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JAMES A.
CIBS & AVUPoTRIARS
fie, BIOLOGY OF THE FROG
-The
Rana pipiens. Upper figure, ordinary resting attitude; lower figure, crouch-
ing position. (From photographs by Mr, F. M. Abbott.)
t, VIE:
BIOLOGY OF THE FROG.
BY, 4
OK”
SAMUEL J. HOLMES, Pu.D.
ASSISTANT PROFESSOR OF ZOOLOGY IN THE UNIVERSITY
OF WISCONSIN
SECOND EDITION
Nets Bork
THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., Lt,
1907
Adul rights reserved
CoPpyRIGHT, 1906,
By THE MACMILLAN COMPANY.
Set up and electrotyped. Published May, 1906. Reprinted
July, 1907.
Norwood Jpress
J. 8. Cushing Co. — Berwick & Smith Co.
Norwood, Mass., U.S.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. » Anura.
I THE AMPHIBIA IN GENERAL
eS)
THE APODA
The Apoda, or Ceecilians, 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 . : . - Serenide.
Both jaws with teeth. Fore and hind limbs
present.
Gills persistent. No eyelids or maxillary bones Proteide,
Gills usually absent in the adult. Maxillary
bones present.
Eyes with lids. A : : , . Salamandride.
Eyes devoid of lids . = ‘ ‘ . Amphiumide.
The Proteide 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. anguinus, is found
only in the caves of Austria. This species occurs in deep
cool water in regions of complete darkness. Its eyes, like
4 THE BIOLOGY OF, THE FROG CHAP,
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, Z7yphlomolge rathbunt, closely allied
to Proteus, was found only a few years ago in Texas, where
it was discovered in water thrown up from an artesian well.
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 Proteidz
FIG. 1.— Proteus anguinus. Front view of the mouth in the upper left
corner. (After Gadow, Cambridge Natural History.)
are the “mud puppies” or “ water dogs,’ which belong
to the genus Necturus. WVectwrus maculosus 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 Sirenide is represented by two genera, Siren
and Pseudobranchus, both of which are confined to North
I THE AMPHIBIA IN GENERAL 5
America. Each genus contains but a single species. The
larger of these, Szven /acertina, 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-
FIG. 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. s¢viazus, occurs in Georgia and
Florida.
The Amphiumide include forms of quite diverse appear-
ance, which are sometimes placed in distinct families. The
genus Amphiuma is represented by a single species, 4. 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-
de. ‘The former is represented by the large “ hellbender,”
6 THE BIOLOGY OF THE FROG CHAP,
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,’ is certainly remarkable.
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 4m. Nat., Vol. 16, 1882.
I THE AMPHIBIA IN GENERAL °
tail to and fro, and give other undoubted signs of vitality.”
The giant salamander of Japan, J/egalobatrachus maximus,
is closely related to the preceding species. ‘The largest
specimens known exceed five feet in length.
The Salamandride 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. Vertebrz am-
phiccelous. Toes 4-5 . - . - Amblystomatine,
BB. Parasphenoid with teeth. |
C. Vertebrze opisthoccelous. Toes 5. Tongue
largely free . : : ; , . Desmognathine.
CC. Vertebree amphiccelous. Tongue small
and largely free. - : . LPlethodontine.
AA. Series of palatal teeth in two fonattnainal
series diverging behind. Parasphenoid
toothless é ° ° : : . Salamandrine.
The subfamily Amblystomatine is represented in this
country mainly by the two genera Amblystoma and Chon-
drotus. Amblystoma contains quite a large number of
species. They are mostly of considerable size and _ fre-
quently spotted in color. ‘They are very retiring in their
habits, and are not often seen except in the spring, when
they go to the water to breed. Their eggs 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 Amblystoma (probably “g77num) 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 larve of A. Hgrinum 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 Desmognathine 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
Ny, United States. The
| species live con-
cealed in the day-
time under stones
or insheltered nooks
where the air is
moist. The female
FIG. 4.— Desmognathus Suscus. Female with of D. fuscus lays her
egg-mass. (After Wilder.)
<< =
'€ & a
eggs in two long
strings which she wraps around her body after having resorted
to a suitable hiding place. Another representative of this
subtamily is 7yphlotriton speleus, a blind species found in a
cave in Missouri.
I THE AMPHIBIA IN GENERAL 9
The Plethodontine 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’ to have the
peculiar habit of laying its eggs in holes high up in the
branches of live-oak trees.
The Salamandrine are mainly found in the Old World.
The well-known fire salamander of Europe, Sa/amandra
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 Jarva, in which they attain
a remarkable degree of development. The large size of
1 Univ. of Calif. Publications, Zodlogy, Vol. 2, 1904.
fe) 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 7° c7zs-
tatus at this time possesses a high serrated crest above the
-
\ \ “a”
ct qd \ y \
Coa ie sad dda |
on
fart,
EAQy
SARWAN
oP ity
4
= Wal WW .
PNM Yr a cs wn : WV \ RR an Sm
oN Ee aha
eu ee cake dude Wo,
> 455 4, 1uttan ye TER Pp ipky 2 NY v7
Qik Saas x By ed FNS By af 33 /
» tf »
awe oF jai ally s Hs (ie: he
FIG, 5.— Triton cristatus. 1, female; 2, male as he appears during the
breeding season. (After Gadow.)
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
(Diemyctylus 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, mznza-
I THE AMPHIBIA IN GENERAL It
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 1o to June. The eggs are laid in
small nests attached to masses of vegetation, or wrapped
within leaves of aquatic plants.
THE ANURA
The Anura, or tailless Amphibia, have a short, broad
body, with well-developed hind legs fitted for jumping.
They are divided by Gadow as follows : —
A. Tongue absent : - - . Aglossa.
AA, Tongue present (Plenermiisa
B. Halves of the shoulder girdle overlapping
in the middle line (4rczfera).
C. Sacral diapophyses dilated.
D. Terminal phalanges not claw shaped.
Ribs present. Upper jaw with teeth . Déscoglosside.
No ribs. Upper jaw withteeth . . Lelobatide.
No ribs. ‘Both jaws without teeth . Lufonide.
DD. Terminal phalanges claw shaped, usually
supporting adhesive disks oat oh ceed aati,
CC, Sacral diapophyses cylindrical , . Cystignathide.
BB. Halves of the shoulder girdle meeting in
the middle line and forming a me-
dian bar (Fzrmisternia).
C. Sacral diapophyses dilated é : . Lugystomatide.
CC. Sacral diapophyses cylindrical : . Ranide.
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
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 the young Pipa is quite fully formed, it breaks out and
makes its escape.
The Discoglosside 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, AZvzes
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 larve are about
ready to escape, the male takes to the water and frees him-
self of the mass.
The Pelobatide include but two American 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 hol-
brookt, 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.!
The Bufonide, 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 avery 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, 4m, Nat., Vol. 16, and Hargitt, 4m. Nat., Vol. 22.
14 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. Kirkland? and
Garman,” 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. Buckland® 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 Bufo kentginosus
in Massachusetts, according to Kirkland, is in April; in
Ithaca, New York (Gage),' from the middle of April to May ;
in Ana 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 2. /entginosus 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? 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 guercicus, which ranges from North Caro-
lina to Florida, is B. Zentginosus, 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 . Acr?zs.
BB. Toes scarcely webbed. Tympanum distinct . Chorophilus,
AA. Disks round, conspicuous. Fingers somewhat
webbed. Skin roughened. Arboreal . . Ayla.
Acris is represented by a single species, 4. grylus, 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. A. versicolor, so-called on account of its
remarkable change in color, is onc of the most common and
largest of the North American species, 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 Hylidz possess singular devices for carrying
the eggs. In Ayla 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,
c
18 THE BIOLOGY OF THE FROG CHAP.
and where they undergo development as far as the tadpole
stage.
The Engystomatide contain but one North American
species, Hxgystoma carolinense, which is found in the South-
ern States, from South Carolina to Texas. The large family
Cystignathide is represented on this continent by only three
species, which are confined to Mexico and Florida.
The Ranide, 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 RF. agilis, 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 plicz 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
i 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, &. grylo, 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 plicz 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, 2. sy/vatca is usually found in damp woods
often far from water. It occasionally occurs at a consider-
able elevation, one specimen having been taken by Mr.
Allen near the top of Mount Bartlett, New Hampshire, at
an altitude of twenty-five hundred feet. This frog, says Mr.
Allen, “is commonest in the beech woods and so closely
resembles in color the dead beech leaves, that not infre-
quently, even after having seen one jump, it is with diffi-
culty distinguished from the background. When frightened
it takes prodigious leaps in an erratic course, and usually
escapes into some hole or under a log. At night, while
walking in a damp spot in the woods, I found numbers of
them congregated in the path, where they had probably
come to. feed. ... Rarely have I heard them utter'a
sound in the summer, though occasionally, when in the
woods at night, I have detected their faint, rasping ‘ craw-
aw-auk.’ ”
Rana pipiens, the leopard frog. — This is perhaps the most
common of all the North American species of Rana. Its
ground color is green marked with rather large black
I THE AMPHIBIA IN GENERAL od §
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.”
Rana 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 plice. 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 NorthAmerica. Art. “Amphibia” in the
Riverside Natural History.
Duméril et Bibron. Erpétologie Générale ou Histoire compléte des
Reptiles.
Durigen, 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
des 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. Expériences pour servir a |’ Histoire de la généra-
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,
1896-1904.
Howes, G. B. Atlas of Practical Elementary Zodtomy, 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 Zodlogy,
1900.
Vogt und Yung. Lehrbuch der praktischen vergleichenden Anato-
mie, 2 Bd.
i), 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 skin
must be kept moist in order that cutaneous respiration may
take place. As soon as the integument becomes dry, as it
quickly does if the frog is exposed to 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 keép 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 sylvatca, is commonly
found in woods miles away from any pond orstream. 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 sey-
11 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, Rana fusca 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 #. fusca would devour large snails
(Helix hortensis and H. nemoratis) 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
Diirigen 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 CHAP.
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?
have found that the
protrusion is brought
about by the pressure
of the lymph in the
Fic. 6. — Figure showing the tongue of the large sublingual lymph
frog in three different positions. (After :
WWiedorshein) sac.; /“Dhis. tay “ie
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
1 Hartog, Ann. Nat. Hist., May, (7), 7, 1901.
2 Gaupp, Azat. Anz., Ig, 1901.
rT 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 Mana 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,
which is then pushed upward by the strokes of the hind legs.
The fore legs are also used in causing the body to move
from side to side, and unequal movements of the hind legs
are employed for the same purpose. Bendings of the body
are also used to help steer the course of the animal. The
hind legs usually make a stroke at the same instant, but the
frog not infrequently uses them alternately, especially when
struggling near an obstacle.
Attitude when Floating on the Surface. — When frogs
are kept in water beyond their depth, they spend a 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-
im tH ABITS 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
30 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
1 HABITS AND NATURAL HISTORY OF THE FROG 31
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 theirmaximum amount of air. Ananimalsuchasa 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.
He tfABIDS 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 isa 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.
b
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)? or May (Funke).* The size of the liver in
1 Gaule, Arch. ves. Phys., Bd. 81, Igoo.
2 Leonard, Arch. Anat. u. PAys., phys. Abth. Suppl., 1881.
3 Funke, Denkschr., Wien Akad. math. nat. Cl. Bd. 68, 1go0.
it= 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
esculenta) that this organ changed but little during the
winter months, but suffered a marked diminution in size just
before and during the breeding period in the late spring.
After this there is a gradual increase in the size of the fat
body until fall, when it reaches its maximum.
The advent of the breeding season is marked by great
changes in the reproductive system, both in the gonads, or
organs which produce the sex cells, and the various 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 CHAD,
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
papille 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
1 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 pipiens, 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 ina 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 kurope, 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 falla 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-
feria bl’ SAND 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
larvee 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 ¢ap-
ture an unwary specimen alive; but, for the most part, it
probably makes use of frogs killed by some other means.
Certain species of Glossiphonia (Clepsine), among the leeches,
live upon frogs and turtles ; but they do not require a very
large quantity of food, since one meal may suffice to keep
them alive for over a year. Like higher animals, frogs are
attacked by mosquitoes, but it is uncertain how much incon-
venience arises from this source.
40 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 along period. ‘The larve 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 larve 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. vee.
. “SS oN
a
EA
OOF,
Ore
sen
>
xX
Uy)
€
oy)
sia
Serene
ane LOO.
"Cag;
FiG. 19.—Sagittal section through a frog embryo. B&B, blastoccel or segmen-
tation cavity; “LP, lip of blastopore; #/, outer or epidermic layer of
ectoderm; “AZ, 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 ’gt,' Assheton ’94,” Morgan ’97.°) 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. Jour. Mic. Sct., Vol. 32, 1891.
2 Assheton, /éid., Vol. 37, 1894.
3 Morgan, “ The Development of the Frog’s Egg,” 1897.
96 THE ‘BIOLOGY+OF, THE PROG CHAP.
thick. The outer of these layers is the ectoderm ,; the inner,
the extoderm. 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
FIG. 20.—Sagittal section throuzh a frog embryo. B, blastoccel; AP,
dorsal lip of blastopore; 4/7’, ventral lip of blastopore; “, epidermic
layer of ectoderm; £4, inner or, nervous layer of ectoderm; “4, hypo-
blast or entoderm; 7}; 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
Vv 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 berinning of the cw/om, or
Fic. 21. — Transverse section through the middle of a frog’s embryo. CAH,
notochord; /, ectoderm; 4/7, mesoderm; VG, neural groove; VP,
neural plate; 7, 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 ofochori?’, a structure forming the beginning of the
H
98 THES BIOLOGY, (OF (THE SEROG CHAP.
vertebral column, and occurring in the embryo, when not
also present in the adults, of all vertebi.te animals.
It
WS
Ae
Cy
‘a aH
5 a
Cae 488
ie He
eet) = tea
Ae EY
C)
Ss
3
e =
FIG. 22. — Transverse section through a frog embryo before the closure of
the medullary or neural folds.
C, coelom or body cavity; CH, noto-
chord; ££, epidermic layer of ectoderm; AA, nervous layer of ecto-
derm; 1/7, mesoderm; M/Z, outer or somatic mesoderm; A/A, inner or
splanchnic mesoderm; NVC, neural groove; VL, dorsal root of spinal
nerve; VS, spinal cord; 7, archenteron; W/, liver diverticulum; YF,
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!
has recently studied the formation of the notochord in Bufo
lentiginosus and Rana 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 medi#ary 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, Bzo/. Bull., Vol. 4, 1903.
100 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
D
FIG. 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, P, later stages; £, 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 ven¢va/ sucker of the
Vv THE DEVELOPMENT OF THE FROG 101
tadpole. Subsequently these depressions meet in front and
become converted into a U-shaped groove. In the poste-
rior pair of plates, the gi// flares, 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
An
Sy
B
FIG, 24.—Embryos. Gf, gill plate; Gs, Gs’, two gill slits ; S, suckers; SP,
sense plate; Az, anus. (From Morgan, after Schultze.)
Gs Gs’
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 zzmer 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.
av 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 canals centrais. ‘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 cerebe/um, 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 meduda.
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 /obes, whose cavities or ventri-
cles communicate with the median canal, which becomes
narrowed by the thickening of its walls, and forms the aguwe-
duct of Sylvius, or iter a tertio ad quartum ventriculum. The
floor of the midbrain forms the crwra cerebrt.
104 THE: BIOLOGY, OF THEW ROG 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-
/amt, the roof remains thin and membranous, and the cavity
becomes the ¢“hird 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 z2fwndibulum, which extends downward. It
comes into close contact with another structure, the pituztary
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 vena 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 dazeral 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.
O eres
OADOAKS
er-eleeel. oe. "an
Neslossees QE
. me PH
LV
FIG. 25.— Sagittal sections through two embryos. In A the blastopore is
overarched and there is the beginning of the proctodzeum or anal invagi-
nation. In B the proctodzum has met and fused with an evagination of
the archenteron. A, anus; FB, forebrain; AZ, hindbrain; LI, liver
diverticulum; M2, midbrain; NV, notochord; N7, neurenteric canal;
PD, proctodzeum; PH, pharynx; PN, pineal body; P7, pituitary body.
(From Morgan, after Marshall.)
105
106 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. AR, archenteron; A/S, meso-
blastic somites; VV, notochord; VS, neural crest; 4/7, medullary tube;
PR, pronephros; SN, 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
Vv THE DEVELOPMENT OF THE FROG 107
arise as outgrowths from it. ‘The first of these to be formed
is the “ver, 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 é7/e duct, which is produced
by a lengthening of the neck of the original outgrowth. A
lateral outgrowth of the bile duct forms the gal 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 d/adder arises as an outgrowth of the ventral side
of the alimentary canal, near the posterior end ; its lining,
therefore, is of entodermic origin.
The Zvmgs 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 /avyzx, the mouth
of the depressed portion going to form the g/o//#s of the
adult.
The gt// sits 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
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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 lamella separate and form a pouch which
communicates with the pharynx. In most forms the hyo-
mandibular cleft forms the Hwséachian 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 “hyroid 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 ¢hymus, 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-
1 fe) 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 fatera/ plate. ‘The former becomes divided trans-
versely into a number of blocks called syotomes, 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 coeelom. The inner or splanchnic layer becomes closely
V THE DEVELOPMENT OF THE FROG III
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 dayer 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 perzzoneum, 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,
che THE BIOLOGY" OF THE EROG 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 celom. Owing to
FIG. 28.— A, B, C, three stages in the development of the heart. , endo-
thelium ; PZ, pericardium; PA, 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 éruncus
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 lacunz or spaces between the cells of
Vv THE DEVELOPMENT’ OF THE FROG 1 8)
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," 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 (zephrostomes) 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 J/Zalpighian 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. Zo6l, Harvard, Voi. 21, 1891.
I
114 THE BIOLOGY OF SHE FROG CHAP.
IN HM
wee HB
: HI
BR? H2
a
CX FD
he ee
@!
ie
%
=
Sesee
mt
FIG. 29.-— Horizontal section through an advanced embryo. AR, archen-
teron; BR1, BR2, BR3, branchial arches; 1, H2, /7, gill slits; AB,
hyoid slit; 4717, hyomandibular cleft; A7Y, hyoid arch; /J, infundibu-
lum; OF, olfactory pit; OS, optic stalk; #, pronephros; S, segmental
duct. (From Morgan, after Marshall.)
Vv THE DEVELOPMENT OF THE FROG II5
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 Miillerian duct was formerly |
supposed to arise by a splitting of the segmental duct, but
according to MacBride,' Marshall,” Gemmill,? and. more
recently Hall,* it develops quite independently of that
structure.
The reproductive organs first appear as ridges of the
peritoneum near the base of the mesentery (Marshall).
As the genital ridges increase in size they become 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.
2 Marshall, ‘‘ Vertebrate Embryology.”
3 Gemmill, Arch. f. Anat. u. Phys., Phys. Abth., 1897.
4 Hall, Bull. Mus. Comp. Zobl, Harvard, Vol. 45, 1904.
116 THE BIOLOGY. OF -THE FROG CHAP. ‘V
somite, collect around the notochord, forming a tubular in-
vestment. From the dorsal side of this mass ridges or folds
grow up and surround the spinal cord. The mesoderm covy-
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 vertebrze ; they soon become cartilaginous,
and finally ossify. The cartilaginous sheath grows inward
at the ends of the vertebree, 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 fen
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 Pana temporaria. 1, tadpole just hatched; 2, 3, successively
older tadpoles seen trom one side; 4, a slightly older tadpole seen from the dorsal 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 ro, successively older stages;
9, specimen with the ventral body wall removed, showing the coiled intestine and oulis ri,
both pairs of legs free; 12, 13, 14, successive stages in the resorption of the tail; 15, adult
frog. A,anus; AZ, hind leg; A, gills; AZ, gill opening; 1, mouth; XN, nasal opening;
O, eye; SN, ventral sucker. (From Weysse’s “Synoptic Text Book of Zodlogy,” after
Leuckart and Nitsche.)
117
118 THE BIOLOGY OF THE FROG CHAP.
pairs of zz¢erna/ 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 sfzvac/e. 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 papillz, 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 cel!s
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 papille 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 alge 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. Ay
general elementary description of the development of the frog, based
mainly on the work 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. Zo6l. Soc. London, 1891.
Camerano, L. Observationi sui girini degli Anfbi anuri. Boll.
Mus. Torino, 8, 1893.
Copeland, E. B. Heterogeneous Induction in Tadpoles. Science,
Nl. S:5\.V Gly 13-1900,
Hinkley, M.H. On Some Differences in the Mouth Structure of
Tadpoles of the Anurous Batrachians found at Milton, Mass. Proc.
Bos. Soc. Nat. Hist., Vol. 21,
Ryder, J. A. The “ Ventral Sucker” or “Sucking Disks” of the
Tadpoles of Different Genera of Frogs and Toads. Am. Nat., Vol. 22.
VI HISTOLOGY OF THE FROG 121
CHAPTER VI
HISTOLOGY OF THE FROG
SINncE 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 “@sswes which
are aggregations of cells of similar character bound together
by means of an ¢néercellular substance. In the bodies of
animals the classes of tissues commonly distinguished are
the following : —
1. Lpithehal.
2. Connective.
3. MLuscular.
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 epfithetal 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-
lum are distinguished according to the shapes of the cells.
122 THE BIOLOGY OF THE FROG CHAP.
An excellent example of flaztened 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 Kana pipiens. many places, as in the outer skin,
5s, stoma cell. Ss
there may be all transitional stages
between columnar epithelium and squamous epithelium.
Layers such as this which are several cells deep are called
stratified epithehum.
In some parts of the body there occurs a peculiar variety
called cehated epitheium 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. Ifa 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
124 THE? BIOLOGY) OF THE 2 ROG 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 curllike
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
ASS Mo their form and in
hw ee Zeid the appearance of
Sa their cytoplasm ;
FIG. 32.— Fibrous connective tissue from the usually they are
frog. c, connective tissue corpuscles; e, elas-
tic fibers; zw, 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 lacunze 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 sexzdon which is very dense and
VI SEISLOLOGY 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 adenozd, 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 Aya/ine 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 lacune, scat-
— tered irregularly through the matrix; in some cases minute
channels have been observed connecting the neighboring
lacunz 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
bones of the limbs, be-
tween the vertebrae and
at the ends of their trans-
verse processes, at the
tip of the urostyle, in the
pubis of the pelvic gir-
dle, in the hyoid and the
cartilages of the larynx,
and at both ends of the
sternum; it forms the
ee See hase from the head of the basis uhthe cranial
emur. c, cells; c’, cells in process of di-
vision; c. s, empty cell space; , matrix. the central axis of the
(After Parker and Parker.) lower jaw.
Calcified cartilage, which contains a deposit of lime salts
in the matrix, occurs in the suprascapula, the pelvis of old
frogs, and at the ends of some of the larger bones of the
limbs ; viz. the heads of the humerus and the femur.
The structure of dove 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, —
VI HISTOLOGY OF THE FROG 127
compact bone, which is very firm and dense, and spongy or
cancellous bone, which is made up of plates and bars forming
a structure which is comparatively loose and lacking in
strength. The latter is found within the center of the verte-
bre 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
with marrow, and
the outer surface
is covered by a
layer of perzoste-
wm, which is simi-
lar in structure to
the pericondrium Fic. 34. — A part of a cross section of the femur of
. he frog. c¢, canaliculi; Zc, lacunee; dm, lamellee:
rrounding the '* eae Pig ak
ene m, marrow cavity. (After Parker and Parker.)
cartilage. The
bony substance is arranged in concentric layers, or Zamelle,
which contain numerous /acune@, in which lie the Jone cells.
From the lacune fine branching tubes, or canaticul, 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 os/eodlasts, 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
128 THE BIOLOGY “OF THE SEROG 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 s¢zazed, or so-called voluntary,
and the wastriated, 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 f477//e@, which are con-
FIG. 35.—Unstriated muscle < :
fibers from the intestine of sidered by most investigators to
the frog. zw, nucleus. (After be the contractile elements of the
Howes.
ey 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
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 s¢v7ated 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
sarcolemma, which
is best seen in
places where the
contents of the
fiber are crushed
or broken apart. Fic. 36.— A, part of a fresh muscle fiber of a frog ;
Each fiber of vol- B, the same after treatment with distilled water
‘ followed by methyl green. 4, light bands; d,
untary muscle is to dark bands; 2, nuclei; s, sarcolemma showing
be regarded as gq more clearly where the fiber is broken, (After
Parker and Parker.)
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 nueleus
civides 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
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 sav-
costyles or fibrille, which extend the length of the cell. The
fibrille, 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 fibrillze
into bundles, the mzscle columns, which are separated from
each other by a thicker layer of sarcoplasm than that be-
tween the fibrillz. ;
The appearance of cross striation is brought about by the
division of the fibrille into segments, or sarcomeres. ‘The
sarcomeres are separated from each other by a very fine
dark line known as Avrause’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 “ine 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 fibrille.
The muscle fibers of the heart differ from both of the
above classes. ‘They are cross-striated, but each fiber con-
tains but a single nucleus. Each muscle cell is furnished
with branches which connect with the branches of contigu-
ous muscle cells, so that the whole mass forms a sort of
network.
The tissue of the nervous system consists of nerve fibers,
and nerve, or ganglion cells. Each nerve is composed of
usually a large number of nerve fibers, held together by con-
nective nerve tissue and surrounded by a common sheath.
A typical nerve fiber presents the following parts: a central
strand, or axzs cylinder; a sheath of fatty substance around
this called the medullary sheath, or white substance of
Schwann ; and a delicate external membrane, veurzlemma,
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 zucisures 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
fibrille, 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
132 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. Inthe 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
FIG. 37. — Nerve cells and fibers of 4 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; /.S, incisure of Schmidt;
m.s, medullary sheath; 2, nucleus; #/, neurilemma; JV.A, node of Ran-
vier; #., 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 fibrillze 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 anadbolism,; the opposite, or tearing- -
down processes by which it is resolved into simpler sub-
stances are known as afaboiism. If an organism grows, it is
evident that the anabolic side of the process must predomi-
wat) TELE WIGESEIVE SYSTEM AND ITS, FUNCPIONS. ».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
VII 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 100° C. destroys the action of ferments entirely.
The substances which may serve as food are the profeds,
fats, carbohydrates, water, salts of various kinds, and a few
other substances not failing 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 fas 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.
138 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
Fic. 38. — Alimentary canal of Rana es-
culenta, A, opening of the rectum into
the cloaca, C7; Du, duodenum; D,
ileum; t, boundary between the lat-
ter and the large intestine, R; AA,
urinary bladder; JZ, stomach; Zz,
spleen; Oe, esophagus; /y, pylorus.
(After Wiedcrsheim.)
inner surface is thrown into
longitudinal folds which ex-
tend also into the stomach.
There is no sharp line of
demarcation separating the
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
gastro-hepato-duodenal liga-
ment), which extends to the
duodenum and liver. The
wall of the stomach is much
vi 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 serouscoat or serosa, and it is formed by the peritoneum.
Within the serosa is a thicker layer, the swdserosa, 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 cercular muscles
which becomes thicker toward the pylorus. Internal to the
circular muscles is a layer of connective tissue, the szé-
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 mucose, 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 ina 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 CHAP.
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; 4, from pyloric
end; m, mouth; 2, neck; 4, 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 large 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 mucose.
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
mucose 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, Aepsz7, which acts upon the
proteids, converting them into soluble Aepftones. 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 ita
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
VII THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 143
eranular 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
Eclises 2. “Wp 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
more, deeply. . .\.
ifthe 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- Frc. 40.—Showing changes in the gastric glands
plasm, and it is of the frog. A, gland from a hungry frog
; ‘ which had not been fed for five days. The
sometimes pla ced cell outlines are indistinct and the granules are
near the outer bor- scattered throughout the cells. B, gland three
; hours after a meal; the granules have disap-
der of the cells. The peared along the inner border of the cells;
return to the normal lumen of the gland visible. C, gland twenty-
. five hours after a heavy meal; the cells are
er PCarance begins shrunken and not so full of granules. (After
about the fifth hour, —_ 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 fora 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;
vit > THE DIGESTIVE SYSTEM AND. ITS FUNCTIONS - 145
it is then enly 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; z.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 tke
larger part of thecell. . .. 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
L
146 THE BIOLOGY:,OF GHi- 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 of the mucous membrane of the esophagus causes
a disappearance of the granules from that region. ‘The con-
clusions of Nussbaum that in normal hungry frogs the cells of
the esophageal glands have an outer clear 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
vit 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 fepsinogen, 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 thanin the cardiacend. 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
148 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 zZewm, 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 muscle fibers ; then comes a
vi 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 mucosze 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 god/e¢ 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).
Pe Bl Be’
FiG. 41. — Part ofa cross section of the small intestine of the frog. 47, blood
vessels; cg, goblet cells; ef, ordinary epithelial cell; e¢.s, submucosa;
m.c, circular muscles; ./, longitudinal muscles; fe, 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,
vai THE DIGESTIVE SYSTEM AND ITS FUNCTIONS I51
the folds lose their regular arrangement, and in the posterior
third they assume a longitudinal direction.
The /arge 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 (Na,COs), and it
contains three ferments : s#eapsiu, which causes a splitting of
fats into fatty acid and glycerin ; amylopsin, which converts
starch into sugar ; and ¢vps7n, 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 dz/e
FIG. 42.— Liver and pancreas of frog. De,
common bile duct; Dey, cystic ducts; DA,
Dhl, hepatic ducts, which with the cystic
ducts combine to form the common bile
duct; G, gall bladder; Z, £1, Z2, Z3, lobes
of the liver turned forwards; Lp, hepato-
‘ duodenal ligament; 4/, stomach; P, pan-
creas; /1, pancreatic ducts entering the
common bile duct; /y, pylorus. (After
Wiedersheim.)
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 leit
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 the
pericardium, ventral
body wall, dorsal body wall, and the stomach and intestine.
The bile duct is formed by the confluence of the hepatic
Vite TEE DIGESTIVE SYSTEM AND ITS FUNCTIONS. 2-153
ducts leading from the lobes of the liver. The ga// 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 dle 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 dz/e 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 porfa/ 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 3oth, the cells become
granular throughout.” The decrease of granules was found,
as a rule, to be accompanied by an increase in the glycogen
in the cells, and vice versdé. 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
vil THE DIGESTIVE SYSTEM AND ITS FUNCTIONS — 155
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
B
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 22° C. The
proteid granules are scattered uniformly throughout the cell. Much the
same appearance is presented by the hepatic cells of a frog in summer.
C, cells taken from a frog starved for a long time in summer. The cells
are shrunken and the 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 g/ycogen,
a carbohydrate, having the same empirical formula as starch,
C,H,O;. 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,
Vil 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
Mpril, £1.12, per cent
It may thus be seen that the relative amount of pigment
contained in the liver increases through the winter, then
diminishes in the spring after the period of feeding.
The same observer found that in winter and early spring
the average size of the secreting cells and also their nuclei
was smallest in early spring, and increased during the
summer as is shown in the following table : —
160 THE: BIOLOGY ‘OF “THE FROG CHAP.
NOVEMBER | DECEMBER APRIL JUNE JULY
Average diameter |.0292 mm.,| .0162 mm.| .oI2._ mm.| .cI72 mm.| .0274 mm.
of cells
Average diameter
of nuclei .006 mm.| .0044 mm.| .0076 mm.| .0065 mm.| .c0o65 mm,
Similar measurements by Funke gave results approximately
the same as those obtained by Miss Leonard. The minimum
size of the cells in 2. fzemporaria according to Funke occurs
in May, the maximum in July and August. In &. esculenta
the minimum falls in June and the normal size is reached
two or three months later, but there is no well-defined period
of maximum growth. In both species the minimum size of
the liver cells as well as the liver as a whole occurs at the
time of breeding.
The fat content of the liver was found by Funke to vary
in an irregular manner both in &. esculenfa and RX. femporaria.
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 &. zemporaria the amount of fat in the liver is
very small compared with that in 2. esculenta, 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 percent 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 muchincreased 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 (2. escu/enda) 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
M
“762 THE BIOLOGY OF THE, £ROG 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 : —
MALE FEMALE
PAYS 6 LSPA ice hem Moth af tor Beek | ete 05775 0430
VC PGIEAEY,§ Suaksiitces Silhes Mets ee te 0436 .0382
Manche aig icrei tra id wands SRS ar ee .0502 .0348
Jor yh Le eae Nn ah eee eae re Paneer .0370 0323
EA alent er ee re tet. st lesye ch’ os Mics ae que were .0370 .0232
| FEU Sm VNR a tt hd Ne PE ae 0244 .0214
Hilly: oteetee! Capes heee tea te es Pete enna rhe .0317 0311
ECS pga OR em eae, Mee on a .0360 .0360
DODtSMIO To Vc ste crite, ee rec glee Nhat oe .0710 .0509
DCLODEED | ter ia eM) ltt Nae Gee fied cian 0571 0574
IMO VEMRDET ey Wor) cul itee Ver Nyae pf oma a ghee 50725 05882
Wecemibens. Ma. Caen} Mipyet ests. poeete we .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
vil 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, ‘l. 112, 1891.
Dewevre. Note sur la fonction glycogenique chez la grenouille
d@hiver. C. R. hebd. Soc. Biol., Paris, 1892.
164 THE BIOLOGY OF THE @#ROG CHAP.
Eberth, C.I. Die Pigmentleber der Frésche und die Melamie.
Virchow’s Archiv, Bd. 40, 1867.
Grutzner, P. Ueber Bildung und Ausscheidung von Fermenten.
Arch. ges. Phys., Bd. 20, 1879.
Grtiitzner 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 Einfluss der Jahreszeit auf der Leberzellen von
kana temporaria. Arch. Anat. u. Phys., phys. Abth. Suppl. Bd., 1887.
Moraczewski, W. von. Die Zuzammensetzung des Leibes von
hungernden und blutarmen Fréschen. Arch. Anat. u. Phys., Suppl. Bd.
_ 1900.
Nussbaum, M. Ueber den Bau und die Thatigkeit der Driisen.
Arch. milk. Anat. Bd. 23, 15, 16,ands21.
Oppel, A. Lehrbuch der vergleichenden mikroskopischen Anatomie.
Partsch, C. Beitrage zur Kentniss des Vorderdarmes einiger Amphi-
bien und Reptilien. Arch. mik. Anat., Bd. 14, 1877.
Stolkinow. Vorginge 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 Frésche erleidet. Bericht. Verh. K6nigl.
sachs. Ges. Wiss. Leipzig, Math.-phys. Cl., 1850.
VIII THE VOCAL AND RESPIRATORY ORGANS 165
CHAPTER: VIIt
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 /arvynx, situ-
ated just below the pharyngeal cavity at the beginning of the
entrance into the lungs. ‘lhe larynx opens into the pharynx
through the slit-like g/otts 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 crzcozd 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
166 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
FIG. 44.— Respiratory organs of the frog. A, ventral aspect. The right
lung, 7. /zg, 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. av, arytenoid cartilage; 4. hy,
main part of the hyoid; .2/, glottis; 2. tv. c, laryngo-tracheal chamber;
p. ¢. hy, posterior horn of hyoid; v. cd, vocal cord. (After Howes.)
the lungs. The aryéenoid cartilages are a pair of semilunar
valves, which rest upon the cricoid cartilage ; their upper
CL” Ca? A PB
eee ~ % =
y i RY
FIG. 45.— Cartilages of the larynx of the frog. A, from above; B, from
the side; Ca, arytenoid cartilage; C. /1 to C. 4, cricoid cartilage; P,
expansion of the cricoid; .Sf, spinous process of the cricoid; * * *,
prominences of the arytenoids. (After Wiedersheim.)
edges form the lateral margins of the glottis; .they afford
VII 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 cords, extending longitudi-
nally across the larynx. They may easily be seen from above
by spreading apart the two sides of the glottis, or from below
by removing the membranous floor of the 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 reénforce 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-
168 THE BIQLOGY, OF) EFRE- 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 afveo, 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-
fenta 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
VIII 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
(ng
Be gue fem
Fic. 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.z,external nares; 7, glottis; gv/, gullet; zm, internal
nares; /vg, lung; oéfs, olfactory chamber; fx, 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
VIII 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 the nares is an essential part of
the mechanism for filling the lungs. It was formerly thought
that the nares were closed by special muscles attached to
the valves, but it was shown by Gaupp that this function is
performed through raising the tip of the lower jaw, thus
elevating the premaxillaries and thereby closing these 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.
172 THE BIOLOGY ‘OF{ THE FROG CHAP.
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: (1) external respiration, or the exchange
Vil THE VOCAL AND RESPIRATORY ORGANS 275
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 whensubmerged in water at o° to 13° C. for several days.
At a higher temperature frogs tend to come to the surface
oftener for air, and if prevented from doing so, they may
die of asphyxiation. The skin functions as a respiratory
organ both in water and in air. If the nostrils of a frog be
plugged with wax, the animal may be kept alive in cool air
for several days.
The experiments of several investigators have shown that
more carbon dioxide is given off through the skin than through
the lungs. Klug found that the ratio of CO, 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
vit THE VOCAL AND RESPIRATORY ORGANS 175
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 CO, from both the lungs and skin compared under
various conditions. In Rana fusca at a temperature of 20° C.
the average ratio of oxygen income to CO, output in several
experiments on frogs taken at different times of year was,
in pulmonary respiration, O, 105 : CO,45 ; in cutaneous
respiration O, 52 : CO, 129. It is thus evident that in the
lungs the oxygen taken in is greatly in excess of the CO,
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 CO, eliminated through the lungs.
The respiratory quotients (/¢. ratio of O, to CO.) in the two
species at 20° C. are as follows :—
CuTANEOUS PULMONARY
RESPIRATION RESPIRATION
R. fusca. Deiter wees wade” a Fa iy peat sO 2.48 r.q. 32.00;
NGM ACTITE Rtn Vig Gk See os a 1.92 f£.q. 232 fq
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 (PROG CHAP.
and reaches its minimum in the winter, the ratios of oxygen
absorption at a temperature of 20° C. being as follows:
spring, 134.5 ; summer, 82; winter, 54. The output of CO,
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 CO, 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 CO, 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 CO, output in frogs exposed to
differently colored lights. The ratios of CO, production under
vr THE, VOCAL AND RESPIRATORY ORGANS E77
violet, yellow, and red light were found to be as 114, 103, and
roo respectively. Inred light there is but little more CO, 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 Athmungsmechanismus des Frosches. Arch. Anat.
u. Phys., phys. Abth., Suppl. Bd., 1900.
Berg, W. Untersuchungen iiber die Hautathmung des Frosches.
Inaug. Diss. Dorpat, 1868. ;
Bert, P. Des movements respiratoires chez les Batrachiens et les
Reptiles. Jour. Anat. et Phys., T. 6, 1869. Legons sur la physiologie
comparée de la respiration, Paris, 1870.
Bohr. Ueber die Haut- und Lungenathmung der Friésche. Skan-
dinav. Arch. f. Phys., Bd. 10, 1899.
Dissard, A. Influence du milieu sur la respiration chez la gre-
nouille. C. R. Ac. Sci., Paris, T. 116, 1893:
Gaupp, E. Zur Lehre von dem Athmungsmechanismus beim Frosch.
Arch. Anat. u. Phys., Anat. Abth., 1896.
Klug. Ueber die Hautathmung des Frosches. Arch. Anat. u. Phys.,
phys. Abth., 1884.
Krogh, A. On the Cutaneous and Pulmonary Respiration of the
Frog. Skandinav. Arch. f. Phys., Bd. 15, 1904.
Martin, H. N. The Normal Respiratory Movements of the Frog.
Jour. Phys., Vol. 1, 1878-1879.
N
178 THE BIOLOGY OF. [THE FROG CHAP.
Milne-Edwards, H. De la influence des agens physiques sur la vie,
Paris, 1824. Lecons sur la physiologie et l’'anatomie comparée de
Vhomme et des animaux, 1857-1865.
Moleschott and Fubini. Sull’ influenza della luce mista e chromatica
nell’ esalazione di acido carbonico per l’ organismo animale. Atti 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. ee '
IX THE SKIN 179
CHAPTER IX
THE SKIN
q
External Characters. — The skin is an organ of unusual
importance in the life of the frog, because, in addition to the
functions which it commonly performs among other animals,
it has a number of special functions which are peculiar to
the Amphibia, and which, in most cases, reach their fullest
development among the Anura. 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 plicze, 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 papillz often
occur, especially on the dorsal side of the body, some of
which, the tactile papillz, are permanent ; others, the sexual
papilla of the female, occur only during the breeding period.
Histological Structure. — The skin is composed of two
principal layers, the epzdermzs, and the cortum, 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 BIGLOGY 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 corneum) become very
broad and thin. It is the stratum corneum that is shed during
the molting process. ‘The gradual change in shape between
the cells of the inner and outer surfaces of the epidermis
is due to the fact that there is a continual 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 corlum. 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?
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 stellate
1 Loeb and Strong, Am. Jour. Anat., Vol. 3, p. 275, 1904.
1X 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 papillze
mentioned above.
182 THE BIOLOGY OF THE FROG CHAP,
a
' 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 potson glands. While Heidenhain,
Nicoglu, and others regard these two types of gland as
specifically distinct, other investigators (Calmels, Leydig,
Sezcesny, Junius) consider them as different phases in the
development of one and the same gland. However this
may be, the glands of the frog’s skin may be grouped into
two classes which are structurally and functionally different,
and we shall describe them separately without regard to the
question as to whether they are genetically connected.
The mucus glands are smaller and much more abundant
than the poison glands, and are found over practically the
entire surface of the body. In some places they are so thick
that they nearly touch. In Rana fusca, according to Engel-
mann, they average about sixty to each square millimeter of
surface. ‘Their ducts are narrow, and lined with a layer of
small flattened epithelial cells. The body of the gland is
lined with epithelial cells which form a single layer except
near the opening of the neck, where there are two layers.
It is this epithelium which forms the mucus which is dis-
charged into the lumen of the gland, and poured out through
the neck over the surface of the skin. The appearance of
the secreting epithelium varies greatly in different glands.
In some cases, more often in the smaller glands, the epi-
thelial cells are low, clearly marked off from each other, and
from the large lumen of the gland, and contain nuclei which
take up a large part of the cell. In other glands the cells
are elongated so that they fill a large part of the lumen, the
nucleus is relatively small, and situated near the base of the
cell, and numerous granules occur toward the free ends..
During secretion these granules swell up, and become con-
184
THE BIOLOGY OF THE’ FROG
CHAP.
verted into a transparent substance which 1s 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
Fic. 47.— Cross section of the skin
of the frog. YD, dermis or cutis; Z,
epidermis; 4.v, blood vessel; c.g7,
cutaneous gland cut through the
center; ¢.g/’, the same from one
side; d, duct of gland; 2.74 hf,
h.f’'’, horizontal fibers of connec-
tive tissue; 4.7, outer or horny layer
of the epidermis; #./, Malpighian
layer of the epidermis; 42, pigment
cells. (After Howes.)
and angular.
occur, and it is quite certain
that the differences are due
to the age of the glands, and
their different states of secre-
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
rounded and now wrinkled
tion.
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 plice.
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 M.G
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 ¢ells 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
186 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 &. arvaks,
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 papilla 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 papillz in the female of Rana
Jusca during the breeding period has already been sufficiently
‘Ix THE SKIN 187
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 arvats (R. oxy-
rrhinus) it has been described by Steenstrup and by Siebold.
In Rana fusca 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 Rana fusca, and Smith has observed
a blue coloration of the throat which he regards as charac-
teristic of the female of that species at this time.
The skin of the male of Raza 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
Or, the: eye.
With the exception of the swollen first finger of the male
and the dermal papille 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.
188 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: 4lack pigment cells (melanophores), 77terference
cells (leucophores), golden pigment cells (xanthophores,
xantholeucophores), and in some species of frogs ved fig-
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
hes
ae
Lt Y 4
FIG. 49.— Pigment cells from the frog, in different states of extension.
(From Verworn’s “ General Physiology.’’)
GR
T
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 form, and they lie a little above the black cells, which
constitute a sort of dark background. Their golden color is
due to a fatty pigment or lipochrome, which is sometimes
diffused throughout the cell and at other times aggregated
into large drops (Biedermann). This pigment is soluble in
alcohol, chloroform, and ether, giving a golden yellow solu-
tion which turns to a yellowish green when very dilute.
The same substance, according to Kiihne, produces the
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
the cells.
In
The yellow pigment is here uniformly distributed through-
llow pigment is here aggregated into small masses with
7
The ye
, Skin of a golden-yellow specimen.
gray specimen.
o. — Skin of Hyla.
B, skin of a
out the cells.
FIG. 5
IgI
(From Gaupp, after Biedermann.)
192 THE BIOLOGY OF THE FROG CHAP.
Jusca 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
Ix THE SKIN 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 eee the result of which is to produce
Ge Ge EE BS Stee TE SSE 2 a he Ps See ee
BEG Se BS OO a OCGA Ce Qa S. iy & DS
RneGe Ct : wees
B Bes RsO Sia. O08 @ 826 6 468 SO CO: 28
g J z 7
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.
fe)
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
oe 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 27°C. Changes of temperature
affect the concentration of pigment even in isolated pieces
of skin.
Various chemical substances affect the chromatophores,
some causing a contraction, others an expansion of the pig-
ment. Carbon dioxide produces a darkening of the skin ;
carbon monoxide, on the other hand, causes the skin to turn
pale. Chloroform and some other anesthetics 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 &. ages and Hyla by Werner.
Biedermann has shown that color changes are influenced
in a remarkable way by contact stimuli. Specimens of
1X 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 4 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
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
Ascherson. Ueber die Hautdriisen der Frésche. Arch. Anat. u
Phys., 1840.
Bert, P. Venin cutané de la grenouille. C. R. Soc. Biol. (8), T. 2,
1885.
Biedermann, W. Zur Histologie und Physiologie der Schleimse-
cretion. Sitzb. d. k. Ak. Wiss. Math.-nat. Cl., Bd. 94, Abth. 3, 1886,
Wien, 1887; Ueber den Farbenwechsel der Froésche. Arch. ges. Phys.,
Bd. 51, 1892.
Boulenger,G. A. The Poisonous Secretion of Batrachians. Nat.
Sel, Vols, 1602,
Donaldson, H. H. Onthe Absorption of Water by Frogs. Science,
Hys:, Vol, 43, TOOT.
Drasch, 0. Beobachtungen an lebenden Driisen, etc. Arch. Anat.
u. Phys., phys. Abth., 1889.
Dutartre, A. Sur les changements de couleur chez la grenouille
commune (ana esculenta). C. R. Hebdom. Ac. Sci., T. 3, 1890.
Ehrmann, S. Zur Physiologie der Pigmentzellen. Cent. f. Phys.,
Bd. 5, 1891.
Engelmann, T. W. Die Hautdriiseh des Frosches. Arch. ges.
Phys., Bd. 5, 1872.
Ix THE SKIN 199
Gadow, H. Color in Amphibia. Proc. Roy. Inst. Great Britain,
1902.
Harless, E. Ueber die Chromatophoren des Frosches, Zeit. wiss.
Zool., Bd. 5, 1854.
Heidenhain, M. Die Hautdriisen der “ Amphibien.” Sitzb. Wiirzb.
phys.-med. Ges., 1893.
Huber, 0. Ueber Brunstwarzen bei Rana temporaria, Zeit. wiss.
Zool., Bd. 45, 1887.
Junius, P. Ueber die Hautdriisen des Frosches. Arch. mik. Anat.,
Bd. 47, 1896.
Leydig, F. Ueber die allgemeinen Bedeckungen der “ Amphibien.”
Arch. mik. Anat., Bd. 12, 1876; Die anuren Batrachier der deutschen
Fauna, 1877; Integument briinstiger Fische und Amphibien. Biol.
Cent., Bd. 12, 1892.
Ueber das Blau in der Farbe der Thiere, Zool. Anz., Bd. 8, 1885;
Blaufarbiger Wasserfrosch. Zool. Garten, Bd. 33, 1892.
Overton. 39 Thesen iiber die Wasser6konomie der Amphibien, etc.
Verh. phys.-med. Ges. Wiirzburg, Bd. 36.
Pfitzner, W. Die Epidermis der Amphibien. Morph, Jahrb., Bd.
6, 1880.
Reid, W. Osmosis Experiments with Living and Dead Membranes.
Jour. Phys., Vol. 11, 1890.
Reid and Hambly. On the Transpiration of Carbon Dioxide
through the Skin of the Frog. Jour. Phys., Vol. 18, 1895.
Seeck, O. Ueber die Hautdriisen einiger Amphibien. Inaug. Diss.
Dorpat, 1891.
Steinach, E. Ueber Farbenwechsel bei niederen Wirbelthieren
bedingt durch directe Wirkung des Lichtes auf die Pigmentzellen.
Gent. f.Phys.,. Bd. 5, 1891.
Stieda, L. Ueber den Bau der Haut des Frosches. Arch, Anat. u.
Phys., 1865. ,
Stirling, W. On the Extent to which Absorption can take
Place through the Skin of the Frog. Jour. Anat. and Phys., Vol. 11,
1877. |
Stricker und Spina. Untersuchungen iiber die mechanischen Leis-
tungen der acinosen Driisen, Sitzb. Ak. Wiss. Math.-nat. Cl., Bd. 8o.
Abth. 3, 1879, Wien, 1880.
Townson, R. Observationes physiologicee de Amphibiis, Gottingz,
795°
200 THE. BIOLOGY: GP THE -FROG CHAP.
Werner, F. Ueber die Verainderung 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, 1893. .
Wittich, W. von. Die griine Farbe der Haut unserer Frésche, ihre
physiologische und pathologische Veranderungen. Arch. Anat. u. Phys.,
1854; Entgegnung auf Herr Harless’ : “ Ueber die Chromatophoren
des Frosches.” Ibid., 1854.
x THE EXCRETORY SYSTEM 201 |
CHAPTER 0X
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. |
202 THE “BIOLOGY .OF THE ROG CHAP.
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.
FIG. 52.— Male urinogenital organs.
Ao, Aorta; Cl, cloaca; Cv, postcaval
vein; FA, fat bodies; Ho, testes;
Ur, ureters opening into the cloaca at
S, S’; Vr,renal veins, (After Wieder-
sheim.)
The duct of the kidney,
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 wriniferous 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-
x THE EXCRETORY SYSTEM 203
rounding membrane, or Lowman’s capsule. ‘The artery,
vas afferens, 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
FIG. 53. — A urinifer-
ous tubule. c, col-
lecting tubule. m,
Malpighian body;
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
7, uriniferous tubule
leading from the lat-
ter to the collecting
tubule. (After Nuss-
baum.)
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
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 xephrostomes,
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
FIG. 54.— Diagram of a kidney show- peculiar one and occurs
ing the ureter and collecting tubules. only in the Anura. ‘The
C, collecting tubules; Z, longitudi- ae
nal canal of Bidder; .S, seminal lower Amphibia preserve
vesicle; 7, testis; U, ureter; VE, the typical arrangement of
vasa efferentia.
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-
x 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
S
vasa efferentia are is
. . =) i
Originally out- pan
ae
growths of the z |
o UO
walls of the Mal-
pighian —corpus- ’
cles which be-
come connected
with the _ testis.
In some species
(Rk. esculenta)
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 efferentia
lead into a longi-
tudinal canal (Bidder’s canal) which runs near the median
edge of the kidney.
In Rana fusca, according to Beissner, this canal is con-
1 ‘dayain ‘7 !sajnqn} snosasrutin ‘7 ‘awojso1ydau
po [elayel “7 | AaUpry JO aovyzins [esiop ‘J
‘
f
4
Oo
‘UIDA [e}I0d [euaI
NV’ + Apoq uerysidjeyy ‘Wy: Aaupry jo a
i
‘[euvo slapplq “gq ‘Bory ay} Jo Aauply oy} Jo uOoas ssoso v Jo Wesel
-JOa[00 ‘9
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-
1 lished between the vasa effe-
rentia and the collecting tubules,
and the spermatozoa, therefore,
are not found in the Malpighian
” "we. bodies and functional renal tu-
BiG. 30.57 Diagramto illustrate’) iules,.: Bidder’s canal oceurena
the course of the spermatozoa
through the kidney of Rava the kidneys of both sexes, but
Jusca. a, ampulla; ¢,collect- its function in the female is not
ing tubule; 7, Bidder’s longi-
tudinal canal; 4, uriniferous known.
tubule; , ureter; ve, vas In many of the lower verte-
ame Drop es a sa abEates (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 zeva/ arteries, which rise from the urino-
genital arteries, or direct from the aorta, and (2) the vena/
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
(arteriz recte), 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,
(NH,).CO, 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
4
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
x 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
B
FIG. 57.— Diagram of the bladder and rectum of the frog; A, from the
side; B, from below; 4&7, bladder; C, cloaca; R, rectum; S, sphincter
muscle; U, ureter; U7, 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
P
210 THE BIOLOGY: OF THE FEOG 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,’ “la prétendue .
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, destinée a
l’exhalation cutanée, semble étre 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, “ Erpétologie générale.”
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, 1808.
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. /c., Bd. 17, 1878.
Ueber die Entwickelung der samenableitenden Wege bei den Anuren.
Zool. Anz., Bd. 3, 1880. Ueber die Endigung der Wimpertrichter in
der Niere der Anuren. /c., Bd. 3, 1880. Ueber den Bau und die Thatig-
keit der Driisen. Arch. mik. Anat., Bd. 27, 1886. See also Anat. Anz.,
Bd. 1; 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 ovarvves, and in the male as /esées,
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 s¢ratum 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 seca folicul. After the
sPeREPRODUCIIVE-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
efeater part. of
the body cavity,
become reduced
to small wrinkled
organs, containing
the minute ova for
the following year.
The ovitducts
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
which extends to
the dorsal body
wall outside of the
mesovaria. Ante-
FIG. 58.— Urinogenital organs of a female frog.
NV, kidneys; Od, oviduct; Of, its opening into
the ccelom; Ov, ovary; P, opening of the ovi-
duct into the cloaca; .S, 5’, openings of the
ureters; U#, uterine dilatation of the oviduct.
(After Wiedersheim.)
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 u¢erus;
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 ; z.c. 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).
me 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 “ss are rounded or ovoid
organs lying ventral to the kidneys. Like the ovaries, they
are surrounded by peritoneum, which is extended dorsally
as a double membrane, the mesorchium, to the dorsal side
of the body cavity, where it becomes continuous with the
general ccelomic lining. The vasa efferentia, or ducts of
the testes, consist of a variable number of slender tubes,
which extend within the mesorchium to the inner margin
of the kidney, where they connect with Bidder’s canal. ‘The
vasa efferentia often branch and anastomose more or less,
in a way which varies greatly in different individuals.
The testis is made up essentially of a mass of tubules,
together with blood vessels and nerves, and a small amount
of connective tissue binding the tubules together. The whole
is surrounded by a connective tissue membrane, the /unzca
albuginea, outside of which is the peritoneum. ‘Toward the
outer portion of the testis the tubules extend radially, and
end blindly next to the tunica albuginea. Near the point
where the vasa efferentia enter they become coiled irregu-
larly. The vasa efferentia form a network within the testis,
into which the tubules open at their inner ends. Each
tubule possesses an outer membrana propria and an inner
216 THE BIOLOGY (OF “THE, EROG CHAP,
lining of cells, some of which (sfermatogonia, 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
FIG. 59.— A, cross section of one of the tubules of the testis; sf, bundles of
spermatozoa; ¢.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 FR. 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
fall and less rapidly during the winter. In Rana esculenta,
according to Ploetz, the testes vary little in size in different
months. This is, perhaps, due to the fact that during most
of the year all stages of spermatogenesis may be met with
in some of the tubules. The interstitial substance between
the tubules increases in Rava fusca from March to Septem-
ber. There is a storage of fat and pigment during this
period which later disappears (Ploetz, Friedmann). In
Rana esculenta there is most interstitial substance around
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.
218 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. Histogenése de la Glande Genitale Femelle chez Rana
temporaria. Arch. de Biol., T. 17, 1901.
Funke, R. Ueber die Schwankungen des Fettgehaltes der Fettfih-
renden Organe im Kreislauf des Jahres. Denkskr. Ac. Wiss. Math.-
nat. Cl., Bd. 68, 1900, Wien.
Giglio-Tos, E. Sur les corps gras des Amphibies. Arch. Ital. d.
Biol... 25, 2090,
Ploetz, A. J. Die Vorginge 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.
xll INTERNAL SECRETION AND DUCTLESS GLANDS 219
CHAPTER XII
INTERNAL SECRETION AND THE DUCTLESS GLANDS
Tue idea of internal secretion was first brought into
prominence by Brown-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.
x11 INTERNAL SECRETION AND DUCTLESS GLANDS 221
The Spleen. — The sfécen 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 AzZus. 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
222 THE BIOLOGY OF THE FROG
CHAP.
convert the proteid-splitting enzyme of that organ into an
active form. Doubt has, however, been recently thrown upon
FIG. 60. — Diagram show-
ing the position of the
thyroid glands, ¢; J,
lateral process of hyoid
cartilage; 7¢, thyro-
hyoid process of hyoid.
tion in Rana pipiens.
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
FIG. 61. — Part of a cross section of the thy-
roid of Rana pipiens. e, epithelial layer of
vesicles ; 2, colloidal substance in vesicle.
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-
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 ii
the disease called myxcedema or
cretinism, caused by the atrophy
of the thyroid, is often much
helped’) or. even. cured: by the
administration of thyroid extract.
The substance to which the thy-
roid owes its important function is Rice ie cee
a proteid with which a compara- position of the thymus, 7A.
tively large amount of iodin is in “2”, depressor mandibulze
combination. Treupel found that mere , es en
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 /zymus is a small, oval organ, some-
what reddish in color, situated behind the tympanic mem-
brane under the depressor mandibulz muscle. As in most
higher forms, the thymus diminishes in size with age. Maurer
found that in Rana esculen¢a 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. Hammar, however, failed to con-
firm these results. He found that both thymus glands may
be removed from the frog without injury, and concludes that
the results obtained by Abelous and Billard were the effects
of accidental infection.
The Pseudothyroid and the Epithelial Bodies. — The
pseudothyroid and the epithelial bodies are organs of similar
structure and origin. ‘They are derived from the modifica-
tion of the epithelium of the gill slits of the larva and are
therefore products of the entoderm. The two pgeudo-
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
xu 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
nharynx 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 larvee
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 adrena/ 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
x11 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 l’action antitoxique des capsules surrenales.
C. R. Soc. Biol., 1895.
Abelouset Billard. Recherches sur les fonctions du thymus chez
la grenouille. Arch. Phys. Norm. et Path. Année, 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
aprés la destruction des capsules surrenales, /c., 1891. Toxicite de l’ex-
trait alcoholique du muscle de grenouilles prives de capsules surrenales,
/.c.. 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, /.c. (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.
Rey, med. Suisse romand., T. 16, 1896.
228 THE BIOLOGY OF THE FROG CHAP.
Hammar, J. A. Ist die Thymusdriise beim Frosch ein lebenswich-
tiges Organ? Arch. ges. Phys., Bd. 110, 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 Schilddriise und Thymus bei den
Amphibien. Anat. Anz., Bd. 3, 1888.
Maurer, F. Schilddriise, Thymus, und Kiemenreste der Amphibien.
Morph. Jahrb., Bd. 13, 1888. Die Epidermis und ihre Abkémmlinge,
1895.
Moore and Vincent. The Comparative Chemistry of the Suprarenal
Capsules. Proc. Roy. Soc., London, Vol. 62, 1897.
Oliver and Schafer. 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.
xl THE SKELETON 229
CHAPTER XIII
THE SKELETON
In the skeleton, or bony framework, of the frog we com-
monly distinguish two main divisions, the axza/, consisting
of the skull and vertebra, 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 vesceral 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 prodtic
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.c.hy,
anterior cornu of hyoid; actd, acetabulum ; AST, astragalus; 6.Ay, basi-hyal; C, calear;
CAL, calcaneum; EX.OC, exoccipital; FE, femur; /07, fon', fontanelles; FR.PA, fronto-
parietals; HU, humerus; IL, illum; J/X, maxilla; odf.cf, oliactory capsule; of.p7, otic-
process; 7.c.Ay, posterior cornu of hyoid; PAX, premaxilla ; PR.OT, prootic; RA.UL,
radio-ulna; SP.ETH, sphenethmoid 42 SO, squamosal; S.SCP, supra-scapula; és, sus-
pensorium; TI.FI, tibio-fibula; 77. Z7, transverse Process ; UST, urostyle; V.]; cervical
vertebra; V.g, sacral vertebra; VO, vomer; I-V, digits. B, the fourth vertebra seen from
in front. a.zyg, anterior zygapophysis ; cz, centrum , 7m, lamina; 2.s5p, neural spine ; pd,
pedicle; ¢.pr, transverse Process. (From Parker and Haswell’s Zodlogy, slightly
altered from Howes.)
230
THE SKELETON 31
CHAP. XIII
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 parasphenotd, which is in the shape of a dagger with-
(‘somoPy WO. pasajye APYSIs ‘ASO[QOT S[[aMSeF] pue JoyIeg Wo1y) “IaWoA ‘OY
‘sodgys ‘Zs ‘jesowenbs ‘Os ! prlowyjoueyds 'HLa'ds + ;eonf-ojerpenb 7 / ‘QO + prosA1a\d
‘Od :onooid ‘Loud $ $eyrxeutsid ‘yd +2olped ‘pag : ploay jo nusoo 10194sod ‘Aya g
‘prousydseied ‘ygs'hd ‘ayeipenb-oyejed ‘wb yng ‘auyered ‘7pqg :ssaeoid oN0 ‘«g¢70
‘amnsdvo A10j0BI0 ‘Za:/o ! a]Apuoo |vpdI9090 ‘w7'70 : SaAIaU YJUIAVS PUR YYY Y} 1O0fF UatUIOF
‘L‘S au Suawesoy ondo ‘2 ‘au tyeseu ‘PAY + el[IxewW ‘yyy +:sauod UeI[axOIW-e}UNU “HOW
‘W {yejoued-ojuoy ‘ro yy ‘wnuseu uoweiloy ‘ove sof :eydioooxe ‘D0°xXa | AIvjuap
‘TN {e[[aumnjoo ‘Too !proky jo Apoq ‘dy"g + ploay Jo Nutoo JoMajyue Ayry —*sjeyideo
oe UI SaUOg sUvIqUIAW JO ASOY} | YOIY} Ul SauoO ase|I}ivo Jo soWeVN “pulyaq Woy ‘DO
‘ prody puv Mef JaMO] OY} SUTMOYS ‘APIs Je] 94} WOT; “_ + (aunsy JO Yo]) opis yYSsIA ay} Woy
peaowlad SoUOG SULIqUIOW oy} YIM ‘YyywoUaq WOY WY =‘ vLev4ogua, DUBY JO |[INYS— ‘bg “OIA
90°X3 hkyso
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 sagz¢ta/
suture. Fach 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 zasa/s are two narrowly triangular bones, lying above
the nasal capsules ; their bases, which lie near each other in
the middle line, are separated from the fronto-parietals by a
small part of the roof of the ethmoid.
The vomers lie ventral to the nasal capsules; each has
three outer processes, between the two posterior of which
occur the internal nares; the ventral surface bears the
vomerine teeth.
Suspensorium and Jaws. — The jaws are attached to the
cranium by means of an intermediate suspensory apparatus
in which the following separate bones are to be distin-
guished : —
(1) The tympanic (sguamosa/), 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 prodtic, while the anterior end extends obliquely
downward in front. Below the tympanic lies (2) the pzery-
XIII THE SKELETON 233
goil, 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 guadrato-jugals. These are short bones, devoid of
teeth, articulating behind with the pterygoid and tympanic,
and joining the maxillary in front by an oblique suture. The
maxillaries are the largest bones of the upper jaw; they
connect with the Aremaxil/aries 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 zvztermaxillaries
or premaxillaries are the two small bones which form the
apex of the maxillary arch; they are furnished with teeth
and are produced backward on the upper side into the
facial processes which are instrumental in closing the nares
in respiration.
The lower jaw, or mandibular arch, is composed of a
central core, called Meckel’s cartilage, which is partly sur-
rounded by two membrane bones. The bone at the proxi-
mal end is called the angu/are, or angulo-splenial. Meckel’s
cartilage runs in a groove along the outer side of this bone
and widens out at the posterior end, where it forms the
facet for articulation with the suspensorium above.
CES
(Ss
K
NN
connected close to the hip joint by the ¢ransverse tac vein,
which passes dorsally to the femur and enters the body cavity
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-
ly
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; azt.ad, anterior abdominal vein; dz, duodenum;
du', artery to same; 9, gastric vein; £.0l, gall bladder; @v’, dv’, right
and left lobes of the liver respectively; %, portal vein; fc, pancreas; sé,
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 sendsits branches. The renal
portal receives the dorso-/uméar 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 dulbi cords, 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 Briicke studied the structure
and action of the frog’s heart in detail and explained the
mechanism by which the two kinds of blood were kept .
278 THE, BIOLOGY OF THE FROG CHAP.
separate. Briicke’s observations were extended and in most
points confirmed by Sabatier in 1873. ‘The interpretation
of the latter author has been followed by Gaupp in his
recent revision of Ecker’s “ Anatomie des Frosches.”
When the auricles contract, the blood from the left auricle,
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 cords. 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 cisterna magna, which extends above
most of the dorsal side of the body cavity.
XV SEE, 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 4ymph hearts and also through the ciliated nephro-
stomes on the ventral surface of the kidney which lead from
Fic. 78.— Lymph sacs of Rava. The dark lines indicate where the septa
extend between the skin and the body. a, abdominal lymph sac; 4,
lateral brachial lymph sac; c, crural lymph sac; d, dorsal lymph sac;
J, femoral lymph sac; Z, 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. bBeitrage zur vergleichenden Anatomie und Physi-
ologie des Gefasssystems, 1. Ueber die Mechanik des Kreislaufes bei
den Amphibien. Denkskr. d. k. Akad. Wiss. math.-wiss. Cl., Bd. 3,
Wien, 1852.
Dekhuysen, M.C. Ueber das Blut der Amphibien. Verh. Anat.,
(Ges... 6 Vers, 1802.
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. Himatologische Studien, 1. Ueber die Blutbildung
von Fréschen. Virchow’s Archiv, Bk. 143, 1896.
Sabatier, A. Etudes sur le cceur et la circulation centrale dans la
série des Vertébrés. Ann. Sci. Nat. (5), T. 18, 1873.
Térdék, 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
Tue 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 cerebro-spina/, consisting of the
spinal cord and brain; the feviphera/, 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 jilum terminale, 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 vertebree to which they correspond ; the
bundle of roots thus formed, together with the f7/um ‘ermt-
uale, is known as the cauda equina.
XVI
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 se pa-
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 pra mater and
arachnoidé of the
higher
brates ; only here
and there does it
present a division
verte-
THE NERVOUS SYSTEM
285
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. az,
atlas, or first vertebra; az, auditory capsule;
é.s, brachial enlargement of the cord; fz, filum
terminale; 2.¢7, prodtic ganglion (trigeminus, or
Gasserian ganglion of many authors); 45, lum-
bar enlargement of cord; mz (V’’’), mandibular
branch of fifth nerve; mx (V’'’), maxillary branch
of trigeminus nerve; my, myelon, or spinal cord;
za, right nasal sac; za’, left nasal bone; %.c,
neural canal; o/., olfactory nerve; oph (V’),
ophthalmic branch of fifth nerve; //, optic nerve.
(After Howes.)
286 THE BIOLOGY OF THE FROG CHAP.
into two lamella. 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
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 canals cen-
traits, lined by a single
FIG. 80.— Cross section through the verte-
bral column, and spinal cord. showing the
origin of the spinal nerves. c.c, central
canal; cz, centrum; d./, dorsal fissure;
dm, dura mater; dz, dorsal root of
nerve; g.#72, gray matter; 2x, ganglion of
dorsal root; #.a, neural arch; #.sf, neural
spine; f.7, pia mater (the reference line
layer of epithelial cells.
This canal is the rem-
nant of the lumen
formed by the closing
over of the edges of
should stop at the margin of the cord) ;
Z, nerve trunk; 77.pr, transverse process;
v.f, ventral fissure; wm, white matter.
(After Howes.)
the medullary groove
during development ;
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 cornua 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
FIG. 81. — Diagram of the spinal cord showing the paths taken by nervous
impulses. The direction of the impulses is indicated by arrows. ¢.c¢,
central canal; co/, collateral fibers; c.cor¢, cell in the cerebral cortex;
c.g, smaller cerebral cell; d.c, cells in dorsal horn of gray matter; @7,
dorsal root; g, ganglion of dorsal root; g.c, ganglion cell in dorsal
ganglion; 2.m, gray matter; d/, muscie; w.c, cell in medulla ob-
longata; m.f, motor fiber; S,skin; s.f, sensory fiber; sf.c, spinal
cord; v.c, cells in ventral horn of gray matter; v7, 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 &. fusca), 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 jvs¢ nerve emerges between the first and second
vertebre ; 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 vertebree. 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.
i? y
og > te
LEE py
ig RA cl aS
ry Let —
ss
aE
9
ie EAECATALICLCEL HT CELE
v
a
es HR HFT SATE
336 THE BIOLOGY OF THE FROG CHAP,
“masses of similar crystals which are commonly called the
otolths.
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 ampulle. The
whole labyrinth is filled with a fluid called the exdolymph,
and it is surrounded by another fluid, the Aex7/ymph, 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 ovals,
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 ¢ympanic
membrane, which is readily seen at the side of the head
behind the eye. ‘This membrane is nearly circular in form
and is attached by its outer margin to a ring of cartilage, the
annulus tympanicus. Near its middle it gives attachment
to the outer head of a rod, the columella, which extends
across the cavity of the middle ear, and joins the small 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.
MF,
XVII, THE SENSE ORGANS 337
The ean 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
‘ Z
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 (A. escu/enta) 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 330
“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 clamitans)
was found to respond to sounds varying in pitch between
fifty and ten thousand vibrations per second.
The reaction time of frogs to visual stimuli is also 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 Voreille interne chez la grenouille. Arch. Phys. Norm. et Path. (5),
¥.745 1802:
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. Zc., 1882.
Landois, H. Kénnen Frésche héren? 25 Jahresber. westfal. Prov.
Vien. Loy:
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.
xvil THE SENSE ORGANS 341
Plateau, E. Sur la vision des poissons et des amphibies. Ann.
Wate, oci-(5.),,b.275. 1967.
Retzius,G. Das Gehérorgan der Wirbelthiere, 1. Stockholm, 1881.
Schrader, M. Zur Physiologie des Froschgehirns. Arch. ges.
Phys., Bd. 41, 1887.
Steiner, J. Die Functionen des Centralnervensystems und ihre
Phylogenese. Leipzig, 1888.
Yerkes, R. M. Inhibition and Reinforcement of Reactions in the
Frog, Rana clamitans. Jour. Comp. Neur. and Psych., Vol. 14, 1904.
The Instincts, Habits, and Reactions of the Frog. Monogr. Suppl.
Psych. Rev., Vol. 4, 1903. The Sense of Hearing in Frogs. Jour.
Comp. Neur. and Psych., Vol. 15, 1905.
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-
XVII 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
sut 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
XVIII INSTINCTS AND 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 aman. 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 phototacic or heliotropic. ‘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 Aana 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
XVIII 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 60° or
over. Frequently the frog assumed an almost erect posi-
tion, by means of placing the fore feet against the side of
fhe jars”
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 uncovy-
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 phofopathy. 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 25° C. the positive response
becomes considerably accelerated, the frog moving more
quickly and more directly toward the light end of the box.
Above 30° C. the movements become irregular, owing to
the predominant effect of heat. When the 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 weil
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-
XVUI INSTINCTS AND TROPISMS 351
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 77iton 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 10° 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
To ¢Geand a.”
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.
XIX 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
2A
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
XIX THE INTELLIGENCE OF THE FROG 358
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
Fic. 94.— Labyrinth used in studying the formation of habits by the frog.
From A the frog entersat #. G, glass plate; /, partition; 7, red surface ;
W, white surface; 7, 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
rogs 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 4 the box is divided so that a choice of two
paths is given. If the frog passes to the night, 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 , 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 4 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., 1894.
Brehm. Thierleben. Bd. 7.
Jourdain, S. De Vintelligence des batraciens. C.R. Ass. frang.
Av. Sci., 2gme 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 Reactions in the Frog, Rana clamitans. Jour. Comp. Neur.
and Psych., Vol. 14, 1904.
INDEX, OF AUTHORS
Abbott, 13, 19, 21, 353, 358.
Abelous, 224, 227; Abelous and Bil-
lard, 224, 227; Abelous and Lang-
lois, 226, 227.
Ach, 337, 349.
Adami, 210, 2II.
Albertoni, 318.
Allen, 15, 20, 21.
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.
Bolir;, 176,177.
Bolau, 227.
Boll, 322.
Born, SI.
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; 16,,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.
Beker! 22, 140,252, 270.
Eidam, 44.
Ehrmann, 180, 192, 198.
Engelmann, 181, 183, 184,
198.
Ewald, 190, 337.
Farrington, 211.
Fatio, 187.
Field; 113.
185,
Fisher-Sigwart, 22, 25, 38, 46, 49, 50,
52, 54, 59, 85.
Fixsen, 70.
Fraisse, 55.
Frank], 211.
Frear, 6.
359
360 INDEX
Fuchs, 282. Klug, 174, 177.
Funke, 34, 160, 218. Knauer, 26, 324, 357, 358.
Knauthe, 57.
Gadow, 3, 7, 9, II, 22, 199. Krawzoff, 311.
Gage, II, 15, 22. Krogh, 168, 175, 177.
Garman, 14. Krukenberg, Igo.
Gaskell, 318. Kiihne, Igo.
Gaule, 34, 162, 227, 282.
Gaupp, 26, 139, 171, 177, 185, 222, | Labbé, 43.
224, 225, 278, 285. Lahousse, 154.
Gemmill, 115. Landois, 338, 340.
Giglio-Tos, 217, 218. Langendorff, 35, 319; and Mozeik,
Girard, 337, 349. 161.
Goltz, 303, 305-307, 312, 313, 318, | Langley, 142-148, 154, 161, 164.
337, 340°. Laudenbach, 340.
Graber, 346, 352. Leidy, 40-42.
Griitzner, 142, 145, 146, 164. Leonard, 34, 154, 159, 160, 164.
Guth, 332. Leydig, 22, 183, 186, 187, 189, Igo,
199.
Hall, 115. Lister, 195.
Hammar, 224, 228. Loeb, J., 316, 319, 352.
Hardesty, 318. Loeb, L., and Strong, 180.
Harless, 181, 192, 199, 318. Loeser, 310, 319.
Hartog, 26. Lombard, 254.
Hay, 22. Loos, 41.
Heidenhain, M., 164, 181, 183, 199. Luchsinger, 161.
Heidenhain, R., 149, 150, 164. Lyon, 340.
Hensen, 337-
Hering, 228. Macallum, 282.
Heubel, 319. McBride, 115.
Hinckley, 17, 120. Marquis, 263, 282.
Hirschberg, 326, 333, 340. Marshall, 22, 95, 109, II5, I19,
Hoffmann, 22. 204.
Holbrook, 22. Martin, 177.
Howes, 22, 149. Massart, 260.
Huber, 36, 199. Maurel and Lagriffe, 56.
Huxley, 22. Maurer, 109, 223, 225, 228, 269.
Hyrtl, 269. Mayer, 181, 224, 228, 277.
Mendelsohn, 319.
Jordan, D. S.., 17, 22. Merzbacher, 319.
Jordan, E. O., 11. Milne-Edwards, 178.
Junius, 183, 185, 199. Mivart, 22.
Moleschott and Fubini, 176, 178.
Kalm, Ig. Moore and Vincent, 226, 228.
Kato, 310, 311, 319. Moraczewski, 164.
King, 99. Morgan, 45, 95, 99, 119.
Kirkland, 14, 15. Miiller-Erzbach, 57.
INDEX 361
Neumann, 217, 263, 282. Seeck, 199.
Nickerson, 41, 42. Sewell, 142, 145, 147, 148.
Nicoglu, 181, 183. Sirotinin, 319.
Nothnagel, 319. Slonaker, I5.
Nussbaum, 51, 142, 146,148, 164, 208, | Spallanzani, 22, 48, 55, 82, 85, 305,
ONE, 217: 307.
Steenstrup, 186, 187.
Oliver and Schafer, 227, 228. Steinach, 47, 49, 195, 199, 332-
Oppel, 150, 154, 164. Steiner, 195, 308, 311, 319, 337, 341-
Overton, 199. Stejneger, 19.
Stieda, I99.
Paneth, 149. Stilling, 225.
Parker, G. H., 350, 352. Stirling, 199.
Parker, W. K., 254. Stolkinow, 164.
Partsch, 142, 148, 164. Stossich, 4T.
Pfitzner, 181, 199. Stricker and Spina, 199.
Pfluger, 47, 51. Swiecicki, 142, 164.
Plateau, 333, 34I.
Ploetz, 35, 217, 218.
Ponfick, 154, 22!.
Pouchet, 189.
Tarchanoff, 218.
Torok, 282.
Toldt, 217.
Torelle, 346, 349, 352.
Townson, 59, 199.
Regnault and Reiset, 178.
Treupel, 223, 228.
Reid, 197, 199, and Hamby, Ig9.
Retzius, 341.
Ritter, 9.
Robinson, 95.
Roésel von Rosenhof, 22.
Ryder, 120.
Valatour, 139, 149.
Van Genderen Stort, 331.
Verworn, 60.
Volkmann, 319.
Vulpian, 319.
Sabatier, 269, 278, 382.
Schleiden, 82, 121. Weber, 159.
Schlosser, 319. Wedenski, 178.
Schrader, 308-311, 316, 319, 337, 341. | Werner, 196, 200.
Schultze, F. E., 181. Wiedersheim, 71.
Schultze, M., 331. Wittich, Von, 161, 190, 192, 200.
Schultze, O., 86. Wright, 41.
Schultze, P., 139. Wyman, 320.
Schwann, 82, 121.
Sezesny, 183. Yerkes, 319, 338, 341, 355, 358.
INDEX OR SUBJECTS
Absorption of food, 156 ; of water, | Auricle, 73, 264, 279.
58, 187, 197.
Acanthocephali, 41.
Acetabulum, 243.
Acris, 17; A. gryllus, 17, 45.
Adipose tissue, 125.
Adrenal body, 202, 225.
Afferent-nerves, 300.
Aglossa, II.
Alveoli, 168.
Alytes obstetricans, 12.
Amblystoma, 7, 8, 46;
matinee, 7.
Amphiuma, Amphiumidae, 5.
Ampulla of ear, 335.
Anabolism, 134.
Angulare, angulo-splenial, 233.
Annulus tympanicus, 336.
ATTA 2 51.
Anus, 64, 74, IOI.
Aorta, 264, 270.
Aortic arches, I12.
Apoda, 3.
Aqueduct of Sylvius, 103.
Aqueous humor, 326.
Arachnoid, 285.
Archenteron, 95.
Arcifera, I1.
Area centralis of retina, 331.
Artery, 268. For particular arteries,
see 268-272.
Arytenoid cartilage, 165.
Ascaris entomelas, 40.
Astragalus, 230.
Atdas, 229.
Auditory organ, 63, 334; nerve, 298,
330.
Amblysto-
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.
Bloods. r2r 258 :
262; vessels, 268.
Body cavity, see coelom.
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.
Bufonide, II, 13.
Bufo quercicus, 16.
Bulbus cordis, 264, 277.
Bullfrog, 18, I9, 72, 215, 216.
corpuscles, 258,
Calcaneum, 230.
Canal, central of cord, 103, 286;
semicircular, 335, 337-
363
364 INDEX
Canaliculi, 127. Coracoid, 240.
Capillaries, 269, 280. Corium, 179.
Carbohydrates, 137. Cornea, 324.
Cardiac gland, I4o. Corpus adiposum, 77, 217.
Carotid artery, 269; gland, 269, 278. | Corpuscles, of blood, 258, 262; of
Carpus, 242. lymph, 262.
Cartilage, 125; cartilage bones, 235. | Cranial nerves, 295.
Cauda equina, 284. Cranium, 229.
Cell theory, 82, 121. Cricket frog, 17.
Centrum, 237. Cricoid cartilage, 165.
Cerebellum, 103, 291, 293, 296. Croaking, 31, 302.
Cerebral hemispheres, cerebrum, 103, } Crura cerebri, 103, 293.
291, 293, 294, 309. Cryptobranchide, 5.
Chiasma optic, 294, 296. Cryptobranchus, 5, 6, 39, 59-
Choanee, 69. See also Nares. Crystalline lens, 326.
Chondrotus, 7. Cuticle, 122,179,
Choroid coat of eye, 237; plexus, | Cuticula dentis, 72.
103, 293. Cutis, 179.
Chorophilus, 17, 46. Cystignathidee, 11, 18.
Chromatophores, 180, 188. Cytamceba, 43.
Cilia, 122.
Ciliated epithelium, 71, 122. Dactylosoma, 43.
Circulation, 258. Dentale, 234.
Cisterna magna, 79, 280. Dentine, 72.
Clavicle, 240. Dermal plicz, 67.
Cleavage, 89. Desmognathine, 7, 8.
Clepsine, 39. Desmognathus fuscus, 8.
Cloaca, 64, 74. Development, 81.
Coagulation, 261. Diemyctylus viridescens, Io.
Coccidium, 43. Digestion, 136, 142, 148, I51, 156.
Codonocephalus mutabilis, 42. Digestive organs, 134.
Ceecilians, 3. Diplodiscus subclavatus, 42.
Coelom, 73, 97, 110. Discoglossidz, I1, 12.
Ccelomic fluid, 79. Distomum, 41, 42.
Cohnheim’s fields, 130. Diving, 29.
Cold, effects of, 55-58, 85, 158, 176, | Drepanidium, 43.
196; influence on blood, 262, 351. | Drum, 63.
Color changes, 37, 58, 187-197. Duct, bile or gall, 73, 152; ‘cystie;
Columella, 63, 336. 153; hepatic, 152.
Commissures of cord, 286. Ductus endolymphaticus, 335.
Compensatory motions, 308. Duodenum, 74, 148.
Condyles, occipital, 229. Dura mater, 285.
Cones of retina, 329.
Conjunctiva, 325. Ear, 63, 334; development of, 102.
Connective tissue, 121,123; subcuta- | Echinorhynchus, 41.
neous, 182. Ectoderm, 96, 103.
Copulation, 47, 52, 305. Efferent nerves, 300.
INDEX
Egg laying, 44, 48, 50, 214.
Eggs, 81, 85.
Elastic tissue, 124.
Embryology, 81.
Enamel, 72.
Endolymph, 336.
Enemies, 38.
Engystoma carolinense, 18.
Engystomatidz, 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, 2209.
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.
Fibrillze, 128.
Fibrin, 261.
Fibrinogen, 262,
Fibrous tissue, 123.
Fibulare, 244.
Filum terminale, 284.
Fissures, of cord, 286; of brain, 294.
Fontanelles, 236.
Food, 24, 119, 134-137.
365
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.
Gangiion, 284, 289; Bidder’s, 299;
Dogiel’s, 299; Gasserian, 296;
jugular, 298; prodtic, 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, ror.
Gland, cardiac, 140; carotid, 269,
278; ductless, 220; esophageal,
141; gastric, 140; mucus, 183;
poison, 183; pyloric, 141; thymus,
Tog, 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.
Hemoglobin, see Hemoglobin.
Heemogregarina magna, 43.
Hezmosporidia, 43.
Hearing, 338.
Heart, 73, 111, 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
Hermaphroditism, 59-61.
Hibernation, 53.
Hind-brain, 103, 291; limb, 119.
Histology, 121.
Holostomum nitidum, 42.
Humerus, 239-241.
Hyaloklossia Lieberktihni, 43.
Hyla, 17, 194; goeldii, 17; picker-
ingii, 46; versicolor, 17.
Hylidee, 11, 16.
Hyoid, 70, 234.
Hyomandibular,
297.
Hypnotism, 59-61.
Hypoblast, see Entoderm.
Hypoglossal nerve, 289.
Hypophysis, 294.
cleft, I09; nerve,
Ileum, 74, 148.
Ilium, 243.
Infundibular lobe, 294.
Infundibulum, 104.
Infusoria, parasitic, 42.
Inscriptiones tendine, 255.
Insertion of muscle, 247.
Inspiration, 170.
Instinct, 342.
Intelligence, 353.
Intermaxillary gland, 71.
Intervertebral foramina, 238.
Intestine, 74, 148.
Invagination, 94.
Iris, 320.
Ischium, 243.
Iter, 103, 293.
Jaw, 69, 232, 233, 236.
Jelly, 83.
Karyamoeba renis, 43.
Karyophagus ranarum, 43.
Katabolism, 134.
Kidneys, 77, 202.
Krause’s membrane, 130.
Labyrinth, 335.
Labyrinthodonts, 2.
INDEX
Lacunz, I25, 127.
Lankestrella, 43.
Larva, 102, 116.
Larynx, 107, 165.
Lateral plate, 110; ventricle,
294.
Laverania, 43.
Lens of eye, 326, 333.
Leopard frog, 20.
pipiens.
Leptothera Ohlmacheri, 43;
43-
Leucocytes, 150, 154, 22I, 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, 20%, 207.
Lungs, 74, 107, 167.
Lymph, 258, 262, 281; hearts, 281;
spaces, 187, 280.
104,
See also Rana
rane,
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, IIo,
Mesogaster, 138.
Mesorchium, 77, 215.
Mesosternum, 240.
Mesovarium, 212.
INDEX*
Metabolism, 134.
Metacarpus, 242.
Metamorphosis, 116.
Metatarsus, 244.
Mid-brain, 103, 291, 293.
Molybdis Entzi, 43.
_Monostomum, 42.
Mouth, 68, 116.
Mucigen, 144.
Mucosa, 139, 149.
Miillerian duct, 115.
Muscle fibers, 128-130,
Muscles, kinds of, 247; of body wall,
255, 250; of eye, 333; of hind leg,
248-255; seasonal changes of, 34;
of throat, 257.
Muscularis mucose, 139.
Myotome, Ito.
Myxosporidia, 43.
Nares, 63, 69, Io2.
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, II5.
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, I19.
367
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, 4I.
Palatine, 233.
Pancreas, 74,107, I5I, 219.
Pancreatic duct, 107, 151; juice, 151.
Papillz, 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.
Pelobatide, 11, 12.
Pelvic girdle, 242.
Pepsin, 142, 147.
Pepsinogen, 147.
Peptones, 142.
Pericardium, 73, I1I, 264.
Perichondrium, 126.
Perilymph, 336.
Periosteum, 127.
Peritoneum, 78, III, 139, 148, 213.
Pes, 65.
Phalanges, 242.
Phaneroglossa, II.
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.
Optic chiasma, 294; lobe, 103, 293, | Pleistophora Danilewskyi, 43.
368
Plethodon, 9; Plethodontine, 7, 9.
Plexus, Auerbach’s, 299; brachial,
289; coeliac, 299; ischio-coccygeal,
291 ; lumbo-sacral, 290; Meissner’s
299; Sciatic, 290; solar, 299;
urogenital, 299.
Poison glands, 183.
Polar bodies, 86.
Portal 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;
Segmental duct.
Prootic, 229.
Proteide, 3.
Proteids, 137.
Proteus, 3, 351.
Pseudobranchus, 4.
Pseudothyroid, 109, 224.
Pterygoid, 232.
Pubis, 243.
Pulmocutaneous arch, artery, 268,
272.
Pulmonary artery, 272;
275.
Pulvinar rostrale, 68.
Pyloric glands of stomach, 141.
Pylorus, 74, 138.
duct of, see
Vein, 9275,
Quadrate, 235.
Quadrato-jugal, 233.
Radiale, 242.
Radio-ulna, 241.
Rana, 18, 194.
Rana agilis, 18.
Rana arvalis, 45, 187.
Rana Catesbiana, 18, I9, 41, 215,
216.
INDEX
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, Ig.
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, I5I.
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, g; maculosa, 9.
Salamandride, 3, 7.
Salamandrine, 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
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, Ioo.
Serosa, 139.
Sex differences, 186, 306; recognition,
305; organs, 35, 74, I15, 212.
Sexes, proportions of, 51.
Sheath of Schwann, 131.
Shoulder girdle, 238.
Sinus venosus, 265; beating of, 279.
Sirenide, 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, 11g.
Spiral valve, 267.
Splanchnic layer, I1o.
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, too, 118.
Sulcus marginalis, 68.
Suprascapula, 239.
2B
369
Surinam toad, 12.
Suspensorium, 232, 236.
Sympathetic system, 284, 298.
Symphysis, public and ischial, 243.
Systemic arch, 270.
Tadpoles, 39, 116-119.
Teenia dispar, 41.
Tarsus, 244.
Taste, organs, 322; sense of, 25, 322.
WPeeth, 68,71, 12S.
Temperature, effects of, 55-58.
Tendon, 124.
Testis, 77, 215.
Tetracotyle crystallina, 42.
Thalamencephalon, 104,
function of, 311.
Thalamus, 104, 311.
Thigmotaxis, 351.
Thymus, I09, 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, I12, 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 spelzeus, 8.
291, 293;
Urea, 207, 219.
Ureter; 115.,202:
Urinary bladder, 77, 209; tubules,
TI5, 202, 208.
Urodela, 3, 165.
Urostyle, 237, 238.
Uterus, 77, 214.
Utriculus, 335.
340 INDEX
Valvula parodoxica, 278. Voice, 31, 165, 302.
Vas afferens, 203. Vomer, 232.
Vas deferens, 205. Vomerine teeth, 69, 232.
Vasa efferentia, 115, 204, 215.
Veins, 265, 272-277; anterior abdom- | White matter of cord, 287.
inal vein, 73, 276. Wolffian body, 113; duct, 115.
Vena cava, 265, 274. Wood frog, 20, 354; see also Rana
Vent, 74. sylvatica.
Ventricle of heart, 73, 264, 279.
Ventricles of brain, 103, 291, 293, | Xantholeucophores, 188.
294. Xanthophores, 188.
Vertebree, 116, 237. Xiphisternum, 239.
Vertebral plate, IIo.
Vesicula seminalis, see Seminal ves- | Yolk, 86.
icle. Yolk plug, 95.
Visual green, 329; purple, 329.
Vitelline membrane, 85, 89. Zaitha, 39.
Vitreous humor, 326. Zygapophyses, 237.
Vocal cords, 167; sacs, 69, 167. Zymogen, IsI.
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 bovuk gives us a much-needed text-book for
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investigation by pointing out the wide field the frog’s egg still offers to
embryological 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... . he 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 /ndependent.
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
“T have greatly admired your recent text-book of Entomology.
There is only one word to express my admiration of it, — Splendid.” —
Professor S. W. WILLISTON, University of Kansas.
PAB VEAC MILLAN “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. W. Conn, Wesleyan University.
“T think it will supply a need, not before met, of a brief but clear
and interesting outline for classes in Zodlogy. I have never seen its
equal and intend to introduce it next year.”
— H. D. DENsMorg, Beloit College.
A Synoptic Text-book of Zoology
For Colleges and Schools
By ARTHUR WISSWALD WEYSSE
Instructor in Zodlogy at the Massachusetts Institute of Technology,
and Associate Professor of Physiology at Boston University
Cloth 8Svo $4.00 net
This book includes in a single volume of convenient size all that is
necessary for an elementary course in zodlogy 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 zodélogy 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 fragmentarily, as in the majority of the elementary books, which
are for the most part adapted to children only ; thus, the purpose has
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Hence the title of the work.
THE. MACMILEAN COMPAAR.
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