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THE BIOLOGY OF THE FROG

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Rana pipiens. Upper figure, ordinary resting attitude ; lo

ing position. (From photographs by Mr. F. M. Abbott.)

figure, crouch-

THE

BIOLOGY OF THE FROG

BY

SAMUEL J. HOLMES, Ph.D.

ASSISTANT PROFESSOR OF ZOOLOGY IN THE UNIVERSITY
OF WISCONSIN

SECOND EDITION

THE MACMILLAN COMPANY

LONDON: MACMILLAN & CO., Ltd.
1907

All rights reserved

Copyright, 1906,
By THE MACMILLAN COMPANY.

Set up and electrotyped. Published May, 1906. Reprinted
July, 1907.

Norbjoorj ]$xwz
J. 8. dishing Co. — Berwick & Smith Co.

Norwood, Muss., U.S.A.

«

J
A

PREFACE

The present book is the outgrowth of a course of
lectures delivered during the past six years at the Uni-
versity of Michigan. This course with the accompanying
laboratory work was based on the frog, which was chosen
as a convenient form with which to introduce students to
a knowledge of the morphology, physiology, and life his-
tory of vertebrate 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

vi PREFACE

getting a fairly adequate idea of the whole life of any one
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
subject of so many scientific investigations. One seldom
picks up a volume of a physiological journal without find-
ing 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 bio-
logical research. In fact, most of what is known in certain
departments 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 Wie-
derscheim'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.

PREFACE vii

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

PREFACE TO THE SECOND
EDITION

In the present issue of this work little change has been
made except to correct a few errors which, in spite of care-
ful revision, had crept into the previous one, I hope that
now with the aid of my critics, whose services are gratefully
acknowledged, the book has been rendered free from any
serious mistakes.

CONTENTS

CHAPTER

I. The Amphibia in General and Frogs in Particular

II. The Habits and Natural History of the Frog

III. External Characters of the Frog

IV. Preliminary Account of the Internal Stricture
V. The Development of the Fro<

VI. Histology of the Frog

VII. The Digestive System and its Function

VIII. The Vocal and Respiratory Organs

IX. The Skin .

X. The Excretory System

XI. The Reproductive Organs

XII. Internal Secretion and t

XIII. The Skeleton . ' .

XIV. The Muscular System
XV. The Circulatory System

XVI. The Nervous System .

XVII. The Sense Organs

XVIII. Instincts and Tropisms as related to Reflex

Action

XIX. The Intelligence of the Frog

and the Fat Bodies
he Ductless Glands

INDEX 359

PAGE
I

23
62

68

81

121

134
165

179
201
212
219
229
246
258
283
321

342
353

THE BIOLOGY OF THE FROG

CHAPTER I
THE AMPHIBIA IN GENERAL AND FROGS IX PARTICULAR

The frog is a representative of the class of vertebrate
animals known as the Batrachia, or Amphibia. This group
is intermediate in many features of structure and habit be-
tween the fishes and the reptiles and presents the following
general characters : The skin is usually smooth and moist,
and, in all the recent Amphibia, with the exception of a few
of the Apoda, entirely devoid of scales. The limbs, unlike
those of fishes, are typically of the five-toed or pentadactyle
type. The skull articulates with the first vertebra by two
occipital condyles and is usually composed of few bones.
Lungs are present except in rare instances, as in certain
lungless salamanders, where they have been secondarily lost.
The heart has three chambers, a ventricle and two auricles.
The aortic arches are symmetrical on the two sides. There
is no amnion nor allantois, although there is a homologue
of the latter in the urinary bladder. Most of the species
undergo a metamorphosis, the young living in the water and
breathing by means of gills. The latter disappear as the
lungs become functional, except in some of the lower Am-
phibia, in which they are retained throughout life.

The members of this class as a rule are aquatic or semi-
aquatic in habit, a circumstance which suggested the name
of the class. Even the most terrestrial of the species gener-

2 THE BIOLOGY OF THE FROG chap.

ally keep in a damp habitat. Very few of them can endure
any considerable amount of drought. Even when they live
far away from water during most of the year, these animals
nearly always go to the water in the spring to deposit their
eggs.

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 which caused the name Labyrinthodont
to be given to the group.

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.

I THE AMPHIBIA IX GENERAL 3

THE APODA

The Apoda, or Ccecilians, are creatures of wormlike form,
entirely 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 integument. The
eyes are small or absent. 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 the lower members of the group retain their
gills in the adult state. The Urodela are divided by Gadow
into four families as follows : —

Jaws without teeth. No hind limbs . . . Sirenidce.
Both jaws with teeth. Fore and hind limbs
present.
Gills persistent. No eyelids or maxillary bones Proteidcz.
Gills usually absent in the adult. Maxillary
bones present.

Eyes with lids SalamandricUe.

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 Typhlomolge,
are confined to North America. The remaining genus, Pro-
teus, represented by a single species, P. angninus, is found
only in the caves of Austria. This species occurs in deep
cool water in regions of complete darkness. Its eyes, like

THE BIOLOGY OF THE FROG

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.

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

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

are the "mud puppies" or "water dogs," which belong
to the genus Necturus. Nee turns maeulosus is the most
abundant species. It occurs in the northern and eastern
part of the United States, west of the Alleghanies, and is
especially abundant in the region of the Great Lakes. Its
general color is brown above, marked with darker spots, 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 Sirenidse is represented by two genera, Siren
and Pseudobranchus, both of which are confined to North

I THE AMPHIBIA IX GENERAL 5

America. Each genus contains but a single species. The
larger of these, Siren laceriina, is found in the rivers and
ponds of the Southern States, from Texas to North Caro-
lina. The body is long and snakelike in appearance. The
fore legs are very short and situated close behind the exter-

FlG. 2. — Siren lacertina. (From the Cambridge Natural History.)

nal 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. striates, 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,
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 Cryptobranchi-
dae. The former is represented by the large " hellbender,"

6

THE BIOLOGY OF THE FROG

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.

m

Fig. 3. — Amphiuma means. (From the Cambridge Natural History.)

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,
"when it began to open its big mouth, vigorously sway its
1 Am. Nat., Vol. 16, 1882.

! THE AMPHIBIA IN GENERAL 7

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.

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-

phicoelous. Toes 4-5 .... Ambly stomatin*.
BB. Parasphenoid with teeth.
C. Vertebrae opisthocoelous. Toes 5. Tongue

largely free Desmognathina.

CC. Vertebrae amphicoelous. Tongue small

and largely free Plethodontina.

AA. Series of palatal teeth in two longitudinal
series diverging behind. Parasphenoid
toothless ...... Salamandrin(Z.

The subfamily Amblystomatinae is represented in this
country mainly by the two genera Ambly stoma and Chon-
drotus. Ambtystoma 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 are usually
attached to twigs or stems of grass, and are found in
rounded or irregular masses. Each egg is surrounded
by a very thick coat of jelly. Amblystomas are among the
very first amphibians to lay their eggs. Eggs of a species
of Ambly stoma (probably tigrinum) have been taken near

8

THE BIOLOGY OF THE FROG

CHAP.

Ann Arbor, Michigan, at the following dates : March 15,
1892 ; March 26, 1895 > March 29, 1896 ; March 13, 1897 ;
March 28, 1905.

The larvae of A. tigrinum were formerly considered a
separate species, the axolotl, which was placed in a distinct
genus, Siredon, among the perennibranchiate urodeles.
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 recently 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 con-
cealed in the day-
time under stones
or in sheltered nooks
where the air is
moist. 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 representative of this
subfamily is Typhlotriton spelceus, a blind species found in a
cave in Missouri.

FlG. 4. — Desmognathus fuscus. Female with
egg-mass. (After Wilder.)

I THE AMPHIBIA IN GENERAL 9

The Plethodontinae form a large group, which is mainly
confined to America. The species of Plethodon, Spelerpes,
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 California species,
Autodax 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
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 sus-
picion."

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
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

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

io THE BIOLOGY OF THE FROG chap.

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

Fig. 5. — Triton cristatus. 1, female; 2, male as he appears during the
breeding season. (After Gadovv.)

head and body, and is marked with conspicuous colors.
After the breeding season the dorsal crest becomes greatly
reduced and the coloration becomes more dull. 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
(yDiemyctylus 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, minia-

, THE AMPHIBIA IN GENERAL «

fus 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 ANTJRA

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

. Aglossa.
A. Tongue absent •

AA Tongue present (Phaneroglossa).

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

Ribs present. Upper jaw with teeth . Discoglosstd*.
No ribs. Upper jaw with teeth . . Pelobatid*.
No ribs. Both jaws without teeth . Bujomda.
DD. Terminal phalanges claw shaped, usually

supporting adhesive disks . . H}didz.
CC. Sacral diapophyses cylindrical
BB. Halves of the shoulder girdle meeting in
the middle line and forming a me-
dian bar (Firmistemia).
C. Sacral diapophyses dilated
CC. Sacral diapophyses cylindrical

The Aglossa include only a few aberrant forms character-
ized by the absence of a tongue and the fact that the
Eustachian tubes open by a single median aperture in the
posterior side of the palate. The most noteworthy member

Cystignathidtz.

Engystomatidcc.
Rani da.

12 THE BIOLOGY OF THE FROG chap.

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. After the eggs are laid and fertilized, they be-
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 Jhe 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, Alytcs
obstetricans. In the breeding season the male clasps the
female in the usual way, and when the egg strings are ex-
truded, he tangles them around his hind legs and carries
them about with him. When the young larvoe are about
ready to escape, the male takes to the water and frees him-
self of the mass.

The Pelobatidse include but two x\merican genera,
Scaphiopus and Spea. These forms are commonly known
as the spade-foot frogs, on account of the peculiar horny
appendage which occurs 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 hoi-
brooki, which is found in the southern and eastern parts of
the United States, is very capricious in making its appear-
ance. After rains in the spring or summer the spade-foot
frogs come out in great numbers and lay their eggs, making

I THE AMPHIBIA IN GENERAL 13

a great clamor with their song. Then they disappear, and
may not again show themselves for several years.1

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 produces fatal
effects. The abundance of this secretion affords the toad
very efficient protection, and not many animals have the
hardihood to attack the creature. In addition to the poison
the skin secretes mucus, as in other amphibians, although
not in great quantity.

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
becomes lighter in color. In a dark environment it be-
comes 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

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

i4 THE BIOLOGY OF THE FROG chap.

then withdrawing it along with the insect to which it adheres.
Angleworms are seized by the jaws and stuffed into the
mouth by the fore legs. Toads are very useful in destroying
large numbers of injurious insects, and hence deserve all
possible encouragement and protection. Kirkland1 and
Garman,2 who have carefully examined the contents of the
stomachs of a large number of toads, find that the variety of
insects devoured 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 lie
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
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 numer-
ous stories of live toads found embedded in rocks or sealed
up in trees ; but while many of them seem to be quite well
authenticated, they do not give evidence of sufficiently care-
ful investigation to compel belief. Buckland3 has shown
that toads may live without food, when sealed up in blocks
of limestone, for over a year ; toads imprisoned in the lime-

1 Kirkland, Bull. No. 46, Mass. Agric. Exp. Sta.

2 Garman, Bull. No. 91, Ky. Exp. Sta.

3 Buckland, " Curiosities of Natural History."

I THE AMPHIBIA IN GENERAL 15

stone 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 Bnfo kntiginosus
in Massachusetts, according to Kirkland, is in April \ in
Ithaca, New York (Gage),1 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. kntiginosus are much
smaller than the females. During the breeding season they
frequently utter a peculiar shrill 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."

When toads are handled, and often even when approached,
they swell their bodies with air. Slonaker 2 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

1 Gage, Proc. Am. Ass. Adv. Sci., Vol. 47, 1898.

2 Proc. Indiana Ac. Sci., 1900.

16 THE BIOLOGY OF THE FROG chap.

most of them are confined to the Western States. About the
only species which occurs east of the Mississippi, with the
exception of Bufo quercicus, which ranges from North Caro-
lina to Florida, is B. lentiginosus, which is widely distributed
and quite abundant.

The Hylidae, 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 sticky
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
is often regarded as indicative of approaching rain. It
is heard frequently 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

I THE AMPHIBIA IN GENERAL 17

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 . Chorophilus.
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, with a dark triangular patch between the eyes, with
the middle of back and head bright green or reddish brown.
There are variations in color in the specimens of different
regions and a considerable power of color change in the
individuals themselves. This species is usually found along
the banks of ponds and swamps. Its note resembles that
of a cricket.

Hyla is represented by over a hundred species, ten of
which occur in North America. The species are mainly
found in trees. H. versicolor, so-called on account of its
remarkable change in color, is one of the most common and
largest of the North American spec/es, reaching a length of
two inches. The eggs of this species are deposited singly
or in small clusters on grass growing near the water's edge.
The breeding period in Massachusetts, according to Miss
Hinkley, is in the early part of May.

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 female, the skin being produced into a fold
which borders the egg mass. Nototrema, the " marsupial
frog " of South America, has a large pouch opening near the
posterior end of the back, in which the eggs are received,

18 THE BIOLOGY OF THE FROG chap.

and where they undergo development as far as the tadpole
stage.

The Engystomatidae contain but one North American
species, Engystoma carolinense, which is found in the South-
ern 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, include the Firmisternia with
cylindrical sacral diapophyses. The family comprises nu-
merous genera, only one of which, the typical genus Rana,
is found in North America. This genus contains about
one hundred and forty species, which are found in all of
the continents of the globe, although occurring only in the
extreme northern parts of South America and Australia.
There are fourteen North American species, two of which,
R. temporaria and R. agi/is, occur also in Europe. Only a
few of the better-known forms are here treated of. Full
description of the species may be found in Cope's " Batrachia
of North America."

Rana catesbiana, the bullfrog. — This is by far the largest
of 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 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 dermal plicae behind the eyes are indistinct. The tym-
panum 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

j THE AMPHIBIA IN GENERAL 19

bank of some pond or stream. It makes for the water very
quickly when alarmed, and usually skims along the surface
for several yards before diving below. According to Kalm,
it may leap to a distance of three yards, but Abbott, 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 roaring 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-op!* 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 clamitans. — This species has a nearly uniform green

20 THE BIOLOGY OF THE FROG chap.

or brownish color above, marked only with small irregular
black spots. The dermal plicae are conspicuous. The hind
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 Canada
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, R. 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 ' cravv-
aw-auk.' "

Ranapipiens, 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

I THE AMPHIBIA IN GENERAL 21

blotches edged with whitish. The legs are crossed above
with black bars which may or may not be interrupted in the
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."

Rami palustris, the pickerel frog. —This species re-
sembles 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.

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

Diirigen, B. Deutschlands Amphibien und Reptilien, 1897.

22 THE BIOLOGY OF THE FROG chap.

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

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

Hay, 0. 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
dee Thierreichs, Bd. VI, 2.

Holbrook, J. E. North American Herpetology.

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

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

Rosel von Rosenhof. Historia naturalis ranarum nostratium, 1758.

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

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 Wiederscheim. Anatomie des Frosches, auf Grund
eigener Untersuchungen durchaus neu bearbeitet von Dr. Ernst Gaupp,
1 896- 1 904.

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.

HABITS AND NATURAL HISTORY OF THE FROG 23

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 Ameri-
can 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 akin
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 a 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. Any one who has walked along
the margin of a pond or stream must have observed that

24 THE BIOLOGY OF THE FROG chap.

when a frog is started up it almost invariably makes a jump
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 into the water 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. Fre-
quently, 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 vege-
tation, 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. In Europe the water frog,
R. esculenta, is decidedly aquatic in 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. Large frogs have no
sentimental scruples against devouring their smaller relatives.
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 sev-

ii HABITS AND NATURAL HISTORY OF THE FROG 25

eral large worms one after the other, often seizing a new
worm before having finished the one it is attempting to
swallow. According to Fischer- Sigwart, Ranafusca will de-
vour large May beetles, employing both fore limbs to push
the rough legs of the insect into such a position that the
prey can be forced down the throat, an operation 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 ac-
customed to that form of diet by being fed with specimens
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 devour-
ing six large specimens in succession. According to Fischer-
Sigwart, this frog does not ordinarily devour snails although
Dlirigen reports Rana muta as swallowing specimens of
Helix as well as species of mollusks devoid of shells. Bees
and wasps are eaten with avidity notwithstanding 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 it is seized is readily swallowed.
Both the sense of taste and the sense of smell are apparently
obtuse ; if anything is taken into the mouth, it usually con-
tinues its course down the alimentary canal. Objects of a
too objectionable nature 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 with

26

THE BIOLOGY OF THE FROG

which the tongue is covered enables it to adhere to the
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
animal may be jerked out of the water. According to
Knauer, frogs and toads have the power of ejecting indigest-
ible 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 ac-
complished was, until
recently, but inade-
quately understood.
Hartog ! 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

• ! Hartog, Ann. Nat. Hist., May, (7), 7, 1901.

2 Gaupp, Anat. Am., 19, 1901.

1 -'

FlG. 6. — Figure showing the ton^i
frog in three different positions
Wiedersheim.)

of the
(After

ii HABITS AND NATURAL HISTORY OF THE FROG 27

to be raised up and thrown forward. " If," says Hartog,
" we inject with melted cocoa butter colored with car-
mine or alkanet, and keep up the pressure until the mass
sets, we find that it fills an enormous 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 ramification of the intrin-
sic 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

28 THE BIOLOGY OF THE FROG chap.

spread apart, and as the web between them affords a con-
siderable resistance to passing through the water, this mo-
tion gives the body a forward impulse. The fore limbs are
held back against the body, after the stroke, and if the 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 swimming, taking
strokes sometimes together and sometimes alternately. To
a certain extent they aid in propelling the animal forward,
but they are also employed, as in locomotion 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,
wrhich 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 consider-
able 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, there-

II HABITS AND NATURAL HISTORY OF THE FROG 29

fore, 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
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
condition of the hind limbs is of service in enabling the
animal to suddenly draw itself downward whenever danger
threatens 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 accomplished. 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 downward
through the water the animal has to reverse its position, as
an extension of the hind legs in its normal resting attitude
would tend to throw it out of the water. The first move-
ment is that of withdrawal from the surface, which is accom-
plished by suddenly bringing the hind legs forward, thus
giving the body a backward impulse. This brings the hind

3o THE BIOLOGY OF THE FROG chap.

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 shoot 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 readi-
ness for a spring. At the moment the frog leaves the sur-
face, 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. The operation involves the coordinated
action of several muscles. The position of equilibrium may
be attained by rolling over either to the right or to the left,
and a frog will do now the one and now the other, some-
times hesitating a moment between the two courses. 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 observer a chance of following its
movements. These movements vary a good deal in different
acts, but they commonly occur in about the following way :
If the frog rolls over toward its left side, the right hind leg is
brought dorsally 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
forward so that the thigh lies at a considerable angle from
the body, and this gives the limb a greater purchase in roll-
ing the body over. The left fore leg is brought down along-
side 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 heard. In large
frogs the notes are deeper than in small ones. The notes
of frogs are more often heard during the breeding season,
when they are supposed to serve the purpose of a sex call.
In the summer, however, it is not unusual to hear the
croaking of frogs, especially in the evening. A damp at-
mosphere 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 produced by rubbing the
back or side of the body. After each stroke the frog usu-
ally responds by a croak and then lapses into silence.
Croaking is often caused through accidental contact with
other individuals. 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

32 THE BIOLOGY OF THE FROG chap.

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 severely injured, they
sometimes utter a sort of cry which is called the pain
scream. When seized by a snake or other enemy, many
species of frogs may respond by making this piteous cry.

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 would 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.

ii HABITS AND NATURAL HISTORY OF THE FROG 33

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. Curiously
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 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 produced 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 attempts to avoid further persecu-
tion 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, for instance, is done by man. Its
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 recurrence of the period of
sexual activity, such as the development of the nuptial excres-
cences 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.

34 THE BIOLOGY OF THE FROG chap.

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 furnish-
ing the energy necessary to carry on the various activities of
the organs, but it is drawn upon to contribute to the growth
of the reproductive cells. A part of this material is stored
in the muscles, which during the winter decrease in weight
in relation to the rest of the body. Gaule * 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

1 Gaule, Arch. ges. P/iys., Bd. 81, 1900.

2 Leonard, Arch. . Inat. it. P/iys., phys. Abth. Suppl., 1881.

3 Funke, Deuksckr., Wien Akad. math. nat. CI. Bd. 68, 1900.

ii HABITS AND NATURAL HISTORY OF THE FROG 35

relation to the rest of the body, according to Langendorff,
Ploetz, and Funke, even increases during the winter months.
After 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 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 temporaria and Rana
esculentd) 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 accessory
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

36 THE BIOLOGY OF THE FROG chap.

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
escape of the spermatozoa and then a gradual increase in
size during the summer until 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

ii HABITS AND NATURAL HISTORY OF THE FROG 37

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,
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 predominant
colors of their environment, which are green, the color of
vegetation, and some shade of gray or brown, 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 pipiois, like most of the members of its genus,
possesses a much less 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,

38 THE BIOLOGY OF THE FROG chap.

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
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. Water rats and skunks
catch many frogs, the latter in Europe, according to Fischer-
Sigwart, hunting out the frogs from the hollows in which
they often congregate during the winter. There are a num-
ber of birds which prey upon frogs, such as cranes, herons,
and crows ; but their greatest enemies, next to man, are the
various species of snakes, of which, according to Fischer-
Sigwart, 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 destruc-
tive to the tadpoles.

To a certain extent frogs are preyed upon by other mem-

ii HABITS AND NATURAL HISTORY OF THE FROG 39

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, Bena-
cus, Zaitha, Ranatra, and even the small back-swimmers,
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 source of
food. Mortality among the tadpoles is naturally 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 exposed to many dangers from which
older and larger individuals are exempt. It is very proba-
ble that but a small part of the favored few who reach
maturity and perpetuate their kind 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
Lirge 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 incon-
venience arises from this source.

4o THE BIOLOGY OF THE FROG chap.

Parasites. — The frog, like most of the higher animals,
serves as the host of a large number of parasitic forms, be-
longing 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 larvae that hatch out
feed upon the membranes 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 frogs, although it is not improbable that
they are occasionally found there.

Of the several species of Nematodes found in the frog,
Rhabdonema nigrovenosa, which occurs in several 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 para-
sitic form which is usually found in the lungs. The latter is
hermaphroditic, and produces eggs which give rise to rhab-
ditiform 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, Ascaris entomelas Leidy, occurs in this
country in the lungs of Rana pipiens.

II HABITS AND NATURAL HISTORY OF THE FROG 41

Species of Strongylus, Nematoxys, and Oxysoma are occa-
sionally found in the intestine of frogs, and the ventricle of
the heart and the blood vessels may be infested by species
of Filaria. The hair worm, Gordius, occurs in the frog
among other forms during a part of its larval life.

The Acanthocephali are represented by one species,
Echinorhynchus haruca Rud., parasitic in the intestine of
the frog. Another species, E. lesiniformis Malin, has been
described in an encysted state in the peritoneum.

Cestodes are rarely found in frogs. Toznia dispar
Goeze, which occurs in the intestine of several European
species of Rana, has been found by Leidy * in this country,
in the intestine of Rana pipiens and Bufo /entiginosus.

Frogs harbor several species of Trematodes, which occur
chiefly in the intestine and bladder. Loos 2 describes eight
species of Distomum from frogs found in Europe, and
Stossich3 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. Distomum variegatum Rud.,
according to Leidy, occurs in the lungs of Rana pipiens,
and D. retusam Dies, in the intestine of the same species.
Two forms of D. cygnoides, according to Bensley, are found
in the bladder of Rana pipiens, c/amiians, and catesbiana.
D. ovocaudatum, according to Nickerson, occurs in some
American frogs. Distomum quietum is reported from Rana
pipiens by Stafford, and D. heterostomitm from the same
species by Wright. A new species, Distomum arcanum, has

1 Leidy, " Researches in Helminthology and Parasitology," Smithsonian
Inst., 1904.

2 Loos, " Die Distomen unserer Fische und Fr6sche,"i8Q4.

3 Stossich, " I Distomi degli Anfibi," 1889.

42 THE BIOLOGY OF THE FROG chap.

been recently found in some American species by Nickerson.
The peculiar genus Polystomum, characterized by having a
circle of several distinct suckers at the posterior end of the
body, is often found in the urinary bladder of frogs. Mono-
s to mum ornatum Leidy occurs in the body cavity, and
Holostomum nitidum Leidy and Diplodiscus subclavatus
Dies, in the intestine of Rana pipiens. Monostomum
ellipticum infests the lungs of Rana esculenta, and Tylodel-
p/iis rhachidis has been found in the neural canal of the
same species. Codonocephalus mutabilis Dies., Tetracotyle
crystallina Rud., and several species of Distomum occur in
an encysted state in various parts of the body of several
European frogs.

Of Protozoan parasites there are several noteworthy forms
among the Infusoria. The large multinucleate Opalina com-
monly occurs in the intestine, especially the rectum. O.
ranarum, dimidiata, and intestinalis are found in Rana,
while other species infest other genera of Amphibia. Three
species of Balantidium, viz. entozoon Ehr, elongatum Stein,
and duodeni Stein, inhabit the alimentary canal of Rana, the
first species living chiefly in the rectum. Nyctotherus cordi-
formis Stein and Bursaria intestinalis Ehr. are intestinal
parasites ; the latter was found by Leidy in the intestine of
Rana pipiens.

Of the flagellate protozoa there are two species of Bodo
which infest the intestine. Trypanosoma sanguinis Gruby
occurs in the blood of several European species, and I have
found a closely allied if not identical .species in the blood of
Rana pipiens.

The largest number of protozoan parasites of the frog fait
within the exclusively parasitic class of Sporozoa. The Sporo-
zoa infesting the American species of Amphibia have received
little attention, but it is probable that a large proportion of

ii HABITS AND NATURAL HISTORY OF THE FROG 43

them will prove identical with species found in Europe.
Descriptions of most of these forms may be found in Labbe's
" Sporozoa," and in the excellent chapter on the Sporozoa by
Minchin in Lankester's " Treatise on Zoology," from which
the following list of species has mainly been compiled. The
order Coccidiidea is represented by Hyaloklossia Lieberkiihni,
found in the envelope of the kidney, and the intestinal para-
sites Paracoccidium prevoti recently described by Laveran
and Mesnil, and Coccidium ranarutn Labbe. Molybdis
Entzi, which occurs, like the two preceding species, in Rana
esculenta, is doubtfully united with Coccidium ranai-um by
Minchin, who regards the Karyophagus ranarutn of Labbe as
also but another form of this same species.

The blood parasites belonging to the Haemosporidia are
represented by at least two species which infest frogs.
Lankestrella \_Drepanidium~] ranarum (Lank.) and an allied
species, L. monilis, occur in the red and white blood
corpuscles of Rana esculenta. The forms described as
HcEtnogregarina magna and Cytamceba bacterifera Labbe are
considered by Minchin as anomalous forms of one of the
species of Lankestrella. Dactylosoma \_Laverania~\ rana-
rum (= D. splendens Labbe) is considered by Hintze as but
one form of Lankestrella ranarutn.

Of the Myxosporidia, Leptothera O/ilmaclieri and L. ranee,
occur in the uriniferous tubules of Rana temporaria and
R. esculenta, and Pleistophora Danilewskyi in the muscles of
R. temporaria.

Karyamoeba renis Giglio Tos, which is stated to occur in
the renal epithelium of both the frog and the mouse, is a
sporozoan of uncertain systematic position.

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

44 THE BIOLOGY OF THE FROG chap.

which may spread over a considerable part of the body.
Necturus and other amphibians are liable to attacks from
the same fungus. Eidam l has described a species of fungus,
Basidiobolus ranarum, which inhabits the alimentary canal
of Ra?ia esculenta and R. oxyrhina.

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 crawl back into the mud after the breeding season
is over, and resume for a time their winter sleep. AtDorpat,
Marquis observed that in one year Ranafusca, immediately
after the breeding season, 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. The breeding of Rami
pipiens usually occurs, near Ann Arbor, in the latter half of
April or the first part of May. In general, a late spring
delays the time of breeding, and warm weather, on the other
hand, hastens it. Fischer-Sigwart, who for fourteen years
has carried on an extensive series of observations upon Rana
fusca, found that the difference between the earliest and
the latest period was one month (from February 26 to
March 26) in Barmoosweiher, and one and a half months
(from February 26 to April 15) in Holdenweiher. 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

1 Eidam, " Cohn's Beilrage zur Biologie der Pflanzen," Bd. 4, p. 181.

ii HABITS AND NATURAL HISTORY OF THE FROG 45

early in the season, frogs breed much 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.

The different species of frogs breed at different times.
In Germany Rana fusca breeds generally in March. In
Rana arvalis the association of the sexes commences two
or three weeks later, but the time of the egg laying nearly
coincides, according to Born, with that of the preceding
species, but its duration is somewhat longer. Rana esculenta
lays eggs only in May or June.

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 tanporaria, 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. Morgan1 has recorded the results of four years' observa-
tions of several American species of frogs in the vicinity of
Baltimore, Maryland.

"The first frogs to lay, and among the very first {Acris
gryllus excepted) to appear, are the wood frogs (Rana
sylratica). A few warm days in early spring suffice to bring
them out. The following records 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. The bunches are generally large, four to six
inches in diameter, and contain very many good-sized eggs.
In the same pools it is quite usual to find the firmer egg

1 Morgan, Am. Xat., Vol. 25.

46 THE BIOLOGY OF THE FROG chap.

bunches of Ambly stoma, 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, there-
fore, during the day often quite warm, puddles ; sometimes
in the same pools as the wood frogs, oftener in the ditches
by the side of the road. These two frogs are Hyla picker-
ingii and Chorophilus triseriatus. The eggs of these species
are very similar, and I know of no certain method of distin-
guishing 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 follow-
ing are the records: March 25, April 15, 1890. Eggs of
Bufo lentiginoses 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 tempera-
ture. The period of copulation, or the time during which
males may be found clasping the females, often considerably
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/usca,
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 dura-

ii HABITS AND NATURAL HISTORY OF THE FROG 47

tion of copulation in frogs kept in the terrarium was found
to vary between six and twenty-three days. The less varia-
bility 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, according to the temperature ; in
the terrarium a variation between ten and twenty-eight days
was observed. According 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
fusca, the pairs remaining 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 firmness
of his hold. The body of the female is very much com-
pressed 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 himself 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 inactive, and
will endure unfavorable conditions rather than make any effort
to seek a better situation. Sometimes several males will be

48 THE BIOLOGY OF THE FROG chap.

found clasping different parts of the body of one female,
forming masses, " Begattungsklumpen," 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 fit sea 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 one's finger, 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 over-
comes 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 copula-
tion. 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.

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 discovered
long ago, females killed in the first part of the breeding sea-
son have most of the eggs still in the ovary. Later many
eggs are found in their passage down the oviduct, although
they are not often met with in the body cavity. Females

ii HABITS AND NATURAL HISTORY OF THE FROG 49

killed near the end of their breeding period have nearly all
the eggs in the uterus. At the beginning of the breeding
period the seminal vesicles of the males are empty, or con-
tain very little seminal fluid. Steinach 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 in some specimens partly filled, and
there were yet several males in which the vesicles were
empty on March 9. Copulation begins, 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 some-
what similar to the fertilization of the eggs of most fishes,
where the male who accompanies the female during the breed-
ing 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 the time for extruding the sexual product arrives, 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 to 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 usu-
ally found among the masses of eggs in the breeding places.
This observer has often seen males discharging their fluid

50 THE BIOLOGY OF THE FROG chap.

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 shal-
low 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
entirely accounted for by the tendency of the animals to
seek a similar habitat for breeding. According to Fischer-
Sigwart's observations on Rana fusGci, if only a few pairs
occur in any locality, they get as closely together as possible,
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 lav eggs without

ii HABITS AND NATURAL HISTORY OF THE FROG 51

the embrace of the male has been investigated by Nussbaum1
in Ranafusca. 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 as they are in copulation, but were passed out
slowly, a few at a time, some eggs often being retained until
considerably after the breeding season. In a female killed
late 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 dependent
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,2 in studying the proportion
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. Pfltiger 3 and Griesheim,4 who in-
vestigated the subject with considerable thoroughness, found

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

2 Born, Breslauer artel. Zeitsch, 1881.

3 Pfliiger, Arch.ges. Phys., Bd. 29, 1882.

4 Griesheim, Ibid., Bd. 26, 1881.

52 THE BIOLOGY OF THE FROG CHAP.

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 appearance of
ovaries, and many frogs which would naturally be taken for
females show only male characters at a later period of devel-
opment. 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 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 predomi-
nate 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 appearance
between ovaries and testes. Egg follicles are often found in
the testes of frogs at a later period of development.

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-

Ii HABITS AND NATURAL HISTORY OF THE FROG 53

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.

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 maintain
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 surround-
ings, requires the use of a certain amount of combustible
material for its support. During the summer the frog feeds
voraciously, and when cold weather ensues, its system 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 activities of the
physiological machinery of the body. And in addition to
supplying energy for this purpose, it must afford the sub-
stance 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 follows
the awakening in the spring, the frog is naturally in great
need of food, and it becomes a very voracious feeder. Dur-

54 THE BIOLOGY OF THE FROG chap.

ing 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 betakes
itself to a place of concealment, coming out only at intervals
to obtain food. This period of comparative inactivity 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.

In summer, the frog has food in abundance, and it is less
liable to fall a prey to enemies than during the breeding
period. It can bask in the sun, seek the shade when it is
too warm, or plunge into the water to moisten its skin or
cool its body. It is usually on its guard, however, against
whatever enemies may prey upon it, but from most of these
it frequently has near at hand a ready means of escape. It
is free to enjoy life as best an amphibian can, and to store
up food for passing the winter and for the ripening of the
sexual products for the following spring. To keep alive its
stock, a process which incidentally implies the preservation
of its own body, is the great end of the frog's existence.

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-Sigvvart 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

ii HABIT? AND NATURAL HISTORY OF THE FROG 55

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 Barfurth
carried on several experiments on both old and young tad-
poles, and found that if the legs were cut off from young
specimens these organs would be frequently regenerated,
although slowly. The power of the tadpole to regenerate
missing limbs was found to decrease rapidly with age.

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 400 and 500 below zero
F. and considerably over ioo° F., and yet the tem-
perature of the blood will not vary more than a small
fraction of a degree. 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

56 THE BIOLOGY OF THE FROG chap.

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 mechan-
ism the higher animals are able to keep in an active con-
dition in both winter and summer. With the cold-blooded
animals it is different. Their temperature rises and falls
in correspondence with the temperature of their environ-
ment. In the cold their metabolism is slow, their tem-
perature 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, how-
ever, entirely at the mercy of the environment. Evaporation
from the surface of the body tends to keep the temperature
of the animal in warm weather below that of the surround-
ing atmosphere. And as the weather approaches the
freezing point, the small amount of metabolism in the ani-
mal serves to keep its temperature somewhat above that of
its environment.

The effect of high temperatures on the frog has been
studied by Maurel and LagrifTe.1 At 2 6° to 300 C. the frogs
become active and restless. At 310 to 330 C. they show
evident signs of discomfort. From 340 to 360 C. they jump
about wildly without any apparent sense of direction. At
a temperature of 370 to 390 C. they lose their sense of equi-
librium, and if exposed to a temperature of 390 to 400 C.
they die. If, however, they are exposed only for a short
time at the latter temperature, they may subsequently re-

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

ii HABITS AND NATURAL HISTORY OF THE FROG 57

cover, although they may at first appear as if dead. The
cause of death is probably the coagulation of certain albumi-
nous 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 x found that frogs which
were exposed to a temperature of from — 1° to — 50 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 activity. The
bodily temperature of the frogs in the experiment 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.

Muller-Erzbach 2 performed the experiment of placing
the frog in a dish of water, which was gradually cooled off

1 Knauthe, Zoo/. Am., Bd. 14.

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

58 THE BIOLOGY OF THE FROG chap.

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
afforded to escape from being solidly frozen during the
winter.

It was observed by Knauthe that the color of frogs
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

ii HABITS AND NATURAL HISTORY OF THE FROG 59

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. 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
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, Cryptobranchus.

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
considerable 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 up-

6o

THE BIOLOGY OF THE FROG

CHAP.

right position. According to Verworn,1 the muscles involved
in the righting movements are in a state of tonic contrac-
tion as if these movements were suddenly inhibited. The
breathing movements and the heart beats are at first accel-
erated, but at a later stage their rate falls below the normal
(Heubel), and there is a decreased responsiveness to ex-
ternal stimuli. Dif-
ferent frogs vary
greatly as regards
both the ease with
which they may be
hypnotized, and the
duration of the hyp-
notic state. In some
cases if a frog is sim-
ply placed on its back
without being held,
it may become hyp-
notized after it has
righted itself a few
times, and lie for a
long time in some
phase of the pro-
cess 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

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

FIG. 7. — 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 maintained when the frog is placed on
its back, as is shown in the lower figure.
(Modified from Verworn.)

II HABITS AND NATURAL HISTORY OF THE FROG 61

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, Dume-
ril et Bibron, Diirigen, Fischer-Sigwart, Hay, Gadow, Leydig, Rosel
von Rosenhof, and Spallanzani cited in the references to the preceding
chapter.

62 THE BIOLOGY OF THE FROG chap.

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.

in EXTERNAL CHARACTERS OF THE FROG 63

It corresponds rather to the lower eyelid proper, plus a
nictitating membrane. The latter structure in most animals
in which it occurs is very distinct from both the other eye-
lids. 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 continuation. It is
thinner and more transparent than the lower lid and sepa-
rated 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 commu-
nicates with the inner ear. When the tympanic membrane
is 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

64 THE BIOLOGY OF THE FROG chap.

is slightly movable, and if it be 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 end of 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 in connection with a peculiar
outgrowth of the brain known as the epiphysis or pineal
gland. When the outer or distal portion of this structure
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 ver-
tebrates, including man, in whom it was given by certain
ancient philosophers 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

in EXTERNAL CHARACTERS OF THE FROG 65

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

The two pairs of legs are very different in form as in func-
tion. The fore limbs are short, and consist of three divi-
sions, the upper arm, the forearm, and the manus, or hand.
The hand has four fingers, and the rudiment of an additional
digit on the inner side which can be felt under the 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 digits of the
frog in either pair of limbs. The fore limbs, when the ani-
mal is in a resting position, are held in a peculiar twist ; the
forearm, and to a greater extent the hand, are turned in-
ward, 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 relatively is 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 breeding season is past.

The kind limbs are long and admirably adapted for jump-
ing and swimming, but are of little service in walking, as the
frog can scarcely be said to employ this method of locomo-
tion. Like the fore limbs, the hind limbs are divided into
three parts : an upper portion or thigh, a middle part, cms
or shank, and the J oof, ox pes. The latter is very well devel-
oped and has the ankle remarkably elongated ; there are
five toes and the rudiment of a sixth toe, termed ihe pre-
hallux, which is situated on the inner side of the foot. The

F

66 THE BIOLOGY OF THE FROG chap.

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 articulations
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 be-
tween the toes varies greatly in different species of frogs,
and is a character which is therefore made use of for pur-
poses of classification. In Rana pipiens the web extends
from the last joint of the first toe to the next to the last
articulation of the second toe, then from the last joint oi the
second to the next to the last articulation of the third ; from
the last joint of the third to a little beyond the second joint
of the fourth, and from about the same level on the other
side of this toe to the last joint of the fifth. Different indi-
viduals vary considerably as regards the extension of the
web, especially if they come from different localities, and
the above statements, therefore, must not be expected to
apply to all cases.

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

Ill EXTERNAL CHARACTERS OF THE FROG 6/

the exception of small scattered prominences occurring
mainly on the back and on the dorsal side of the hind legs.
Nothing corresponding to 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 ox 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 entirely
white. The large black pigment spots which occur on the
dorsal side of the body and legs are subject to much varia-
tion 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 to form trans-
verse bands. In addition to the black pigment there are
green and golden colors, which are present in varying
proportions. The changes in color which the skin may
undergo under certain conditions will be discussed in a later
chapter.

68

THE BIOLOGY OF THE FROG

CHAP.

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
l 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. Ex-
ternal to the teeth is a fleshy
fold, or upper Up, and on the
inner side is a groove, the
sulcus marginalise which re-
ceives the lower jaw when it
is closed. Anteriorly this
groove is crossed on each
side by a low elevation, the
pulviriar rostrate; immedi-
ately behind the tip of the
jaw the sulcus is deepened
again, forming the median
sub rostral fossa; on each
side of the pulvinars are the

Fig. 8. — Mouth of the frog widely
opened. E, Eustachian tubes ;
G, glottis ; /, lower jaw ; L, lat-
eral subrostral fossa; M, median
subrostral fossa; A7, posterior
nares ; O, oesophagus ; P, pulvi-
nar rostrale ; S, opening of the
vocal sac ; T, tongue ; tp, tuber-
culum prelingnale.

iv THE INTERNAL STRUCTURE 69

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 mandib-
ular muscles being normally in a state of tonic contraction ;
the tip of the lower jaw is flexible and is capable of being ele-
vated 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 {tuberculum prehnguale) 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, as we
-shall see later, is a necessary feature in relation to the pecul-
iar 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 together. 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 conti-

70 THE BIOLOGY OF THE FROG chap.

nuity with these organs may be shown by passing a bristle
into them or by inflating them by means of a blowpipe.
Posteriorly 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 membrane
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 papillae, of which there are two kinds :
the filiform, which are conical or threadlike in shape, and
the fungiform, which are larger and less numerous 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

iv THE INTERNAL STRUCTURE 71

specimen of R. pipiens I have found as many as six hundred
and forty.

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 esoph-
agus. 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,

7-

THE BIOLOGY OF THE FROG

CHAP.

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
processes of the maxillary and premaxillary bones, to which
they are attached by cement substance. They are embedded
in the mucous membrane of the mouth,
B beyond which they project only for a short
distance. Each tooth is approximately cy-
lindrical in form, tapering slightly toward the
upper end, which is somewhat incurved.
The basal portion of the tooth which is fas-
tened 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
{odontoblasts') are situated that produce
new material for the growth of the tooth.
The greater portion of the crown is com-
posed 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, resistent layer of enamel
which is considerably thickened over the tip. The enamel
shows a stratified structure, but it does not contain the verti-
cal prisms found in higher forms. Outside of the enamel
there is a thin, resistent 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 replace-

FlG. 9. — Teeth of
the bull frog. A,
view of inner
face; B, lateral
face ; c, crown ;
r, root ; /, sec-
tion of lower
jaw; /, pulp
cavity.

iv THE INTERNAL STRUCTURE 73

ment 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
become freed from their attachment to the jaw bone, and
are then cast out. New teeth are produced 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
ventricle, which has a very thick muscular 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 gall duct. Projecting

74 THE BIOLOGY OF THE FROG chap, iv

into the body cavity from in front are two thin, very disten-
sible 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 con-
tract to a very small size.

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
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. The 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 vent. Along its whole
extent the alimentary canal is suspended from the mid-dor-
sal portion of the body cavity by a thin, transparent sheet
of membrane, the mesentery.

In the U-shaped loop between the stomach and intestine
lies an elongated, light-colored organ of irregular shape, the
pancreas. There is a dark red, rounded body, the splee?i,
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

abd.ts

Fig. io. — Organs of a female frog. The alimentary canal has been cut off
at the gullet {gul.) and rectum {ret.), and most of the liver has been re-
moved. The right ovary and fat body are also removed and the ventricle
of the heart has been turned forwards, abd. v, abdominal vein cut and
turned back ; coel. mes, cceliaco-mesenteric artery ; cp. ad, corpus adi-
posum, or fat body; d. ao, dorsal aorta; gul, gullet; /. au, left auricle;
/. lug, left lung; /. ovd, left oviduct; /. ovd' , its opening into the body
cavity; I. ovd", its posterior dilatation or uterus ; /. ovy, left ovary; Ir,
portion of liver ; pt, cv, postcaval vein ; r. au, right auricle ; ret, rectum ;
r. kd, right kidney; r. Ing, right lung; rn.pt, renal portal vein; r. ovd,
right oviduct; r. ovd', its opening into the body cavity; r. ovd", its
posterior dilatation; syst. tr, systemic trunks; u. bl, urinary bladder;
ur, ureter ; v, ventricle. (After Parker and Parker.)

75

76

THE BIOLOGY OF THE FROG

CHAP.

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 con-

d/.iy

SPH.ETH

opt I gl

FIG. ii. — Rana temporaria. Dissection from the left side; the viscera
somewhat displaced, an, anus; b. d, bile duct; b. hy, body of hyoid ;
bl, urinary bladder; bV , its opening into cloaca; c. art, conus arteriosus;
cbhn, cerebellum ; el, cloaca; en. j, centrum of third vertebra; cp. ad,
corpus adiposum ; crb. h, cerebral hemisphere; d. ly. s, dorsal lymph
sinus; du, duodenum; cp. cor, epicoracoid ; ens. t, Eustachian tube;
FR. PA, fronto-parietal ; gl, glottis; gul, gullet; IL, ilium ; IS, ischium ;
kd, kidney; /. an, left auricle; /. big, left lung; //-, liver; M. MCK, mento-
meckelian ; n. a. i, neural arch of first vertebra; olf. I, olfactory lobe;
opt. I, optic lobe ; O. ST, omo- and epi-sternum ; ped, pericardium ; PMX,
premaxilla; pn, pancreas ; p. na, posterior naris ; ///.pubis; ret, rec-
tum ; /-. Ing, right lung; s. int, small intestine; sp. cd, spinal cord; SPH.
ETH, sphenethmoid ; spl, spleen ; si, stomach ; s. v, sinus venosus; Ing,
tongue; ts, testis; ur, ureter; nr' , its aperture into the cloaca; UST,
urostyle ; v, ventricle ; v. ly. s, ventral lymph sinus ; vo. t, vomerine
teeth ; vs. sent, vesicula seminalis.

voluted 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

iv THE INTERNAL STRUCTURE 77

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 ccelom. The walls of the
oviducts are thick and glandular except toward the posterior
end, where they become expanded into thin, very disten-
sible 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
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 attached
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 pos-
terior 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 blow pipe inserted
into the cloaca, it will swell to a large size, and an accurate
idea may be gained of its form and connections.

78 THE BIOLOGY OF THE FROG chap.

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

, u.stsvt** .

. _ ci.cto i I "i scu.ct/.s
iL \^- — Y — ~L^l / y

s.cu.iy.s ^s^^£^^— ^-^rr^C

/~ v.per*

FIG. 12. — Diagram of a cross section of the body of a frog showing the
course of the peritoneum by a dotted line, add. v, abdominal vein ;
d. ao, dorsal aorta; il, ilium; /'///, intestine; kd, kidney; m, m' , muscles
of back and abdomen respectively; vies, mesentery; p. per, p. per' ,
parietal layer of peritoneum; pt. cv, postcaval vein; sk, skin; spy,
spermary or testis; s. cu. ly. s, subcutaneous lymph sac; u. st, urostyle;
v.per, v. per', visceral peritoneum. (After Parker and Parker.)

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 ena-
bling us to get the conditions clearly in mind. The alimen-
tary canal, the liver, the lungs, the gonads, oviducts, bladder,
fat bodies, and other organs are covered with peritoneum,
which usually adheres closely to the surface. The mesen-

iv THE INTERNAL STRUCTURE 79

teries 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 men-
tioned. The peritoneum passes down from the 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 be-
tween 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 body muscles,
but on the dorsal side of the body it is separated from the
wall, forming a large lymph space, the cistema magna, or
subvertebral 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 mentioned, which extends
between 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 pericardium and liver
on the other.

The ccelom is filled with a transparent fluid, the ccelomic
ox 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 coat-
ing, 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 ver-
tebral column and skull and containing the central nervous

8o

THE BIOLOGY OF THE FROG

system. This neural cavity, as we may well call it, extends
farther forward than the coelom, and is separated from the
latter by the bases or centra of the vertebrae. The anterior

portion of the central nerv-
ous system, or brain, lies
in the skull, and is contin-
br ued posteriorly as the spi-
nal cord, which is inclosed
within the vertebral col-
umn. If we make a cross
section through the frog
somewhere near the middle,
we shall find that the body
contains two longitudinal
cavities separated by the
centra of the vertebrae, —
the ccelom 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
ccelom below. And outside
of the muscles, from which
it is separated by large

FIG. 13. — Organs in the neural cavity.
The anterior part of tins cavity con-
tains the brain, which is composed of
the olfactory lobes, olf. I; the cere-
bral hemispheres, crb. h ; the di-
encephalon, diet? ; the optic lobes,
opt. I; cerebellum, cblm ; and me-
dulla oblongata, med. obi. n. c, neu-
ral canal; sp. cd, spinal cord ending
in the filum terminale, f. t; e, eye.
(After Parker and Parker.)

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.

THE DEVELOPMENT OF THE FROG

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 cilia 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 sur-
rounded 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 numer-
ous 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 even the
simplest animal seems almost a miracle, and our wonder is
only increased the more we attempt to understand how and
why the process takes place.

82 THE BIOLOGY OF THE FROG chap.

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 development 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 ox preformation, was championed by such men
as Haller, Leibnitz, Bonnet, and Spallanzani ; it really
amounted to a denial of development. What appears to be
such, according to this interpretation, is simply growth, the
expansion of something preformed. The Abbe Spallanzani,
who made extensive and most excellent studies on the breed-
ing habits of Amphibians, considered the eggs of these ani-
mals to be small embryos of tadpoles whose parts through
the influence of the spermatic fluid were stimulated to growth
and activity. Other writers, among whom C. F. Wolff occu-
pies the most prominent place, regarded the egg as a mass
of simple unorganized material which becomes more and
more complex as development 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 epigensis or evolution 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 evolution, and
started investigators on the road to a truer conception of
development. A farther step in advance was made when

V THE DEVELOPMENT OF THE FROG 83

it was ascertained that the egg is a single cell. Not many
years afterward it was discovered that the spermatozoon is
likewise a single cell, and that in fertilization there is a union
of the nuclei of the ovum and spermatozoon, each contribut-
ing an equal share of chromatin to the nucleus of the fer-
tilized egg, and thence to all the cells of the body of the
embryo. The whole aspect of the problem of development
is very different from what it appeared to the older natural-
ists. However we may regard the modern forms of the
doctrines of evolution 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 multi-
tude of cells of great variety of form and function. This
cell may be enormously complex, containing somehow fea-
tures which represent all the different organs of the body, or
it may be comparatively simple in structure, and the differ-
entiations appearing in the embryo may be the results of the
interactions of its parts and the influence of external condi-
tions. Evolution 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 func-
tion of this jelly is primarily the protection of the eggs. It
keeps them free from dirt, bacteria, and the spores of fungi,

84 THE BIOLOGY OF THE FROG CHAP.

and also from the attacks of aquatic insects and snails, and
various mechanical injuries which would otherwise affect
them. Bernard and Bratuschek1 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 entrance
to the sun's rays, but checking radiation from the egg. The

FIG. 14. — Egg in jelly. (After Schultze.)

heat waves that radiate from the eggs are longer, less refran-
gible,-and pass through the jelly less readily than the shorter
waves, which are abundant in the direct heat of the sun.
Bernard and Bratuschek 2 found in experimenting upon the
jelly that the greater the wave 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

1 Bernard und Bratuschek, " Der Nutzen der Schleimhiillen fur die
Froscheier," Biol. Centrlb., Bd. n, 1891. 2 Ibid.

V THE DEVELOPMENT OF THE FROG 85

property required to keep the eggs warmer than the sur-
rounding 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-Sigwart
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 spermato-
zoa 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 resistance 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
discharge from the ovary are surrounded by a very thin vitel-
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

86 THE BIOLOGY OF THE FROG chap.

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
light-colored spot near the center of the dark cap. The
great 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 albuminous 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 pre-
liminary steps of the process occurring just before the egg
leaves the ovary. The large watery nucleus shrinks, the
chromatin becomes aggregated into definite bodies or chro-
mosomes, a spindle forms at right angles to the surface of the
egg, and half the chromatin, with a small amount of cyto-
plasm, is extruded as the first polar body.

v THE DEVELOPMENT OF THE FROG 87

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, and in the second division, as in
the first, half the chromatin material of the egg is extruded.
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 between the egg and the vitelline
membrane. If the jelly is removed from a freshly laid egg,
and the light spot, or fovea, 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. Whether
the chromosomes in the matured egg are double, so that the
reduction in number is only apparent, or whether there is an
actual loss of half of the chromosomes is a question still
under discussion.

Fertilization. — The act of fertilization consists in the union
of two cells, the ovum from the female and the spermatazoon
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, elon-
gated body consisting of a narrow, pointed head formed
mainly of the nucleus, a short middle piece just behind the
head, and a long, very slender tail. As before described,
the seminal fluid of the frog is shed over the egg masses as
they are extruded into the water from the body of the female.
The spermatozoa which swarm in the seminal fluid are active,
nnd swim about by the movements of the tail, working their
•vay through the jelly of the egg mass until one comes in
contact with an egg. Then the spermatozoon slowly pene-

ss

THE BIOLOGY OF THE FROG

trates the egg substance. The entrance of one spermato-
zoon 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 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 pro-
cess of maturation 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 pro-
nucleus, or the nucleus of the matured ovum.
Sper- The male and female pronuclei approach
and finally fuse into one, which is called the
copulation nucleus. The number of chromo-
somes 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

Fig. 15

matozodn of
Nana esculen-
ta. (After La
V a 1 e 1 1 e St.
George.)

v THE DEVELOPMENT OF THE FROG 89

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 doubtless
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 fertili-
zation the eggs lie in all possible planes, but soon after
fertilization they 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 accompanied
by a karyokinetic division of the nucleus, so that each daugh-
ter cell contains a nucleus derived from the copulation
nucleus of the fertilized egg ; and in all subsequent 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

go

THE BIOLOGY OF THE FROG

G H

FlG. 16. — Segmentation of egg. (After Schultze.) A, two-cell stage,
with the beginning of the second furrow ; B, eight-cell stage, showing
the cross furrow at the animal pole; E, eight-cell stage; C, D, F, G,
sixteen-cell stages, showing variations in the plan of cleavage ; //, thirty-
two-cell stage. (From Morgan's "Development of the Frog's Egg.")

V THE DEVELOPMENT OF THE FROG 91

rarely 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 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 furrows run
nearly vertically and hence at right angles to the third cleav-
age 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 arrangement of
the cells. The fourth cleavage furrows are subject to more
variation at the vegetal pole of the egg, and 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
v^'viaUy follow the rule that each cleavage plane comes in at
right curgi'e-; to the previous cleavage plane. Deviations
from the typical method of cleavage are apparently of little
moment, even in the first few divisions, as such abnormally
dividing eggs may nevertheless produce perfect embryos.

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

92

THE BIOLOGY OF THE FROG

CHAP.

B

\ ■ • y

H

Fig. 17. — Segmentation of the egg and formation of the blastopore. A,
eight-cell stage seen from one side; />', beginning of sixteen-cell stage;
C, thirtv-two-cell stage; D, forty-eight-cell stage; E, F, and G, succes-
sive later stages of cleavage ; H, beginning of the blastopore in the form
of a small crescent ; /, circular blastopore on the vegetative side of the
egg. (After Morgan.)

v THE DEVELOPMENT OF THE FROG 93

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 laver of cells in thickness.

FlG. 18. — Vertical section through the blastula of a frog in different stages.
B, segmentation cavity or blastoccel. (After Marshall.)

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 essentially 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, according to
a well-worn illustration, by pushing in one side of a hollow
rubber ball with the finger. The mouth of the gastrula is
called the blastopore, and this opening naturally becomes

04

THE BIOLOGY OF THE FROG

CHAP.

smaller as the process of in-pushing is completed. The pro-
cess 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 accumulation of yolk at
the vegetal side of the blastula prevents the invagination of
this region from taking place in the typical way. The same

FlG. 19. — Sagittal section through a frog embryo. B, blastoccel or segmen-
tation cavity ; DP, lip of blastopore ; EE, outer or epidermic layer of
ectoderm; EN, inner or nervous layer of ectoderm; Y, yolk cells.
(After Marshall.)

end is reached partly by a process of in-pushing and partly
by the overgrowth of the white pole by the dark. The in-
pushing and overgrowth take 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

V THE DEVELOPMENT OF THE FROG 95

thins out toward the edges, which gradually extend around
the lower pole of the egg. In this way the crescent becomes
converted into a 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.

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 '9 1,1 Assheton '94/ Morgan '97-3) As the archenteron
increases in size, the blastoccel or segmentation cavity neces-
sarily becomes smaller. According to Marshall the former
breaks through into the latter, and the two form one cavity.

The Germ Layers. — The formation of the gastrula pro-
duces a two-layered embryo, each layer being several cells

1 Robinson and Assheton, Quart four. Mic. Set., VoL 32, 1891.

- Assheton, Ibid., Vol. 37, 1894.

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

96

THE BIOLOGY OF THE FROG

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

Y DP'

FIG. 20. — Sagittal section through a frog embryo. B, blastoccel ; BP,
dorsal lip of blastopore ; BP' , ventral lip of blastopore; EE, epidermic
layer of ectoderm ; EN, inner or nervous layer of ectoderm; //.hypo-
blast or entoderm; T, mesenteron or gastrula cavity; Y, yolk plug.
(After Marshall.)

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-
derm on either side are brought near each other in the mid-
dorsal line. The free ventral edges of the masses or sheets

THE DEVELOPMENT OF THE FROG

97

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
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 be -inning of the cos lorn, or

FlG. 21. — Transverse section through the middle of a frog's embryo. CH,
notochord ; E, ectoderm ; M, mesoderm ; AG, neural groove ; NP,
neural plate ; T, mesenteron ; Y, yolk cells. (After Marshall.)

body cavity. It is at first small, but as development proceeds,
it widens out more and more.

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

,,s

THE BIOLOGY OF THE FROG

vertebral column, and occurring in the embryo, when not
also present in the adults, of all vertebi-te animals. It

NO

FIG. 22. — Transverse section through a frog embryo before the closure of
the medullary or neural folds. C, ccelom or body cavity; CH, noto-
chord ; EE, epidermic layer of ectoderm ; EN, nervous layer of ecto-
derm ; M, mesoderm ; ME, outer or somatic mesoderm ;. A/H, inner or
splanchnic mesoderm ; NC, neural groove ; ND, dorsal root of spinal
nerve ; NS, spinal cord ; T, archenteron ; W, liver diverticulum ; Y,
yolk. (After Marshall.)

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
evolution of the race. Whether in the frog it is entodermic

V THE DEVELOPMENT OF THE FROG 99

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 l
has recently studied the formation of the notochord in Bufo
lentiginosus and Rami palustris, and has come to the con-
clusion 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.

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
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
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 folds
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 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 medul-
lary tube is completed it is constricted off from the ecto-
derm above, and the latter becomes continuous over the
mid-dorsal line. Subsequently it develops into the brain
and spinal cord of the embryo.

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

IOO

THE BIOLOGY OF THE FROG

CHAP.

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

FlG. 23.— Development of the embryo. A, yolk-plug stage; B, showing
the medullary folds, the blastopore nearly closed, and below the latter
the invagination which is to form the anus; C, D, later stages; E, the
medullary folds have grown together and covered the blastopore. Above
the anus is the rudiment of the tail. (From Morgan, after Ziegler.)

of thickenings of the ectoderm. The anterior members of
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

THE DEVELOPMENT OF THE FROG

IOI

tadpole. Subsequently these depressions meet in front and
become converted into a U-shaped groove. In the poste-
rior pair of plates, the gill plates, there appear two 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 break through until after

FIG. 24. — Embryos. Gp, gill plate; Gs, Gs', two gill slits ; S, suckers ; Sp,
sense plate ; An, anus. (From Morgan, after Schultze.)

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 communicate 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

102 THE BIOLOGY OF THE FROG chap.

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 be-
comes flattened from side to side, and its upper and lower
edges become produced into a thin expansion, or tail
fin.

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 devel-
oped on either side an invagination or pit of the ectoderm,
which 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.

v THE DEVELOPMENT OF THE FROG 103

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 medul-
lary 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 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
midbrain.

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.

104 THE BIOLOGY OF THE FROG chap, v

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 tha-
lamic 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. It
comes into close contact with another structure, the pituitary
body, which is developed from the ectoderm of the dorsal
wall of the stomodeum. The sides of the thalamencephalon
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 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.

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.

B

FIG. 25. — Sagittal sections through two embryos. In A the blastopore is
overarched and there is the beginning of the proctodeum or anal invagi-
nation. 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 ; PH, pharynx; PN, pineal body; FT, pituitary body.
(From Morgan, after Marshall.)

105

io6

THE BIOLOGY OF THE FROG

CHAP.

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

Fig. 26. — Cross section of a frog embryo. A R, archenteron ; MS, meso-
blastic somites ; AT, notochord ; NS, neural crest ; M, medullary tube;
PR, pronephros ; SNt subnotochordal rod ; SO, SP, somatic and splanch-
nic mesoderm. (From Morgan, after Marshall.)

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

v THE DEVELOPMENT OF THE FROG 107

arise as outgrowths from it. The first of these to be formed
is the liver, which at the beginning appears as an outpocket-
ing of the ventral side near the anterior end. The out-
pocketing becomes folded and branched, being converted
finally into a number of clusters of tubules, all emptying
into the common 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 diver-
ticula become the secreting cells of the liver. The connec-
tive 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
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

chap, v THE DEVELOPMENT OF THE FROG 109

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-
mandibular 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
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-

no THE BIOLOGY OF THE FROG chap.

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
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 ccelom. The inner or splanchnic layer becomes closely

v THE DEVELOPMENT OF THE FROG in

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 ccelom arise inde-
pendently 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 of 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-
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,

H2 THE BIOLOGY OF THE FROG chap.

and the two sides of the pericardial cavity become continu-
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 coelom. Owing to

Fig. 28. — A, B, C, three stages in the development of the heart. E, endo-
thelium ; PE, pericardium ; PH, pharynx; W, wall of heart. (After Mor-
gan.)

its increase in length the heart becomes bent in the form of
an S ; anteriorly it becomes continued into the truncus
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

v THE DEVELOPMENT OF THE FROG 113

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 ccelom.
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 (nep/iros tomes) 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 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

1 Field, Bull. Mus. Comp. Zool. Harvard, Vol. 21, 1891.
I

ii4

THE BIOLOGY OF THE FROG

FIG. 29. — Horizontal section through an advanced embryo. AR, archen-
teron; BR\BR*, BR*, branchial arches; H\H*,H*%g\\\ slits; HB,
hyoidslit; HM, hyomandibular cleft; J/Y, hyoid arch; IN, infundibu-
lum ; OF, olfactory pit; OS, optic stalk; P, pronephros; S, segmental
duct. (From Morgan, alter Marshall.)

v THE DEVELOPMENT OF THE FROG 115

later open into the ccelom by funnel-shaped ciliated mouths
or nephrostomes, 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 out-
let of the pronephros is worked in to form the Wolffian duct
or ureter of the adult. The Mtillerian duct was formerly
supposed to arise by a splitting of the segmental duct, but
according to MacBride } Marshall,- 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 1 Marshall).
As the genital ridges increase in size they become con-
stricted at their points of attachment, and finally hang
supported by a peritoneal membrane. In the male the
testis becomes connected 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 por-
tion of the backbone of the adult frog. Loose mesodermic
cells, or mesenchyme, produced from the periphery of the

1 MacBride, Quart. Jour. Mic. Sci., Vol. 33, 1892.
- Marshall, " Vertebrate Embryology."

3 Gemmill, Arch. f. Atiat. u. Phys., Phys. Abth., 1897.

4 Hall, Bull. Mus. Covip. Zool. Harvard, Vol. 45, 1904.

ii6 THE BIOLOGY OF THE FROG chap, v

somite, collect around the notochord, forming a tubular in-
vestment. t From the dorsal side of this mass ridges or folds
grow up and surround the spinal cord. The mesoderm cov-
ering 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 cartilaginous,
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 within 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

Fig. 30. — Metamorphosis of Fana temporaria. 1, tadpole just hatched ; 2, 3, successively
older tadpoles seen from one side; 4, a slightly older tadpole seen from the aorsal side; 5,
a still older specimen from below; 6, tadpole with the gills covered, leaving only. a small
opening on the left side; 7, first indication of hind legs; 8 and 10, successively older stages;
g, specimen with the ventral bodv wall removed, showing the coiled intestine and gills; 11,
both pairs of legs free ; 12, 13, 14, successive stages in the resorption of the tail; 15, adult
frog. A, anus; HE, hind leg; A", gills; KL, gill opening; M, mouth; N, nasal opening;
O, eye; SiY, ventral sucker. (From Weysse's "Synoptic Text Book of Zoology," after
Leuckart and Nitsche.)

117

n8 THE BIOLOGY OF THE FROG chap.

pairs of internal gills, which are produced by foldings of the
membrane lining the gill slits. Both external and internal
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

v THE DEVELOPMENT OF THE FROG 119

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
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.

Toward 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 ;
animal 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 con-
tained 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. A
general elementary description of the development of the frog, based
mainlv on the w< >r; of Marshall, is contained in Reese's " Introduction to

120 THE BIOLOGY OF THE FROG chap.

Vertebrate Embryology." Lists of the most important literature on the
subject will be found in the works of Marshall and Morgan just referred
to. The following papers deal with the characteristics and metamor-
phoses of tadpoles : —

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

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. Mist., 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.

HISTOLOGY OF THE FROG

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 kinds of epithe-
lium are distinguished according to the shapes of the cells.

122 THE BIOLOGY OF THE FROG chap.

An excellent example of flattened or squamous epithelium
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 the colum-
nar epithelium the cells are elon-
gated perpendicularly to the sur-
face and are usually prismatic in
outline, owing to mutual pressure ;
such epithelium is common in the
fig. 31. — a portion of the mucous layer of the intestine. In
epidermis of Ranapipiens. many piaces, as in the outer skin,

s, stoma cell. . . „ ...

there may be all transitional stages
between columnar epithelium and squamous epithelium.
Layers such as this which are several cells deep are called
stratified epithelium.

In some parts of the body there occurs a peculiar variety
called ciliated epithelium in which the cells are furnished
with cilia at their outer ends. Usually such cells are colum-
nar, but they may be cuboid or even somewhat flattened.
Ciliated epithelium occurs in the mouth and throat of the
frog, in certain 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,

vi HISTOLOGY OF THE FROG 123

and the effect of this common movement is to create a
current which carries small objects away from that region.
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 back-
ward 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

[24

THE BIOLOGY OF THE FROG

CHAP.

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

yield gelatin when
boiled ; they fre-
quently branch, and
when cut across, the
ends do not curl like
those of the white
fibers. Imbedded
in spaces of the ma-
trix here and there
are the connective
tissue corpuscles or
cells. These cells
vary considerably in
their form and in
the appearance of
their cytoplasm ;

FIG. 32. — Fibrous connective tissue from the USuallv thev are
frog, c, connective tissue corpuscles; e, elas-
tic fibers; w, white fibers. (After Parker and branched, and the

Parker.) branches of neigh-

boring cells often unite or anastomose, forming an irregu-
lar network, the meshes of which are filled with the inter-
cellular substance. These processes of the cells run in
canals which allow a circulation of the fluid among the spaces
or lacunae in which the cells lie. White fibrous tissue varies
greatly in consistency and texture in different parts. The
loose tissue binding the muscles together is called areolar
tissue, and is composed of sheets and strands intersecting
each other in all planes. It forms a coating or fascia for
each muscle, and toward the ends of the muscles it is
frequently -modified into tendon which is very dense and

vi HISTOLOGY OF THE FROG 125

inelastic, and mainly composed of fibers, all of which lie in
one direction. The loose tissue of lymphatic glands belongs
to a variety called adenoid, which is composed of an irregular
network of sheets and strands forming a fine meshwork which
supports the 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 pellicle of protoplasm surround 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 connective tissue.
In the clear hyaline cartilage which is the predominant variety
in the frog, the matrix appears transparent and homogene-
ous, 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, scat-
tered 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 the cells
separated by cleavage soon form a partition between each
other which gradually increases in thickness and presses the
cells farther and farther apart. The outer surfaces of car-

126 THE BIOLOGY OF THE FROG chap.

tilages are covered by a layer, or perichondrium, which
consists of an outer fibrous membrane, below which are con-
nective tissue corpuscles, which, as the cartilage grows, sink
into the matrix and become transformed into ordinary car-
tilage cells. Hyaline cartilage occurs at the ends of the
es bones of the limbs, be-

** p( f$r^ tween the vertebrae and

gp vj xi$'?Q at the ends of their trans-

™@7' -^9 /S«®7§) verse processes, at the

' ,£% (*'/*. ™' ^ tip of the urostyle, in the

rf^® 0 , 9 '<$&'' &~c pubis of the pelvic gir-
T^S" ^0) '®® >■■'.'*■ dle,inthehyoidandthe

® % 8\ ^f§> 0Q—-~cs cartilages of the larynx,
' /. •.,:■•**«? and at both ends of the

° sternum ; it forms the
FIO. 33— Cartilage from the head of the basig f he cranium and
femur, c, cells ; c , cells in process of di-
vision ; c. s, empty cell space ; m, matrix, the central axis of the
(After Parker and Parker.) lower iaw

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-
ever, 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, —

'.•I

HISTOLOGY OF THE FROG

127

compact bone, which is very firm and dense, and spo?igy or
cancellous bone, which is made up of plates and bars forming
a structure which is comparatively loose and lacking in
strength. The latter is found within the center of the verte-
brae and to a small extent within some of the long bones. A
good example of
compact bone
may be obtained
by making a cross
section of the fe-
mur. The central
part of the bone is
hollow and filled c--/"> '■ _y ":" * ';* ''^ ^ / 5*S«?W

with marrow, and ^ ' * / * ' , „"'

the outer surface V ^ *• ••■■" ' &***?^\

is covered by a , • . ' ' , , — ' -

layer of perioste- , ''- '*' ■

urn, which is simi- ** /

lar in structure to -' ~s

the pericondrium Fig. 34. — A part of a cross section of the femur of
surrounding the thefro^ '.«*■««*!
cartilage. The
bony substance is arranged in concentric layers, or lamella,
which contain numerous lacuna, in which lie the bone cells.
From the lacunae fine branching tubes, or canaliculi, contain-
ing processes from the bone cells, are given off which extend
in all directions and anastomose with the canaliculi of neigh-
boring spaces.

Bones increase in thickness by the addition of successive
layers to the outside. The osteoblasts, or cells forming the
inner layer of the periosteum, give rise continually to new
bone cells which cause the deposition of new layers of bony
substances between the periosteum and the old bone. New

lc, lacunae; Im, lamellae;
(After Parker and Parker.)

[28

THE BIOLOGY OF THE FROG

CHAP.

layers may also be added from within by a layer of cells lin-
ing 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 spindle-shaped, with a
single nucleus near the center,
which is usually elongated in the
direction of the fiber. The ends of
the fibers are sometimes branched,
but they are more commonly en-
tire. The length of the unstriated
muscle fibers varies greatly ; they
may be very narrow and attenu-
ated, as in the walls of the bladder,
or short and comparatively thick,
as in the walls of the smaller blood
vessels. While the fibers usually
show no cross striation, the cyto-
plasm shows delicate longitudinal
strands, or fibrillce, which are con-
sidered 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 movement.
It occurs in the muscular coats of the alimentary canal, in
the walls of the blood vessels and of many ducts, in the

nu

Fig. 35. — Unstriated muscle
fibers from the intestine of
the frog, nu, nucleus. (After
Howes.)

VI

HISTOLOGY OF THE FROG

129

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 by a
small amount of
unmodified cyto-
plasm. There is a
thin, but well-de-
fined, cell wall, or
sarcokmma, which
is best seen in
places where the
contents of the
fiber are crushed
or broken apart.
Each fiber of vol-
untary muscle is to
be regarded as a
single cell, with nu-
merous nuclei scattered about through its cytoplasm. In
its early stages of development a voluntary muscle cell pos-
sesses but one nucleus. As the fiber grows, the nucleus
divides repeatedly, but as the cytoplasm does not divide at
the same time, there come finally to be numerous nuclei
within the limits of a single cell wall. The cytoplasm shows
both a longitudinal striation, and a cross striation consist-

K

Fig. 36. — 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; dt
dark bands ; n, nuclei ; s, sarcolem ma showing
more clearly where the fiber is broken. (After
Parker and Parker.)

130 THE BIOLOGY OF THE FROG chap.

ing of alternate light and dark bands. The longitudinal
striation is due to the existence of minute strands, the sar-
costyles or fibrillce, which extend the length of the cell, The
fibrillar, which are supposed to represent the contractile ele-
ments of the fiber, are separated by a semi-fluid substance,
the sarcoplasm. There is an arrangement of the fibrillar
into bundles, the muscle columns, which are separated from
each other by a thicker layer of sarcoplasm than that be-
tween the fibrillae.

The appearance of cross striation is brought about by the
division of the fibrillae into segments, or sarcoi?ieres. The
sarcomeres are separated from each other by a very fine
dark line known as Krause's 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 con-
tracted state. The dark bands of the muscle fiber are
composed of material which is anisotropic, or doubly refract-
ing, while the lighter areas on either side of Krause's mem-
brane 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 ap-
pearance 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.

vi HISTOLOGY OF THE FROG 131

Each of the fields shows a dotted appearance, due to the
cut ends of the individual fibrillar.

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 gang/ion cells. Each nerve is composed of
usually a large number of nerve fibers, held together by con-
nective nerve tissue and surrounded by a common sheath.
A typical nerve fiber 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 two
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 pro-
cess of a ganglion cell, and under high magnification is
found to be made up, much like a muscle cell, of very fine
fibrillae, 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

I32

THE BIOLOGY OF THE FROG

CHAP,

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 out-
growth of a nerve or ganglion cell and is always of ectoder-
mic 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

ABC

Fig. 37. — Nerve cells and fibers of M 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 cylinder;
ax.p, axis cylinder process of ganglion cell; I.S, incisure of Schmidt;
m.s, medullary sheath ; n, nucleus ; til, neurilemma; N.R, node of Ran-
vier; p.p, protoplasmic process of ganglion cell.

surrounded with nucleated cells which flatten out and form
the neurilemma ; the white substance appears at a compara-

VI HISTOLOGY OF THE FROG 133

tively late period. The cells forming the sheath of a nerve
are 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
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 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 fibrillar of the nerve
fiber and other processes of the cell. Two kinds of pro-
cesses are commonly distinguished : the axis cylinder pro-
cess, which acquires a sheath and forms a part of a nerve
fiber ; and the protoplasmic processes, often several in
number, 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.

134 THE BIOLOGY OF THE FROG chap.

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 proteids. These are complex
substances containing carbon, oxygen, hydrogen, and nitro-
gen, and, in many cases, also sulphur, phosphorus, calcium,
potassium, sodium, iron, and occasionally other elements.
Living substance, or protoplasm, is of proteid 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 sub-
ject 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 resolved into simpler sub-
stances are known as katabolism. If an organism grows, it is
evident that the anabolic side of the process must predomi-

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 135

nate over the katabolic. If katabolism 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 is
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 the same kind
of food material after it has been eliminated from the
organism, we should find the latter to be much less in

136 THE BIOLOGY OF THE FROG chap.

amount. If now we could measure the energy expended by
the organism by radiating heat and performing work during
the time this 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 phe-
nomena 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.

In order that food material may be assimilated or built
up into the tissues of the bodies, it must be rendered solu-
ble, so that it can pass through the lining of the alimen-
tary 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 contrac-
tions 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 produced in food
by the action of the various digestive fluids. These changes
are mainly of the nature of fermentations caused by sub-
stances 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 associa-
tion with other substances, but it is probable that they are

vn THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 137

some form of proteid. They have the property of causing
chemical changes in other bodies without suffering any, or
at least but very little, destruction of their own substance.
A very minute amount of enzyme will cause the fermenta-
tion 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 ioo° C. destroys the action of ferments entirely.

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

The proteids 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 proteid. 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 ; in other words, there are twice
as many atoms of hydrogen as of oxygen in each carbo-
hydrate molecule. Sugar and starch are examples of this
class of food.

All of these classes of food are acted upon by specific
ferments, 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.

'38

THE BIOLOGY OF THE FROG

CHAP.

The Esophagus and Stomach. — The esophagus is' very
short and remarkably distensible, as is proven by the rela-
tively large animals the frog is capable of swallowing. The

inner surface is thrown into
longitudinal folds which ex-
tend also into the stomach.
There is no sharp line of
demarcation separating the
_M esophagus from the phar-
ynx on the one hand and
from the stomach on the
other. The anterior end
of the stomach is consider-
ably wider than the esopha-
gus, and the organ tapers
gradually to the posterior
or pyloric end, where it is
separated by a constric-
tion, the pylorus, from the
small intestine. The stom-
ach 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 peritoneum, the mes-
ogaster, and from the ven-
tral side arises a second
sheet of peritoneum (the
gas tro-hepato-ditodenal liga-
ment), which extends to the
duodenum and liver. The
wall of the stomach is much

i

Fig. 38. — Alimentary canal of Rana es-
culenta. A, opening of the rectum into
the cloaca, CI; Du, duodenum ; D,
ileum ; f, boundary between the lat-
ter and the large intestine, R ; ///?,
urinary bladder ; M, stomach ; Mz,
spleen ; Oe, esophagus ; Py, pylorus.
(After WiediTsheim.)

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 139

thicker than that of the esophagus or the intestine. The
inner surface is thrown into several longitudinal folds, which
become less prominent posteriorly, and near the pyloric end
entirely disappear.

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, consisting
mainly of connective tissue. This layer has been frequently
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), on the other hand,
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,
according 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 mucosa, 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 invagination of the epithelium lining
the inner surface of the stomach. They are elongated
tubular structures set very closely together, -and frequently

140

THE BIOLOGY OF THE FROG

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 the outer end of the gland the cells are more elongated,
like those of the surface epithelium : behind the clear sub-
stance the cytoplasm of the cells is granular, the nucleus is

Fig. 39.

•Glands of the stomach. A, from cardiac end; B, from pyloric
end ; m, mouth ; n, neck ; b, body of gland.

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 Targe cells con-
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

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 141

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.

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. Occasion-
ally there may be a few polygonal granular cells below
these. In general, however, the pyloric glands may be said
to correspond 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
granular appearance much like the corresponding cells of

142 THE BIOLOGY OF THE FROG chap.

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,
and it contains also a ferment, pepsin, which acts upon the
proteids, converting them into soluble peptones. Neither the
fats nor the carbohydrates undergo digestion in the stomach.
By digesting out the proteid 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 to practically obliter-
ate the central canal. The contents of the cell are uniformly

THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 143

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 gran-
ules they contain
become distinctly
smaller, and the cell
substance stains
jnore deeply. . . .
At the period of
maximum change
the nucleus is much
larger compared
with the cell sub-
stance than it is dur-
ing rest ; it is still
surrounded by fine-
ly granular proto-
plasm, and it is
sometimes placed
near the outer bor-
der of the cells. The
return to the normal
appearance begins
about the fifth hour,

A

Fig. 40. — 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 indistinct and the granules are
scattered throughout the cells. B, gland three
hours after a meal; the granules have disap-
peared 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
Langley.)

so that during the

greater part of the digestive period the formative processes
go on whilst the secretory are still active. In twenty-four
hours the glands have nearly or altogether returned to the
hungry condition." The time and extent of the changes
produced in the glands were found by Langley to vary

144 THE BIOLOGY OF THE FROG chap.

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, 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 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 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 moderately pro-
longed fast. The minimum amount of mucigen is contained
by these cells twelve to eighteen hours after a heavy meal ;

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 145

it is then only with difficulty that the mucous can be dis-
tinguished 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
digestion proceeds, and the whole cell grows smaller. " As
the outer zone increases, the granules in the inner end
become smaller. The diminution in the size of the granules
is very marked in cells in which the outer zone takes up tts
larger part of the cell. . . . Nothing very definite can be said
as to the time after feeding at which the changes in the
esophageal glands occur. When frogs are taken as nearly
as possible alike, and they are treated in the same way, then
the results obtained correspond very closely ; but when such
results are compared with those obtained from frogs which
are older or younger, more or less healthy, or when different
amounts of food are given, then considerable divergences
occur."

" The changes occurring are in each case of the same
nature, but the extent to which these changes take place
varies largely. Hence any estimation made of the time
taken for the first appearance of a clear zone, for its maxi-
mum development, and so on, can only be approximate."

" 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

146 THE BIOLOGY OF THE FROG chap.

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
recovery 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.
Similar changes take place in the cells 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.

Nussbaum has found that a direct stimulation of a partic-
ular part pf the mucous membrane of the esophagus causes
a disappearance of the granules from that region. The coll-
usions of Nussbaum that in normal hungry frogs the cells of
the esophageal glands have an outer clea/ zone, and that
after feeding there is an increase instead of a decrease of
granules, were probably drawn from unhealthy specimens.
Sluggish and unhealthy frogs often show glandular cells with
an outer clear zone, but lively and vigorous specimens have
the cells filled with granules. Hungry frogs with foreign
bodies in the stomach, such as bits of leaf or other objects
swallowed with the food, often show a decrease in the granu-
lar content of the gland cells, owing to the irritation thus set
up. According to Griitzner there is a preliminary increase in
the granules in the esophageal glands for a short time after
feeding, and then a marked decrease, but Langley was able

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 147

to obtain no decisive evidence of such preliminary increase
in granulation although he was not disposed to deny that it
might take place at least to a slight extent.

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
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
has 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, mid-
dle, and pyloric end of the stomach, and the pepsin content
of each estimated, the power of converting proteid was much
the greatest from the piece from the esophagus, and became

i4S THE BIOLOGY OF THE FROG chap.

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 earth-
worm was digested in somewhat less than twenty-four hours,
but if several worms were given, they do not disappear from
the stomach until a longer period. It is very probable that
the temperature of the body is an important factor in deter-
mining the rate of digestion, but I am acquainted with no
observations to that effect.

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 backward
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 intestine, 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 duodenal portion is connected to the
liver and stomach by the previously 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 ?nuscle fibers ; then comes a

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 149

thicker layer of circular muscle fibers, and within this the
submucosa ; the latter is a 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
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 con-
vince 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 due to minute rods. Ac-
cording to Bizzozero the mucous cells do not arise from the
transformation of cells of the ordinary type, as Paneth main-
tains, 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.

150

THE BIOLOGY OF THE FROG

CHAP.

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).

rpr^'

H bl

Fig. 41. — Pnrtof 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.)

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
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,

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 151

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
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
duct into which its ducts enter. Of these there is a princi-
pal 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 externally
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, which causes a splitting of
fats into fatty acid and glycerin ; amylopsin, which converts
starch into sugar ; and trypsin, which converts proteids into

152

THE BIOLOGY OF THE FROG

CHAP.

peptones. The latter differs from pepsin in that it acts in an
alkaline or neutral medium ; in a strongly acid medium its
action is entirely 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 or-
gan is of a dark red-
dish color, and is
divided into a right,
a left, and a middle
lobe. The middle lobe
is small and con-
cealed from view by
the heart. The left
lobe is divided by an
oblique incision into
an anterior and a pos-
terior portion, the lat-
ter occupying the
middle of the poste-
rior part of the liver.
The greater por-
tion of the liver is
covered by a closely
adherent layer of per-
itoneum, which is
continued to form at-
tachments with t h e
pericardium, ventral
body wall, dorsal body wall, and the stomach and intestine.
The bile duct is formed by the confluence of the hepatic

FlG. 42. — Liver and pancreas of frog. Dc,
common bile duct; Dcy, cystic ducts; DA,
DA1, hepatic ducts, which with the cystic
ducts combine to form the common bile
duct; G, gall bladder; L, L\ LP, D\ lobes
of the liver turned forwards ; Lhp, hepato-
duodenal ligament; M, stomach; P, pan-
creas ; P1, pancreatic ducts entering the
common bile duct; Py, pylorus. (After

• Wiedersheim.)

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 153

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 outline, and usually appears
green from the color of the bile seen through its thin walls.
The gall bladder is connected with the cystic 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
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 to much obscure
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 proteid gran-
ules, 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 portal vein, 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.

154 THE BIOLOGY OF THE FROG chap.

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. Ponfick and Leonard
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.

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
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 ceils 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 behavior
of similar granules in other gland cells, Langley considers
the granules in the liver to be concerned in the secretion of
bile. Lahousse finds that granules disappear from the cell

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS

55

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

Fig. 43. — Three phases of the hepatic cells of the frog. A, cells rich in
glycogen taken from a frog during winter. There are numerous pro-
teid granules around the lumen, and several larger fat globules toward
the outer ends of the cells. B, cells poor in glycogen taken from a win-
ter frog that had been kept for ten days at a temperature of 220 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 longtime in summer. The cells
are shrunken and the glycogen has almost disappeared. (From Foster's
Physiology, after Langley.)

hour after feeding the congestion has disappeared, 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

156 THE BIOLOGY OF THE FROG chap.

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 com-
position. Some of its constituents, such as the fatty sub-
stance, cholesterin, and the bile pigments, are simply waste
products, but others play a certain part in digestion. In
higher vertebrates it has been shown that the bile helps to
emulsify fats and facilitates their absorption from the intes-
tine ; it also has a slight power of converting starch into sugar.

Intestinal Digestion and Absorption. — The food when
it is passed from the stomach into the duodenum, possesses
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 proteids which may have
escaped the fermentative action of the pepsin in the stomach
are acted upon by the trypsin of the pancreatic juice and
converted into peptones. The starchy constituents of the
food are converted into sugar mainly by the secretion of the
pancreas, though perhaps also to a slight extent by the bile,
and the fats are partly split into fatty acid and glycerin, and
partly emulsified by the action of the pancreatic juice. The
role of the intestinal juice in the frog is uncertain ; but in
some of the higher vertebrates it has the property of con-
verting starch into sugar.

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 peptones
is absorbed by the capillaries of the blood vessels. Almost

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 157

nothing is known of the routes taken by the different kinds
of absorbed food material in the frog, and little enough of
the courses followed by peptones and sugar in any form.

Probably but a 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
especially adapted for absorption on account of the large
number of folds it contains which give a large amount of
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 resem-
blance 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 throwing the
liver of a recently killed frog into boiling water, then grind-
ing 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.

158 THE BIOLOGY OF THE FROG chap.

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
boiling, a greater amount of glycogen can be obtained from
the liver if it is boiled soon after 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 becomes
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 acts
as a sort of reservoir of food, storing it up in a comparatively
insoluble form when it is in excess, and expending it gradu-
ally 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,

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 159

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 per cent of glycogen than
the liver ; but owing to their much greater bulk their total
glycogen content may exceed that of the liver, although it is
usually less. Smaller quantities of glycogen are found in
the ovaries, central nervous system, and skeleton.

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 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, 1 1.1 2 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 : —

i6o

THE BIOLOGY OF THE FROG

CHAP.

Average diameter

of cells
Average diameter

of nuclei

NOVEMBER

DECEMBER

APRIL

JUNE

.0292 mm.

.0162 mm.

.012 mm.

.0172 mm.

.006 mm.

.0044 mm.

.0076 mm.

.0065 mm.

JULY

.0274 mm.

.0065 mm.

Similar measurements by Funke gave results approximately
the same as those obtained by Miss Leonard. The minimum
size of the cells in R. temporaries according to Funke occurs
in May, the maximum in July and August. In R. escidenta
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. temporaria.
In the first species -the fat content of the liver in many
instances almost entirely disappeared in June. During the
summer fat is stored in the liver, and in the winter it suffers
very little diminution if it does not actually increase in
amount. In R. temporaria the amount of fat in the liver is
very small compared with that in R. eseulenta, and no defi-
nite conclusion could be drawn regarding the general course
of its seasonal changes. According to Langley's observa-
tions 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 compara-
tively small though varying amounts. It is usually present
in minimum amount in September and October."

Miss Leonard found that the relative proportion of blood

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 161

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 that
as the size of the cells of the liver increases, the relative pro-
portion 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 being
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 win-
ter 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 dimin-
ished. Athanasiu, who has investigated the amount of glyco-
gen in the whole body of the frog (J?, esctrienta) at different sea-
sons, 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 maximum (1.43 per cent of the body weight) followed

1 62

THE BIOLOGY OF THE FROG

CHAP.

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 con-
tent of the rest of the body, exceeding the latter in the fall
and early winter, while in the fall 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
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 : —

January
February
March .
April
May
June
July. .
August .
Septemb. r
October
November
December

Male

Female

•05775

.0430

.0436

.0382

.0502

.0348

.0370

.0323

.0370

.0232

.0244

.0214

•0317

.0311

.0360

.0360

.0710

.0509

•05 7 l

•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

vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 163

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 proteids are capable by themselves of form-
ing 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, carbon,
oxygen, and hydrogen.

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 proteids.

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 proteids, 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, 1 891.

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

164 THE BIOLOGY OF THE FROG chap.

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

Griitzner, P. Ueber Bildung und Ausscheidung von Fermenten.
Arch. ges. Phys., Bd. 20, 1879.

Griitzner 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 Labdriisen.
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
daring Secretion. Jour. Phys., Vol. 2, 1880.

Leonard, A. Der Einfiuss der Jahreszeit auf der Leberzellen von
Rana tempo-aria. Arch. Anat. u. Phys., phys. Abth. Suppl. Bd., 1887.

Moraczewski, W. von. Die Zuzammensetzung des Leibes von
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 Anatomic

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

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 welcher
die Leber der Hiihner und der Frosche erleidet. Bericht. Verh. Konigl.
sachs. Ges. Wiss. Leipzig, Math. -phys. CI., 1850.

THE VOCAL AND RESPIRATORY ORGANS 165

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 produc-
tion 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 developed
or entirely absent. It attains its maximum development
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 at-
tached by muscles as well as connective tissue. The skele-
ton 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

1 66

THE BIOLOGY OF THE FROG

CHAP.

loop, the tracheal process, which is bent backward and
serves as a means of attachment for the necks or roots of

ring.

Fig. 44. — Respiratory organs of the frog. A, ventral aspect. The right
lung, r. lug, has been laid open to show the inner surface. In B the
larynx has been cut through the middle, and the right half of the larynx
and right lung are seen from the side, ar, arytenoid cartilage ; b. hy,
main part of the hyoid ; gl, glottis ; l.tr.c, laryngo-tracheal chamber;
p. c. hy, posterior horn of hyoid ; v. cd, vocal cord. (After Howes.)

the lungs. The arytenoid cartilages are a pair of semilunar
valves, which rest upon the cricoid cartilage ; their upper

CIZ.

m

Fig. 45. — Cartilages of the larynx of the frog. A, from above; B, from
the side; Ca, arytenoid cartilage; C. Z1 to C. /4, cricoid cartilage; P,
expansion of the cricoid; Sp, spinous process of the cricoid; * * *,
prominences of the arytenoids. (After Wiedersheim.)

edges form the lateral margins of the glottis; they afford

vin THE VOCAL AND RESPIRATORY ORGANS 167

attachment to muscles by which the glottis may be opened
or closed. The true sound-producing organs consist of a
pair of elastic bands, the vocal cordsy 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 laryngeal 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 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
vocal 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
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 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-

1 68 THE BIOLOGY OF THE FROG chap.

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 surface 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 unstriated
muscle cells which give the lungs their great power of con-
traction. The outer surface of the lungs is coated with
peritoneum.

The area of the inner surface of the lungs of Rana escu-
lenta 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, therefore, 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
expansion of the body wall ; and accompanying the latter
movement there is a brief closure of the nares. The respi-
ratory 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

THE VOCAL AND RESPIRATORY ORGANS

169

movements may continue for quite a long period, 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

offs

pmxL

FlG. 46. — 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;
tng, tongue. (After Parker and Parker.)

them into the buccal cavity as the floor of the mouth is
lowered, and forced out through them as the floor of the
mouth is raised. These oscillating movements perform two
functions : (1) they are subservient to the respiration which
takes place in the mucous walls of the mouth and pharynx,
and (2) by renovating the air discharged into the buccal

170 THE BIOLOGY OF THE FROG chap.

cavity after each expiration from the lungs they enable com-
paratively 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 inciden-
tally 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 immediately 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 after-
wards 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 sub-
jected 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 inspiration 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

vin THE VOCAL AND RESPIRATORY ORGANS 171

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 com-
partively pure air previously in the mouth cavity. The val-
vular arrangement for closing thenares 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 open-
ings. 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 inward
during each gulp of air, thus aiding the process by diminish-
ing the size of the buccal cavity.

I 72

THE BIOLOGY OF THE FROG

According to Baglioni the external aperture of the nares
does not remain closed during the last phases of the elevation
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 apparatus 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 because 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 portion
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 and
more of the latter gas. The process of respiration falls
into two phases : (i) external respiration, or the exchange

vin THE VOCAL AND RESPIRATORY ORGANS 173

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 of the body involves the consumption of oxygen which
is received from the blood, and as the result of the oxidation
of compounds 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 cor-
puscles in combination with hemoglobin. This peculiar
substance has the power of forming a weak 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 proteid compound
containing iron ; it is readily soluble in water and may be
obtained by evaporation from its solution 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. Dur-
ing the winter when the frog lies buried in the mud it becomes

174 THE BIOLOGY OF THE FROG chap.

practically the only respiratory organ. Frogs may be kept
alive when submerged in water at o° to 130 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 COa 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 were put in a chamber divided
by a partition which contained an aperture surrounded by rub-
ber. 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 res-
piration. Experiments of ligating or extirpating the lungs,
removing the skin, tying the cutaneous blood vessels, plung-
ing 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

THE VOCAL AND RESPIRATORY ORGANS

75

conditions. If carbon dioxide is prevented from escaping
through the skin, more 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 out-
put of C02 from both the lungs and skin compared under
various conditions. In Ranafusca at a temperature of 200 C.
the average ratio of oxygen income to C02 output in several
experiments on frogs taken at different times of year was,
in pulmonary respiration, 02 105 : C02 45 ; 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 through
the skin and relatively less C02 eliminated through the lungs.
The respiratory quotients (7 e. ratio of 02 to C02) in the two
species at 200 C. are as follows : —

Cutaneous

Respiration

Pulmonary
Respiration

R. fusca

R. esculenta

2.48 r.q.
1.92 r.q.

.32 r.q.
.32 r.q.

Influence of External Conditions upon Respiration. —

The respiratory functions of 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 breed-
ing season ; then it rapidly decreases during the summer,

176 THE BIOLOGY OF THE FROG chap.

and reaches its minimum in the winter, the ratios of oxygen
absorption at a temperature of 200 C. being as 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, therefore, the skin becomes relatively
more important in respiration 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 pro-
duced 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 refrangi-
ble 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

Vlli THE VOCAL AND RESPIRATORY ORGANS 177

violet, yellow, and red light were found to be as 114, 103, and
100 respectively. In red light there is but little more C02 out-
put than in the dark. The influence of heat was excluded in
the experiments by passing the light through a vessel of water.
As frogs which are placed in the light become restless and
excited and frequently make efforts to go toward the source
of illumination, it is probable that these differences in respi-
ration 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
differently colored lights is not inconsistent with this inter-
pretation, since phototaxis still occurs in frogs from which the
eyes have been removed.

REFERENCES

Baglioni. S. Zur Athmungsmechanismusdes Frosches. Arch. Anat.
u. Phys., phys. Abth., Suppl. lid., 1900.

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

Bert, P. Des movements respiratuires chez les Batrachiens et les
Reptiles. Jour. Anat. et Phys., T. 6, 1869. Lecons sur la physiologie
eomparee de la respiration, Paris, 1S70.

Bohr. Ueber die Haut- und Lungenathmung der Frosche. Skan-
dii av. Arch. f. Phys., Bd. 10, 1S99.

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. I, 1S78-1879.
N

178 THE BIOLOGY OF THE FROG chap.

Milns-Edwards, H. De la influence des agens physiques sur la vie,
Paris, 1824. Lecons sur la physiologie et l'anatomie comparee de
I'homme et des animaux, 1857— 1865.

Moleschott and Fubini. Sull' influenza della luce mista e chromatica
nell' esalazione di acido carbonico per 1' organismo animale. Atli dell'
Acad. Torino, 15, 1879.

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

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

ix THE SKIN 179

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 A num. As in most of the Amphibia,
the skin of the frog is smooth and moist ; it is very loosely
attached to the underlying musculature by thin bands of con-
nective tissue, which separate the large subcutaneous lymph
spaces. It is everywhere very tough, but it is considerably
thicker on the dorsal side of the body than it is below. In
certain regions it presents special thickenings ; 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 swelling at the base of the first
finger of the arm, the protuberance over the sixth toe or pre-
hallux, 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.

180 THE BIOLOGY OF THE FROG chap.

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 corneuni) 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 production 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 regions all of the cells of the epi-
dermis may contain small pigment granules. Ermann 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 l
have come to the conclusion that the chromatophores 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 stelhte
1 Loeb and Strong, Am. Jour. Anat., Vol. 3, p. 275, 1904.

ix THE SKIN 181

cells, which, according to Mayer, arise from the modifica-
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.

1 82 THE BIOLOGY OF THE FROG chap.

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

ix THE SKIN 183

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
mucus glands and the poison glands. While Heidenhain,
Nicoglu, and others regard these two types of gland as
specifically distinct, other investigators (Calmels, Leydig,
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 Ranafusca, 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-

1 84

THE BIOLOGY OF THE FROG

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

occur, and it is quite certain
that the differences are due
to the age of the glands, and
their different states of secre-
tion. Changes in the form
of the cells, however, are pro-
duced to a certain extent by
the contraction 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 function of the muscle
cells is the expulsion of the
secretion of the gland. The
glands of the skin are in con-
stant motion (Ascherson, En-
gelmann), as may be seen by
an examination of the glands
in the web of the foot. They
change not only in size, but
also in form, being now

FUj. 47. — 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.gl' t the same from one
side ; d, duct of gland ; h.f, h.f\
h.f", horizontal fibers of connec-
tive tissue ; h.l, outer or horny layer
of the epidermis ; m.l, Malpighian
layer of the epidermis ; pg, pigment
cells. (After Howes.)

rounded and now wrinkled
and angular. Contraction 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

IX

THE SKIN

185

mucus glands, and less uniformly distributed over the sur-
face of the body. They are more numerous on the dorsal
side of the body and hind legs, and they are especially
abundant, and unusually large, in the lateral dermal plicae.
According to Junius, they occur on all parts of the skin,
although they may be comparatively scarce in certain situa-
tions. Like the mucus glands they possess a muscular and
a connective tissue coat outside the layer of epithelium.

M. G

V

M. G

-"^sT*

P. G

■mm*

FIG. 48. — 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.)

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
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

1 86 THE BIOLOGY OF THE FROG chap.

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
within one minute ; another frog of the same species which
was inoculated with the poison 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 fusca, 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 function of 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 sufficiently

ix THE SKIN 1,87

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 anjalis (R. oxy-
rrhinus) it has been described by Steenstrup and by Siebold.
In Rci7iafusca Falio described a blue coloration appearing
on the throat during the breeding period. Leydig found
that this color disappeared soon after the animal was taken
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 Ranafitsca, 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,
assuming what Leydig designates a " quammig-quappiges
Ansehen." 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.

i88 THE BIOLOGY OF THE FROG chap.

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. Ac-
cording to 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,
xantholencophores), 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

IX

THE SKIN

189

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

Fig. 49. — Pigment cells from the frog, in different states of extension.
(From Verworn's " General Physiology.")

again like an Amoeba. While such movements undoubtedly
occur in the pigment cells of many of the lower animals, the
majority of investigators consider that the movement of the
pigment in the chromatophores of the frog takes place along
preformed 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

190 THE BIOLOGY OF THE FROG chap.

invisible except when containing pigment. Virchow, Von
Wittich, and Biedermann think that the changes in the
chromatophores may involve both of these factors. The
question is still not certainly decided.

The cells which give the skin its golden and green colors
form a layer immediately beneath the epidermis. Unlike the
black chromatophores they are usually rounded or polygo-
nal in fori*, 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 Ktihne, produces the
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.

The golden cells usually contain an additional source of
color in the form of the so-called interference granules, or
the iridescent pigment of Leydig. These granules, according
to Ewald and Krukenberg, are composed of guanin. They
are soluble in caustic soda or potash and present an appear-
ance of cross striation (Biedermann). By transmitted light
they are brown or gray, but in reflected light they are
usually blue.

The interference cells are stellate chromatophores which
are mainly confined to the subcutaneous tissue of the ventral
side of the body, where they produce the light color charac-
teristic of that region. They contain guanin granules like
those in most of the golden cells.

Red stellate pigment cells have been described in Rana

IX

THE SKIN

191

192 THE BIOLOGY OF THE FROG chap.

fusca by Von Wittich. They occur in the corium, and were
observed to undergo changes in the distribution of their
pigment like those of the black chromatophores.

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
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 SKIX

193

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

Fig. 51. — 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 chromatophores
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 pigment from the black
chromatophores has surrounded the yellow cells, giving the skin a very
dark color. (From Gaupp, after Ehrmann.)

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,
o

194 THE BIOLOGY OF THE FROG chap.

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 golden
color may predominate. The role of the contraction and
expansion of the golden pigments is not accurately deter-
mined. 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
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

ix THE SKIN 195

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 removed
from the body, the areas covered were found to be consid-
erably darker than those exposed to the light. Specimens
of Hyla in which certain parts were shaded while other 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.

The influence of the nervous system upon the chromato-
phores is shown by the experiments of several investigators.
Destruction of the optic thalamus causes the skin to become
much darkened (Steiner, Biedermann). Stimulation of the
medulla causes the skin to assume a lighter color. The skin
of the leg may be made to turn pale through stimulation of
the sciatic nerve. Biedermann has shown that color changes
may be brought about both through the spinal nerves and
the sympathetic system. If the spinal nerves supplying the
leg be cut, the skin of the leg may still respond to changes in

196 THE BIOLOGY OF THE FROG chap.

the central nervous system provided the sympathetic nerves
which accompany the blood vessels remain uninjured.

The condition of the pigment cells is profoundly influenced
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.

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 270 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. Carbon dioxide produces a darkening of the skin ;
carbon monoxide, on the other hand, causes the skin to turn
pale. Chloroform and some other anaesthetics as well as
certain irritants, such as croton oil and cantharides, cause
an expansion of the pigment on the parts of the skin to
which they are applied. Dryness tends to cause the skin
to turn pale, while immersion 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

THE SKIN

197

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 different in
the two cases. If frogs are held in the hand for some time,
the skin turns paler • this may in part be a reaction to tem-
perature, 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. The skin does
not function in absorption like a dead membrane. The
facility with which fluids pass through the skin from without
inwards is quite different from that with which they pass in
the reverse direction. According to Reid a five per cent
sugar solution in distilled water passes through the skin more
rapidly from within outward than from without inward ; but
if the same percentage of sugar is dissolved in a normal
salt solution, the fluid will pass more rapidly from without

\i

198 THE BIOLOGY OF THE FROG chap.

inward. Chloroform and other depressants decrease the
rate of passage of fluid from without and increase its rate
of passage from within. These differences in the rate of
the transmission of fluids in different directions tend to dis-
appear after the skin dies.

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.

REFERENCES

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Phys., 1840.

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

Biedermann, W. Zur Histologic und Physiologic der Schleimse-
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Wien, 1887; Ueber den Farbenwechsel der Frosche. Arch. ges. Phys.,
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Boulenger, G. A. The Poisonous Secretion of Batrachians. Nat.
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Donaldson, H. H. On the Absorption of Water by Frogs. Science,
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Drasch, 0. Beobachtungen an lebenden Driisen, etc. Arch. Anat.
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Dutartre, A. Sur les changements de couleur chez la grenouille
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Ehrmann, S. Zur Physiologie der Pigmentzellen. Cent. f. Phys.,
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Engelmann, T. W. Die Hautdrusen des Frosches. Arch. ges.
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Gadow, H. Color in Amphibia. Proc. Roy. Inst. Great Britain,
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Harless, E. Ueber die Chromatophoren des Frosches. Zeit. wiss.
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Heidenhain, M. Die Hautdriisen der " Amphibien." Sitzb. Wiirzb.
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Steinach, E. Ueber Farbenwechsel bei niederen Wirbelthieren
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Phys., 1865.

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

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

Townson, R. Observationes physiologicae de Amphibiis, Gottingae,
«795-

20o THE BIOLOGY OF THE FROG chap.

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

43, J893.

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 Chromatophoren
des Frosches." Ibid., 1854.

THE EXCRETORY SYSTEM 201

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 execretes 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.

THE BIOLOGY OF THE FROG

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,

FK

Cy Ao or ureter, is joined at

about the posterior third
or fourth of the outer
margin • it then 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 kid-
ney is flatter than the
dorsal and is traversed
longitudinally by the yel-
lowish adrenal body. The
kidneys are covered by
peritoneum only on the
ventral surface with the
exception of a very short
space where this mem-
brane is folded in over
the edges.

The kidney may be
regarded as a compound, tubular gland, made up of a
large number of coiled uriniferous -tubules. Each urinif-
erous tubule begins in a Malpighian body near the ven-
tral surface. A Malpighian body consists of two parts,
a knot of blood vessels, the glomerulus, and a sur-

FlG. 52. — Male urinogenital organs.
Ao, Aorta ; CI, cloaca ; Cv, postcaval
vein; FK, fat bodies; Ho, testes;
Ur, ureters opening into the cloaca at
S, S ; Vr, renal veins. (After Wieder-
sheim.)

THE EXCRETORY SYSTEM

203

rounding membrane, or Bowman's capsule. The artery,
vas affe?-ens, entering the capsule breaks up into several
capillaries which, after forming a few coils, emerge as the
efferent blood vessel from the same opening. Bowman's
capsule is an exceedingly thin membrane ; there is an inner
fold closely applied to the glomerulus which is continuous
with the outer wall at the point where the blood vessels enter
the capsule. 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 effe-
rent vessels. 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 urinif-
erous tubule. The very thin cells of
this wall shade off gradually into cells
of columnar epithelium 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
one of the collecting canals which extend transversely across
the dorsal surface of the kidney from the inner margin to

Fig. 53. —A urinifer-
ous tubule, c, col-
lecting tubule, m,
Malpighian body ;
t, uriniferous tubule
leading from the lat-
ter to the collecting
tubule. (After Nuss-
baum.)

204

THE BIOLOGY OF THE FROG

CHAP.

the ureter. The tubules are held together by connective
tissue which forms a support also for the numerous blood

vessels with which the kid-
ney is supplied.

The ventral surface of
the kidney is furnished
with numerous ciliated
funnels, the nephros tomes,
whose expanded ends open
into the ccelom. At their
other end the nephrostomes
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 nephrostomes 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 condi-
tion, as Marshall has found, occurs also in the early stages
of the life of the frog, but later the nephrostomes lose their
original connection with the tubules and become united
secondarily 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-

FlG. 54. — Diagram of a kidney show-
ing the ureter and collecting tubules.
C, collecting tubules ; L, longitudi-
nal canal of Bidder; S, seminal
vesicle ; T\ testis ; U, ureter ; VE,
vasa efferentia.

THE EXCRETORY SYSTEM

205

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 erTerentia are
originally out-
growths of the
walls of the Mal-
pighian corpus-
cles which be-
come connected
with the testis.
In some species
(R. escu lento)
the Malpighian
bodies, which give
rise to these out-
growths, still pre-
serve their original
function, and dur-
ing the period of
sexual activity
spermatozoa may
be seen in them
as well as along
the whole length
of the renal tu-
bules which arise
from them. The
vasa erTerentia
lead into a longi-
tudinal canal {Bidder s canal) which runs near the median
edge of the kidney.

In Ranafusca, according to Beissner, this canal is con-

C/3

206 THE BIOLOGY OF THE FROG chap.

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 fusca there is a compara-
tively direct connection estab-
lished between the vasa effe-
rentia and the collecting tubules,
and the spermatozoa, therefore,
are not found in the Malpighian
bodies and functional renal tu-
nc 56. - Diagram to illustrate ^x^ Bidder's canal occurs in

the course of the spermatozoa

through the kidney of Rana the kidneys of both sexes, but
fusca. «, ampulla; c, collect- its function in the female is not

ing tubule; /, Bidder's longi-
tudinal canal; t, uriniferous known.

tubule; u, ureter; v.e, vas in many of the lower verte-

efferens. (Modified from . .„, . . .

Beissner.) brates (Elasmobranchs, Am-

phibia) the kidney is divided
into an anterior, or sexual portion, and a posterior, or ex-
cretory portion. The frog presents only the beginning of
such a differentiation. The vasa efferentia are connected
with the anterior part of the kidney, but the excretory func-
tion of this region is still retained. The course of the sper-
matozoa through the kidney varies considerably in different
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 condition.

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

x THE EXCRETORY SYSTEM 207

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
(arteriae rectae), and also to the glomeruli (vasa afferentia).
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 capil-
lary 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 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 solu-
ble in water. Urea represents the final product of the break-
ing 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 is given to
the blood by a process of internal secretion. The kidney
also excretes several salts such as the chlorides, sulphates,
and phosphates of sodium, potassium, calcium, and magne-
sium, and numerous other substances in smaller proportions.

The specific roles of the glomeruli and tubules in renal
excretion has long been a matter of dispute. It is certain

208 THE BIOLOGY OF THE FROG chap.

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 func-
tion of the tubules is to absorb the excess of water and
certain other materials which pass down the lumen. By
other physiologists it has 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 in
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 by 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 EXCRETORY SYSTEM

209

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 estab-
lished on a firm basis. The subject is one that demands
renewed investigation.

The Bladder. — The bladder is a thin-walled, bilobed sac
attached to the ventral side of the cloaca, just below the

s ..._

Fig. 57. — Diagram of the bladder and rectum of the frog; A, from the
side; B, from below; Z>/, bladder; C, cloaca; R, rectum ; S, sphincter
muscle ; U, ureter ; Ut, uterus. (Modified from Gaupp.)

openings of the ureters. It arises as an outpushing of the
ventral wall of the cloaca like the allantois of the embryos
of the higher vertebrates, with which it is regarded as homolo-
gous. 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

210 THE BIOLOGY OF THE FROG chap.

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
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 amphibia,
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 " la pretendue
vessie urinaire des Grenouilles, des Rainettes et des Cra-
pauds, ainsi que celle des Salamandres, est une sorte de
citerne ou une humeur aqueuse, presque pure, destinee a
l'exhalation cutan£e, semble etre apportie, soit par les veins
sanguines, soit par les lymphatiques." Subsequent investiga-
tions by Davy, Nussbaum, and Adami have shown that there
is no doubt that the fluid contained in the bladder is derived
from the kidneys, since it contains urea and other sub-
stances characteristic of renal excretion.

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.

1 Dumeril, " Erpetologie geneiale."

x THE EXCRETORY SYSTEM 211

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
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.

Beissner, H. Der Bau der samenableitenden Wege bei Rana fusca
und Rana esculenta. Arch. mik. Anat., Bd. 53, 1898.

Farrington, 0. C. The Nephrostomes of Rana. Trans. Conn. Ac.
Sci., Vol. 8, 1892.

Frankl, 0. Die Ausfuhrwege der Harnsamenniere des Frosches.
Zeit. wiss. Zool., Bd. 63, 1897. See also Arch. mik. Anat., Bd. 51, 1898.

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 denAnuren.
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 Thatig-
keit der Driisen. Arch. mik. Anat., Bd. 27, 1886. See also Anat. Anz.,
Bd. I ; Zool. Anz., Bd. 20 ; and Arch. mik. Anat., Bd. 51.

212 THE BIOLOGY OF THE FROG chap.

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 thcca folliculi. After the

xi REPRODUCTIVE ORGANS AND THE FAT BODIES 213

eggs reach their full development they break through the
follicle and the outer wall of the ovary, and are discharged
into the body cav-
ity. When the
eggs are all ex-
truded, the ova-
ries, which before
had filled the
greater part of Qd—--
the body cavity,
become reduced
to small wrinkled
organs, containing
the minute ova for
the following year.

The oviducts
are a pair of con-
voluted tubes ex-
tending the length
of the body cavity
on either side of
the middle line.
They are sur-
rounded by peri-
toneum which is
continued dorsally
to form a sup-
porting membrane FlG' 58- — Urindgenital organs of a female frog.
A", kidneys; Od, oviduct; Ot, its opening into
\R nicn extends to the ccelom ; Ov, ovary ; P, opening of the ovi-
the dorsal body duct into tne cloaca; S, S', openings of the

,•■ . ., r , ureters; Ut, uterine dilatation of the oviduct.

wall outside of the (After Wiedersheim>)
mesovaria. Ante-
riorly, each oviduct opens by a wide mouth, or ostium, into

214 THE BIOLOGY OF THE FROG chap.

the body cavity near the base of the lung. At the posterior
end it enlarges to form the thin-walled, very distensible uterus;
the openings of the two uteri lie close together on the dorsal
wall of the cloaca. With the exception of the uterus, and
a short space at the anterior end, the oviducts 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 gelati-
nous 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 dis-
charged, the outer membrane of the cells is burst (Lebrun),
and the contents, which formed a greater part of the bulk
of the 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. During
this passage they receive their coats of jelly, after which they
collect in the uteri, whose walls they greatly distend. Here
they may remain for several days, the length of time depend-
ing upon the presence or absence of the male (see p. 51).

xi REPRODUCTIVE ORGANS AND THE FAT BODIES 215

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
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 use-
less, 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 mesorcJiium, to the dorsal side
of the body cavity, where it becomes continuous with the
general ccelomic lining. The rasa 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 me mb rana propria and an inner

2l6

THE BIOLOGY OF THE FROG

CHAP.

lining of cells, some of which {spermatogonia, spermatocytes,
and spermatids) represent stages in the formation of sperma-
tozoa ; others form the so-called " follicle-cells," and the
flattened cells described by Bertacchini, which lie next to

FlG. 59. — A, cross section of one of the tubules of the testis ; s/>, bundles of
spermatozoa ; t.e, epithelial lining of the tubule. B, stages in the de-
velopment of spermatozoa. (After Parker and Parker.)

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
of their discharge from the body. The seminal receptacle
is poorly developed in Rana pipiens and R. Catesbiana. In
the European species R. fusca it becomes very large and
divided into a number of compartments.

Corresponding to the various stages in the development

xi REPRODUCTIVE ORGANS AND THE FAT BODIES 217

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 attain
their maximum, after which they decrease in size during the
fill and less rapidly during the winter. In Rana esculentat
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 Septem-
ber. There is a storage of fat and pigment during this
period which later disappears (Ploetz, Friedmann). In
Rana escuknta there is most interstitial substance around
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 which accompany these changes
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.

2i8 THE BIOLOGY OF THE FROG chap.

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
differentiation 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 und 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.

Funke, R. Ueber die Schwankungen des Fettgehaltes der Fettfuh-
renden Organe im Kreislauf des Jahres. Denkskr. Ac. Wiss. Math.-
nat. CI., Bd. 68, 1900, Wien.

Giglio-Tos, E. Sur les 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.

xii INTERNAL SECRETION AND DUCTLESS GLANDS 219

CHAPTER XII

INTERNAL SECRETION AND THE DUCTLESS GLANDS

The idea of internal secretion was first brought into
prominence by Brovvn-Sequard, who found that extracts of
the testis of mammals when injected into the blood produce
a marked stimulating effect upon the organism. According
to this investigator, the testis produces some substance which
passes into the circulation. Such a process is termed inter-
nal secretion, in contrast to the production of substances
which are conveyed to the outside of a gland by a duct,
as in the secretion of saliva or bile. In recent years the
subject of internal secretion has become one of the most
important and fruitful fields of physiological research.

All of the cells of the body give off substances into the
blood, or lymph, but only in a comparatively few cases has
any definite physiological function of these products been
discovered. Two important internal secretions, sugar and
urea, are formed by the liver, the former arising from the
glycogen which is stored in the hepatic cells. The pancreas
produces, in addition to the pancreatic juice, an internal
secretion which is of even greater importance to the organ-
ism. Removal of the pancreas from one of the higher
animals results in the production of diabetes, which soon
terminates fatally. In this disease there is an abnormal
amount of sugar in the blood ; under ordinary circumstances
the undue production of this substance is prevented through
the agency of some secretion which is given off from the
pancreas into the general circulation. If the duct of the

220 THE BIOLOGY OF THE FROG chap.

pancreas be tied so as to destroy the ordinary function of
this organ, there is no abnormal production of sugar, and the
animal may live for a long time. A large part of the pan-
creas may be removed, or the whole organ removed, and a
part of it grafted in some other part of the body without
producing fatal effects. The animal may also be kept alive,
even after complete extirpation of the pancreas, if extracts
of this organ are injected into the blood. So long as the
body receives substances formed by the pancreas it may be
kept alive, but when these are completely withdrawn fatal
effects quickly follow.

In nearly all vertebrate animals there are several organs
the function of which was for a long time unknown. Many
of them were regarded as rudiments of organs useful once,
but now functionless. This was the case with certain small
structures such as the thyroid, hypophysis, and adrenal
bodies. It is now known that certain of these organs, far
from being useless rudiments, are absolutely essential to the
maintenance of life. Most of these organs belong in the
category of " ductless glands," so called because they have
no duct or external outlet. The way in which they function
has been a matter of dispute. We know that they act by
producing internal secretions which are given off into the
blood, and it is held by some that these substances destroy
poisons which are produced by the other tissues and which
would cause the death of the organism if allowed to accumu-
late. Others regard these secretions as affording the
stimuli needful to the discharge of the functions of other
organs. In certain cases, the latter interpretation seems
to be borne out ; but this does not prove that the internal
secretions of other organs do not possess antitoxic proper-
ties, and in fact there seems to be good evidence, in some
instances, that such is the case.

Xii INTERNAL SECRETION AND DUCTLESS GLANDS 221

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
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 absorbing 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.

According to some investigators the spleen produces an
internal secretion which acts upon the pancreas so as to

THE BIOLOGY OF THE FROG

CHAP.

,-*. R

Fig. 60. — Diagram show-
ing the position of the
thyroid glands, t; /,
lateral process of hyoid
cartilage ; t.p, thyro-
hyoid process of hyoid.

tion in Rana pipiens.

convert the proteid-splitting enzyme of that organ into an

active form. Doubt has, however, been recently thrown upon
this conclusion.

The Thyroid Glands. —The thy-
roid glands of the frog are com-
pletely 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
described some thyroid tissue (ac-
cessory thyroid) on the ventral side
of the hyoglossus muscle, and I have
been able to confirm this observa-
The tissue of the thyroid shows a

unique structure, being composed of a mass of rounded

follicles united by a

small amount of con-

nective tissue in

which there is a rich

supply of blood ves-
sels. Each follicle

is a perfectly closed

sac lined by a sin-
gle 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.
The thyroid of the frog, like that of the higher vertebrates,

has been found to secrete a substance rich in iodin (iodo-

e-

FlG. 61. — Part of a cross section of the thy-
roid of Rana pipiens. e, epithelial layer of
vesicles ; m, colloidal substance in vesicle.

xii INTERNAL SECRETION AND DUCTLESS GLANDS 223

thyrin, thyroiodin). Little is known of its function in the
frog. In the higher vertebrates its removal is followed in
nearly all cases by fatal effects. Removal of only a part
of the gland, as a rule, creates but little disturbance. Life
may be maintained for a considerable period after complete
removal of the thyroid, by giving injections of extracts of the
gland into the blood. In man
the disease called myxcedema or
cretinism, caused by the atrophy

of the thyroid, is often much S* Vfc— J **~yW^*Th.
helped or even cured by the
administration of thyroid extract.
The substance to which the thy-
roid owes its important function is ,

r FIG. 62. — Diagram showing the

a proteid with which a compara- position of the thymus," ih.
tively large amount of iodin is in D»l< depressor mandibuiae

, . rr, ir 1 .1 . muscle; Ty, tympanum.

combination. I reupel found that

frogs from which both thyroids were removed lived only two
or three days, but he was not entirely certain that the result
might not be due to effects of the operation other than the
loss of the parts in question.

The Thymus. — The thymus is a small, oval organ, some-
what reddish in color, situated behind the tympanic mem-
brane under the depressor mandibuiae muscle. As in most
higher forms, the thymus diminishes in size with age. Maurer
found that in Rana esculenta the thymus attained its maxi-
mum size in specimens of two or three centimeters in 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

224 THE BIOLOGY OF THE FROG chap.

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. Ham mar, 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.

The Pseudothyroid and the Epithelial Bodies. — The
pse it do thyroid 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
reddish 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 inter-
nal 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 propericardial
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

xii INTERNAL SECRETION AND DUCTLESS GLANDS 225

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
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
recently been discovered by Gaupp between the coracoid
and procoracoid portions of the pectoral girdle. Its mode
of origin has not been traced, but it is found in young larvae
with external gills. It probably does not belong in the
category of epithelial bodies, although it bears a certain
resemblance to them in internal structure.

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 cells only
in the winter and spring. The " cortical cells " are derived
Q

226 THE BIOLOGY OF THE FROG chap.

from the peritoneum, while the " central cells " are generally
regarded as modifications of the cells of the sympathetic
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 scattered through the cortical cells in an
irregular manner.

Abelous and Langlois found that if both adrenals of the
frog were destroyed, the operation 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 consider-
ably longer period than would otherwise have been possible.
It is well known that the adrenals produce an internal secre-
tion 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 experimented 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 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 and fibrillary twitchings

xii INTERNAL SECRETION AND DUCTLESS GLANDS 227

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 condition."
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.

REFERENCES

Abelous, J. E. Sur Paction antitoxique des capsules surrenales.
C. R. Soc. Biol., 1895.

Abelous et Billard. Recherches 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 grenouilles
apres la destruction des capsules surrenales, I.e., 1 89 1. Toxicite de l'ex-
trait alcoholique du muscle de grenouilles prives de capsules surrenales,
/.<:., 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.

Baber, E. C Researches on the Minute Structure of the Thyroids.
Phil. Trans., 1881, part 3.

Bolau, H. Glandula thyroidea und Glandula Thymus der Amphibien.
Zool. Jahrb. Abth. f. Anat., Bd. 12, 1899.

Gaule, A. Biological Changes in the Spleen of the Frog. Jour.
Morph., Vol. 8, 1893.

Gonfrin. Recherches physiol. sur le fonction du glandes surrenales.
Rev. med. Suisse romand., T. 16, 1806.

228 THE BIOLOGY OF THE FROG chap.

Hammar, J. A. 1st die Thymusdriise beim Frosch ein lebenswich-
tiges Organ? Arch. ges. Phys., Bd. no, p. 337.

Herring, P. T. The Action of Pituitary Extracts on the Heart of
the Frog. Jour. Phys., Vol. 31.

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 Suprarenal >
Capsules. Proc. Roy. Soc, London, Vol. 62, 1897.

Oliver and Schaf er. On the Physiological Action of the Extract of
the Suprarenal Capsules. Jour. Phys., Vol. 16, 1894.

Treupel, J. Stoffwechseluntersuchung bei einem mit " Iodothyrin "
(Thyroiodin) behandelten Falle von Myxoedem und Mittheilung einiger
Thierversuche mit Iodothyrin (Thyroiodin). Miinchener med. Wochen-
schr. 43 Jahrg., 1896.

Xiii THE SKELETON 229

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 posterior
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 articulate 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 prod tic
bones, each of which forms a ring-like lateral projection on
each side of the skull, which incloses the inner ear. Anteri-
orly 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, which

Fig. 63. — 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;
names of cartilage bones in thick; those of membrane bones in italic capitals, a.chy,
anterior cornu of hyoid; actb, acetabulum ; AST, astragalus; b.liy, basi-hyal ; C, calcar;
CAL, calcaneum; EX.OC, exoccipit.il; FE, femur; /'on, foil', fontanelles ; FR.PA, fronto-
parietals; HU, humerus; IL, ilium; MX, maxilla ; olf.cp, olfactory capsule; ot.pr, otic-
process ; p.c.hv, posterior cornu of hyoid ; /'A/A, premaxilla; I'K.ot, prodtic ; ra.ul,
radio-ulna; SP.ETH, sphenethmoid ; SQ, squamosal; s.sci', supra-scapula; sus, sus-
pensorium; TI.FI, tibio-fibula ; tr.jr, transverse process; UST, urostyle; V.l, cervical
vertebra; V.9, sacral vertebra; I'O, vomer; I-V, digits. B.the fourth vertebra seen from
in front, a.zyg, anterior zygapophysis ; en, centrum ; lin, lamina ; n.sp, neural spine ; pd,
pedicle; tr.'pr, transverse process. (From Parker and Haswell's Zoology, slightly
altered from Howes.)

230

CHAP. XIJI

THE SKELETON

231

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 with-

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out any handle ; the lateral portions underlie the two prootics.
The fronto-parietal bones form most of the roof of the

232 THE BIOLOGY OF THE FROG chap.

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
elements are 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 open-
ings 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 : —

(i) 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 ptery-

xin THE SKELETON 233

goid, a triradiate bone, the inner limb of which attaches
to the outer side of the prootic, while the two outer limbs
diverge, the one running beneath the long stem 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 quadrate- 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
tnaxillaries are the largest bones of the upper jaw ; they
connect with the premaxittaries in front, and the quadrato-
jugals behind ; they are furnished with teeth throughout
their length. On the upper side each bears a frontal pro-
cess which is overlapped by the nasal. The intermaxittaries
or prcmaxillarics 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 proxi-
mal end is called the angulare, or angulo-splefiial. 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 annulare bears a

234 THE BIOLOGY OF THE FROG chap.

prominence, the coronoid process, which gives attachment to
the muscles for closing the jaw.

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 correspond-
ing 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 cormia, 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 produced
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 origi-
nal cartilaginous basis become converted into bone. The

XIII

THE SKELETON

235

various bones which constitute the skull may be divided as
regards their origin into cartilage bones, which result from

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the ossification of cartilage, and membrane bones, which are

236 THE BIOLOGY OF THE FROG chap.

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, fenestras paiie-
ta/es, farther back.

The suspensortum 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 is
continued forward beneath the maxillary as far as the pala-
tine bone, where a transverse process joins 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 openings 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.

xin THE SKELETON 237

in which ossifications of membranes, or dermal bones, be-
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 rela-
tions 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 centruniy a basal portion, which is oval in cross
section and concave in front for the reception of the cen-
trum 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 car-
tilaginous epiphyses. Both the posterior and anterior mar-
gins 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 zygapophyses
are smooth and allow a limited gliding movement when the
spinal column is bent from side to side.

238

THE BIOLOGY OF THE FROG

The first vertebra, or atlas, differs from the others in hav-
ing no transverse processes, in the absence of the anterior
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
zygapophyses, and the posterior surface of the centrum bears

a pair of prominences
for articulation 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 sur-
face possesses .a pair
of cavities for articula-
tion with the ninth
vertebra. The verte-
bral canal is small and
triangular 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 i?ttervertebral
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

s.sc/>

SC/>

ep.ccr

Fig. 66. — Transverse section of shoulder
girdle, cor, coracoid ; ep.cor, epicoracoid ;
gl, gleniod cavily; hu, humerus; sop. scap-
ula; s.scp, supra-scapula ; v. 3, third ver-
tebra. (After Parker and Parker.)

XIII

THE SKELETON

239

portion, the suprascapular which is composed of cartilage
which is more or less calcified at the base. The supra-

FlG. 67. — Middle part of the shoulder girdle of the frog from below. Co,
coracoid ; Co', epicoracoid ; CI, clavicle; Ep, episternum ; G, glenoid
cavity; Fe, fenestra; AC, cartilage between scapula and clavicle; Kn,
xiphisternum ; m, junction of epicoracoids ; S, scapula; .S/, sternum.
(After Wiedersheim.)

scapula articulates below with the long scapula, which is
oblong and constricted in the middle ; the posterior sfde 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
gle noid part from a ventral aero mial portion.

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

240 THE BIOLOGY OF THE FROG chap.

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
stout bone constricted in the middle, and broadly expanded
at its inner end ; at its outer extremity it forms a portion of
the glenoid fossa. Between the two ends of the coracoid,
and extending forward between the clavicles, are the two
epicoracoid cartilages, which are usually more or less calcined
along the ventral side of their line of junction.

The epistemum lies in front of the epicoracoids and con-
sists of a basal piece of bone and a terminal piece of carti-
lage, 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 SKELETON

241

af-

file humerus ; its proximal end or head articulates with the
glenoid cavity of the pectoral girdle ; the distal or lower
extremity has a
rounded articular eBSfond

prominence in the
middle, on either
side of which is
a small projection,
or condyle. A
large crest, the
deltoid ri dge, ex-
tends from the «
head of the hume-
rus to about the
middle of the ven-
tral side. At the
distal end of the
humerus there is
a ridge above ,
each of the two p
condyles ; the in-
ner ridge is much

larger in the male Fig. 68. — Longitudinal sections of the larger bones
.1 • 1 r 1 of the limbs. A, humerus; B, radio-ulna; C,

tnan in tne temale. femur; D> tibio.fibula- ^, condyle ;/ foramen

The skeleton for artery; yf, fibula; kd,head; m, marrow; ol.

Of the forearm o^cranon ; p bony partition; ™ radius; sh

shaft; ti, tibia; ul, ulna. (After Parker and
Consists Of the Parker.)

radio-ulna, which arises from the fusion of two origi-
nally distinct bones, the radius and ulna, the line of
union between which is very marked, especially at the distal
end. The postaxial or ulnar part is produced 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

242

THE BIOLOGY OF THE FROG

IZ-

distal end of the radio-ulna is widened, and ends in two
epiphyses, one for each of the component bones.

The carpus, or wrist of the frog, contains six bones arranged
in two rows. In the proximal row the ulnare and radiate
are situated at the ends of the ulna and radius, respectively,
and at the inner or preaxial side of the radiale is the centrale.
In the distal row the first carpal
occurs just behind the rudimentary
thumb ; the second carpal, a very
small bone, lies behind the second
digit ; the outer carpal is of rela-
tively 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 proxi-
mal part of the hand. The first meta-
carpal is rudimentary and usually car-
tilaginous in the female, but in the
male it is larger, and becomes calci-
fied or even ossified. The other
metacarpals are elongated cylindrical
bones, somewhat expanded at the two
ends ; the inner one of the four is stouter in the male than
in the female, 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

Fig. 69. — Pelvic girdle
of the frog from the
right side. G, acetab-
ulum ; //, ilium ; Is,
ischium ; P, pubis.
(After Parker and
Parker.)

xiii THE SKELETON 243

the large, transverse processes of the ninth or sacral verte-
bra. 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 ante-
rior spine of the pelvis. The ilium forms the anterior part
of the cup, or acetabulum, which receives the head of the
f£mur.

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 repre-
sented by the ischium ; it forms the posterior part of
the acetabulum, and extends forward dorsally as far
as the top of a prominence, 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, cor-
responding to the humerus of the fore limb. .Below this is
the tibio-fibula, corresponding to the radio-ulna ; and follow-

244 THE BIOLOGY OF THE FROG chap.

ing the tibio-fibula the tarsus and foot, corresponding to the
carpus, and hand. The femur is an elongated, cylindrical,
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
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-
ale, the postaxial one next to the fibula, the fib u tare ; 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
tarsalia (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.

xin THE SKELETON 245

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 I, 1881.

Parker and Bettany. The Morphology of the Skull, London, 1887.

246 THE BIOLOGY OF THE FROG chap.

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 investigation
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 demonstrable 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 and increases within certain limits, with the increase
of the intensity of the stimulus. 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

xiv THE MUSCULAR SYSTEM 247

a dense fibrous band. The end of a muscle which is fastened
to a relatively immovable part is called the origin ; the more
movable end is the insertion. The contraction of a muscle
has the effect of drawing the origin and insertion nearer to-
gether. The gastrocnemius muscle, which may be taken as
an illustration, has its origin by two heads, one from a ten-
dinous 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 adductor. 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 niandibulce, 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 cap-
able 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

24? THE BIOLOGY OF THE FROG chap, xiv

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
following muscles may be seen on the ventral side of the
thigh : —

The sartorius, a narrow muscle, having its origin on the
ileum, 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 t]?e 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, /ongus, a narrow muscle arising fro'm*the
ventral part of the ileum 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 exposed
along the preaxial side of that muscle. It pulls the thigh
forward and ventrally.

The trkeps femoris, a very large muscle covering the

sbml

p.cJiy
o.sl

Fig. 70. — Muscles of the
frog seen from the ventral
side. On the left side (right of figure)
some of the muscles have been partly re-
moved, add.brev, adductor brevis ; add.long,
adductor longus ; add.mag, adductor magnus ; del,
deltoid ; ext.cr, extensor cruris ; ext.trs, extensor tarsi,
or tibialis anticus brevis; FE, femur; gn.hy, genio-hyoid ; gstr, gastrocnemius; hy.gl,
nvo-glossus; ins.ten, inscriptionestendinae; I. alb, linea alba ; my.hy, mylo-hyoid; obl.int,
transversus ; obl.ext, obliquus externus ; o.st, omosternum ; p.c.hy, posterior cornu of
hvoid; pet, pectoralis ; petti, pectineus ; per, peroneus ; rcf.abd, rectos abdominis; red.
irit.maj, rectus internus major, or gracilis major; rect.int.min. rectus internus minor, or
graci.is minor; sar, sartorius; sb.mt, submentalis ; sem.ten, semitendinosus ; tib.ant,
tibialis anticus longus; fib. post, tibialis posticus; TI.FI, tibio-fibula; vast.int, vastus
internus, caput anticum, or cruraiis; x.st, xiphisternum. .(After Parker and Haswell.)

249

250 THE BIOLOGY OF THE FROG chap.

whole front of the thigh. It arises by three heads. The
anterior head {caput antic um or c nivalis) arises from
the anterior border of the acetabulum at the junction of the
ileum and pubis. The middle head {caput medium) is a
short, flat muscle which arises from the ventral side of the
ileum near the middle and joins the fascia of the rest of the
muscle at about the proximal third of the anterior margin
of the thigh. The posterior head (caput posticum, vastus
externus, gluteus magnus) arises from the posterior end of
the crest of the ileum ; 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 poste-
rior 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 semitcndinosus and joining
the posterior side of the head of the tibio-fibula. Near the
middle the muscle is crossed by an oblique tendinous in-
scription. 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 900 with the femur,
the action of the gracilis is to flex it still more. If, how-
ever, the crus is partly extended so that it makes a con-
siderably greater angle thango0 with the femur, the contraction
of the gracilis still further extends it. The difference of the

xiv THE MUSCULAR SYSTEM 251

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
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 semimembranosus, 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 attach-
ment from the dorsal half of the posterior margin 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-fibula 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 gracilis, flexes or
extends the leg according to whether it is in a flexed or an
extended position.

The ileo-jibularis, a slender muscle lying between the
semimembranosus and the posterior head of the triceps
femoris. It arises from the ileum, 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 semitendinosus, 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

5°

252 THE BIOLOGY OF THE FROG chap.

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 to 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 dor-
sal crest of the ileum from the anterior third to within a
short distance from the posterior end. It extends back-
ward, 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 ilfeum, 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 ileum to be attached to the median sur-
face. Its insertion extends from the hip joint to about the
middle of the femur. This muscle is broad and flat, 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
internus, ileo-femoralis, quadratics f e mo ris, 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

xiv THE MUSCULAR SYSTEM 253

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 450 of a
continuation of the long axis of the cms.

The tibialis anticus longus is a narrow muscle on the
front of the cms. It arises by a long, narrow tendon from
the distal end of the femur. Distally it divides into two
parts which are inserted into the proximal ends of the tibi-
ale and fibulare. It acts as an extensor of the leg and a
flexor of the foot.

The peroncus 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 plan-
tar surface downward ; when the foot is strongly flexed, it
acts as a flexor, bringing it up close to the cms ; it also pulls
the ankle postaxially.

The extensor cruris is a slender muscle lying on the

254 THE BIOLOGY OF THE FROG chap.

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 femoris,
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
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
flex 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
together, and pulling it backward and extending or unfold-
ing its different parts. Any muscle, such as the iliacus
interims, which pulls the femur backward, brings about a
flexion of the crus through the tension produced on the

i " Contributions to Medical Research, dedicated to V. C. Vaughan,"
p. 260, 1903.

xiv THE MUSCULAR SYSTEM 255

flexors of the back of the thigh. A flexion of the cms
stretches the tibialis antic us longus on the anterior side of
the cms 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 ten-
dency becoming stronger the farther back the thigh is pulled.
Extending 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 movement
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. On
the ventral side of the body the large rectus abdominis ex-
tends 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 transversely
and divide it into segments.

The obliquus extemus is a large muscle covering most of
the sides of the body. It originates on the sides of the
ileum and dorsal fasciae above, and is inserted into the sides
of the rectus abdominis. Its fibers extend obliquely upward
ami forward.

256 THE BIOLOGY OF THE FROG chap.

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 ven-
tral 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 arising
from the coracoid and epicoracoids. All three parts are in-
serted 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 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,

xiv THE MUSCULAR SYSTEM 257

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 surface
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
frcm the outer portion of the prootic bone and inserted into
the hyoid. The anterior muscle is the largest, and is attached
ventrally to the lateral margin or the body of the hyoid near
the middle. The three posterior petrohyoids diverge ven-
trally 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
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.

258 THE BIOLOGY OF THE FROG chap.

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 of 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.
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 cell contains
a large percentage of hemoglobin, 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 Amceba.
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

XV

THE CIRCULATORY SYSTEM

259

greatly in different cells. In some cases, especially in the
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 cyto-
plasm is relatively
small and forms a
narrow, irregular
envelope around
the spherical nu-
cleus. The cyto-
plasm of the larger
corpuscles is
relatively much
greater in amount.
By virtue of
their amoeboid
movements the
white corpuscles
have the power
of independent
locomotion and
they even pass
through the deli-
cate walls of the
capillaries into the
spaces between
the other cells of the body. They are not confined, there-
fore, 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.

Fig. 71. — Blood corpuscles of the frog. a,b,c,
red blood corpuscles; a and b, young stages;
c, mature corpuscle ; d-h, spindle cells in differ-
ent stages ; d, early stage ; e, a somewhat later
stage ; f, g, h, typical spindle cells ; i-l, forms of
leucocytes ; /', very early stage ; /, an older stage ;
k, cell with lobed nucleus ; /, cell with four nuclei.
(After Dekhuyzen.)

260 THE BIOLOGY OF THE FROG chap.

One important property of the white cells is their power
of ingulfing small bodies, which are taken in much as
an Amoeba takes in its food. Bacteria are devoured in
this way, and the leucocytes thus afford the body a meas-
ure of protection against these organisms, which are con-
stantly being introduced into the system in one way
or another, and might, if unchecked, be productive of
serious if not fatal effects. An irritation set up in any region
causes leucocytes to be attracted to the spot in large num-
bers. 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 movements of the
leucocytes in such cases is the presence of substances given
off by the bacteria which exercise a chemotactic effect upon
these wandering cells, causing them to congregate about the
center of diffusion. The chemotactic influence of such sub-
stances is shown by the following experiments first performed
by Massart. If a fine capillary tube sealed at one end be
filled with a culture fluid containing the bacterium Staphy-
lococcus 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 pro-
duced by the bacteria are apparently the cause of the inva-
sion 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 organ-
ism. In the degeneration of the tail of the tadpole the

XV THE CIRCULATORY SYSTEM 261

removal of skeletal and muscular tissues is effected in large
part through the agency of the white corpuscles.

The spindle cells, as their name implies, 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 leucocytes, and during the
springtime acquire hemoglobin and become transformed
into red corpuscles. They are normal constituents of the
blood at all times of year and are usually colorless ; only in
the spring do they transform into red cells. Soon after the
blood is shed they run together into masses and play an
important role in producing the clot.

The plasma, or fluid portion of the blood, is of very com-
plex constitution ; it contains fats, sugar, and numerous pro-
teids in solution, a considerable number of salts, various
products arising from the breaking down of tissues through-
out the body, several gases, chiefly oxygen, nitrogen, and
carbon dioxide, and many other substances not included in
any of the above classes. While all but a small part of the
oxygen is carried by the hemoglobin of the red blood cor-
puscles, the other materials of the blood are contained in a
state of solution in the 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 remaining 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

262 THE BIOLOGY OF THE FROG chap.

hand, if blood be drawn into a vessel whose sides are smeared
with oil, it may be kept from clotting for a comparatively
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 ioo° C, its power
of clotting is destroyed. The formation of clot is de-
pendent in some way upon the presence of calcium salts in
the plasma ; for if these are precipitated out, clotting maybe
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 trans-
forms the soluble compound into an insoluble form. The
formation of clot has the function of preventing indefinite
bleeding after an injury has been received, the contact with
foreign bodies causing the clot to form and thereby check-
ing further loss of blood.

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 mainly confined to 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
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.

xv THE CIRCULATORY SYSTEM 263

As Bizzozero and Torre have shown, the principal 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
concerned in the formation of new corpuscles, undergo
rapid multiplication.

While most of the corpuscles, both red and white, arise
in the marrow of the bones, 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 stages of the red
corpuscles have been witnessed by a number of observers.
The spindle cells from which red corpuscles are derived
undergo indirect division, at least after they have acquired
hemoglobin. Their transformation into red corpuscles,
according to Neumann, occurs only in the spring, after they
acquire hemoglobin, but they are present in the blood,
although in less numbers, during the whole year. The
mature red corpuscles do not divide. The very youngest
stage of the red cells is represented, according to Neumann,
Dekhuyzen, and others, by small leucocytes with a relatively
large nucleus. Transitional stages occur between these and
the spindle cells, and between the latter and the young red
cells. In their young stages, the red cells have a more or
less circular but irregular form, comparatively little hemo-
globin, and a relatively large irregular nucleus. However
different the various forms of corpuscles may be in their

264 THE BIOLOGY OF THE FROG chap.

adult condition, they arise from cells of a very similar,
although perhaps 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 ccelom, although originally
continuous with it in early development. The pericardium
is composed of two layers, the parietal, forming 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 pericar-
dial 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; (i) 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 ob-
liquely across the right auricle ; anteriorly it is continued
into the thinner-walled truncus arteriosus from which it is
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.

On the dorsal side of the heart is the triangular, thin-

XV

THE CIRCULATORY SYSTEM

265

walled sinus venosus ; at the two anterior angles of the sinus
the precaval veins, or anterior vena cava, enter; the poste-

eangl m ^

au,.v.V

FIG 72 -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, aunculo-ven-
tricular valves; b,b', 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) ;
carJr, carotid trunk; Lau, left auricle; /^.a, lingual artery; /.*, longi-
tudinal or spiral valve ; pul.cu.tr, pulmo-cutaneous trunk; pui.v, open-
ing of pulmonarv veins; r.au, right auricle; s.au.ap, smu-auncular
aperture; sptaur, inter-auricular septum; z;, z/', valves; *, ventricle.
(From Parker and Haswell's Zoology.)

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.

266

THE BIOLOGY OF THE FROG

CHAP.

CCZ

'.CU

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
blood separated. By removing the ventral wall of the
auricles, ventricles, and bulbus, most of the features of the

internal structure may be
exposed to view. The
interauricular septum is
so situated that the right
auricle is much larger
than the left. In the
right auricle, close to the
septum, is the large
sinu-auricular aperture,
through which blood en-
ters from the sinus veno-
sus. 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 pulmonary 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 the ventricle
by a large opening, the auriculo-ventrkular aperture, which is

pt.C

Fig. 73. — Heart seen from the dorsal
side with the sinus venosus opened up.
ao, aortic trunk; ait, right auricle; au" ,
left auricle; ca, carotid trunk; p. en,
pul mo-cutaneous trunk; pr.c, precaval
vein ; pt.c, postcaval vein ; p.v, pul-
monary vein ; s.v, sinus venosus ; v,
ventricle; va", sinu-auricular valves.
(After Howes.)

xv THE CIRCULATORY SYSTEM 267

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 important
structure known as the spiral valve. It consists of a longi-
tudinal 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 ven-
tricle. 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 union of the two
branches to the middle of the anterior enlargement of the
spiral valve. Each of the two branches of the truncus is

268

THE BIOLOGY OF THE FROG

etc—/

FIG. 74. — Diagram of the arterial system of the
frog, seen from the ventral side, ao" , aortic
arch; an', right auricle ; an", left auricle ; br,
brachial artery; c.c, carotid; c.gl, carotid
gland; c.il, common iliac; 'cce, cceliaco-mes-
enteric; cce', coeliac; cu, cutaneous; d.ao,
dorsal aorta; fm, femoral; gt gastric; h,
haemorrhoidal ; hp, hepatic; hy, epigastrico-
vesical; k, kidney; /, lingual; Ig" , left lung;
;;/, anterior mesenteric; m.i, posterior mesen-
teric; oc, occipital; pc' , pancreatic; p.cu,
pulmo-cutaneous ; ////.pulmonary; re, renal ;
sc, sciatic; sp, splenic ; //.a, truncus arterio-
sus ; ts, testis ; v, vertebral. (After Howes.)

divided by two
septa into three
compartments. The
anterior compart-
ments, which lead
to the common
carotid arteries,
both enter the un-
paired division of
the truncus to the
right of the septum.
The middle com-
partments, which
lead to the aorta,
open into the im-
paired portion of
the truncus on
either side of the
septum.

The posterior
compartments,
which are con-
tinued into the
pulmo-cutaneous
arches, join each
other and open by
a common aperture
into the bulbus
cordis behind the
valves at its ante-
rior end.

The Arteries. —
The arteries, o r

XV THE CIRCULATORY SYSTEM 269

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 capilla-
ries 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 net-
work which serves as an impediment to the flow of the
blood. As this organ becomes somewhat distended 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
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).

The internal carotid proceeds outward and dorsally from
the carotid gland, and then forward and somewhat toward

2-jo THE BIOLOGY OF THE FROG chap.

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
a?rh, 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 arter-
ies 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 some-
times 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 back-
ward 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 back-
ward beneath the vertebral column. At the point of meet-
ing of the two systemic trunks, but usually more on the left

xv THE CIRCULATORY SYSTEM 271

than on right, the large cceliaco-mesenteric artery is given off
which supplies the alimentary canal and its appendages. It
divides into an anterior branch, or ceeliac artery, and a pos-
terior 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, sup-
plying the right side of the stomach and pancreas, and the
hepatic, which, after giving a branch to the anterior branch
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 urinqgenital arteries are four to six small arteries which
arise from the ventral side of the aorta, and are distributed
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, supply-
ing 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

272 THE BIOLOGY OF THE FROG chap.

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 ileum, and supplies the hind
limbs.

The third branch of the truncus, the pulmo- cutaneous
artery, passes outward and dorsally, and gives off posteriorly
the pulmonary artery to the lungs; it is then continued as
the great cutaneous artery, which passes upward and forward,
dividing behind the tympanic membrane into three branches :
(i) the auricu/aris, 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 distributed to the skin of the side of
the body. With the exception 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 cutaneous artery, the largest branches of which are dis-
tributed 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 cavse, which enter the
anterior angles of the sinus, are each formed through the
junction of three branches, the external jugular, the innomi-
nate, and the subclavian. The most anterior of these, the
external jugular, passes forward, where it receives branches

Fig. 75. — Venous system of the frog seen from the dorsal side, add, ab-
dominal vein; br, brachial vein; cd, cardiac, or vena bulbi cordis; ds.
Imb, dorso-lumbar ; du, duodenal ; extju, external jugular ; fin, femoral;
gs, gastric; hp, hepatic; hp.pt, hepatic portal ; int. intestinal; int.ju,
internal jugular; kd, kidney; Lou, left auricle; Ing, lung; tor, liver;
ms.cu, musculo-cutaneous ; pr.cv, precaval ; pt.cv, postcaval ; pul, pul-
monary; pv, peivic; r.au, right auricle; rn, renal veins; rn.pt, renal
portal; sc, sciatic; spl, splenic; spin, spermatic; s.v, sinus venosus;
ts, testis; ves, vesical veins. (From Parker and Haswell's Zoology.)
T 273

274 THE BIOLOGY OF THE FROG chap.

from the tongue, hyoid, thyroid, and pseudo-thyroid glands
and floor of the mouth.

The innominate vein, or second branch of the anterior
vena cava, passes laterally and divides into the internal
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 between the kidneys,
and runs forward ventral to the dorsal aorta to the tapering
posterior end of the sinus venosus. Near the latter it
receives the 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 of 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

XV

THE CIRCULATORY SYSTEM

275

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

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276

THE BIOLOGY OF THE FROG

CHAP.

behind the crest of the ileum. The femoral vein branches
in front of the base of the thigh into two parts, one of which
passes ventrally and joins its fellow of the opposite side to

FIG. 77. — 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; agastric vein; g.bl, gall bladder; lv' , Iv" , right
and left lobes of the liver respectively; p, portal vein; pc, pancreas; st,
stomach. (After Howes.)

form the anterior abdominal vein, which runs forward in
the middle of the ventral body wall ; the other branch, the
external iliac, passes forward, and dorsally, and joins the sci-
atic vein to form the common iliac or renal 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

xv THE CIRCULATORY SYSTEM 277

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 sys-
tem which carries venous blood to the liver. The latter con-
sists 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 abdominal vein, just before it enters the liver,
receives a small branch, the vena bulbi cordis, from the bul-
bus 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 the 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 Rriicke 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

278 THE BIOLOGY OF THE FROG chap.

separate. Brlicke'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,
which has 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 contrac-
tion. When the ventricle first contracts, the wall of the bul-
bus 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 impedi-
ments 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 con-
traction of the ventricle, when the pure blood is passing out,
there is a contraction of the bulbus cordis. This brings the

xv THE CIRCULATORY SYSTEM 279

wall of the bulbus against the free edge of the spiral valve
and prevents the blood from flowing over into the left or pul-
monary side of this division of the heart. The blood is pre-
vented 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 arches lies behind the valves at the anterior end
of the bulbus, it can receive no blood when the communica-
tion between 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.

The heart of the frog may beat for hours, or, under fav-
orable 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 observa-
tion with the microscope by tying the frog down to a small
piece of board, and spreading its toes apart so that the web

280 THE BIOLOGY OF THE FROG chap.

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 to
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
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 a connective
tissue.

The subcutaneous lymph spaces are especially well devel-
oped ; they are separated from each only other 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 cistern a magna, which extends above
must of the dorsal side of the body cavity.

THE CIRCULATORY SYSTEM

281

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-
stomes on the ventral surface of the kidney which lead from

FlG- 7s. _ Lymph sacs of Ran a. The dark lines indicate where the septa
extend between the skin and the body. a, abdominal lymph sac ; b,
lateral brachial lymph sac; c, crural lymph sac; d, dorsal lymph sac;
/ femoral lymph sac ; /, lateral lymph sac ; /, pectoral lymph sac ;
s, submaxillary lymph sac. (Modified from Gaupp.)

the ccelom into the renal veins. The anterior lymph hearts
are situated just behind the transverse processes of the third
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 prevent
the blood from passing into the lymph heart when it becomes
relaxed. At the opposite end there are ostia (but appar-
ently no valves) through which the lymph enters the heart

282 THE BIOLOGY OF THE FROG chap.

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
heart, nor is there unison between the beats of the lymph
hearts on the two sides of the body.

REFERENCES

Briicke, E. Beitrage zur vergleichenden Anatomie unci Physi-
ologic des Gefasssystems, I. Ueber die Mechanik des Kreislaufes bei
den Amphibien. Denkskr. 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.

Fuchs, E. Beitrag zur Kentniss des Froschblutes und der Frosch-
lymphe. Virchow's Archiv, Bd. 71, 1877.

Gaule, J. Beobachtungen iiber die farblosen Elemente des Frosch-
blutes. Arch. Anat. u. Phys., phys. Abth., 1880.

Macallum. Studies on the Blood of Amphibia. Trans. Canadian
Inst., Vol. 2, 1892.

Marquis, C. Das Knochenmark der Amphibien in den verschie,
denen Jahreszeiten. Inaug. Diss., Dorpat, 1892.

Neumann, E. Hamatologische Studien, 1. Ueber die Blutbildung
von Froschen. Virch >w's Archiv, Bk. 143, 1896.

Sabatier, A. Etudes sur le cceur et la circulation centrale dans la
serie des Vertebres. Ann. Sci. Nat. (5), T. 18, 1873.

Torok, L. Die Theilung der rothen Blutzellen bei Amphibien.
Arch. f. mik. Anat., Bd. 32, 1888.

xvi THE NERVOUS SYSTEM 283

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 sys-
tem 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 behav-
ior 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

284 THE BIOLOGY OF THE FROG chap.

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 sup-
plied with nerves which are connected with the central
nervous system, and through this channel connections may
be established between any two or more parts of the organ-
ism. In this way there is rendered possible the coordina-
tion of movements in different parts of the body, and the
ability of the organism to act as a whole in relation to
external things.

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 foramina.
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 correspond ; the
bundle of roots thus formed, together with the filum termi-
nate, is known as the cauda equina.

THE NERVOUS SYSTEM

285

Both the cord
and the brain are
surrounded by
membranes which
are designated by
Gaupp as fol-
lows : Externally
is the dura mater,
consisting of two
layers sepa-
rated by a lymph
space {interdural
space) ; the outer
layer of this is
pigmented and
closely applied to
the inner surface
of the cranium
and neural canal;
the inner layer is
devoid of pigment
and lies close to
the brain and
cord. Within the
dura mater is a
thin vascular layer
corresponding to
the pia mater and
arachnoid of the
higher verte-
brates ; only here
and there does it
present a division

Fig. 79. — The central nervous system of the frog.
The roof of the skull and vertebral column re-
moved to show the brain and spinal cord, at,
atlas, or first vertebra; au, auditory capsule;
b.s, brachial enlargement of the cord; f.tm, filum
terminate ; g.tr, prootic ganglion (trigeminus, or
Gasserian ganglion of many authors) ; Ls, lum-
bar enlargement of cord; mn {V"), mandibular
branch of fifth nerve ; mx ( V"), maxillary branch
of trigeminus nerve; my, myelon, or spinal cord;
iia, right nasal sac; na' ', left nasal bone; n.c,
neural canal; ol.n, olfactory nerve ; oph (V),
ophthalmic branch of fifth nerve; //, optic nerve.
(After Howes.)

286

THE BIOLOGY OF THE FROG

CHAP.

into two lamellae. This layer is very closely applied to the
central nervous system, and is continued into various fissures
of the brain, and the ventral fissure of the spinal cord.

A cross section of the cord shows it to be composed
mainly of ganglion cells and nerve fibers. The central part

of the cord is formed

dm.
Tr.pr)
pm.

grt.

4

This canal is the rem-
nant of the lumen

of gray matter which
consists chiefly of gan-
, glion cells and non-
medullated nerves.
Near the center of the
gray mass is a small
canal, the canalis cen-
Fig. 80. — Cross section through the verte- traits, lined by a single

bral column, and spinal cord showing the , , p •.!_,• 1 ,1

origin of the spinal nerves. c.c, central layer 0t ePltrielial cells-

canal; en, centrum; d./\ dorsal fissure;

d.m, dura mater; d.r, dorsal root of

nerve; g.m, gray matter; gn, ganglion of

dorsal root ; n.a, neural arch ; n.sp, neural formed by the closing

spine; p.m, pia mater (the reference line over of the edges of

should stop at the margin of the cord) ;

/, nerve trunk ; Tr.pr, transverse process; tne medullary groove

v./, ventral fissure; w.m, white matter, during development"

(After Howes.) . ° . V , .

at its anterior end it
widens out into the ventricles of the brain.

At the sides the gray matter is produced both dorsally
and ventrally into the dorsal and ventral coniua or horns.
The gray matter on the two sides of the cord is connected
both above and below the central canal by means of the
dorsal and ventral gray commissures, 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 sepa-
rates the right and left columns of white matter. From the

XVI

THE NERVOUS SYSTEM

287

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 tapering outer ends of

C.corl

FlG. 81. — Diagram of the spinal cord showing the paths taken by nervous
impulses. The direction of the impulses is indicated by arrows, c.c,
central canal ; col, collateral fibers; c.cort, cell in the cerebral 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, ganglion cell in dorsal
ganglion ; g.m, gray matter ; M, muscle ; m.c, cell in medulla ob-
longata ; m.ft 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; w.m, white matter. (After Parker and Parker.)

the cells lining the central canal ; these 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.

The cells of the gray matter give off processes by means

288 THE BIOLOGY OF THE FROG chap.

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
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 pro-
cesses 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 ventral or
motor roots of the nerves as well as with the cells of the gray

xvi THE NERVOUS SYSTEM 289

matter on both sides of the cord. The ventral roots of the
spinal nerves are broader and consist of several isolated
strands.

The Spinal Nerves. — The frog possesses but ten pairs of
spinal nerves. The tadpole has a much larger number
(twenty-two in R. fused), but the posterior ones disappear
with the degeneration of the tail. There is also a pair of
nerves which appears in the embryo in front of what is the
first pair of spinal nerves of the adult, but we shall continue
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 composed as well as the
sensory fibers of the peripheral portion 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 branches supply the ventral
musculature and limbs ; a short communicating nerve con-
nects 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

u

290

THE BIOLOGY OF THE FROG

CHAP.

br.ftl

aci.pl

Sj.A

which are distributed 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 transversus mus-
cles and gives some twigs to
the skin.

The fourth, fifth, and
sixth nerves are small and
are distributed mainly to the
skin and muscles of the wall
of the abdomen.

The seventh, eighth, and
ninth nerves pass almost
directly backward and anas-
tomose with each other to
form the lumbosacral, or
sciatic plexus. The seventh
nerve, before it enters the
plexus, gives off the ileo-
hypogastric nerve, which is

distributed to the muscles
of the abdomen. The cru-
ralis nerve is given off from

I-X, spinal nerves ; Ao,

FIG. 82. — Spinal nerves and sympa-
thetic system of the frog, the right
side seen from below. Only the
ventral branches of the spinal
nerves shown. Sympathetic system in black,
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;
Sp.A, splanchnic, or cceliaco-mesenteric artery; Sy, sympathetic cord;
Sy.c, commissures between sympathetic and spinal nerves ; Sy.g,
sympathetic ganglia; Ust, urostyle; Vl-V$, centra of vertebrae; Vgt
vagus nerve. (From Parker and Parker.)

xvi THE NERVOUS SYSTEM 291

the plexus ventral to the posterior portion of the ileum ;
it is distributed to the muscles of 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
ileum, passing between the pyriformis and posterior head of
the triceps extensor muscle and extending down the back
of the thigh ; a short distance proximal to the knee it
divides into the tibialis nerve and the perotieus. The
former extends along the posterior or flexor side of the
leg, and enervates 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 ileo -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 ?nid-brain ;
the thalamejicephalon ; and the fore-brain, which consists of
the cerebral hemispheres and olfactory lobes.

The medulla oblongata is formed by a widening of the an-
terior end of the spinal cord. On its dorsal side is situated

2S/i

—Sped

ClfZ

w zxx
Cpt.ch inf , i

J m pa v
Cer.H fcrAf cpl v ^ ch Lv2

1 €cm pmCcml /

A fed. cbl

1 iy )>u

FIG. 83. — Brain of frog. A, dorsal side; B, ventral side; C, left side; D, in vertical
longitudinal section through the middle. Cb, cerebellum ; Lcr.H, cerebral hemispheres ;
ch.plx^ anterior, and ch.plx- posterior, choroid plexus; com, commissures connecting
the right and left halves of the brain; Cr.C, crura cerebri; Di, diencephalon, or thala-
mencephalon; for. M, foramen of Monro; i, iter, or aqueduct of Svlvius ; inr, infun-
dibulum; Med.obl, medulla oblongata; Olf.I, olfactory lobe; ()//./,' optic lobe ; opl.v\
optic vesicle; pin, pineal body; pit, pituitary bodv ; Sp.cd. spinal cord; z$, third ven-
tricle; v*, fourth ventricle; l-X, cranial nerves;' / Sp, 2 Sp, first and second spinal
nerves. (From Parker and Parker's Zo8logv, after Gaupp and Osborn.)

2y2

XVI

THE NERVOUS SYSTEM

29-

t—Afed. oil

the wide, more or less triangular fourth ventricle, which com-
municates 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 con-
tinuous with the ventral fissure of
the cord.

The cerebellum consists of a
small transverse fold at the ante-
rior margin of the fourth ventricle.
In most other vertebrates the cere-
bellum 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 cerebellum. Their Fig. 84.
cavities, the optic ventricles, com-
municate with each other and also
with the channel between the third
and fourth ventricles. 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
behind and the cerebral hemi-
spheres in front. Its roof is thin and lined with a vascular
membrane, the anterior choroid plexus ; it bears two out-

\-Sp.cd

Diagram of a hori-
zontal section of a frog's brain.
c.c, central canal ; Cer. H,
cerebral hemisphere ; Di, dien-
cephalon, or thalamencepha-
lon ; for. M, foramen of
Monro ; /', iter ; Lat.v, lateral
ventricle ; Med.obl, medulla
oblongata; Nv. i, first, or ol-
factory nerve ; Olf. I, olfactory
lobe ; Olf.v, olfactory ven-
tricle ; Opt.l, optic lobe ; Opt.v,
optic ventricle ; Sp. cd, spinal
cord ; iA, third ventricle ; lA,
fourth ventricle. (After Ecker
and Wiedersheim.)

294 THE BIOLOGY OF THE FROG chap.

growths in the mid-dorsal line, the paraphysis, a vascular
outpocketing of the epithelium of the roof, and a short dis-
tance behind 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 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.

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 of
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 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

XVI

THE NERVOUS SYSTEM

295

separated from the latter by a shallow lateral sulcus. Unlike
the cerebral hemispheres, they are closely fused together in
the mid'dle 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

FIG. 85. — Diagram of the distribution of the fifth, seventh, ninth, and tenth
cranial nerves, the first spinal nerve, and the anterior part of the sympa-
thetic. Ao, systemic arch of aorta ; br.pl, brachial plexus ; D.Ao, dor-
sal aorta ; du, duodenum ; H , heart ; Hy, hyoid with hy1 anterior and hy-
posterior horns; L, lung; N, nasal bone; On, orbit; Pul, pulmonary
artery; Sp.A, splanchnic, or coeliaco-mesenteric artery; St, stomach;
Sy, sympathetic ; //, cut end of optic nerve ; V1, ophthalmic, V2, maxil-
lary, and Vs, mandibular branch of fifth, or trigeminus nerve; VII1,
palatine, and VII2, hyomandibular branch of facial ; IX, glossopharyn-
geal nerve ; X, vagus with Xcd, cardiac, Xgas, gastric, Xlar, laryngeal,
and Xpul, pulmonary branches; / Sp, first spinal nerve, or hypoglossal;
2 sp-j sp, second to fifth spinal nerves. (From Parker and Parker's
Zoology, slight'y modified from Howes.)

296 THE BTOLOGY OF THE FROG chap.

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 optic, or second pair of cranial nerves, arise from the
thalamencephalon, and, after crossing in the chiasma, emerge
from the skull through foramina in the sides of the chondro-
cranium, 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 from 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 troch/earis, 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

xvi THE NERVOUS SYSTEM 297

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 head. The
maxillo-mandibular nerve runs outward behind the eye ; it
soon divides into the maxillaris superior and the maxillaris
inferior, or mandibular. The former runs forward and out-
ward below the eye and supplies the upper lip and adjacent
structures. The latter supplies the principal muscles for
moving the lower jaw, the lower lips, and skin of the lower
side of the mouth. The trigeminus is a mixed nerve, partly
sensory and partly motor.

The sixth nerve, the ab due ens, 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, ox facial, nerve arises from the medulla, closely
behind the fitth, 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

298 THE BIOLOGY OF THE FROG chap.

to the nasal chambers and anterior portion of the roof of
the mouth. The second branch, the hyo mandibular, 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 mandibularis interims,
which runs forward close to the mandible, and the hyoideus,
which innervates the subhyodeus muscle and the skin in the
region of the throat. The seventh nerve, like the fifth, con-
tains both sensory and motor fibers.

The eighth, or auditory, nerve is distributed entirely to the
inner ear.

The ninth, ox 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 foramen
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 accessorius,
which is a small branch supplying the cucullaris muscle,

xvi THE NERVOUS SYSTEM 299

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 com mini leans) from each of the spinal
nerves, and where the two join there is a ganglionic en-
largement. The eighth spinal nerve, however, is connected
with the sympathetic by two communicating branches, and
the 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,
intestine, liver, pancreas, spleen, ovaries, oviducts, and
kidneys. 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

300 THE BIOLOGY OF THE FROG chap.

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 {AuerbacJVs and Meissner's
plexuses) ; in the heart (Remak's, Bidder's, DogiePs 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. If the acid is placed near the middle of the body,
both hind feet may be employed to remove it. The vigor
of the response 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 con-
tortions. 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.

xvi THE NERVOUS SYSTEM 301

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 nerve be cut, stimulation of a sensory nerve produces
no effect. If, however, the distal end of the part in connec-
tion with the cord is irritated, muscular contractions will be
produced. If the ventral or motor end 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 forward
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

302 THE BIOLOGY OF THE FROG chap.

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 tc 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
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.

xvi THE NERVOUS SYSTEM 303

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
stroked, and they croak only once after each stroke on the
back or 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 physio-
logical experiment," says Goltz, " which succeeds in so cer-
tain 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, sev-
eral 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

304 THE BIOLOGY OF THE FROG chap.

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
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 readi-
ness with which the swelling reflex is initiated. When the
animal is seized and there is a constant pressure stimulus,
there is a repeated swelling of the body and accompanying

xvi THE NERVOUS SYSTEM 305

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 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
vertebrae, 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 re-
moved, 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
x

306 THE BIOLOGY OF THE FROG chap.

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
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

xvi THE NERVOUS SYSTEM 307

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, but
there is not sufficient evidence for adopting this view, and
the only reason for accepting it is that other explanations of
sex recognition do not suffice.

There is so much variation in the behavior of different
individuals, and so many factors that may determine whether
the clasping reflex may or may not persist in particular cases,
that it is hazardous to draw conclusions from single experi-
ments. Differences in tactile stimulation may play a role in
sex recognition, and differences in the movements of the two
sexes may be another factor. The experiments of Goltz,
although very instructive and suggestive, have not succeeded
in showing sex recognition to be the result of any one cause,
and it is not improbable that it may depend upon several
factors, each of which is not entirely indispensable.

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
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.

308 THE BIOLOGY OF THE FROG chap.

Compensatory Motions. — If a frog is rotated on a circu-
lar disk with its head pointing away from the center, the head
of the animal will turn opposite the direction of rotation.
The turning of the head is often followed by locomotion 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
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 possi-
bilities and the limits of the development of its intelligence.

xvi THE NERVOUS SYSTEM 309

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
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-

310 THE BIOLOGY OF THE FROG chap.

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 opera-
tion these results were, observed is not stated. Since so
many of the functions supposed to have their seat in the
cerebral hemispheres are shown to be performed by other
parts of the brain, it is not definitely established just what
roles these organs really play. In higher animals they are
the seat of intelligence and voluntary control ; and it is not
improbable that whatever intelligence the frog may possess
may have a similar local habitation. But this conclusion,
probable as it may be, cannot be regarded as established by
experiment.

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
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 hemispheres be 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

xvi THE NERVOUS SYSTEM 311

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 gen-
eral insensibility to touch (Loeser). Since the optic nerves
enter the thalamencephalon, removal of this part eauses
complete blindness ; the frog no longer avoids obstacles
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 hemispheres, but
it responds only by movements of the head, lowering that
part when the anterior part of the body is raised and rais-
ing it when the reverse movement is performed (Steiner,
Schrader). If the animal is rotated on a horizontal disk
it responds by turning ; soon after the operation it shows no
compensatory motions ; later, it responds by turning the
head opposite to the direction of rotation, and, after com-
plete recovery, performs circus movements of the body in
the same direction as those of the head. The same move-
ments 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

3i2 THE BIOLOGY OF THE FROG chap.

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 disappeared. Frogs with the brain removed as far
back as the medulla locomote normally, perform the croak-
ing reflex, breathe regularly, and show the usual compensatory
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
removed 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 later,
somewhat more than the cerebellum alone was removed.
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

xvi THE NERVOUS SYSTEM 313

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 coor-
dinated 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
removed 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.

If the anterior portion {pars commissuralis) of the medulla
is removed, the frog becomes possessed of an uncontrol-
lable 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

3H THE BIOLOGY OF THE FROG chap.

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 flattened
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 combination 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
seventh nerves originate is destroyed. These nerves form
the afferent and efferent channels 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

xvi THE NERVOUS SYSTEM 315

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 when the
medulla is cut across just behind the cerebellum and the parts
in front of the cut entirely removed. They are not destroyed
if the medulla is also cut across at the tip of the calamus
scriptorius, but if the region between these two cuts is re-
moved, 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 removed 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
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

316 THE BIOLOGY OF THE FROG chap.

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
experiments 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 independ-
ent 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
one reflex acts at the same time as the stimulus for
setting free a second reflex. The taking of food may serve

THE NERVOUS SYSTEM

317

as in 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, com-
plicated 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 inhibiting
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 appearance.
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. Locomo-
tion 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 locomotion 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 the reflex

318 THE BIOLOGY OF THE FROG chap.

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.

Bechterew, W. Ueber die Function der Vierhiigel. Arch. ges.
Phys., Bd. 33, 1884.

Bickel, A. Ueber den Einfluss der sensibelen Nerven und den
Labyrinth auf die Bewegung der Thiere. Arch. ges. Phys., Bd. 67,
1897. Beitrage zur Riickenmarksphysiologie 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 Ruckenmark des Frosches. Arch.
Anat. u. Phys., phys. Abth., 1882.

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 Willkur-
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, 1898 ; continuation by Donaldson and
Schoemacher, I.e., Vol. 10, 1900.

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 iiber den Nervenmechanismus, welcher.
wahrend der Begattung der Frosche, thatig ist. Cent. med. Wiss., Bd. 3,
1865. Weiteres iiber den Nervenmechanismus, etc., I.e., Bd. 4, 1866.
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 Parthien des
Riickenmarks der Amphibien. Arch. Anat. u. Phys., 1846.

xn THE NERVOUS SYSTEM 319

Heubel, E. Das Krampfcentrum des Frosches und seine Yerhalten
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. Yersuche am Grosshirn des Frosches. Inaug. Diss.
Berlin, 1886.

Langendorff, 0. 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, 1 900.

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 liber Reflexe. Sitzber. d. k.
preuss. Ak. wiss. Berlin, 1882 und 1883.

Merzbacher, L. Ueber die Beziehungen der Sinnesorgane zu den
Reflexbewegungen 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 iiber die Hemmung von Reflexem
Arch. Anat. u. Phys., phys. Abth., 18S0.

Schrader, M. Zur Physiologie des Froschgehirns. Arch. ges. Phys.,
Bd. 41, 1887.

Sedgwick, W. T. On the Variations of Reflex Excitability in the f^
Frog induced by Changes of Temperature. Studies Biol. Lab. Johns
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Sirotinin. Die punktformige begrenzte Reizung des Froschriicken-
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Steiner, J. Die Functionen der Centralnervensystem und ihre
Phylogenese. Abth. I, Untersuchungen iiber die Physiologie des
Froschgehirns, Braunschweig, 1885.

Volkmann, A. W. Ueber Reflexbewegungen. Miller's Archiv,
1838.

Vulpian. Lecons sur la physiologie du systeme nerveux. Paris,
1866.

Wyman, J. Anatomy of the Nervous System of Rana pipiens.
Smithsonian Contributions to Knowledge, Vol. 5, 1853.

320 THE BIOLOGY OF THE FROG chap.

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-
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xvii THE SENSE ORGANS 321

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
being 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 enlarged
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
Rana fusca, according to Huber, possess a structure in
some respects resembling that of the touch bodies.

Y

322

THE BIOLOGY OF THE FROG

CHAP.

The skin of the frog is an organ of unusual sensibility. It
is sensitive not only to touch, chemical stimuli, and differences

Fig. 86. — 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.)

of temperature, but it is affected in no small degree by
light. It is also readily influenced by changes in the mois-
ture of the air, as is indicated by the behavior of the animal
under different conditions of the atmosphere.

Sense Organs 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 papiliae 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
these organs is covered by cylindrical epithelial cells between
which are scattered various forms of elongated cells which

XVII

THE SENSE ORGANS

323

are connected usually by means of branching processes,
with the ramifications of the supplying nerve.

The Olfactory Organs. — The seat of smell is in the nasal
cavity, the walls of which are supplied with the olfactory

d b

» Plex. subepitbel.

1
Plex. subbas.

FIG. 87. — Section through a fungiform papilla. A bundle of nerve fibers
enters the papilla from below, breaking up into first the subbasal plexus,
and second the subepithelial plexus around the bases of the cells of
columnar epithelium. The latter plexus forms connections with the
branches of the sensory cells, a, free nerve endings ; b, c, d, nerve cells ;
e,f, cells connected with the subepithelial plexus. (From Gaupp, after
Niemack.)

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

324

THE BIOLOGY OF THE FROG

CHAP.

are very much elongated and cylindrical inform ; (2) basal
cells (Krause), which are of stellate form and lie next to the
connective tissue ; and (3) the ol-
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 nu-
cleus occurs, 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 connected 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 cavi-
ties in the sides of the head called
the orbits, within which they are
freely movable. The outer or ex-
posed portion of the eye is cov-
ered by a transparent membrane
known as the cornea; the remain-
ing 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

Fig. 88. — Cells of the olfac-
tory epithelium. On the left
two sensory cells with cili-
ated outer ends, enlarged
nuclei, and fine basal fibers.
On the right two interstitial
cells. (After Dogiel.)

xvii THE SENSE ORGANS 325

outer fibrous membrane, and an inner layer of hyaline car-
tilage ; on the side next to the cranium it is perforated 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
tipper 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 eye-
ball. What appears as the lower lid is really made up of
two elements, the lower lid proper, and the nictitating mem-
brane. 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 por-
tion and an upper thinner part, which is folded in behind the
former when the eye is uncovered. The nictitating mem-
brane 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 ourselves, 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 produc-
ing a tension on the tendon of the nictitating membrane
causes the latter to be pulled up, although one might easily

326 THE BIOLOGY OF THE FROG chap.

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 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 are 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 oph-
thalmoscope 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 scle?-otie coat, which has been described

XVII

THE SENSE ORGANS

327

above, the choroid, and the retina. The choroid is a vas-
cular, pigmented layer lying next to the sclerotic. Next

Cam. anter

Cam. post

Corp. cil.
Ora optica

Ketina
Chorioidea

FlG. 89. — 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 Sclerce, cartilaginous and fibrous I
layers of the sclerotic coat ; Zon.cil, zonula ciliaris. (From Gaupp, after
G. Retzius.)

within the choroid lies the retina which is continued ante-
riorly over the posterior surface of the iris as far as the 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 tissue.

328 THE BIOLOGY OF THE FROG chap.

The optical portion of the retina which lines the whole pos-
terior region of the eye and extends as for forward as the
ciliary body, is a complex structure formed mainly of nervous
elements. It is the part of the eye which is especially sen-
sitive to light and upon which images of external objects are
thrown. In addition to the outer pigmented epithelium it is
composed, as in the higher vertebrates, 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 : —
i. 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 narrow 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 por-
tions 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.

THE SENSE ORGANS

329

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 of the retina, al-

the light must pass through all

though

of the other layers before reaching it.
The rods, which are larger and more
numerous than the cones, are very much
elongated cells; their outer ends are cy-
lindrical 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 surface. The
nucleus lies in the part of the cell on the
inner side of the outer limiting mem-
brane. 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 for-
mer are more abundant and have a much
longer cylindrical segment, which is per-
meated by a peculiar reddish purple pig-
ment which is known as visual purple. The
green rods have a short, cylindrical seg-
ment which is followed by an elongated
narrow neck ; the cylindrical portion con-
tains a green substance (visual green).

The cones have a conical outer seg-
ment which shows a

Fig. 90. — Kods from
the retina of the
frog : a green rod
on the right, a
red one on the
left, a, outer seg-
ment; b, refractive
body; c, inner seg-
ment; d, intermedi-
ate disk; e, inner
enlargement con-
taining the nucleus;
/, external limiting
membrane. (From
Gaupp, after Greeff.)

cross striation like the corresponding

33°

THE BIOLOGY OF THE FROG

CHAP.

portion of the rods. This is followed by a refractive lens-
shaped body, beyond which the cone becomes constricted.

Fig. qi. — Sections through the outer part of the frog's retina with its pig-
mented epithelium. The figure on the left is taken from a frog which had
been exposed to daylight for five hours. The processes of the three outer
pigment cells extend inward to the external limiting membrane, and the
pigment they contain has streamed inward, 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 fortv-eight hours in
darkness. The pigment 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 nucleus lies in an enlargement on the inner side of the
outer limiting membrane. The inner end of the cone, like
that of the rods, runs out into a fiber which enters the outer
reticular layer. The cones vary in size and form, as many

xvii THE SENSE ORGANS 331

as three varieties being distinguished (M. Schultze, van
Genderen Stort). Some of these are movable, expanding

Fig. 92. — Inner surface of the inner half of the eye of Rana esculenta, show-
ing the area centralis as a light-colored band above the papilla optica
where the optic nerve enters. (After Gaupp.)

in 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 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 contraction

332 THE BIOLOGY OF THE FROG chap.

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 pig-
ments are restored, even in the extirpated eye. It is prob-
able 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
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, admitting
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 gives it its
bright yellow spots. Near the margin of the pupil there is a
ring of smooth muscle cells forming a sphincter pupitta which
causes the pupil to contract. Strong light causes a contrac-
tion 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 sensi-

XVI1 THE SENSE ORGANS 333

tive surface of this layer is thereby stimulated and impulses
are 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 particu-
lar distance from the eye. Images of objects at other dis-
tances are not clearly outlined, and there consequently
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 percep-
tion of the outline or the distance of the portentous thing
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,

334

THE BIOLOGY OF THE FROG

CHAP.

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 eyes

FlG. 93>_ Right Membranous labyrinth of the frog from the inner side.
aa, ae, ap, anterior, external, and posterior ampulla; au, aperture of
utricul'us ; ca, ce, cp, anterior, external, and posterior semicircular canals ;
cus, utriculosaccular canal ; de, ductus endolymphaticus ; /, lagena ; mu,
ms, mu, macula acustica of the recessus utriculi, sacculus, and pars
neglecta respectively; pb, pars basilaris ; p/,ppb, papilla acustica lagenae
and basilaris; raa, rap, rs, rn, rl, rb, 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, utnculus. (After G. Retzius.)

are pushed outward by the contraction of the levator bulbi,
a broad sheet of muscle running obliquely across the ventral
side of the orbit.

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

xvn THE SENSE ORGANS 335

the sensory apparatus ; the other, the middle ear, being com-
posed of accessory structures for the transmission of sound
waves to the sensitive part of the organ.

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 utri cuius, and a lower, smaller part, the sac cu-
ius. 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 utriculus
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 furnished 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, 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

336 THE BIOLOGY OF THE FROG chap.

masses of similar crystals which are commonly called the
otoliths.

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 abun-
dant 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 audi-
tory capsule is perforated by an aperture, the fenestra ova/is,
which is closed with a small plug of cartilage.

The accessory auditory apparatus consists of the structures
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 communicates
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 tytnpanicus. 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 car-
tilage 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 sensa-
tion of sound.

xvii THE SENSE ORGANS 337

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 them-
selves, 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). When stimu-
lated, 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 dis-
puted by Steiner. If the labyrinth is destroyed only on one
side, the frog takes an asymmetrical attitude. If the opera-
tion 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 sel-
dom 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

338 THE BIOLOGY OF THE FROG chap.

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 with {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 provo-
cation, whereas this response is rarely elicited from normal
specimens. Locomotion may take place in a normal man-
ner, but the muscular tonus seems to be low, and the animal
quickly becomes fatigued. The compensatory motions are
unaffected.

Frogs have a well-developed sense of hearing, although
they do not usually manifest it by any clearly evident sign.
In croaking, one frog frequently responds to the croak of
another individual, so that one often hears a pair answering
each other by regularly alternating notes. If, after a concert
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 listen-
ing 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

xvii THE SENSE ORGANS 339

" it is never possible to get near to any frogs in the same
region after one has jumped in." The splash sound is sig-
nificant 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."

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 cla?fiita?is)
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 influ-
enced by sound. Yerkes found that frogs which were placed
in a glass aquarium so surrounded that the movements of the
observer could not be detected, would jump vigorously at

340

THE BIOLOGY OF THE FROG chap.

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 pres-
ence by some sort of noise before they come into the frog's
field of vision. If the frog prepares 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 semi-circulaires
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.

xvn THE SENSE ORGANS 341

Plateau, E. Sur la vision des poissons et des amphibies. Ann.
Nat. Sci. (5),T. 7, 1867.

Retzius, G. Das Gehororgan der YVirbelthiere, T. Stockholm, 1 881.

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.

342 THE BIOLOGY OF THE FROG , chap.

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 char-
acter, often in a very striking degree. They are far from
being mere random responses to stimuli ; notwithstanding
the fact that they may take place independently of intelli-
gent 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 con-
sciousness 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 it, buries its prey
in the nest as provision for its future offspring. The wasp
is utterly unconscious 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 activ-
ity, which, although of no service to the individual 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 con-

xviii INSTINCTS AND TROPISMS

343

duct, even were it capable of so doing. 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 shaping the creature's con-
duct. It is truly a marvelous thing that an animal should
be endowed with the power of successfully adapting its con-
duct to a complex environment without any perception of
the consequences of its actions. How can the existence
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 more intimately
connected 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 con-
tinually 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

344

THE BIOLOGY OF THE FROG chap.

not commonly call instinctive, but which are as well adapted
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 purposive 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, we 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 es-
cape 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 methods which
cooperate to effect its escape, both of which are reflexly
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

xvin INSTINCTS AXD TROPISMS 345

the brachial region of the spinal cord. It is a tendency in-
dependent of the higher nervous centers, and it is brought
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 move-
ments 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 per-
form, 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, diving,
crawling, burrowing, righting movements, but the number 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 incapable 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

346 THE BIOLOGY OF THE FROG chap.

manner by placing their bodies so that they face the region
of strongest illumination, and frequently also moving toward
it. Animals which orient themselves to light in this manner
are said to be phototactic or heHotropic. Those which move
toward the light are called positive ; those which move
away from it negative. This orientation is generally re-
garded 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 which 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 re-
sults 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

xvin INSTINCTS AND TROPISMS 347

with their heads pointing toward the light. The same
result occurred when the heat rays were eliminated by
passing the light through a vessel of water before it entered
the 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 6o° or
over. Frequently the frog assumed an almost erect posi-
tion, 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 ;
yet 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 was covered with
black cloth, and the frog placed near the black perpendicular

348 THE BIOLOGY OF THE FROG chap.

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 uncov-
ered 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
response, in which the animal puts itself in line with the
direction of the rays, affords a good illustration of photo-
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 col-
lect 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-

xviii INSTINCTS AND TROPISMS 349

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
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 temper-
ature to a marked degree. At 250 C. the positive response
becomes considerably accelerated, the frog moving more
quickly and more directly toward the light end of the box.
Above 300 C. the movements become irregular, owing to
the predominant effect of heat. When the temperature is
lowered, the positive reaction becomes less decided, and
according to Miss Torelle, when a temperature of 8° C. is
reached, the animal becomes negatively phototactic both in
air and in water. The evidence cited in favor of this con-
clusion is, however, not convincing, inasmuch as other reac-
tions, 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 all animals thus far investigated, it is the blue and
violet rays that are the most influential in evoking the photo-
tactic response ; the effectiveness of the other colors of the
spectrum diminishes in order from blue to red. If frogs are
placed in a box illuminated 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,

350 THE BIOLOGY OF THE FROG chap.

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 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
Dr. Parker has recently shown, 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 ex-
posed. 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
was introduced for comparison, it was found that in most in-
stances 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 interference of the cover-
ing 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 pene-
trated the covering of dead skin, that effected the orienta-
tion.

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 were 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-

INSTINCTS AND TROPISMS

35i

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 con-
siderable period, usually terminated by a jump toward the
light." This reaction occurs when the exposed portions of
the brain are 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 stimulation by light. In the
urodeles Triton cristatus and Proteus anguinus it has been
shown that the skin is similarly sensitive to light.

Thigmotaxis. — Many animals tend to remain in situa-
tions which afford contact stimuli over a considerable
surface of the body. Such forms are called positively
thigmotactic \ those which avoid contact are called nega-
tively thigmotactic. The tendency so common among
insects and worms to crawl under stones and lie under
crevices, is to a great extent the manifestation of a thigmo-
tactic 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 away 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 formed 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
were kept was lowered to io° C. and below. When this
was done, all the frogs responded, either by flattening their

352 THE BIOLOGY OF THE FROG chap.

bodies against the stone floor, or by creeping under the

rocks usually kept there. It therefore seems that the

frog is stereotropic [thigmotactic] in temperatures between
io° C. and 40 C."

REFERENCES

Graber, V. Grundlinien zur Erforschung des Helligkeits- und Far-
bensinnes der Thiere. Prag 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 Reac-
tions of the Frog to Light. Am. Jour. Phys., Vol. 10, 1903.

Torelle, 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.

THE INTELLIGENCE OF THE FROG 353

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 appearence 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 inge-
nuity, but it was of no avail. Unless the food could be
easily reached by making the exertion of a single leap, the

354

THE BIOLOGY OF THE FROG chap.

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 happen
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."

Frogs, when liberated, are generally credited with making
for water by the nearest route, but Abbott finds that this is
by no means always the case. Frogs tend to keep near

THE INTELLIGENCE OF THE FROG

355

water if they can perceive it, even when they cannot get
into it, although they may easily reach another body of
water at a somewhat greater distance. Abbott buried a
pail of water to the brim in dry ground within fifty yards of
a running brook, and covered it with a sieve. He then
" took seven frogs of three species and placed them on the

FIG. 94. — Labyrinth used in studying the formation of habits by the frog.
From A the frog enters at E. G, glass plate ; P, partition ; P, red surface ;
W, white surface ; T, tank. (After Yerkes.)

sieve, which was about half an inch above the surface of the
water. Here five of them remained during the whole day,
exposed to the glare and heat of a cloudless summer day.
The evaporation from the water beneath barely kept them
alive ; and yet within so short a distance was a running
brook, with all the attractive features of ideal frog life."
An animal like a dog, after finding that it could not get
at the water in the pail, would waste little time in making
for the brook ; but the frog is so limited in its resources,
that such a course is out of reach of its powers.

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. 94)
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
A. At the other end of the box an opening at one side leads

356 THE BIOLOGY OF THE FROG chap.

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 cardboard 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.
If, after the frog had learned to go to the water by the
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 con-
fused and frequently take the wrong route. Yerkes comes
to the conclusion 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

xix THE INTELLIGENCE OF THE FROG 357

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.

Toads are generally credited with greater intelligence
than frogs. Their space perceptions are quite well devel-
oped, since they find their way back to their regular habita-
tions 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 what 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

358 THE BIOLOGY OF THE FROG chap, xix

emotional feelings. While there is often rivalry among
them for the possession of food, their struggles are peace-
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.

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 in-
terests 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., 1 894.

Brehm. Thierleben. Bd. 7.

Jourdain, S. De l'intelligence des batraciens. C. R. Ass. franc.
Av. Sci., 29me Sess., 1900.

Knauer, F. K. Beobachtungen an Reptilien und Amphibien in
der Gefangenschaft. Wien, 1875.

Yerkes, R. M. The Instincts, Habits, and Reactions of the Frog.
Monogr. Suppl. Psychol. Rev., Vol. 4, 1903. Inhibition and Reinforce-
ment of Re anions in the Frog, Nana damitans. Jour. Comp. Neur.
and Psych., Vol. 14, 1904.

INDEX OF AUTHORS

Abbott, 13, 19, 2i, 353, 358.
Abelous, 224, 227 ; Abelous and Bil

lard, 224, 227; Abelous and Lang

lois, 226, 227.
Ach, 337. 34o.
Adami, 210, 211.
Albertoni, 318.
Allen, 15, 20, 2i.
Angelluchi, 322.
Ascherson, 184, 198.
Assheton, 95, 102.
Athanasiu, 161.

Baber, 227.

Baglioni, 172, 177, 304.

Barfurth, 55, 120, 161.

Bechterew, 318.

Beer, 333, 340.

Beissner, 205, 311.

Bensley, 41.

Berg, 177.

Bernard and Bratuschek, 84.

Bert, 177, 198.

Bethe, 322.

Bibron, 21.

Bickel, 318.

Biedermann, 190, 192, 194, 195-198.

Billard, 224, 227.
Birge, 318.

Bohr, 176, 177.

Bolau, 227.

Boll, 322.

Born, 51.

Bouin, 218.

Boulenger, 14, 21, 120, 198.

Brehm, 21, 358.

Breuer, 337.

Brown-Sequard, 219.

Briicke, 192, 277, 282.
Buckland, 14.

Calmels, 183.
Camerano, 120.
Ciaccio, 181.
Contejean, 163.
Cope, 18, 21, 23.
Copeland, 120.

Dale, 318.

Danilewsky, 318.

Dekhuyzen, 263, 282.

Dewevre, 163.

Dissard, 177.

Donaldson, 58, 188, 198, 318.

Drasch, 198.

Diirigen, 21, 25.

Dumeril, 21, 210.

Dutartre, 195, 198.

Eberth, 154, 159, 164, 181, 192.

Ecker, 22, 149, 252, 278.

Eidam, 44.

Ehrmann, 180, 192, 198.

Engelmann, 181, 183, 184, 185,

198.
Ewald, 190, 337.

Farrington, 211.

Fatio, 187.

Field, 113.

Fisher-Sigwart, 22, 25, 38, 46, 49, 50,

52, 54- 59. 85.
Fixsen, 70.
Fraisse, 55.
Frankl, 211.
Frear, 6.

359

360

INDEX

Fuchs, 282.
Funke, 34, 160, 218.

Gadow, 3, 7, 9, 11, 22, 199.

Gage, ii, 15, 22.

Garman, 14.

Gaskell, 318.

Gaule, 34, 162, 227, 282.

Gaupp, 26, I39, I7I, I77, 185, 222,

224, 225, 278, 285.
Gemmill, 115.
Giglio-Tos, 217, 218.
Girard, 337, 340.
Goltz, 303, 305-307, 312, 313, 318,

337. 34°-
Graber, 346, 352.
Griitzner, 142, 145, 146, 164.
Gutli, 332.

Hall, 115.

Hainmar, 224, 228.

Hardesty, 318.

Harless, 181, 192, 199, 318.

Hartog, 26.

Hay, 22.

Heidenhain, M., 164, 181, 183, 199.

Heidenhain, R., 149, 150, 164.

Hensen, 337.

Hering, 228.

Heubel, 319.

Hinckley, 17, 120.

Hirschberg, 326, 333, 340.

Hoffmann, 22.

Holbrook, 22.

Howes, 22, 149.

Huber, 36, 199.

Huxley, 22.

Hyrtl, 269.

Jordan, D. S., 17, 22.
Jordan, E. O., 11.
Junius, 183, 185, 199.

Kalm, 19.

Kato, 310, 311, 319.
King, 99.
Kirkland, 14, 15.

Klug, 174, 177.
Knauer, 26, 324, 357, 358.
Knauthe, ^y.
Krawzoff, 311.
Krogh, 168, 175, 177.
Krukenberg, 190.
Kiihne, 190.

Labbe, 43.

Lahousse, 154.

Landois, 338, 340.

Langendorff, 33, 319; and Mozeik,

161.
Langley, 142-148, 154, 161, 164.
Laudenbach, 340.
Leidy, 40-42.

Leonard, 34, 154, 159, 160, 164.
Leydig, 22, 183, 186, 187, 189, 190,

199.
Lister, 195.

Loeb, J., 316,319, 352.
Loeb, L., and Strong, 180.
Loeser, 310, 319.
Lombard, 254.
Loos, 41.
Luchsinger, 161.
Lyon, 340.

Macallum, 282.

McBride, 115.

Marquis, 263, 282.

Marshall, 22, 95, 109, 115, 119,

204.
Martin, 177.
Massart, 260.
Maurel and Lagriffe, 56.
Maurer, 109, 223, 225, 228, 269.
Mayer, 181, 224, 228, 277.
Mendelsohn, 319.
Merzbacher, 319.
Milne-Edwards, 178.
Mivart, 22.

Moleschott and Fubini, 176, 178.
Moore and Vincent, 226, 228.
Moraczewski, 164.
Morgan, 45, 95, 99, 119.
Muller-Eizbach, 57.

INDEX

36i

Neumann, 217, 263, 282.

Nickerson, 41, 42.

Nicoglu, 181, 183.

Nothnagel, 319.

Nussbaum, 51, 142, 146,148, 164, 208,

211, 217.

Oliver and Schafer, 227, 228.
Oppel, 150, 154, 164.
Overton, 199.

Paneth, 149.
Parker, G. H., 350, 352.
Parker, W. K., 254.
Partsch, 142, 148, 164.
Pfitzner, 181, 199.
Pfluger, 47, 51.
Plateau, 333, 341.
Ploetz, 35, 217, 218.
Ponfick, 154, 221.
Pouchet, 189.

Regnault and Reiset, 178.

Reid, 197, 199, and Hamby, 199.

Retzius, 341.

Ritter, 9.

Robinson, 95.

Rosel von Rosenhof, 22.

Ryder, 120.

Sabatier, 269, 278, 382.

Schleiden, 82, 121.

Schlosser, 319.

Schrader, 308-311, 316, 319, 337, 341.

Schultze, F. E., 181.

Schultze, M., 331.

Schultze, O., 86.

Schultze, P., 139.

Schwann, 82, 121.

Sczesny, 183.

Seeck, 199.

Sewell, 142, 145, 147, 148.

Sirotinin, 319.

Slonaker, 15.

Spallanzani, 22, 48, 55, 82, 85, 305,

3°7-
Steenstrup, 186, 187.
Steinach, 47, 49, 195, 199, 332.
Steiner, 195, 308, 311, 319, 337, 341.
Stejneger, 19.
Stieda, 199.
Stilling, 225.
Stirling, 199.
Stolkinow, 164.
Stossich, 41.
Strieker and Spina, 199.
Swiecicki, 142, 164.

Tarchanoff, 218.
Tordk, 282.
Toldt, 217.

Tordle, 346, 349, 352.
Townson, 59, 199.
Treupel, 223, 228.

Valatour, 139, 149.
Van Genderen Stort, 331.
Verworn, 60.
Volkmann, 319.
Vulpian, 319.

Weber, 159.
Wedenski, 178.
Werner, 196, 200.
Wiedersheim, 71.
Wittich, Von, 161,
Wright, 41.
Wyman, 320.

190, 192, 200.

Yerkes, 319, 338, 341, 353, 358.

INDEX OF SUBJECTS

Absorption of food, 156 ; of water,

58, 187, 197.
Acanthocephali, 41.
Acetabulum, 243.
Acris, 17; A.gryllus, 17,45.
Adipose tissue, 125.
Adrenal body, 202, 225.
Afferent nerves, 300.
Aglossa, 11.
Alveoli, 168.
Alytes obstetricans, 12.
Amblystoma, 7, 8, 46; Amblysto-

matinae, 7.
Amphiuma, Amphiumidae, 5.
Ampulla of ear, 335.
Anabolism, 134.
Angulare, angulo-splenial,233.
Annulus tympanicus, 336.
Anura, 2, n.
Anus, 64, 74, 101.
Aorta, 264, 270.
Aortic arches, 112.
Apoda, 3.

Aqueduct of Sylvius, 103.
Aqueous humor, 326.
Arachnoid, 285.
Archenteron, 95.
Arcifera, 11.

Area centralis of retina, 331.
Artery, 268. For particular arteries,

see 268-272.
Arytenoid cartilage, 165.
Ascaris entomelas, 40.
Astragalus, 230.
Atlas, 229.
Auditory organ, 63, 334; nerve, 298,

336-

Auricle, 73, 264, 279.
Autodax lugubris, 9.
Axis cylinder, 131, 288.

Balantidium, 42.

Basidiobolus ranarum, 44.

Batrachoseps, 9.

Belostoma, 39.

Benacus, 39.

Bidder's canal, 205.

Bile, 152, 155; capillaries, 153; duct,

107, 152.
Bladder, 107, 209.
Blastoccel, 93.
Blastopore, 93.
Blastula, 93.
Blood, 121, 258; corpuscles, 258,

262; vessels, 268.
Body cavity, see ccelom.
Bone, structure of, 126; cells, 127.
Bowman's capsule, 203.
Brachial plexus, 289.
Brain, 80, 103, 291 ; functions of,

308; influence on cord, 302.
Branchial arches, 118 ; clefts, 109, 118.
Breeding habits, 45, 50.
Brow spot, 64, 294.
Buccal cavity, 69.
Bufo lentiginosus, 15, 41, 46, 99.
Bufonidas, 11, 13.
Bufo quercicus, 16.
Bulbus cordis, 264, 277.
Bullfrog, 18, 19, 72, 215, 216.

Calcaneum, 230.

Canal, central of cord, 103, 286;
semicircular, 335, 337.

363

364

INDEX

Canaliculi, 127.

Capillaries, 269, 280.

Carbohydrates, 137.

Cardiac gland, 140.

Carotid artery, 269; gland, 269, 278.

Carpus, 242.

Cartilage, 125; cartilage bones, 235.

Cauda equina, 284.

Cell theory, 82, 121.

Centrum, 237.

Cerebellum, 103, 291, 293, 296.

Cerebral hemispheres, cerebrum, 103,
291,293, 294, 309.

Chiasma optic, 294, 296.

Choanae, 69. See also Nares.

Chondrotus, 7.

Choroid coat of eye, 237; plexus,
103, 293.

Chorophilus, 17, 46.

Chromatophores, 180, 188.

Cilia, 122.

Ciliated epithelium, 71, 122.

Circulation, 258.

Cisterna magna, 79, 280.

Clavicle, 240.

Cleavage, 89.

Clepsine, 39.

Cloaca, 64, 74.

Coagulation, 261.

Coccidium, 43.

Codonocephalus mutabilis, 42.

Ccecilians, 3.

Ccelom, 73, 97, no.

Ccelomic fluid, 79.

Cohnheim's fields, 130.

Cold, effects of, 55-58, 85, 158, 176,
196; influence on blood, 262, 351.

Color changes, 37, 58, 187-197.

Columella, 63, 336.

Commissures of cord, 286.

Compensatory motions, 308.

Condyles, occipital, 229.

Cones of retina, 329.

Conjunctiva, 325.

Connective tissue, 121, 123; subcuta-
neous, 182.

Copulation, 47, 52, 305.

Coracoid, 240.

Corium, 179.

Cornea, 324.

Corpus adiposum, 77, 217.

Corpuscles, of blood, 258, 262; of

lymph, 262.
Cranial nerves, 295.
Cranium, 229.
Cricket frog, 17.
Cricoid cartilage, 165.
Croaking, 31, 302.
Crura cerebri, 103, 293.
Cryptobranchidas, 5.
Ciyptobranchus, 5, 6, 39, 59.
Crystalline lens, 326.
Cuticle, 122, 179.
Cuticula dentis, 72.
Cutis, 179.

Cystignathidae, n, 18.
Cytamoeba, 43.

Dactylosoma, 43,

Dentale, 234.

Dentine, 72.

Dermal plicae, 67.

Desmognathinas, 7, 8.

Desmognathus fuscus, 8.

Development, 81.

Diemyctylus viridescens, 10.

Digestion, 136, 142, 148, 151, 156.

Digestive organs, 134.

Diplodiscus subclavatus, 42.

Discoglossidae, n, 12.

Distomum, 41,42.

Diving, 29.

Drepanidium, 43.

Drum, 63.

Duct, bile or gall, 73, 152; cystic,

153 ; hepatic, 152.
Ductus endolymphaticus, 335.
Duodenum, 74, 148.
Dura mater, 285.

Ear, 63, 334; development of, 102.
Echinorhynchus, 41.
Ectoderm, 96, 103.
Efferent nerves, 300.

INDEX

3*

Egg laying, 44, 48, 50, 214.

Eggs, 81,85.

Elastic tissue, 124.

Embryology, 81.

Enamel, 72.

Endolymph, 336.

Enemies, 38.

Engystoma carolinense, 18.

Engystomatidae, 11, 18.

Entoderm, 96, 106.

Enzymes, 136.

Epicoracoid, 240.

Epidermis, 122, 179.

Epigenesis, 82.

Epiphysis, 64, 104, 293.

Episternum, 240.

Epithelial bodies, 224.

Epithelium, 71, 121, 122.

Esophagus, 70, 74, 138, 147.

Ethmoid, 232.

Eustachian tube, 63, 69, 109, 336.

Evolution, 82.

Excretion, 197, 201.

Excretory organs, 113, 201.

Exoccipitals, 229.

External characters, 62.

Eyelids, 62, 325 ; muscles, 333.

Eyes, 62, 102, 324, 350.

Fascia, 124.

Fat body, 77, 217.

Fear, 310, 356, 357.

Female, organs of, 212 ; pronucleus,

88.
Femur, 243.
Fenestra ovalis, 336.
Ferments, 136.
Fertilization, 48-50, 87.
Fibrillse, 128.
Fibrin, 261.
Fibrinogen, 262.
Fibrous tissue, 123.
Fibuiare, 244.
Hilum terminale, 284.
Fissures, of cord, 286; of brain, 294.
Fontanelles, 236.
Food, 24, 119, 134-137.

Foramen, intervertebral, 238; mag-
num, 229; of Monro, 104, 294;
ovale, 229.

Forebrain, 104, 291.

Fourth ventricle, 103, 291.

Fronto-parietal, 231.

Gall bladder, 107, 153.

Ganglion, 284, 289; Bidder's, 299;
Dogiel's, 299; Gasserian, 296;
jugular, 298 ; prootic, 296 ; Re-
mak's, 299; sympathetic, 299.

Ganglion cells, 131-133.

Gastric juice, 142.

Gastrocnemius, 246, 252, 255.

Gastro-hepato-duodenal ligament,
138.

Gastrula, 93.

Genital organs, 115, 212; ridge, 115.

Germ layers, 95.

Gill, 116, 118; arches, 118; clefts or
slits, 107, 116; plate, 101.

Gland, cardiac, 140; carotid, 269,
278; ductless, 220; esophageal,
141; gastric, 140; mucus, 183;
poison, 183,; pyloric, 141 ; thymus,
109, 223 ; thyroid, 109, 222.

Glenoid fossa, 239.

Glomerulus, 113, 202.

Glossiphonia, 39.

Glottis, 70, 107, 165.

Glycogen, 157.

Goblet cells, 149.

Gonads, 212.

Gray matter, 286.

Haemoglobin, see Hemoglobin.
Haemogregarina magna, 43. *
Haemosporidia, 43.
Hearing, 338.
Heart, 73, ill, 264.
Heat, effects of, 55-58, 349.
Helix, 25.

Hemoglobin, 173, 258.
Hensen's line, 130.
Hepatic artery, 153, 271; ducts, 152;
portal system, 153, 277; vein, 274,

366

INDEX

Hermaphroditism, 59-61.
Hibernation, 53.

Hind-brain, 103, 291; limb, 119.
Histology, 121.
Holostomum nitidum, 42.
Humerus, 239-241.
Hyaloklossia Lieberkiihni, 43.
Hyla, 17, 194; goeldii, 17; picker-

ingii, 46; versicolor, 17.
Hylidae, 11, 16.
Hyoid, 70, 234.
Hyomandibular, cleft, 109; nerve,

297.
Hypnotism, 59-61.
Hypoblast, see Entoderm.
Hypoglossal nerve, 289.
Hypophysis, 294.

Ileum, 74, 148.
Ilium, 243.

Infundibular lobe, 294.
Infundibulum, 104.
Infusoria, parasitic, 42.
Inscriptiones tendinae, 255.
Insertion of muscle, 247.
Inspiration, 170.
Instinct, 342.
Intelligence, 353.
Intermaxillary gland, 71.
Intervertebral foramina, 238,
Intestine, 74, 148.
Invagination, 94.
Iris, 326.
Ischium, 243.
Iter, 103, 293.

|aw, 69, 232, 233, 236.
jelly, 83.

Karyamceba renis, 43.
Karyophagus ranarum, 43.
Katabolism, 134.
Kidneys, 77, 202.
Krause's membrane, 130.

Labyrinth, 335.
Labyrinthodonts, 2.

Lacunae, 125, 127.

Lankestrella, 43.

Larva, 102, 116.

Larynx, 107, 165.

Lateral plate, no; ventricle, 104,

294.
Laverania, 43.
Lens of eye, 326, 333.
Leopard frog, 20. See also Rana

pipiens.
Leptothera Ohlmacheri, 43; ranae,

43-
Leucocytes, 150, 154, 221, 258, 280.
Leucophores, 188.
Light, effects of, 176, 194; reactions

to- 345-
Limbs, 65.
Linea alba, 255.
Lips, 68.

Liver, 73, 107, 152, 157-163, 201, 207.
Lungs, 74, 107, 167.
Lymph. 258, 262, 281 ; hearts, 281 ;

spaces, 187, 280.

Male, instincts of, 31, 46-52; organs

of, 215 ; pronucleus, 88.
Malpighian body or corpuscle, 113,

202, 205.
Mandible, 233.
Marrow, 127, 263.
Marsupial frog, 17.
Maturation of egg, 86.
Maxillary bone, 233; teeth, 233.
Meckel's cartilage, 233-234.
Medulla oblongata, 284, 291, 313.
Medullary folds, 99 ; sheath, 131.
Megalobatrachus, 5 ; M. maximus, 7.
Melanin, 188.
Melanophores, 188.
Membrane bones, 235.
Mento-meckelian bones, 234.
Mesentery, 74, 148.
Mesoblast, mesoderm, 96, no.
Mesogaster, 138.
Mesorchium, 77, 215.
Mesosternum, 240.
Mesovarium, 212,

INDEX

367

Metabolism, 134.

Metacarpus, 242.

Metamorphosis, 116.

Metatarsus, 244.

Mid-brain, 103, 291, 293.

Molybdis Entzi, 43.

Monostomuni, 42.

Mouth, 68, 116.

Mucigen, 144.

Mucosa, 139, 149.

Mullerian duct, 115.

Muscle fibers, 128-130.

Muscles, kinds of, 247; of body wall,

255. 256; of e>e- 333'. of hind leg.

248-255 ; seasonal changes of, 34 ;

of throat, 257.
Muscularis mucosae, 139.
Myotome, no.
Myxosporidia, 43.

Nares, 63, 69, 102.
Nasal bone, 232 ; capsule, 232.
Necturus, 3, 4.
Nematoxys, 41.
Nephrostome, 113, 204.
Nerve, 131; cells, 132.
Nervous system, 80, 195, 283.
Neural arch, 237 ; spine, 237.
Neurenteric canal, 105.
Neurilemma, 131.
Neuroglia, 287.

Nictitating membrane, 63, 325.
Nodes of Ranvier, 131.
Nostrils, 63, 102.
Notochord, 97, 115.
Nototrema, 17.
Nucleus, of egg, 85.

Occipital condyle, 229.

Odontoblasts, 72.

(Esophagus, see Esophagus.

Olecranon, 241.

Olfactory capsule, 232; lobe, 291,

294; nerve, 295, 324; organ, 323.
Opalina, 43.
Operculum, 118, 119.
Optic chiasma, 294; lobe, 103, 293,

311; nerve, 104, 296; thalamus,

104 ; ventricle, 293.
Orbit, 62.
Osteoblasts, 127.
Osteoclasts, 73.
Otoliths, 336, 337.
Ova, 81, 212.
Ovary, 74, 212.
Oviduct, 76, 213.
Oviposition, 44, 48, 50, 214.
Oxysoma, 41.

Palatine, 233.

Pancreas, 74, 107, 151, 219.
Pancreatic duct, 107, 151 ; juice, 151.
Papillae, of skin, 179, 186, 321 ; of

tongue, 70, 322.
Parabasal, parasphenoid, 231.
Paracoccidium Prevoti, 43.
Paraphysis, 293.
Parasites, 40.
Parietal nerve, 294.
Pectoral girdle, 238.
Pelobatidae, 11, 12.
Pelvic girdle, 242.
Pepsin, 142, 147.
Pepsinogen, 147.
Peptones, 142.
Pericardium, 73, in, 264.
Perichondrium, 126.
Perilymph, 336.
Periosteum, 127.

Peritoneum, 78, in, 139, 148, 213.
Pes, 65.

Phalanges, 242.
Phaneroglossa, 11.
Pharynx, 138.
Photopathy, 348.
Phototaxis, 346.
Pia mater, 285.
Pigment, 67, 154, 188, 221 ; cells, 154,

188.
Pineal gland, 64, 104.
Pipa americana, 12.
Pituitary body, 104.
Plasma, of blood, 262.
Pleistophora Danilewskyi, 43.

368

INDEX

Plethodon, 9; Plethodontinas, 7, 9.
Plexus, Auerbach's, 299; brachial,

289 ; coeliac, 299 ; ischio-coccygeal,

291 ; lumbo-sacral,29o; Meissner's

299; sciatic, 290; solar, 299;

urogenital, 299.
Poison glands, 183.
Polar bodies, 86.
Porta! system, hepatic, 153, 277;

renal, 277.
Portal vein, 153, 277.
Postbranchial body, 225.
Prehallux, 65, 244.
Premaxilla, 233.
Primitive groove, 99.
Procoracoid, 239.
Procoracoidal body, 225.
Proctodeum, 106.
Pronephros, 113; duct of, see

Segmental duct.
Prootic, 229.
Proteidae, 3.
Proteids, 137.
Proteus, 3, 351.
Pseudobranchus, 4.
Pseudothyroid, 109, 224.
Pterygoid, 232.
Pubis, 243.
Pulmocutaneous arch, artery, 268,

272.
Pulmonary artery, 272; vein, 273,

275.
Pulvinar rostrale, 68.
Pyloric glands of stomach, 141.
Pylorus, 74, 138.

Quadrate, 235.
Quadrato-jugal, 233.

Radiale, 242.
Radio-ulna, 241.
Rana, 18, 194.
Rana agilis, 18.
Rana arvalis, 45, 187.
Rana Catesbiana, 18, 19, 41, 215,
216.

Rana clamitans, 19, 20, 41, 339, 346.

Rana esculenta, 24, 35, 41-45, 47, 51,

60, 160, 168, 175, 186, 205, 206, 216,

223. 338. 346-
Rana fusca, 25, 44-48, 50, 51, 59, 70,

87, 175, 183, 186, 187, 205, 206, 216,

289, 321.
Rana grylio, 19.
Rana halecina, 46.
Rana muta, 25.
Rana oxyrhina, 44, 187.
Rana palustris, 21, 99.
Rana pipiens, 20, 23, 24, 27, 31, 37-

44, 66, 67, 71, 215, 216, 222, 346.
Rana sylvatica, 20, 24, 45, 354.
Rana temporaria, 18, 34, 35, 36, 41,

43- 45- 49. 160.
Ranatra, 39.
Rectum, 74, 151.
Reflex action, 300, 305, 314.
Regeneration, 54.
Renal portal system, 206, 276 ; renal

veins, 206, 274.
Reproductive organs, 35, 74, 115, 212.
Respiration, 168-177.
Respiratory movements, 168.
Retina, 104, 327.
Rhabdonema nigrovenosa, 40.
Righting movements, 30.
Rods and cones of retina, 329.

Sacculus, 335.

Saccus endolymphaticus, 335.
Sagittal fissure, 294 ; suture, 232.
Salamandra atra, 9; maculosa, 9.
Salamandridae, 3, 7.
Salamandrinae, 7, 9.
Sarcolemma, 129.
Sarcomeres, 130.
Sarcoplasm, 130.
Scaphiopus Holbrooki, 12.
Scapula, 239.
Schmidt, incisures of, 131.
Sciatic nerve, 292; plexus, 291.
Sclerotic coat, 324, 326.
Seasonal changes, 33, 159, 188, 214.
217, 262.

INDEX

369

Secretion, 142, 154, 183, 207 ; internal,
219.

Segmental duct, 113, 114.

Segmentation cavity, 93.

Segmentation of egg, 89.

Semicircular canals, 335, 337.

Semilunar valves, 267.

Seminal vesicle, 77, 216.

Sense organs, 321 ; plate, 100.

Serosa, 139.

Sex differences, 186, 306 ; recognition,
305 ; organs, 35, 74, 115, 212.

Sexes, proportions of, 51.

Sheath of Schwann, 131.

Shoulder girdle, 238.

Sinus venosus, 265 ; beating of, 279.

Sirenidae, 3, 4.

Siren lacertina, 5.

Skin, 66, 173, 179, 321, 350; absorp-
tion by, 58, 59; color changes of,
37, 58, 187-197 ; respiration of,
173 ; shedding of, 59.

Skull, 229.

Smell, sense of, 25, 324.

Spea, 12.

Spermary, see Testis.

Spermatozoa, 83, 85, 87, 216.

Sphenethmoid, 232.

Spinal cord, 80, 103, 284; reflexes of,
300.

Spinal nerves, 104, 284, 289, 300.

Spiracle, 118, 119.'

Spiral valve, 267.

Splanchnic layer, no.

Spleen, 74, 221.

Sporozoa, 42, 43.

Squamosal, 232.

Sternum, 240.

Stomach, 74, 138.

Stomodeum, 106.

Subcutaneous lymph spaces, 280;
tissue, 179.

Submucosa, 139, 149.

Subserosa, 139.

Sucker, ico, 118.

Sulcus marginalis, 68.

Suprascapula, 239.
2B

Surinam toad, 12.
Suspensorium, 232, 236.
Sympathetic system, 284, 298.
Symphysis, public and ischial, 243.
Systemic arch, 270.

Tadpoles, 39, 116-119.

Taenia dispar, 41.

Tarsus, 244.

Taste, organs, 322; sense of, 25, 322.

Teeth, 68,71,118.

Temperature, effects of, 55-58.

Tendon, 124.

Testis, 77, 215.

Tetracotyle crystallina, 42.

Thalamencephalon, 104, 291, 293 ;

function of, 311.
Thalamus, 104, 311.
Thigmotaxis, 351.
Thymus, 109, 223.
Thyroid, 109, 222.
Tibiale, 244.
Tibio-fibula, 243.
Toads, 13-16, 59, 211, 357.
Tongue, 26, 70 ; sense organs of, 322.
Touch corpuscles, 321 ; papillae, 321.
Triton, 10, 351.
Tropisms, 342.
Truncus arteriosus, 112, 264.
Trypanosoma, 42.
Trypsin, 151.

Tuberculum prelinguale, 69.
Tunica albuginea, 215.
Tylodelphis rhachidis, 42.
Tympanic bone, 232 ; membrane, 63,

69. 109. 336-
Typhlomolge, 3, 4.
Typhlotriton spelaaus, 8.

Urea, 207, 219.

Ureter, 115, 202.

Urinary bladder, 77, 209; tubules,

115, 202, 208.
Urodela, 3, 165.
Urostyle, 237, 238.
Uterus, yj, 214.
Utriculus, 335.

37<>

INDEX

V.ilvula parodoxica, 278.

Yas after ens, 203.

Vas deferens, 205.

Vasa efferentia, 115, 204, 215.

Veins, 265, 272-277 ; anterior abdom-
inal vein, 73, 276.

Vena cava, 265, 274.

Vent, 74.

Ventricle of heart, 73, 264, 279.

Ventricles of brain, 103, 291, 293,
294. *

Vertebrae, 116, 237.

Vertebral plate, no.

Vesicula seminalis, see Seminal ves-
icle.

Visual green, 329 ; purple, 329.

Vitelline membrane, 85, 89.

Vitreous humor, 326.

Vocal cords, 167 ; sacs, 69, 167.

Voice, 31, 165, 302.

Vomer, 232.

Vomerine teeth, 69, 232.

White matter of cord, 287.
Wolffian body, 113; duct, 115.
Wood frog, 20, 354; see also Rana
sylvatica.

Xantholeucophores, 188.
Xanthophores, 188,
Xiphisternum, 239.

Yolk, 86.
Yolk plug, 95.

Zaitha, 39.
Zygapophyses, 237.
Zymogen, 151.

The Development of the Frog's Egg

AN INTRODUCTION TO EXPERIMENTAL EMBRYOLOGY
By THOMAS HUNT MORGAN, Ph.D.

Professor of Biology in Bryn Mawr College. Author of " Evolution and
Adaptation," " Regeneration," etc.

Cloth 8vo $1.60 net

"Professor Morgan's bo k gives us a much-needed text-hook for
both student and instructor, and it should stimulate and greatly aid
investigation by pointing out the wide field the frog's egg still offers to
embryulogical research." — American Journal of Science.

" A clear, succinct, and comprehensible account of all the known
phases of the fertilization and development of the frog's egg. . . .
That the statements are clear and intelligible as possible the reader
may feel sure. . . . The medical student should master it. The gen-
eral student or the reader who is interested in the matter of the physi-
cal basis of heredity will find here the fundamental facts regarding the
first beginnings of life and the structure of the egg as well as the sperm
cell." — The Independent.

A Text=book of Entomology

Including the Anatomy, Physiology. Embryology, and Metamorphoses of In-
sects, for Agricultural and Technical Schools and Colleges as well as for
Working Entomologists.

By ALPHEUS S. PACKARD, M.D., Ph.D.

Professor of Zoology and Geology, Brown University

Cloth 8vo $4.50 net

" I have greatly admired your recent text-book of Entomology.
There is only one word to express my admiration of it, — Splendid." —
Professor S. W. WlLLlSTON, University of Kansas.

THE MACMILLAN COMPANY

64-66 FIFTH AVENUE, NEW YORK

A MANUAL OF ZOOLOGY

By T. JEFFERY PARKER

Professor of Biology in the University of Otago, Dunedin, N.Z., and

WILLIAM A. HASWELL

Professor of Biology in the University of Sidney, N.S.W.

Revised and adapted for the use of American schools and colleges

Cloth 12mo $1.60 net

" There has been long felt a great need in this country of a good
Zoology adapted for our schools. ... So far as I can judge from ex-
amination of the book, it seems to me to be well arranged and planned
and likely to be very useful in our colleges."

— H. \V. Conn, Wesleyan University.
" I think it will supply a need, not before met, of a brief but clear
and interesting outline for classes in Zoology. 1 have never seen its
equal and intend to introduce it next year."

— H. D. Densmore, Beloit College.

A Synoptic Text=book of Zoology

For Colleges and Schools
By ARTHUR WISSWALD WEYSSE

Instructor in Zoology at the Massachusetts Institute of Technology,
and Associate Professor of Physiology at Boston University

Cloth 8vo $4.00 net

This book includes in a single volume of convenient size all that is
necessary for an elementary course in zoology in our American colleges
suitable for the general student and at the same time a proper intro-
duction to extended treatises, or the books distinctively on comparative
anatomy, for the student who wishes to pursue the subject further.
There is at present no other book on zoology that exactly meets this
need or that treats the subject in a way suited to the adult student who
is, to some extent, capable of thinking for himself and forming his own
judgments. Hence the treatment here is new; the selection of facts
and their arrangement differ from that of existing text-books; in fact,
this is not a text-book as that term is properly used to-day when applied
to extended treatises, usually in more than one volume, which enter
into the subject exhaustively. Hence the whole subject is epitomized;
the fundamental facts of the science, not the theories of the author,
are presented to the student; each subject is handled synoptically, not
treated fragmentary, as in the majority of the elementary books, which
are for the most part adapted to children only ; thus, the purpose has
been to present a resume, readable it is hoped, which shall be suited to
the large class of students for whom such a treatment is needed.
Hence the title of the work.

THE MACMILLAN COMPANY

64-66 FIFTH AVENUE, NEW YORK

3 2044 072 209 703

Date Due

JUL 1 3 1952

J/Ui 1 n lOQfl

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