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THE FROG:
AN INTRODUCTION TO ANATOMY.
HISTOLOGY, AND EMBRYOLOGY.
THE FROG:
AN INTRODUCTION TO
ANATOMY, HISTOLOGY, AND
EMBRYOLOGY.
4
6
ty 1 LIBRA
m.
BY THE seal
A. MILNES MARSHALL, M.D..D.Sc., M.A, F.R.S.,
FORMERLY FELLOW OF ST. JOHN’S COLLEGE, CAMBRIDGE ; PROFESSOR IN THE
VICTORIA UNIVERSITY ; BEYER PROFESSOR OF ZOOIOGY
IN OWENS COLLEGE
EDITED BY
G. HERBERT FOWLER, B.A., Pu.D.,
LATE BERKELEY FELLOW OF THE OWENS COLLEGE, MANCHESTER; ASSISTANT
PROFESSOR OF ZOOLOGY, UNIVERSITY COLLEGE, LONDON
SIXTH EDITION
REVISED AND ILLUSTRATED
NEW YORK
MACMILLAN AND CO.
1896
Med. >1\
PREFACE TO THE FIRST EDITION.
Tue Owens College Course of Elementary Biology, which
forms part of the scheme of study prescribed by the Victoria
University, is of a rather more extended and comprehensive
nature than the courses held elsewhere under the same name;
and experience has shown me that there is want of a book
that will guide and direct the student through the practical
part of his work, the whole ground of which is covered by no
one of the existing manuals. It is to meet this want that the
present little work has been prepared.
This first instalment of the work consists of an Introduction,
containing practical instruction in the methods employed in
biological investigation ; followed by the application of these
methods to the examination, both anatomical and histological,
of an actual animal. For this purpose the frog has been
selected as being easy to obtain, convenient to dissect, and a
fairly typical example of the great group of Vertebrate animals.
Where, from its small size or for other reason, the frog has
proved unsuitable, other animals have been substituted for it.
For convenience of reference, and in order to definitely
stamp the practical character of the work, directions for dis-
section, etc., have throughout been printed in italics,
It is not expected that the student should do the whole of
iv PREFACE
the work here given the first time he goes over it. The dis-
section of the muscles and of the cranial nerves should only be
attempted if time remain after the other work is completed.
In preparing this first part I have received very valuable
assistance from Dr. Hartog, Demonstrator of Biology in the
College, and from my friend and pupil Mr.C. H. Hurst. Iam
also much indebted to Prof. Gamgee and to Mr. Waters for the
important help they have given me in the histological portions.
OWENS COLLEGE,
August, 1882.
PREFACE TO THE SIXTH EDITION.
A Few additions and alterations, based upon a continuous
working knowledge of the book since its first issue, have been
made in this edition. Professor Marshall had so complete a
grasp of the difficulties which confront a student of Elementary
Zoology, and so great a sense of proportion and arrangement,
that no substantial alteration could increase the educational
value of his work.
Two new figures have been added, Figs. 30 and 37. For the
second of these, taken from Professor Marshall’s “ Vertebrate
Embryology,” I am indebted to Messrs. Smith, Elder & Co.
UNIVERSITY COLLEGE,
Lonpon, 1896.
CONTENTS.
INTRODUCTION.
Apparatus required. Dissection. Drawing. Use of the
Microscope. Preparation of Microscopical Objects.
Section cutting. Table of Histological Processes
CHAPTER I.
GENERAL ANATOMY OF TIE FROG.
External Characters. Buccal Cavity. Abdominal Viscera.
Peritoneum. Digestive Organs
CHAPTER II.
THE VASCULAR SYSTEM OF THE FROG.
The Heart. The Veins. The Arteries. The Structure of the
Heart. Microscopical Examination of Blood
CHAPTER III.
THE SKELETON OF THE FRoG.
The Axial Skeleton. The Appendicular Skeleton
CHAPTER IV.
Tue Muscucar SYSTEM OF THE FRoG.
Muscles of Trunk. Muscles of Head. Muscles of Hind-
limb : ; :
CHAPTER V.
Tue NERvous SYSTEM OF THE FROG.
The Central Nervous System. The Peripheral Nervous
System. Histology of Nerves
PAGE
I-14
15-23
24-39
40-54
55-67
68-86
vili CONTENTS
CHAPTER VI.
THE Eye anpD Ear.
The Eye of the Frog. The Eye of the Sheep or Ox. OEE
of the Eye. The Ear ofthe Frog. ; . 87-95
PAGE
CHAPTER VII.
THE REPRODUCTIVE ORGANS AND THE CLOACA.
The Male Frog. The Female Frog . ‘ i ‘ ¥ 96-98
CHAPTER VIII
DEVELOPMENT OF THE FRoG.
General Account. Formation of the Egg. Maturation of the
Egg. Fertilisation. Segmentation, Formation of the
Germinal Layers. Development of the Nervous System.
Development of the Sense Organs. Development of the
Alimentary Canal. The Gill Clefts and Arches. The
Vascular System. The Muscular System and the Cceelom.
Development of the Skeleton. Development of the
Urinary System . : 7 : ‘ ; 99-148
CHAPTER IX,
ELEMENTARY HISTOLOGY,
Epithelium. Glands, Muscle. Connective Tissue. Carti-
lage. Bone ‘ ‘ 3 . 149-160 -
INDEX : . F ‘ 161-167
INTRODUCTION.
I.—LIST OF APPARATUS REQUIRED.
Tue following apparatus is recommended to the student of
Elementary Biology :
1. Two or three scalpels or dissecting knives of different
sizes.
2. Two pairs of forceps, one large and one small. Both pairs
should be straight, and should have the tips roughened in order
to secure a firmer hold.
3. Two pairs at least of scissors; one pair large and strong,
for cutting bone and other hard tissues; the other pair small,
for fine dissections. A second small pair may have the blades
bent at an angle (elbow scissors). In selecting scissors be care-
ful to see that they cut quite up to the points of the blades.
4. A pair of stout needles, firmly mounted in handles.
5. A pair of the finest sewing needles, mounted in handles:
only about a quarter of an inch of the needle should project.
They are used for teasing histological preparations.
6. A seeker, 7.¢., a blunt needle mounted in a handle, and
bent at an angle half an inch from the end.
7. A metal blow-pipe: and a glass cannula with india-rubber
cap.
a. A pocket lens, containing two or three lenses mounted in
a handle, and giving when combined a magnifying power of at
least six diameters.
9. Slides and coverslips, for mounting microscopical speci-
mens. The coverslips should be the thinnest sold (No. 1).
Square covers are easier to handle than circular.
10. A blank note-book, for drawing in; an HB pencil, and
a piece of indian-rubber.
11. A cheap pair of compasses, for measuring the dissections.
A
Ne
DISSECTION AND DRAWING
II—ON DISSECTION.
The object of dissection is to separate the several parts and
organs from one another, so as to define their boundaries and
display clearly their mutual relations. Dissection consists
mainly in removing the “connective tissue” which binds the
several parts together.
The following rules should be carefully attended to:
1. Pin down the animal firmly to the dissecting board.
Never attempt to dissect a specimen that is not so fixed.
2. In pinning out a dissection stick the pins in, not vertically,
but obliquely, so that their heads do not get in the way or
obscure the dissection.
3. Never cut away anything until you are quite certain what
it is you are removing.
4, Put the part you are dissecting slightly on the stretch;
é.g., when dissecting the bloodvessels or nerves of the throat,
distend it by passing a small roll of paper or the handle of a
seeker down the cesophagus; or when dissecting the muscles of
the leg, pin out the leg in such a position as to stretch the
muscles you are cleaning.
5. In cleaning bloodvessels or nerves always dissect along
them and not across them ; and avoid laying hold of them with
the forceps. Similarly when cleaning muscles, dissect along
their fibres and not across them.
6. Fine dissections should be done under water, which sup-
ports the parts and greatly facilitates the operation. A stream
of water allowed to play gently on the dissection from time to
time is often a valuable aid.
7. The dissection of muscles, and still more of nerves, is
greatly aided by placing the specimens in‘spirit for a day before
dissecting.
8. Keep your instruments clean and sharp. Be careful not
to blunt your fine scissors or scalpel by using them for cutting
hard parts.
9. If you get in a muddle, stop and wash the dissection
thoroughly under the tap before proceeding further.
TII—ON DRAWING.
It is absolutely essential to draw your dissections, and this
must on no account be omitted. Keep a separate book for
THE USE OF THE MICROSCOPE 3
your drawings, and draw every dissection you make. Do not be
discouraged if you find it difficult at first: you will never
regret time spent on it.
The following rules will be useful to those who have not
learnt to draw systematically :
1. Make your drawing to scale, i.¢., either the exact size of
the natural object, or half or double or treble that size, as the
case may be, remembering always that a drawing can hardly be
made too large.
2. In commencing a drawing, first determine by careful
measurement the positions of the principal points, and sketch
in lightly the whole outline before finishing any one part.
3. If the object you are drawing is bilaterally symmetrical,
draw a faint line down the middle of your paper, and sketch in
the left-hand half first ; by measuring from your median line
it will be very easy to make the two halves symmetrical.
4, Name on your drawing the several parts shown, and mark
also the scale adopted. If your drawing be of the natural size
mark it thus— x 1; if it be double the size of the object mark
it x2; if half the size, x 4, and so on.
5. Draw on one side of the page only: and write an explana-
tion of your drawing on the opposite page.
6. Always make your drawing in pencil first, since much
clearer outlines can be obtained with pencil than with chalk,
but for complicated drawings coloured pencils are very useful,
and water-colour paints still better. Keep certain colours for
particular organs or tissues; ¢.g., when drawing the skeleton
colour the cartilage blue, the cartilage bones yellow, and the
membrane bones either red or white ; when drawing the blood-
vessels colour the arteries red and the veins blue.
7. Draw only what you see.
IV.—THE USE OF THE MICROSCOPE.
The microscope consists essentially of a stand and a body, the
latter of which bears at its ends the lenses by which the magni-
fying power is obtained.
The stand is an upright pillar, the lower end of which is
attached to a heavy foot to ensure steadiness. A little way
above the foot the stand supports a horizontal plate—the stage
—on which the object to be examined is placed. The stage is
4 THE USE OF THE MICROSCOPE
perforated in the middle by a hole, the size of which can be
varied by means of diaphragms. Through this hole light is
directed on the object to be examined by means of a mirror
attached to the stand below the stage. Above the stage the
stand supports a vertical tube, in which the body of the micro-
scope slides up and down.
The body is a tube, in the upper end of which is placed a
combination of lenses, known as the eyepiece, while to the lower
end is screwed another combination of lenses—the objective.
A microscope is usually provided with a couple of eyepieces
and a couple of objectives of different magnifying power. An
objective magnifying only a small number of times is called a
low power ; one magnifying many times (200 diameters or more),
a high power. Similarly eyepieces are spoken of as high or low
according to their magnifying power.
In order that an object may be seen clearly the objective
must be at a certain definite distance from the object, this
distance varying with different objectives, and toa slight extent
with different observers. The higher the power employed the
closer must the objective be brought to the object. As the
position of the object on the stage of the microscope is fixed,
this distance is regulated by moving the body of the microscope
up and down in the tube in which it slides.
This process of focussing is effected in two ways:
(1) By simply sliding the body up and down by hand in the
stand, or by screwing it up and down with a rack and pinion,
according to the type of microscope employed. This is known
as the coarse adjustment. The sliding of the body should be
performed with a slightly screwing motion, and can only be used
when low powers are being employed.
(2) With high powers the objective has to be brought so
close to the object that a more delicate method of adjustment
is necessary. This fine adjustment is effected by a screw witha
milled head placed at the top of the vertical pillar forming the
stand. By turning the head from right to left, in the direction
of the hands of a watch, the body of the microscope is lowered
and the objective brought nearer to the object: by turning in
the reverse direction the objective is raised.
Tn using the microscope attend to the following rules:
1. Always examine an object first with the low power. Having
adjusted the eyepiece and objective, direct the light up the tube
THE USE OF THE MICROSCOPE 5
of the microscope by means of the mirror, and then place the
object on the stage. Twist down the body until the objective
is about a quarter of an inch from the cover-glass; look down
the microscope, and gradually twist the body up until the
object becomes visible. Focus accurately by means of the fine
adjustment screw.
2. When using a high power begin with the objective close to
the cover-glass, and then focus with the fine adjustment. It
will facilitate the process if, while focussing with the right
hand, you move the object about slightly with the left hand.
3. Take extreme care never to let the objective touch the
cover-glass ; and never to touch or allow any dirt to get on the
face of the objective. The face of an objective cannot be cleaned
without doing harm to it. :
4. Should by any chance a drop of glycerine get on the face
of the objective, wash it carefully with water from a wash-
bottle, and wipe it gently with a silk handkerchief or piece of
chamois leather. Should Canada balsam be allowed to get on
the objective, do not attempt to clean it yourself, but hand it
at once to the assistant.
5. See that the body of the microscope slides smoothly in its
tube. If it does not, remove it, and clean it by rubbing with a
few drops of olive oil: wipe off the oil before replacing the body
in the tube.
6. Keep both eyes open when looking through the micro-
scope: a very little practice will enable you to do this, and it
will save you much fatigue. Also get into the habit of using
either eye.
7. With a high power, use a small diaphragm: the amount
of light will be somewhat diminished, but the clearness and
definition of the object much increased.
8. When examining an object, keep one hand on the fine
adjustment, and keep screwing it up and down slightly the
whole time: in this way parts of the object at different depths
are brought into focus successively, and a clearer idea of the
object is obtained.
9. If the object appears dim or dirty, find out where the
fault lies in this way :
While looking down the microscope, turn round the eyepiece
with your right hand. If the dirt turns round too, remove and
clean the eyepiece. If the fault is not in the eyepiece, move
6 THE PREPARATION OF MICROSCOPICAL OBJECTS
the slide about gently ; if the dirt moves with the slide, remove
the slide and clean it. If the dirt does not move with either
the eyepiece or the slide the fault is almost certainly in the
objective, which should be removed and examined ; if dirty, it
must be cleaned very carefully with a piece of silk or chamois
leather.
V.—THE PREPARATION OF MICROSCOPICAL OBJECTS.
In mounting microscopical objects be careful that your slides
and coverslips are thoroughly clean. Slides should be labelled
as soon as they are prepared, and should be kept in a box or
cabinet in which they lie flat.
A. Methods of Mounting.
There are various media in which objects may be mounted.
The method of procedure is much the same with all. Put a
small drop of the fluid in the middle of the slide, place the
object in the middle of the drop, and arrange it with needles in
the position desired. Then place the cover-glass carefully on
the top, letting it rest by one edge on the slide and supporting
the opposite edge by a needle: withdraw the needle gradually
so as to let the cover-glass down slowly, and drive out any air-
bubbles there may be in the fluid. If any air-bubbles still
remain, leave them alone, as they will probably work out by
themselves. Be careful not to use too large a drop of your
mounting medium, and above all things be careful not to let
any of it get on the top of the cover-glass; should this happen,
the cover-glass must be removed at once and the specimen
mounted afresh with a clean one.
The most important mounting media are the following :
1. Normal Salt Solution : a 0°75 per cent. solution of com-
mon salt in water. This is very useful in the examination of
fresh specimens of animal tissues, as, unlike water, it has
practically no action on them. It cannot be used, however,
for making permanent preparations.
2. Glycerine can be used either pure or diluted with its own
bulk of water. If the preparations are intended to be per-
manent, a narrow ring of cement must be painted round the
edge of the cover-glass to fix it to the slide. For permanent
THE PREPARATION OF MICROSCOPICAL OBJECTS 7
preparations it is better to use glycerine jelly ; a drop of this
should be melted on the slide, and the object transferred to it
from glycerine: ring with cement as before.
3. Canada Balsam is the most generally useful medium for
permanent preparations, as requiring no cement. Specimens
that are to be mounted in balsam must first be deprived of all
water they may contain by placing for an hour or so in absolute
alcohol, and should then, before mounting, be soaked for a few
minutes in oil of cloves or turpentine in order to clear them,
z.e., render them permeable by the balsam. Canada balsam, if
too thick, may be diluted with chloroform, turpentine, or
benzole.
B. Teasing.
The object of teasing is to separate the several parts of a
tissue or organ from one another in order to show their minute
structure.
The fragment to be teased should be placed on a slide in a
drop of the medium in which it is to be mounted, and then
torn up into shreds by means of a couple of needles held one
in-each hand. The process is often greatly facilitated by
placing the slide on a piece of black paper, which renders the
particles easier to see. When torn up as finely as possible, a
cover-glass is placed on as before. The two rules to be borne
in mind in teasing are the following :
1. Take a very small fragment to commence with; hold it
with one needle, and tear it with the other.
2. Tease it as finely as you can. Your object is to separate
the component parts from one another.
C. Maceration.
The process of teasing is in many cases facilitated by pre-
viously macerating the specimen, #.e., soaking it in some fluid,
which, while preserving the individual cells, tends to loosen
them from one another. The most important macerating fluids
are as follows :
1, Ranvier’s Alcohol: a mixture of one part of strong spirit
with two parts of water. The specimen should be put fresh
into the mixture and allowed to remain twenty-four hours or
more.
8 THE PREPARATION OF MICROSCOPICAL OBJECTS
2. Miiller’s Fluid; a solution of bichromate of potash with
a little sodic sulphate in water.
D. Staining.
Various reagents are employed for the purpose of staining
preparations; some of these merely dye the whole preparation
more or less uniformly, but the most useful ones are those which
stain certain parts of the cells only, or at any rate stain these
much more strongly than the other parts (selective stains). The
most important are the following :
1. Hematoxylin. There are various preparations of hema-
toxylin, or logwood, used in microscopical work: the best is that
proposed by Delafield. It is prepared thus : dissolve 4 grammes
of crystallised hematoxylin in 25 cubic centimetres of strong
alcohol ; add this solution to 400 c.c. of a saturated. solution of
ammonia alum, and expose to the light in an unstoppered bottle
for 3to 4 days. Filter, add 100 ¢.c. glycerine and 100 c.c. of 90
per cent, alcohol.
The specimens, which must be perfectly free from all trace
of acid, should be cut into small pieces, and passed through
weak spirit to water. They should then be left in the hema-
toxylin in a covered vessel or stoppered bottle for from one to
twelve hours, according to the size of the specimen and the
depth of staining desired, and then brought up through water
and weak spirit, and left in strong spirit for some hours before
mounting. Hematoxylin stains the nuclei of cells much more
strongly than the other parts.
2. Borax-Carmine, This, which is perhaps the most gener-
ally useful of all the stains in ordinary use, is prepared as
follows. Dissolve 2 parts of carmine and 4 parts of borax in
100 parts of water: add an equal volume of 70 per cent.
alcohol ; let the mixture stand for a couple of days, and then
filter.
Specimens may be left in borax-carmine for from one to
twenty-four hours, or even for two or three days: on removal
they should be placed in acid-aleohol—.e., 70 per cent. alcohol
to which a few drops of hydrochloric acid have been added—
until they become a bright scarlet colour, when they should be
transferred to 70, and then to 90 per cent. alcohol, in which
latter they may be left till required. The time of immersion in
THE PREPARATION OF MICROSCOPICAL OBJECTS 9
acid-alcohol will vary, according to the nature and size of the
specimen, from a quarter of an hour up to a day or more.
3. Picro-Carmine is a very useful, and to a certain extent a
differential stain, as it colours the several tissues different
tints. It may be prepared thus. Dissolve 1 gramme of carmine
in 4 cc. of liquid ammonia and 200 c.c. of distilled water. Add
5 grammes of picric acid; shake the mixture well for some
minutes, and then decant from the excess of acid. Leave the
decanted liquid for some days, stirring it occasionally; then
evaporate it to dryness, and to every 2 grammes of the dried
residue add 100 c.c. of distilled water.
Picro-carmine answers best with specimens preserved in 70
per cent. alcohol. They should be left in the stain for a day, and
‘then placed in 70, and afterwards in 90 per cent. alcohol. Some
specimens give better results when washed freely with water
on removal from the picro-carmine, and then placed in 1 per
cent. acetic acid for an hour before transferring to alcohol.
4. Magenta stains very rapidly but diffusely: the colour
also is not permanent.
5. Silver Nitrate. A 4 per cent. solution in water stains the
intercellular substance, which binds together the several cells of
a tissue, much more strongly than the cells themselves, and is
therefore chiefly used when we wish to render prominent the
outlines of the individual cells. The specimens should be placed
fresh in the silver solution for from two minutes to a quarter
of an hour, then washed thoroughly with distilled water, and
exposed to the light until stained sufficiently deeply, when they
may be mounted in glycerine. Such preparations are rarely
permanent, as the reduction of the silver, to which the staining
is due, continues until the specimens ultimately become too
dark to be of any use.
6. Osmic Acid. A 1 per cent. solution of osmic acid in water
forms an extremely useful staining reagent. It is especially use-
ful for the detection of fat, which is stained by it a dark brown
or black colour. Specimens, which must be quite fresh, should
only be left in it a few minutes, and may then be mounted in
glycerine, or else washed, dehydrated, and mounted in balsam.
7. Acetic Acid. Although not strictly a staining agent, in-
asmuch as it does not colour the specimens, acetic acid may
10 PRESERVING AND HARDENING
conveniently be mentioned here, as it is used for the same pur-
pose as the true stains, i.¢., for the sake of rendering certain
parts of the cells especially distinct. Acetic acid, of which a
1 per cent. solution is employed, causes the protoplasm of cells
to swell up and become transparent, and brings the nuclei into
special prominence. It is used with fresh specimens.
VION PRESERVING AND HARDENING.
The reagents in common use for killing and preserving small
animals are valuable also from their power of “ fixing” the
tissues, z.¢., of coagulating the protoplasm of the cells. The
objects to be attained are to effect this coagulation quickly,
before the tissues can undergo any alteration ; and thoroughly,
i.e., throughout the whole thickness of the object to be hardened.,
They are as follows:
1. Alcohol. Specimens may be placed at once in 70 per
cent. alcohol; and thence transferred after a couple of days to
90 per cent., in which they may be left till required.
2. Osmic Acid. For this purpose a 1 per cent. solution in
water is used: it acts almost instantaneously, and so allows no
change to occur in the tissues; it has also the merit of staining
the tissues as well as hardening them. It can, however, only
be employed when the specimens are very small, as it hardens
the surface layers so rapidly that it is unable to penetrate
beyond a very slight depth. A few minutes’ immersion is
usually sufficient.
3. Corrosive Sublimate. This is by far the best general
reagent for killing and fixing small animals. A saturated solu-
tion in water is employed, in which the object is placed for
half an hour or more. After removal it is thoroughly washed
with water or weak alcohol, and then transferred to 70 per
cent. alcohol before staining.
4. Chromic Acid. 70%
B. Staining.
50%
+
30%
Water
Borax Hematoxylin
Carmine or Picro-Carmine Silver Nitrate
> Water <—
)
30 %
50%
Acid spirit —___-+ 70%
C. Section cutting 4 % 7
and Mounting.
Absol. Alec.
Turpentine
or Benzole
+ + YF
Melted Canada Stron i
g Glycerine
Paraffin, Balsam. Glycerine. Jelly.
CHAPTER I.
GENERAL ANATOMY OF THE FROG.
Fic. 1.—The Common Frog (Rana temporaria) (from Ecker).
A. External Characters.
Lay the frog on a board before you ; note, and make drawings
showing the following points :
1. The division into head, trunk, and limbs ; and the absence
of neck and tail.
2. The two great surfaces.
a. The dorsal surface, or back, is directed upwards when
the frog is in the natural position.
b. The ventral surface, or belly, is directed downwards
towards the ground.
16 GENERAL ANATOMY OF THE FROG
3. The skin is moist and smooth; and devoid of hairs, scales,
and claws. The colour of the skin is variable in different
specimens and at different times: it is mottled on the dorsal
surface, paler on the ventral.
4, The head is flat and triangular, with a blunt apex directed
forwards.
At the sides of the head are the eyes, which are large and
prominent. Each eye has two eyelids, of which the upper is
thick, pigmented, and almost immovable, while the lower is
semi-transparent and freely movable. W101
Behind the eye on either side is an obliquely placed elongated
patch of a dark colour, in the middle of which is a circular
area—the tympanic membrane—supported by a firm marginal
ring.
5. The limbs. There are two pairs of limbs, fore and hind ;
each limb being composed of three segments.
a. The Fore limb presents the following divisions :
i, Arm.
il, Forearm.
ii. Hand, with four digits, corresponding to the four
fingers of man; the thumb being very small and
inconspicuous. In the male frog there is a thicken-
ing along the inner edge of the first digit, specially
developed at the breeding season.
b. The Hind limb is much longer than the fore limb, and
divided into the following parts :
i, Thigh.
li. Leg.
iii. Foot, with five toes webbed together. The shortest
toe corresponds to the big toe of man,’ and the
longest to his fourth toe.
6. External apertures: or openings on the surface of the
body.
a. Median apertures.
j. The Mouth is a wide horizontal slit.
ii. The Cloacal aperture is a small hole at the posterior
end of the body, between the legs: it lies slightly
on the dorsal surface, just behind the bony projec-
tion formed by the posterior end of the urostyle.
THE BUCCAL CAVITY 17
b. Paired apertures.
i. The Nostrils or anterior nares are two small open-
ings on the dorsal surface of the head, close to its
anterior end.
B. The Buccal Cavity.
Open the mouth to its full extent : note the wide buccal or mouth
cavity, of which the hinder part or pharyna is continued back into
the esophagus. Note also the following structures :
1. On the Roof of the Mouth.
a. The Teeth.
i. The maxillary teeth are a row of fine teeth, attached
round the edge of the upper jaw.
ii. The vomerine teeth are two small patches of sharp
teeth in the fore part of the roof of the mouth
and near the middle line.
b. The posterior nares are two small holes lying to the
outer sides of and slightly in front of the two patches
of vomerine teeth.
Pass bristles through the nostrils, and see that they
come out through the posterior nares into the buccal
cavity.
c. The Eustachian tubes or recesses are a pair of much
larger holes, at the sides of the posterior part of the
buccal cavity. Each hole opens into a slightly dilated
chamber—the tympanic cavity—which is closed ex-
ternally by the tympanic membrane already seen on
the surface of the head.
Perforate the tympanic membrane on one side with a
needle, and pass a bristle or seeker through the hole and
down the Eustachian tube into the mouth.
d. Two rounded prominences at the sides of the roof of
the mouth are caused by the eyeballs.
Press down one of the eyes with your finger, and note
that it can be made to project very considerably into the
buccal cavity.
2. On the Floor of the Mouth.
a. The lower jaw, which is devoid of teeth, forms a bony
margin to the floor of the mouth: the rest of the floor
B
18 GENERAL ANATOMY OF THE FROG
is soft and fleshy, but is slightly stiffened by a car-
tilaginous plate—the body of the hyoid.
b. The tongue, which is thin and fleshy, is attached to
the front part of the floor of the mouth, and has its
free bilobed end turned backwards towards the throat.
Turn the tongue forwards with the forceps.
c. The glottis, or aperture of the larynx, is a longitudinal
slit in the floor of the posterior part of the mouth,
and is stiffened laterally by the arytenoid cartilages.
Pass bristles through the glottis into the lungs. If
any difficulty is experienced in finding the glottis
snip through the angles of the mouth with scissors,
so as to allow the mouth to be opened more widely.
C. The Abdominal Viscera.
Lay the frog on its back under water, and fasten it down to
the dissecting board by pins through the limbs. Cut through the
skin, along the middle line, the whole length of the ventral sur-
face. Separate the skin from the underlying parts, noticing its
very loose attachment to these parts, and the large space—a lymph
cavity—beneath it. Turn the flaps of skin outwards, and pin
them back. Notice:
a. The muscles of the body-wall.
b. The pectoral or shoulder girdle: a bony arch running
across the body, opposite the fore limbs.
Pinch up with forceps the muscular body-wall, and cut through
it into the body-cavity or celom with scissors a little to one side
of the median line, being careful not to injure the anterior abdo-
minal vein which runs along the inner surface of the body wall
in the middle line. ;
Continue the cut backwards to the hinder end of the body, and
forwards to the jaw, cutting through the pectoral girdle with strong
scissors, and taking care not to injure the parts beneath.
Note on the inner surface of the larger flap the anterior abdo-
minal vein, and carefully dissect this from the flap. Pull the two
flaps apart, cutting through them transversely at their posterior
ends to facilitate the process, and turn them back so as to display
the viscera.
ABDOMINAL VISCERA 19
Inflate the lungs with a blow-pipe through the glottis, and inflate
the bladder through the cloacal aperture.
Note and draw the general arrangement of the viscera,showing
the following structures :
1. The heart, enclosed in the pericardium, is situated in the
middle ventral line, and in the natural condition of the parts
is covered by the pectoral girdle and the sternum.
2. The liver is a large reddish-brown bilobed organ, behind
and at the sides of the heart.
3. The lungs are two thin-walled elastic sacs at the sides of
the heart : they lie dorsal to the liver, and are often hidden by it.
Note the bristles already passed into the lungs through the glottis.
4. The small intestine is a light-coloured convoluted tube ;
in the middle line behind is the much wider large intestine.
5. The bladder is a thin-walled bilobed sac at the posterior
end of the body cavity.
Fic. 2.—A diagrammatic transverse section across the posterior
part of the body of a female frog.
a, urostyle; 4, muscles of body wall; d, large intestine; d.a, dorsal
aorta; i, ilium; 2, lymph space between the skin and the muscular
body wall; 7, spinal nerves ; 0, kidney; ov, oviduct ; 4, peritoneum ;
s, skin; ¢, fold of skin at groin; z, ureter ; v, posterior vena cava.
6. In the female frog note, in addition to the above parts,
a. The ovaries: two large bodies of irregular shape, each
consisting of a mass of spherical black and white
eggs, like small shot.
20 GENERAL ANATOMY OF THE FROG
b. The oviducts: two long, very much convoluted tubes with
thick white walls, lying at the sides of the body cavity.
7. In the male frog note,
a. The testes: a pair of ovoid bodies of a pale yellow
colour, attached to the dorsal wall of the body cavity.
D. The Peritoneum.
Notice the thin pigmented membrane—the peritoneum—
which lines the body cavity. Trace this to the mid-dorsal line,
where it is reflected downwards as a double layer—the mesen-
tery—which embraces at its edge the alimentary canal, and
binds its several coils together. (See Fig. 2.)
Notice also that all the abdominal viscera are really outside
the peritoneum, which forms a closed sac into which the viscera
are as it were pushed from without.
E. The Digestive Organs.
= (-
_ FIG. 3.—General view of the viscera of the male frog, from the
right side.
a, stomach 4, bladder; c, small intestine; cZ, cloacal aperture ;
d, large intestine ; ¢, liver; /, bile duct ; & gall bladder; 4, spleen;
z, lung; &, larynx ; 7, fat body; m, testis; 7, ureter; o, kidney;
Pp. pancreas ; 7, pelvic symphysis ; s. cerebral hemisphere ; sf, spinal
cord ; Z, tongue; 7, auricle; «7, urostyle; v, ventricle; v.s, vesicula
seminalis; ww, optic lobe; «+, cerebellum; y, Eustachian recess;
z, nasal sac. ;
DIGESTIVE ORGANS 21
Turn the liver forwards, and note the stomach lying beneath its
left lobe. Pass the handle of a seeker through the mouth and
down the esophagus into the stomach.
[ff the specimen be a female, remove the ovaries and oviducts
completely, taking cure not to damage the alimentary canal. |
1. The Alimentary Canal.
a. The esophagus isa short wide tube leading from the
buccal cavity to the stomach.
b. The stomach is a wide tubular sac about an inch and a
half in length : it is narrower behind, and separated
from the duodenum by a distinct pyloric constriction.
Cut open the stomach longitudinally along its left side, and
wash out its contents: note the handle of the seeker already in-
serted through the mouth; also the longitudinal folds of the
mucous membrane lining the stomach, which increase the extent
of its surface. i
c. The duodenum is the first part of the intestine, rather
more than an inch in length : beyond the pylorus it is
bent back so as to lie parallel to the stomach. At its
further end it is continuous with the small intestine.
d. The small intestine is a slender convoluted tube about
four and a half inches long, opening at its distal end
by a small orifice into the large intestine.
e. The large intestine is a short straight tube about an
inch and a quarter long: it is very much wider than
the small intestine, and opens behind to the exterior
at the cloacal aperture.
f. The cloaca in the frog is continuous with the large
intestine, into it open the renal and genital ducts
as well as the bladder: it will be described more
fully when considering the urinary and reproductive
organs. (See Chap. VIII.)
2. The Liver.
The liver is a large reddish-brown organ, divided into right
and left lobes, connected together by a narrow bridge of liver-
tissue. Of the two lobes the left one is much the larger, and
is again subdivided into two.
22 GENERAL ANATOMY OF THE FROG
a. The gall-bladder is a small spherical sac lying between
the right and left lobes of the liver.
b. The bile duct is a slender tube leading from the liver
and gall-bladder to the duodenum, into which it opens
about half an inch beyond the pylorus, and on the
inner or concave side of the loop formed by the duo-
denum and stomach. The distal half of the bile duct
traverses the pancreas: it has rather thick white walls
and is easy to see; the upper half is more slender and
more difficult to trace.
To trace the bile duct turn the liver forwards so that the point
of attachment of the gall-bladder is clearly seen ; and slightly
stretch the duodenum by a pin passed through the pylorus. De-
termine the position of the two ends of the bile duct from the
description given above, and dissect with a scalpel along the line
thus indicated.
To see the opening of the bile duct, slit up the first three
quarters of an inch of the duodenum along its convex border and
wash out tts contents : squeeze the gall-bladder so as to drive the
bile along the duct into the duodenum: note the point at which
it enters, and insert a bristle through the opening into the duct.
Notice also the strong wavy transverse folds of the mucous mem-
brane of the duodenum.
3. The Pancreas.
The pancreas is a whitish irregularly lobed mass lying in the
loop between the stomach and duodenum, and best seen by
turning the whole loop forwards. The pancreatic ducts are
numerous and open into the bile duct, which passes through
the pancreas to reach the duodenum.
Cut through the mesentery along its attachment to the intestine :
uncotl the intestine, leaving it attached at both ends, and spread
it out on your dissecting board: measure the lengths of the
several portions and draw them to scale.
F. Other Abdominal Viscera.
1. The Kidneys are two flat elongated oval bodies of a red
colour attached to the dorsal body-wall, close to the middle line,
one on each side of the backbone or vertebral column. They
lie in the large lymph space behind the peritoneum, and, like
ABDOMINAL VISCERA 23
the other viscera, are outside the abdominal ccelomic cavity.
(See Fig. 2, p. 19.)
a. The ureters, or ducts of the kidneys, are a pair of white
tubes arising from the outer edges of the kidneys at
about a quarter of their length from their hinder ends,
and running back to open into the dorsal wall of the
cloaca, opposite the opening of the bladder.
In the male frog a pouch-like dilatation, the
vesicula seminalis, is present on the outer side of
each ureter, close to its opening into the cloaca.
b. The adrenal bodies are small yellowish-red patches on
the ventral surface of the kidneys.
c. The corpora adiposa, or fat bodies, are two bright yellow
tufts of flattened processes attached to the dorsal wall
of the body cavity; they vary much in size, and
usually come to the surface just behind the liver.
2. The Spleen is a small round dark-red body lying in the
mesentery, opposite the commencement of the large intestine.
CHAPTER IT.
THE VASCULAR SYSTEM OF THE FROG.
Tue vascular system is a closed system of tubes or vessels
filled with blood, and ramifying through all parts of the body :
its main parts are: (1) the heart, which by its contractions is
continually driving the blood round and round the system of
vessels: (2) the arteries, which are the vessels taking the
blood from the heart to all parts of the body: (3) the veins,
which carry the blood from those parts back to the heart: and
(4) the capillaries, a system of very small vessels connecting
the arteries and veins together.
A. The Heart. :
Pin down the frog on its back wnder water and open the body
cavity as before, taking special care to preserve the anterior ab-
dominal vein. Dissect this vein carefully from the body wall.
In freeing the pectoral girdle from the underlying muscles take
care not to injure the neighbouring bloodvessels.
Open the pericardial cavity and dissect the pericardium from
the heart and the roots of the great vessels, examine and draw the
heart in situ, showing its several divisions.
1. The divisions of the heart. .
i, The auricles form the anterior and dorsal division
of the heart: they are thin-walled and appear
dark in colour owing to the blood being seen
through their walls. On close examination the
division into right and left auricles can be seen.
ii. The ventricle is posterior to the auricles : it is paler
in colour owing to the greater thickness of its
walls; and is conical in shape, with the apex
pointing backwards.
iii, The truncus arteriosus is a cylindrical body arising
from the right anterior border of the ventricle,
and running obliquely forwards across the
auricles.
THE VEINS 25
Lift up the ventricle and turn its apex forwards so as to
expose the sinus venosus.
iv. The sinus venosus is a thin-walled sac, lying dorsal
to the ventricle and behind the auricles; it
receives the three large vene cave.
2. The pulsation of the heart.
a. Note that the contractions of the heart continue some
time after the frog has been killed, or even after the
heart is completely removed from the body.
b. Note the character of the heart’s pulsations: a regularly
alternating series of contractions and dilatations.
c. Note further that in each contraction or systole of the
heart all four divisions of the heart contract, but not
simultaneously. The sinus venosus contracts first,
then the two auricles, then the ventricle, and finally
the truncus arteriosus.
B. The Veins.
from the right side.
a, stomach ; a.v, Matinee abdominal vein ; 4, bladder ; 4.v, brachial
yein; c.d, cloacal aperture ;~c.v, cardiac vein; d, large intestine ;
é, liver ; e.v, external jugular yein ; fv, femo al vein ; g, gall- -bladder ;
a spleen j 46, en peli ae 2.0, inno’ n; 7-v, interna)
ein ; left_pelvic_vein; 7.v, wilted le rere
Ganey: ro ia atic Sy poral vet vein ; 7, right pelvic vein; ~v,
xenal_portal vein ; Sy Sue yenosus ; s.c, sciatic vein ; s.v, subclavian
vein; Z, tongue; 4a, truncus arteriosus; #, right auricle ; vy, Ta
tricle ; v.v, vesical veins,
Fic. 4.—Diagrammatic e of the venous system of the frog,
26 THE VASCULAR SYSTEM OF THE FROG
The veins should be dissected before the arteries, because, as
a rule, they lie nearer the surface and are therefore met with
first. The veins are further distinguished from the arteries by
their larger size and darker colour, due to the blood being seen
more clearly through their thinner walls.
Dissect from the ventral surface. In cleaning a vein take
hold with the forceps, not of the vein itself but of the tissue sur-
rounding it ; and take especial care not to prick the vein, as by
doing so you allow the blood to escape and obsewre the dissection,
and also render the vein itself difficult to see owing to the loss of
colour. Always dissect along and not across « bloodvessel, and
pin out the parts so as tu stretch it slightly.
I. Veins opening into the Sinus Venosus.
a. The right anterior vena cava is a large vein opening
into the right side of the sinus venosus, and returni
to it the blood from the right side of the head and
body, and from the right fore limb. It is formed by
the union of three veins,
A. The external jugular vein is formed by
i. The lingual vein, from the floor of the mouth
and the tongue.
ii, The mandibular vein, from the margin of the
lower jaw.
In close connection with the ventral surface of
each external jugular vein is a small round
vascular body, the thyroid gland.
“ 2. The innominate vein is formed by
A. The internal jugular vein, returning blood
from the interior of the skull, which it leaves
by an aperture at the posterior border of
the orbit.
“ii. The subscapular vein, a small vein from the
‘back of the arm and shoulder.
73. The subclavian vein, the largest of the three,
is formed by
«i. The brachial vein, from the fore limb.
vii. The musculo-cutaneous vein: a very large
vein returning blood from the skin and
THE VEINS
bo
b
i
muscles of the side and hack of the body,
and of the head as far forwards as the nose.
4b. The left anterior vena cava corresponds in its course
and branches to the right one.
J* The posterior vena cava is a median vein which, com-
mencing between the kidneys, runs forward, dorsally
to the liver, to open into the posterior end of the
sinus venosus. It returns to the heart the blood from
the liver and from the kidneys, and indirectly from
other viscera and from the hind limbs, It receives
the following veins :
Vi. The right and left hepatic veins, from the liver:
these open into the posterior vena cava just before
it joins the sinus venosus.
vw ii. The renal veins, from the kidneys: of these there
- are four or five on each side, which open into, or
rather form by their union, the posterior vena
cava, The most anterior of these receive the
veins from the fat bodies.
~ iii, The ovarian veins (in the female), or spermatic
veins (in the male); returning blood from the
ovaries or testes. They are usually four or five
in number on each side, and open into the pos-
terior vena cava between the renal veins.
II. Vein opening into the Left Auricle.
/a. The pulmonary vein is formed by the union of the
right and left pulmonary veins, returning to the
heart the blood from the right and left lungs respec-
tively. Each pulmonary vein runs along the inner
side of its lung.
III. The Portal Systems.
A portal vein is one which, returning blood from the capil-
laries of some part, breaks up before reaching the heart into a
second set of capillaries within some other organ; these again
unite to form a vein which carries the blood to the heart. In
the frog there are two portal systems, one supplying the
kidneys, and the other the liver,
28 THE VASCULAR SYSTEM OF THE FROG
Ze a. The renal portal system.
Trace back the anterior abdominal vein to the hinder end of the
body, where it will be seen to be formed by the union of the two
pelvic veins. Follow back the pelvic vein of one side to the base
of the hind limb ; here it will be seen to be one of two branches into
which the femoral vein, the large vein returning blood from the
hind limb, divides. The other branch of the femoral vein is the
renal portal vein, which is to be followed: to the outer side of the
kidney.
1. The right renal portal vein is the dorsal branch of
the right femoral vein : it runs forwards along the
outer side of the kidney and ends in numerous
branches in its substance, It receives the follow-
ing branches :
i. The right sciatic vein, from the muscles and
skin of the back of the thigh, joins the
renal portal vein close to its commencement,
before it reaches the kidney.
ii. The right dorso-lumbar veins are small veins
from the dorsal wall of the body, and, in the
female, from the oviduct: they join the
renal portal vein opposite the kidney.
2. The left renal portal vein corresponds in its course
and branches to the right vein.
b. The hepatic portal system.
This is formed partly by the anterior abdominal vein, which
brings to the liver blood from the hind limbs; and partly by
veins returning blood from the alimentary canal.
. The anterior abdominal vein is a median vein
formed by the union of the two pelvic veins, the
ventral branches of the femoral veins. It runs
forwards along the middle line of the ventral
body-wall to the level of the liver, where it leaves
the body-wall and divides into right and left
branches, which enter the right and left lobes of
the liver respectively. During its course it
receives the following veins :
i. Vesical veins, from the bladder.
ii, Parietal veins, from the ventral body-wall.
THE ARTERIES 29
iii. A cardiac vein, from a network of vessels on
the truncus arteriosus.
YY 2. The hepatic portal vein is a wide vein which runs
in the mesentery and joins the anterior abdo-
minal vein at its point of division into right and
left branches ; giving off, before doing so, a branch
to the left lobe of the liver. It carries to the
_liver the blood from the walls of the alimentary
canal, and is formed by the union of the follow-
ing veins:
j. The gastric vein, from the stomach.
ii. Intestinal veins, from the whole length of the
intestine, both small and large.
iii. The splenic vein, from the spleen: this usually
joins one of the intestinal veins.
C. The Arteries.
Fic. 5.—Diagrammatic figure of the arterial system of the male
frog, from the right side.
a, stomach; 4, nostril; c, small intestine; ¢.a, carotid artery;
cg, Carotid gland; c.#, coeliaco mesenteric artery ; ¢.7, cutaneous
artery; d, large mtestine ja, Cecsal aorta J Fem %, spleen ;
f.a, hepatic artery; 7, right lung; Za, Timgual artery; m, testis;
o, kidney ;' 0.2, gccipito-vertebral_artery; #.@, pulmonary artery ;
r, pelvic girdle; s, sternum; s.@, subclavian artery; 5.c, sciatic artery;
Z, tongue; 4a, i ; #.a, urogenital arteries; v, ven-
tricle ; 1, carotid arch ; 2, systemic arch >~3putmre-curateons arch.
~—SE ee —_—<—<—<——— ———
30 THE VASCULAR SYSTEM OF THE FROG
Dissect as for the veins. Pass a roll of paper or plugs of
cotton-wool down the esophagus, so as to distend it and stretch
the aortic arches. Clean carefully the aortic arches, commencing
at the truncus arteriosus ; and follow the several arteries to their
distribution, removing the veins and other structures which over-
lie them. Note the division of the truncus arteriosus in front
into right and left branches, each of which again divides into
three aortic arches—the carotid arch, the systemjc arch, and the
ma a arch. oh Fake wan O.
I. The Carotid Arch is the most anterior of the three arches:
it runs round the side of the csophagus, and is connected
dorsally with the second or systemic arch ; its chief branches
are as follows :
1. The lingual artery is a small artery supplying the
tongue. Immediately beyond the origin of the lingual
artery the carotid arch presents a small spongy
swelling, the carotid gland.
2. The carotid artery runs round the side of the cesopha-
gus to its dorsal surface: it is connected with the
systemic arch by a short branch, the ductus Botalli,
which in the adult frog is usually impervious; and
then turns forwards beneath the base of the skull,
dividing in front into the two following vessels :
i. The external carotid artery, supplying the roof and
sides of the buccal cavity, and the orbit.
ii. The internal carotid artery, which enters the skull
and supplies the brain.
II. The Systemic Arch, the middle arch of the three, runs
somewhat obliquely round the oesophagus to the dorsal surface,
and unites with its fellow of the opposite side about the level of
the anterior ends of the kidneys to form the dorsal aorta: near
the level of the posterior ends of the kidneys the aorta divides
into the two iliac arteries. The branches of the systemic arch
are as follows: ;
a. Branches given off before the union of the two arches.
1. The laryngeal artery is a small branch arising from the
inner side of the systemic arch near its origin from
the truncus arteriosus, and supplying the larynx.
2. The esophageal arteries are one or two branches arising
THE ARTERIES 31
from the upper part of the arch and entering the
dorsal wall of the esophagus.
3. The occipito-vertebral artery is a short branch arising
from the dorsal part of the arch: it runs upwards
immediately in front of the transverse process of the
second vertebra, and divides into two:
i, The occipital artery: which runs forwards, supply-
ing the side of the head and jaws.
ii. The vertebral artery: a large artery which runs
back alongside of and above the vertebral column,
and gives branches to the muscles of the body-
wall and to the spinal cord.
4, The subclavian artery: arises from the arch immedi-
ately behind the occipito-vertebral artery, and runs
outwards, supplying the shoulder and fore-limb.
b. Branches given off after the union of the two arches to
form the dorsal aorta.
1. The celiaco-mesenteric artery is a large median artery
arising immediately beyond the point of union of the
two arches, or sometimes from the left arch just
before the union, and supplying the stomach and
intestines. Its branches are as follows:
i. The celiac artery: which divides into
a. The gastric artery, supplying the stomach.
B. The hepatic artery, supplying the liver and
gall-bladder.
ii. The mesenteric artery: which divides into
a. The anterior mesenteric artery, supplying the
proximal part of the intestine.
B. The posterior mesenteric artery, supplying
the distal part of the intestine.
y. The splenic artery, supplying the spleen.
2. The urinogenital arteries are four to six small arteries
which arise from the ventral surface of the aorta
between the kidneys, and immediately divide into
right and left branches, supplying the kidneys, the
reproductive organs and ducts, and the fat bodies.
3. The lumbar arteries are small paired lateral branches
supplying the body-walls.
32 THE VASCULAR SYSTEM OF THE FROG
4, The hemorrhoidal artery is a small median artery
arising from the hinder end of the aorta, and supply-
ing the large intestine.
c. Branches formed by the division of the aorta.
1. The iliac arteries are the two large arteries formed by
the division of the aorta, and supplying the hind-
limbs. Each gives off a hypogastric artery, which
supplies the bladder, giving epigastric branches to
the ventral body-wall, and then continues as the
sciatic artery down the leg, giving off branches to
the muscles and skin of the thigh, and dividing at the
knee into peroneal and tibial arteries supplying the
leg and foot.
III. The Pulmo-cutaneous Arch is the hindmost of the three
aortic arches: it divides about the level of the carotid gland
into the following branches:
J. The cutaneous artery is a large artery which at first
runs forwards and upwards and then turns backwards,
supplying the skin of the back along the whole length
of the body, and sending smaller branches to the sides
of the head and to the skin of the ventral surface.
2. The pulmonary artery runs with somewhat sinuous
course along the outer side of the whole length of
the lung, giving off branches into its walls.
D. The Structure of the Heart.
Having completed the dissection of the bloodvessels, cut them
across, about half an inch from the heart ; remove the heart com-
pletely, and dissect it carefully under water. It is well to cut the
vessels of unequal lengths on the two sides, as this will facilitate
the recognition of the sides of the heart during the dissection,
Place the heart at first with the dorsal surface upwards.
1. The Sinus Venosus (Fig. 4, p. 25) is a thin-walled sac on
the dorsal surface of the heart; it is triangular in shape, with
the apex directed backwards. Into its anterior angles the right
and left anterior vene cave open, and into its posterior angle
or apex the posterior vena cava.
Cut away with scissors the dorsal wall of the sinus venosus so
as to expose its cavity : wash out any contained blood.
THE HEART 33
The sinu-auricular aperture (Fig. 6, SV) leading from the
sinous venosus to the right auricle, is a transversely oval opening,
arded by imperfect anterior and posterior valves, in the
ventral wall of the sinus venosus, close to its anterior end, and
very nearly in the median plane.
Fic. 6.—The frog's heart seen from the ventral surface, and dis-
sected so as to show its structure. The ventral walls of the truncus
arteriosus, and of the auricles and ventricle have been removed. (From
a‘drawing by Dr. Hurst.)
‘A, auriculo-ventricular aperture and one of its valves ; B, aperture
leading from ventricle to truncus arteriosus, with one of its valves ;
C, left carotid arch; C’, style passed down right carotid arch into the
truncus arteriosus; LA, left auricle; P, left pulmo-cutaneous arch ;
P’P’, style, passed down right pulmo-cutaneous arch into the truncus
arteriosus; PV, opening of pulmonary vein into left auricle: RA, right
auricle; S, left systemic arch; S/, style passed down right systemic
arch into the truncus arteriosus; SV, opening from sinus venosus into
right auricle; V, ventricle.
? Cc
34 THE VASCULAR SYSTEM OF THE FROG
2. The Auricles. Zurn the heart over, with its ventral sur-
face upwards. Cut away the ventral wall of both auricles with
jine scissors, taking care not to damage the truncus arteriosus
which lies across the right auricle. Wash out the blood from the
auricles.
a. The right auricle (Fig. 6, RA) is the larger of the two.
It has thin walls, thickened by muscular strands
which form interlacing reticular ridges on its inner
surface. In the dorsal wall of the auricle, very near
the median plane of the heart, is the aperture from
the sinus venosus already described (Fig. 6, SV).
b. The left auricle (Fig. 6, LA) is smaller, sometimes
much smaller, than the right auricle, which it resem-
bles in the structure of its walls. In its dorsal wall,
very close to the sinu-auricular aperture, is the opening
of the pulmonary vein (Fig. 6, PV).
c. The interauricular septum is the thin partition between
the right and left auricles. It is much thinner than
the walls of the auricles, and is placed somewhat
obliquely, the left auricle lying rather more dorsally
than the right. The septum ends with a free posterior
edge, opposite the auriculo-ventricular aperture.
Cut away with scissors the ventral wall of the ventricle, taking
care not to damage the truncus arteriosus.
3. The Ventricle (Fig. 6, V) is conical in shape with the
apex backwards, and has a small central cavity, with thick
spongy walls. The spongy character is due to great develop-
ment of a reticulum of interlacing muscular strands similar to
those of the auricles: the true outer wall of the ventricle is no
thicker than that of the auricles, and the meshes of the sponge-
work are really part of the cavity of the ventricle, and are filled
with blood.
The auriculo-ventricular aperture lies at the base of the
ventricle, and rather to the left side. It is guarded by valves
(Fig. 6, A) which hang into the ventricle, and are tied down at
their edges by fine tendinous threads; and it is divided by the
free lower edge of the interauricular septum into right and
left divisions, admitting blood from the right and left auricles
respectively.
THE HEART 35
Cut away carefully, with fine scissors, the ventral wall of the
truncus arteriosus so as to expose its cavity and the contained
valves.
4. The Truncus Arteriosus consists of two parts; a proximal
part or pylangium, which is a single vessel arising from the
ventricle ; and a distal part or synangium, which consists of the
basal parts of the aortic arches closely united together.
a. The pylangium (Fig. 6) is a short tube arising from
the right-hand ventral corner of the anterior end of
the ventricle: it has thick muscular walls and is
widest about the middle of its length.
The opening from the ventricle to the pylangium
(Fig. 6, B) is guarded by three semilunar pocket
valves.
The opening from the pylangium to the synangium
is also guarded by three semilunar valves which are
of very unequal size, a large right one, a small left
one, and a still smaller dorsal valve.
The spiral valve is a longitudinal ridge, projecting
into the cavity of the pylangium: it commences at
the left side of the ventricular aperture and runs
forwards somewhat spirally along the dorsal wall of
the pylangium to its anterior end, where it fuses
with the large right valve of the three between the
pylangium and the synangium. The ventral edge of
the spiral valve is free and rounded, and the valve is
much wider at its anterior than at its posterior end.
b. The synangium is the distal part of the truncus
arteriosus. In its dorsal wall immediately beyond
the valves separating it from the pylangium, is an
aperture (Fig. 6, P’) leading to the right and left
pulmo-cutaneous arches, P,P’. Beyond this the
synangium contains a wide cavity continued right
and left into the two systemic arches—8,S’. The
cavity is partially divided by a vertical tongue-like
projection from its dorsal wall: on the ventral
surface of this tongue are two small openings, very
close together, which lead into the right and left
carotid arches, C,C’.
Cut across the aortic arches, just beyond the division of the
36 THE VASCULAR SYSTEM OF THE FROG
truncus into right and left branches, and note that though each
branch is apparently a single vessel its cavity is really divided
into three vessels corresponding to the three aortic arches. Pass
bristles down these aortic arches, and note the points at which they
severally open into the truncus arteriosus.
E. The Lymphatic System.
The lymphatic system forms an accessory part of the vascular
system. Its main divisions are as follows:
1. The lymphatic vessels are a series of thin-walled tubes,
very variable in diameter and irregular in shape, which
traverse all the parts and organs of the body and are
in free communication with the veins. They are of
small size, and can only be recognised with the
microscope.
2. The lymph sacs are large irregular spaces communi-
cating with the lymphatic vessels. The most
important are the following :
a. The subcutaneous lymph sacs are the large
cavities between the skin and the muscles,
which have already been seen when remov-
ing the skin. They are separated from one
another by narrow septa of connective tissue,
which bind the skin to the underlying body-
wall.
b. The abdominal lymph sacs are the large
spaces along the dorsal surface of the body-
cavity, ventral to the kidneys, and between
the peritoneum and the body-walls. (See
Fig. 2, p.17.) The body-cavity itself also
communicates with the lymphatic system
through small openings or stomata in the
peritoneum.
3. The lymph hearts are two pairs of small globular con-
tractile sacs placed at points where the lymphatic
vessels communicate with the veins. They are quite
transparent.
a. The anterior lymph hearts lie immediately
behind the transverse processes of the third
BLOOD 37
vertebra, and beneath the shoulder girdle :
they open into the subscapular veins.
b. The posterior lymph hearts lie at the sides of
the urostyle, close to its hinder end. They
communicate by short vessels with the
femoral veins. Their pulsations can easily
be seen in a pithed frog.
4, The spleen has been already referred to (p. 23).
F. Microscopic Examination of Blood.
I. Frog's Blood.
1. Normal.
Place on a slide a small drop of blood from the heart of a frog ;
dilute it with a drop of normal salt solution (0°75 per cent.) ; put
on a thin cover-glass, and run a ring of oil round the edge to pre-
vent evaporation : examine with the high power.
Blood consists of a colourless fluid, the liquor sanguinis
or plasma, in which float the blood corpuscles. These
corpuscles are of two kinds.
i. Red corpuscles. These are very numerous, pale
red or yellowish red in colour, and of a flattened
oval shape, with rounded edges and a central
bulging, the nucleus. The flattened shape is best
seen when a corpuscle turns edgeways. They
measure 0°0235 mm. in length by 0:0145 mm. in
width ; or about 755 X rey Of an inch.
ii, White corpuscles. These are much fewer in number
and of smaller size: they are colourless, granular,
subspherical in shape, and exhibit “ ameceboid”
movements. Sketch one half a dozen times at
intervals of half a minute.
2. Action of acetic acid on blood.
Place a fresh drop of blood on a clean slide: add a drop of
acetic acid : cover, and examine with the high power: note the
changes produced.
i. Red corpuscles: the nuclei become much more
apparent than before, and the red colour dis-
appears.
38 THE VASCULAR SYSTEM OF THE FROG
ii, White corpuscles: become clearer, and show nuclei,
sometimes more than one in a single corpuscle.
II, Human Blood.
1. Normal.
Prick the tip of your finger, and place a small drop of the
blood on a slide: add a drop of normal salt solution, cover, and
examine as before. Note the following points :
i. Red corpuscles. These, which are much smaller
than in frog’s blood, are in the form of circular
biconcave discs with rounded edges, but no nuclei.
They have a tendency to run together into rou-
leaux, like piles of coins. Their average diameter
is 0-008 mm., or about z555 of an inch.
ii. White corpuscles. These are very similar to those
of the frog: they are slightly larger than the
red corpuscles, averaging about 0°01 mm., or
zeoo Of an inch in diameter: their ameboid
movements are not well seen unless the slide is
warmed.
2. Action of acetic acid.
Treat with acetic acid as before: note that, unlike the frog’s
blood, no nuclei are visible in the red corpuscles.
G. Circulation of the Blood in the Web of a Frog’s Foot.
The web uniting the toes of the frog’s foot is so thin and
transparent, that with the microscope the blood in it can readily
be seen coursing along the capillaries.
Examine a frog prepared to show the circulation in the web of
the foot. Note the following points :
1. With a low power.
a. The irregularly branched pigment cells to which the
colour of the frog’s skin is due.
b. The fine meshwork of bloodvessels along which the
blood can be seen flowing. These bloodvessels are
of three kinds.
i. The arteries, carrying blood to the web, are dis-
CIRCULATION OF BLOOD 39
tinguished by the fact that when they divide,
the direction of flow of the blood is from the
larger trunk to its branches.
- ii. The capillaries form a close network of very small
and thin-walled vessels, along which the blood
flows from the arteries to the veins.
iii. The veins, carrying the blood away from the web
back towards the heart, are distinguished from
the arteries by the fact that the blood in them
flows from smaller to larger vessels.
2. With a high power : note the following points :
a. The walls of the arteries and veins are much thicker
than those of the capillaries, which latter are often
difficult to see.
b. The white corpuscles have a marked tendency to creep
along the sides of the vessels, while the red corpuscles
rush far more rapidly along the middle of the stream :
this is seen best in the small arteries.
c. The variations in calibre of the small arteries and
capillaries: whilst under observation an artery or
capillary may be seen to change its size to a consider-
able extent.
d. The indefinite character of the capillary circulation.
Owing to changes of size in adjacent vessels, the
direction of flow of the blood in a given capillary may
become reversed.
e. The elasticity of the red corpuscles: seen best when
they are turning the corners of the capillary net-
work.
f, The tendency of the white corpuscles to migrate through
the walls of the capillaries into the tissues outside.
This is much increased by the application of some
irritant substance, such as a drop of weak acid, to the
web.
CHAPTER IIT.
THE SKELETON OF THE FROG.
THE skeleton, which forms the hard internal parts of the frog,
is composed partly of cartilage and partly of bone. It forms a
framework giving definite shape to the body, and precision to
the movements ; and serves also to protect from injury some of
the more important and delicate organs, notably the central
nervous system, the sense organs and the heart. In the early
stages of its development the skeleton consists entirely of car-
tilage; in the adult this primary cartilaginous skeleton is replaced
to a greater or less extent by bone. Bone may also be developed
in places where there was no pre-existing cartilage, and is then
called membrane-bone, in contradistinction to the former kind,
or cartilage-bone, which replaces cartilage. Membrane-bones
arise in the first instance as ossifications in the dermis or deeper
layer of the skin: in many fish they retain this primitive
position, but in the frog and most higher vertebrates they sink
below the skin and graft themselves on to the more deeply placed
cartilaginous skeleton. Cartilage may also become calcified,
t.¢., have calcareous salts deposited in its matrix, without in any
way taking on the character of true bone.
The skeleton may conveniently be divided into (1) the axial
portion, including the skull and the vertebral column: and
(2) the appendicular portion, including the limbs, and the
limb-girdles which attach them to the body.
Examine the prepared skeletons, and make careful drawings to
scale of the several parts. In your drawings colour the cartilage
blue, the cartilage-bones yellow, and the membrane-bones white or
red, Prepare skeletons for yourself by dipping the parts in hot
water, and carefully brushing away the soft tissues until the
skeleton is clean.
THE abies OF THE FROG
Fic. 7.—The skeleton of the frog, seen from the dorsal surface ; the
left suprascapular and scapular have been removed.
a, astragalus; c, caleaneum; d, suprascapular; e¢, exoccipital ;
¥, femur; /, frontoparietal ; g, metacarpals; 2, humerus ; 2, ilium; 2,
metatarsals ; Z, carpus; , maxilla; , nasal; 0, pro-otic; 4, pterygoid ;
pm, premaxilla; g, ‘‘quadratojugal”’; 7, radio-ulna; s, squamosal;
se, sphenethmoid ; s.v, sacral vertebra ; ¢, tibio-fibula; ~, urostyle.
41
42 HE SKELETON OF THE FROG
A. The Axial Skeleton.
I. The Vertebral Column or “ backbone.”
This is a bony tube which surrounds and protects the spinal
cord; it consists of an anterior part, which is divided trans-
versely into nine rings or vertebra, and a posterior unsegmented
portion of about equal length—the urostyle. At the sides of
the tube, between the successive vertebra, are the intervertebral
foramina through which the nerves pass out from the spinal
cord to the various parts of the body.
a, Structure of a vertebra. Haxamine one of the vertebra,
say the third, more closely: draw it, showing the
Sollowing points :
i. The vertebra is a bony ring; the spinal cord lying
during life in the central neural canal.
ii. The centrum or body is the thickened ventral por-
tion of the ring: it articulates with the centra of
the vertebre in front of and behind it ; and forms
the floor of the neural canal.
iii. The neural arch consists of the lateral and dorsal
portions of the ring; and forms the sides and
roof of the neural canal.
iv. The spinous process or neural spine is a small blunt
median process, projecting upwards and back-
wards from the top of the neural arch.
. The transverse processes are a pair of large processes
projecting horizontally outwards from the point
of union of centrum and neural arch.
vi. The articular processes or zygapophyses, on the
anterior and posterior borders of the neural arch,
articulate with corresponding processes on the
vertebre in front and behind, and so serve to
link the vertebre together.
a. The anterior articular processes, or prezyg-
apophyses, face upwards and slightly in-
wards.
8. The posterior articular processes, or post-
zygapophyses, face downwards and slightly
outwards.
<
; {HE VERTEBRAL COLUMN 43
ines
ME rane el cr ar Taal
G j i i alu
lira
{i
|
sc
Fic, 24.—Longitudinal vertical section through a frog embryo at a
- later stage in the formation of the mesenteron.
H, invaginate hypoblast ; M, mesoblast ; MN, mesenteron ; N, noto-
chord ; SC, segmentation cavity; YP, yolk plug, filling up the blasto-
pore,
free from pigment, but much distended by food-yolk, which is
present in such quantity as to render them comparatively inert.
The former are the epiblast cells; the latter may con-
veniently be spoken of as the lower layer cells or yolk-cells.
The epiblast shows almost from the first a distinction into
two layers; the most superficial cells being somewhat cubical
in shape and closely applied side by side so as to form a con-
tinuous and deeply pigmented layer ; while the deeper cells are
112 DEVELOPMENT OF THE FROG
more spherical, less strongly pigmented, and loosely arranged
in a layer two or more cells deep.
The epiblast cells continue to increase by division, and very
early, owing apparently to multiplication of the cells at the
margin of the layer, seem to spread over the lower or yolk-cells.
Owing to the difference in colour of the two halves of the egg,
the various stages of this process can be readily followed, the
black epiblast cells seeming to spread over and gradually enclose
the almost white yolk-cells, until a small round patch alone
Fic. 25.— Longitudinal vertical section through a frog embryo show-
ing the completion of the mesenteron. i
B, blastopore ; EE, epidermic layer of epiblast ; EN, nervous layer of
epiblast ; H, invaginate hypoblast ; M, mesoblast ; MN, mesenteron ;
N, notochord,
remains uncovered. It is doubtful how far this process is due
to actual growth of the epiblast, rather than to differentiation
of lower layer cells.
This apparent spreading of the epiblast does not take place
equally fast all round its margin, and at one place the epiblast,
instead of extending over the yolk-cells, bends inwards into the
latter, and grows into the interior of the egg. This place is
visible externally as a sharply defined horizontal or slightly
crescentic groove, concave downwards, bounded above by the
small black epiblast cells, and below by the large white yolk-
cells. As the epiblast continues spreading over the rest of the
FORMATION OF THE GERMINAL LAYERS 113
yolk this groove becomes horseshoe-shaped, and a little later
circular.
The egg has now the appearance shown in Fig. 24, the
epiblast covering the whole surface except a circular patch,
where alone the yolk-cells are visible from the surface. This
circular aperture in the epiblast is called the blastopore; it is
situated at what will become the posterior end of the embryo;
and it is bordered by a distinct rim or lip, round which the
epiblast turns inwards into the interior of the egg. The cir-
cular plug of yolk-cells filling up the blastopore is spoken of as
the yolk plug.
The structure of the egg, or rather of the embryo at a
slightly later stage, is shown in Fig. 25, which represents a
vertical section passing through the middle of the blastopore.
The epiblast covers the whole surface except at the blastopore.
From the lip of the blastopore a layer of cells appears to grow
into the egg concentrically with its surface.
This layer is called the hypoblast: it appears to be formed
partly by an ingrowth of cells round the whole margin of the
blastopore, partly by differentiation of yolk-cells; it extends
much more rapidly dorsally than ventrally, so that while on the
upper or dorsal surface it extends to the anterior end of the
embryo, at the sides and below it only extends a very short
way as yet. From the mode of its formation it is necessarily
continuous with the epiblast all round the lip of the blasto-
pore.
Between this ingrowing layer of hypoblast and the yolk-cells
there is a space. This is a very narrow chink near the blasto-
pore, but further forwards it dilates on the dorsal surface
to form a cavity of some size (Fig. 25, MN), wider from side to
side than it is dorso-ventrally. (Cf. Fig. 26.)
This cavity, which is named the mesenteron, is the future
alimentary canal: it communicates with the exterior through
the blastopore, though the aperture is reduced to a narrow
chink and is almost stopped up by the yolk plug. The per-
manent mouth and anus are not yet formed.
During the process of formation of the mesenteron the
segmentation cavity gets pushed out of place. It has been
asserted to ultimately open into and form part of the mesen-
teron.
The Notochord. Along the roof of the mesenteron in the
H
114 DEVELOPMENT OF THE FROG
mid-dorsal line a rod-like thickening of the hypoblast is formed
at a very early stage. This is the notochord (Figs. 24 to 26, N),
which serves to slightly stiffen the back of the embryo, and is
for some time the only skeleton which it possesses.
It very early splits off from the roof of the mesenteron, except
at its hinder end, where it remains for some time in continuity
with both hypoblast and epiblast at the lip of the blastopore.
The Mesoblast. Between the epiblast and hypoblast a third
or intermediate layer of cells, the mesoblast, is soon established.
It is formed by differentiation of the surface hypoblast and
yolk-cells as a separate layer, lying immediately beneath the
epiblast, but quite distinct from it. It extends all round the
embryo except along the mid-dorsal line, where the space
between the epiblast and hypoblast is occupied by the notochord.
It is, for a time, incomplete in front, opposite the segmentation
cavity, but soon grows in from the sides so as to fill up the
deficiency.
The cells of the mesoblast become early arranged in two
parallel layers or sheets, which separate slightly from each other,
so as to leave between them a narrow space, which later on
becomes the body cavity or celom. (Cf. Fig. 26.) In many
specimens the mesoblast cells are from the first arranged in
two layers; and in some cases the mesoblast may be distinctly
seen to arise as a pair of flattened hollow outgrowths from the
hypoblast, a little way in front of the blastopore, and at the
sides of the notochord : these outgrowths spread in all directions,
and their cavities become the body cavity or ceelom. Such a
mode of origin of the mesoblast is known to obtain in some
lower vertebrates, but it is uncertain as yet how far it is general
in the case of the frog.
Growth of the hypoblast. The hypoblast is formed in the
first instance from the epiblast by invagination at the lip of the
blastopore: its after growth is effected, however, mainly at the
expense of the yolk-cells, with which it is in contact. By
growth at its margin it gradually creeps round until it extends
all round the embryo, the yolk-cells forming part of its
ventral wall.
The two forms of hypoblast may be named, according to
their modes of origin, invaginate hypoblast and yolk hypoblast
respectively. They may be easily distinguished in the early
stages of development by the fact that the invaginate hypoblast,
THE NERVOUS SYSTEM 115
like the epiblast, from which it is derived, consists of small
cells containing pigment: while the yolk hypoblast cells are
larger and have no pigment. The invaginate hypoblast forms
the notochord and the roof of the alimentary canal, while the
yolk hypoblast gives rise to its sides and floor. The line between
the two is at first a fairly sharp one, especially at the hinder
end, near the blastopore: later on it disappears.
Fate of the germinal layers. From one or other of the three
germinal layers—epiblast, mesoblast, and hypoblast—all parts
of the embryo are formed.
The epiblast, or outer layer, gives rise to the epidermis
covering the body generally, and to the various glandular and
other structures derived from the epidermis; to the nervous
system, both central and peripheral; to the olfactory and
auditory epithelium, to the retina and lens of the eye, and to
the other sensory organs; to the epithelial lining of the mouth
and anus (stomodeum and proctodeum); and to the pineal
and pituitary bodies.
The hypoblast, or inner layer, gives rise to the epithelium
lining the alimentary canal and its various diverticula, including
the glands of the cesophagus, stomach, and intestine, the lungs,
the bladder, the bile ducts, gall bladder, pancreatic ducts, and
the hepatic cells of the liver and the secreting cells of the
pancreas; the notochord is also formed from hypoblast.
From the mesoblast, or middle layer, are derived all struc-
tures between the epiblast and hypoblast; 7.e., the connective
tissue, muscles, skeleton (except the notochord), bloodvessels
and lymphatics; and also the peritoneum, and the urinary and
reproductive organs.
F. Development of the Nervous System.
It is convenient from the point we have now reached to deal
with the several systems one by one. The nervous system is a
suitable one to commence with, as it appears at a very early
stage of development, and plays an important part, especially
in the younger stages, in determining the shape and proportions
of the embryo.
The epiblast consists almost from the first of two layers, the
distinction between which is already established at the close of
segmentation. (Fig. 23.) Of these the upper or epidermic
layer is a single stratum of closely fitting cubical cells; while
116 DEVELOPMENT OF THE FROG
the lower or nervous layer consists of ovoid or spherical cells,
more loosely compacted, and two or three deep. It is from the
latter that the nervous system is developed.
The first trace of the nervous system is seen about a week
after fertilisation, when the embryo is still spherical and the
blastopore has become much reduced in size and difficult to see.
(Cf. Fig. 26.)
The dorsal surface of the embryo now flattens slightly, and
along the flattened area the nervous layer of the epiblast
thickens to form the neural plate, which is wide in front but
Fic, 26,—Transverse section through a frog embryo during the for-
mation of the neural canal. .
C, ccelom ; EE, epidermic layer of epiblast ; EN, nervous layer of
epiblast; H, hypoblast; M, mesoblast ; ME, somatopleuric layer of
mesoblast ; MH, splanchnopleuric layer of mesoblast ; MN, mesenteron ;
N, notochord; NF, neural fold; NG, neural groove ; Y, yolk cells.
narrows posteriorly towards the blastopore. Slightly raised
ridges, the neural folds, soon appear, bordering the sides of the
neural plate; and a longitudinal neural groove is formed along
its dorsal surface in the median line, extending forwards from
the blastopore.
A transverse fold connects the anterior ends of the neural
folds together, slightly raising up the anterior end of the neural
plate. The neural folds now grow rapidly: the groove between
THE NERVOUS SYSTEM 117
them deepens, and the folds becoming more and more prominent
bend in towards each other (Fig. 26) and finally meet and fuse,
thereby converting the neural groove into a tube.
The neural folds first meet about the junction of the head
and trunk of the future tadpole, from which point the fusion
extends rapidly in both directions, forwards and backwards.
The last point at which fusion occurs is a little distance behind
the anterior end of the tube, at the place where the pineal body
will appear later.
In front, the neural tube ends blindly ; at its posterior end
Fic. 27.—Longitudinal vertical section through a frog embryo
shortly before the closure of the blastopore. Length of the embryo
2°5mm. X 30.
B, blastopore ; BF, fore-brain; BH, hind-brain; BM, mid-brain ;
H, hypoblast; L, liver; M, mesoblast; MN, mesenteron; N, noto-
chord; NC, neurenteric canal; P, ingrowth of epiblast to form pituitary
body; PD, proctodzeum ; R, rectal diverticulum of mesenteron ; S, cen-
tral canal of spinal cord ; Y, yolk-cells.
it opens to the exterior at the blastopore, and is in free com-
munication with the mesenteron. (Cf. Fig. 27.) The short
channel of communication between the neural tube and the
mesenteron, 7.¢., between the nervous system and the alimen-
tary canal, is spoken of as the neurenteric canal; it is only
118 DEVELOPMENT OF THE FROG
present for a short time, and closes up before the tadpole
hatches.
The neural tube, formed in this way, soon separates from the
surface epiblast, and by thickening of its walls and other
changes becomes converted into the central nervous system ;
the anterior part forming the brain, and the posterior part the
N
s
ne
L. roy A
v c
Fic, 28.—Longitudinal vertical section through the anterior end of
a tadpole shortly after the time of hatching. Length of the tadpole
8 mm.
‘A, auricle of heart ; BF, fore-brain ; BH, hind-brain ; BM, mid-brain;
C’, pericardial cavity; CV, vesicle of cerebral hemispheres ; |, infundi-
bulum ; L, liver; N, notochord ; O, depression of floor of fore-brain
from which the optic vesicles arise; OE, c:sophagus; P, pituitary
body; PN, pineal body; S, central canal of spinal cord: SD, stomo-
dzeum ; T, truncus arteriosus ; V, ventricle; Y, yolk-cells.
spinal cord. The lumen or cavity of the tube persists as the
central canal of the spinal cord and the ventricles of the brain.
The Brain. At the time of its first appearance the brain is
bent at right angles about the middle of its length ; the axis of
THE BRAIN 119
the anterior portion being vertical, and that of the posterior
portion horizontal. (Fig. 27.) The posterior portion, or hind-
brain, BH, is wide from side to side, and has moderately thick
sides and floor, but a thin roof; it is continuous behind with
the spinal cord.
The anterior or vertical portion has walls of nearly uniform
thickness in all parts. It is divided by a slight constriction,
cP
AD cB i
1 o SP OPN
BH BM
| j
i Hu il Cn TASS Se
BF
(ke
c ¥ }
ARNIS
er Gy « : \:
C2 RES gg “|e BB
occas T
cllUy «CA
i
Fic. 29.—Longitudinal vertical section through the head and
anterior part of the body of a tadpole about the time of appearance of
the hind legs. Length of tadpole, 12mm. X 14.
A, auricle of heart; AD, dorsal aorta; BB, basi-branchial cartilage ;
BF, fore-brain; BH, hind-brain ; BM, mid-brain; C, coelom or bedy
cavity ; C’, pericardial cavity; CH, cerebral hemisphere ; CB, rudi-
mentary cerebellum; CP, choroid plexus of fourth ventricle; CP’,
choroid plexus of third ventricle; F, pharynx; G, stomach; H, lung;
|, infundibulum; J, horny jaws; kK, lip; L, liver ; N, notochord ;
O, depression of floor of fore-brain from which the optic nerves arise ;
OE, cesophagus; P, pituitary body; PN, pineal body; §, central
cana! of spinal cord ; T, truncus arteriosus ; V, ventricle.
best marked at the sides, into an upper or posterior part, the
mid-brain, BM, which forms the angle of the bend and lies
opposite the anterior end of the notochord; and a lower and
120 DEVELOPMENT OF THE FROG
larger portion, the fore-brain, BF, which is produced laterally
into a pair of hollow outgrowths, the optic vesicles.
The further development of the brain is illustrated by Figs.
28 and 29. It will be seen that the rectangular bending of the
brain, which is known as the cranial flexure, and which was so
prominent a feature in the earlier stage, is no longer obvious ;
a closer comparison of the figures will show, however, that this
straightening of the brain, or rectification of the cranial flexure,
is apparent rather than real, and is brought about partly by the
development of the cerebral hemispheres, which grow upwards
and forwards from the fore-brain, and still more largely by the
formation of the mouth and the growth forwards of the face
and lips, which cause the brain to take a much less prominent
share in determining the shape of the head.
The hind-brain, BH, has undergone but little change in
Fig. 28, except an increase in thickness of its floor and sides.
At the stage represented in Fig. 29 it is separated from the
mid-brain on the dorsal surface by a well-marked groove, im-
mediately behind which the roof of the hind-brain is thickened
transversely to form the cerebellum, CB. The cavity of the
hind-brain remains as the fourth ventricle, the roof of which
is very thin and thrown into numerous transverse folds, CP,
which hang down into the ventricle, and between the layers of
which lie the bloodvessels of the choroid plexus of the ventricle.
The mid-brain, BM, thickens on its floor to form the crura
cerebri. Its roof grows out laterally into a pair of hollow ovoid
processes, the optic lobes; and its cavity persists as the
aqueductus Sylvii, or iter a tertio ad quartum ventriculum.
The fore-brain, BF, becomes the thalamencephalon of the
adult ; its cavity becomes the third ventricle, which by thick-
ening of its walls to form the optic thalami is reduced to a
vertical cleft, very narrow from side to side. Its floor is pro-
duced downwards and backwards into a hollow sac-like diverti-
culum, the infundibulum, |, in connection with which is the
pituitary body. In front of the infundibulum is a transverse
ridge projecting into the ventricle, and formed by the roots of
the optic nerves.
The roof of the fore-brain remains thin; a little behind the
middle of its length the pineal body, PN, arises as a median
hollow diverticulum, Figs. 28 and 29; this is formed at the spot
where the final closure of the neural tube took place, and is at
THE SENSE ORGANS 121
first directed backwards ; in the later stages it grows forwards
and forms a rounded vesicle connected with the brain by a long
pigmented stalk ; when the skull develops it cuts off the vesicle
from the stalk, the former remaining as a small rounded body
outside the skull, while the stalk persists as a slender pigmented
tract within the cranial cavity.
In front of the pineal body, and at the anterior end of the
fore-brain, the roof is thrown into folds which hang down into
the ventricle forming a choroid plexus, CP’, similar to that in
the medulla.
The anterior end of the fore-brain grows forwards as a median
thin-walled cerebral vesicle, from which at a slightly later stage
the cerebral hemispheres, CH, arise as a pair of hollow out-
growths ; the foramina of Monro being the apertures of com-
munication between the lateral ventricles or cavities of the
hemispheres, and the third ventricle. The anterior ends of the
hemispheres grow forwards as the olfactory lobes, which become
fused together in the median plane.
The peripheral nervous system. ‘The cranial nerves and the
dorsal roots of the spinal nerves are formed from the deeper or
nervous layer of the epidermis. They appear to arise as lateral
outgrowths from the edges of the neural plate, and may be
recognised at a very early stage, while the neural groove is still
shallow and open ; they are, therefore, at their first appearance
continuous with the brain or spinal cord.
The ventral roots of the spinal nerves arise later than the
dorsal ones, as outgrowths from the cord near its ventral sur-
face. They are at first independent of the dorsal roots, but
soon become connected with these.
G. Development of the Sense Organs.
The organs of special sensation are developed from the deeper
or nervous layer of the epiblast, and become connected with
their respective nerves at a very early stage of their formation.
The derivation of the sense organs from the epiblast is
explained by the fact that they are concerned with the appre-
ciation of the presence and nature of external objects, and are
therefore necessarily formed on the surface of the body. They
may be regarded as specially modified portions of the epidermis.
The Nose. The olfactory organs appear at a very early stage
as paired thickenings of the nervous layer of the epiblast at
122 DEVELOPMENT OF THE FROG
the anterior end of the head, in the angles between the fore-
brain and the optic vesicles. A pitting-in of the surface,
involving both layers of the epiblast, soon appears in each of
these thickenings, and the pits so formed become the nasal
sacs; the mouths of the pits forming the nostrils or anterior
nares, and the epiblast lining the pits giving rise to the olfac-
tory epithelium.
From the inner or deeper end of each olfactory pit a diverti-
culum, at first solid, but soon becoming hollow, grows down-
wards to the roof of the pharynx, into which it opens,zas the
posterior nares, very shortly after the formation of the mouth
opening.
Q
FG. 30.—Half sections in the transverse plane of a tadpole 10 mm.
long (left half) and of a tadpole r2 mm. long (right half). X 35.
BF, fore-brain ; OD, outer wall of optic cup (pigment layer of adult
retina); OC, inner wall of optic cup (remainder of adult retina) ;
OL, lens, attached to epiblast in younger tadpole, but forming a
hollow vesicle at the later stage; TP, pharynx; Q, sucker. (G. H.F.]
The Eye. The eye differs from the other sense organs, inas-
much as the lens alone is formed directly from the surface epi-
blast, while the sensitive part of the eye, or retina, arises as an
outgrowth from the brain. The optic vesicles have already been
described as arising at a very early period as lateral outgrowths
from the fore-brain; these soon become constricted at their
necks so as to be connected with the brain by narrow stalks,
which ultimately become the optic nerves.
The outer surface of each optic vesicle, which is at first in
close contact with the surface epiblast, soon becomes flattened
(Fig. 30, left half), and then thickens so greatly as almost to
THE SENSE ORGANS 123
obliterate the cavity of the vesicle. At the same time a thick-
ening of the deeper or nervous. layer of the surface epiblast
takes place opposite the optic vesicle; this grows rapidly and
forms a spherical body, projecting inwards from the surface ;
this is at first solid, but soon becomes hollow and breaks away
completely from the surface epiblast; it becomes later on the
lens of the eye, and may be spoken of as the lens vesicle. _
Partly in consequence of the ingrowth of the lens vesicle,
and partly through growth of the optic vesicle itself, this latter
becomes pitted on its outer surface, and so converted into a cup
—the optic cup—with double walls; the inner wall being the
thickened and originally outer wall of the optic vesicle, and the
outer wall of the cup being the original inner o1 deeper part of
the wall of the vesicle. The lip of the cup is incomplete below,
owing to the presence of a slit, the choroidal fissure, through
which mesoblastic elements penetrate into the interior of the
eye.
From the optic cup and lens vesicle the adult eye is derived
in the following way : The lens becomes solid, owing to thicken-
ing of its inner wall, which proceeds so far as to finally oblite-
rate the cavity. The optic cup enlarges considerably; it remains
in contact with the lens at its edge or lip, but elsewhere is
separated from it by a space which becomes the posterior
chamber of the eye, and in which the vitreous humour is
formed. The inner wall of the optic cup gives rise to the
retina, the rods and cones growing out from its outer surface ;
while the outer and thinner wall of the optic cup forms the
layer of pigment cells in which the rods and cones are imbedded.
The choroid and sclerotic coats are formed from the. mesoblast
surrounding the optic cup.
The eye develops very slowly, and throughout the tadpole
stage of existence is in a very rudimentary and imperfect
condition.
The Har. The ears are developed as a pair of pit-like in-
vaginations of the nervous layer of the epiblast at the sides of
the hind-brain. The invaginations do not involve the epidermic
or surface layer of the epiblast, so that the auditory pits do not
open to the exterior.
The mouths of the pits very early narrow and close; and the
auditory vesicles so formed separate from the epiblast and lie
in the mesoblast at the sides of the head. The vesicle becomes
124 DEVELOPMENT OF THE FROG
the vestibule of the adult ear; the semicircular canals arising
as outgrowths from it.
Throughout the tadpole stage of existence there is no further
modification ; but shortly after the metamorphosis the hyoman-
dibular cleft, which has at no period opened to the exterior,
is stated to widen somewhat and form the Eustachian passage,
while the layer of integument closing its outer end becomes the
tympanic membrane. There is some reason, however, for
thinking that the Eustachian passage develops independently
in the frog, and not from the hyomandibular cleft. The colu-
mella, which has been described with the skull, is formed stil]
later. (Cf. Fig. 10, p. 48.)
Special Sense Organs. During the tadpole stage, while the
animal is leading an aquatic life, special sense organs in the
form of small epidermal papille, supplied by branches of the
trigeminal and pneumogastric nerves, are found arranged in
rows along the body, and round the eyes, and in other parts of
the head. These are lost at the time of the metamorphosis.
The mouth of the tadpole is also provided with small rounded
papille, which are probably organs of taste. (See Fig. 29.)
H. Development of the Alimentary Canal.
The alimentary canal is developed in three lengths: (1) the
mesenteron, which is formed by splitting apart of the yolk-cells
as described above; this gives rise to nearly the whole length
of the alimentary canal; and from it are developed the gill slits,
the lungs, the thyroid, the liver, the pancreas, and the bladder;
as well as the notochord; (2) the stomodeum, which is a
pitting-in at the anterior end of the body, from which the
mouth and pituitary body are formed ; and (8) the proctodeum,
which is a similar pitting-in at the hinder end of the body to
form the anal or cloacal opening.
From the mode of their formation it follows that the mesen-
teron is lined by hypoblast, and the stomodzum and proctodeum
by epiblast.
1. The mesenteron. The early development of the mesen-
teron has already been described.
The anterior end of the mesenteron, in the head region, is
considerably dilated from the first : and at the hinder end of the
embryo a similar, though much smaller, expansion takes place.
In this way (¢f. Fig. 27), the mass of the food-yolk becomes
THE ALIMENTARY CANAL 125
confined to the ventral portion of the body.region, not extend-
ing into either the head or the tail.
The hypoblast, which is a definite layer of cells, at first con-
fined to the roof of the mesenteron, gradually spreads round its
sides until it encloses the whole of the food-yolk, and the
alimentary canal is completed as a tube, which from the first is
slightly convoluted. When the tadpole begins to feed, the
alimentary canal lengthens rapidly, and becomes coiled in a
spiral manner. Except at the anterior end, in the gill-bearing
region, it is of approximately uniform diameter throughout.
During the metamorphosis, the alimentary canal shortens con-
siderably, and the distinction between stomach, small intestine,
and large intestine, is definitely established.
The liver is recognisable at a very early stage (Fig. 27) as a
ventral and backwardly directed diverticulum of the anterior
part of the mesenteron, forming the anterior boundary of the
mass of food-yolk. In the later stages the walls of the diver-
ticulum thicken, and become thrown into folds between which
the vascular mesoblast makes its way: the diverticulum itself
persists as the bile duct, and the gall bladder arises as an out-
growth from this.
The pancreas is developed as a pair of hollow outgrowths
from the mesenteron, behind the liver: in the later stages the
ducts shift so as to open into the bile duct instead of directly
into the intestine.
The bladder arises shortly before the metamorphosis as a
ventral outgrowth from the hinder end of the mesenteron, which
soon becomes bifid at its distal blind end.
The post-anal gut is an extension of the hinder end of the
mesenteron into the base of the tail, which appears as this latter
is developed : it becomes solid after a short time, and later on
disappears altogether. It is perhaps to be regarded as formed
by a mechanical drawing out of the intestine by the outgrowing
tail.
The lungs. Immediately behind the gill-bearing region or
pharynx, the alimentary canal narrows very considerably ; its
sides become folded inwards, and the two folds meeting each
other divide the canal into a dorsal tube or cesophagus, and a
ventral one which forms the laryngeal chamber: from this
latter the lungs arise as thin-walled lateral outgrowths, They
appear first in young tadpoles of about 8 mm. length, 7.e., some
126 DEVELOPMENT OF THE FROG
time after hatching, but shortly before the opening of the
mouth. About the time that the lungs first appear, in tadpoles
of about 8 mm. length, the esophagus, which up to this time
has been tubular, becomes solid, and remains so until a short
time after the formation of the mouth. The meaning of this
curious point has not been ascertained.
2. The stomodeum. At the stage represented in Fig. 27,
shortly after closure of the neural canal, a conical ingrowth, P,
of the nervous layer of the epidermis is formed at the anterior
end of the body immediately below the fore-brain: from this
ingrowth the pituitary body is developed, and a slight depres-
sion of the surface epiblast opposite its base, marks the position
of the stomodeum.
At the time of hatching, this depression is a small shallow
pit, separated from the anterior end of the mesenteron by a thin
septum. Soon after hatching, in tadpoles of about 9 mm.
length, this septum becomes perforated, and the alimentary
canal communicates with the exterior through the stomodeal
pit. After the perforation is effected, the lips with the whole
anterior part of the face grow forwards rapidly ; the horny jaws
are formed, and the tadpole begins to feed vigorously. (Cf. Figs.
28 and 29.)
The pituitary body (Figs. 27 to 29, P) is formed from the
ingrowing stalk of epiblast described above: this rapidly
elongates, growing backwards between the brain and the roof
of the mesenteron until it reaches the infundibulum ; its hinder
end now becomes tubular, gives off a few lateral diverticula;
separates from the stalk, which soon disappears, and becomes
applied to the ventral surface of the hinder end of the infundi-
bulum to form the pituitary body.
3. The proctodzum or anal invagination appears before the
stomodeum. Shortly before the neural folds have met to
form the neural tube, the proctodeum is visible as a small
median depression of the epiblast at the hinder end of the
embryo, a little way below the blastopore. The cells lining it
are rather strongly pigmented, and slightly larger than the
surrounding epiblast cells.
From the hinder end of the mesenteron a rectal diverticulum
(lig. 27, R) extends downwards towards the proctodeum; a
little later, and some time before the tadpole hatches, the two
structures meet; perforation occurs; and the definitive anal or
THE GILL CLEFTS AND ARCHES 127
cloacal opening is formed. For a short time the blastopore
and the proctodeum are both open; but very shortly after
completion of the proctodzeum the blastopore closes finally.
I. The Gill Clefts and Arches.
Some little time before the tadpole is hatched a series of
vertical ridge-like thickenings appear on the sides of the head
and neck. These are the visceral arches, and are six in number
on each side.
The most anterior is the mandibular arch, and gives rise
later on to the lower jaw; the second is the hyoid arch;
and the succeeding four are the first, second, third, and fourth
branchial arches respectively.
About the time of hatching the external gills grow out as
branching and richly ciliated processes from the outer surfaces
of the first and second branchial arches, and a little later from
the third branchial arches as well (Fig. 31).
At the same time, the hypoblastic epithelium at each side of
the buccal cavity becomes thrown into folds, which extend out-
wards towards the surface of the neck as paired outgrowths, lying
between the visceral arches. Of these outgrowths or pouches,
which are known as visceral clefts, there are five on each side.
The most anterior one is the hyomandibular cleft, and lies
between the mandibular and hyoid arches: its outer end lies
very close to the surface of the neck, though it does not actually
open to the exterior.
The four hinder visceral clefts perforate the skin about the
time of formation of the mouth opening, 7.e., in tadpoles of about
9 mm. length, and open to the exterior as the gill clefts. These
are slit-like openings lying between the hyoid and first branchial,
first and second branchial, second and third branchial, and third
and fourth branchial arches respectively ; and are known as the
first, second, third, and fourth branchial clefts.
From the hyoid arches a pair of opercular folds arise, which
grow back over the external gills, and the branchial arches and
clefts. The two opercular folds meet below the neck in the
mid-ventral line, and enclose the gills in a branchial chamber.
The hinder borders of the opercular folds fuse with the body-
wall except at one place on the left side, where a spout-like
opening remains, through which the branchial chamber con-
municates with the exterior.
128 DEVELOPMENT OF THE FROG
As the opercular folds develop, the external gills gradually
shrivel up, and are replaced functionally by the internal gills.
These latter are delicate thin-walled vascular tufts, arranged in
a double row along the ventral half of each of the first three
branchial arches, and in a single row on the fourth branchial
arch.
The inner borders of the branchial arches are thickened, and
produced into processes which unite to form a kind of filtering
apparatus, or sieve, through which the water, taken in through
the mouth or nose, is strained before being passed over the gills
into the branchial cavity and so out.
K. The Vascular System.
The heart is at first a straight tube developed in the meso-
blast of the ventral wall of the pharynx. This soon lengthens,
becomes twisted into an § shape, and divided by transverse
constrictions into chambers. (Cf. Figs. 28, 29, and 32.) 'The
auricle is at first single, but later becomes divided by the down-
growth of a septum from its dorsal wall.
While the tadpole is breathing by means of gills, its circula-
tion is in all essential repects that of a fish. The venous blood,
returned from the body generally, enters the posterior end of
the heart, or sinus venosus: from this it passes into the second
or auricular chamber, thence to the ventricle, and from that to
the truncus arteriosus. From this latter arise on each side the
aortic arches, which carry the venous blood to the gills to be
aérated : from the gills the blood is collected by efferent vessels,
which unite above the alimentary canal to form the dorsal aorta,
which by its branches distributes the arterialised blood to all
parts of the body.
1. The Circulation during the time the tadpole is breathing
by external gills.
The arrangement of the bloodvessels, and the course of the
circulation in a 64 mm. tadpole, at a time when the external
gills are in full activity, but the internal gills have not yet
formed, is shown in Figs. 31 and 32.
The truncus arteriosus, on reaching the anterior end of the
pericardial cavity, divides at once into right and left branches.
Each of these again divides into two, the afferent vessels for the
first and second branchial arches, AF, and AF,, which carry
THE VASCULAR SYSTEM 129
blood into the external gills and their branches: from these the
blood passes through short wide capillary loops into the efferent
branchial vessels, EF, and EF,, which carry it, now aérated, to
S eo
KA A
Fic. 31.—Diagrammatic figure of the head and fore part of the body
of a 63 mm. tadpole, showing the arrangement of the branchial vessels
as seen from the ventral surface. The heart has been removed. x 33.
A, dorsal aorta; AF, AFe, AF3, afferent branchial vessels of the
first, second, and third branchial arches; AP, pulmonary artery;
AR, anterior cerebral artery; CA, anterior commissural artery ;
CP, posterior commissural artery : the arterial circle formed by these
commissural vessels with the carotid arteries surrounds the infundi-
bulum of the brain: EF, EFe, EF3, EF4, effcrent branchial vessels of
the first, second, third, and fourth branchial arches; EH, efferent
hyoidean vessel; EM, efferent mandibular vessel; GE, external gill;
GM, glomerulus; KA, segmental or archinephric duct; KP, head
kidney or pronephros; K§,, KS3, first and third nephrostomes of
pronephros ; RT, truncus arteriosus.
the dorsal aorta in the roof of the pharynx. The dorsal aortz
of the two sides run forwards as the carotid arteries, AC, to
supply the head and brain, and also run backwards in the roof
of the pharynx, the aortz of the two sides meeting and uniting
I
130 DEVELOPMENT OF THE #ROG
about the junction of head and body to form the single
systemic aorta which supplies arterial blood to all parts of the
body. :
Besides the complete sets of afferent and efferent branchial
vessels in the first and second branchial arches, similar vessels,
roe, EF4 ERs EF2 EFi EH ap EM
GM \ \ | AT
cere Li;
o 4:4
AR
LV.4
vb
VH
‘
Fic, 32.—Diagrammatic figure of the head and fore-part of the body
of a 64 mm. tadpole, showing the heart, aorta, and vessels of the
branchial arches from the right side. The external gills have been
removed, xX 40.
A, dorsal aorta; AB, basilar artery; AC, carotid artery; AFi, AFo,
AF3, afferent branchial vessels of first, second, and third branchial
arches; AP, pulmonary artery; AR, anterior cerebral artery; AT,
anterior palatine artery; EFy, EFe, EF3, EF4, efferent branchial vessels
of first, second, third, and fouith branchial arches; EH, efferent
hyoidean vessel; EM, efferent mandibular vessel; GM, glomerulus ;
LV4, lacunar afferent vessel of fourth branchial arch; RA, auricle;
RV, ventricle; VD, Cuvierian vein; VH, hepatic veins; WK, vein of
sucker; VY, hyoidean vein ; YM, mandibular vein.
RA
\
AFS AF2 ary “VK
RV VY
as yet incompletely developed, are present in the hinder arches
as well.
In the third branchial arch, there is a short afferent branch,
THE VASCULAR SYSTEM 131
AF, from the afferent vessel of the second branchial arch, which
as yet ends blindly. There is also a well-developed efferent
vessel, EF,, which opens into the dorsal aorta.
In the fourth branchial arch there is no afferent vessel, but
an efferent vessel, EF,, is present, opening into the dorsal
aorta. From this efferent vessel, just before it reaches the
aorta, a backwardly directed branch arises, which will become
later the pulmonary artery, AP.
In front of the first branchial arch, vessels are present in the
hyoid and mandibular arches, which clearly belong to the same
category as the branchial vessels, but which never attain full
development, probably owing to the fact that no gills are formed
on these arches. Efferent branches, EH, EM, opening into the
dorsal aorta, are present in both hyoid and mandibular arches ;
but these have no connection with the heart, as there are no
afferent vessels corresponding to them.
The condition of the bloodvessels, while the tadpole is
breathing by external gills, may be summarised thus :—Com-
plete systems of afferent and efferent vessels, connecting the
heart with the aorta through the gill capillaries, are present in
the first and second branchial arches, and at a stage slightly
later than that shown in Fig. 32 in the third branchial arch as
well. A similar set of vessels, but incomplete, is present in the
fourth branchial arch : and vessels formed on the same plan, but
still less complete, and showing signs of degenerative changes,
are present in the hyoid and mandibular arches.
There are thus six sets of branchial vessels on each side of
the pharynx: of these, three, in the first, second, and third
branchial arches, are complete; one, in the fourth branchial
arch, is incomplete; and two, in the hyoid and mandibular
arches, are rudimentary.
2. The Circulation during the time the tadpole is breathing
by internal gills.
On the formation of the internal gills, additional loops of
communication are formed in the gill tufts between the afferent
and efferent vessels of the first, second, and third branchial
arches, and also a series of similar loops between the afferent
and efferent vessels of the fourth branchial arch. The vessels
in the hyoid and mandibular arches undergo further retrograde
changes, and need not be described in detail.
132 DEVELOPMENT OF THE FROG
Fic. 33—A 12 mm, tadpole dissected from the ventral surface
to show the heart, the internal gills, the branchial vessels, and the
head kidneys and their ducts. The tail, which is about double the
length of the head and body, has been removed. X 22,
A, dorsa] aorta; AF,, AF3, afferent branchial vessels of first and
third branchial arches; AL, lingual artery; CG, carotid gland ; EA,
junction between afferent and efferent branchial vessels of first
branchial arch; EF), EF3, efferent branchial vessels of first and third
branchial arches; GM, glomerulus; KA, archinephric or segmental
duct; KM, Wolffian tubules; KP, pronephros or head kidney ;
KS, KS3, first and third nephrostomes of head kidney; LI, upper
lip ; Lu, lower lip ; LP, hind limb ; OA, aperture of opercular cavity;
OP, opercular cavity; R&, sinus venosus; RT, truncus arteriosus ;
RV, ventricle ; TC, cloaca ; TO, cesophagus, cut short; TR rectal spout.
THE VASCULAR SYSTEM 133
In tadpoles of 12 mm. length, in which the internal gills are
fully established, and the external gills are shrivelling up, the
condition of the bloodvessels is shown in Figs. 33 and 34.
The truncus arteriosus divides at once into right and left
branches, which run straight outwards in the floor of the
A GM AB AU EF3 EFz CP EF, cA
|
|
VO AFs RB RA RV
Fic. 34.—A diagrammatic figure of the head and neck of a 12 mm.
tadpole from the right side to show the heart and branchial vessels,
The gills and the gill capillaries are not represented. x 35.
A, dorsal aorta; AB, basilar artery; AFi, AF2, AF4, afferent
branchial vessels of first, second, and fourth branchial arches; AL,
lingual artery ; AP, pulmonary artery; AR, anterior cerebral artery;
AS, posterior palatine artery; AT, anterior palatine artery; AU,
cutaneous artery; AY, pharyngeal artery; CA, anterior commissural
vessel; CG, carotid gland ; CP, posterior commissural vessel; EF,
EF, EF3, EFy, efferent branchial vessels of first, second, third, and
fourth branchial arches; GM, glomerulus; RA, right auricle; RB,
left auricle; RT, truncus arteriosus; RV, ventricle; VD, Cuvierian
vein; VH, hepatic vein ; VI, posterior vena cava ; VP, pulmonary vein.
Ake RT
pharynx. Each of these branches divides, after a short course,
into three vessels, and the hindmost vessel again intotwo. In
this way the four afferent branchial vessels, AF,, AF,, AF;,AF,,
134 DEVELOPMENT OF THE FROG
of the first, second, third, and fourth branchial arches respectively
are formed.
Each afferent vessel runs outwards and upwards in its own
arch. The efferent branchial vessels lie immediately in front
of the corresponding afferent vessels, with which they are
connected by very numerous capillary loops in the substance of
the internal gills, and not shown in the figures. At their upper
ends the efferent vessels open, as before, into the dorsal aorta,
Fig. 34.
The venous blood in the heart is driven by the contraction of the
ventricle into the truncus arteriosus, and then along the afferent
branchial vessels, through the capillary loops of the gills, in
which it gets aérated, to the efferent branchial vessels; and
thence to the dorsal aorta, and so all over the body.
The lungs are by this time of considerable size: they receive
blood by the pulmonary arteries, AP, which, as already noticed,
are branches from the efferent vessels of the fourth branchial
arches, and therefore contain blood which has already passed
through the gill capillaries. The blood from the lungs is
returned direct to the heart by two pulmonary veins which
unite and open into the left auricle, the single auricular cavity
of the earlier stage being by this time divided by a vertical
septum into right and left auricles.
One other point of great importance remains to be noticed
in the arrangement of the branchial vessels of the tadpole.
The afferent and efferent vessels of each arch at first com-
municate only through the gill capillaries: but in tadpoles of
about 12 mm. length each efferent vessel becomes directly
connected at its ventral end with the corresponding afferent
vessel, Fig. 84. These direct connections are situated ven-
trally to the gills, so that the blood in any one of the
afferent branchial vessels has two paths open to it: it may
either (1) continue along the afferent vessel, and then reach
the efferent vessel by passing through the connecting loops
afforded by the gill capillaries; or (2) it may pass at once
into the efferent vessel through the direct communication,
and so reach the dorsal aorta without having passed through
the gill at all.
So long as the tadpole is breathing by gills, these direct com-
munications between afferent and efferent vessels, though
present in all four branchial arches, are so small that practi-
THE VASCULAR SYSTEM 135
cally no blood passes through them, and all the blood is com-
pelled to pass through the gills to reach the aorta.
3, The Changes in the Circulation at the time of the Meta-
morphosis.
At the time of the metamorphosis, however, when the
anterior limbs are protruded, and the tail begins to shorten,
these direct communications enlarge, so that an increasing
amount of blood takes the direct short passage, and reaches the
aorta without having passed through the gills. Additional
work is thus thrown on the lungs and skin, which consequently
receive a larger supply of blood: the gills rapidly atrophy,
though remnants of them usually persist, in a functionless con-
dition, until the end of the first year; and the change from the
gill-breathing to the air-breathing condition is completed.
The further changes necessary to convert the circulation into
that of the adult are slight. Of the four aortic arches present
at the metamorphosis (Fig. 34), the first, in the first branchial
arch, persists as the carotid arch of the adult frog; the lingual
artery is a branch from the ventral end of the efferent vessel of
the arch, and is present from an early stage of development
(Fig. 33); and the external and internal carotid arteries are
already present. The carotid gland, CG, is not, as sometimes
stated, a persistent portion of a gill, but is formed by further
elaboration of the direct communication between the afferent
and efferent branchial vessels of the first branchial arch.
The second aortic arch, in the second branchial arch, becomes
the systemic arch of the frog. Its dorsal end remains connected
with the carotid arch, though the connection may in the adult
become closed and ligamentous. (Cf Fig. 5, p. 29.)
The third aortic arch, in the third branchial arch, loses its
connection with the aorta, and finally disappears altogether.*
The fourth aortic arch, in the fourth branchial arch, also
* According to another view, of fairly general acceptance, the follow-
ing is the scheme of arterial arches in the frog:
Arch. Embryonic. Adult.
I. sis Mandibular sti 2
Il. diva Hyoidean aie 2
II. sib First branchial iia Carotid arch.
IV. ... Second branchial... Systemic arch.
Vv. ae Third branchial A Cutaneous artery.
VI. .. Fourth branchial .... Pulmonary artery.
136 DEVELOPMENT OF THE FROG
loses its connection with the aorta, but persists as the pulmo-
cutaneous arch of the adult, from which both pulmonary and
cutaneous arteries arise.*
L. Development of the Muscular System and the Celom.
The splitting of the mesoblast into outer or somatopleuric,
and inner or splanchnopleuric layers has already been described.
(Cf. Fig. 26, p. 116.)
In the body the mesoblast becomes very early divided on each
side into (1) a vertebral plate, which is dorsally situated, and
lies alongside of the spinal cord and notochord; and (2) a
lateral plate, which surrounds the side of the body.
The vertebral plate very early becomes divided transversely
into muscle-segments or myotomes, which form a row of hollow
and somewhat cubical bodies, lying along each side of the spinal
cord, and separated from each other by connective tissue septa.
Later on, the walls of the myotomes thicken considerably,
especially the inner walls, and become converted very largely
into muscles ; while the cavities become obliterated.
The myotomes may be well seen in the tail of the tadpole,
where they form the great lateral sheets of muscle on each side
of the tail, by which the swimming movements are effected.
Owing to the transparency of the tail, their arrangement can
be very readily made out; the septa dividing the successive
myotomes from each other are not transverse, but > shaped,
with the angles directed forward towards the head.
The lateral plates are also in part converted into muscle;
the two layers, somatopleuric and splanchnopleuric, remain
comparatively thin, but the space between them widens out
considerably, and becomes the body cavity or celom. This at
first consists of two separate halves, right and left; but, owing
to the splitting of the mesoblast extending down to the mid-
ventral line, the cavities.of the two sides soon became con-
tinuous. The anterior portion of the ccelom is very early shut
off from the hinder part as the pericardial cavity. (Cf. Figs.
28 and 29.)
The outer or somatopleuric layer of mesoblast, with the epi-
blast, forms the body-wall of the adult; the inner or splanchno-
pleuric layer, with the hypoblast, forms the wall of the
alimentary canal and its diverticula. The cells covering the
* See note, p. 135.
THE SKELETON 137
free surfaces of both layers, z.¢., the cells lining the body cavity,
become the peritoneum, or ccelomic epithelium, from which, as
we have already seen, the ovaries and testes are formed.
M. Development of the Skeleton.
1, The Vertebral Column.
The earliest skeletal structure, and for,a time the only one,
is the notochord, the development of which from the hypoblast
of the mid-dorsal wall of the mesenteron has already been
described. It forms a cellular rod extending from the blasto-
pore to the pituitary body ; and as the tail is formed, it extends
back into it. The notochord consists of vacuolated cells, filled
with fluid, and-is invested by a delicate structureless sheath.
About the time of appearance of the hind legs, a delicate
skeletal tube, at first soft but soon becoming cartilaginous, is
formed round the notochord from the mesoblast. This tube
grows upwards at the sides of the spinal cord, as a pair of longi-
tudinal ridges, with which a series of cartilaginous arches, which
appeared at the sides of the spinal cord at a slightly earlier
stage, very soon become continuous.
By the appearance of transverse lines of demarcation, the
cartilaginous sheath of the notochord becomes cut up into a
series of nine vertebre, followed by a posterior unsegmented
portion, which later becomes the urostyle. This transverse
division does not affect the notochord, which remains as a
continuous structure until the complete absorption of the tail
at the end of the metamorphosis.
Shortly after the metamorphosis thin rings of bone, slightly
constricted in their centres, so as to be hourglass-shaped in
section, are developed in the membrane investing the cartila-
ginous sheath of the notochord: these correspond with the nine
vertebre already present, and form the first rudiments of the
vertebral centra.
In the intervertebral regions, between the successive bony
rings, annular thickenings of the cartilaginous sheath occur,
which grow inwards so as to constrict and ultimately obliterate
the notochord. Each of these intervertebral rings becomes,
after the metamorphosis, divided into an anterior and a posterior
portion, which fuse with the bony centra of adjacent vertebrae,
and ossify to form their articular ends.
From the circumference, and from the articular ends of each
138 DEVELOPMENT OF THE FROG
vertebra, ossification gradually spreads inwards; but a small
portion of notochord persists in the middle of each centrum
for a long time, or even throughout life.
The vertebre are not placed opposite the myotomes, but
alternate with these ; so that each vertebra is acted on by two
myotomes on each side, one pulling it forwards, and the other
backwards. ,
The transverse processes are at first independent of the
corresponding vertebre, but very early fuse with them. They
extend into the septa between the myotomes, and probably
correspond to the ribs of other vertebrates.
The urostyle is the part of the axial skeleton behind the
vertebre ; it is not divided into vertebrae at any stage in
development.
The anterior end of the notochord, imbedded in the base of
the skull, is gradually encroached on by the cartilage and bone
around it, and ultimately completely absorbed.
2. The Skull.
The skull of the tadpole consists almost entirely of cartilage ;
none of the bones of the skull, with the exception of the para-
sphenoid, appearing until nearly the time of the metamorphosis.
In the adult frog, this cartilaginous skull is replaced to a con-
siderable extent by cartilage-bone; while other bones primitively
distinct, and probably of dermal origin—the membrane-bones—
graft themselves on to it.
The three morphologically distinct elements of which the skull
consists (cf. p. 43) may with advantage be described separately.
a. The Cranium or brain case. This in its fully formed con-
dition is an unsegmented cartilaginous tube, enclosing the brain:
it is developed as follows :
In the front part of the head a pair of longitudinal cartilagi-
nous bars, the trabecule cranii, appear in tadpoles of about 10
mm. length: these grow back alongside of the notochord as a
pair of horizontal parachordal rods. :
The hinder ends of the trabecule are some little distance
apart, and between them is a space in which the pituitary body
lies. In front of this pituitary fossa, the trabecule unite to
form a plate of cartilage, which underlies the anterior end of the
brain, and is produced into blunt processes at its outer angles.
The parachordals grow rapidly: they extend inwards so as to
THE SKULL 139
meet each other both above and below the notochord, which
they now completely surround. The two parachordals soon fuse
together to form the basilar plate, which, with the trabecule,
forms a firm cartilaginous floor to the brain case. At their
hinder ends the parachordals grow upwards to form the side
walls of the cranium, and a little later bend inwards so as to
meet each other above the brain, and complete the occipital
part of the cranium. Further forwards the pituitary foramen
becomes closed by a thin plate of cartilage, and the lateral
margins of the parachordals and trabecule grow upwards so as
to form the side walls of the skull, the roof remaining im-
perfect in this region.
The first bone to be developed is the parasphenoid. The
exoccipitals, the frontals and parietals, which are at first
separate, and other bones soon follow; and by the time the
metamorphosis is complete and the tail absorbed, all the bones
of the adult cranium are present, except the sphenethmoid,
which does not appear till some months later.
b. The Sense Capsules. The cartilaginous auditory capsules
appear in tadpoles of about 12 mm. length as thin shells of
cartilage investing the auditory vesicles. They are at first
quite independent of the cranium, but before the completion of
the opercular folds they fuse with the upgrowing parachordals
to form part of the side walls of the skull. The pro-otic appears
about the time of completion of the metamorphosis.
The optic capsules are thin shells of cartilage, forming part
of the sclerotic coats of the eyes. They arise about the same
time as the auditory capsules; and, unlike the other sense
capsules, they remain distinct from the cranium throughout life,
in order to secure mobility of the eyeballs.
The olfactory capsules are from their first appearance very
closely connected with the anterior ends of the trabecule, which
grow up between them to form the median vertical internasal
septum. They develop later than the auditory and optic capsules.
c. The Visceral Skeleton. This consists of a series of carti-
laginous hoops developed within the visceral arches, and forming
a framework which surrounds and stiffens the walls of the
pharynx. Each hoop consists of right and left halves, which
are independent at their dorsal ends, but fused or closely con-
nected ventrally. There are in all six of these hoops or bars
forming the oral (mandibular) arch, hyoidean arch, and the
140 DEVELOPMENT OF THE FROG
four branchial arches respectively; and they develop in order
from before backwards.
i. The oral (mandibular) bar, which is the largest of the
series, lies at first parallel to the others, i.e. perpendicular
to the long axis of the body. It very early, however, under-
goes important changes, and by the time that the external gills
are developed, and before the appearance of the opercular folds,
it has altered its direction, and now runs almost horizontally
forwards, parallel to and below the trabecule.
It soon unites with the trabecule, both behind and in front
of the eyeball, the latter union being effected by a short trans-
verse bar of cartilage—the palato-pterygoid. In front of the
palato-pterygoid, the most anterior part of the oral bar becomes
segmented off as a short rod of cartilage, which is directed
upwards and forwards in the lower lip; it is known as Meckel’s
cartilage, and forms the basis of the lower jaw or mandible.
That part of the oral bar with which this segment articulates
will give rise to the quadrate of the adult. In connection with
the lips two pairs of small labial cartilages appear, serving to
support the horny jaws of the tadpole.
In the later stages the subocular or quadrate portion of the
oral bar acquires a very close connection at its hinder end with
the auditory capsule, and changes its direction, so that in place
of running horizontally forwards, it now runs downwards and
forwards. This change, which may be described as a rotation
backwards of the bar, causes lengthening of the palato-pterygoid
bar and of. Meckel’s cartilage; these latter become respectively
the basis of the upper and lower jaws of the tadpole, which
are completed later on by the development of the membranous
pterygoid, squamoégal, maxilla and other bones.
This rotation backwards of the distal end of the quadrate,
with corresponding lengthening of the upper and lower jaws,
proceeds rapidly during and after the metamorphosis, so that
the quadrate, instead of being directed downwards and forwards,
soon runs vertically downwards, and later on downwards and
backwards as in the adult. (Cf. Fig. 10, p. 48.)
ii. The hyoid bar also undergoes important changes. At
first it is a wide band of cartilage placed nearly vertically in
the side wall of the pharynx, immediately behind the oral bar.
When the mandibular arch becomes horizontal the hyoid forms
a broad stout bar of cartilage, articulating at its upper end with
THE URINARY SYSTEM 141
the subocular part of the oral bar, and connected at its ventral
end with the hyoid bar of the other side by a small median
basi-hyal plate in the floor of the mouth.
At the commencement of the metamorphosis the hyoid bar
becomes narrower, and begins to extend upwards towards the
auditory capsule; and by the end of the metamorphosis this
upper part of the hyoid has become the long slender anterior
cornu of the hyoid, which acquires a loose connection at its
upper end with the cranium and with the quadrate cartilage.
The development of the columella is imperfectly known. It
consists of two elements, one of which—the stapes—is a small
plate of cartilage partially filling a hole, the fenestra ovalis,
which appears in the lower and outer wall of the auditory cap-
sule about the time that the opercular folds are growing back
over the gills. The other portion of the columella is a small
rod, partly cartilage, partly bone, which does not appear till
some months after the completion of the metamorphosis, and
which fuses with the stapes at its inner end, while its outer
end becomes connected with the tympanic membrane (cf. Fig.
10, p. 48); this outer element of the columella is commonly
regarded as formed from the uppermost part of the hyoid arch,
but appears to be really quite independent of it in the frog.
iii. The branchial bars are at first simple flattened rods of
cartilage, independent of one another, but becoming early con-
nected with a median basi-branchial cartilage, which appears in
the floor of the mouth between the ventral ends of the first two
pairs of bars.
As the hind-legs appear,the branchial bars of each side coalesce
with one another both at their dorsal and their ventral ends:
they also become strongly curved, and together form a complex
basket-work supporting the gills. Later on, as the gills begin to
shrink, the branchial bars become more slender : their dorsal ends
disappear, while their ventral ends fuse with the basi-hyal and
basi-branchial cartilages, and together give rise to the body of
the hyoid and its posterior cornua. s
N. The Development of the Urinary System.
1. General Account.
The excretory organs of the tadpole, during the early stages
of its existence, are the head kidneys or pronephra. These
are a pair of globular organs imbedded in the dorsal wall of the
142 DEVELOPMENT OF THE FROG
body at its anterior end, immediately behind the constricted
neck region (Figs. 833 and 35 KP). Hach head kidney is a
Fic. 35.—A 40 mm. tadpole dissected from the ventral surface to
show the heart, the branchial vessels, and the head kidneys and
Wolffian bodies. The tail has been cut off. x 5.
A, dorsal aorta; AF., AF3, afferent branchial vessels of first and
third branchial arches; AL, lingual artery; CG, carotid gland; EF),
EF3, efferent branchial vessels of first and third branchial arches ; F, fat
body; GM, glomerulus; KA, archinephric or segmental duct; KM,
Wolffian body; KP, pronephros or head kidney, now degenerating ;
LA, fore-limb, still within opercular cavity; LI, upper lip; LJ, lower
lip; LP, hind-limb; OR, genital ridge; RT, truncus arteriosus ; RV,
ventricle; TC, cloaca ; TO, cesophagus, cut short; TR, cloacal spout.
THE URINARY SYSTEM 143
convoluted tube with glandular walls, opening into the body
cavity by three ciliated mouths or nephrostomes (Fig. 33, KS),
and continued back along the dorsal wall as the archinephric
or segmental duct, KA, to the hinder end of the body, where
it joins with the corresponding duct of the opposite side, and
opens into the cloaca.
The head kidneys and their ducts are well developed in the
tadpole at the time of hatching: they subsequently increase
considerably in size, and are the sole excretory organs of the
tadpole during its early stages. In tadpoles of about 12 mm.
length the adult kidneys or Wolffian bodies (Fig. 33, KM),
begin to form in the hinder part of the body as a series of paired
tubules, which grow towards and open into the segmental duct.
These Wolffian tubules rapidly increase in number, as well as
in size and complexity, and become bound together by connec-
tive tissue to form the compact Wolffian bodies or kidneys of
the fully formed tadpole (Fig. 35, KM). At the same time the
head kidneys diminish in size, and undergo degenerative changes,
and by the time of the metamorphosis (Fig. 86) have almost
completely disappeared. The Wolffian bodies persist as the
kidneys of the frog; and by a series of further changes the
ureters and generative ducts of the adult become established.
2. The Head Kidney and its duct.
In tadpoles of about 34 mm. length, z.e., some time before
hatching, a pair of longitudinal grooves appear along the inner
surface of the somatopleure, extending from the neck to the
hinder end of the body, and lying a little distance to the right
and left of the notochord. The lips of each groove soon meet
and fuse so as to convert the groove into a tube or duct. The
closure of the tube takes place from behind forwards, and at the
anterior end is effected imperfectly, three holes or nephrostomes,
one behind another, being left, through which the tube opens
into the body cavity. As the embryo grows, the anterior end
of the duct becomes convoluted and twisted on itself to form a
ball, the three nephrostomes becoming at the same time
lengthened out into short tubes. This convoluted mass is the
head kidney or pronephros. The hinder part of the duct is
the archinephric or segmental duct; it remains straight, or
nearly so, and shortly before the tadpole hatches acquires an
opening into the cloaca.
144 DEVELOPMENT OF THE FROG
At the time of hatching, the excretory organs thus consist on
each side of (1) a head kidney, which is a convoluted tube
lined by a glandular epithelium, and opening into the anterior
end of the body cavity by three ciliated openings, the nephro-
Fic. 36.—A tailed frog, near the close of the metamorphosis,
dissected from the ventral surface to show the kidneys and repro-
ductive organs, x 4.
A, dorsal aorta; F, fat body; GM, glomerulus; KA, archinephric
or segmental duct; KM, Wolffian body; KP, head kidney, dis-
appearing ; KU, ureter; O, mouth; OR, genital ridge; RV, tip of
ventricle ; TO, cesophagus, cut short.
THE URINARY SYSTEM ' 145
stomes; and (2) the archinephric or segmental duct, which is
the posterior part of the tube, and runs back along the dorsal
body-wall nearly straight to the cloaca, into which it opens.
The head kidney is closely surrounded by, indeed almost
imbedded in, the posterior cardinal vein (Fig. 37, VC), and it
is from the blood of this vein that the epithelial cells of the
NS
Fic. 37-—Transverse section through the body of a tadpole at the
time of hatching ; the section passing through the second pair of the
nephrostomes, and the third pair of'myotomes. xX 50. (From
Marshall's ‘‘ Vertebrate Embryology.”’)
A, aorta; C, ccelom or body-cavity; CH, notochord; CJ, sub-
notochordal rod; GM, glomerulus; KP, segmental or archinephric
duct ; KS, second nephrostome of left side ; ME, somatopleuric layer
of mesoblast ; MH, splanchnopleuric layer of mesoblast; ML, myo-
tome; NL, lateral line branch of pneumogastric nerve; NS, spinal
cord; T, intestinal region of mesenteron; VC, posterior cardinal
vein ; VH, hepatic vein ; W, liver diverticulum.
head kidney tubules separate the excretory matters, which are
then passed down the duct to the exterior.
The head kidney continues to increase in size, the tubules
becoming still more convoluted, and lateral diverticula arising
K
146 DEVELOPMENT OF THE FROG
from their sides, until the tadpole is about 12 mm. in length,
and the hind-limbs are just commencing to appear. It remains
stationary for a time and then, in tadpoles of about 20 mm.
length, begins to degenerate: the tubules become obstructed ;
some of them become collapsed, others for a time irregularly
dilated : the whole organ steadily diminishes in size, and in
tadpoles of 40 mm. (Fig. 35, KP) is not more than half its
former size. It now shrinks rapidly, and at the time of the
metamorphosis (Fig. 36, KP) has almost disappeared, all three
nephrostomes having closed up, and the organ being reduced to a
few small pigmented and irregularly twisted tubules, which have
separated from the duct, and which soon disappear completely.
Opposite the head kidney an irregular sacculated outgrowth,
the glomerulus, arises from the aorta on each side (Figs. 31
to 37, GM): this appears first about the time of hatching, and
its development keeps pace with that of the head kidney. It
lies immediately opposite the nephrostomes, and very close to
these, though not touching them. It begins to diminish in size
about the same time as the head kidney. At the time of the
metamorphosis (Fig. 36, GM) it is very small, and after the first
year it can no longer be recognised. Its close relation to the
head kidney, and the fact that its growth and subsequent
degeneration keep pace with those of. the head kidney, point
to a close physiological connection between the two organs,
though it is not easy to imagine what precise function the
glomerulus subserves.
3. The Wolffian Body.
The Wolffian body, or kidney, first appears in tadpoles of
from 10 to 12 mm. in length. It arises on each side as a series
of small solid masses of mesoblast cells lying along the inner
side of the segmental duct, between this and the aorta (Figs.
33 and 35). They develop from behind forwards, the hindmost
pair being a short distance in front of the cloaca, and the most
anterior ones about three segments behind the head kidney.
These solid masses soon become elongated into twisted rods,
which then become tubular, and growing towards the segmental
duct meet and open into it. At their opposite ends these
Wolffian tubules, as they are termed, dilate into bulb-like
expansions, which become doubled up by ingrowth of little
knots of bloodvessels, derived from the dorsal aorta, and so
THE URINARY SYSTEM 147
form Malpighian bodies. From the necks of the Malpighian
bodies, short solid rods of cells grow towards the peritoneal
epithelium and fuse with it. These rods soon become hollow,
and open into the body cavity by ciliated funnel-shaped mouths
or nephrostomes: their opposite ends break away from the
Wolffian tubules and open directly into the renal veins on the
ventral surface of the kidney. The Wolffian tubules rapidly
increase in number; they also branch freely, and so give rise to
a complicated system of glandular tubules, which, when bound
together by bloodvessels and connective tissue, form the
Wolffian body or kidney of the frog. The nephrostomes persist :
and in the adult frog as many as 200 or more are present on
the ventral surface of the kidney, as minute funnel-like ciliated
openings, leading by short tubes into the renal veins.
4. The Wolffian and Miillerian ducts.
So far we have only described one duct on each side, the
segmental duct, which acts as the excretory duct first of the
head kidney, and then of the Wolffian body as well. We have
now to consider in what way the ureters and generative ducts
of the adult frog are formed.
About the time of the metamorphosis the head kidney,
which has become rudimentary, separates completely from the
duct, which now ends blindly a short distance in front of the
Wolffian body.
A little later, after completion of the metamorphosis and
entire disappearance of the tail, this anterior end of the
segmental duct, in front of the Wolffian body, becomes divided
somewhat obliquely into two; an anterior part, which is now
isolated from the Wolffian body, and will be called the
Miillerian duct ; and a posterior part, the Wolffian duct, which
is simply the posterior part of the original segmental duct, and
receives the Wolflian tubules of the kidney.
The Miillerian duct becomes connected in front with the
peritoneal epithelium, and acquires an opening into the anterior
end of the body cavity. At its hinder end it grows back along
the outer side of the Wolffian duct to the cloaca, into which it
opens. So far the changes are the same in bothsexes. In the
male frog the Miillerian duct persists in this condition through-
out life, and may be recognised as a slender longitudinal streak
lying in the thickness of the peritoneum a short distance to the
148 DEVELOPMENT OF THE FROG
outer side of the kidney, and extending some distance in front
of it. In the female frog the Miillerian duct becomes the
oviduct, the anterior opening being carried forward first as a
groove, and then by closure of the lips as a tube, to the position
characteristic of the peritoneal opening of the adult oviduct;
while the posterior part becomes greatly convoluted and acquires
thick glandular walls : the hindmost part of the oviduct remains
thinner walled, but of much greater capacity.
The Wolffian duct becomes in both sexes the ureter. In
the female frog it undergoes no further change of importance,
In the male frog the hinder end of the Wolffian duct becomes
dilated into a much-branched glandular enlargement, the
vesicula seminalis.
5. The Vasa Efferentia.
In both sexes at an early stage, as the Malpighian bodies
are forming in the Wolffian body, those nearest to the genital
ridges give off tubular branches from their capsules into the
ridges.
In the female frog these tubules are said to expand very
greatly, and to give rise to the chambers or cavities present
in the adult ovary: but the point is not established with
certainty.
In the male frog these tubules become the vasa efferentia:
they become connected with the spermatic tubules, and, as at
their other ends they open into the Wolffian tubules, they form
passages along which the spermatozoa can get from the testis
to the Wolffian duct or ureter, and so out.
CHAPTER IX.
ELEMENTARY HISTOLOGY.
WHEN examined under the microscope, all the different tissues
and organs of the body are found to consist of elementary bodies
called cells and of an intercellular substance, connecting the
several cells together ; in much the same way as a wall is built
of bricks cemented together with mortar. These cells, of which a
white blood corpuscle is a typical example, vary much in shape,
size, and structure in different tissues, but are to be considered
as fundamentally equivalent to one another. The intercellular
substance varies very much in quantity; it may be almost
absent, so that the several cells are practically in contact with
one another; or it may be so abundant as to separate them
widely ; it is to be viewed as formed by the cells, and, therefore,
as secondary in importance to these.
When drawing histological preparations, it is well to look out
for, and draw, a few red blood corpuscles, to the same scale as the
vest of the drawing. The blood corpuscles form most useful:
standards of measurement, as their dimensions are already known
(p. 87).
A. Epithelium.
Epithelium consists of cells placed side by side so as to form
layers, which form the surface covering, or epidermis, of the
body, and line the alimentary canal, the blood vessels, and the
various internal cavities of the body. It may be defined as a
continuous sheet of cells lining a free surface. At the external
apertures of the body, the epidermis is directly continuous with
the epithelial lining of the internal cavities.
The layers may be one or more cells in thickness; in the
former case the epithelium is said to be simple, in the latter
stratified.
Epithelium is of different kinds, according to the shape and
structure of its component cells.
150 ELEMENTARY HISTOLOGY
I. Squamous Epithelium. In this the component cells are
flattened parallel to the surface they cover ; if the epithelium
is stratified, the flattening is most marked in the superficial
cells.
a. Isolated Cells.
Scrape gently the inside of your cheek with the handle of a
scalpel, and put the scrapings on a slide ; cover, and examine
with a high power ; draw, showing the following points :
i. The cells are large, flattened and scale-like in shape,
often slightly curled up at their edges.
ii, The nucleus is oval and granular, and lies near the
middle of the cell; it may be rendered more dis-
tinct by acetic acid or magenta.
b. Cells in situ; cast skin of newt.
Take a small piece of the prepared specimen, which has been
stained in hematoxylin, and then, after treatment with alcohol,
cleared with oil of cloves. Mount. the specimen in balsam ;
cover, and examine with the high power.
i. The cells are flattened, and fitted together at their
edges, like a mosaic, to form a continuous layer.
Each cell has a large nucleus near its centre.
II. Columnar Epithelium. This consists of elongated rod-
like cells, placed vertically to the surface on which they rest.
If a columnar epithelium is stratified the columnar character is
most marked in the superficial cells.
a. Isolated cells: from the small intestine of the frog ;
isolated by maceration for 24 hours in Ranvier’s
alcohol, and stained with picro-carmine.
Mount a drop of prepared specimen in glycerine ; paint a
ving of cement round the cover-glass ; and examine with the high
power.
i. The cells, which often remain side by side in little
groups, are columnar in shape, with nuclei near
their inner or deeper ends.
b. Cells in situ.
Take w prepared section of dog’s stomach which has been stained,
4
EPITHELIUM 151
and then cleared in oil of cloves. Mount in balsam, and examine
with the high power.
i. The superficial layer consists of long narrow
columnar cells, packed together side by side, with
nuclei at their inner or deeper ends.
III. Ciliated Epithelium. In this the cells, which are usually
columnar, bear at their free ends tufts of exceedingly fine hair-
like processes—cilia—which, when living, exhibit active lashing
movements.
a. Isolated cells. From trachea of rabbit: isolated by
maceration for 24 hours in Ranvier’s alcohol; stained
with picro-carmine, and scraped into glycerine.
Mount a small drop of the prepared specimen in glycerine ;
paint a ring of cement round the cover-glass ; examine with the
high power, and note :
i. The shape of the cells: their nuclei; and the tuft
of cilia at one end of each cell.
b. Cells in situ: ciliary movement.
Snip off a small piece of epithelium from the roof of the
mouth of a freshly killed frog, near the eyeball; mount in normat
salt solution, and add a small drop of gamboge water to render
the movements more clearly visible ; examine with the high power :
note :
i. The currents due to the ciliary motion.
ii. The movements of the individual cilia: best seen
when the specimen is beginning to die, and the
movements to slacken in speed.
IV. Stratified Epithelium. This is characterised by the
epithelium being several cells in thickness.
Take a prepared section of esophagus of rabbit, or of conjunc-
tiva of rabbit or pig, which has been hardened in chromic acid,
stained, and cleared in oil of cloves. Mount in balsam, examine
with the high power, and note :
i. The stratification of the epithelium.
ii. The transition from the deeper spherical or columnar
cells to the superficial squamous cells.
152 ELEMENTARY HISTOLOGY
B. Glands.
A gland consists essentially of a layer of epithelial cells
secreting some special fluid. The epithelial surface may be flat,
but is more usually folded or pitted, often in a very complicated
manner, so as to increase the extent of the secreting surface.
I. Simple Glands. In simple glands the epithelial surface
is increased by simple pit-like depressions, whose mouths serve
to discharge the secretion on the free surface.
Take a prepared section of large intestine of rabbit which has
been hardened in chromic acid, stained, and cleared in oil of
cloves. Mount in balsam, and examine first with the low power,
then with the high. Note the following points :
i. The glands are simple tubular depressions of the
surface.
ii. The glandular epithelium lining the pits is a single
layer of short columnar granular cells, many of
which are swollen to form goblet cells.
II. Compound Glands. In compound glands each depression
instead of being a simple pit is itself subdivided or branched,
often in a very complicated manner. There are two chief
varieties: (1) tubular glands, in which the several sub-
divisions are tubular, and of tolerably uniform diameter
throughout : and (2) racemose glands, in which the blind ends
of the pits are dilated into globular chambers or alveoli, to which
the special glandular epithelium is usually confined.
a. Compound tubular glands. Take a prepared section of
kidney of frog: mount in balsam, and examine with both
low and high powers.
i. The tubular gland cavities are cut at various angles.
If cut transversely a tube appears as a circular
ring: if cut obliquely, as a more or less elongated
elliptical ring: if cut longitudinally, as two
parallel rows of epithelial cells.
ii. The gland cells form a single layer of cubical granular
cells, lining the tubes.
iii. The Malpighian bodies are spherical dilatations on
the tubes, into which project little knots of capillary
153
GLANDS
yyy
cM
cc
co
GB
diac end of
e car
Fic. 38.—Section through mucous membrane of th
a dog’s stomach. X I4o.
B, bloodvessel; CC, cubic or peptic cells; CM, columnar cells ;
CO, ovoid cells ; GB, fundus or bottom of gland cavity ; GC, gland
cavity cut across ; GM, mouth of gland; MC, circular muscle fibres;
ML, longitudinal muscle fibres; P, connective tissue layer between
the mucous membrane and the outer muscular walls of the stomach,
154
ELEMENTARY HISTOLOGY
bloodvessels. Their structure is most readily made
out in specimens in which the bloodvessels have
been injected with a coloured substance to make
them more distinct. ‘
III. Gastric Glands. The glands of the stomach are well
adapted for a more minute examination of the histology of
glands.
Examine again with a high power the section of the cardiac end
of the dog’s stomach already used for columnar epithelium.
1. Characters of the glands. The gastric glands are good
examples of simple or slightly branched tubular glands.
They are deep, but very narrow, cylindrical pits,
imbedded vertically in the wall of the stomach, with
their open mouths discharging into its cavity. The
glands are lined by epithelial cells, and are set very
close together side by side. In the microscopical
’ sections, some of the glands may be seen cut along
their entire length; but in most cases, owing to the
glands being not quite straight, or the plane of section
being oblique to the surface of the stomach, the tubes
will be cut more or less obliquely, or even trans-
versely.
2. Characters of the gland cells. There are three distinct
kinds of epithelial cells found at different parts of
the length of the gland.
i. Columnar cells, arranged in a somewhat radiate
manner round the mouths of the glands, and
extending a short way down the tubes.
- ii, Cubic cells, or peptic cells, lining the deeper parts
of the glands and the greater part of their length :
these are cubical granular cells with centrally
placed nuclei.
iii. Ovoid cells: large oval cells with large nuclei:
these are less numerous than the other two forms,
and occur most abundantly a short way below the
mouths of the glands. They lie along the sides of
each gland, outside the cubical cells, and are said
to secrete the acid of the gastric juice.
MUSCLE 155
C. Muscle.
Tn muscular tissue the component cells are much elongated
and, in the higher forms, very highly specialised. Muscular
tissue is of two kinds: (1) striated, or voluntary, of which all
muscles that are under the control of the will consist : and (2)
non-striated or involuntary, forming those muscles over whose
contractions the will has no direct control. The muscular tissue
of the heart, which though involuntary is striated, forms the
chief exception to this rule.
I. Striated, or Voluntary Muscle.
a. Crab’s muscle. Shred in glycerine a small piece of crab’s
muscle that has been hardened in alcohol ; cover, and
examine with both low and high powers : note :
i, The elongated fibres of which the muscle consists.
Each fibre is a single cell, and is enclosed in a
delicate elastic sheath —the sarcolemma —which
will be visible in but few cases ; it is most readily
seen at places where the fibre has been torn
across.
ii. The alternate light and dark bands with which the
muscle fibres are marked transversely, and from
which the name, striated muscle, is derived.
iii, The readiness with which the fibres split up longi-
tudinally into fibrils.
b. Frog’s muscle. Shred yently a piece of fresh frog's nuuscle
in normal salt solution : cover, and examine with the
high power : note:
i, The transverse striations.
ii. The sarcolemma: best seen by slightly crushing
the specimen.
iii. The nuclei in the fibres : seen on addition of acetic
acid.
II. Non-striated, or Involuntary Muscle.
Take a prepared specimen of frog’s bladder which has been
macerated in Ranvier’s alcohol for 24 hours ; pencilled with a fine
brush to remove the epithelium of the inner surface ; stained, and
cleared with oil of cloves. Mount in balsam, and examine with
low and high powers : note:
156 ELEMENTARY HISTOLOGY
i. The bands of muscular fibre.
ii. The formation of each band by a number of
elongated, fusiform, nucleated muscle-cells.
iii. The absence of transverse striation in the muscle.
D. Connective Tissues.
Under the name “ connective tissue” are included various
tissues whose functions are mainly passive, and which serve to
support, strengthen and bind together the various organs and
parts of the body. Histologically the connective tissues consist
of elements of four kinds, united together in very varying pro- -
portions in different situations: (1) white fibrous tissue; (2)
yellow elastic tissue ; (3) connective tissue corpuscles, which are
comparatively slightly altered cells, usually branched; and (4)
ground substance, or intercellular substance.
I. White Fibrous Tissue. This consists of a number of fine
transparent fibres of a more or less cylindrical shape, and
with a very characteristic wavy outline; between the
fibres are connective tissue cells, usually in small num-
bers. The fibres are arranged side by side in bundles,
and each fibre presents a number of longitudinal fibrillar
striations. The cellular origin of white fibrous tissue is
difficult to determine. The fibres are believed to be
formed by modification of the intercellular matrix rather
than from the bodies of the cells themselves.
a. Tendon of rat’s tail. Pull out a small piece of tendon
jSrom the tail of a rat; place it on a slide.in a drop of
normal salt solution ; spread it out with needles, cover
and examine with low and high powers: note:
i. The fibres, with wavy outlines.
ii. The fibrille, indicated by longitudinal wavy stria-
tions within the fibres.
Add a drop of acetic acid to the preparation : note that
iii. The fibres swell up and become transparent.
iv. Longitudinal rows of tendon cells, with nuclei,
become visible between the fibres.
II. Yellow Elastic Tissue. This consists of fine branching
homogeneous fibres, with great power of resisting
CONNECTIVE TISSUES 157
chemical reagents ; the fibres are formed from an inter-
cellular matrix, and not from cells directly.
a. Ligamentum nuche of ox. Tease finely a small shred
in water ; examine with low and high powers : note:
i. The branching fibres, with very sharp outlines.
ii. The tendency of the branches to anastomose with
one another and so form networks.
iii, The tendency of the fibres and branches to curl up
at their broken ends.
Add a drop of acetic acid : note that
iv. No alteration whatever is produced in the fibres.
v. No nuclei appear.
III. Areolar tissue. This is a meshwork composed of both
white fibrous and elastic tissues.
a. Subcutaneous tissue of mammal. Take a freshly killed
rat, and snip off a small piece of the loose fibrous tissue
which connects the skin with the subjacent parts ; stretch
it till quite flat with a pair of needles, breathing con-
stantly upon it to make it adhere to the slide ; cover,
and examine with low and high powers : note :
i. The meshwork, composed of white fibrous tissue
with wavy outlines, mingled with which are
branched elastic fibres.
Add acetic acid : note that
ii. The white fibrous tissue swells up and becomes
transparent.
iii. The elastic tissue is unaltered.
iv. Connective tissue corpuscles, with nuclei, become
visible.
IV. Adipose tissue. This consists of a fine network of
vascular connective tissue, in the meshes of which are
at cells, 7.e., connective tissue corpuscles in which large
quantities of fatty or oily matter have accumulated.
a. Omentum of rabbit or kitten. Mount a small piece of
fresh omentum in normal salt solution ; protect it from
158 ELEMENTARY HISTOLOGY
the pressure of the cover glass ; examine with low and
high powers : note :
i, The vascular connective tissue meshwork, in which
lie the fat cells.
ii. The fat cells: large, spherical, or from mutual
pressure polyhedral, cells; distended with fatty
matter, and with their nuclei near the surface.
b. Osmic Acid specimen.
Note the reduction of the osmic acid by the fat, which *
becomes stained a dark brown or black colour.
E. Cartilage.
In cartilage or gristle the intercellular substance, which in
most other tissues is only present in small quantity, is greatly
increased so as to far exceed in bulk the cells which it connects
together, The intercellular substance forms a dense translucent
matrix resembling an extremely stiff jelly, in which are im-
bedded the cartilage cells, either singly or in groups. In young
cartilage the intercellular substance is much less abundant, and
the cells consequently closer together than in older or more
mature specimens.
Cartilage when free from other tissue is called hyaline carti-
lage, from the clear or glassy appearance of the matrix, in
contradistinction to fibro-cartilage, in which the matrix is
fibrous from admixture with white fibrous or elastic tissues.
I. Hyaline Cartilage,
a. Cartilage of newt. Take a small piece of cartilage from
the shoulder girdle of a newt ; scrape away genily any
muscle on other tissue that may adhere to it ; mount in
normal salt solution, and examine with low and high
powers.
i. The intercellular matrix is either hyaline or faintly
granular.
ii, The cartilage cells are imbedded in the matrix;
each cell is nucleated, and occupies a cavity or
lacuna in the matrix. In places the cells are in
groups of twos or fours owing to recent division.
BONE 159
Wash the specimen thoroughly in water ; stain with carmine,
and mount as a permanent preparation in glycerine ; examine
with the high power, and note that
iii. The cell nuclei are stained deeply, and the matrix
very slightly ; the layer of matrix immediately
surrounding each cell—the capsule—stains more
deeply than the other parts.
b. Articular cartilage. This forms caps covering the
ends of those bones which fit together to form mov-
able joints; the caps act as elastic cushions to break
the force of shocks.
Mount in balsam a prepared section of articular cartilage from
the head of the femur, the section being made perpendicular to the
articular surface ; examine with low and high powers.
i, The matrix is hyaline or faintly granular.
ii. The cartilage cells. Towards the free surface the
cells and cell groups become gradually flattened,
and arranged parallel to the surface.
F. Bone.
Bone consists of a dense fibrillar intercellular matrix, in
which are imbedded cells which lie in cavities connected with
one another by fine branching canals. The matrix is richly
impregnated with inorganic salts, chiefly phosphate and car-
bonate of lime, which form about two-thirds by weight of the
substance of the bone, and give it its great hardness and
strength. The matrix, with its contained bone-cells, is arranged
in concentric layers or lamelle, around tubular passages, the
Haversian canals, in which lie the bloodvessels, which pene-
trate the bone in great numbers. A Haversian canal with its
contained bloodvessels, and its surrounding layers of matrix
and cells, are together spoken of as a Haversian system.
1. Examine with both low and high powers prepared trans-
verse sections of a long bone.
i. The Haversian systems form the greater part of
the bone, and are readily recognised by the con-
centric arrangement of the lamelle, and the
central canals.
160
ii,
iii.
iv.
vi.
vii.
ELEMENTARY HISTOLOGY
The interstitial lamelle fill up the spaces between
the Haversian systems. They form parts of circles
which are in many cases of much larger radius
than the circles of the Haversian systems.
The lacune are the spaces in the matrix in which
the bone-cells lie. In sections of dried bone the
lacune appear black, through being filled either
with air or with dirt.
The canaliculi are very fine branching canals con-
necting the lacune together ; they are occupied
while the bone is living by branching processes
of the bone-cells. At the outer part of each
Haversian system, some of the canaliculi are
looped, opening at both ends of the loop into
the same lacuna.
. The large central medullary cavity of the bone is
occupied during life by the marrow, which
consists of adipose tissue, with very numerous
bloodvessels and large nucleated reddish marrow
cells.
The peripheral or circumferential lamelle are a
series of concentric lamelle parallel to the surface
of the bone, and forming its most superficial
layer.
The perimedullary lamell2 are a series of concen-
tric lamelle lining the central medullary cavity
of the bone.
INDEX.
A
ABDOMINAL VISCERA, 18-20
Acetabulum, 52
Adipose tissue, 157
Adrenal bodies, 23
Afferent nerves, 68
Alimentary canal, 21
development of, 124-127
Ampulla, 95
Ankle, 53
Annulus tympanicus, 48
Aorta, 30, 129
Aortic arch, 30-32, 129 seq.
Aperture, cloacal, 16, 96, 126
external, 16, 17
Aponeurosis, 55
Apparatus, 1
Appendicular skeleton, 49-54
Aqueductus Sylvii, 73, 120
Aqueous humour, 88, 89
Archinephric duct, 143
Areolar tissue, 157
Arteries, 29-32
Artery, anterior mesenteric, 31
carotid, 30
celiac, 31
ceeliaco-mesenteric, 31
cutaneous, 32
dorsal aorta, 30
epigastric, 32
external carotid, 30
gastric, 31
hemorrhoidal, 32
hepatic, 31
hypogastric, 32
iliac, 30, 32
internal carotid, 30
Artery, laryngeal, 30
lingual, 30
lumbar, 31
mesenteric, 31
occipital, 31
occipito-vertebral, 31
cesophageal, 30
peroneal, 32
posterior mesenteric, 31
pulmonary, 32
sciatic, 32
splenic, 31
subclavian, 31
tibial, 32
urino-genital, 31
vertebral, 31
Articular cartilage, 158
process, 42
Atlas, 43
Auditory capsule, 43, 45, 189
organ, 94, 95
Auricle, 23, 34
Axial skeleton, 42-49
Axis cylinder, 85
B
BaCKBONE, 42
Basi-branchial, 141
Basi-hyal, 141
Basilar plate, 139
Bile-duct, 22, 125
Bladder, gall, 22, 125
urinary, 19, 97, 98, 125
Blastocoele, 109
Blastopore, 113, 126
Blind spot, 91
162 INDEX
Blood, 37-39 C
Body-cavity, 18, 114, 136
Bone, structure of, 159, 160 CALcAR, 54
angulo-splenial, 47 Canal, semicircular, 95
astragalus, 53 Canaliculi, 160
calcaneum, 53 Capillaries, 23, 39
carpal, 51 Capsule, auditory, 43, 45
clavicle, 50 olfactory, 43, 45
columella, 49, 95,141 Cardiac plexus, 77
coracoid, 50 Carotid arch, 30
dentary, 48 Cartilage, 158, 159
exoccipital, 44 Cartilage-bone, 40
femur, 53 Cauda equina, 76
fronto-parietal, 45 Cavities of brain, 72, 73
girdle, 44 Cell, 149
humerus, 51 Central canal of cord, 86
ilium, 52 Centrum, 42
ischium, 53 Cerebellum, 72, 120
maxilla, 47 Cerebral hemisphere, 70, 121
mento-Meckelian, 48 vesicle, 121
metacarpal, 52 Chiasma optic, 73
metatarsal, 54 Choroid, 88, 90, 91
nasal, 45 plexus, 71, 120, 121
omosternum, 50 Choroidal fissure, 123
os cruris, 53 Cilia, 151
palatine, 46 2 Ciliary movement, 151
parasphenoid, 45, 139 muscle, 90
phalanges, 52, 54 nerves, 90
precoracoid, 50 processes, 90
premaxilla, 47 vessels, 90
pro-otic, 45 Circulation of blood, 24, 38,
pterygoid, 46 39
pubes, 53 o in tadpole, 128-135
“quadratojugal,” 47 Cloaca, 21, 96-98
radio-ulna, 51 Cloacal aperture, 16, 127
scapula, 50 Cochlea, 94
sphen-ethmoid, 44, 139 Ceelom, 114, 136, 137
squamosal, 47 Columella, 49, 95, 141
sternum, 50 Condyle, occipital, 44
suprascapula, 50 Conjanctiva, 88
tarsal, 53, 54 Connective tissue, 156-
tibio-fibula, 53 158
vomer, 45 Contraction of muscle, 55
Brachial plexus, 75 Coracoid foramen, 50
Brain, 70-74 Cornea, 87, 88
development of, 118-121 Corpora, adiposa, 23
Branchial arch, 127 seg. Cranial flexure, 120
chamber, 127 nerves, 77-84, 121
cleft, 127 Cranium, 43-49, 138
Buccal cavity, 17, 18 Crura cerebri, 74, 120
INDEX
D
DEHYDRATION, 11
Development, 99-148
general account, 99-103
detailed account, 103-148
of nervous system, 115-121
of sense organs, 121-124
of alimentary canal, 124-127
of gill arches and clefts, 127,
128
of circulatory system, 128-
136
of ceelom, 136, 137
of skeleton, 137-141
of urinary system, 141-148
Digestive organs, 20-22
Dissection, 2
Drawing, 2, 3
Duct, bile, 22, 125
Ductus endolymphaticus, 94
Duodenum, 21
E
Har, 84, 85, 123
Efferent nerves, 68
Egg, 99
fertilisation of, 106, 107
formation of, 103, 104
segmentation of, 107-111
Elastic tissue, 156, 157
Epiblast, 111 seq.
epidermic layer of, 115
nervous layer of, 115
Epicoracoid, 50
Epiphysis, 51
Episternum, 50
Epithelium, 149-151
ciliated, 151
columnar, 150
glandular, 152-154
squamous, 150
stratified, 151
Eustachian passage, 17, 95, 124
External characters, 15-17
Bye, 87-93, 122, 123
frog, 87, 88, 92, 93
ox, 88-91
F
FAT-BODY, 23, 103
Fat-cells, 157, 158
Female organs, 97, 98
pronucleus, 106
Fenestra ovalis, 49, 95
Fertilisation, 106, 107
Fibrous tissue, 156
Filum terminale, 74
Fissure of cord, 86
Fontanelle, 44
Food-yolk, 101, 104, 110
Foramen, intervertebral, 42
magnum, 44
of Monro, 73, 121
Fore-brain, 120, 121
Fore-limb, 16, 51, 52
Fourth ventricle, 73, 120
G
GALL BLADDER, 22, 125
Gasserian ganglion, 79
General anatomy, 15-23
Genital plexus, 77
ridge, 103
Germinal layers, 111-113
spot, 104
vesicle, 104
Gill arches, 127, 128
clefts, 101, 127
Gills, external, 100, 127
internal, 101, 128
Gland, 152-154
carotid, 30, 135
compound, 152
gastric, 154
racemose, 152
simple, 152
thyroid, 24
tubular, 154
Glenoid cavity, 50
Glottis, 18
Grey matter, 86
H
H2MORRHOIDAL PLEXUS, 77
Hallux, 54
163
164 INDEX
Hardening, 10, 11
Haversian system, 159
Head, 16
kidney, 141, 143-146
Heart, 19, 24, 25, 32-35
development of, 128
pulsation of, 25
Hepatic plexus, 77
portal system, 27-29
Hind-brain, 119
Hind-limb, 16, 53, 54
Histology, 84-86, 91-93, 149-160
Hyaline cartilage, 158
Hyoid, 18, 43, 48, 49
arch, 127, 140, 141
Hyomandibular cleft, 124, 127
Hypoblast, 114, 115
I
IMBEDDING, 11, 12
Impregnation, 106, 107
Infundibulum, 73, 120
Insertion of muscle, 55
Intercellular substance, 149
Intestine, 19, 21, 150
Iris, 87, 88
Iter, 73, 120
J
JAW, 43, 46-48, 101, 139, 140
lower, 17, 47, 48
upper, 46, 47
K
KIDNEY, 22, 141-148
L
LABIAL CARTILAGE, 140
Lacuna of bone, 160
Laryngeal chamber 125
Lateral plate, 136
ventricle, 73, 121
Law of Recapitulation, 102
Lens, 87, 89, 91, 122, 123
Ligamentum nuche, 157
Limbs, 16, 101, 102
skeleton of, 51-54
Linea alba, 56
Lips, 101
Liquor sanguinis, 37
Liver, 19, 21, 125
Lower layer cells, 111
Lung, 19, 102, 125
Lymphatic system, 36, 37
Lymph heart, 36, 37
sacs, 36
M
MACERATION, 7, 8
Male organs, 96, 97
pronucleus, 106
Malpighian body, 147, 152
Mandibular bar, 47, 48, 127, 139,
140
Marrow, 160
Maxillary bar, 46, 47
teeth, 17
Meckel’s cartilage, 47, 140
Medulla oblongata, 72
Medullary cavity, 160
sheath, 84, 85
Medullated nerves, 84
Membrane-bone, 40
Mesenteron, 113-115, 124
Mesentery, 20
Mesoblast, 114, 115
Mesosternum, 51
Metamorphosis, 102, 135
Metasternum, 51
Methods, hardening, 10-11
imbedding, 11, 12
macerating, 7, 8
mounting, 6, 7
section-cutting, 12, 13
staining, 8-10
table of histological, 14
teasing, 7
Microscope, 3-6
Mid-brain, 119, 120
Migration of blood corpuscles, 39
Mounting media, 6,7
Mouth, 16-18
Miillerian duct, 147
Miiller’s fibres, 93
Muscles, of head, 58-61
165
Muscles, retrahens scapule, 57
sartorius, 62
of hind-limb, 61-67
of trunk, 56-58
adductor brevis, 65
adductor longus, 62
adductor magnus, 62
biceps, 64
ciliary, 90
cucullaris, 57
depressor palpebrze inferioris,
60
depressor mandibulz, 57
extensor cruris, 67
extensor dorsi communis, 58
gastrocnemius, 66
geniohyoid, 58
glutzeus, 58, 64
hyoglossus, 59
ilio-psoas, 65
infraspinatus, 57
intertransversales, 58
involuntary, 155
latissimus dorsi, 57
levator anguli scapule, 57
levator bulbi, 60
masseter, 60
mylohyoid, 58
non-striated, 155
obliquus externus, 56
inferior, 61
internus, 56
superior, 61
obturator, 66
pectineus, 65
pectoralis, 56
peroneus, 67
petrohyoid, 59
pterygoideus, 59
pyriformis, 64.
quadratus femoris, 66
rectus abdominis, 56
rectus anticus femoris, 64
rectus externus, 60
rectus inferior, 61
rectus internus, 60
rectus internus major, 62
rectus internus minor, 62
rectus superior, 60
retractor bulbi, 61
semimembranosus, 64
semitendinosus, 65
sternohyoid, 58
striated, 155
structure of, 155
submandibular, 58
temporalis, 59
tibialis anticus, 67
tibialis posticus, 66
triceps extensor femoris, 62
vastus externus, 64
vastus internus, 64
voluntary, 155
Muscular system, 55-67
Myotome, 136
N
NARES, ANTERIOR, 17, 122
posterior, 17, 122
Nephrostome, 143, 147
Nerve, abducens, 80
auditory, 82
brachial, 75
cells, 84-86
ciliary, 90
coccygeal, 76
coraco-clavicular, 75
cranial, 77-83, 121
crural, 76
facial, 81, 82
fibres, 84, 85
glosso-pharyngeal, 82
hypoglossal, 75
ileo-hypogastric, 76
motor oculi, 79
olfactory, 78
optic, 78
pathetic, 79
peroneal, 76
pneumogastric, 83
radial, 75
roots, 86
sciatic, 76
spinal, 74-77, 121
sympathetic, 77, 84
tibial, 76
trigeminal, 79, 80
166 INDEX
Nerve, ulnar, 75
vagus, 83
Nervous system, 68-86
development of, 115-121
histology of, 84-86
Neural arch, 42
canal, 42
fold, 116
groove, 116
plate, 116
spine, 42
tube, 117
Neurenteric canal, 118
Neuroglia, 86
Nodes of Ranvier, 85
Non-medullated nerves, 85
Nose, development, 121
Nostril, 17
Notochord, 113
Oo
OCCIPITAL CONDYLE, 44
Occipital-atlantal membrane, 43
Csophagus, 21
Olecranon process, 51
Olfactory capsule, 43, 45, 139
lobe, 70, 121
organ, 121
Operculum, 101, 127
Optic capsule, 139
chiasma, 73
cup, 123
lobe, 71, 120
thalami, 71, 120
vesicle, 122, 123
Ora serrata, 90 —
Origin of muscle, 55
Ovary, 19, 97, 103
Oviduct, 20, 98
Ovisac, 98
P
PALATO-PTERYGOID, 140
Pancreas, 22, 125
Parachordal, 138
Pectoral girdle, 18, 49, 50
Pelvic girdle, 52, 53
Pericardial cavity, 19, 136
Periganglionic glands, 77
Perimedullary lamelle, 160
Perineurium, 84
Periotic capsule, 94
Peripheral lamelle, 160
nervous system, 74-84, 121
Peritoneum, 20, 137
Pia mater, 70
Pineal body, 71, 120
Pituitary body, 74, 126
Polar bodies, 106
Pollex, 52
Portal system, 27, 29
Post-anal gut, 125
Post-axial surface, 61
Preaxial surface, 61
Presternum, 50
Primitive sheath, 84, 83
Proctodeum, 124, 126
Pronephros, 141-146
Pronucleus, 106
Pulmo-cutaneous arch, 32
Pupil, 87, 88
Pylorus, 21
Q
QUADRATE, 47, 140
R
REAGENTS, HARDENING, 10,11
macerating, 7,8
mounting, 6,7
staining, 8-10
Recapitulation, law of, 102
Renal plexus, 77
portal system, 28
Reproductive organs, 96-98
Retina, 88, 92, 93
Rhinal process, 45
Rods and cones, 93
Rules for drawing, 2,3
for dissection, 3
for use of microscope, 3-6
8
SaccuLus, 94
Sacrum, 43
Crown 8vo, vi”
BIT
LEGTI"
«lun Cavity, 109, 113
nucleus, 107
of the egg, 107-111
Semicircular canal, 95
Sense capsules, 45, 139
organs, 121, 124
Sheath of Schwann, 85
Shoulder girdle, 18
Sinus venosus, 25-27, 32, 33
Skeleton, 40, 54
appendicular, 49-54
axial, 42-49
Skin, 16
Skull, 43-49
development of, 138-141
Somatopleure, 136
Solar plexus, 77
Spawn, 99
Spermatozoa, 96, 99, 106
Splanchnopleure, 136
Spleen, 23
Splitting of mesoblast, 114, 136
Spinal cord, 74, 85, 86
development of, 115-118
ganglia, 77
nerves, 74-77
Spinous process, 42
Staining reagents, 8-10
Stapes, 141
Sternum, 50
Stomach, 21
Stomodzum, 124, 126
Subcutaneous tissue, 157
Subocular bar, 140
Sucker, 101
Suspensorium, 47
Suspensory ligament, 91
Sympathetic nervous system, 77,83
Symphysis, 52
Systemic arch, 30-32
Systole, 25
T
TASTE PAPILLA, 124
Teasing, 7
Teeth, 17, 46, 47
Tendo Achillis, 66
Tendon, 156
Testis, 20, 96, 103
Thalamencephalon, 71, 120
Third ventricle, 71, 73, 120
Thyroid gland, 26
Tongue, 18
Trabecule cranii, 138
Transverse process, 42
Truncus arteriosus, 24, 35
Tuber cinereum, 73
Tympanic cavity, 17, 95
membrane, 17, 95, 124
U
UPPER LAYER CELLS, 111
Ureter, 23, 96-98, 148
Urostyle, 42, 43, 138
Utriculus, 94
Uvea, 90
Vv
VaS DEFERENS, 96, 97
Vasa efferentia, 96, 148
Vascular system, 24-39
development of, 128-136
Vein, 24-29, 39
anterior abdominal, 28
anterior vena cava, 26, 27
brachial, 26
cardiac, 29
dorso-lumbar, 28
external jugular, 26
femoral, 28
gastric, 29
hepatic, 27
hepatic portal, 29
innominate, 26
internal jugular, 26
intestinal, 29
lingual, 26
mandibular, 26
rousculo-cutaneous, 26
ovarian, 27
parietal, 28
pelvic, 28
167
168 “Ny EX
Vein, portal, 27-29
posterior vena cava, 27
pulmonary, 27
renal, 27
renal portal, 28
sciatic, 28
splenic, 29
spermatic, 27
subclavian, 26
subscapular, 26
vesical, 28
Ventral fissure, 74
Ventricle, of brain, 73, 118-121
of heart, 24, 34
Vertebre, 42,43
Vertebral column, 42, 43
development of, 137, 138
plate, 136
Vesical plexus, 77
Vesicula seminalis, 23, 96, 148
Vestibule, 94
Viscera, abdominal, 18-23
Visceral arches, 127, 189-141
clefts, 127
skeleton, 139-141
Vitelline membrane, 104
Vitreous humour, 88
~tiganglionic glands, 77
~edaullary lamelle, 160
~ium, 84.
WHITE FIBROUS 160. ‘
matter, 86 ,
Wolffian body, 143, 146, 147
duct, 147, 148
Wrist, 51
x
XIPHISTERNUM, 51
Y
YELLOW ELASTIC TISSUE, 156
Yolk, 101, 104, 110, 111
cells, 111
hypoblast, 114
plug, 113
Z
ZONULE OF ZINN, 91
Zygapophysis, 42
Printed by BALLANTYNE, Hanson & Co.
London and Edinburgh.
Crown 8vo, viii, 363 pp., cloth.
BIOLOGICAL
LECTURES AND ADDRESSES
DELIVERED BY THE LATE
ARTHUR MILNES MARSHALL,
M.A., M.D., D.Se., F.R.S.
EDITED BY
C. F. MARSHALL, M.D., B.Sc., F.R.C.S.
With 37 Illustrations, 8vo, cloth.
LECTURES ON
THE DARWINIAN THEORY
DELIVERED BY THE LATE
ARTHUR MILNES MARSHALL,
M.A., M.D., D.Sc, F.R.S.
EDITED BY
C. F. MARSHALL, M.D., B.Sc. F.R.C.S.
NEW YORK: MACMILLAN & CO.