of iodine. Treupel found that
frogs from which both thyroids were removed lived only
two or three days, but he was not entirely certain that the
result was not due to the effects of
A B the operation rather than to the

loss of the parts in question.
^ n Most of the experimental inves-

tigations on the functions of thyroid
"V Jj^ in the Amphibia have been per-

' I formed upon the larvae. This work

i was initiated by the spectacular re-
Fig. 75. — A, Tadpoles fed suits obtained by Gudernatsch in

°of '$niLB'£LV% feeding tadPoles on the thyroid
given thyroid. Both glands obtained from mammals.

GvEatsS) (Aft6r Thyr0id feedinS caused a marked
inhibition of growth combined with

a very early metamorphosis, so that the tadpoles began
to transform into frogs long before they had reached their
usual size. Some of the frogs thus obtained were diminu-
tive creatures not much larger than house flies. The thy-
roid feeding, therefore, hastened differentiation while it
checked growth.

The experiments of Gudernatsch led Dr. B. M. Allen to
ascertain the effect of removing the thyroids in very young
tadpoles. The loss of the thyroids produces little effect on
development until the stage at which the hind legs begin to
grow, when further differentiation ceases and the tadpoles
fail to undergo metamorphosis. The larvae, however, con-

INTERNAL SECRETION AND ENDOCRINE GLANDS 231

Dm.

tinue to grow, but while the differentiation of most organs
is inhibited, the sex organs continue to develop even more
rapidly than in normal tadpoles. The male larvse may
produce mature spermatozoa, and the female larvse may
show a considerable growth of the egg cells in the ovary.

If larvae having no thyroids are fed upon the thyroid they
may undergo metamorphosis. Swingle has shown that vari-
ous compounds of iodine, or food impregnated with iodine,
may cause metamorphosis in thyroidectomized tadpoles,
and that iodine and its compounds check growth and
accelerate differentiation much as
is done in thyroid feeding. He
concludes that iodine per se is the
essential substance involved in
causing metamorphosis. How-
ever, iodine and most of its com-
pounds are unable to compensate
for thyroid deficiency in the mam-
mals, and they do not cause the
other physiological effects pro-
duced by thyroxin. The physio-
logical influence of thyroxin on the
mammals and its effect upon the development of frogs and
toads depend apparently on different mechanisms. Both
involve the influence of iodine, but in the mammals this
element must form a part of a particular compound, where-
as this is unnecessary for its effect upon the metamorphosis
of amphibians.

The Thymus. — The thymus is a small, oval organ, some-
what reddish in color, situated behind the tympanic mem-
brane under the depressor madibula) muscle. As in most
higher forms, the thymus diminishes in size with age.
Maurer found that in Rana esculenta the thymus attained
its maximum size in specimens of two or three centimeters in

Fig. 76. — Diagram showing
position of the thymus,
Th. Dm, depressor man-
dibular muscle ; Ty, tym-
panum.

232

THE BIOLOGY OF THE FROG

length. In old frogs (7 to 8 cm.) the organ is much smaller
and shows marks of degeneration in structure.

The thymus has essentially the structure of a lymphoid
gland. In its fine network of adenoid tissue lie numerous
small, rounded cells. There are also several large cells of
concentric structure concerning whose origin and significance
there has been much discussion, but of whose function
nothing positive is known.

It is probable that blood corpuscles are produced to a
certain extent in the thymus (Mayer). According to Abe-
lous and Billard, if both thymus glands of the frog are
removed, the animal soon dies, after a period of great mus-
cular weakness, ulceration of the skin, and a variety of other
pathological symptoms. Hammar, however, failed to con-
firm these results. He found that both thymus glands may
be removed from the frog without injury, and concludes that
the results obtained by Abelous and Billard were the effects
of accidental infection.

According to Adler, when the thymus is removed from
young frog tadpoles, the gonads increase in size much be-
yond those of normal larvae. This conclusion is contradicted
by B. M. Allen who finds in a larger series of tadpoles than
Adler worked with that removal of the thymus has no ap-
preciable effect upon the gonads. Allen holds that the
thymus exerts no marked influence upon either growth or
metamorphosis, nor does it have any effect upon the devel-
opment of the thyroid. On the other hand, the removal of
the thyroid, according to Rogers, checks the usual degener-
ation of the thymus which follows metamorphosis, and
prevents it from migrating to its definitive position. This
result is probably just an incident of the general effect of
the removal of the thyroid in checking differentiation.

It was claimed by Gudernatsch that frog larvae fed upon
thymus grow well, but metamorphose little, if at all. Thy-

INTERNAL SECRETION AND ENDOCRINE GLANDS 233

mus feeding was held to produce effects the reverse of those
obtained by feeding thyroid. Repetition of Gudernatsch's
experiments under varied circumstances have not entirely
confirmed his findings and doubt has been cast on his inter-
pretation of his results.

The Pseudothyroid and the Epithelial Bodies. — The
pseudothyroid and the epithelial bodies are organs of similar
structure and origin. They are derived from the modifica^
tion of the epithelium of the gill slits of the larva and are
therefore products of the entoderm. The two pseudo-
thyroids are the largest of these. They are rounded red-
dish bodies, lying on either side of the posterior portion
of the hyoid cartilage. They were formerly mistaken
for the thyroids, but they possess a very different in-
ternal structure, which is essentially that of a lymphoid
gland.

The epithelial bodies are small, rounded structures usually
more than two in number on each side and somewhat vari-
able in position, but generally situated near the pseudothy-
roids. As an organ probably belonging in the same cate-
gory as the preceding may be mentioned the prepericardial
body, which is a transverse oval organ lying ventral to the
hyoglossus muscle between the thyroids. It possesses a
lymphoid structure and is larger in young than in old frogs
(Gaupp). From its mode of development Maurer classes
the carotid gland also among the epithelial bodies, but its
structure in the adult shows no resemblance to that of the
organs described above.

Another epithelial derivative, but one having a quite dif-
ferent structure from the rest, is the post branchial body, a
paired organ lying beneath the mucus membrane of the
pharynx on either side of the glottis. Each organ, accord-
ing to Maurer, consists of a group of four to six small folli>
cles, lined by cylindrical epithelium, which sometimes bears

234

THE BIOLOGY OF THE FROG

cilia. Its structure resembles that of the thyroid glands,
but the follicles contain a thin fluid instead of a colloidal
substance (Maurer).

A small lymphoid organ, the procoracoidal body, has
been discovered by Gaupp between the coracoid and pro-
coracoid portions of the pectoral girdle. Its mode of origin
has not been traced, but it is found in young larvae with ex-
ternal gills. It probably does not belong in the category
of epithelial bodies, although it bears a certain resemblance
to them in internal structure.

Little is known concerning the functions of any of these
bodies. B. M. Allen has shown that after the removal of
the thyroids of toad larvae the bodies he designates as para-
thyroids increase greatly in size. They do not take on the
peculiar structure of thyroid tissue although their growth
suggests that they may compensate in a measure for the loss
of thyroid function. Schulze has fed frog tadpoles on the
parathyroids of cattle and reports that there followed an
inhibition of growth and an acceleration of metamorphosis
similar to that which follows thyroid feeding.

The Adrenal Bodies. — The adrenal bodies are thin bands
of a golden yellow color extending along the middle of the
ventral surface of the kidneys. They consist essentially of
small solid groups of cells which lie close to the branches of
the renal veins. Among the ordinary epithelial cells com-
posing the main bulk of the organs are scattered cells of
larger size and often of brownish color. The former, accord-
ing to Stilling, correspond to the peripheral or cortical cells
of the adrenals of mammals, and the latter to the central
cells. A third type of cell, which is characterized by its
granular contents, and its taking an intense red color when
stained in eosin, was found by Stilling to occur only during
the summer months. On the other hand, the ventral portion
of the adrenals was found to contain numerous lymphoid

INTERNAL SECRETION AND ENDOCRINE GLANDS 235

cells only in the winter and spring. The "cortical cells" are
derived from the peritoneum, while the "central cells" are
generally regarded as modifications of the cells of the sym-
pathetic ganglia. In the higher vertebrates the central cells
form a single mass which is surrounded by a definite cortex,
but in the frog they are scat-
tered through the cortical cells
in an irregular manner.

Abelous and Langlois found
that if both adrenals of the
frog were destroyed, the oper-
ation was soon followed by
fatal effects; but if only one
adrenal was destroyed, the
animal would continue to live.
If after the destruction of both
adrenals portions of one of
the bodies were transplanted
in the dorsal lymph space, life
was maintained for a consid-
erably longer period than
would otherwise have been
possible. Fatal results follow-
ing the removal of both ad-
renals have recently been reported by Gayda who finds
that frogs succumb after increasing muscular weakness and
lack of heat production in eight to twelve days. It is well
known that the adrenals produce an internal secretion upon
which the life of the organism is dependent. This material
(adrenalin, epinephrin) may be extracted from the bodies
and its physiological action tested. It has been much experi-
mented with among higher animals/ and is now used to a
considerable extent in medicine and surgery. It has the
property of greatly increasing blood pressure by causing a

Fig. 77. — Kidneys and adrenal
bodies. ad, adrenals ; r.v.,
renal veins ; ur, ureters ; v.c,
vena cava.

236

HIE BIOLOGY OF THE FROG

strong contraction of the smooth muscle fibers of the blood

vessels.

Experiments on the effects of the extract of the adrenals
of the frog show that this substance has much the same
properties as among mammals. When injected into the
blood of a mammal, it produces a marked rise in blood pres-
sure; and, on the other hand, injection of the extract from
the mammalian gland into the frog produces very marked
results, which may be fatal if the dose is large. Moore and
Vincent found that " after injection of a glycerin extract
equivalent to about .5 g. of the fresh gland into the dorsal
lymph sac, paralysis immediately
comes on. . . . With larger doses there
are spasms of fibrillary twitchings in
various parts." With smaller doses
(.3 g. of the fresh gland) Oliver and
Schafer found a similar paralysis, but
it came on more slowly. After half an
hour the animal appeared to be
"nearly, if not quite, in a normal con-
dition. "

The Hypophysis, or Pituitary Body.

— The hypophysis, or pituitary body
is an organ consisting of two chief
parts having a quite different origin.
The so-called pars anterior, or an-
terior lobe, arises from the ectoderm
of the dorsal side of the mouth cavity. The pars posterior,
or posterior lobe, first appears as an outgrowth from the
floor of the third ventricle of the brain. In the higher verte-
brates the two parts become very intimately united. They
are less closely associated in the frog, and the so-called pars
anterior comes to lie posterior to the true posterior lobe. In
addition to the anterior and posterior lobes there is a para

--p.inT.

Fig. 78. — Hypophysis
of Rana catesbiana,
ventral side, ant.,
anterior lobe ; p. int.,
pars intermedia ;
p.n. pars nervosa ;
p.t., pars tuberalis ;
t.c, tuber cinereum

(inf undi bulum) .

(After Swingle.)

INTERNAL SECRETION AND ENDOCRINE GLANDS 237

intermedia between the two, and a pars tuberalis, both of
which arise from the original outgrowth from the roof of the
mouth.

In the higher vertebrates complete removal of the pitui-
tary speedily results in death. The two parts of the gland
secrete distinct hormones having very different functions.
According to Robertson, the anterior lobe produces a sub-
stance, tethelin, which has a marked stimulating effect upon
growth, a fact correlated with the circumstance that over-
growth of the pituitary may lead to gigantism and a
relatively large size of the hands, feet and head. From the
posterior lobe a substance, pituitrin, has been extracted
which exerts a strong stimulating influence on unstriated
muscle and on the functions of several glands.

The pituitary is an organ of several functions also in the
Amphibia. The fatal effects reported as resulting from the
removal of the pituitary of the frog do not necessarily
follow. Hogben found that frogs could be kept alive for a
considerable period even after the removal of both lobes.
A few hours after the operation the animals exhibited a re-
markable and persistent pallor. The melanophores were
strongly contracted, and the xantholeucophores, on the other
hand, were in a state of extension. Ordinary influences,
such as cold or darkness, which cause frogs to become dark
had no appreciable effect on the operated animals. The in-
jection of extracts of the pituitary of the frog or the com-
mercial preparation from the posterior lobe of the ox was
found to cause the skin to resume its dark color.

In the frogs from which only the anterior lobe of the
pituitary was removed these effects were not produced.
It has been clearly established that the hypophysis produces
some substance which has a powerful influence upon the
pigment cells as has been illustrated in the chapter on the
skin. The results of these investigations are quite in har-

238

THE BIOLOGY OF THE FROG

mony with those obtained previously by extirpating the
hypophysis of tadpoles. Adler removed the hypophysis
from frog larva? and found that metamorphosis was delayed.
Soon afterward B. M. Allen and P. E. Smith, working
independently, removed the hypophysis from young tad-
poles and observed a reduc-
tion of growth, an inhibition
of metamorphosis and a re-
markable paling of the skin,
so that the tadpoles ap-
peared whitish in color.
There was also a reduced de-
velopment of the thyroid
glands. The pale color of
the larvae is due in part to
the reduction of black pig-
ment, but chiefly to the con-
traction of the melanophores
and the expansion of the
more superficial xantholeu-
cophores. There seems to be
little effect on the develop-
ment of the sex glands. If
the stunted larvae with no
hypophysis are fed with an-
terior lobe substance from
cattle, or if an anterior lobe is engrafted from another larva,
growth to normal size may be resumed.

According to Allen it is the secretion of the intermediate
lobe which is responsible for the distribution of pigment
as is indicated by the fact that extracts of beef in intermedi-
ate lobe will produce normal pigmentation in the operated
larvae. Grafting the intermediate lobe, or the intermediate
and posterior lobes of the frog into larvae without a hy<

Fig. 79. — Normal tadpole to the
left. To the right a tadpole
after removal of the hypophy-
sis. (After P. E. Smith.)

INTERNAL SECRETION AND ENDOCRINE GLANDS 239

pophysis produces a similar effect, but transplanting the
posterior lobe alone does not cause a restoration of the dark
color. The experiments of Hogben with adult frogs, how-
ever, indicate that the posterior lobe has the greatest in-
fluence upon the chromatophores. Whether these different
conclusions are due to experimentation at different ages
is at present uncertain.

The Sex Hormones. — Among the organs of internal secre-
tion must be ranked the gonads which, while not essential
for the maintenance of the life of the individual, exercise,
as in higher vertebrates, a considerable influence upon the
physiological state of the body. As we have already seen
in Chapter II, the reproductive organs are subject to marked
seasonal changes, and along with these, there are corre-
sponding modifications in other organs. The clasping pro-
pensities of the male are evoked in the spring at a time
when the testes are much enlarged. Nussbaum, Meisen-
heimer and Harms have shown that if the testes are re-
moved some months before the breeding season, the clasping
reflex is largely inhibited. If the operation is performed
during the breeding season the clasping may continue much
as usual. The removal of the gonads requires a considerable
time to produce its full effect.

If a testis is grafted into the body of a castrated frog
the clasping reflex will return. Injection of the extract of
the testes, or even the ovary, will cause a recurrence of
the clasping propensity, but the effect lasts only for a few
days. Extracts of most other organs failed to elicit the
same response, but Steinach found that extracts of the brain
of male frogs in the breeding season would evoke the clasp-
ing reflex in castrated males. If the extract was made from
the brain of a male that had been castrated it failed to
produce the same result. The conclusion drawn was that
the testicular hormone is taken up by the substance of the

240

THE BIOLOGY OF THE FROG

brain and thereby checks the usual inhibitory action of that
organ over the clasping reflex which, as experiments have
shown, can be elicited any time of year by cutting the
spinal cord in two near the medulla.

The development of the thumb of the male is also as-
sociated with the sex cycle and, according to Nussbaum,
if male frogs are castrated early the enlargement of the
thumb is prevented. Nussbaum and Meisenheimer find
that if parts of the testis are grafted into the body of a
castrated frog the enlargement of the thumb will again occur
in the breeding period. Similar results were obtained by
injecting extracts from the testis into castrated frogs.
These conclusions have been challenged by G. Smith who
finds that frogs "completely castrated Dec, 1911, after ac-
quiring perfectly smooth thumbs, without a trace of papillae
on them in the summer and autumn of 1912, grew papillae
again in January, 1913, without any experimental treatment
at all," and he concludes that "no evidence exists sufficient
to prove that injection of testes or ovarian extracts, or im-
plantation of testes or ovaries can call forth the growth
of papillae on the thumb of a castrated frog." According
to Harms the thumb of male frogs tends to undergo its
usual seasonal changes even after castration, but these
changes are less marked than in normal individuals. While
it seems clearly established that the testis produces an in-
ternal secretion which is responsible for the appearance of
the clasping reflex, it is not proven that the same hormone
affects the development of the thumb and seminal vesicles,
even if the changes in these organs may be stimulated by a
testicular secretion. The seminal vesicles, which are much
more fully developed in the breeding season, are much less
developed, according to Nussbaum, in castrated frogs. Other
investigators are not convinced that the observed changes
are caused by castration, and hold that the changes de-

INTERNAL SECRETION AND ENDOCRINE GLANDS 241

scribed would have occurred in normal animals. Despite
the several studies on the relation of sex hormones to
secondary sexual characters in the frog, the subject is still
in a state of considerable confusion.

The Pineal Gland. — We have already briefly described
the pineal gland in Chapters III and V. Observations and
experiments on higher vertebrates indicate that this struc-
ture is an organ of internal secretion. Pineal tumors are
frequently associated with rapid growth and a precocious
development of the sex organs, and the extirpation of the
pineal gland has been followed by increased growth and a
more rapid development of the testes. The functions of the
pineal gland are still very imperfectly understood. Little
is known of its function in the frog except that feeding
tadpoles on pineal extract causes a paling of the skin, due
to a strong contraction of the melanophores. The effect
is temporary and apparently does not occur in adult frogs
(M'Cord and Allen1).

Internal Secretions as a Means of Functional Correla-
tion.— From what has been said it is evident that«internal
secretions play an important role in securing the coordina-
tion of functions of the various organs of the body. They
act as regulative agents, making possible the partial control
of one organ by another independently of the central
nervous system. Organs through their internal secretions
may act and react upon each other, and in this way bring
about the harmonious functioning of the different parts
which is essential to the life of the whole.

1 M'Cord and Allen, Jour. Exp. ZooL, Vol. 23, p. 207, 1917.

242

THE BIOLOGY OF THE FROG

REFERENCES

Abelous, J. E. Sur Faction antitoxique des capsules surrenales,
C. R. Soc. Biol., 1895.

Abelous et Billard. Rocherches sur les fonctions du thymus chez
La grenouille. Arch. Phys. Norm, et Path. Annee, 28, Ser. 5, T. 8,
1896.

Abelous et Langlois. Note sur les fonctions des capsules surre'
nales chez la grenouille. C. R. Soc. Biol., 1891. La mort des gren-
ouilles apres la destruction des capsules surrenales, I.e., 1891. Toxicite
de l'extrait alcoolique du muscle de grenouilles prives de capsules
surrenales, I.e., 1892. Recherches experimentelles sur les fonctions de
capsules surrenales de la grenouille. Arch. Phys. Norm, et Path. (5).
T. 4, 1892. Sur les fonctions des capsules surrenales, I.e. (5), T. 4,
1892.

Adler, L. Metamorphosestudien an Batrachierlarven. Arch. Entw.-
mech., Bd. 40, 1, 1914.

Allen, B. M. The Results of the Extirpation of the Anterior Lobe
of the Hypophysis and of the Thyroid of Rana pipiens Larvae.
Science, n.s. Vol. 44, 755, 1916. For other papers on amphibian en-
docrines see Biol. Bull., Vol. 32, 117, 1917; Vol. 36, 405, 1919; Jour.
Exp. Zool., Vol. 24, 499, 1918; Vol. 30, 189, 201, 1920.

Aron, M., and Alfonsi, N. Recherches sur l'histogenese, etc., des
ilots pancreatiques endocrines des Batraciens. C. R. Soc. Biol., T.
91, 609, 1924.

Baber, E. C. Researches on the Minute Structure of the Thyroids.
Phil. Trans., 1881, part 3.

Bolau, H. Glandula thyroidea und Glandula Thymus der Am-
phibien. Zool. Jahrb. Abth. f. Anat., Bd. 12, 1899.

Eidmann, H. Ueber Wachstumsstorungen bei Amphibienlarven.
Arch. Entw.-mech., Bd. 49, 510, 1921.

Gaule, A. Biological Changes in the Spleen of the Frog. Jour.
Morph, Vol. 8, 1893.

Gayda, T. La produzione di calore nella rana in diverse con-
dizioni sperimentali. Nota V. Arch. Sci. Biol., T. 4, 93, 1923.

Gonfrin. Recherches physiol. sur le fonction des glandes surrenales.
Rev. med. Suisse romand., T. 16, 1896.

Gudernatsch, J. T. Feeding Experiments on Tadpoles I. Arch.
Entw.-mech., Bd. 35, 456, 1912. Feeding Experiments on Tadpoles II
Am. Jour. Anat., Vol. 15, 431, 1914.

INTERNAL SECRETION AND ENDOCRINE GLANDS 243

Hammar, J. A. 1st die Thymusdriise beim Frosch ein lebenswich-
tiges Organ? Arch. ges. Phys., Bd. 110, p. 337.

Harms, W. Hoden- und Ovarialinjektionen bei Rana fusca-Kas-
traten. Arch. ges. Physiol., Bd. 133, 27, 1910. Korper und Keim-
zellen. Berlin, 1926. (Full bibliography.)

Herring, P. T. The Action of Pituitary Extracts on the Heart of
the Frog. Jour. Phys., Vol. 31.

Hogben, L. T. The Pigmentary Effector System. Edinburgh and
London, 1924.

Mayer, S. Zur Lehre von der Schilddruse und Thymus bei den
Amphibien. Anat. Anz., Bd. 3, 1888.

Maurer, F. Schilddruse, Thymus, und Kiemenreste der Amphibien.
Morph. Jahrb., Bd. 13, 1888. Die Epidermis und ihre Abkommlinge,
1895.

Moore and Vincent. The Comparative Chemistry of the Supra-
renal Capsules. Proc. Roy. Soc, London, Vol. 62, 1897.

Nussbaum, J. Hoden und Brunstorgane des braunen Landfrosches.
Arch. ges. Physiol., Bd. 126, 1909.

Oliver and Schafer. On the Physiological Action of the Extract of
the Suprarenal Capsules. Jour. Phys., Vol. 16, 1894.

Romeis, B. Die Wirkung der Verfiitterung frischer Thymus auf
Froschlarven. Arch. mik. Anat., Bd. 104, 273, 1925.

Schulze, W. Versuche liber den Einfluss endokriner Drusensub-
stanzen auf die Morphogenie. Arch. Entw.-mech., Bd. 48, 489, 1921.

Schwartz, A., and Bricka, M. L'action de Finsuline sur la gly-
cemie, etc., des grenouilles. C. R. Soc. Biol., T. 91, 1428, 1925.

Sharpy-Schafer, E. The Endocrine Glands, 2 vols. London,
1924-1926.

Sklower, A. Das incretorische System im Lebenscyclus der
Frosche (Rana temporaria). Zeit. vergl. Physiol., Bd. 2, 474, 1925.
(Full bibliography.)

Smith, G. The Effect of Castration on the Thumb of the Frog
(Rana fusca). Zool. Anz., Bd. 41, 623, 1913.

Smith, P. E. Experimental Ablation of the Hypophysis in the
Frog Embryo. Science, n.s., Vol. 44, 280, 1916. See also Anat. Rec,
Vol. 11, 57, 1916. The Pigmentary, Growth, and Endocrine Dis-
turbances Induced in the Anuran Tadpole by the Early Ablation of
the Pars Buccalis of the Hypophysis. Am. Anat. Mems. no. 11, 1920.

Swingle, W. W. Thyroid Transplantation and Anuran Meta-
morphosis. Jour. Exp. Zool., Vol. 37, 219, 1923. (Contains references
to author's previous paper.<0

244

THE BIOLOGY OF THE FROG

Treupel, J. Stoffwechseluntersuchung bei einem mit "Iodothyrin"
(Thyroiodin) behandelten Falle von Myxoedem und Mittheilung
einiger Thierversuche mit Iodothyrin (Thyroiodin). Munchener
med. Wochenschr. 43 Jahrg., 1896.

Uhlenhuth, E. The Internal Secretions in Growth and Develop-
ment of Amphibians. Am. Nat., Vol. 55, 193, 1921. (Bibliography.)

CHAPTER XIII

THE SKELETON

In the skeleton, or bony framework, of the frog we com-
monly distinguish two main divisions, the axial, consisting
of the skull and vertebrae, and the appendicular, composed
of the limbs and their girdles or supports. We shall begin
our description with the skull.

Bones of the Cranium. — In the skull we may distinguish
the cranium, or part inclosing the brain and principal sense
organs, and the visceral skeleton, which forms the jaws and
hyoid arch. The cranial portion of the skull is relatively
small, and is narrowest in the central part, between the very
large spaces, or orbits, which lodge the eyes. At the pos-
terior end is a large aperture, the foramen magnum, through
which the spinal cord passes. On either side of this opening
are the exoccipital bones, which are separated from each
other above and below by a small piece of cartilage. At
the sides of the foramen magnum these bones bear a pair
of rounded prominences, the occipital condyles, which ar-
ticulate with the atlas, or first vertebra. Just external to
each condyle is a small aperture for the exit of the vagus
nerve.

At the sides and in front of the exoccipitals lie the prodtic
bones, each of which forms a ring-like lateral projection on
each side of the skull, which incloses the inner ear. An-
teriorly each prootic is perforated by a large aperture,
through which pass the 5th, 6th, and 7th cranial nerves.
On the outer side there is an opening, the foramen ovale,

245

246

Til E BIOLOGY OF THE FROG

Olfactory capsule^
Sphenethmoicl^
FoataBelles^

Anterior
corau ot

hyoid
Fontanelles I

/Preiiiaxllla
-Vomer

Neural spine zygopophysls
Lamina v

Sacral vertebra

Urostyle-

Acetabulum

Calcaneum

Astragalus

Fig. 80. — A, skeleton of Rana temporaria. The left limbs, left
shoulder girdle and membrane bones of the left side of the skull
are removed. Cartilaginous parts dotted. / — F, digits. B, the
fourth vertebra seen from in front. (From Hegner, slightly altered
from Howes.)

THE SKELETON

247

which is plugged with cartilage against which abuts the
inner end of the columella of the ear.

In the ventral side of the skull is a large bone, the para-
basal, or parasphenoid, which is in the shape of a dagger

Fig. 81. — Skull of Rana temporaria. A, from beneath, with the mem-
brane bones removed from the right side (left of figure) ; B, from
the left side, showing the lower jaw and hyoid ; C, from behind
Names of cartilage bones in thick ; those of membrane bones in
italic capitals, a.c.hy, anterior cornu of hyoid ; b.hy, body of hyoid ;
col, columella ; DNT, dentary ; ex.oc, exoccipital ; for.mag, foramen
magnum ; FR.PA, fronto-parietal ; m.mck, mento-meckelian bones ;
MX, maxilla ; NA, nasal ; nv. 2, optic foramen ; nv. 5, 7, foramen for
the fifth and seventh nerves ; oc.cn, occipital condyle ; olf.cp, olfac-
tory capsule ; ot.pr, otic process ; PAL, palatine ; pal.qu, palato-
quadrate ; PA.SPH, parasphenoid ; p.c.hy, posterior cornu of hyoid ;
ped, pedicle ; PMX, premaxilla ; pr.ot, prootic ; PTG, pterygoid ;
QU.JU, quadrato-jugal ; sp.etii, sphenethmoid : SQ, squamosal;
stp, stapes; VO, vomer. (From Newman, slightly altered from
Howes. )

without any handle; the lateral portions underlie the two
prootics.

The fronto-parietal bones form most of the roof of the
skull. Along the middle line they are united by the sagittal
suture. Each represents two bones, a frontal and a parietal,
and in the early stages of the development of the skull these

248

THE BIOLOGY OF THE FROG

elements arc separate, but subsequently they fuse into a
single bone.

The anterior end of the cranium is surrounded by a bony
ring, the ethmoid (or sphenethmoid) bone. This is over-
lapped by the fronto-parietals above and the parabasal be-
low, and is separated from the prootics behind by quite a
long interval of unossified cartilage. The anterior part of
the ethmoid is widened out and divided into two chambers
by a median vertical partition. The expanded portion
forms the posterior wall of the nasal cavity; the latter may
be seen to communicate with the cranial cavity by a pair
of small openings through which the olfactory nerves pass.
The rest of the nasal capsules are formed mainly by
cartilage.

The nasals are two narrowly triangular bones, lying above
the nasal capsules; their bases, which lie near each other in
the middle line, are separated from the fronto-parietals by
a small part of the roof of the ethmoid.

The vomers lie ventral to the nasal capsules; each has
three outer processes, between the two posterior of which
occur the internal nares; the ventral surface bears the
vomerine teeth.

Suspensorium and Jaws. — The jaws are attached to the
cranium by means of an intermediate suspensory apparatus
in which the following separate bones are to be distin-
guished:—

(1) The tympanic (squamosal), a T-shaped bone, the
main limb of which extends outward and backward to the
angle of the jaws; the posterior end of the cross piece
articulates with the prootic, while the anterior end extends
obliquely downward in front. Below the tympanic lies
(2) the pterygoid, a triradiate bone, the inner limb of wThich
attaches to the outer side of the prootic, while the two
outer limbs diverge, the one running beneath the long stem

THE SKELETON

249

of the tympanic to connect with the posterior end of the
upper jaw, the other extending forward and joining the
upper jaw near its middle. The tympanic and pterygoid
are separated from each other by a strand of hyaline
cartilage.

(3) The palatines are slender, rodlike bones on the lower
side of the cranium, which extend from the anterior end
of the ethmoid to the upper jaw.

The upper jaw, or maxillary arch, is composed of three
pairs of bones. The posterior portion of the arch is formed
by the quadrato-jugals. These are short bones, devoid of
teeth, articulating behind with the pterygoid and tympanic,
and joining the maxillary in front by an oblique suture.
The maxillaries are the largest bones of the upper jaw; they
connect with the premaxillaries in front, and quadrato-
jugals behind; they are furnished with teeth throughout
their length. On the upper side each bears a frontal process
which is overlapped by the nasal. The intermaxillaries or
premaxillaries are the two small bones which form the apex
of the maxillary arch ; they are furnished with teeth and are
produced backward on the upper side into the facial
processes which are instrumental in closing the nares in
respiration.

The lower jaw, or mandibular arch, is composed of a
central core, called Meckel's cartilage, which is partly sur-
rounded by two membrane bones. The bone at the
proximal end is called the angulare, or angulo-spleniaL
Meckel's cartilage runs in a groove along the outer side
of this bone and widens out at the posterior end, where
it forms the facet for articulation with the suspensorium
above. A short distance in front of the articulation
the angulare bears a prominence, the coronoid process,
which gives attachment to the muscles for closing the
jaw.

250

THE BIOLOGY OF THE FROG

The dentale lies on the outer side of the distal end of the
angulare, overlapping Meckel's cartilage, which there runs
between these two bones. The apical portion of the man-
dibular arch consists of two short movable elements, the
mento-meckelian bones, which result from the ossification
of the distal portions of Meckel's cartilage. They underlie
the premaxillaries, and when they are raised, cause a corre-
sponding elevation of the latter bones.

The Hyoid. — The branchial skeleton of the frog is com-
posed of the hyoid cartilage and its processes. The body
of the hyoid is a large flat plate of hyaline cartilage, more
or less quadrate in general outline, lying in the floor of the
buccal cavity. Its anterior margin, where the base of the
tongue is inserted, is strongly concave. At the anterior end
of the body arise the anterior cornua, which are long, slen-
der rods of cartilage, which pass backward and upward on
either side of the throat and join to the prootic bones of the
skull.

On the inner side of the base of each cornu is a short
anterior process. The alary processes, flattened, distally
expanded plates of cartilage, arise just behind the anterior
cornua. The postero-lateral angles of the body are pro-
duced to form the postero-lateral processes. The thyroid
processes diverge from the middle part of the posterior
margin of the body and lie on either side of the larynx,
which they help to support. They are the only parts of the
hyoid apparatus to become ossified.

The Cartilaginous Cranium. The skull of the frog is
composed of cartilage to a much greater extent than in the
higher vertebrates. Only here and there does its original
cartilaginous basis become converted into bone. The various
bones which constitute the skull may be divided as regards
their origin into cartilage bones, which result from the
ossification of cartilage, and membrane bones, which are

THE SKELETON

251

252

THE BIOLOGY OF THE FROG

developed from membranes. The latter are more superficial
in position and may be stripped off from the rest of the
skull leaving a sort of cartilaginous box with ossifications
here and there which represent the so-called cartilage bones.
In the cranium only the exoccipitals, the prootics, and the
ethmoid are cartilage bones, the fronto-parietal, parabasal,
nasals, and palatines are developed from membranes, and
may readily be separated from the underlying cartilage.
The cartilaginous cranium forms an almost complete case
for the brain; the roof, however, is incomplete, there being a
large oblong opening, or fontanelle, the fenestra frontalis,
near the middle, and a pair of smaller openings, fenestra
parietales, farther back.

The suspensorium and jaws also have a cartilaginous
basis which is continuous with that of the cranium proper.
The bones of these parts, with the exception of the quadrato-
jugal and the mento-meckelian bones, are developed from
membrane. A cartilaginous bar runs between the pterygoid
and tympanic to the angle of the upper jaw, whence it ie
continued forward beneath the maxillary as far as the pala^
tine bone, where a transverse process joirfe the posterior
end of the nasal capsule. The exposed cartilage on the
side of the cranium is perforated by a large foramen for
the optic nerve and by smaller (finings, for the third and
fourth nerves. The angulare and dentary of the lower
jaw are membrane bones applied to the cartilaginous core,
or Meckel's cartilage. The thyroid processes of the hyoid
are developed from cartilage.

The frog's skull represents a type between the skulls of
the lower fishes and those of the higher vertebrates. In
such forms as sharks and skates the cranium and its various
appendages are entirely composed of cartilage. Higher up
in the scale we meet with fishes, such as some of the ganoids,
in which ossifications of membranes, or dermal bones, be-

THE SKELETON

253

come applied to the cartilaginous cranium, while the latter
remains in great part unossified. In the birds and mam-
mals ossification of the original cartilaginous basis of the
cranium has become almost complete, and the bones that
are developed from membrane enter into more intimate
relations with the cartilage bones than they do in the frog,
the whole forming a structure which is very firm and
compact.

The Vertebral Column. — The vertebral column of the
frog consists of ten bones of which the first nine are verte-
brae proper, the tenth, or urostyle, being a long rodlike
bone extending from the ninth vertebra to the apex of the
pelvic girdle. A typical vertebra of which we may take the
third as an example consists of the following parts: —

(1) The centrum, a basal portion, which is oval in cross
section and concave in front for the reception of the centrum
of the preceding vertebra and convex posteriorly.

(2) The neural arch, which incloses the neural canal, in
which is lodged the spinal cord. The neural arch is pro-
duced in the mid-dorsal line into a projection called the
neural spine, and at the sides it bears a pair of elongated
transverse processes which extend almost at right angles to
the body; the tips of these processes are furnished with
cartilaginous epiphyses. Both the posterior and anterior
margins of the neural arch bear a pair of short articulating
processes or zygapophyses, by means of which the successive
vertebrae are joined together; the articular faces of the
anterior articulating processes look upward and inward, and
are covered by the posterior zygapophyses of the preceding
vertebra, the articulating faces of which look downward
and outward. The opposed faces of the adjoining zygap-
ophyses are smooth and allow a limited gliding movement
when the spinal column is bent from side to side.

The first vertebra, or atlas, differs from the others in

254

THE BIOLOGY OF THE FROG

having no transverse processes, in the absence of the an-
terior zygapophyses, and in having in front a pair of oval,
concave facets for articulating with the occipital condyles
of the skull. The ninth vertebra has the transverse
processes very strong and directed obliquely backward; it

has no posterior zyga-
pophyses, and the pos-
terior surface of the cen-
trum bears a pair of
prominences for articu-
lation with the urostyle.

The urostyle\ is ele-
vated on the dorsal side
into a prominent keel
which extends nearly to
the posterior end. The
anterior surface posses-
ses a pair of cavities for
articulation with the
ninth vertebra. The vertebral canal is small and triangu-
lar in outline. There is a pair of small openings through
the sides of the urostyle near the anterior end for the exit
of the last pair of spinal nerves.

The centra of the vertebrae are joined together by means
of pads of hyaline cartilage; connecting ligaments extend
along both the ventral and the dorsal surfaces of the centra ;
and the arches and neural spines are joined by ligaments.
The spinal nerves make their exit through the intervertebral
foramina, between the sides of the neural arches.

The Pectoral Girdle and Sternum. — The pectoral girdle
is a bony arch which gives support to the fore limbs. The
upper end of the girdle is formed by a flat, distally expanded
portion, the suprascapula, which is composed of cartilage
which is more or less calcified at the base. The supra-

Fig. 83. — Transverse section of shoul-
der girdle, cor, coracoid ; ep.cor, epi-
coracoid ; gl, glenoid cavity ; hu, hu-
merus ; scp, scapula ; s.scp, supra-
scapula; v.S, third vertebra. (After
Parker and Parker.)

THE SKELETON

255

scapula articulates below with the long scapula, which is
oblong and constricted in the middle; the posterior side of
the lower end forms part of the glenoid fossa, which receives
the head of the humerus. A notch occurs at the glenoid
fossa, dividing the lower end of the scapula into dorsal or
glenoid part from a ventral acromial portion.

Fig. 84. — Middle part of the shoulder girdle of the frog from below.

Co, coracoid ; Co', epicoracoid ; CI, clavicle ; Ep, episternum ; G,
glenoid cavity ; Fe, fenestra ; KC, cartilage between scapula and
clavicle ; Kn, xiphisternum ; m, junction of epicoracoids ; S, scapula ;
St, sternum. (After Wiedersheim.)

From the lower end of the scapula two bars extend
toward the middle line. The anterior of these consists of
a bar of hyaline cartilage, the procoracoid, in front of, and
partly inclosing which is a membrane bone, the clavicle.
At its outer end the clavicle is bent forward, and applied to
a forward projection of cartilage, the acromion, at the lower
end of the scapula. The posterior bar, the coracoid, is a

256

THE BIOLOGY OF THE FROG

stout bone constricted in the middle, and broadly expanded
at its inner end; at its outer extremity it forms a portion of
thr glenoid fossa. Between the two ends of the coracoid,
and extending forward between the clavicles, are the two
epicor "coid cartilages, which are usually more or less calci-
fied along the ventral side of their line of junction.

The episternum lies in front of the epicoracoids and
consists of a basal piece of bone and a terminal piece of
cartilage, which has an almost circular expansion at the
anterior end.

The sternum proper resembles the episternum in general
shape, and in being composed of a proximal piece of bone
(mesosternum) , and a distal expanded piece of cartilage.
The latter has a notch in the middle of its posterior margin,
which receives the anterior abdominal vein just before it
leaves the body wall. Both the sternum and the episternum
are capable of a limited vertical movement, especially at
the flexible cartilaginous ends.

In addition to forming a place of attachment for the
muscles which move the fore limbs, the pectoral arch pro-
tects the various internal organs, such as the lungs, heart,
etc., and serves to maintain the general form of the body.
It also gives attachment to the ventral muscles which draw
back the hyoid, and depress the floor of the mouth, and for
these purposes the distal ends of the sternum and epister-
num are flattened and expanded, their cartilaginous consis-
tency enabling them to accommodate themselves to the
changes produced in the body wall.

The Fore Limbs. — The upper bone of the fore limbs is
the humerus; its proximal end, or head articulates with the
glenoid cavity of the pectoral girdle; the distal, or lower
extremity has a rounded articular prominence in the middle,
on either side of which is a small projection, or condyle. A
large crest, the deltoid ridge, extends from the head of the

THE SKELETON

257

ft

humerus to about the middle of the ventral side. At the
distal end of the humerus there is a ridge above each of the
two condyles; the inner ridge is much larger in the male
than in the female.

The skeleton of
the forearm consists
of the radio-ulna,
which arises from
the fusion of two
originally distinct
bones, the radius
and ulna, the line of
union between which
is very marked, es-
pecially at the distal
end. The postaxial
or ulnar part is pro-
duced backward at
its upper end to form
the olecranon, which
fits over the rounded
end of the humerus
at the joint of the
elbow. The distal
end of the radio-
ulna is widened, and
ends in two epi-
physes, one for each
of the component bones.

The carpus, or wrist of the frog, contains six bones ar-
ranged in two rows. In the proximal row the ulnare and
radiate are situated at the ends of the ulna and radius, re-
spectively, and at the inner or preaxial side of the radiale is
the centrale. In the distal row the first carpal occurs just

Fig. 85. — Longitudinal sections of the larg-
er bones of the limbs. A, humerus ; B,
radio-ulna ; C, femur ; D, tibio-fibula.
cn, condyle ; /, foramen for artery ; fi,
fibula ; hd, head ; ra, marrow ; ol, ole-
cranon ; p, bony partition ; ra, radius ;
sh, shaft; ti, tibia; ul, ulna. (After
Parker and Parker.)

25S

THE BIOLOGY OF THE FROG

behind the rudimentary thumb; the second carpal, a very
small bono, lies behind the second digit; the outer carpal
is of relatively large size, and is formed by the fusion of
three originally distinct bones one for each of the three
outer digits of the hand. Beyond the wrist are the five
metacarpals, which form the skeleton of the proximal part
of the hand. The first metacarpal is rudimentary and
usually cartilaginous in the female,
but in the male it is larger, and be-
comes calcified or even ossified. The
other metacarpals are elongated cyl-
indrical bones, somewhat expanded at
the two ends; the inner one of the four
is stouter in the male than in the fe-
male, and bears a prominent crest on
the inner margin. The metacarpals,
except the first, are succeeded by the
phalanges, of which each of the two
outer digits have three, while the two
inner ones have but two.

The Pelvic Girdle.— The pelvic
girdle which supports the hind limbs
is a V-shaped mass of bone, the apex
of which lies at the posterior end of
the skeleton, and receives the tip of
the urostyle; the two anterior ends are united to the large,
transverse processes of the ninth or sacral vertebra. Each
half of the pelvic girdle is composed of three elements,
the ilium, the ischium, and the pubis. The first named
is a long bone, attached to the ninth vertebra in front,
and meeting its fellow behind; on the dorsal side is a
high thin crest, at the anterior end of the ventral side of
the junction of the two ilia is a prominence called the an-
terior spine of the pelvis. The ilium forms the anterior

Fig. 86.— Pelvic girdle
of the frog from the
right side. G, ace-
tabulum ; II, ilium ,
Is, ischium ; P, pu-
bis. (After Parker
and Parker.)

THE SKELETON

250

part of the cup, or acetabulum, which receives the head of
the femur.

The pubis is a triangular mass of hyaline, or in older
frogs calcified, cartilage on the ventral side of the pelvic
girdle; it forms a part of the lower side of the acetabulum.

The posterior portion of the pelvic girdle is represented
by the ischium; it forms the posterior part of the acetabulum,
and extends forward dorsally as far as the top of a promi-
nence, the posterior spine of the pelvis, which is situated
above, and a little behind the acetabulum; here it is fused
with the ilium; the suture between these bones can best be
seen in young frogs. The lines of union of the pubic and
ischial bones of opposite sides are known respectively as the
pubic and ischial symphyses.

The pelvic girdle of the frog is remarkable on account of
the elongation of the ilia, the reduction of the pubis and
ischium, and the intimate fusion of the two latter with the
posterior expanded part of the ilium to form an almost
circular mass.

The Hind Limbs. — The skeleton of the hind limb of the
frog is constructed on essentially the same plan as that of
the fore limb. It consists of an upper bone, the femur,
corresponding to the humerus of the fore limb. Below this
is the tibio- fibula, corresponding to the radio-ulna; and
following the tibio-fibula the tarsus and foot, corresponding
to the carpus, and hand. The femur is an elongated, cylin-
drical, very slightly sigmoid bone; it articulates by
its expanded and rounded head with the acetabulum above,
forming the hip joint.

The tibio-fibula is an elongated, very slightly bent bone
somewhat expanded and flattened at either end, where it is
marked on both sides by a groove which indicates its forma-
tion from two bones, the tibia and the fibula, which were
originally separate. The tibia is preaxial in position and

260

THE BIOLOGY OF THE FROG

corresponds to the radius of the forearm; the fibula is post-
axial and corresponds to the ulna. Near the middle the
tibio-fibula is perforated by a foramen for the anterior tibial
artery.

The tarsus of the frog is peculiarly modified in that the
proximal portion is much elongated and consists of but two
bones; the preaxial bone next to the tibia is called the tibi-
als, the postaxial one next to the fibula, the fibulare; the
two bones are united at their ends, inclosing a narrowly
oval space between them. The centrale is represented by
a small bone on the preaxial side of the distal end of the
tibiale; at its distal end it supports the prehallux. The
tarsalgia (which correspond to the carpalia of the hand) are
much reduced both in size and number. There is a small
first tarsal behind the base of the first metatarsal bone.
Behind the second and third metatarsals is a small bone
which represents the fused second and third tarsalia. The
fourth and fifth tarsalia are absent. There are five meta-
tarsal bones, all of which are elongated and cylindrical.
Of the phalanges the first and second toes contain two each,
the third and fifth toes three each, and the fourth toe four.
The small prehallux is composed usually of two pieces.

REFERENCES

Parker, W. K. On the Structure and Development of the Skull of
the Common Frog. Phil. Trans., Vol. 161, 1871. On the Structure
and Development of the Skull in the Batrachia. Phil. Trans., Vol.
166, part 1, 1881.

Parker and Bettany. The Morphology of the Skull, London, 1887.

CHAPTER XIV

THE MUSCULAR SYSTEM

The function of muscle is the production of movement
through contraction. The muscles of the frog retain their
vitality for a long time after they have been removed from
the body, and they are consequently well adapted for
physiological experiments. The large gastrocnemius, or
calf muscle, of the frog is so favorable a one for investiga-
tion that much of what is known of the general physiology
of muscular activity is derived from a study of this object.
When a muscle contracts, it increases in thickness as it de-
creases in length. Contraction may be brought about, as
is readily demonstrated with a fresh muscle, by a variety
of causes, since it follows upon the application of nervous,
thermal, mechanical, chemical, or electrical stimuli. The
response to stimulation in voluntary muscle takes place
very quickly but in involuntary muscle the response is much
slower.

Most of the muscles are attached by one or both ends to
bones. The attachment in some cases is direct, in others
it is by means of a tendon, which is a band of very tough,
inelastic connective tissue. The outer surface of a muscle
is covered by a connective tissue membrane, or fascia,
which is more or less elastic. The tendons of many muscles
are formed by a continuation of the fascia, which becomes
thicker toward the end of the muscle, where it graduates
into a dense fibrous band. The end of a muscle which is
fastened to a relatively immovable part is called the origin;

261

262

THE BIOLOGY OF THE FROG

the more movable end is the insertion. The contraction of
a muscle has the effect of drawing the origin and insertion
nearer together. The gastrocnemius muscle, which may be
taken as an illustration, has its origin by two heads, one
from a tendinous band extending from the lower end of the
femur to the tibio-fibula, the other by a narrow tendon
which joins the tendon of the triceps on the anterior side
of the thigh. At its lower extremity the muscle is inserted
by means of a strong tendon which passes over the ankle
joint and spreads out over the lower or plantar surface of
-—the foot. When the muscle contracts, it may produce two
„. movements. It may straighten out, or extend the foot, and
hence is spoken of as an extensor of that member. It may
also bend the leg upon the thigh at the knee, a movement
which is called flexion. The gastrocnemius is designated,
therefore, as an extensor of the foot and a flexor of the leg.
A muscle which draws the limb posteriorly toward the long
axis of the body is called an adductor, one which pulls it
in the opposite direction an abductor. Then there are
rotators, which cause a limb to rotate about its long axis;
levators, which raise a part, such as the muscles which raise
the lower jaw; and depressors, which produce the opposite
movement, such as the depressor mandibulce, which lowers
the jaw. The kind of movements a limb may make is
dependent on the nature of its joints and the number and
attachments of its muscles; and the actions which the frog
as a whole is capable of performing are dependent in a
similar manner upon the organization of its skeletal and
muscular systems, although the order and combinations of
the movements are directed by impulses from the central
nervous system.

The action of muscles may be well illustrated by a study
of some of the muscles of the hind leg. If the skin be
stripped off from the leg and the frog laid on its back, the

THE MUSCULAR SYSTEM

263

following muscles may be seen on the ventral side of the
thigh: —

The sartorius, a narrow muscle having its origin on the
ilium, just in front of the pubis. It crosses the thigh ob-
liquely and is inserted by a tendon a short distance below
the head of the tibia. When it contracts, it flexes the leg
on the thigh and pulls the whole limb forward and ventrally.

The adductor magnus, a large thick muscle lying behind
the sartorius, which it crosses at its lower end. Its origin
is from the pubis and ischium, and it receives a small slip
which originates on the tendon of one of the heads of the
semitendinosus. It is inserted into the distal end of the
femur. Its action is to bend the thigh ventrally and to pull
it anteriorly or posteriorly according to the position of the
limb. When the thigh is pulled forward so that a line con-
necting the centers of the origin and insertion of the muscle
lies in front of the head of the femur, the contraction of the
adductor magnus has the effect of moving the limb still
farther forward while pulling it ventrally. When the thigh
is bent back so that the head of the femur lies in front of a
line connecting the centers of the origin and insertion, the
contraction of the muscle has the opposite effect. The
adductor magnus thus acts as an adductor or an abductor
according to circumstances.

The adductor longus, a narrow muscle arising from the
ventral part of the ilium just dorsal to the head of the
sartorius; distally it joins the adductor magnus. It is partly
covered by the sartorius, but a small portion of it is ex-
posed along the preaxial side of that muscle. It pulls the
thigh forward and ventrally.

The triceps femoris, a very large muscle covering the
whole front of the thigh. It arises by three heads. The
anterior head (caput anticum or cruralis) arises from the
anterior border of the acetabulum at the junction of the

?64 THE BIOLOGY OF THE FROG

THE MUSCULAR SYSTEM

265

ilium and pubis. The middle head (caput medium) is a
short, flat muscle which arises from the ventral side of the
ilium near the middle and joins the fascia of the rest of
the muscle at about the proximal third of the anterior mar-
gin of the thigh. The posterior head (caput posticum, vastus
externus, gluteus magnus) arises from the posterior end of
the crest of the ilium; distally its fibers join those of the
cruralis or anterior head. The whole muscle is inserted by
a very strong tendon which passes over the knee and joins
the upper end of the tibio-fibula. A tendinous connection
passes also to one head of the gastrocnemius. The action
of the whole muscle is to extend the crus and draw the whole
leg forward.

The gracilis major, a large muscle lying along the pos-
terior side of the ventral surface of the thigh. It arises
by a small tendon from the posterior margin of the ischium.
Its distal tendon divides into two parts, the one passing
beneath the tendon of the sartorius to be inserted into the
preaxial side of the proximal end of the tibia, the other
passing dorsal to the tendinous insertion of the semitendi-
nosus and joining the posterior side of the head of the tibio-
fibula. Near the middle the muscle is crossed by an oblique
tendinous inscription. The gracilis major pulls the femur
backward and either flexes or extends the crus according to
the position of the latter in relation to the femur. If the
crus is flexed so that it makes an angle of less than 90°
with the femur, the action of the gracilis is to flex it still
more. If, however, the crus is partly extended so that it
makes a considerably greater angle than 90° with the femur,
the contraction of the gracilis still further extends it. The
difference of the action of the muscle under these different
conditions depends upon the change in the fulcrum which
is brought about by the bending of the leg.

The gracilis minor is a slender muscle which arises from

206

THE BIOLOGY OF THE FROG

ta long.

a tendon behind the ischiac symphysis. Distally it joins the
tendon of insertion of the gracilis major. The muscle is

also attached to the
skin of the posterior
side of the thigh. Its
action is similar to
that of the gracilis
major.

The two following
muscles appear on
the dorsal side of the
thigh: the semi-
membranous, a large
muscle lying on the
posterior side of the
dorsal surface of the
thigh just above the
gracilis major. It
arises by a broad
fleshy attachment
from the dorsal half
of the posterior mar-
gin of the ischium.
It is inserted by a
short tendon which
passes beneath the
tendon of origin of
the gastrocnemius
into the proximal
end of the tibio-fib-
ula behind the knee.
There is an oblique tendinous inscription running across
this muscle as in the gracilis major. The semimembranosus
adducts the thigh, or pulls it backward, and, like the graci-

Fig. 88. — Muscles of the hind leg of the
frog seen from the dorsal side. c.a.,
cm., c.p., caput anticum, caput medium,
and caput posticum of the triceps ex-
tensor femoris ; gast, gastrocnemius ; il,
ilium ; il. ext., iliacus externus ; il. fib.,
ilio-fibularis ; per, peroneus ; py, pyrifor-
mis ; sent, semimembranosus ; /, a, tendo
Achillis : t. a. long, tibialis anticus lon-
gus ; ust, urostyle.

THE MUSCULAR SYSTEM

267

Us, flexes or extends the leg according to whether it is in a
flexed or an extended position.

The ilio-fibularis, a slender muscle lying between the
semimembranosus and the posterior head of the triceps
jemoris. It arises from the ilium, just behind the posterior
end of the dorsal crest, and it is inserted into the proximal
end of the fibula. It draws the thigh dorsally and flexes
the leg.

The semAtendinosus, a slender muscle which is covered by
the gracilis major. It arises by two tendinous heads from
the ischium. The ventral head passes between the dorsal
and ventral heads of the adductor magnus and affords a
point of attachment for the small third head of the latter
muscle. The two heads unite near the middle of the thigh ;
the muscle is inserted by a narrow tendon into the preaxial
side of the proximal end of the tibia. It adducts the femur
and flexes the leg.

The pyriformis, a short, slender muscle extending from
the tip of the urostyle tc a short distance beyond the head
of the femur. It lies between the semimembranosus and
the posterior head of the triceps. It pulls the urostyle to
one side and draws the femur dorsally.

The iliacus externus arises on the outer side of the dorsal
crest of the ilium from the anterior third to within a short
distance from the posterior end. It extends backward,
passing between the middle and posterior heads of the
triceps to be inserted on the posterior side of the head
of the femur. It rotates the femur forward.

The iliacus internus arises from the ventral border of
the ilium, a little in front of the anterior spine. In the
anterior position of its broad origin its fibers run below the
lower margin of the ilium to be attached to the median
surface. Its insertion extends from the hip joint to about
the middle of the femur. This muscle is broad and flat,

268

THE BIOLOGY OF THE FROG

and extends between the middle and anterior heads of the
triceps ventral to the iliacus externus. It draws the thigh
forward.

There are several smaller muscles around the head of the
femur; viz. the pectineus, obturator externus, obturator
iti tenuis, ilio-femoralis, quadratus jemoris, and gemellus,
for a description of which the student is referred to Ecker's
"Anatomy of the Frog." The muscles of the leg, or crus,
are as follows: —

The gastrocnemius which has been mentioned above is
the largest muscle of the leg. It arises by two heads, the
larger one from a tendinous arch which extends from the
posterior side of the distal end of the femur to the head
of the tibio-fibula; the other head joins the tendon of the
triceps. The muscle tapers distally where it is inserted by
a very strong tendon, which passes over the ankle joint and
spreads over the plantar surface of the foot. It acts as a
flexor of the leg and an extensor of the foot.

The tibialis posticus lies on the posterior side of the leg
beneath the gastrocnemius. It arises from the greater por-
tion of the length of the tibio-fibula and ends in a tendon,
which, running in a groove on the inner side of the distal
end of the tibia, passes between the ends of the tibio-fibula
and the bones of the ankle to be inserted into the anterior
side of the proximal end of the tibiale. When the foot is
flexed, it acts as an extensor and pronator, and when the
foot is fully extended, it flexes it to within about 45° of a
continuation of the long axis of the crus.

The tibialis anticus longus is a narrow muscle on the
front of the crus. It arises by a long, narrow tendon nam
the distal end of the femur. Distally it divides into two
parts which are inserted into the proximal ends of the
tibiale and fibulare* It acts as an extensor of the leg and a
ilexor of the foot.

THE MUSCULAR SYSTEM

269

The peroneus lies on the postaxial side of the preceding
muscle, partly covered by gastrocnemius. It arises by a
narrow tendon from the distal end of the femur. One part
of the muscle is inserted into the distal end of the fibula,
the other passes over the ankle joint and joins the outer
angle of the head of the fibulare. The muscle acts to
extend the leg; when the foot is partially extended, it acts
to extend it still farther, and pronates it, i.e. turns its
plantar surface downward; when the foot is strongly flexed,
it acts as a flexor, bringing it up close to the crus; it also
pulls the ankle postaxially.

The extensor cruris is a slender muscle lying on the
anterior side of the leg, partly covered by the tibialis anticus
longus. It arises by a slender tendon, from the distal end
of the femur. It is inserted directly upon the bone, along
the anterior side of the tibio-fibula. It extends the leg.

The tibialis anticus brevis is a short muscle lying close
alongside of the extensor cruris. It arises from the distal
third of the tibio-fibula where it is attached directly to the
bone. Distally it ends in a tendon which is inserted in the
proximal end of the tibiale. It flexes the foot.

There are numerous smaller muscles for moving the dif-
ferent parts of the foot, whose description we shall omit.

When a muscle works in company with others, its action
is often quite different from that produced when it acts
alone. This fact has been well illustrated by Dr. Lombard,1
who has worked out the double action of the muscles of the
frog's leg in a very thorough manner. The triceps jemoris,
for instance, when working alone acts as an extensor of the
leg; but since it pulls the thigh forward at the same time,
it causes the flexor muscles on the back of the thigh to
become tense, and thereby indirectly brings about a flexion

1 "Contributions to Medical Research, dedicated to V. C. Vaughan/5
n 200. 1903.

'270

THE BIOLOGY OF THE FROG

of the leg. The muscles of the frog's leg are so arranged
that any force which pulls the leg forward causes the crus to
Ilex against the thigh and the foot to flex against the crus,
and any force which pulls the leg backward extends the crus
and foot. The movements of the frog's leg in jumping and
swimming consist mainly in alternately bringing the leg up
against the side of the body, folding the various parts to-
gether, and pulling it backward and extending or unfolding
its different parts. Any muscle, such as the iliacus internus,
which pulls the femur backward, brings about a flexion
of the crus through the tension produced on the flexors of
the back of the thigh. A flexion of the crus stretches the
tibialis anticus longus on the anterior side of the crus and
thereby brings about a flexion of the foot. If a muscle
pulls the femur backward, the triceps femoris is put on the
stretch: this tends to straighten the leg, the tendency be-
coming stronger the farther back the thigh is pulled. Ex-
tending the leg causes the gastrocnemius to be put on a
strain, and the tension is increased through the pull on the
tendon this muscle receives from the lower end of the
triceps. Through the pull on the gastrocnemius the foot is
extended. Other muscles are of course brought into play
in this process, but what has been said will illustrate the
principle on which coordination of movements employed in
jumping or swimming is effected. The relative intensity of
the nervous impulses distributed to the various muscles
is an important factor in determining the kind of move-
ment the limb will make, but the basis for the unity
of action of the parts in the ordinary movements of the
limb lies in the structural arrangement of the bones and
muscles.

As an extended treatment of the remaining portions of
the muscular system lies beyond the scope of this work, only
a few of the more noteworthy muscles will be described.

THE MUSCULAR SYSTEM

271

On the ventral side of the body the large rectus abdominis
extends from the pubis, to which its fibers converge behind,
to the sternum in front. Its two halves are separated in the
middle line by the linea alba, and there are five connective
tissue septa (inscriptiones tendince) which cross it trans-
versely and divide it into segments.

The obliquus externus is a large muscle covering most of
the sides of the body. It originates on the sides of the
ilium and dorsal fasciae above, and is inserted into the sides
of the rectus abdominis. Its fibers extend obliquely upward
and forward.

The transversus is a broad muscle lying beneath the ex-
ternal oblique and forming the innermost muscular layer of
the body wall. Its fibers run for the most part transversely.
Anteriorly some of its fibers are inserted into the esophagus
and closely overlie the pericardium; other fibers attach to
the coracoid and xiphisternum ; the posterior portion of the
muscle is inserted into a flat tendon which extends dorsally
to the rectus abdominis to the mid-ventral line. All of these
muscles have the general effect of contracting the body
cavity.

The cutaneous pectoris, a paired muscle lying on the
ventral side of the anterior part of the body. Posteriorly
it is attached to the body wall, and it is inserted anteriorly
into the skin between the fore legs.

The pectoralis major, a large muscle on either side of the
anterior part of the body. It is composed of three parts,
an abdominal portion arising from the sides of the anterior
half of the rectus abdominis, a middle portion arising from
the sternum and xiphisternum, and an anterior portion aris-
ing from the coracoid and epicoracoids. All three parts are
inserted near together on the ventral crest of the humerus.

The submaxillary muscle extends transversely across the
floor of the buccal cavity, between the two rami of the

•j 72

THE BIOLOGY OF THE FROG

lower jaw. It raises the floor of the buccal cavity in
respiration.

The submentalis is a small muscle lying in the anterior
angle of the lower jaw. Its fibers run transversely, and by
their contraction raise the tip of the jaw and thereby bring
about the closure of the nares in respiration.

The subhyoid, a small muscle arising from the anterior
corner of the hyoid near the skull and joining ventrally the
posterior margin of the submaxillary. It raises the hyoid.

The following muscles are attached to the hyoid and take
part in the movements of respiration: The geniohyoid,
extending from near the anterior angle of the jaw to the
thyroid and posterior lateral processes of the hyoid. It
draws the hyoid forward and upward. In connection with
sternohyoid it lowers the jaw.

The sternohyoid, attached posteriorly to the dorsal sur-
face of the sternum and coracoids, where it joins the rectus
abdominis, of which it may be regarded as an extension.
Anteriorly it is inserted into the lower surface of the hyoid
and its thyroid processes. It draws the hyoid backward
and lowers it, thus enlarging the buccal cavity.

The omohyoid, extending from the scapula to the lateral
portion of the ventral surface of the hyoid. It draws the
hyoid backward.

The petrohyoids, a series of four slender muscles arising
from the outer portion of the prootic bone and inserted into
the hyoid. The anterior muscle is the largest, and is at-
tached ventrally to the lateral margin or the body of the
hyoid near the middle. The three posterior petrohyoids
diverge ventrally to be inserted upon the thyroid process
of the hyoid. The petrohyoids raise the hyoid apparatus
and pull it forward.

The hyoglossus arises from the thyroid processes and
ventral surface of the body of the hyoid. The two halves

THE MUSCULAR SYSTEM

273

of this muscle converge toward the middle line as they
pass forward. Anteriorly the single muscle thus formed
bends backward around the concave anterior margin of the
tongue, where it breaks up into numerous branches.

CHAPTER XV

THE CIRCULATORY SYSTEM

The principal functions of the circulatory system are to
carry food material and oxygen to all parts of the body, and
to remove the carbon dioxide and other waste products of
tissue metabolism to the organs where they are eliminated.
These functions are discharged by means of two fluids, the
blood and the lymph. We shall describe the blood first.

The Blood. — The blood of the frog consists uf a fluid, the
plasma, in which there occur numerous free cells, or corpus-
cles. The corpuscles are of three kinds: red corpuscles
(erythrocytes) , white corpuscles (leucocytes), and spindle
cells (thrombocytes) . The red corpuscles are elliptical in
outline and have an oval nucleus in the center. When seen
on edge they appear flattened and slightly bulging in the
center where the nucleus lies. The nearly transparent
cytoplasm of the c6ll contains a large percentage of hemo-
globin, which gives it a yellowish color. Blood appears red
only when a layer of considerable thickness is seen.

The white corpuscles, unlike the red ones, vary^exceed-
ingly both in size and form. The outline of the white cor-
puscle undergoes changes much like those of an Amoeba.
The changes are slow, but they may easily be shown by
making outline sketches of the corpuscle at intervals of one
or more minutes. The cytoplasm of the white corpuscles is
nearly colorless, but it often contains granules of various
sizes and staining reactions. The form of the nucleus varies
greatly in different cells. In some cases, especially in the

274

THE CIRCULATORY SYSTEM

275

smaller leucocytes, it is nearly spherical. In other cells it is
very irregular in outline and may be deeply constricted in
several places, and not infrequently it may be divided into
two or more distinct nuclei. In the smallest leucocytes the
amount of cytoplasm is relatively small and forms a
narrow, irregular en-
velope around the
spherical nucleus.
The cytoplasm of
the larger corpuscles
is relatively much
greater in amount.

By virtue of their
amoeboid move-
ments the white cor-
puscles have the
power of independ-
ent locomotion and
they even pass
through the delicate
walls of the capil-
laries into the spaces
between the other
cells of the body.
They are not con-
fined, therefore, to
the blood vessels like
the red cells, but

may be found in almost every part of the organism. And
they often pass out of what are, strictly speaking, the
limits of the body into the mouth and alimentary canal.

One important property of the white cells |s_their power
of ii^ulfing small Bodies^ which are taken in much as
an Amoebatakesni its food. Bacteria are devoured in

Fig. 89. — Blood corpuscles of the frog, a, ft,
c, red blood corpuscles ; a and b, young
stages ; c, mature corpuscle ; d-h, spindle
cells in different stages ; d, early stage ;
e, a somewhat later stage ; /, g, h, typical
spindle cells ; i-l, forms of leucocytes ; t,
very early stage ; j, an older stage ; k,
cell with lobed nucleus ; 7, cell with four
nuclei. (After Dekhuyzen.)

276

THE BIOLOGY OF THE FROG

this way, a.udJJiiJiaicocytes thus jifford the body a measure
ciT^rotec.tion against_these organisms, which are^ constantly
being introduced into the systerri in one way or another^,
and might, if unchecked, be productive of serious if not
fatal ^gf£c£ts. An irritation set up in any region causes
leucocytes to be attracted to the spot in large numbers.
The introduction of bacteria into any part of the body
is followed by the invasion of that part by leucocytes, and
it frequently happens that the bacteria are devoured by
these cells before they gain a strong foothold. It is very
probable that one important factor which causes the move-
ments of the leucocytes in such cases is the presence of
substances given off by the bacteria which exercise a chemo-
tactic effect upon these wandering cells, causing them to
congregate about the center of diffusion. The chemotactic
influence of such substances is shown by the following ex-
periments first performed by Massart. If a fine capillary
tube sealed at one end be filled with a culture fluid con-
taining the bacterium Staphylococcus pyogenes albus, and
introduced into one of the large lymphs spaces under the
skin, it will be found in the course of ten or twelve hours
that a swarm of leucocytes have made their entrance into
the open end of the tube. If a similar tube filled only with
the culture medium be introduced, no leucocytes will be
found to enter it. The substances produced by the bacteria
are apparently the cause of the invasion of the tube by the
wandering cells.

Besides protecting the body against the germs_of disease,
the^white^cells take part in the removal of tissue which has
become broken down and no longer of service to the or-
ganismT" In the degeneration of the tail of the tadpole the
removal of skeletal and muscular tissues is effected in large
part through the agency of the white corpuscles.

The spindle cells (thrombocytes), as their name im-

THE CIRCULATORY SYSTEM

plies, are spindle shaped, but their general outline is subject
to considerable variation. They are from one-half to two-
thirds the length, and from one-third to one-half the breadth
of the red cells. They possess a certain power of amoeboid
movement, especially in the young state. The spindle cells,
according to Neumann, are developed from the small leu-
cocytes. They are normal constituents of the blood at all
times of year and are usually colorless. Soon after the
blood is shed they run together into masses and disappear.

ThQjplasma, or fluid portion oiih^Ugod^s of very com-
ple^L^nstitufi^rflt contains7ats7sugar, and numerous pro-
teins in solution, vitamines, hormones, a considerable num-
ber of salts, various products arising from the breaking down
of tissues throughout the body, several gases, chiefly oxygen,
nitrogen, and carbon dioxide, and many other substances
not included in any of the above classes. WhHe__all but a
smaU_]i>art of the oxygen is carried by the hemoglobin ot
the red blood corpuscles, TlTe other materials of the blood
are^ontamecTin a state of solutira Jiijthe plasma^ Under
certain conditions the plasma coagulates, or gives rise to a
solid substance called fibrin, which, with the corpuscles
which become entangled in it, forms the clot. The remain-
ing portion of the plasma is called the serum. Clotting of
blood is brought about by means of contact with foreign
bodies. Powdered substances, filter paper, or any objects
which bring a large amount of surface in contact with the
fluid greatly facilitate the precipitation of fibrin. On the
other hand, if blood be drawn into a vessel whose sides are
smeared with oil, it may be kept from clotting for a com-
paratively long time. Cold greatly checks clotting, the
process being delayed almost indefinitely if blood is kept
near the point of freezing. If blood be heated to near
100° C, its power of clotting is destroyed. The formation
of clot is dependent in some way upon the presence of

278

THE BIOLOGY OF THE FROG

calcium salts in the plasma; for if these are precipitated out,
clotting may be prevented. The process of clotting is due
to the formation of fibrin from a substance called fibrinogen
which exists in a state of solution in the plasma. The
change is due, like the formation of cheese in milk, to a
ferment which transforms the soluble compound into an
insoluble form. The formation of clot has the function of
preventing indefinite bleeding after an injury has been re-
ceived, the contact with foreign bodies causing the clot to
form and thereby checking further loss of blood.

Accordin^jto^several observers^the spindle cells play an
i^riiDorla^^

quickly disintegrate when brought into contact with glass or
most other substances, but Tait and Green have shown that
if blood is kept in contact with paraffin after it is drawn
the spindle cells retain their normal form. If they are re-
moved the blood does not coagulate even in contact with
glass. "Spontaneous coagulation of centrifuged frog's
plasm is due wrholly to disintegration of the contained
thrombocytes. " Contact, according to this interpretation,
produces coagulation of the blood by causing the destruction
of the spindle cells which liberate a substance which acts on
some of the dissolved proteins of the plasma.

The Lymph. — The lymph is a colorless fluid, devoid of
red corpuscles, but furnished with numerous leucocytes. Its
plasma coagulates, but not quite so readily as that of blood.

The Production of New Corpuscles. — The corpuscles of
the blood, after functioning for a certain time, die and are
replaced by new cells. The process of regeneration of new
corpuscles does not take place uniformly throughout the
year as in higher animals, but is most active in the spring
and early summer. During the late fall, winter, and early
spring there is a period of inactivity in the production of
new cells. Only after the breeding period, when the frog

THE CIRCULATORY SYSTEM

279

begins to take food, and store up nutriment in the body
is there a rapid regeneration of the blood. The process
reaches its maximum in one or two weeks, after which the
production of new corpuscles suffers a gradual diminution
until fall.

As Bizzozero and Torre have shown an important seat of
the formation of new corpuscles is in the marrow of the
bones. Marquis found that the marrow undergoes periodic
changes corresponding to the changes in the blood. In
the late spring and early summer, during the period of
most active renewal of blood cells, the marrow assumes a
lymphoid character; during the summer it gradually ac-
cumulates fat, and retains a fatty character during the fall
and winter. During the spring, the fat is more or less
completely resorbed, and the lymphoid cells, which are con-
cerned in the formation of new corpuscles, undergo rapid
multiplication.

After hibernation the chief seat of the formation of
new corpuscles, according to Jordan and Speidel, is the
spleen. Red, white, and spindle cells are produced in this
organ, and certain large cells (macrophages) in the spleen
engulf and destroy the old blood corpuscles. In the tad-
pole blood corpuscles, according to Jordan and Speidel,
are produced chiefly in the kidneys.

The mature red corpuscles do not divide. Most observers
hold that the red, white and spindle cells are all derived
from the small leucocytes (lymphocytes) since various
gradations occur between the latter and the different kinds
of mature corpuscles.

While most of the corpuscles, both red and white, arise
in the marrow of the bones and in the spleen, it is certain
that both kinds of cells are also produced by the division
of preexisting cells in the circulating blood. This is notably
the case in the white corpuscles; and divisions of the young

280 THE BIOLOGY OF THE FROG

stages of the red corpuscles have been witnessed by a num-
ber of observers. The spindle cells undergo indirect division,
even after they have acquired hemoglobin. In their young
stages, the red cells have a more or less circular but irregu-
lar form, comparatively little hemoglobin, and a relatively
large irregular nucleus. However different the various
forms of corpuscles may be in their adult condition, they
arise from cells of a very similar, if not identical, structure.

The Structure of the Heart. — The heart of the frog is
situated in the anterior part of the body cavity, ventral
to the liver. It lies within a sac, the pericardium, whose
cavity is completely cut off from the coelom, although
originally continuous with it in early development. The
pericardium is composed of two layers, the parietal, form-
ing the outer wall of the sac, and the visceral, which closely
invests the heart. The two layers are continuous, passing
into each other in the region of the truncus arteriosus below
and the anterior venae cavse above. The relation is such
as would be produced if the heart were pushed into the
hollow pericardial sac from in front, although as a matter
of fact it is not brought about in this way.

When the pericardium is opened on the ventral side, the
following parts come into view; (1) The conical ventricle.
with its apex pointing backward; this part of the heart has
very thick muscular walls and appears paler than the rest.

(2) The auricles lie immediately in front of the ventricle.
The auricles are thin-walled and are separated from each
other internally by a septum, but from the outside they
present only a faint indication of this division; they are
clearly separated from the ventricles by the coronary sulcus.

(3) The bulbus cordis, lying in front of the right side of
the ventricle; it is a thickened muscular tube, extending
obliquely across the right auricle ; anteriorly it is continued
into the thinner-walled truncus arteriosus from which it is

THE CIRCULATORY SYSTEM

281

demarcated by a sulcus. (4) The truncus arteriosus is
somewhat narrower than the bulbus; it soon divides into
two diverging trunks, which give rise to three arteries, the
common carotid, the aorta, and the pulmo- cutaneous.

Fig. 90. — The heart of a frog cut open and seen from the ventral side.
a, a', bristle passed into the left carotid trunk ; au.v.v, auriculo-ven-
tricular valves ; b, 7/, bristle passed into the left systemic trunk ;
c, c\ bristle in left pulmo-cutaneous trunk; car.a, carotid artery;
car.gl, carotid gland; cart, bulbus cordis (conus arteriosus of some
authors) ; car.tr, carotid trunk; l.au, left auricle; Ig.a, lingual ar-
tery ; l.v, longitudinal or spiral valve ; pul.cu.tr, pulmo-cutaneous
trunk ; pul.v, opening of pulmonary veins ; r.au, right auricle ; s.au.
ap, sinu-auricular aperture ; spt.aur, inter-auricular septum ; v, v\
valves; vt, ventricle. (From Parker and Haswell's Zoology. )

On the dorsal side of the heart is the triangular, thin-
walled sinus venosus; at the two anterior angles of the sinus
the precaval veins, or anterior vence cavce, enter; the poste-

282

THE BIOLOGY OF THE FROG

rior apex receives the large postcaval vein, or posterior vena
cava. In front of the anterior margin of the sinus is the
pulmonary vein, which empties into the left auricle.

The internal structure of the heart presents a complicated
and beautifully adapted mechanism for propelling the blood
always in one direction and keeping the pure and impure
ccz blood separated. By re-

moving the ventral wall of
^p.cu the auricles, ventricles, and
bulbus, most of the features
of the internal structure
may be exposed to view.
The inter auricular septum
is so situated that the right
auricle is much larger than
the left. In^the right aur-
icle, close to the^epfum, is
the large sinu- auricular^
aperture, through which
blood enters from the sinus
venosus. It is a transverse
oval opening guarded by
valvular lips on the ante-
rior and posterior sides,
which prevent the blood
that has entered from the
sinus from being forced back again when the auricles
contract. The left auricle receives blood from the pul-
monary vein through a small opening near the septum
slightly anterior to the sinu-auricular aperture; there is
no valve at this point, but since the vein perforates the
wall obliquely, the pressure caused by the contraction of
the auricle serves to close the opening and thus prevents
the backward flow of the blood. Both auricles empty into

Fig. 91. — Heart seen from the dor-
sal side with the sinus venosus
opened up. ao, aortic trunk ; au,
right auricle ; au", left auricle ;
ca, carotid trunk ; p.cu, pulmo-
cutaneous trunk ; pr.c, precaval
vein ; pt.c, postcaval vein ; p.v,
pulmonary vein ; s.v, sinus veno-
sus ; v, ventricle ; va", sinu-auric-
ular valves. (After Howes.)

THE CIRCULATORY SYSTEM

283

the ventricle by a large opening, the auriculo- ventricular
aperture, which is divided by the interauricular septum.
This opening is guarded by four valves, two large ones on
the dorsal and ventral edges, and a small valve at either
end; small fibers extend from the ventricular wall to be
inserted into the free edges of the valves; they prevent the
edges of the valves from being turned back into the auricles
when the ventricle contracts, and thus keep the blood from
flowing from the ventricle into the auricles. The ventricle
possesses in addition to the central cavity at its base a
number of fissure-like chambers in the thick muscular wall ;
these chambers are separated from each other by muscular
partitions and extend outwardly nearly to the periphery of
the ventricle; they receive a large part of the blood that
passes through the heart, and have the important function,
as will be seen later, of preventing the mixing of the blood
that comes in from the two auricles.

The opening from the ventricle into the bulbus cordis is
guarded by three semilunar valves. They are in the form
of pockets, open anteriorly, whose walls may be pressed
down when blood is passing out of the ventricle, but when
blood tends to pass the other way, they fill and prevent
its return. In front of these valves is a peculiar and im-
portant structure known as the spiral valve. It consists of
a longitudinal fold attached along the dorsal wall of the
bulbus, its ventral edge lying free. At its posterior end it
is attached to the left side of the bulbus, near the opening
into the ventricle. It passes obliquely across the bulbus,
and widens out at its anterior end into a cup-like valve.
Two smaller valves occur at the same level. In front of
these valves is the unpaired portion of the truncus, which
is partly divided by a ridge extending from the point of
anion of the two branches to the middle of the anterior
enlargement of the spiral valve. Each of the two branches

2S4

THE BIOLOGY OF THE FROG

of the truncus is divided by two septa into three compart-
ments. The anterior compartments, which lead to the com-
mon carotid arteries, both enter the unpaired division of
the truncus to the right of the septum. The middle com-
partments, which lead to the aorta, open into the unpaired
portion of the truncus on either side of the septum.

The posterior compartments, which are continued into
the pulmo-cutaneous arches, join each other and open by
a common aperture into the bulbus cordis behind the valves
at its anterior end.

The Arteries. — The arteries, or vessels which carry blood
away from the heart, have thicker walls than the veins,
and after death are usually almost devoid of blood. The
ultimate ramifications of the arteries lead to a system of
minute capillaries, through the very thin walls of which
there is an exchange of products between the blood and the
tissues. From the capillaries the blood flows into the veins,
by which it is returned to the heart. All of the blood vessels
with the exception of the capillaries are provided with coats
of unstriated muscle, by the contraction and relaxation of
which their caliber may be diminished or increased.

The arterial system begins in the truncus arteriosus,
which, after its bifurcation, splits up into three pairs of
arteries which are symmetrically disposed on either side of
the middle line. The anterior of these three arteries, the
common carotid, soon divides into the lingual, or external
carotid, and the larger internal carotid. The lingual runs
forward, giving branches to the thyroid, pseudo-thyroid,
various muscles of the hyoid apparatus, and tongue. At the
junction of the internal and external carotid, but principally
upon the former, there is an oval enlargement known as
the carotid gland. Internally the carotid gland contains a
spongy network which serves as an impediment to the flow
of the blood. As this organ becomes somewhat distended

THE CIRCULATORY SYSTEM

285

after each pulsation of the heart and then slowly contracts
(Hyrtl, Sabatier), it also serves to equalize the blood flow,
especially in the internal carotid. The carotid gland is

Fig. 92. — Diagram of the arterial system of the frog, ventral view.
(After Howes.)

developed in the larva through the anastomoses of vessels
connecting the afferent and efferent arteries of the first gill
arch; between the blood vessels are cells derived from the
epithelium of the gill slits (Maurer).

286

THE BIOLOGY OF THE FROG

The internal carotid proceeds outward and dorsally from
the carotid gland, and then forward and somewhat toward
the middle line to the base of the skull. In front of the
lateral process of the parasphenoid bone it gives off the
palatine artery, which courses forward along the roof of the
mouth, a little farther forward the cerebral carotid arises,
entering the skull in the region of the orbit and sup-
plying the brain. The third branch, the ophthalmic, passes
forward and supplies the eye and some of the neighboring
parts.

The second branch of the truncus arteriosus, or systemic
arch, passes outward and then dorsally around the alimen-
tary canal, meeting its fellow of the opposite side to form
the dorsal aorta. Near its origin each systemic arch gives
off a small laryngeal artery, which supplies the larynx and
certain muscles of the hyoid. The small esophageal arteries
are given off at about the level of the second vertebra;
they are distributed to the dorsal side of the esophagus.
The occipito-vertebral artery arises slightly beyond, or
sometimes in common with, the last and passes forward
across the transverse process of the second vertebra; it then
divides into two branches which run above the transverse
processes of the vertebrae near the centra. The posterior
branch, or vertebral artery, extends backward along the
spinal column. The anterior branch, or occipital artery,
runs forward to the head, giving branches to the upper and
lower jaw, orbit, and nose.

The large subclavian artery arises immediately behind
the occipito-vertebral, and passes laterally, giving branches
to the shoulder and body wall, and then, as the brachial
artery, supplying the arm.

The two systemic arches unite at about the level of the
sixth vertebra to form the dorsal aorta, which proceeds
backward beneath the vertebral column. At the point of

THE CIRCULATORY SYSTEM

287

meeting of the two systemic trunks, but usually more on the
left than on the right, the large cceliaco-mesenteric artery is
given off which supplies the alimentary canal and its ap-
pendages. It divides into an anterior branch, or coeliac
artery, and a posterior branch, the anterior mesenteric
artery. The former, after giving off the left gastric artery,
which goes to the left side of the stomach, divides into the
right gastric artery, supplying the right side of the stomach
and pancreas, and the hepatic, which, after giving a branch
to the anterior lobe of the pancreas, is distributed to the
liver. The anterior mesenteric artery supplies the small
intestine, spleen, cloaca, and anterior portion of the rectum.

The urinogenital arteries are four to six small arteries
wThich arise from the ventral side of the aorta, and are dis-
tributed to the reproductive organs, fat bodies, and kidneys.
They vary greatly in their mode of origin, but typically
they arise by very short median trunks which divide into
right and left branches.

The lumbar arteries are small vessels, one to four in num-
ber on either side, which arise from the dorsal side of the
aorta and are distributed to the body wall.

The posterior mesenteric artery is a small vessel given off
from near the posterior end of the aOrta. It supplies the
posterior portion of the rectum and, in the female, the
median dorsal wall of the uterus.

At its posterior end the dorsal aorta divides into two
large iliac arteries, which are distributed mainly to the hind
limbs. A short distance behind the bifurcation each iliac
artery gives off a branch which divides into an epigastric
artery, supplying the ventral body wall, and a recto-vesical
artery to the rectum and bladder. A small artery arising
from the iliac close to the above supplies the seminal vesicles
in the male, and the lateral part of the uterus in the female.

A short distance beyond the foregoing the femoral artery

2SS

THE BIOLOGY OF THE FROG

is given off to the skin and muscles of the anterior part of
the thigh. Distal to the origin of the femoral artery the
iliac artery is continued as the sciatic, which passes out of
the body cavity, along with the sciatic nerve, just behind the
posterior end of the crest of the ilium, and supplies the hind
limbs.

The third branch of the truncus, the pulmo-cutaneous
artery, passes outward and dorsally, and gives off pos-
teriorly the pulmonary artery to the lungs; it is then con-
tinued as the great cutaneous artery, which passes upward
and forward, dividing behind the tympanic membrane into
three branches: (1) the auricularis, which supplies the
tympanum and gives branches to the thymus, lower jaw,
pharynx, and hyoid, and anastomoses with branches of the
occipital and internal carotid; (2) the dorsalis, which is
mainly distributed to the skin of the back; and (3) the
lateralis, which passes outward and is extensively dis-
tributed to the skin of the side of the body. With the ex-
ception of a few minor branches the distribution of the
pulmo-cutaneous artery is such as to bring the blood into
regions where it may undergo oxygenation. The blood
which does not go to the lungs passes into the great cutane-
ous artery, the largest branches of which are distributed to
the skin, where much of the respiration of the frog takes
place; the buccal cavity and pharynx, which are also
respiratory organs, receive blood from the same source.

The Veins. — With the exception of the blood coming
from the lungs, all of the blood is returned to the heart
through the three large venous trunks which enter the sinus
venosus. The two anterior venae cavae, which enter the an-
terior angles of the sinus, are each formed through the junc-
tion of three branches, the external jugular, the innominate,
and the subclavian. The most anterior of these, the external
jugular, passes forward, where it receives branches from

THE CIRCULATORY SYSTEM

289

External Jugular -

Left auricle

Right I
auricle

Internal
Jugular

Hepatic
Gastric
Hepatic portal —
Duodenal "

Intestinal

Splenic'

Renal <

Renal portal.

*J*recaval

Pulmo-^
nary

Sinus "
venosus

n

Brachial
vein

Lung *
Cardiac vein^"
Liver Musculocutaneous
-Abdominal vein
_ -Postcaval

-Testis

m

-Spermatic
-Kidney

Dorso-lumbar
vein

Vesical veins-

-Abdominal vein

" Renal portal

Femoral, Sciatic Pelvic
Fig. 93. — Venous system of the frcg seen from the dorsal side. (From
Hegner, slightly altered from Parker and Haswell's Zoology.)

the tongue, hyoid, thyroid, and pseudothyroid glands and
floor of the mouth.

The innominate vein, or second branch of the anterior
vena cava, passes laterally and divides into the internal

290

THE BIOLOGY OF THE FROG

jugular, which receives blood from the brain and various
parts of the head, and the subscapular, which runs along the
outer side of the fore limb and receives branches from the
shoulder.

The subclavian, or posterior branch of the anterior vena
cava, is formed by the confluence of the brachial, which is
distributed to the fore limb, and the large cutaneous vein,
which is extensively distributed over the side of the body
and head. The latter returns most of the blood carried by
the cutaneous artery.

The large posterior vena cava arises^£tj&ecrithe kidneys,
and runs forward ventral to the dorsal aorta to the tapering
posteriorZj^rid of~Trie~~^mus venosus. Near the latter it
receives thg_large but very_-short hepatic veins from the
liver.

The renal veins, four to six in number on each side, lead
from the kidneys into the posterior vena cava; the veins
from the reproductive organs (spermatic or ovarian, accord-
ing to the sex of the animal) lead either directly into the
vena cava or first into the renal veins. The vena cava also
receives one or two branches on either side from the fat
bodies.

The blood from the hind limbs does not empty directly
into the posterior vena cava as in the higher vertebrates, but
it is forced to pass through a second system af capillaries
before reaching that vessel. Two large veins convey the
blood from the hind legs, the sciatic, which runs along the
post-axial side of the thigh, and the femoral, which courses
along the dorsal and anterior side of the thigh and passes
under the iliac bone into the body cavity. These two veins
are connected close to the hip joint by the transverse iliac
vein, which passes dorsally to the femur and enters the body
cavity behind the crest of the ilium. The femoral vein
branches in front of the base of the thigh into two pans, one

THE CIRCULATORY SYSTEM

291

292

THE BIOLOGY OF THE FROG

of which passes ventrally and joins its fellow of the opposite
side to form the anterior abdominal vein, which runs for-
ward in the middle of the ventral body wall; the other
branch, the external iliac, passes forward, and dorsally, and
joins the sciatic vein to form the common iliac or renal

Fig. 95. — The hepatic portal system, showing its relations to the
stomach, intestine, pancreas, and liver, a', branch from the anterior
abdominal to the portal vein ; ant.ab, anterior abdominal vein ; du,
duodenum; du' artery to same; g, gastric vein; g.bl, gall bladder;
lv\ Iv", right and left lobes of the liver respectively ; p, portal vein ;
pc, pancreas; st, stomach. (After Howes.)

portal vein, which runs forward along the outer margin of
the kidneys, into the substance of which it sends its
branches. The renal portal receives the dorso-lumbar vein,
from the body wall, and in the female several branches from
the oviducts. The system of veins which lead blood to the
kidney is known as the renal portal system. There is also a
hepatic portal system which carries venous blood to the

THE CIRCULATORY SYSTEM

293

liver. The latter consists of (1) the anterior abdominal
vein, which receives blood from the femoral veins, bladder,
and ventral body wall, and (2) the portal vein, which
carries blood from the stomach, intestine, spleen, and
pancreas, the terminal portion passing through the latter
organ to empty into the left lobe of the liver. The ab-
dominal vein, just before it enters the liver, receives a small
branch, the vena bulbi cordis, from the bulbus cordis; the
other parts of the heart are devoid of special blood vessels.

The Action of the Heart.-^-In the beating of the heart,
which may readily be observed in a frog that has recently
been killed, the contraction first occurs in the sinus venosus;
and this is followed by successive contractions of the
auricles, ventricle,^and bulbus. As we have seen, the ar-
rangement of th§>Valves of the heart is such as to keep
the blood flowing through these parts in the order named.
Although the frog does not possess a complete double circu-
lation, such as occurs in birds and mammals, in which the
systemic and the pulmonary circulations are entirely sepa-
rated, the impure and the oxygenated blood are, neverthe-
less, not allowed to completely mix, but are kept more or
less apart and sent out to different parts of the body. It
was formerly held that the blood from the two auricles was
completely mingled in the ventricle, but Mayer showed in
1835 that if the tip of the ventricle be cut off, two blood
streams, a dark and a red, issue from the cut end. Later
(1851) the noted physiologist Briicke studied the structure
and action of the frog's heart in detail and explained the
mechanism by which the two kinds of blood were kept
separate. Briicke's observations were extended and in most
points confirmed by Sabatier in 1873. The interpretation
of the latter author has been followed by Gaupp in his
recent revision of Ecker's "Anatomie des Frosches."

When the auricles contract, the blood from the left auricle,

204

THE BIOLOGY OF THE FROG

which lias come in from the pulmonary vein and is therefore
oxygenated, is forced into the left side of the ventricle, while
the impure blood from the right auricle, which comes
through the sinus venosus, pours into the right side and
middle portion of the ventricle. The blood from these
different sources is prevented from becoming mixed by
being received into the slit-like chambers in the ventricular
wall. During the contraction of the ventricle the impure
blood lying near the opening of the bulbus naturally passes
out first, while the pure pulmonary blood from the left side
is forced out only toward the close of the ventricular con-
traction. When the ventricle first contracts, the wall of the
bulbus cordis is relaxed, and the impure blood flows freely
over the edge of the spiral valve into the left compartment,
whence it is free to issue into the pulmo-cutaneous arches
through their common opening. Now the blood is under less
pressure in the pulmo-cutaneous arches than in the others,
because its route is shorter and there are no impediments to
its flow. In the carotid arches the blood meets with a
partial obstruction in the carotid gland, and at the outer
ends of the systemic arches there is a small valve (valvula
paradoxica), which also tends to retard its flow. The blood
first issuing from the heart takes the line of least resistance,
namely, the pulmo-cutaneous arches, and is forced through
the first two pairs of arches only when it has no easier
avenue of escape. Toward the close of the contraction of
the ventricle, when the pure blood is passing out, there is a
contraction of the bulbus cordis. This brings the wall of
the bulbus against the free edge of the spiral valve and pre-
vents the blood from flowing over into the left or pulmonary
side of this division of the heart. The blood is prevented
from access to this side anteriorly by valves, so there is now
no course open to it but through the carotid and systemic
arches. Since the common opening of the pulmo-cutaneous

THE CIRCULATORY SYSTEM

295

arches lies behind the valves at the anterior end of the
bulbus, it can receive no blood when the communication be-
tween the two sides of the bulbus is cut off. In this way the
impure blood first sent out of the heart goes mainly to the
lungs and skin, where it is purified, while the purer blood
passing out toward the close of the contraction of the heart
is sent to the various other parts of the body.
fThe heart of the frog may beat for hours, or, under favor-
able conditions, even for days, after it has been removed
from the body. Even isolated parts of the heart, such as
the sinus venosus, auricles, or ventricle, may continue beat-
ing, although not with the same rhythm. If the heart is
removed so as to leave the sinus venosus within the body,
the auricles and ventricle beat with a rate less than the nor-
mal, but the sinus continues to beat with nearly the same
rhythm as before. If the sinus is removed with the rest of
the heart, the beating of the whole heart is more rapid than
that of the auricles and ventricle when removed alone. It
is apparently the sinus venosus which sets the rhythm for
the beating of the other parts of the heart. After the heart
has ceased to beat spontaneously it may be caused to resume
its activity by the application of a stimulus. ^)

Circulation in the Web of the Foot. —The web of the
frog's foot affords a classical object for the study of the
capillary circulation. It may easily be prepared for obser-
vation with the microscope by tying the frog down to a small
piece of board, and spreading its toes apart so that the web
is stretched across a notch or hole through which light may
be passed from below. The toes may be held in position
by small pieces of thread tied to the tips and fastened by
their other ends to the board.

In a web thus prepared the blood may be seen flowing
rapidly in the small veins and arteries, and more slowly in
the capillaries. The red corpuscles will be found to become

296

THE BIOLOGY OF THE FROG

elongated and narrowed as they thread their way slowly
through the small capillaries. The leucocytes often creep
slowly along the walls of the vessels, and may be seen to
stop frequently, and sometimes to migrate through the capil ■
lary walls. In the arteries a pulsation due to the beating of
the heart may be observed; the caliber of the arteries often
changes, owing to the contraction of the muscle fibers of
their walls.

The capillary circulation may also be easily studied in the
tail of the tadpole.

The Lymphatic System. — The lymphatic system of the
frog is remarkable on account of the abundance and large
size of the lymph spaces in various parts of the body. There
are no well-defined lymphatic vessels such as occur in the
mammals; the lymph flows in irregular spaces between and
within the different organs; the larger spaces are lined by
flattened endothelial cells, but are entirely devoid of a
muscular coat, and usually, also, of a lining of connective
tissue.

The subcutaneous lymph spaces are especially well devel-
oped; they are separated from each other only by the nar-
row septa of connective tissue by which the skin is here
and there attached to the underlying muscles. One of the
largest of the lymph spaces within the body is the subverte-
bral lymph sinus, or cisterna magna, which extends above
most of the dorsal side of the body cavity.

The lymph spaces of the body stand in communication so
that there is a flow of lymph from the one to the other, but
of the course of the flow, if there be a constant one, little is
known. There is a flow of lymph into the blood through
the four lymph hearts and also through the ciliated nephro-
stomies on the ventral surface of the kidney which lead from
the ccelom into the renal veins. The anterior lymph hearts
are situated just behind the transverse processes of the third

THE CIRCULATORY SYSTEM

297

vertebra, and empty into the vertebral vein, which flows into
the internal jugular. The posterior lymph hearts lie on
either side of the tip of the urostyle, and empty into the
transverse iliac vein. All of the lymph hearts pulsate regu-
larly, and pump the lymph from the lymph spaces with
which they communicate into the blood. At their openings
into the veins there is a pair of semilunar valves which pre-
vent the blood from passing into the lymph heart when it
becomes relaxed. At the opposite end there are ostia (but

Fig. 96. — Lymph sacs of Rana. The dark lines indicate where the
septa extend between the skin and the body, a, abdominal lympli
sac ; 6, lateral brachial lymph sac ; c, crural lymph sac ; d, dorsal
lymph sac ; /, femoral lymph sac ; I, lateral lymph sac ; p, pectoral
lymph sac; s, submaxillary lymph sac. (Modified from Gaupp.)

apparently no valves) through which the lymph enters the
heart from the lymph sacs. The lymph hearts are furnished
with a muscular coat composed of a network of bundles of
striated muscle fibers.

The beating of the lymph hearts may readily be observed
in a recently killed frog. Often the pulsations of the pos-
terior lymph hearts may be seen beneath the skin) but they
are easily demonstrable in a very satisfactory manner by
removing the integument on each side of the end of the
urostyle. Their pulsations have no relation to those of the

298

THE BIOLOGY OF THE FROG

heart, nor is there unison between the beats of the lymph
hearts on the two sides of the body.

REFERENCES

Brticke, E. Beitrage zur vergleichenden Anatomie und Physic
ologie des Gefasssystems, 1. Ueber die Mechanik des Kreislaufes bei
den Amphibien. Denkschr. d. k. Akad. Wiss. math.-wiss. CI., Bd. 3,
Wien, 1852.

Dekhuysen, M. C. Ueber das Blut der Amphibien. Verh. Anat.,
Ges., 6 Vers., 1892.

Frankel, L. Zur Blutbildung beim Frosche {Rana esculenta).
Folia Haematol., Bd. 17, 1, 1913.

Friedsohn, A. Zur Morphologie des Amphibienblutes. Arch. mik.
Anat., Bd. 75, 435, 1910.

Fuchs, E. Beitrag zur Kentniss des Froschblutes und der Frosch-
lymphe. Virchow's Archiv, Bd. 71, 1877.

Gaule, J. Beobachtungen liber die farblosen Elemente des Frosch-
blutes. Arch. Anat. u. Phys., phys. Abth, 1880.

Jordan, H. E., and Speidel, C. C. An Experimental Study of the
Spleen of the Frog, Rana pipiens. Anat. Rec, Vol. 25, 155, 1923.
Studies on Lymphocytes. Am. Jour. Anat., Vol. 32, 155, 1923.

Macallum. Studies on the Blood of Amphibia. Trans. Canadian
Inst, Vol. 2, 1892.

Marquis, C. Das Knochenmark der Amphibien in den verschied-
enen Jahreszeiten. Inaug. Diss, Dorpat, 1892.

Neumann, E. Hamatologische Studien, 1. Ueber die Blutbildung
von Froschen. Virchow's Archiv, Bd. 143, 1896.

Nottin, L. J. Some Facts Concerning the Spindle Cells of the
Frog's Blood. Quart. Jour. Exp. Physiol, Vol. 14, 84, 1924.

Sabatier, A. Etudes sur le cceur et la circulation centrale dans la
serie des Vertebres. Ann. Sci. Nat. (5), T. 18, 1873.

Tait, J, and Green, F. The Spindle Cells in Relation to Coagu-
lation of Frog's Blood. Quart. Jour. Exp. Physiol, Vol. 16, 141, 1926.

Torok, L. Die Theilung der rothen Blutzellen bei Amphibien
Arch. f. mik. Anat, Bd. 32, 1888.

CHAPTER XVI

THE NERVOUS SYSTEM

The frog has the power not only of performing a large
number of complicated movements, but of adapting its
actions to the various elements of its environment. The
initiation and control of these movements are dependent
upon the reception of stimuli either from within or without
the organism and the transfer of the impulses thus arising
to the muscles which by their contraction bring about the
required actions. When the frog withdraws its foot when
it is irritated, or snaps at a moving insect, it is performing an
act of an adaptive nature in response to an external stimu-
lus. It is evident that the actions of the frog in relation to
external stimuli and the coordination of activities going on
in different parts of the organism necessitate some highly
specialized means for the transfer and direction of impulses,
and it is with these functions that the nervous system is
especially and primarily concerned. But the nervous system
has another important function, inasmuch as it affords the
means for the accumulation of the effects of experiences
whereby the animal is enabled to profit by its former be-
havior and modify its conduct to suit new situations. This
latter power forms the basis of intelligence, a faculty rather
feebly developed in the frog, it is true, but, as we shall see
later, a not unimportant element in the life of the animal.

The nervous system has often been compared to a system
of telegraph wires by means of which any one part of a
country may be put into communication with any other

299

300

THE BIOLOGY OF THE FROG

part. The nerves correspond to the wires, and the ganglia
to the central stations where messages may be transferred
from one line to another. All parts of the body are supplied
with nerves which are connected with the central nervous
system; and through this channel connections may be estab-
lished between any two or more parts of the organism. In
this way there is rendered possible the coordination of move-
ments in different parts of the body, and the ability of the
organism to act as a whole in relation to external objects.

The nervous system is composed of three rather closely
associated divisions: the cerebrospinal, consisting of the
spinal cord and brain; the peripheral, consisting of the
spinal and cranial nerves; and the sympathetic.

The Spinal Cord. — The spinal cord of the frog is short
and somewhat flattened. It presents two enlargements, one
in the brachial region, where the large nerves to the fore
limbs are given off, and one farther back, where the large
nerves originate which supply the hind legs. Behind the
posterior enlargement the cord tapers to a narrow thread,
the filum terminate, which extends into the urostyle. At its
anterior end the cord widens gradually into the medulla
oblongata, the posterior division of the brain. Both the
dorsal and the ventral sides of the cord are divided by a
median fissure. At the sides of the cord the roots of the
spinal nerves are given off ; each nerve arises from a dorsal
and a ventral root which combine just after they emerge
from the vertebral canal through the intervertebral fora-
mina. The roots of the posterior spinal nerves are much
elongated, inasmuch as the shortening of the cord brings
their origin far in front of the vertebrae to which they corre-
spond; the bundle of roots thus formed, together with the
filum terminate, is known as the cauda equina.

Both the cord and the brain are surrounded by membranes
which are designated by Gaupp as follows: Externally is the

THE NERVOUS SYSTEM

301

aura mater, consist-
ing of two layers sep-
arated by a lymph
space (inter dural
space) ; the outer
layer of this is pig-
mented and closely
applied to the inner
surface of the cra-
nium and neural ca-
nal; the inner layer
is devoid of pigment
and lies close to the
brain and cord.
Within the dura ma-
ter is a thin vascular
layer corresponding
to the pia mater and
arachnoid of the
higher vertebrates ;
only here and there
does it present a di-
vision into two lam-
ellae. This layer is
very closely applied
to the central ner-
vous system, and is
continued into vari-
ous fissures of the
brain, and the ven-
tral fissure of the
spinal cord.

A cross section of
the cord shows it to

Fig. 97. — The central nervous system of
the frog. The roof of the skull and ver-
tebral column removed to show the brain
and spinal cord, at, atlas, or first verte-
bra ; an, auditory capsule ; b.s, brachial
enlargement of the cord ; f.tm, filium ter-
minate ; g.tr, prootic ganglion (trigem-
inus, or Gasserian ganglion of many
authors) ; l.s, lumbar enlargement of
cord; mn (F'"), mandibular branch of
fifth nerve; mx (V"), maxillary branch
of trigeminus nerve ; my, myelon, or spi-
nal cord ; na, right nasal sac ; na', left
nasal bone; n.c, neural canal ; oJ.n. olfac-
tory nerve; oph (I7'), ophthalmic branch
of fifth nerve; II, optic nerve. (After
Howes.)

302

THE BIOLOGY OF THE FROG

be composed mainly of ganglion cells and nerve fibers. The
(HMitral part of the cord is formed of gray matter which
consists chiefly of ganglion cells and non-medullated nerves.
Near the center of the gray mass is a small canal, the
canalis centralis, lined by a single layer of epithelial cells.
This canal is the remnant of the lumen formed by the clos-
ing over of the edges of the medullary groove during devel-
opment; at its anterior
end it widens out into
the ventricles of the
brain.

At the sides the gray
matter is produced
both dorsally and ven-
trally into the dorsal
and ventral cornua or
horns. The gray mat-
ter on the two sides of
the cord is connected
both above and below
the central canal by
means of the dorsal
and ventral gray com-
missures, which consist
chiefly of non-medullated nerve fibers. Just below the
ventral gray commissure is a conspicuous oblique crossing
of medullated fibers in the white matter, the ventral white
commissure. Below the white commissure is the ventral
fissure, which separates the right and left columns of white
matter. From the shallow dorsal fissure there extends a
narrow septum as far as the dorsal gray commissure. The
nervous elements of the cord are bound together by stellate
neuroglia cells and by processes which arise from the taper-
ing outer ends of the cells lining the central canal; these

Fig. 98. — Cross section through the ver-
tebral column, and spinal cord show-
ing the origin of the spinal nerves, c.c,
central canal ; cn, centrum ; d.f, dor-
sal fissure ; d.m, dura mater ; d.r, dor-
sal root of nerve ; g.m, gray matter :
gn, ganglion of dorsal root ; n.a, neu-
ral arch ; n.sp, neural spine ; p.m, pia
mater (the reference line should stop
at the margin of the cord) ; t, nerve
trunk ; Tr.pr, transverse process ; v.f,
ventral fissure ; w.m, white matter.
(After Howes.)

THE NERVOUS SYSTEM

303

processes branch repeatedly, and some of them extend to
the periphery of the cord.

The white matter of the cord is composed mainly of
medullated fibers. Most of these run longitudinally. Iso-
lated ganglion cells appear, but there seems to be no regu-
larity in their distribution. Strands of gray matter, largely
ependyma fibers, radiate from the central part of the cord
to the outer surface.

Fig. 99. — Diagram of the spinal cord showing the paths taken by ner-
vous impulses. The direction of the impulses is indicated by ar-
rows, cc, central canal ; col, collateral fibers ; c.cort, cell in the cere-
bral cortex ; eg, smaller cerebral cell ; d.c, cells in dorsal horn of
gray matter ; d.r, dorsal root ; g, ganglion of dorsal root ; g.c, gan-
glion cell in dorsal ganglion ; g.m, gray matter ; M, muscle ; m.c,
cell in medulla oblongata ; m.f, motor fiber ; &, skin ; s.f, sensory
fiber ; sp.c, spinal cord ; v.c, cells in ventral horn of gray matter ;
v.r, ventral root of nerve ; ic.m, white matter. (After Parker and
Parker.)

The cells of the gray matter give off processes by means
of which connections become established between different
parts of the cord. In the broad ventral cornua there are
several ganglion cells of unusual size from which processes
arise which form the axis cylinders of the fibers of the ven-
tral roots of the spinal nerves; other processes from these
cells cross to the opposite side of the cord in the ventral

304

THE BIOLOGY OF THE FROG

white commissure, and still other processes branch irregu-
larly in both the gray and white matter of the same side.
Scattered about through most of the gray substance are the
commissural cells which give off axis cylinder processes
which cross to the opposite side of the cord in the ventral
gray commissure and then give off branches which run in
the white matter both anteriorly and posteriorly; protoplas-
mic processes are also given off which connect with similar
processes from other cells in the gray matter in the same
side. Other cells give off axis cylinder processes, which run
in both directions in the white matter of the same side of
the cord. Still other cells occur whose axis cylinder proc-
esses divide, the one branch going into the white matter of
the same side of the cord, the other crossing through the
ventral gray commissure to the white matter of the opposite
side. Finally there are numerous cells whose processes do
not enter the white matter, but branch and connect with
cells in the gray matter of the same or the opposite side.

A cross section through a region where the spinal nerves
are given off shows the fibers of the dorsal root passing
through the dorso-lateral portion of the white matter to
enter the gray substance in a narrow bundle. Most of the
fibers of the dorsal roots are processes of cells lying in the
spinal ganglion. Each fiber as it enters the cord gives off
branches which run in opposite directions. Connections are
made with processes of the large cells which supply the ven-
tral or motor roots of the nerves as well as with the cells of
the gray matter on both sides of the cord. The ventral roots
of the spinal nerves are broader and consist of several
isolated strands. J'

The Spinal Nerves. — The frog possesses but ten pairs of
spinal nerves. The tadpole has a much larger number
(twenty-two in R. fusca), but the posterior ones disappear
with the degeneration of the tail. There is also a pair of

THE NERVOUS SYSTEM

305

nerves which appears in the
embryo in front of what is the
first pair of spinal nerves of
the adult, but we shall con-
tinue to speak of the latter as
the first pair. Each spinal
nerve arises from the cord by
a dorsal and a ventral root
which unite just outside the
inter - vertebral foramina
through which they emerge.
Near its junction with the
ventral root each dorsal root
bears a ganglion whose cells
give rise to most of the fibers
of which that root is com-
posed as well as the sensory
fibers of the peripheral por-
tion of the nerve. At the
outer end of the ganglion each
nerve divides into a dorsal and
a ventral branch. Each of
these contains both sensory
and motor fibers. The dorsal
branches divide into several
nerves which supply the skin
and muscles of the dorsal side
of the body; the ventral

Fig. 100. — Spinal nerves and
sympathetic system of the
frog, the right side seen from
below. Only the ventral
branches of the spinal nerves
shown. Sympathetic system
in black. / — X, spinal nerves ;
Ao, systemic arch of aorta ;
br.pl, brachial plexus ; C, calcareous, --bodies," around the spinal
ganglia; D.Ao, dorsal aorta; fern, femoral nerve; II. A, iliac artery;
sc, sciatic nerve ; sci.pl, sciatic plexus ; Sk, skull ; 8p.A, splanchnic,
or coeliaco-mesenteric artery ; Sy, sympathetic cord ; Sy.c, commis-
sures between sympathetic and spinal nerves; Sy.g, sympathetic
ganglia ; TJst, urostyle ; V^V0, centra of vertebrae ; Vg, vagus nerve.
(From Parker and Parker.)

306

THE BIOLOGY OF THE FROG

branches supply the ventral musculature and limbs; a short
communicating nerve connects each ventral branch with
the trunk of the sympathetic system. The distribution of
the spinal nerves, exclusive of their dorsal rami, is as
follows: —

The first nerve emerges between the first and second
vertebrae, its principal branch, the hypoglossal, innervates
the tongue and several of the muscles attached to the hyoid;
a short communicating branch joins the second nerve.

The second pair of nerves emerges between the second
and third vertebrae. This pair, which is of large size, forms
with branches received from the first and third pairs the
brachial plexus, from which the nerves arise which are dis-
tributed to the fore limb and muscles of the shoulder.

The third pair of nerves, after giving a branch to the
brachial plexus, supplies the anterior part of the external
oblique and Iransversus muscles and gives some twigs to the
skin.

The fourth, fifth, and sixth nerves are small and are dis-
tributed mainly to the skin and muscles of the wall of the
abdomen.

The seventh, eighth, and ninth nerves pass almost directly
backward and anastomose with each other to form the
lumbosacral, or sciatic plexus. The seventh nerve, before
it enters the plexus, gives off the ilio-hypogastric nerve
which is distributed to the muscles of the abdomen. The
cruralis nerve is given off from the plexus ventral to the
posterior portion of the ilium; it is distributed to the
muscles pf the abdomen and skin of the anterior part of the
thigh. The largest nerve coming from the plexus is the
sciatic, which emerges from the body cavity just behind the
posterior end of the crest of the ilium, passing between the
pyriformis and posterior head of the triceps extensor muscle
and extending down the back of the thigh ; a short distance

Fig. 101. — Brain of frog. A, dorsal side ; B, ventral side ; C, left
side ; D, in vertical longitudinal section through the middle. 06,
cerebellum ; Cer.H, cerebral hemispheres ; ch.plx1 anterior, and
ch.plx2 posterior, choroid plexus ; com, commissures connecting the
right and left halves of the brain ; Cr.C, crura cerebri ; Di, dien-
cephalon, or thalamencephalon ; for.M, foramen of Monro ; i, iter, or
aqueduct of Sylvius ; inf, infundibulum ; Med.obl, medulla oblon-
gata; Olf.l, olfactory lobe; Opt.l, optic lobe; opt.v, optic vesicle;
pin, pineal body ; pit, pituitary body ; Sp.ed, spinal cord ; v 3, third
ventricle ; v*, fourth ventricle ; I-X, cranial nerves ; 1 Sp, 2 8p, first
and second spinal nerves. (From Newman, slightly modified from
Parker and Parker's Zoology.)

SOS

THE BIOLOGY OF THE FROG

proximal to the knee it divides into the tibialis nerve and the
peroni us. The former extends along the posterior or flexor
side of the leg, and innervates the gastrocnemius, tibialis
posticus, and numerous muscles of the plantar surface of the
foot. The peroneus runs under the tendon of the triceps and
extends along the extensor surface of the cms, giving
branches to the peroneus muscle, the tibialis anticus, and the
muscles on the extensor surface of the foot. )

The tenth nerve with a branch from the ninth forms the
ischio-coccygeal plexus, from which branches are given off to
the bladder, cloaca, oviducts, and posterior lymph hearts.
The tenth nerves are of small size and emerge from small
foramina in the sides of the urostyle near the anterior end.
An eleventh spinal nerve sometimes occurs. When present
it emerges from the urostyle behind the opening for the
tenth and joins the ilio-coccygeal plexus. All of the plexuses
are subject to considerable variation in different individuals.

The Brain. — The brain is composed of the following
parts taken in order from behind forward: the medulla
oblongata, or hind-brain; the cerebellum; the mid-brain;
the thalamencephalon; and the fore-brain, which consists of
the cerebral hemispheres and olfactory lobes.

The medulla oblongata is formed by a widening of the
anterior end of the spinal cord. On its dorsal side is situated
the wide, more or less triangular fourth ventricle, which
communicates by its tapering posterior end with the central
canal of the cord. Its roof is thin and thrown into folds
bearing the posterior choroid plexus of blood vessels. At
the sides the medulla gives rise to several pairs of
cranial nerves. Its lower surface is divided by a median
fissure which is continuous with the ventral fissure of the
cord.

The cerebellum consists of a small transverse fold at the
anterior margin of the fourth ventricle. In most other

THE NERVOUS SYSTEM

309^

Cerr//

vertebrates the cerebellum is an organ of considerable size,
but in the frog it is almost rudimentary.

The optic lobes, which form the dorsal part of the mid-
brain, are large rounded bodies
lying just in front of the cerebel-
lum. Their cavities, the optic
ventricles, communicate with each
other and also with the channel
between the third and fourth ven-
tricles. Below the optic lobes are
the crura cerebri, which extend
from the medulla to the cerebral
hemispheres and form the floor of
the mid-brain.

The thalamencephalon is in the
region between the optic lobes be-
hind and the cerebral hemispheres
in front. Its roof is thin and lined
with a vascular membrane, the
anterior choroid plexus; it bears
two outgrowths in the mid-dorsal
line, the paraphysis, a vascular
outpocketing of the epithelium of
the roof, and a short distance be-
hind the latter, the epiphysis, a
hollow, thin-walled canal which
terminates blindly at its anterior
end. A small parietal nerve runs
along the dorsal surface of the
epiphysis and extends forward
over the paraphysis and then

passes through the sagittal suture of the skull to end in the
brow spot. The epiphysis, which originally was continuous
with the brow spot, becomes constricted off from it in early

-Med. oil

Sp.cct

Fig. 102. — Diagram of a
horizontal section of a
frog's brain, c.c, central
canal ; Cer.H, cerebral
hemisphere ; Di, dien-
cephalon, or thalamen-
cephalon ; for.M, fora-
men of Monro ; i, iter ;
Lat.v, lateral ventricle ;
Med.obl, medulla oblon-
gata ; Nv.l, first, or
olfactory nerve ; Olf.l,
olfactory lobe ; Olf.v, ol-
factory ventricle ; Opt.l>
optic lobe ; Opt.v, optic
ventricle ; Sp.cd, spinal
cord ; v3, third ventricle ;
v4, fourth ventricle. (Af-
ter Ecker and Wieders-
heim.)

310

THE BIOLOGY OF THE FROG

larval life. On the ventral side of the thalamencephalon is
the optic chiasma, or crossing of the nerves which go to the
eyes. In the frog all of the fibers cross to the opposite side,

Fig. 103. — Diagram of the distribution of the fifth, seventh, ninth,
and tenth cranial nerves, the first spinal nerve, and the anterioi
part of the sympathetic. Ao, systemic arch of aorta ; br.pl, brachial
plexus ; D.Ao, dorsal aorta ; du, duodenum ; H, heart ; Hy, hyoid
with hy1 anterior and ky2 posterior horns ; L, lung ; N, nasal bone ;
On, orbit; Pul, pulmonary artery; Sp.A, splanchnic, or coeliaco-
mesenteric artery ; St, stomach ; Sy, sympathetic ; II, cut end of
optic nerve ; V1, ophthalmic, F2, maxillary, and Vs, mandibular
branch of fifth, or trigeminus nerve ; VIP, palatine, and VIP, hyo-
mandibular branch of facial ; IX, glossopharyngeal nerve ; X, vagus
with Xcd, cardiac, Xgas, gastric, Xlar, laryngeal, and Xpul, pul-
monary branches ; 1 Sp, first spinal nerve, or hypoglossal ; 2 sp-5 sp,
second to fifth spinal nerves. (From Parker and Parker's Zoology,
slightly modified from Howes.)

Just behind the optic chiasma is the infundibular lobe,
a flattened bilobed structure, emarginate posteriorly and
divided by a median longitudinal groove. It is formed cf

THE NERVOUS SYSTEM

311

nervous tissue and contains a cavity which is continuous
with the third ventricle.

The hypophysis cerebri lies behind and partly covered by
the infundibular lobe. It is composed of an anterior and
posterior part; the former is divided into a median and two
lateral portions, the posterior part is flattened and more or
less quadrate in outline. Genetically the true anterior part
of the hypophysis has no connection with the brain, but
arises as an outgrowth from the roof of the stomodeum.

The cerebral hemispheres are elongated bodies lying in
front of the thalamencephalon; they taper somewhat ante-
riorly and are separated from each other by the sagittal
fissure. Their cavities, the lateral ventricles, communicate
with the third ventricle by the foramen of Monro; anteriorly
the lateral ventricles extend into the olfactory lobes.

The olfactory lobes lie just in front of the cerebral hemi-
spheres, of which they are but the continuation. They are
separated from the latter by a shallow lateral sulcus. Un-
like the cerebral hemispheres, they are closely fused together
in the middle line; on the ventral side, however, they show
a well-marked median fissure. Anteriorly they give off the
olfactory nerves.

The Cranial Nerves. — There are ten pairs of nerves in
the frog which arise from the brain and are known conse-
quently as cranial nerves.

The first nerves, counting from before backward, are
the olfactory. They arise from the olfactory lobes by two
roots, the anterior one emerging from the front end, the
posterior running along the ventral side of the lobe nearly
to its posterior end. The olfactory nerves pass through
small foramina in the ethmoid bone and are distributed to
the walls of the nasal chambers.

The opticy or second pair of cranial nerves, arise from the
thalamencephalon, and, after crossing in the chiasma,

312

THE BIOLOGY OF THE FROG

emerge from the skull through foramina in the sides of the
chondrocranium, and are distributed to the eyes. Both the
olfactory and the optic nerves are purely sensory.

The third nerve, oculo-motor , is small and arises from
the ventral surface of the crura cerebri. It emerges from the
skull through a small foramen near the opening for the
optic nerve, and innervates four of the muscles of the eye-
ball, the rectus superior, rectus inferior, rectus medialis, and
obliquus inferior. After giving off a branch to the rectus
superior, the third nerve becomes connected with the ciliary
ganglion; this ganglion also receives a branch from the
ophthalmic division of the fifth nerve, and gives off the small
ciliary nerves to the retractor bulbi muscle and the tunics
of the eye.

The fourth nerve, the trochlears , is very small; it arises
from the dorsal side of the brain, between the optic lobes
and the cerebellum. It leaves the skull through a special
foramen a little above the optic nerve and is distributed to
the superior oblique muscle of the eye. Both the third and
the fourth nerves are exclusively motor.

The fifth nerve, the trigeminus or trifacial, is one of the
largest of the cranial nerves. It arises from the sides of the
anterior end of the medulla by a pair of roots which unite
in a large prootic ganglion before leaving the skull. This
ganglion, which corresponds in part to the Gasserian gan-
glion of higher forms, is connected with the sixth and seventh
nerves and also the sympathetic. Two branches of the tri-
geminus, the ophthalmic and the maxillo-mandibular ,
emerge from the ganglion, and leave the skull by a foramen
in the anterior part of the prootic bone. The former runs
along the dorsal side of the orbit, passes through a foramen
in the posterior wall of the nasal capsule, and divides into
branches, most of which emerge from the nasal capsule
again, and are distributed to the skin of the anterior part of

THE NERVOUS SYSTEM

313

the head. The maxillo-mandibular nerve runs outward be-
hind the eye ; it soon divides into the maxillaris superior and
the maxillaris inferior, or mandibular. The former runs
forward and outward below the eye and supplies the upper
lip and adjacent structures. The latter supplies the prin-
cipal muscles for moving the lower jaw, the lower lips, and
skin of the lower side ol the mouth. The trigeminus is a
mixed nerve, partly sensory and partly motor.

The sixth nerve, the abducens, arises from the ventral side
of the medulla. It joins the prootic ganglion and emerges
through the same opening as the fifth nerve, to be distributed
to the lateral or external rectus and retractor bulbi muscles
of the eye.

The seventh, or facial, nerve arises from the medulla,
closely behind the fifth, in company with which it leaves the
skull after emerging from the prootic ganglion. It soon
divides into two branches; the first, the palatine, courses
along the ventral side of the orbit, just above the mucous
membrane of the mouth. At the anterior end of the orbit it
gives off a branch which extends laterally and joins the
maxillary branch of the fifth nerve. The main nerve passes
forward to the nasal chambers and anterior portion of the
roof of the mouth. The second branch, the hyomandibular,
runs outward and then backward, around the auditory
capsule, passing over the columella, and, after receiving a
branch from the glossopharyngeal, runs outward, gives some
twigs to the ear and muscles of the lower jaw, and then
divides near the angle of the jaw into the mandibulars in-
ternus, which runs forward close to the mandible, and the
hyoideus, which innervates the subhyoideus muscle and the
skin in the region of the throat. The seventh nerve, like the
fifth, contains both sensory and motor fibers.

The eighth, or auditory, nerve is distributed entirely to the
inner ear.

314

THE BIOLOGY 01' THE FROG

The ninth, or glossopharyngeal, nerve arises from the
sides of the medulla from a group of roots in common with
the vagus; these roots emerge from the skull through a fora-
men in the exoccipital external to the condyle and enter the
large jugular ganglion. Shortly after its emergence from
this ganglion the trunk of the glossopharyngeal bears a small
ganglionic swelling and then soon divides, the one branch
passing forward to join the hyomandibular division of the
facial, the other running forward in a sinuous course along
the floor of the mouth and innervating the mucous mem-
brane of the tongue and pharynx.

The tenth (vagus or pneumogastric) nerve emerges from
the jugular ganglion generally by two trunks; the small
anterior trunk, the ramus auricularis, is distributed to the
region of the tympanum; the main nerve passes backward,
and, after giving off some small branches to the muscles
of the shoulder, becomes distributed to the larynx, esopha-
gus, stomach, lungs, and heart. Both the glossopharyngeal
and vagus contain sensory and motor fibers. The acces-
sorius, which is a small branch supplying the cucullaris
muscle, becomes in the higher vertebrates an independent
cranial nerve.

The Sympathetic System. — The main trunks of the
sympathetic system consist of a nervous strand on either
side of the spinal column. Anteriorly each trunk begins
in the prootic ganglion, from which it extends backward
within the cranial cavity, leaving the skull by the jugular
foramen in common with the vagus. It receives a branch
from the jugular ganglion, which enters the first ganglionic
enlargement of the main trunk. Each trunk receives a
branch (ramus communicans) from each of the spinal
nerves, and where the two join there is a ganglionic enlarge-
ment. The eighth spinal nerve, however, is connected with
the sympathetic by two communicating branches, and the

THE NERVOUS SYSTEM

315

ninth nerve by three and sometimes four. The last ganglion
of the sympathetic chain consists of the ninth and tenth
ganglia fused into one; it receives, besides the branches from
the ninth spinal nerve, a single small branch from the tenth.

A part of the fibers from the sympathetic trunks enter
the spinal nerves by way of the communicating rami ; other
fibers form independent sympathetic nerves. From the an-
terior portion of the trunks branches are given off to the sub-
clavian and occipito-vertebral arteries and anterior ends of
the oviducts. Farther back (from third to sixth ganglion)
several nerves are given off which unite to form the coeliac
or solar plexus, in which several ganglionic masses occur.
From this plexus nerves are distributed to the stomach, in-
testine, liver, pancreas, spleen, ovaries, oviducts, and kid-
neys. Farther back several branches anastomose to form
the urogenital plexus, which supplies the kidneys, ovaries,
oviducts, and testes. The ganglion cells of the sympathetic
system are often situated very far from their point of origin.
They are found embedded along the course of certain nerves
such as the vagus and facial; in the walls of the intestine
(Auerbach's and Meissner's plexuses) ; in the heart
(Remak's, Bidder's, DogieVs ganglia and several smaller
groups of cells); in the walls of the bladder; and in the
skin.

Reflex Action. — The reflex actions of the spinal cord of
the frog may be illustrated by the following experiment:
Cut the spinal cord of a frog across just behind the medulla
and, after destroying the brain, suspend the animal upon
a hook passed through the upper jaw. Now pinch one of the
toes; the hind foot will be drawn up to the body. If a piece
of blotting paper saturated with dilute acetic acid be placed
on one side of the body, the hind leg of that side will be
brought forward and the acid wiped away with the foot. Tf
the acid is placed near the middle of the body, both hind

316 THE BIOLOGY OF THE^ROG

feel may be employed to remove it. /rThe vigor of the re-
sponse depends upon the strength of the stimulus, a weak
stimulus producing only a slight movement, while a very
strong stimulus will throw the animal into violent contor-
tions. Similar responses may be evoked from the fore limbs,
and if acid be placed on the side of the body some distance
in front of the hind legs, both fore and hind limbs may be
employed to remove the irritant. These actions are of a
very definite and mechanical type, and, if the frog is in good
condition, follow inevitably upon the application of the
stimulus. They are termed reflex actions on account of a
certain analogy with the reflection of light which may be
thrown back upon its original source by a mirror. The
spinal cord nerves contain two kinds of fibers, the afferent
or sensory, which conduct impulses from the periphery to
the cord, and efferent (usually motor fibers), by means of
which impulses are carried outward to other organs. What
happens during a simple reflex action of the cord, then, is
this: Impulses set up by a stimulation of the sensory nerve
endings of the skin travel along the sensory fibers of a spinal
nerve to the cord, entering it through a dorsal root. The
afferent fibers as they enter the cord divide and run for a
distance both anteriorly and posteriorly, and then give off
collaterals, which branch in the gray matter. Some of these
form connections with the cells of the ventral roots of the
spinal nerves of the same side of the cord. In the simplest
case impulses reaching these cells through the collaterals are
transmitted to the axis cylinders of the motor nerves arising
from them and pass out through the ventral root and down
the same spinal nerve to the muscles of the leg, causing them
to contract. The spinal cord here serves as a medium of
communication between sensory and motor nerves. If a
dorsal root be cut, stimulation of a sensory nerve produces
no effect. If, however, the distal end of the part in connec-

THE NERVOUS SYSTEM

317

tion with the cord is irritated, muscular contractions will be
produced. If the ventral or motor root of the nerve be cut,
reflex action in the part supplied by that nerve is destroyed.
If the cut end of the nerve is irritated, the muscles to which
it is distributed will contract, but stimulation of the end
connected with the spinal cord will have no effect.

The impulses passing through the cord are not limited to
a single path, but they may take any one or more of several
routes. They may cross by way of the gray commissures to
the opposite side of the cord and become transferred through
the branches of the gray commissural fibers to the ventral
roots of that side, or they may pass across by means of the
white commissures. They may also pass backward or for-
ward along the cord either in the gray matter or in the white.
In this way a stimulus applied to any part of the body may
give rise to movements not only on the two sides, but also in
regions in front of and behind the point stimulated. The
stronger the stimulus, the greater is the part of the body
involved in the response. A weak stimulus applied to the
foot produces movement only in the member stimulated,
while a much stronger stimulus may cause a movement of
the opposite leg as well as other parts of the body. If acid
be placed on one side of the body and the leg of that side
held, the leg on the opposite side is sometimes brought
around to wipe the acid away. This apparently intelligent
act is obviously dependent upon the passage of impulses
from one side of the cord to the other. And it requires a
stronger stimulus to bring it about than is necessary to
produce a simple unilateral reflex.

These reflex actions of the spinal cord are of a purposive
nature; they bring the limbs away from injurious stimuli
and remove irritating substances from the body. They are
dependent upon the organization of the animal, the neuro-
muscular mechanism being such that a stimulus to any part

31S

THE BIOLOGY OF THE FROG

of the body brings about the appropriate actions for remov-
ing or escaping from the source of injury.

The reflex actions of a frog may be checked or modified
by impulses from the brain. A frog with its brain in con-
nection with the cord will not respond to stimulation in the
same regular and unvaried manner as a brainless individual.
Its actions are much less mechanical and more spontaneous
and uncertain of prediction. The influence of the brain is
rendered possible by means of nerve trunks which pass from
the brain down the spinal cord and form connections with
the neurones of the spinal nerves. If the anterior end of
the spinal cord is strongly stimulated at the same time a
stimulus is applied to the foot, the withdrawal of the latter
may be entirely prevented. In this way the ordinary reflex
actions of the cord are continually checked and modified by
impulses from the higher nervous centers.

The Croaking Reflex. — If the side of a frog be stroked
with the finger, the animal often responds by croaking. The
same reaction frequently occurs when the frog is picked up
in the hand. It takes place more often in the male frog
than in the female, especially during the breeding period,
when the croaking mechanism is very readily put into
activity. A frog that is seized may continue to croak vigor-
ously for some time, and when it has ceased to croak it may
be induced to continue by rubbing its side with the finger.
The croaking reaction, however, is very variable, and in
certain individuals it may not be performed at all. And the
same individual reacts quite differently at different times,
according to what seems its own caprice.

It is quite otherwise with frogs from which the cerebral
hemispheres have been removed. It was found by Goltz
that animals thus operated on croak with mechanical regu-
larity whenever one strokes their side or back. They never
croak when the hind legs or ventral side of the body is

THE NERVOUS SYSTEM

319

stroked, and they croak only once after each stroke on the
back o: side. The experiment succeeds well with both sexes,
but the croak of the male is naturally much louder than that
of the female. "I know of scarcely any physiological ex-
periment," says Goltz, "which succeeds in so certain and
regular a manner as this croaking experiment. ... As I was
delivering a discourse upon this subject before the meeting
of the naturalists at Hannover, in the year 1865, I placed
upon the table, for purposes of demonstration, several frogs
operated upon several months previously, which I had
brought with me from Konigsberg. In the course of the
session I asked Herr Von Wittich, who was present in the
audience, how often each of the frogs, which were resting
silently and still, should croak. The answer five times was
given, and each of the frogs, upon being given five strokes,
croaked exactly five times, to the evident gratification of
the auditors, whereupon they all lapsed into silence."

The croaking of a brainless frog is a reflex set in opera-
tion by a particular localized external stimulus. In a normal
specimen this reflex is subject to control from the higher
nerve centers. It may be entirely checked by impulses
down the spinal cord from the brain. The normal frog
shows a spontaneity and freedom of action which is largely
destroyed in individuals which have recently lost their
cerebral hemispheres. The latter often croaks without any
apparent exciting cause, while a brainless frog croaks only
upon the application of an external stimulus to certain parts
of the body.

The utility of the croaking reflex is not apparent. It has
been suggested by Baglioni that, since in croaking air is
pushed back and forth between the mouth and the lungs,
the swelling thus produced enables the animal to push
against anything that seizes it and make its escape, the
sound being merely an incidental accompaniment of the

320

THE BIOLOGY OF THE FROG

process. Any one who picks up a frog in the hands may
readily convince himself that the croaking movements are a
means of enabling the creature to slip from his grasp. In
fact, these movements often occur in the female without
producing any sound whatever. It is a significant fact that
the reflex is evoked by stimuli upon those parts which, as
the body swells, press against whatever seizes the animal.
It is not brought about by seizure of the head or hind legs;
the swelling in such cases would be of no avail. The fact
that the croaking reflex is brought about by gently stroking
the side or back may indicate nothing but the extreme
readiness with which the swelling reflex is initiated.
When the animal is seized and there is a constant pres-
sure stimulus, there is a repeated swelling of the body and
accompanying croaks. If the animal is stroked once on
the side, it croaks once and then stops because the stimulus
is stopped.

The Clasping Reflex and the Recognition of Sex. — The

tendency of the male frog to clasp the female is one of the
strongest of its instincts, but it appears only for a short time,
during the breeding period in the spring. At this time the
male will clasp another male frog, one's fingers, or in fact
almost any object that is placed between its fore legs, but
objects other than females are after a time rejected, while
the duration of ordinary copulation is commonly several
days.

The clasping reflex may be evoked any time of the year
by cutting through the medulla or, according to Busquet, by
simply removing the cerebellum. Steinach found that the
higher nerve centers exercise an inhibitory influence over the
clasping reflex except during the breeding season when the
internal secretion of the testes acts upon the higher centers
so as to check their usual inhibitory function. Whether the
seat of the normal inhibition is in the cerebellum or in the

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321

mid-brain, as held by Baglioni, or perhaps in more than
one center, is not definitely established.

The clasping efforts of the male frog afford a typical
illustration of instinctive action; nevertheless they occur
in entire independence of the higher nerve centers. The
Abbe Spallanzani showed that a male frog may have its
head cut off during copulation without ceasing to cling
tenaciously to the female. Goltz went still farther and cut
off the head of a male, then cut the body through between
the third and fourth vertebrae, and removed the viscera from
the body cavity; the section of the frog that remained after
these operations consisted of the first three vertebra, the
pectoral girdle, and the fore legs. Yet when the skin of
the inner surfaces of the fore legs was rubbed with the
fingers, this segment would show the same clasping efforts
as a normal male frog. The clasping of the male frog is,
therefore, a reflex action whose center lies in the brachial
region of the spinal cord. If the skin on the inner surfaces
of the fore legs and on the breast be removed, the clasping
reflex is destroyed. It is probable, therefore, that the reflex
is initiated through the stimulation of the sensory organs
of these regions of the body.

Notwithstanding the; almost mechanical character of this
spinal reflex, the frog discriminates between two such similar
objects as the males and females of its own species. When
a number of frogs of both sexes are placed together during
the breeding season, the males will be found to be clasping
females instead of other males. The question naturally
arises: How does the male frog distinguish the female
from one of his own sex? That it is not, as in many
animals, through the sense of smell, was shown by Goltz
by cutting through the olfactory lobes of several male
specimens; after this operation the mutilated males were
placed among several females, and in a short time they were

322

THE BIOLOGY OF THE FROG

all in copulation with members of the other sex. Then
several males which were blinded were placed among the
females, with the same result as before. That it is not the
sound produced by the female that reveals her sex, was
shown by Goltz by placing several females, which had been
rendered incapable of using their voice, among the males.
These were as readily seized as normal females. After the
destruction of both smell and sight, males were found to
be still able to distinguish the females, although there was
apparently a certain diminution of their ardor. The senses
of sight, smell, and hearing are not, therefore, the exclusive
or indispensable means of sex recognition, whatever be the
part they play under normal conditions. Neither are the
higher nerve centers necessary. Goltz cut through the skull
of a male so as to cut off the cerebral hemispheres and
eyes; the specimen almost immediately clasped a female
that was presented to it, while a male that was offered was
rejected.

The form of the body differs in the two sexes, especially
when the female carries a large mass of eggs, but this alone
does not enable the male to distinguish the female. Goltz
found that if the bodies of males were filled with flesh and
sewed up so as to resemble the form of the females, they
would be clasped for a short time and then rejected. The
bodies of freshly killed females, on the other hand, were
held for a long time, the tension or cramp of the muscles
increasing, the longer the bodies were held. Goltz has sug-
gested that there is something that emanates from the body
of the female that acts as a special excitant to the male, and
some later observations support this view.

Banta, who has studied the mating behavior of the wood
frog, finds that the males during the breeding season swim
about actively in search of mates while the females are rela-
tively less active. Males tend to seize almost any moving

THE NERVOUS SYSTEM

323

object that comes near. "Any individual which moves
within a radius of several feet of another male is likely to
be tested by him. The male thus approached sometimes
swims away and sometimes actively resists but often pays
no attention to the aggressor and the latter turns back,
frequently without coming near enough to touch the male,
and almost always the aggressor gives up the attack after
the very beginning of an attempt to grasp the stranger with
the fore-legs. . . . Seldom is an attempt made upon any but
a moving individual. . . . Even a female may usually avoid
pursuit as long as she remains quiet. On the other hand
any small moving object at the surface of the water is most
certain to be approached by an eager male. The writer
twice observed the male approach a speckled tortoise when
the latter thrust its head out of the water."

The different behavior of males to the two sexes of their
species may be in part due to the peculiar reactions of the
female, but, as Banta observed, "The readiness with which
the attempt of a male to pair with another male is given
up on near approach, the keenness of the male's pursuit
after once approaching very near or touching a female, and
the discrimination between a dead male and a dead female
particularly in cases in which, to the human eye, the latter
is indistinguishable in size, color and general features from
a male, suggests that a chemical sense is involved in final
sex-recognition though one experiment designed to test this
hypothesis was unsuccessful."

Sex discrimination in frogs is not very precise. Males
are sometimes held by other males for a long time, and
copulation is not infrequently known to occur between the
males of different species. Spallanzani records a male toad
that was carrying around an individual of his own sex that
had died some days previously and was in an advanced state
of decay. Whether or not a male will clasp an object other

324

THE BIOLOGY OF THE FROG

than the female depends upon the length of time he has
been separated from the sexual embrace. If a male is
recently torn from a female, he is very sluggish, and will
clasp the fingers or almost any object that is presented to
him. If left for a time, he becomes more active and shows
little tendency to clasp all sorts of objects. Male frogs
which at first would be seized are now usually rejected, or
held but for a short time; but if a female is presented, she
is seized with eagerness.

Compensatory Motions. — If a frog is rotated on a circu-
lar cisk with its head pointing away from the center, the
head of the animal will turn opposite the direction of rota-
tion. The turning of the head is often followed by locomo-
tion in the same direction, so that the frog keeps circling
around in one direction while the disk is rotating in another.
If the frog is placed on a board, and tilted either up or down,
the head is turned opposite the direction of motion. Com-
bined movements of rotation and tilting are followed by
corresponding combined movements of the head, the result
being in all cases to keep the head as nearly as possible in
its original position. Such movements are therefore called
compensatory motions. They take place in response to very
slight changes in the position of the body, and they occur
with remarkable regularity, apparently independently of the
animal's volition. They are performed after the destruction
of all parts of the brain in front of the medulla, or even after
the removal of the anterior part of the latter organ, provided
the injury does not extend so far back as the trigeminus
group of nerves. If the semicircular canals are destroyed
or the auditory nerves cut, compensatory motions, according
to Schrader, are no longer performed, but Steiner finds that
compensatory motions of the head at least still take place.

The Functions of the Brain. — The brain is the great
center of communication between the principal organs of

THE NERVOUS SYSTEM

325

sense and the rest of the body; through it are effected the
numerous coordinations between a great variety of stimuli,
sights, sounds, odors, etc., and the appropriate muscular
actions which enable the animal to adjust itself to the en-
vironment. The number and nature of the connections
established in the central nervous system determine to a
large degree the character of the animal's instincts, and the
possibilities and the limits of the development of its intelli-
gence.

The brain of the frog is in one respect a favorable object
for the study of function, inasmuch as large parts of it may
be removed without causing the death of the animal. The
shock effects following operations are much less in cold-
blooded animals than in birds and mammals; the subjects
recover more quickly, and will endure the removal of much
larger portions of the brain. But even in the frog the after-
effects of certain operations wear off very slowly. There is
danger in removing parts of the brain, of attributing results
to the loss of the part in question, which may be due to
injuries produced' by the operation. The behavior of the
animal some months after being operated upon is often quite
different from its behavior before complete recovery. For
this reason different investigators of the functions of the
frog's brain have arrived at contradictory results. It is only
after waiting until the shock effects of the removal of a part
of the brain have completely worn away, that we can arrive
at a correct idea of the normal role of that part in the life of
the animal.

Cerebral Hemispheres. — It is frequently stated that a frog
which has lost its cerebral hemispheres loses nearly all its
spontaneity; that it remains sitting in one position for
an indefinite time, if not disturbed; and that it takes no food
unless it is forced into its mouth. While it is still able
to hop and swim, avoid obstacles placed in its path, and

326

THE BIOLOGY OF THE FROG

perform many other movements in an apparently normal
manner, it is said to lose the power of originating actions
independently of outer stimuli. The experiments of Schra-
der showed the incorrectness of these commonly accepted
conclusions. The symptoms so frequently ascribed to loss
of the cerebral hemispheres are those which follow when a
part of the thalamencephalon is also removed or subjected
to severe injury. Schrader found that if the cerebral hemi-
spheres were carefully removed without injuring the thalami,
and the animal kept some months after the operation, spon-
taneity of action is by no means lost; the animals move
about from land to water, bury themselves in the mud on the
approach of winter, and snap at flies which move within
their reach. Their conduct, in fact, resembles very closely
that of normal frogs; and in just what respect it differs
Schrader does not make clear. After loss of the cerebral
hemispheres all fear, according to Kato, disappears, and the
frog no longer croaks spontaneously, but how long after the
operation these results were observed is not stated.

In higher animals the cerebral hemispheres are the seat
of intelligence and voluntary control. Dr. T. C. Burnett
removed the cerebral hemispheres from several frogs and
endeavored to ascertain if the animals could learn the
simple things which ordinary frogs are able to learn. He
found that the decerebrate frogs were unable to learn a very
simple path to water, nor would they avoid a path in which
they repeatedly received a strong electric shock. Removal
of the cerebral hemispheres seems, therefore, to affect the
frog much as it does a bird or a mammal in destroying the
powrer of associative memory.

Removal of one cerebral hemisphere is not followed by
so severe shock effects as the removal of the entire cere-
brum. The frog thus operated upon is, according to Kato,
more active, it springs more readily, and manifests fear upon

THE NERVOUS SYSTEM

327

seeing the approach of large objects. It tends at first to
avoid obstacles placed in its path by turning away from
the operated side, but in four days after the operation its
behavior, according to Loeser, becomes entirely normal.

If the parietal region of the cerebral hemisphere is stim-
ulated by a very weak current of electricity, movements of
the limbs are brought about on the opposite side of the
body, and usually also, although to a less extent, on the same
side, whereas stimulation of other parts of the cerebral
hemispheres produces no such effect (Krawzoff, Kato). No
definite localization of motor functions such as has been
established in the brains of the higher mammals has yet
been worked out.

Thalamencephalon. — Removal of the thalamencephalon
along with the cerebral hemispheres results, as before re-
marked, in an almost complete loss of spontaneous move-
ments. The animal rests quietly, and seldom makes a move
unless in response to some external stimulus, and shows a
general insensibility to touch (Loeser). Since the optic
nerves enter the thalamencephalon, removal of this part
causes complete blindness; the frog no longer avoids obsta-
cles placed in its path, or shows any reaction involving the
power of vision. When placed on a piece of wood that is
slowly tilted upward it will not climb to the upper side and
balance there, like a frog that has lost its cerebral hemi-
spheres, but it responds only by movements of the head,
lowering that part when the anterior part of the body is
raised and raising it when the reverse movement is per-
formed (Steiner, Schrader). If the animal is rotated on a
horizontal disk it responds by turning; soon after the opera-
tion it shows no compensatory motions; later, it responds
by turning the head opposite to the direction of rotation,
and, after complete recovery, performs circus movements of
the body in the same direction as those of the head. The

328

THE BIOLOGY OF THE FROG

same movements are also performed after loss of the optic
lobes. The loss of the power to balance the body on the
edge of a tilted board is apparently permanent.

The Optic Lobes. — According to Steiner the center for
coordinated motion is in the optic lobes. Frogs from which
this part of the brain was removed were found by Steiner to
swim with alternate movements of the legs instead of com-
bined strokes. Schrader showed, however, that this was the
result of shock effects, and that frogs carefully operated
upon and given time to recover, performed coordinated
jumping and swimming movements remarkably well. The
center for locomotion cannot, therefore, be located in the
optic lobes. Removal of the optic lobes was found by Loeser
to produce forced movements, an abnormal retention of
urine, and a slight loss of sight, but later these symptoms
gradually disappear. Frogs with the brain removed as far
back as the medulla locomote normally, perform the croak-
ing reflex, breathe regularly, and show the usual com-
pensatory motions.

The optic lobes exercise an inhibiting influence on the
reflex activity of the spinal cord. Removal of the optic
lobes results in an increased irritability of the cord, while
stimulation of the optic lobes may greatly check spinal
reflexes. If one optic lobe is stimulated, the spinal reflexes
on the opposite side of the body are inhibited, while those
on the same sides are not noticeably affected (Langendorff).

The Cerebellum. — The cerebellum of the frog is so small
compared with that of most other vertebrates, that it would
not be expected to have an important function. Steiner
found that frogs from which the cerebellum had been re-
moved were apparently unaffected by the operation, and
other investigators have confirmed this conclusion. Goltz,
however, found that coordinated locomotion was disturbed
after extirpation of the cerebellum, but, as was admitted

THE NERVOUS SYSTEM

329

later, somewhat more than the cerebellum alone was re-
moved. Recently, however, Loeser has arrived at results
similar to those of Goltz. After removal of the cerebellum,
according to Loeser, "the jumps became very unsteady and
were weakened in force, but not much reduced in rate. Most
of the time the frog lay stretched in contact with the floor
of the vessel and covered with moss. The unsteady and
weakened condition of the limbs persisted in a marked
degree two and a half months after the operation (especially
when the frog was greatly excited). " It seems not improb-
able that the discrepant results of the different investigators
of this subject may be mainly due to differences in the
amount of nervous material removed by the operation.

The conclusion of Goltz that the optic lobes and the
cerebellum contain the centers for locomotion cannot be
sustained, since Schrader and others have shown that co-
ordinated locomotion is still possible after the cerebellum
and all parts of the brain lying in front of it are destroyed.
According to Steiner, if the cerebral hemispheres are re-
moved along with the cerebellum, the frog shows certain
symptoms that do not appear after loss of the cerebral hemi-
spheres alone. Frogs deprived of both these parts of the
brain were found, when attempting to jump from one object
to another, either to fall short of the goal, or to jump too
far, whereas animals from which the cerebral hemispheres
alone were removed were able to adjust more accurately the
length of their spring.

The Medulla. — A frog with the brain removed as far
back as the medulla is still capable of performing regular
leaps and swimming movements of the hind limbs. When
thrown on its back it rights itself, and it still performs com-
pensatory motions when tilted or rotated. Breathing is
normal, and the animal swallows pieces of food that are
placed in its mouth.

330

THE BIOLOGY OF THE FROG

If the anterior portion (pars commissuralis) of the
medulla is removed, the frog becomes possessed of an un-
controllable tendency to move about, whereas if the medulla
is intact, the movements of the animal are sluggish. It
creeps about, restlessly, coming to rest only when it has
arrived in some protected nook or corner. Locomotion is
effected mainly by creeping, but the frog is nevertheless
capable of springing in the ordinary manner. In the water
it swims by alternate movements of the limbs. The resting
position assumed is not quite normal, the body is more flat-
tened than usual, and the hind limbs are not drawn up to
the sides. The breathing and swallowing reflexes are still
normal, but the croaking reflex is no longer performed. The
reflex of snapping at food is not destroyed, although in the
absence of connection with the eyes it is not performed in
response to visual stimuli. If a piece of meat is rubbed
against the frog's nose, the animal snaps at it and uses the
fore legs to stuff it into the mouth. The same reaction may
be brought about by using the finger, but after the finger is
seized and it is found that the object is too large to be stuffed
into the mouth, the frog begins to reject the morsel, and uses
the fore legs to push it away. Truly a remarkable com-
bination of reflexes!

The snapping reflex is also brought about by contact
with the fore legs, or even other parts of the body. If sev-
eral frogs, whose brains are cut through just behind the
cerebellum, be placed together in a box, they frequently
snap at each other when they accidentally come into con-
tact, as if they were defending themselves against attack.
Flies creeping over the nose are usually not snapped at, but
the stimulus produces another reflex, the fore leg being
brought forward to brush the insect away.

The snapping reflex is prevented if the facial nerve is cut,
or if the region of the medulla from which the fifth and

THE NERVOUS SYSTEM

331

seventh nerves originate is destroyed. These nerves form
the afferent and efferent channls respectively of the nervous
impulses involved in the snapping reflex. The extrusion of
the tongue, which goes along with this process, is dependent
upon the hypoglossus nerve, which has its origin further
back in the medulla. Destruction of the root of this nerve
prevents the extrusion of the tongue, but the snapping of the
jaws may still occur. The two parts of the reflex of seizing
food may therefore take place independently, although they
are normally almost always associated.

The swallowing reflexes persist after destruction of the
center for snapping. They are dependent upon the vagus
group of nerves, and disappear only when the region of the
medulla is injured from which the roots of the vagus group
arise. Normal breathing movements still occur w^hen the
medulla is cut across just behind the cerebellum and the
parts in front of the cut entirely removed. They are not de-
stroyed if the medulla is also cut across at the tip of the
calamus scriptorius, but if the region between these two cuts
is removed, respiratory movements entirely cease. This
region includes, therefore, the centers for the movements of
respiration.

The posterior region of the medulla contains those parts
of the brain most essential for the maintenance of life. So
long as this region remains, the animal may live for a long
time. Removal of the medulla farther back than the pars
commissuralis results in making locomotion more difficult,
although it is still possible when the brain is cut in two as
far back as the tip of the calamus scriptorius. The restless-
ness of the animal disappears and the body tends to assume
an abnormal attitude.

There is no center for coordinated locomotion in the
medulla. Disturbances of locomotion begin with the fore
limbs. If the medulla is cut across at the tip of the calamus

332

THE BIOLOGY OF THE FROG

scriptorius, the animal sinks on its breast, and the fore limbs
are for a considerable time helpless, although the hind limbs
are capable of performing vigorous coordinated movements.
The reason for this is that the injury lies so near the
region from which the nerves of the fore limbs arise that
the movements of these members are very naturally affected.

The Segmental Character of the Functions of the
Nervous System. — The functions of the central nervous
system of the frog bear out the segmental theory of the
action of the higher nerve centers which was first suggested
by Schrader and more fully elaborated by Loeb. The
nervous system of such forms as worms, crustaceans, and
insects consists of a series of pairs of ganglia connected by
a double nerve cord, there being typically one pair to each
segment of the body; and as a rule each pair of ganglia
forms the center of the movements of the parts of the
segment in which it lies. Schrader at the close of his
paper on the functions of the brain of the frog gives expres-
sion to the segmental theory as follows: "The series of ex-
periments we have given teaches us that the central nervous
system of the frog can be divided into a series of sections,
each of which is capable of performing an independent
function. It brings the central nervous system of the frog
into closer relation with the central nervous system of the
lower forms, which consists of a series of distinct ganglia
that are connected by commissures. It speaks against the
absolute monarchy of a single central apparatus and against
the existence of different kinds of centers, and invites us to
seek for the centralization in the many-sided coupling of
relatively independent stations."

Loeb has attempted to show that "the more complicated
instincts are for the most part nothing but a series of seg-
mental reflexes." "I am inclined/' he says, "to recommend
using the word chain-reflexes whereby the performance of

THE NERVOUS SYSTEM

333

one reflex acts at the same time as the stimulus for
setting free a second reflex. The taking of food may serve
as an illustration of such a chain-reflex. The optic reflex
of the moving fly produced the snapping reflex; the contact
of the mouth epithelium with the fly produces the swallow-
ing reflex. Each of these reflexes is purely segmental. By
taking into account the act of transmission, complicated
acts can thus be resolved into a few segmental reflexes."

The action of the lower nerve centers is much influenced
by the inhibitory action of the parts lying in front of them.
That a frog with the optic thalamus or optic lobes removed
loses much of its spontaneity is due not to the fact that these
parts contain the centers for locomotion, but to the inhibit-
ing influence of these organs on the lower nervous centers.
This is shown by the fact that if the brain is removed still
farther back so as to include the anterior part of the
medulla, spontaneous movements will again make their ap-
pearance. One of the most important functions of the
higher nerve centers is that of checking the movements of
the lower centers that would otherwise occur. In this way
the frog's actions are subject to a control which makes them
more adequately subservient to the needs of the animal.
Locomotion may be effected by the nerve centers of the
spinal cord, but when locomotion is set up, and when
stopped, and how it is directed are determined by impulses
from the higher nerve centers in response to sights, sounds,
etc., from the organs of special sense. The higher centers of
the brain are comparable to the captain of a steamer who
issues orders to the man running the engine when to start
and when to stop, and who has his hand on the wheel so as
to guide the course of the vessel. The machinery of locomo-
tion works away by itself, but it works blindly. The captain
sees and avoids the obstacles in the way and determines the
direction and the limits of the journey. In a similar manner

334 THE BIOLOGY OF THE FROG

the reflex machinery of the lower nerve centers of the frog is
guided by those parts whose organs of sense put them into
more intimate connection with the outer world.

REFERENCES

Albertoni, P. Experiences sur les centres nerveux du crapaud.
Arch. Ital. de Biol., T. 9, 1888.

Baglioni, S. Physiologie des Nervensystems. Winterstein, Handb.
vergl. Physiol., Bd. 4, 1913.

Banta, A. M. Sex Recognition and Mating Behavior of the Wood
Frog, Rana sylvatica. Biol. Bull., Vol. 26, 171, 1914.

Bechterew, W. Ueber die Function der Vierhiigel. Arch. ges.
Phys., Bd. 33, 1884.

Bickel, A. Uebei den Einfluss der sensibelen Nerven und den
Labyrinth auf die Bewegung der Thiere. Arch. ges. Phys., Bd. 67,
1897. Beitrage zur Rlickenmarksphysiologie der Amphibien und
Reptilien, I.e., Bd. 71, 1898.

Birge, E. A. Note on the Functions of the Spinal Cord of the
Frog. Am. Month. Micr. Jour., Vol. 2. Die Zahl der Nervenfasern
und der motorische Ganglienzellen im Rlickenmark des Frosches.
Arch. Anat. u. Phys., phys. Abth., 1882.

Burnett, T. C. Some Observations on Decerebrate Frogs with
Especial Reference to the Formation of Associations. Am. Jour.
Physiol., Vol. 30, 80, 1912.

Dale, H. H. Observations ... on Possible Efferent Fibers in the
Dorsal Nerve Roots of the Toad and Frog. Jour. Phys., Vol. 27.

Danilewsky, B. Ueber die Hemmung der Reflex- und Willklir-
bewegungen. Arch. ges. Phys., Bd. 24, 1881.

Donaldson, H. H. Observations on the Weight and Length of the
Central Nervous System and the Legs in Bullfrogs of Different Sizes.
Jour. Comp. Neur., Vol. 8, 1S98; coDtinuation by Donaldson and
Schoemacher, I.e., Vol. 10, 1900.

Edinger, F. Die Leistungen des Ze.ntralnervensystems beim
Frosch. Referate. Zeit, allg. Physiol., Bd. 15, 15, 1913. (Bibliography.)

Gaskell, W. H. On the Rhythm of the Heart of the Frog and the
Nature of the Action of the Vagus. Phil. Trans., Vol. 173, part 4.

Goltz, F. Einige Versuche liber den Nervenmechanismus, welcher.
wahrend der Begattung der Frosche, thatig ist. Cent. med. Wiss., Bd.
3, 1865. Weiteres liber den Nervenmechanismus, etc., I.e., Bd. 4, 1866

THE NERVOUS SYSTEM

335

Beitrage zur Lehre von den Funktionen der Nervencentren des
Frosches, I.e., Bd. 4, 1866. Beitrage zur Lehre von den Nervencentren
des Frosches. Berlin, 1868.

Hardesty, I. The Number and Arrangement of the Fibers form-
ing the Spinal Nerves of the Frog (Rana virescens). Jour. Comp.
Neur., Vol. 9. Further observations, I.e., Vol. 10, 1899-1900.

Harless, E. Ueber die Funktionen verschiedener Partien des
Riickenmarks der Amphibien. Arch. Anat. u. Phys., 1846.

Heubei, E. Das Krampfcentrum des Frosches und seine Verhalten
gegen gewisse Arzneistoffe. Arch. ges. Phys., Bd. 9, 1874. Ueber
die Abhangigkeit des von wachen Gehirnzustandes ausseren Erregun-
gen. Ibid., Bd. 14, 1887.

Kato, T. Versuche am Grosshirn des Frosches. Inaug. Diss.
Berlin, 1886.

Langendorff, O. Die Beziehung des Sehorganes zu den reflexhem-
menden Mechanismen des Froschgehirns. Arch. Anat. u. Phys., 1887.
Ueber Reflexhemmung. Ibid., p. 96.

Loeb, J. Comparative Physiology of the Brain and Comparative
Psychology. New York, 1900.

Loeser, Wm. A Study of the Functions of Different Parts of the
Frog's Brain. Jour. Comp. Neur. and Psych., Vol. 15, 1905.

Mendelsohn, M. Untersuchungen iiber Reflexe. Sitzber. d. k.
preuss. Ak. wiss. Berlin, 1882 und 1883.

Merzbacher, L. Ueber die Beziehungen der Sinnesorgane zu den
Renexbewegungen des Frosches. Arch. ges. Phys., Bd. 81, 1900.
Untersuchungen iiber die Regulation der Bewegung der Wirbelthiere,
1, Beobachtungen an Froschen. Ibid., Bd. 88, 1902.

Nothnagel, H. Bewegungshemmende Mechanismen im Riicken-
marke des Frosches. Cent. med. Wiss., Bd. 7, 1869.

Schlosser, W. Untersuchungen liber die Hemmung von Reflexen.
Arch. Anat. u. Phys., phys. Abth., 1880.

Schrader, M. Zur Physiologie des Froschgehirns. Arch. ges. Phys.,
Bd. 41, 1887.

Sedgwick, W. T. On the Variations of Reflex Excitability in the
Frog induced by Changes of Temperature. Studies Biol. Lab. Johns
Hopkins Univ., Vol. 2, 1882.

Sirotinin. Die punktformige begrenzte Reizung des Froschriicken-
marks. Arch. Anat. u. Phys., phys. Abth., 1887.

Steiner, J. Die Functionen der Centralnervensystem und ihre
Phylogenese. Abth. 1, Untersuchungen iiber die Physiologie des
Froschgehirns, Braunschweig, 1885.

336

THE BIOLOGY OF THE FROG

Volkmami, A. W. Ueber Reflexbewegungen. Muller's Archiv,

1S3S.

Viilpian. Legons sur la physiologie du systeme nerveux. Paris,
1S66.

Wyman, J. Anatomy of the Nervous System of Rana pipiens.
Smithsonian Contributions to Knowledge, Vol. 5, 1853.

Yerkes, R. M. Inhibition and Reinforcement of Reactions in the
Frog, Rana clamitans. Jour. Comp. Neur. and Psych., Vol. 14, 1904.
Bahnung und Hemmung der Reactionen auf tactile Reize durch akus-
tische Reize beim Frosche. Arch. ges. Phys., Bd. 107, 1905.

CHAPTER XVII

THE SENSE ORGANS

The sense organs are the means through which stimuli are
received from the outer world. They are always connected
with the central nervous system by nerve fibers which carry
the impulses derived from the various objects in the envi-
ronment which act upon the organism. The sense organs in
general are specialized so that they are exceedingly sensitive
to certain forms of stimulation, and but very slightly so to
others. The eye is affected ordinarily only by light, the
auditory organ only by sound, the olfactory organ only by
contact with certain substances. The specific functions of
the sense organs are determined by their structure, each be-
ing specialized in relation to some particular source of
stimulation from the outside world.

Sense Organs of the Skin. — The skin is richly supplied
with sensory nerve endings both in the epidermis and in the
corium. The epidermis is supplied by fine fibers which re-
peatedly branch and form a sort of network between the
cells. The free ends of the ultimate branches are often en-
larged into a disk (Retzius). The corium contains special
end organs, the so-called touch corpuscles, which lie under
small papillae of the epidermis. Each touch corpuscle is com-
posed of a small heap of flattened cells between which are
found the terminal branches of the nerve with which each
organ is supplied. The temporary papillae of the female of
liana fusca, according to Huber, possess a structure in
some respects resembling that of the touch bodies.

337

THE BIOLOGY OF THE FROG

The skin of the frog is an organ of unusual sensibility. It
is sensitive not only to touch and chemical stimuli, but it is
affected in no small degree by light. A. H. Morgan has
shown that the frog's foot reacts differently to warm and
to cold water, and that the reactions are destroyed by re-
moving the skin of the foot. Not improbably it has different
sense organs for heat and cold. The skin is also readily in-
fluenced by changes in the moisture of the air; as is indi-

Fig. 104. — Touch corpuscle of the frog with its supplying nerve. The

branches of the nerve may be seen ramifying between the flattened
cells of the sense organ. (From Gaupp, after Merkel.)

cated by the behavior of the animal under different con-
ditions of the atmosphere.

Sense Orga-ns of the Mouth. — In the mouth there are in
addition to the nerve endings in the general epithelium
numerous sense organs which have been supposed to be
concerned with the sense of taste. They occur on the
flattened surfaces of the fungiform papillae of the tongue and
also on the floor and roof of the mouth, where they assume
a more rounded form although possessing essentially the
same structure in both regions (Bethe). The surface of

THE SENSE ORGANS

339

these organs is covered by cylindrical epithelial cells between
which are scattered various forms of elongated cells which
are connected usually by means of branching processes,
with the ramifications of the supplying nerve.

a c a d b c a

Fig. 105. — Section through a fungiform papilla. *A bundle of nerve
fibers enters the papilla from below, breaking up into first the sub-
basal plexus, and second the subepithelial plexus around the bases
of the cells of columnar epithelium. The latter plexus forms con-
nections with the branches of the sensory cells, a, free nerve end-
ings ; 6, c, d, nerve cells ; e, f, cells connected with the subepithelial
plexus. (From Gaupp, after Niemack.)

The Olfactory Organs. — The seat of smell is in the nasal
cavity, the walls of which are supplied with the olfactory
nerves. The nasal cavity is lined by a mucous membrane
which is composed of a connective tissue substratum and
an outer layer of olfactory epithelium. In the regions to
which the olfactory nerves are distributed the epithelium
consists of three kinds of cells: (1) interstitial cells, which

340

THE BIOLOGY OF THE FROG

are very much elongated and cylindrical in form; (2) basal
cells (Krause), which are of stellate form and lie next to the
connective tissue; and (3) the o£-
factory cells. The latter, which
represent the true sensory cells, are
very variable in form. They are
usually very long and narrow, much
swollen out where the nucleus oc-
curs, and furnished at the outer
free end with a tuft of fine cilia.
At their inner ends they are drawn
out into a fine process which is con-
nected with the olfactory nerve.

Knauer found that ill-smelling
substances held near the nostrils
caused the frog to turn away the
head, but whether the olfactory
sense plays an important role in the
life of the animal is not known.

The Eyes. — The eyes of the frog
are nearly spherical in form, and
they are lodged in large cavities in
the sides of the head called the or-
bits, within which they are freely
movable. The outer or exposed por-
tion of the eye is covered by a trans-
parent membrane known as the cor-
nea; the remaining portion, which
constitutes about two thirds of the
surface, is formed by the sclerotic
coat. This is an opaque, white
covering mainly inclosed within the orbit; it is composed
of an outer fibrous membrane, and an inner layer of hyaline
cartilage; on the side next to the cranium it is perforated

Fig. 106.— Cells of the ol-
factory epithelium. On
the left two sensory
cells with ciliated outer
ends, enlarged nuclei,
and fine basal fibers.
On the right two inter-
stitial cells. (After
Dogiel. )

THE SENSE ORGANS

341

by an aperture; the optic foramen, through which the optic
nerve enters the eye.

At its outer margin the cornea is continuous with a mem-
brane, the conjunctiva, which passes backward a short dis-
tance (about 2 mm.) over the sclerotic, and is then reflected
over the inner surface of the upper and lower eyelids. The
upper eyelid is a narrow, thick fold extending from the
upper edge of the orbit. It has no power of independent
motion, but may be raised or lowered by the movements of
the eyeball. What appears as the lower lid is really made
up of two elements, the lower lid proper, and the nictitating
membrane. In most animals these folds are quite distinct,
but in the frog the nictitating membrane is continuous with
the upper portion of what corresponds to the true lower
eyelid of other forms. The lower lid consists of a lower
thick portion and an upper thinner part, which is folded in
behind the former when the eye is uncovered. The nicti-
tating membrane is a thin semilunar fold somewhat more
transparent than the upper portion of the lower eyelid, from
which it is separated by a slight furrow. At the anterior
and posterior ends of its slightly thickened upper margin it
is attached to the two ends of a tendon which passes behind
the eyeball, thus encircling the greater part of the eye. The
closing of the eye of the frog does not occur through the
independent movements of the eyelids as it does in our-
selves, but is a consequence of the retraction of the eye
within the orbit. If one watches the closing of the eye of
the frog, it will be seen that every time the lids are drawn
together the eye sinks into the head. The withdrawal of
the eye by producing a tension on the tendon of the nictitat •
ing membrane causes the latter to be pulled up, although
one might easily gain the impression that the eye is pressed
into the head by the closure of the lids. The opening of the
eye, on the other hand, while generally accompanied by the

342

THE BIOLOGY OF THE FROG

protrusion of the eyeball through the contraction of the
levator bulbi muscle, is partly effected by means of an
independent muscle, the depressor membrance nictitantis. In
the closure of the eye the lower lid and nictitating membrane
arc pulled over all but a small part of the exposed surface.

Through the transparent cornea may be seen the colored
iris, in the center of which is an oval aperture, the pupil.
Just behind the pupil lies the transparent crystalline lens,
which, unlike the lens of higher animals, is of a nearly
spherical form, although somewhat more flattened on the
anterior face. It is made up of concentric layers like the
coats of an onion. On its outer surface is a single layer of
epithelial cells, the remaining portion consisting of trans-
parent fibers whose general direction is parallel with the
optical axis of the eye. The whole lens is surrounded by
a very delicate transparent capsule which is attached along
the outer margin to the ciliary body by means of radiating
fibers.

The cavity of the eye is divided by the lens and its sup-
porting fibers into an outer chamber which contains a trans-
parent watery fluid, the aqueous humor, and a larger inner
chamber filled by a more solid transparent substance known
as the vitreous humor. The vitreous humor is permeated
by numerous fibers which form a sort of supporting network
which contains free cellular elements and numerous blood
vessels. The circulation in the latter as seen by the
ophthalmoscope has been described by several observers and
with especial fullness by Hirschberg and Schleich.

The wall of the posterior chamber of the eye is formed
of three tunics, the sclerotic coat, which has been described
above, the choroid, and the retina. The choroid is a vas-
cular, pigmented layer lying next to the sclerotic. Next
within the choroid lies the retina which is continued ante-
riorly over the posterior surface of the iris as far as the

THE SENSE ORGANS

343

pupil. It is composed of two principal layers, an outer thin
layer of pigmented cells, and an inner much thicker layer
which forms the sensitive portion of the retina. Over the
ciliary body and posterior side of the iris the retina is much
thinner than it is elsewhere and does not contain nervous

Sclera*

Fig. 107. — Section through the optical axis of the eye of the frog.
Cam.anter, outer chamber containing the aqueous humor; Cam.
post, inner chamber containing the vitreous body; Corp.cil, ciliary
body; Corp.vitr, vitreous body; P.cartil, and P. fibrosa Sclera?, car-
tilaginous and fibrous layers of the sclerotic coat; Zon.cil, zonula
ciliaris. (From Gaupp, after G. Retzius.)

tissue. The optical portion of the retina which lines the
whole posterior region of the eye and extends as far forward
as the ciliary body, is a complex structure formed mainly of
nervous elements. It is the part of the eye which is
especially sensitive to light and upon which images of
external objects are thrown. In addition to the outer pig-

344

THE BIOLOGY OF THE FROG

mented epithelium it is composed, as in the higher verte-
brates, of nine layers, the latter forming the inner or thicker
stratum of this structure. These layers taken in order
from within outward are as follows: —

1. The inner limiting membrane, a thin supporting mem-
brane lying next to the vitreous humor. It is connected
with elongated cells (Miiller's fibers) which extend to the
outer limiting membrane and give off branches which form
a sort of supporting network for the nervous elements of the
retina.

2. A layer of nerve fibers, formed by the ramifications of
the optic nerve.

3. The inner ganglionic layer, formed by comparatively
large ganglion cells whose processes extend into the preced-
ing and following layers.

4. The inner reticular layer, formed by a network of
nerve fibers.

5. The inner nuclear layer, composed of several layers of
ganglion cells whose processes extend into both of the
adjacent layers.

6. The outer reticular layer, a narrowr layer formed by a
network of nerve fibers derived from the inner nuclear layer
and the layer of rods and cones.

7. The outer nuclear layer contains the nucleated portions
of the cells forming the rods and cones and a few isolated
ganglia.

8. The outer limiting membrane, a very thin supporting
membrane, perforated in numerous places by the cells of the
outer layer.

9. The layer of rods and cones.

The latter layer lies next to the pigmented epithelium of
the retina, and processes from the pigmented cells extend
between the rods and cones. It forms the receptive surface

THE SENSE ORGANS

345

d 1

b — I

1—1

of the retina, although the light must
pass through all of the other layers be-
fore reaching it. The rods, which are
larger and more numerous than the cones,
are very much elongated cells ; their outer
ends are cylindrical and show a cross
striation which marks their division into
transverse disks, into which they may
be separated by treatment with certain
reagents. Behind the cylindrical portion
comes a lens-shaped, refractive body with
a flat inner and a very convex outer sur-
face. The nucleus lies in the part of the
cell on the inner side of the outer limit-
ing membrane. The inner end of the cell
tapers into a fiber which extends into the
outer reticular layer. The rods are of
two kinds, the red and the green. The
former are more abundant and have a
much longer cylindrical segment, which
is permeated by a peculiar reddish pur-
ple pigment which is known as visual
purple. The green rods have a short,
cylindrical segment which is followed by
an elongated narrow neck; the cylindri-
cal portion contains a green substance
(visual green).

The cones have a conical outer seg-
ment which shows a cross striation like
the corresponding portion of the rods.
This is followed by a refractive lens-
shaped body, beyond which the cone becomes constricted.
The nucleus lies in an enlargement on the inner side of the
outer limiting membrane. The inner end of the cone, like

Fig. 108.— R ods
from the retina
of the frog : a
green rod on the
right, a red one
on the left. a,
outer segment ;
fo, refractive
body ; c, inner
segment ; d, in-
termediate disk ;
ey inner enlarge-
ment containing
the nucleus ; ?,
external limiting
membrane.
(From Gaupp,
after Greeff.)

346

THE BIOLOGY OF THE FROG

that of the rods, runs out into a fiber which enters the outer
reticular layer. The cones vary in size and form, as many
as three varieties being distinguished (M. Schultze, van
Genderen Stort). Some of these are movable, expanding in

Fig. 109. — Sections through the outer part of the frog's retina with
its pigmented epithelium. The figure on the left is taken from a
frog which had been exposed to daylight for five hours. The proc-
esses of the three outer pigment cells extend inward to the external
limiting membrane, and the pigment they contain has streamed in-
ward, surrounding the rods and cones nearly to their base. The
cones are strongly retracted, so that their outer segments lie near
the limiting membrane. The figure on the right is taken from a
frog that had been kept for forty-eight hours in darkness. The pig-
ment in the processes of the pigment cells is drawn back toward
the center of the cells, and the cones are extended outward between
the rods. The nuclei of the rod and cone cells are shown below
the limiting membrane. (From Gaupp, after Van Genderen Stort.)

the dark until they may reach the periphery, and contract-
ing under the influence of light. Ordinarily most of the
cones do not reach the outer surface of the retina.

The retina is marked by a thickening or papilla where
the optic nerve enters. Above this papilla there extends

THE SENSE ORGANS

347

in an antero-posterior direction a thickened portion of the
retina known as the area centralis, which probably marks
the region of most acute vision, like the yellow spot in the
eye of man. The cones are relatively more numerous in
this region than in other parts (1 c. : 2.4 r. instead of 1 c. : 3
or 4 r., as elsewhere), and the rods are of relatively small
size.

The action of light upon the retina results in the produc-
tion of several visible changes. In addition to the contrac-

Fig. 110. — Inner surface of the inner half of the eye of Rana escu-
lenta, showing the area centralis as a light-colored band above the
papilla optica where the optic nerve enters. (After Gaupp. )

tion of the cones which has been mentioned above, there is
a bleaching of the pigment in the outer ends of the rods;
this takes place more rapidly in the visual purple than in
the green pigment. After a short exposure in the dark both
pigments are restored, even in the extirpated eye. It is
probable that the chemical changes produced in the retinal
pigments through the influence of light, play some part in
the stimulation of the sensitive cells. Light also affects the
cells of the outer pigmented layer of the retina, causing the

Papilla
optica

Area .
centralis

34S

THE BIOLOGY OF THE FROG

pigment to stream out into the processes which extend
between the rods and cones. With intense illumination the
pigment may spread as far as the inner limiting membrane,
in from ten to fifteen minutes, which is about the time which
it takes the visual purple to disappear under the same
conditions (Boll, Angellucci).

The amount of light falling upon the retina is regulated
by the iris which acts as a sort of screen or curtain, ad-
mitting the rays only through its central aperture, the pupil.
The iris lies directly in front of the lens. Its outer margin
is joined to the ciliary body behind and the juncture of the
sclerotic and cornea in front. It contains numerous black
pigment cells and several golden ones which give it its
bright yellow spots. Near the margin of the pupil there is a
ring of smooth muscle cells forming a sphincter pupillce
which causes the pupil to contract. Strong light causes a
contraction of the pupil and consequently a diminution
of the amount of light that falls on the retina, thus checking
the excessive stimulation of the sensitive part of the eye.
This effect is produced, at least in part, through the direct
action of the light upon the muscle cells of the iris, since it
occurs in eyes that have been removed from the body
(Steinach, Guth).

The optical arrangements of the eye of the frog are such
as to throw images of objects upon the retina. The sensitive
surface of this layer is thereby stimulated and impulses
carried through the optic nerve to the brain, there giving
rise to sensations of sight. The eye of the frog has no
power of accommodation for viewing both near and distant
objects such as our own eyes have. The lens cannot either
be changed in form or brought nearer to or farther from
the retina, so that only objects are in focus which are at a
particular distance from the eye. Images of objects at other
distances are not clearly outlined, and there consequently

THE SENSE ORGANS

349

results imperfect vision. In the air the frog is myopic, or
near-sighted (Plateau, Hirschberg, Beer) ; i.e. it can see
clearly only objects which are near at hand. In the water,
on the contrary, it is hypermetropic, or far-sighted. The
myopia of the frog is evidently an advantage to the animal
in securing food, as it is important to see clearly objects
which are sufficiently near to be snapped at. It is evident
that the frog responds to the perception of movement much
more than of form, and for that purpose images which are
somewhat indefinite and blurred will suffice. Large moving
objects mean enemies, and consequently the necessity of
getting out of the way, even if there be no clear perception
of the outline or the distance of the portentous creature
that is coming. —

The frog can be said to possess the power of binocular
vision only in a very small degree, since, owing to the fact
that the eyes are so laterally situated, their fields of vision
overlap anteriorly only to a very slight extent.

The eyes may be moved in various directions by means
of special muscles. Four of these are the straight or recti
muscles, — a superior, an inferior, an anterior or median,
and a posterior or lateral rectus muscle, which roll the eye-
ball upward, downward, forward, and backward respec-
tively. There are two oblique muscles, the superior oblique,
which rotates the eye around its optic axis so that its upper
margin turns forward, and an inferior oblique, which pro-
duces the reverse movement. The eye is pulled into the
orbit by means of the strong retractor bulbi muscle, which
arises from the angle between the lateral and median por-
tions of the parasphenoid bone and is broadly inserted upon
the posterior and median sides of the eyeball. The ever
are pushed outward by the contraction of the levator bulb
a broad sheet of muscle running obliquely across the ventr
side of the orbit.

350

THE BIOLOGY OF THE FROG

The Ear. — In the ear of the frog there are two sets of
organs which are fundamentally different in origin
and in function. The one constitutes the inner ear
which forms the sensory apparatus; the other, the mid-
dle car, being composed of accessory structures for the
transmission of sound waves to the sensitive part of the
organs.

The inner ear is composed of the membranous labyrinth,
which lies within the auditory capsule, which is formed by
the prootic and exoccipital bones of the skull. The laby-
rinth is a complicated sac formed originally by invagination
of the ectoderm of the surface of the body from which it is
subsequently constricted off. It is divisible into an upper
portion, the utriculus, and a lower, smaller part, the saccu-
lus. The former is an oblong sac lying nearly horizontal.
It gives rise to the three semicircular canals, which lie in
planes approximately at right angles to each other. These
canals are membranous tubes embedded within the cartilage
of the auditory capsule and communicating with the utricu-
lus at each end. There is an anterior canal which lies in a
vertical plane, a lateral canal which lies in a nearly horizon-
tal plane on the outer side of the utriculus, and a posterior
canal which extends transversely nearly at right angles
to the two others. At one end each of the canals is fur-
nished with an enlargement, or ampulla, which contains
an important sensory apparatus.

The sacculus, or lower division of the labyrinth, is an
irregular oval sac which projects downward and forward
from the utriculus, with which its interior is connected
through an aperture. There are four small outpocketings
from the base of the sacculus, and from the median side
there is given off a narrow tube, the ductus endolymphaticus,
which extends dorsally where it penetrates the cranial
cavity and ends in a large sac, the saccus endolymphaticus.

THE SENSE ORGANS

351

which is filled with a milky white fluid containing numerous
white crystals of carbonate of lime. The utriculus, the sac-
culus, and the larger outpocketings of the latter contain
masses of similar crystals which are commonly called the
otoliths.

Fig. 111. — Right membranous labyrinth of the frog from the inner
side, aa, ae, ap, anterior, external, and posterior ampulla ; au, aper-
ture of utriculus ; ca, ce, cp, anterior, external, and posterior semi-
circular canals ; cus, utriculosaccular canal ; de, ductus endolyim
phaticus ; I, lagena ; mu, ms, mn, macula acustica of the recessus
utriculi, sacculus, and pars neglecta respectively ; pb, pars basilaris ;
pi, ppb, papilla acustica lagense and basilaris ; raa, rap, rs, rn, rl, rbt
branches of auditory nerve to anterior ampulla, posterior ampulla,
sacculus, macula neglecta, lagena, and pars basilaris respectively ;
s, sacculus ; ss, sp, sinus utriculi superior and posterior ; u, utricu-
lus. (After G. Retzius.)

The different parts of the ear are all supplied by the
branches of the auditory nerve. In the regions of the nerve
endings the epithelium contains sensory cells with hairlike
processes at their outer ends. These are especially abundant
and well developed in the walls of the ampullae. The
whole labyrinth is filled with a fluid called the endolymph,
and it is surrounded by another fluid, the perilymph, which
fills the space between the labyrinth and the wall of the
auditory capsule. On the outer side the wall of the auditory

352

THE BIOLOGY OF THE FROG

capsule is perforated by an aperture, the fenestra ovalis,
which is closed with a small plug of cartilage.

The accessory auditory apparatus consists of the struc-
tures which in higher forms constitute the middle ear, there
being nothing in the frog which corresponds to the external
ear of mammals. The cavity of the middle ear communi-
cates through the Eustachian tube with the mouth cavity
near the angle of the jaw. It is closed externally by the
tympanic membrane, which is readily seen at the side of
the head behind the eye. This membrane is nearly circular
in form and is attached by its outer margin to a ring of
cartilage, the annulus tympanicus. Near its middle it gives
attachment to the outer head of a rod, the columella, which
extends across the cavity of the middle ear, and joins the
small cartilage lying over the fenestra ovalis. It is through
this rod that the vibrations which are produced by the sound
waves impinging upon the tympanic membrane are carried
to the inner ear. Here they set the contents of the labyrinth
into vibration and thus stimulate the sensory end organs of
the auditory nerve. The nervous impulses set up in this
way are conveyed to the brain, where they give rise to the
sensation of sound.

The ear is not only concerned in the perception of sound,
but it has a very important function in the relation to the
maintenance of equilibrium. Removal of the labyrinth on
both sides of the body is followed by loss of ability to main-
tain an upright position. Frogs upon which this operation
is performed no longer balance themselves on a tilted board
so readily as a normal frog will, and when thrown upon the
back, they lie there for a long time, and finally right
themselves, if at all, only with the greatest difficulty
(Goltz). They are able to swim through the water in a
tolerably straight course, but very frequently with the
ventral side up, which a normal frog does not do (Breuer).

THE SENSE ORGANS

353

When stimulated, they jump about irregularly and move
the fore and hind legs in a disconnected and uncoordinated
manner (Girard, Ewald). According to Girard and Schrader
there is a complete loss of compensatory motions, but this
is disputed by Steiner. If the labyrinth is destroyed only
on one side, the frog takes an asymmetrical attitude. If
the operation is performed on the left side, the head is
slightly inclined to the left, the right fore limb is held
straighter than the left (Girard), the body is bent toward
the operated side, and the right hind foot is more extended
than the left and seldom drawn up to the body (Ewald).
The same attitudes are assumed, according to Ewald, for a
year after the operation.

After injury to one of the horizontal canals, the frog
tends to move in a circle; injury to the vertical canals causes
abnormal movements in a vertical direction (Hensen).

If the otoliths are removed from both of the auditory
organs, the behavior of the frog, according to Ach, differs
from that of a normal individual in several particulars. If
the brow of the injured animal is rubbed with the finger,
the creature will close its eyes, bow down its head, elevate
its back, and remain in this attitude, with its muscles in a
state of tonic contraction often for half an hour. A normal
frog of the species experimented wTith (R. esculenta) does
not show this response except in a slight degree, and then
only after it has become fatigued. Frogs devoid of otoliths
were found to utter the so-called "pain cry" upon slight
provocation, whereas this response is rarely elicited from
normal specimens. Locomotion may take place in a normal
manner, but the muscular tonus seems to be low, and the
animal quickly becomes fatigued. The compensatory mo-
tions are unaffected.

Frogs have a well-developed sense of hearing, although
they do not usually manifest it by any clearly evident sign.

354

THE BIOLOGY OF THE FROG

In croaking, one frog frequently responds to the croak of
another individual, so that one often hears a pair answering
rack other by regularly alternating notes. If, after a con-
cert of frogs has been silenced by some one who intrudes too
near their haunts, one individual ventures to resume its
croaking, it is speedily followed by one after another of its
comrades. Landois tells of a tree frog which he kept in
captivity which would give an answering cry every time
that he would imitate its note. Yerkes observed that when
he caused a frog to croak by rubbing its sides, the other
frogs under observation occasionally gave signs of attention
by straightening up and raising the head as if listening.
The same observer noticed, when carrying on experiments
with frogs in a labyrinth, that the animals often gave signs
of hearing the sound made when other individuals jumped
into the water. They would "straighten up and hold the
listening or attentive attitude for some seconds. As the
animals could not see one another, there is good evidence
of their ability to hear the splash made by a frog when it
strikes the water." This, according to Yerkes, explains
the fact that "it is never possible to get near to any frogs
in the same region after one has jumped in." The splash
sound is significant to them and puts them on their guard.

On the other hand, many other sounds, varying greatly
in loudness and pitch, do not elicit any marked response.
"One may approach to within a few feet of a green frog or
a bullfrog and make all sorts of noises without causing it to
give any signs of uneasiness. Just as soon, however, as a
quick movement is made by the observer, the animal jumps.
. . . Sounds like the splash of a plunging frog, or the croak
or pain scream of another member of the same species,
serve as warnings, but the animals do not jump into the
water until they see some sign of an unusual or dangerous
object."

THE SENSE ORGANS

355

It must not be inferred that frogs do not hear a great
Variety of sounds simply because they give manifest signs
of attending to only a few sounds in which they have some
particular interest. Yerkes found that frogs which give no
other signs of perceiving sound, show a difference between
the rates of their respiratory movements before and after
the sound is made. The sound of a tuning fork, falling
water, a shrill whistle, the ringing of a bell, and other noises
were employed. Some of these produced little or no effect.
The shrill whistle and the ringing of a bell caused a decrease
in the rate of respiration, owing perhaps to fear, while the
sound of falling water caused the rate of respiratory move-
ments slightly to increase. The green frog (Rana clamitans)
was found to respond to sounds varying in pitch between
fifty and ten thousand vibrations per second.

The reaction time of frogs to visual stimuli is also in-
fluenced by sound. Yerkes found that frogs which were
placed in a glass aquarium so surrounded that the move-
ments of the observer could not be detected, would jump
vigorously at a small red card which was moved near
them. If a tuning fork was sounded just before the card was
presented, it "became evident that the sound put the frog
on the alert, and, when the object came into view, it jumped
at it more quickly and a greater number of times than when
the visual stimulus was given without the auditory. . . .
When the red card was shown, it was often several seconds
before the frog would notice it and attempt to get it, but
when the sound also was given, the animal usually noticed
and jumped toward the moving card almost immediately. "
It is probable that this habit of getting into readiness for
a spring upon hearing a sound near by is of value to the
frog, since insects and other creatures that serve as food
often manifest their presence by some sort of noise before
they come into the frog's field of vision. If the frog pre-

356

THE BIOLOGY OF THE FROG

pares himself he is more apt to seize his prey when it
appears.

REFERENCES

Ach, N. Ueber die Otolithenfunction und den Labyrinthtonus.
Arch. ges. Phys., Bd. 86, 1901.

Beer, T. Die Accommodation des Auges bei den Amphibien. Arch,
ges. Phys., Bd. 73, 1898.

Girard, H. Recherches sur la fonction des canaux semicirculaires
de l'oreille interne chez la grenouille. Arch. Phys. Norm, et Path.
(5), T. 4, 1892.

Goltz, F. Ueber die physiologische Bedeutung der Bogengange des
Ohrlabyrinths. Arch. ges. Phys., Bd. 2, 1870. Beitrage zur Lehre von
den Funktionen der Nervencentren des Frosches. Berlin, 1868.

Hirschberg, J. Zur Vergleichenden Ophthalmoskopie. Arch. Anat
u. Phys., phys. Abth., 1882. Zur Dioptrik und Ophthalmoskopie der
Fisch- und Amphibienaugen. I.e., 1882.

Landois, H. Konnen Frosche horen? 25 Jahresber, westfal. Prov.
Ver., 1897.

Laudenbach. Zur Otolithenfrage. Arch. ges. Phys., Bd. 77, 1899.

Lyon, E. P. A Contribution to the Comparative Physiology of
Compensatory Motions. Am. Jour. Phys., Vol. 3, 1899.

Morgan, A. H. The Temperature Senses of the Frog's Skin.
Jour. Exp. Zool., Vol. 35, 83, 1922.

Plateau, E. Sur la vision des poissons et des amphibies. Ann.
Nat. Sci. (5), T. 7, 1867.

Retzius, G. Das Gehororgan der Wirbelthiere, 1. Stockholm, 1881.

Schrader, M. Zur Physiologie des Froschgehirns. Arch, ges
Phys., Bd. 41, 1887.

Steiner, J. Die Functionen des Centralnervensystems und ihre
Phylogenese. Leipzig, 1888.

Yerkes, R. M. Inhibition and Reinforcement of Reactions in the
Frog, Rana clamitans. Jour. Comp. Neur. and Psych., Vol. 14, 1904.
The Instincts, Habits, and Reactions of the Frog. Monogr. Suppl.
Psych. Rev., Vol. 4, 1903. The Sense of Hearing in Frogs. Jour.
Comp. Neur. and Psych., Vol. 15, 1905.

CHAPTER XVIII

INSTINCTS AND TROPISMS AS RELATED TO REFLEX

ACTION

We have already treated of some of the reflex actions of
the frog and have shown that they exhibit a purposive
character, often in a very striking degree. They are far
from being mere random responses to stimuli, notwithstand-
ing the fact that they may take place independently of
intelligent control. It is generally recognized that there
is a close connection between reflex actions and instincts,
it being, in fact, very difficult to draw the line between them.
The behavior of an animal is usually called instinctive when
it takes place without previous instruction and with no
consciousness of the end to which it is directed. A digger
wasp, for instance, makes a nest of a particular type in
the ground, catches only certain species of insects, which it
stings in the ventral ganglia, so as to produce paralysis
without causing death, and, after depositing an egg upon its
prey, buries it in the nest as provision for its future off-
spring. The wasp is utterly unaware of the significance of
its complex behavior; of the wriggling grub which it has
labored so industriously to provision it knows nothing and
cares less; it is impelled by blind impulses to a particular
line of activity, which, although of no service to the in-
dividual wasp, is essential to the continuance of the race.
It is guided neither by previous experience nor by imitation,
and has no basis for drawing any conclusion regarding the
utility of its conduct, even were it capable of so doing.

357

358 THE BIOLOGY OF THE FROG

Such behavior affords a typical illustration of instinct; and
throughout the animal kingdom it is instinct which is the
dominant element in conduct.

The behavior of the frog is almost entirely made up of
instinctive actions. Nature has equipped this animal with
the means of getting through the world without relying, to
any great extent, upon the lessons of experience. The frog
has intelligence of a rudimentary sort, to be sure, but it
plays a very subordinate role in directing the creature's
activities. It is truly a marvelous thing that an animal
should be endowed with the power of successfully adapting
its conduct to a complex environment without any percep-
tion of the consequences of its actions. How can the exist-
ence of such a power be explained, or brought into relation
with our knowledge of the other features of the animal's
life?

It was formerly customary to regard instinct as a property
sui generis, something having no necessary affiliation with
the other functions of the organisms, a sort of power with
which animals are mysteriously endowed for their guidance.

It is but another illustration of the effect of increasing
knowledge in bringing different fields of biological inquiry
into closer and more organic connection that the instinctive
behavior of animals is now shown to be intimately con-
nected with their structure and physiological activities.
Instinct is but a phase of the general life process, exhibiting
the same purposiveness that is shown in the activities of
the heart or alimentary canal. All parts of the body are
continually responding to stimuli in ways that are beneficial
to' the organism. When the stomach pours out its secretion
and begins its peristaltic movements upon the receipt of
food and allows the material when digested to escape
through the pylorus, it is performing actions which we do
not commonly call instinctive, but which are as well adapted

INSTINCTS AND TROPISMS

359

to the end achieved as the diving of a frog upon the ap-
proach of an enemy, or its burrowing into the mud in cold
weather. All of these actions are adaptive responses to
stimuli. Those of the stomach we call reflex acts, while the
diving and burrowing into the mud are usually called in-
stincts. The chief distinction between the two is that the
one involves the action merely of a part, while in the other
there is a response by the organism as a whole. There are
so many intermediate types of reaction, however, that it is
no easy matter to decide how some of them should be
classed. If a frog withdraws its foot when its toe is stimu-
lated, we call the act reflex, but how shall we designate the
act of bringing the foot forward to wipe away a drop of acid
from the side of the body? As we have seen, the latter act
may be performed by a frog whose spinal cord is cut across
near the brain; if, therefore, wTe call the action reflex, what
shall we say of the struggles of a frog when, after being
picked up in the fingers, it uses both hind legs to push
against the hand, and at the same time inflates the lungs
with air, causing the body to swell? These struggles to
escape will take place in a frog which has lost the greater
part of its brain, but nevertheless, they would, I think,
generally be regarded as instinctive actions. The use of the
hind limbs and the swelling of the body may be regarded as
two complex reflexes excited by the same cause. The frog
is so organized as to respond to seizure by two inethods
which cooperate to effect its escape, both of which are re-
flexly brought into play.

Many of the more complex instincts of the frog may be
resolved into a series of reflex acts. We have seen that the
tendency of the male frog to clasp the female during the
breeding season depends upon the reflex irritability of the
brachial region of the spinal cord. It is a tendency in-
dependent of the higher nervous centers, and it is brought

360

THE BIOLOGY OF THE FROG

about by certain changes that take place in the organism
during the spring, probably as an indirect result of the
ripening of the sexual products.

Most of the things that the frog does fall into a compara-
tively few categories. Its actions do not show an indefinite
diversity like those of an ape or a human being. It uses its
hind limbs, for instance, in leaping and swimming, and
occasionally for pushing against some object or removing
an irritating substance from the skin. The two former
movements are much alike and the two latter both consist
in bringing the legs forward to the point of stimulation. The
legs of a frog are not moved about in a great variety of
ways for different purposes like the arms of a man. They
have a few simple tricks which they are admirably adapted
to perform, but beyond these the range of their powers is
very limited. The same is true of the fore legs, the head,
and the body as a whole. Movements of the fore and hind
limbs may be combined in various ways, as in leaping, div-
ing, crawling, burrowing, righting movements, but the num-
ber of combined actions employed is much less than in
higher forms. The frog is a sort of mechanism beautifully
adapted to the performance of a number of actions, but its
repertoire is not at all extensive, and it has little power of
improvising new roles. Its modes of response are dependent
upon the way it is organized, for as a machine is constructed
so will it work. A frog is as incapable of leading a life like
that of a cat as a machine made for grinding corn is in-
capable of weaving cloth or printing books. Its kind of life
is laid out for it by the forces that have shaped its bodily
structure.

Reactions to Light. — In addition to responding to vari-
ous objects of sight, frogs react to light in a very peculiar
manner by placing their bodies so that they face the region
of strongest illumination; frequently also they move toward

INSTINCTS AND TROPISMS

361

it. Animals which orient themselves to light in this manner
are said to be phototactic or heliotropic. Those which move
toward the light are called positive; those which move away
from it negative. This orientation is generally regarded as
brought about in a reflex manner through the unequal
stimulation of the two sides of the body, either through the
eyes or other parts of the organism. When the body is
oblique to the rays, it receives more stimulus from the
light on one side than the other, and if the light has any
directive effect upon the animal's movements, it will natu-
rally cause the body to turn until equally illuminated on
the two sides; then, as both sides receive the same amount
of stimulation, the animal tends to go either toward or
away from the light in a straight line.

Graber, who experimented with Rana esculenta, came to
the conclusion that this species is negatively phototactic.
The specimens were placed in a box in wThich were two
compartments, one of which was darkened while the other
was exposed to diffuse daylight. The animals showed a
tendency to collect in the darker of the two compartments.
If the frogs were given a choice between red and blue, they
collected under the red light.

The responses of Rana pipiens and R. clamitans to light
have been studied by Miss Torelle, who arrived at results
quite different from those of Graber. Frogs placed in a
box one half of which was exposed to diffuse light, while
the other half was shaded, moved into the light end of the
box and oriented the body so as to face the incoming rays.
When one half of the box was exposed to direct sunlight,
the frogs first moved into the illuminated area, and then,
after a short time, retreated into the shade, where they sat
with their heads pointing toward the light. The same resull
occurred when the heat rays were eliminated by passing
the light through a vessel of water before it entered the

362

THE BIOLOGY OF THE FROG

box, showing that it was not the heat alone that caused
the frogs to retreat into the shade. If light be admitted
from below, which may be done by making the floor of the
box of glass, the frogs leap into the lighted area as before.
If the whole lower side of the box be exposed, the animal
takes a normal resting position, but if a half or two thirds
of it be covered, the frog moves toward the light and the
body assumes a greater angle to the horizontal, the angle
increasing, the smaller the area through which the light
enters. When light is thrown upon the frog from above,
the anterior part of the body becomes raised. Miss Torelle
found that when a frog was placed in a tall glass cylinder
the bottom and sides of which were covered with black
cloth, "the body was raised so that the fore legs were as
nearly as possible at right angles to the horizontal bottom
of the jar. This made the inclination of the body 60° or
over. Frequently the frog assumed an almost erect position,
by means of placing the fore feet against the side of
the jar."

As these and other experiments show, the frog has a
strong tendency to place its body so as to face the light;
^et notwithstanding its marked orienting response, it mani-
fests a strong proclivity to seek the shade. Frogs placed
out of doors, near the shadows of trees or buildings, soon
hop into the shade and remain there even if they have to
travel at right angles to the rays of light. Miss Torelle
tried the experiment of placing dark objects in the vicinity
of the frog to find if the animal showed any tendency to
approach them. "The side of a large wooden box wras
covered with black cloth, and the frog placed near the black
perpendicular surface. It hopped close to this, remained a
couple of minutes, then moved close to the wall of the
gray-colored building, where it remained at rest in the angle
formed by the wall and the ground. When placed near the

INSTINCTS AND TROPISMS

363

"ancovered box (pine) on the side in full sunlight, there was
no movement toward it. When the box was raised on one
edge and propped, so that the other edge was about four
inches from the ground, the frog moved toward the shadow
thus formed, crept well under the box, placed its body be-
tween the floor and the ground, where it remained with its
head directed outward. A black cloth was fastened close
to the ground in the center of a sun-illuminated area, and a
frog placed near it moved on to it, crept along the edge
as if seeking cover, then hopped off. A second frog also
hopped on to the cloth, but almost immediately moved off.
Apparently a dark surface, brightly illuminated, does not
produce the effect of a shadow or of diffuse light." It may
be, however, that frogs are attracted to such surfaces just
as they are to shadows, but finding different conditions of
stimulation when they get there they do not remain.

It is clear that the frog manifests two quite different
responses in its behavior toward light. The orienting re-
sponse, in which the animal puts itself in line with the
direction of the rays, affords a good illustration of photo- /C
taxis. The proclivity to seek and rest in the shade is more
nearly akin to what is commonly called photopathy. Under
ordinary conditions the frog may be considered as positively
phototactic, but negatively photopathic. Many animals
collect in the shade, not because they are negatively photo-
tactic, but because when they happen to reach the shade in
the course of their moving about, they come to rest there.
The collection of frogs in shady spots may be partly ex-
plained in this way, but there appears to be also a percep-
tion of shaded regions at a distance and a tendency to make
for them, which is not merely a matter of photopathy as
that term is usually employed. It is possible that the
latter peculiarity is not a primary instinctive response, but
a habit acquired by experience. It is known that the frog is

304

THE BIOLOGY OF THE FROG

capable of forming simple associations, and it may learn in
the course of its experiences with light and shade to connect
moving toward the latter with a sense of comfort or respite
from the disagreeable effects of strong light.

The reactions of the frog to light are influenced by tem-
perature to a marked degree. At 25° C. the positive re-
sponse becomes considerably accelerated, the frog moving
more quickly and more directly toward the light end of
the box. Above 30° C. the movements become irregular,
owing to the predominant effect of heat. When the tem-
perature is lowered, the positive reaction becomes less de-
cided, and according to Miss Torelle, when a temperature
of 8° C. is reached, the animal becomes negatively photo-
tactic both in air and in water. The evidence cited in favor
of this conclusion is, however, not convincing, inasmuch as
other reactions, such as the tendency to dive downward and
to crawl under objects, are evoked when the temperature is
lowered to this point. These relations were brought about
both when light came in from above the aquarium, and
when the top and the upper two thirds of the aquarium were
covered by an opaque cloth.

As in most animals thus far investigated, it is the blue
and violet rays that are the most influential in evoking the
phototactic response; the effectiveness of the other colors
of the spectrum diminishes in order from blue to red.
Miss Torelle found that if frogs are placed in a box illumi-
nated through one end with blue light and through the
other with red, they soon gather at the blue end. If they
have the choice between yellow and green, they go toward
the green; in general it may be said that where they are able
to go toward one of two colors of equal intensity they move
to the color lying nearest the violet end of the spectrum.
The same conclusions were drawn by Pearse who experi-
mented upon Rana palustris and Bufo.

INSTINCTS AND TROPISMS

365

The eyes would naturally be regarded as the organs
through which the phototactic response is effected, and in
fact they play an important part in the process, but, as
has been shown by Parker and by Pearse, orientation may
be brought about merely by the photic stimulation of the
skin. In order to determine the role played by the eyes in
phototaxis Dr. Parker covered a frog with the skin of a
somewhat larger individual leaving only the eyes, feet, and
snout exposed. Four specimens thus covered were tested,
and it was found that they "turned toward the light and
jumped toward it much as normal frogs do." When a
normal frog wras introduced for comparison, it was found
that in most instances it "responded more quickly than the
covered one, but the difference was not so great that it
might not have been due to the purely mechanical inter-
ference of the covering skin." When the eyes of the frog
were covered as well as the skin, there was no longer any
response to light, thus showing that it was not the light,
which may have penetrated the covering of dead skin, that
effected the orientation.

That phototaxis may be produced through the skin alone
as well as through the eyes alone was shown by Parker in
the following experiment. Eleven frogs were taken, and
"by a single vertical, transverse cut just behind the eyes,
these organs and the cerebral hemispheres wTere removed
with the snout of the animal. It is well known that frogs in
this condition may with a little care be kept alive for many
weeks, and that the chief difference between these and
normal frogs is the great reduction in spontaneous move-
ments shown by the former." Of these frogs nine showed
an unmistakable phototactic response, turning by the
shortest course to face the light, where they remained "for
a considerable period, usually terminated by a jump toward
the light." This reaction occurs when the exposed por-

366

THE BIOLOGY OF THE FROG

tions of the brain arc covered by an opaque object or
a shadow cast over the head. The skin, therefore, is in
all probability the organ which is sensitive to stimula-
tion by light. Pearse found that eyeless toads would
also orient toward light, but they showed no discrimina-
tion between lights of different colors. A sensitivity to
light is shown by the blinded urodeles Triton, Triturus
and Cryptobranchus, as well as by the naturally blind
Proteus.

Thigmotaxis.— Many animals tend to remain in situa-
tions which afford contact stimuli over a considerable sur-
face of the body. Such forms are called positively thigmo-
tactic; those which avoid contact are called negatively
thigmctactic. The tendency so common among insects and
worms to crawl under stones and lie within crevices, is to
a great extent the manifestation of a thigmotactic response,
although, in some cases, it may be due in part to a negative
phototaxis or photopathy.

Frogs often show a propensity to crawl under stones or
to get between objects, where they remain quiet. The same
tendency seems to be somewhat more marked in toads. It
is apparently stronger when the temperature is lowered.
Miss Torelle in experimenting with frogs placed in a jar
of cold water found that "when a rock was lowered into the
jar in such a way that a small space was formed between it
and the wall of the jar, the frog crawled into this space and
remained there. When a space was formod between the
bottom of the jar and the rock, it crawled into that. This
was tested several times, and was also observed when the
temperature of the water in the aquarium in which the frogs
wrere kept was lowered to 10° C. and below. When this
was done, all the frogs responded, either by flattening their
bodies against the stone floor, or by creeping under the rocks
usually kept there. It therefore seems that the frog is

INSTINCTS AND TROPISMS

367

stereotropic [thigmotactic] in temperatures between 10° C.
and 4° C."

REFERENCES

Graber, V. Grundlinien zur Erforschung des Helligkeits- und Far-
bensinnes der Thiere. Prague and Leipzig, 1884.

Loeb, J. Comparative Physiology of the Brain, and Comparative
Psychology. New York, 1900.

Parker, G. H. The Skin and Eyes as Receptive Organs in the Re-
actions of the Frog to Light. Am. Jour. Phys., Vol. 10, 1903.

Pearse, A. S. The Reactions of Amphnbians to Light. Proc. Am.
Acad. Arts Sci., Vol. 45, 161, 1910.

ToreUe, E: The Response of the Frog to Light. Am. Jour. Phys.,
Vol. 9, 1903.

Yerkes, R. M. The Instincts, Habits, and Reactions of the Frog.
Monogr. Suppl. Psychol. Rev., Vol. 4, 1903.

CHAPTER XIX

THE INTELLIGENCE OF THE FROG

The frog is admirably endowed by nature with a number
of instincts which enable it to cope successfully with most
of the situations that present themselves in the ordinary
course of its life. Its behavior is, to a great extent, stereo-
typed, the result of specific adaptive responses which are
dependent upon its inherited organization. Nevertheless it
shows at least the beginnings of intelligence. It is capable
of learning simple things and of guiding its conduct by the
light of its previous experience. By thus increasing the
range and delicacy of its responses it is able to perfect its
adjustment to its conditions of existence.

Intelligence at its first appearance is very closely asso-
ciated with instinct and rises out of the latter by almost
insensible gradations. A step of fundamental importance
in its development is taken in the acquisition of the power
of forming associations between different experiences.
These associations may be at first of a very simple nature
and formed only after much repetition, but they mark the
important transition from instinctive to intelligent behavior.

Abbott, who has devoted some attention to the intelli-
gence of batrachians, concludes that frogs are exceedingly
stupid. "Hoping," he says, "to find that in the pursuit of
prey, which is principally insects, frogs would display some
intelligence, I tried several experiments to test their in-
genuity, but it was of no avail. Unless the food could be
easily reached by making the exertion of a single leap, the

36*

THE INTELLIGENCE OF THE FROG 369

frogs would go hungry. Subsequently I placed a large fly
upon a piece of thin mica, and surrounded it with a circle
of fine needles, piercing the plate. The fly thus protected
could only be seized by the frog suffering a severe pricking
of the jaws. This I found a frog would suffer indefinitely
in its attempts to secure the fly. In one instance the frog,
which had been fasting for seventy-two hours, continued to
snap at the needle-protected fly until it had entirely skinned
its upper jaw. I concluded from this that the wits of a frog
were too limited to be demonstrated."

Knauer finds that frogs persist for a long time in snapping
at worms from which they are separated by a glass partition
without becoming aware of the futility of their efforts. They
will keep up their endeavors at intervals all day ; how much
more time would be required to convince them that their
efforts are vain is uncertain.

Wood frogs, according to Abbott, exhibit much more
ingenuity in the pursuit of prey than the ordinary aquatic
species. "I have frequently noticed, " says Abbott, "when
I placed flies in the case, that the wood frog singled out
one and approached it in a very stealthy manner, squatting
closely to the moss, hiding behind ferns and dragging itself
along, until it had reached a position suitable for making
a successful leap. If the fly moved, the frog would alter
its position accordingly, and follow up the chase with great
patience and unquestionable skill. At times it would hap-
pen that some one of the smaller batrachians kept in the
case snapped at the coveted prize, when the disgust of the
wood frog would be plainly shown by its manner, but such
an occurrence never led to a quarrel."

Yerkes has studied the power of forming associations in
frogs, and has come to the conclusion that their learning is
slow, but that habits once formed are hard to change. The
frogs experimented with were placed in a labyrinth (Fig.

370

THE BIOLOGY OF THE FROG

112) formed by a box 72 cm. long, 28 cm. wide, and 28 cm.
deep. The frog enters the box through a small opening at
one end .4. At the other end of the box an opening at one
side leads to a tank of water, into which the frog is naturally
desirous of getting. Near A the box is divided so that a
choice of two paths is given. If the frog passes to the right,
its course is blocked off by the partition P. Near the other
end of the box two alternatives are also presented, in that
the frog can go either to the left, where its course is cut off
by the glass plate G, or to the right path, which leads to the
water. The sides of the box were fitted so that colored card-

Fig. 112. — Labyrinth used in studying the formation of habits by the
frog. From A the frog enters at E. G, glass plate ; partition ,
M, red surface; W, white surface; T, tank. (After Yerkes.)

board could be placed in the positions marked W and R, and
the color of the sides of the labyrinth could thus be varied at
will. The partition P was also movable and could be shifted
to the other side of the box so as to reverse the closed and
free passages. A frog entering the box at A usually does
not go at first by the most direct route to the water, but
after several trials it comes to avoid the closed passages and
travels to the water by the shortest route. The frog learns
this path very slowly, as it was found to take from fifty to
one hundred trials before it would take the direct route
without being liable to make a mistake. Associations once
formed, however, were found to persist for over a month,
[f, after the frog had learned to go to the water by the

THE INTELLIGENCE OF THE FROG

371

nearest path, the colored cardboards lining the sides of the
box were exchanged, so that the side that was red before
was made white, the animal would become confused and
frequently take the wrong route. Yerkes comes to the con-
clusion that the frog is guided by color vision as well as by
"complex sensations of turning."

Fear exercises a strong inhibiting effect on the formation

of associations. The frogs experimented with by Yerkes,
although they "gave little evidence of fear by movements,
after being kept in the laboratory for a few weeks, they
were really very timid, and the presence of any strange
object influenced all their reactions. Quiescence, it is to be
remembered, is as frequently a sign of fear as movement,
and one is never safe in saying that the frog is not disturbed
just because it does not jump. The influence of the experi-
menter's presence in the room with the frogs which were
being tried in the labyrinth became apparent when the
animals were tried in a room by themselves. They escaped
much more quickly when alone." If after the frogs had
learned to escape by the nearest route from the labyrinth
into the water they were frightened by being poked about
with a stick, their movements became confused, and they
would as frequently as not take the wrong path.

The experiments of Schaeffer on the feeding reactions
of frogs showed that learning is not necessarily slow. Frogs
were given hairy caterpillars which were seized but quickly
rejected. In some cases even two trials were sufficient to
prevent the frog from snapping at hairy caterpillars although
they would continue to seize other kinds of prey. The
frogs also learned, after a very few trials, to avoid seizing
earthworms which had been treated with a disagreeable
chemical. The habit of avoiding irritating caterpillars was
found to persist for ten days.

It is not surprising that frogs learn to discriminate be-

THE BIOLOGY OF THE FROG

tween different kinds of food more readily than they learn
to take the right path to water. They are continually mak-
ing trials of various moving objects and they have probably
become endowed with an aptitude for associating such
objects with avoiding reactions. An irritating stimulus pro-
duced by a disagreeable caterpillar is apt to make more of
an impression than a simple error in the choice of a path.
Under natural conditions the frog has little experience in
learning routes, but it cannot fail to acquire considerable
experience in the proper choice of food.

Toads are generally credited with greater intelligence
than frogs. Their space perceptions are quite well de-
veloped, since they find their way back to their regular
habitations after making journeys of considerable distance.
They may be readily tamed so that they come toward one
and eat out of the hand, and allow themselves to be stroked
without showing the usual instinctive reaction of bowing
down the head and swelling the body. According to
Knauer, they are endowed with no small amount of curi-
osity. After this observer had placed a glass cage of snakes
near a similar cage containing a number of toads it was
found that each group of animals was apparently contem-
plating the other with much interest. The toads were all
gathered on the side of the cage nearest the snakes, engaged
in wThat seemed to be a close inspection of their neighbors.
Whether the toads' actions were manifestations of curiosity
may be questioned, but they probably indicate at least a
certain power of attention.

The emotional endowment of frogs and toads is meager.
Aside from their sexual impulses, they show little emotional
susceptibility beyond that of fear. It is doubtful if they
ever show anger, which is one of the most primitive of all
emotional feelings. While there is often rivalry among
them for the possession of food, their struggles are peace-

THE INTELLIGENCE OF THE FROG 373

able and betray no ill temper. The " angry and envious
glances" with which, according to Knauer, toads regard one
of their number which is fortunate enough to seize a worm
for which they are all struggling, are more matters of sub-
jective interpretation on the part of the observer than any
real emotional expression on the part of the animals. How
the countenance of the toad is modified to express an angry
glance we are not informed. L. E. Adams 1 comments on
the "pugnacious propensities" of a frog which snapped re-
peatedly at the tip of an umbrella moved near its head.
It seems more probable that this somewhat unusual be-
havior was a manifestation of the food taking response in-
stead of the fighting instinct.

Of sympathy or affection for its kind the frog or the toad
shows no trace. Care for offspring is almost of necessity
absent owing to the methods employed in reproduction,
since the young shift for themselves in entire independence
of their parents. The formation of groups in hibernation is
doubtless brought about either by the animals happening
to get into the same nooks, or through the tendency to seek
the slight degree of warmth afforded by each other's bodies.
The frog and the toad are pure egoists. Their only acts
which have any reference to other members of their species
are those prompted by the blind impulses to reproduction
which nature has implanted in these animals in the interests
of their posterity.

REFERENCES

Abbott, C. C. The Intelligence of Batrachians. Science, Vol. 3,
pp. 66-67. A Naturalist's Rambles about Home, 2d. ed., 1894.
Brehm. Thierleben. Bd. 7.

Jourdain, S. De Intelligence des batraciens. C. R. Ass. franc
Av. Sci., 29me Sess., 1900.

irrhe Zoologist, (4) Vol. 10, 154-5, 1906.

374

THE BIOLOGY OF THE FROG

Kiiauer, F. K. Beobachtuiigen an Reptilien und Amphibien in
der Gefangenschaft. Wien, 1875.

Schaeffer, A. A. Habit Formation in Frogs. Jour. An. Behavior.
Vol. 1, 309, 1911.

Yerkes, R. M. The Instincts, Habits, and Reactions of the Frog
Monogr. Suppl. Psychol. Rev., Vol. 4, 1903. Inhibition and Rein-
forcement of Reactions in the Frog, Rana clamitans. Jour. Comp,
Neur and Psych., Vol. 14, 1904.

INDEX OF AUTHORS

Abbott, 15, 19, 22-25, 368, 369,
373.

Abelous, 242 ; Abelous and Bil-
lard, 222, 242; Abelous and
Langlois, 235, 242.

Ach, 353, 356.

Adami, 213, 216.

Adams, 373.

Adler, 242.

Albertoni, 334.

Allen, B. M., 230, 232, 234, 238,
242.

Allen, G. M., 19, 24, 25.
Angelluchi, 348.
Aron and Alfonsi, 227, 242.
Ascherson, 187, 203.
Assheton, 100, 107.
Athanasiu, 164.

Babak, 180, 203.
Baber, 242.

Baglioni, 175, 180, 319, 321, 334.

Banta, 322, 323, 334.

Banting, 227.

Barfurth, 58, 124, 164.

Bechterew, 334.

Beer, 349, 356.

Beissner, 210, 216.

Berg, 180.

Bernard, C, 225.

Bernard and Bratuschek, 88.

Bert, 180, 188, 203.

Bethe, 338.

Bibron, 25.

Bickel, 334.

Biedermann, 186, 193-197, 201,
203.

Billard, 232, 242.
Birge, 334.
Bizzozero, 152, 279.
Bohr. 179, 180.

Bolau, 242.
Boll, 348.
Born, 55.
Bouin, 224.

Boulenger, 18, 25, 124, 203.

Brehm, 25, 273.

Breuer, 252.

Bricka, 227.

Briicke, 196, 293, 298.

Buckland, 18.

Burnett, 326, 334.

Busquet, 320.

Calmels, 186.
Camerano, 124.
Ciaccio, 184.
Cima, 202.
Clark, 213.
Cope, 22, 25, 27.
Copeland, 124.
Courtis, 19.
Crew, 224.

Dale, 334.
Danilewsky, 334.
Dekhuyzen, 275. 298.
Descartes, 68.
Dewevre, 166.
Dickerson, 22.
Dissard, 180.
Dobell, 46.

Donaldson, 62, 191, 203, 334.
Drake, 28.
Draseh, 203.
Durigen, 25, 29.
Dumeril, 25, 215.
Dutartre, 199, 203.

Eberth, 156, 162, 166, 184, 196,
Ecker, 26, 152, 268, 293.
Edinger, 334.

376 INDEX OF AUTHORS

Ehrmann, 183, 100, 203.

Eidam, 47.

Eidmann, 242.

Emerson and Norris, 48.

Engelmann, 1S4, 186-188, 203.

Ewald, 1D4, 353.

Farrington, 210.
Patio, 189.
Faust, 12, 16.
Field, 117.

Fischer-Sigwart, 25, 29, 50, 53,

54, 58.
Fixsen, 74.
Fraisse, 58.
FrSnkel, 298.
FrankI, 216.
Frear, 8.
Friedsohn, 298.
Fuchs, 203, 298.
Funke, 38, 39, 162, 224.

Gadow, 5, 9. 11, 1°, 26, 203.

Gage, 13, 18.

Garman, 17.

Gaskell, 334.

Gaule, 38, 164, 242, 298.

Gaupp, 30, 143, 174, 180, 188,

228, 233, 293, 300.
Gayda, 227, 235, 242.
Gemmill, 119.
Giglio-Tos, 224.
Girard, 353, 356.
Goltz, 318, 321, 322, 329, 334,

352, 356.
Gonfrin, 242.
Graber, 361, 367.
Griesheim, 55.
Griitzner, 146, 148, 166.
Gudernatsch, 230, 232, 233, 242.
Guth, 348.

Hall, 119.
Hall, F. G., 62.
Hammar, 232, 243.
Hardesty, 335

Harless, 184, 196, 203, 335.
Harms, 239, 240, 243.
Hartog, 30.
Hay, 26.

Ileidenhain, M., 166, 184, 186,
203.

Heidenhain, R., 151, 152, 166.

Hensen, 353.

Herring, 243.

Heubel, 64, 335.

Hinkley, 124.

Hirsehberg, 342, 349, 356.

Hoffmann, 26.

Hogben, 199, 200, 203, 237, 239,

243.
Holbrook, 26.
Holmes, 203.
Hooker, 204.
Howes, 26, 151.
Huber, 40, 204, 337.
Huxley, 26.
Hyrtl, 285.

Jordan, D. S., 20, 26.

Jordan, E. O., 13.

Jordan, H. E., 228.

Jordan and Speidel, 279, 298.

Jourdain, 373.

Junius, 184, 186, 204.

Kalm, 23.

Kato, 310, 311, 319, 326, 32Y5

335.
Kellicott, 123.
King, 102.
Kirkland, 17, 18.
Klug, 177, 180.

Knauer, 30, 340, 369, 373, 374.

Knauthe, 60-62.

Krawzoff, 327.

Krogh, 171, 178-180.

Krukenberg, 194.

Kudo, 47.

Kiihne, 193.

Labbe, 47.

Lahousse, 158.

Landois, 354, 356.

Langendorff, 38, 164, 335; Lan-

gendorff and Mozeik, 164.
Langley, 146-150, 158, 164, 166.
Laudenbach, 356.
Leidy, 44, 45.

Leonard, 38, 156, 162, 163, 166.

INDEX OF AUTHORS S77

Leydig, 26, 186, 189-191, 204.
Lichtenstein, 47.
Lister, 198.

Loeb, J., 332, 335, 367.
Loeb, L., and Strong, 183.
Loeser, 327-329, 335.
Lombard, 269.
Loos, 45.
Luchsinger, 164.
Ludwig. 212.
Lyon, 356.

Macallum, 298.

McBride, 119.

McCord and Allen, 241.

McEwen, 123.

Marquis, 48, 279, 298.

Marshall, 26, 100, 113, 119, 123,

209.
Martin, 180.
Massart, 276.
Maurel and Lagriffe, 60.
Maurer, 113, 114, 231, 233, 234,

243, 285.
Maxwell, 202, 204.
Mayer, 183, 232, 243, 293.
Meisenheimer, 239, 240.
Mendelsohn, 335.
Merzbacher, 335.
Miller, 19, 26.
Milne-Edwards, 181.
Mivart, 26.

Moleschott and Fubini, 179, 181.
Moore and Vincent, 235, 243.
Moraczewski, 166.
Morgan, A. H., 338, 356.
Morgan, T. H., 49, 100, 102, 123.
Miiller-Erzbach, 61.

Neumann, 224, 277, 298.

Nicoglu, 184, 186.

Nothnagel, 335.

Nottin, 298.

Nussbaum, J., 243.

Nussbaum, M., 54, 146, 150, 166,

212, 213, 215, 216, 222, 229,

240, 243.

Oliver and SchiifVr, 236, 243.
Oppel, 152, 157, 166.
Overton, 202. 204.

Paneth, 152.

Parker, G. H., 363, 367.
Parker, W. K., 26, 260.
Parnas, 215, 216.
Partsch, 146, 150, 166.
Pearse, 364-367.
Peter, 213, 217.
Pfitzner, 184, 204.
Pfliiger, 51, 55.
Plateau, 349, 356.
Ploetz, 38, 39, 222-224.
Ponfick, 156, 228.
Pouchet, 192.
Powers, 10.
Przylecki, 202, 204.

Reese, 123.

Regnault and Reiset, 181.

Reid, 202, 204 ; and Hamby, 204.

Retzius, 337, 356.

Richards and Schmidt, 217.

Riddle, 151, 166.

Ritter, 11.

Robertson, 237.

Robinson, 100.

Rogers, 232.

Rosel von Rosenhof, 26.
Romeis, 243.
Ryder, 124.
Rynberk, v., 204.

Sabatier, 285, 293, 298.

Schaeffer, 371, 374.

Schafer, 181.

Schleiden, 86, 125.

Schlosser, 335.

Schmidt, 194, 204.

Schrader, 324, 326, 327, 329, 332,

335, 353, 356.
Schultze, F. E„ 184.
Schultze, M., 346.
Schultze, O., 89, 90.
Schulze, 234, 243.
Schwann, 86, 125.
Schwartz, 227, 243.
Sczcsny, 186.
Sedgwick, 335.
Seeck, 204.

Sewell, 146, 148, 150, 166.
Sharpy-Schafer, 243.

37S INDEX OF

Si rot inm, 335.

Sklower, 243.

Slonaker, 10.

Smith. (J.. 240, 243.

Smith. P. E., 238, 243.

Spallanzani, 26, 52, 58, 86, 89,

321, 323.
Steenstrup, 189.

Steinach, 51, 52, 198, 204, 238,

320, 348.
Steiner, 199, 324, 327-329, 335,

353, 356.
Stejneger, 23.
Stieda, 204.
Stilling, 234.
Stirling, 205.
Stolkinow, 166.
Storer, 26.
Stossich, 45.
Strieker and Spina, 205.
Swiecicki, 146, 150, 166.
Swingle, 56, 91, 92, 231, 243.

Tait and Green, 278, 298.
Tarchanoff, 224.
Toldt, 224.

Torelle, 361, 362, 364, 366, 367.
Torok, 298.

AUTHORS

Townson, 62, 205.
Treupel, 230, 244.

Uhlenhuth, 244.

Valatour, 143, 151.
Van Genderen-Stort, 346.
Verworn, 63, 64.
Virchow, 192.
Volkmann, 336.
Vulpian, 336.

Weber, 162, 166.

Wedenski, 181.

Wenyon, 45.

Werner, 201, 205.

Wiedersheim, 75.

Willem, 181.

Witschi, 56, 91, 92, 223.

Wittich, von, 164, 192, 194, 196

197, 205, 319.
Wolff, 86.
Wright, 26, 64.
Wyman, 336.

Yerkes, 336, 354-356, 369-371

374.
Yoshida, 217.

INDEX OF SUBJECTS

Abs/rrption of food, 158-160; of
water, 62, 198, 201, 202.

Acauthocephali, 44.

Acetabulum, 259.

Acris, 20 ; A. gryllus, 20, 49.

Adipose tissue, 129.

Adrenal body, 207, 234-236.

Afferent nerves, 316.

Aglossa, 13, 14.

Alveoli, 171.

Alytes obstetricans, 15.

Amblystoma, 10, 49 ; Amblysto-
matinse, 9, 10.

Amino acids, 154, 159.

Amphiuma, Amphiumida?, 8.

Ampulla of ear, 351.

Anabolism, 138.

Aneides lugubris, 11.

Angulare, angulo-splenial, 249.

Annulus tympanicus, 352.

Anura, 5, 13.

Anus, 67, 79, 106.

Aorta, 280, 286.

Aortic arches, 117, 122.

Apoda, 5.

Aqueduct of Sylvius, 108, 307.
Aqueous humor, 342.
Arachnoid, 301.
Archenteron, 100.
Arcifera, 13.

Area centralis of retina, 347.
Artery, 284. For particular ar-
teries, see 284-288.
Arytenoid cartilage, 168, 169.
Astragalus, 245.
Atlas, 245, 253.

Auditory organ, 67, 350 ; nerve,

313, 351, 352.
Auricle, 77, 280, 282, 283.
Axis cylinder, 135, 303, 304.
Axolotyl, 10.

Balantidium, 46.
Basidiobolus ranarum, 47.
Batrachoseps, 11.
Belostoma, 42.
Benacus, 42.

Bidder's canal, 209, 210.

Bile, 154, 158; capillaries, 156;

duct, 111, 154, 155.
Bladder, 81, 111, 213-216.
Blastocoel, 98.
Blastopore, 98.
Blastula, 98.

Blood, 39, 125, 274; corpuscles,

39, 274-280; vessels, 283.
Body cavity, see Coelom.
Bone, structure of, 130, 131 ;

cells, 131.
Bowman's capsule, 207, 208.
Brachial plexus, 306.
Brain, 84, 107-110; functions of,

324 ; influence on cord, 318.
Branchial arches, 122 ; clefts,

113.

Breeding habits, 48-55.

Brow spot, 66, 67, 108, 309.

Buccal cavity, 72-75.

Bufo lentiginosus, 18, 45, 50, 102.

Bufonidsp, 13, 16.

Bulbus cordis, 280, 283, 293-295.

Bullfrog, 22, 23, 42, 76, 92, 221.

Calcaneum, 245.

Canal, central of cord, 107, 302;

semicircular, 351, 353.
Canaliculi, 131.
Capillaries, 284, 295, 296.
Carbohydrates, 141.
Cardiac glands, 144.
Carotid artery, 284; gland, 284.

294.
Carpus. 257.

380 INDEX OF

Cartilage, 129; cartilage bones,
250.

Cauda equina, 300.
Cell theory, 86, 125.
Centrum, 253.

Cerebellum, 107, 108,. 311,

Cerebral hemispheres, cerebrum,

108, 308, 325-327.
Chiasma, optic, 311.
Chlamydophris, 46.
Choanal, 73. See also Nares.
Chondrotus, 10.

Choroid coat of eye, 342 ; plexus,

107, 108.
Chorophilus, 49, 50.
Chromatophores, 183, 191.
Chromosomes, 91-93.
Cilia. 126.

Ciliated epithelium, 75, 126.

Circulation, 274.

Cisterna magna, 83, 296.

Clavicle, 255.

Cleavage, 94.

Clepsine, 43.

Cloaca, 67, 74, 79.

Coagulation, 277.

Coecilians, 5.

Ccelom, 77, 101, 115.

Ccelomic fluid, 83.

Cohnheim's fields, 134.

Cold, effects of, 59-62, 88, 89, 164,

200, 364; influence on blood,

277 ; sense of, 338.
Color changes, 40, 41, 62, 191-

201.

Columella, 66, 352.
Commissures of cord, 302.
Compensatory motions, 324.
Condyles, occipital, 245.
Cones of retina, 344-348.
Conjunctiva,, 341.
Connective tissue, 125, 127-129;

subcutaneous, 185.
Copromonas, 46.
Copulation. 51, 56, 321.
Coracoid, 255.
Corium, 182.
Cornea, 340.

Corpus adiposum, 81, 223.

SUBJECTS

Corpuscles, of blood, 274-280; of

lymph, 278.
Cranial nerves, 311.
Cranium, 245.
Cricket frog, 20.
Cricoid cartilage, 168.
Croaking, 35, 318.
Crura cerebri, 108, 309.
Cryptobranchidse, 8.
Cryptobranchus, 8, 42, 63, 366.
Crystalline lens, 342.
Cuticula dentis, 76.
Cutis, 182.

Cistignathidse, 14, 21.
Cytamoeba, 47.

Dactylosoma, 47.

Dentale, 250.

Dentine, 76.

Dermal plica?, 70.

Desmognathinse, 10.

Desmognathus, fuscus, 10, 11.

Development, 85.

Digestion^l40, 145^150^154, 159.

Digestive organs, 141.

Diseoglossidae, 13, 15.

Distomum, 45.

Diving, 33.

Drepanidium, 47.

Drum Of ear, 66.

Duct, bile or gall, 77, 154 ; cystic,

155 ; hepatic, 155.
Ductus endolymphaticus, 351.
Duodenum, 79. 151.
Dura mater, 301.

Ear, 66, 106. 350; Development

of, 106, 107.
Echinorynchus, 44.
Ectoderm, 100.
Efferent nerves, 316.
Egg laying, 48-54, 219, 220.
Eggs, 87, 89, 218.
Eimeria 47.
Embryology, 85.
Enamel, 76.
Endocrine glands, 225.
Endolymph, 351.
Enemies, 42,
Engystomatidae. 14.

INDEX OF

Entamoeba, 46.

Entoderm, 100.

Enzymes, 140.

Epicoracoid, 256.

Epidermis, 183.

Epigenesis, 86.

Epiphysis, 67, 309.

Episternum, 256.

Epithelial bodies, 114, 233.

Epithelium, 75, 125, 126.

Erepsin, 159.

Esophagus, 74, 79, 140.

Ethmoid, 247, 248.

Eustachian tube, 67, 73, 113, 352.
:^olution. 86.
\Excjretion722B2)

Excretory organs, 117, 206.

Exoccipitals, 245.

Eye lids, 65, 341 ; eye muscles,
350.

Eyes, 65, 106, 340-350, 365.

Fascia, 128.

Fat body, 39, 81, 223.

Fear, 36, 371.

Female, organs of, 218 ; pronu-
cleus, 93.
Femur, 259.
Fenestra ovalis, 352.
Ferments, 140.

Fertilization, 52, 53, 87, 92-94.
Fibrillar, 132.
Fibrin, 277.
Fibrinogen, 278.
Fibrous tissue, 127.
Fibulare, 260.
Filum terminale, 300.
Fissures, of cord, 302 ; of brain,
308.

Fontanelles, 252.
Food, 28, 138-141, 165.
Foramen. intervertebral, 254 ;

magnum. 245; of Monro, 110,

311 : ovale, 245.
Forebrain 107, 108, 308.
Fourth ventricle, 108, 308.
Fronto-parietal, 247.

Gall bladder, 77, 111.
Ganglion, 305; Bidder's, 315. Do-
giel's. 315; Gasserian, 312;

SUBJECTS 381

jugular, 314 ; prootic, 312 ;

Remak's, 315 ; sympathetic,

314, 315.
Ganglion cells, 134-137.
Gastric juice, 145.
Gastrocnemius. 261, 262, 268.
Gasfro-hepa to-duodenal ligament,

142.

Gastrophryne carolinensis, 21.

Gastrula, 98-100.

Genital organs, 119, 218; ridge,
119, 225.

Germ layers, 100.

Gill, 120-123 ; clefts or slits, 113,
120; plate, 106.

Gland, cardiac, 144 ; carotid, 284,
294 ; ductless, 225 ; esophageal,
145; gastric, 143; mucus, 186;
poison, 186 ; pyloric, 145 ; thy-
mus, 113, 231; thyroid, 113,
228.

Glenoid fossa, 255.
Glomerulus, 117, 207,' 208.
Glossiphonia, 43.
Glottis, 74, 113, 168.
Glycogen, 160.
Goblet cells, 152. 153.
Gonads, 39, 79, 80, 218.
Gray matter, 302.

Haemoglobin, see Hemoglobin.
Hatteria, 68.
Hearing, 353, 355.
Heart, 77, 115, 116, 280.
Heat, effects of, 59-62, 364.
Helix, 28.

Hemoglobin, 176, 274.

ITensen's line, 134.

Hepatic artery, 156, 287; ducts,

155; portal system, 156, 292,

293; vein, 290.
TTexamita. 46.
Hibernation, 57.

Hind-brain, 107, 308; limb, 259.

Histology, 125.

Hormones, 225, 238.

I Cumerus, 255-257.

Ily_Ia, 20, 198; goeldii, 21; pick-

eringltr49"T^'(u-si('olor, 20.
Hylid«, 13, 19.

3S2 INDEX OF

Hyoid, 74. 250.

Byomandibular, cleft, 113; nerve,
313.

Hypnotism, 63, 64.
Hypoblast; see Entoderm.
Hypoglossal nerve, 306.
Hypophysis, 236, 311.

Ileum, 79, 151.
Ilium, 258.

Infundibular lobe, 310.
Infundibulum, 108.
Infusoria, parasitic, 45, 46.
Inscriptiones tendinse, 271.
Insertion of muscle, 262.
Inspiration, 173.
Instinct, 357.
Insulin, 227.
Intelligence, 368.
Intermaxillary gland, 75.
Internal secretion, 225.
Intervertebral foramina, 254.
Intestine, 79, 151.
Invagination, 98.
Iodine, 230, 231.
Iris, 65, 342, 343.
Ischium, 258.
Iter, 108, 307.

Jaw, 72, 249, 252.
Jelly, 87.

Katabolism, 138.
Kidneys, 81, 207, 279.
Krause's membrane, 134.

Labyrinth, 350.
Labyrinthodonts, 4.
Lacunae, 129, 131.
Langerhans, islands of, 227.
Lankestrella, 47.
Larvae, 105, 120.
Larynx, 113, 168.
Lateral plate, 114 ; ventricle, 110,
311.

Lens of eye, 342, 348.

Leopard frog, 24. See also Rana

pipiens.
Leptotheca ohlmacheri, 47.

SUBJECTS

Leucocytes, 152, 156, 228, 274,
296.

Leucophores, 191.
Light, effects of, 179, 198; reac-
tions to, 360.
Limbs, 68, 69.
Linea alba, 271.
Lipase, 154.

Liver, 38, 77, 154, 158, 160-165.

206, 212.
Lungs, 77, 111, 171.
Lymph, 278, 296, 297; hearts,

296 ; spaces, 182, 296, 297.

Male, instincts of, 35, 36, 50-54;
organs of, 221-223 ; pronucleus,
93 ; Malpighian body or cor-
puscle, 119, 207, 208.

Mandible, 249.

Marrow, 39, 279.

Marsupial frog, 21.

Maturation of egg, 90.

Maxillary bone, 249.

Meckel's cartilage, 249, 250.

Medulla oblongata, 108, 109, 300,
328-332.

Medullary folds, 104; sheath,
135.

Megalobatrachus, 8 ; M. maxi-

mus, 9.
Melanin, 191.
Melanophores, 191, 237.
Membrane bones, 250.
Mento-meckelian bones, 174s

250.

Mesentery, 79, 151.
Mesoblast, mesoderm, 100, 114.
Mesogaster, 142.
Mesonephros, 117.
Mesorchium, 81, 221.
Mesosternum, 256.
Mesovarium, 218.
Metabolism, 138.
Metacarpus, 258.
Metamorphosis, 120.
Metatarsus, 260.
Mid-brain, 108, 308.
Mouth, 72, 106.
Mucigen, 148.
Mucosa, 143, 151, 154.

INDEX OF

Miillerian duct, 119.

Muscle fibers, 131-134.

Muscles, kinds of, 2G1 ; of body
wall, 271 ; of eye, 350 ; of hind
leg, 262-270 ; seasonal changes
of, 38 ; of throat, 272.

Muscularis mucosae, 143.

Myotome, 114.

Myxosporidia, 47.

Nares, 67, 73.

Nasal bone, 248 ; capsule, 248.
Necturus, 6, 7.
Nematoxys, 44.
Nephrostomy 117, 209.
Nerve, 134-137; cells, 134-137.
Nervous system, 84, 107, 299.
Neural arch, 253 ; spine, 253.
Neurenteric canal, 109.
Neurilemma, 135.
Neuroglia, 302.

Nictitating membrane, 66, 340.
Nodes of Ranvier, 135.
Nostrils, 67, 106.
Notochord, 102, 119, 120.
Nototrema, 21.
Nucleus of egg, 89.
Nyctotheres, 46.

Occipital condyle, 245.
Odontoblasts, 76.
(Esophagus, see Esophagus.
Olecranon, 257.

Olfactory capsules, 256 ; lobe,
308, 311; nerve, 311, 339 ; or-
gan, 339.

Opalina, 45.

Operculum, 122.

Optic chiasma, 311: lobe, 108,
300, 328; nerve, 108, 311 : thal-
amus, 108; ventricle, 309.

Orbit, 65.

Osteoblasts, 131.

Osteoclasts, 76.

Otoliths, 351, 353.

Ova, 85, 218.

Ovary, 39, 79. 218.

Oviduct, 39, 79, 219.

Oviposition. 48, 52-55, 220.

Oxysoma, 44.

SUBJECTS 383

Palatine, 249.

Pancreas, 79, 111, 153, 154, 226,
227.

Pancreatic duct, 111, 154 ; juice,
154.

Papillae, of skin, 190, 337; of

tongue, 74, 338.
Parabasal, parasphenoid, 247.
Paraphysis, 309.
Parasites, 43.
Parietal nerve, 309.
Pectoral girdle, 77, 254.
Pelobatidse, 13, 15.
Pelvic girdle, 258.
Pepsin, 146, 150.
Pepsinogen, 149.
Peptones, 146.

Pericardium, 77, 115, 116, 28C.
Perichondrium, 130.
Perilymph, 351.
Periosteum, 131.
Peritoneum, 81, 82, 115, 151,
Pes, 69.

Phalanges, 258, 260.
Phaneroglossa, 13.
Pharynx, 142.
Photopathy, 363.
Phototaxis, 361.
Pia mater, 301.

Pigment, 38, 156, 191, 238 ; cells,

156, 191, 237.
Pineal gland, 67, 108, 241.
Pipa americana, 14.
Pituitary body, 108, 236.
Plasma, 274.

Plethodon, 11 ; Plethodontin:o. 10,
11.

Plexus, Auerbach's, 315; bra-
chial, 306; coeliac, 315: ischio-
coccygeal, 308: lumbo-sacral,
306; Meissner's, 315: sciatic,
306 ; sol a r , 3 15; u rogc nital,
315.

Poison glands. 186.
Polar bodies. 90.

Polystomum, 45.

Portal system, hepatic. 156. 292:

penal, 292.
Portal vein. 156, 292.
Postbranchial body, 114, 233.

384 INDEX OF

Preformation, 8(3.
Preballux, 69, 260.
Premaxilla, 174, 249.
Primitive groove, 104.
Procoracoid, 255.
Procoracoidal body, 234.
Proctodeum, 111.
Pronephros, 117; duct of . see Seg-
mental duct.
Prootic, 245.
Froteidae, 6.
Proteins, 138, 141.
Proteus, 5, 6, 366.
Pseudacris, 20.
Pseudobranchus, 7.
Pseudothyroid, 114.
Pterygoid, 248.
?ubis, 258.

Pulmocutaneous arch, artery,

281, 288.
Pulmonary artery, 288 ; vein, 282.
Pulvinar rostrale, 72.
Pyloric glands, 145.
Pylorus, 81, 142.

Quadrate, 251.
Quadrato-jugal, 249.

Radiale, 257.

Radio-ulna, 257.

Rana, 21.

Rana agilis, 201.

Rana arvalis, 189.

Rana catesbiana, 22, 23, 42, 76,

221, 222.

Rana clamitans, 23, 24, 47, 361.
Rana esculenta, 28, 45, 47, 51,

55, 64. 163, 164, 171, 178, 188,

210, 223, 361.
Rana fusca, 29, 48, 50, 51, 54,

55, 62, 178, 186, 189, 190, 194,

201, 210, 211. 222, 223.
Rana grylio, 23.
Rana halecina. 50.
Rana muta, 29.
Rana, oxyrhina, 47, 189.
Rana palustris, 25, 102, 364.
Rana pipiens, 24, 27, 28, 31, 34,

41, 42, 44, 45, 62, 74, 91, 220,

222. 361.

SUBJECTS

Rana sylvatica, 24, 28, 49,
Rana temporaria, 38, 40, 45, 49,

52, 91, 121, 163, 223.
Ranatra, 42.
Rectum, 79, 153.
Red leg, 48.
Reflex action, 315-322.
Regeneration, 58.
Renal portal system, 211 ; renal

veins, 211.
Reproductive organs, 37, 39, 79,

119, 218.
Respiration, (ITWSCD
Respiratory movements, 171.
Retina, 108, 342.
Rhabdonema nigrovenosa, 44.
Righting movementsr~~34.
Rods and cones of retina, 344-348.

Sacculus, 350, 351.
Saccus endolymphaticus, 351.
Sagittal fissure, 311 ; suture, 247.
Salamandra atra, 12 ; maculosa,

11, 12.
Salamandridse, 6, 9.
Salamandrinse, 10, 11.
Saprolegnia, 47.
Sarcolemma, 133.
Sarcomeres, 133.
Sarcoplasm, 133.
Scaphiopus holbrooki, 15.
Scapula, 255.

Schmidt, incisures of, 135.

Sciatic nerves, 306 ; plexus, 306.

Sclerotic, coat, 340, 342.

Seasonal changes, 37-40, 161-165,
190, 220, 222-224, 278, 279.

Secretion, 145, 158, 186, 212 ; in-
ternal, 225.

Segmental duct, 117-119.

Segmentation cavity, 98.

Segmentation of egg, 94.

Semicircular canals, 351.

Semilunar valves, 283.

Seminal vesicle. 81, 222, 240.

Sense organs, 337 ; plate, 106.

Serosa, 143.

Sex, hormones, 239-241 ; organs,
39, 79, 119, 218; recognition,
320-324.

INDEX OF SUBJECTS

385

Sexes, proportions of, 55.

Sheath of Schwann, 135.

Shoulder girdle, 254.

Sinus venosus, 281 ; beating of,
293.

Sirenidae, 6, 7.

Siren lacertina, 7, 8.

Skin, 70, 176, 182; absorption
by, 62, 201, 202 ; color changes
of, 41, 62, 191-201 ; respiration
of, 176; shedding of, 62.

Skull, 245.

Smell, sense of, 29, 339.

Spermary, see Testis.

Spermatozoa, 87, 89, 92, 222.

Sphenethmoid, 248.

Spinal chord. 84, 107, 300; re-
flexes of, 300.

Spinal nerves, 300, 304.

Spindle cells, 274, 276-278,

Spiracle, 122, 123.

Spiral valve, 283.

Splanchnic layer, 115.

Spleen, 79, 227, 279.

Sporozoa, 47.

Squamosal, 248.

Steapsin, 154.

Stegocephali, 4.

Sternum, 256.

Stomach, 78, 140.

Stomodeum, 111.

Subcutaneous lymph spaces, 182,
296; tissue. 185.

Submucosa. 143, 151.

Subserosa, 143.

Sulcus marginalis. 72.

Suprascapula. 254.

Surinam toad, 14.

Suspensorium, 248, 252.

Sympathetic nervous system, 300,
314, 315.

Symphysis, pubic and ischial,
259.

Synapsis, 91.

Systemic arch, 286.

Tadpoles, 42, 120-123.
Taenia dispar, 44.
Tarsus, 260.

Taste, organ, 338, 339; sense of,

29, 339.
Teeth, 72, 75.

Temperature, effects of, 59-62.
Tendon, 128, 261.
Testis, 81, 221, 222.
Thalamencephalon, 108. 308:

functions of, 327.
Thalamus, 108, 308.
Thigmotaxis, 366.
Thrombocytes, 274, 276-278.
Thymus, 113, 231.
Thyroid, 113. 228.
Thyroxin. 230, 231.
Tibiale, 260.
Tibio fibula, 259.
Toads, 16-19, 63, 216, 366, 372.
Tongue, 30, 74, 75 ; sense organs

of, 338.

Touch corpuscles, 337 ; papillae,
337.

Triton, 12, 13, 366.
Triturus, 13, 366.
Tropisms, 357.

Truncus arteriosus, 116, 117, 280,
281.

Trypanosoma, 46.
Trypsin, 154.
Tuberculosis, 48.
Tuberculum prelinguale, 72.
Tunica albuginea, 221.
Tympanic bone, 248 ; membrane,

66, 73, 352.
Typhlomolge, 6.
Typhlotriton spelaeus, 11.

Ulnare, 257.

Urea, 212.

Ureter, 81. 119, 207.

Urinary bladder, SI, 213; ttt

bules, 117, 207.
Urodela, 5.
Urostyle, 254.
Uterus, 81, 219
Qtriculus, 350,

Valvula paradoxica, 294.
Vas afferens, 2(>7.
Vas deferens. 210.
I Vaaa efferentia, 119, 209. 221

8201

3S6 INDEX OF

Veins, 281, 282, 288-293; an-
terior abdominal vein, 77, 293.
Vena cava, 281, 282, 290.
Vent, 79.

Ventricle of heart, 77, 280, 293.
Ventricles of brain, 108, 309, 311.
Vertebrae, 120, 253.
Vertebral plate, 114.
Vesicula seminalis ; see Seminal
vesicle.

Visual green, 345 ; purple, 345.

Vitamines, 141.

Vitelline membrane, 89, 93.

Vitreous humor, 342.

Vocal cords, 169; sacs, 73, 74,

170.
Voice, 34, 168.

SUBJECTS
Vomer, 248.

Vomerine teeth, 73, 248.

White matter of cord, 302,
Wolffian body, 117 ; duct,
Wood frog, 24, 369; s<
Rana sylvatica.

Xantholeucophores, 191, !
Xanthophores, 191.
Xiphisternum, 25'5.

Yolk, 90.
Yolk plug, 99.

Zaitha, 42.
Zygapophyses, 253o
Zymogen, 154o

v2 ^