mice, voles, squirrels, beavers, cavies, guineapigs is included in that
order of the class of mammals which is called the rodentia, and is
distinguished by the character of the incisor teeth from other orders of
the class.
10. _Questions and Exercises_
1. Describe the venous circulation of the rabbit (with diagrams).
Compare a vein and artery. Compare the distribution of the great
venous trunks with that of the arterial system.
2. Construct a general diagram of the circulation of the rabbit, to show
especially the relation of the portal system, the lymphatics and
lacteals, and the renal circulation to the main blood current.
3. Draw the alimentary canal of the rabbit from memory.
4. What is a villus? Describe its epithelium, and the vessels within it.
Write as explicit an account as you can of the absorbent action of a
villus.
5. Tabulate the alimentary secretions, and their action on the food.
6. What is botryoidal tissue? Where does it occur? What is known
of its functions?
7. Copy Diagram I. (enlarged), and insert upon it the visceral nerves
as far as you can.
8. What are the most characteristic points in the mammalian vertebral
column?
9. Describe cartilage and bone, and compare them with one another.
10. Give an account of the amoeba, and compare it with a typical
tissue cell in a metazoon (e.g., the rabbit).
11. Give a general account of connective tissue. What is tendon?
12. Trace, briefly, the increased modification of tissues in the
vertebrata.
13. Describe, with diagrams, the structure of blood. State the function
of each factor you describe.
14. Compare the pectoral with the pelvic limb and girdle. What other
structures of the adult rabbit display a similar repetition of similar
parts?
15. Draw from memory typical vertebrae from each region of the
vertebral column.
16. What are bilateral symmetry and metameric segmentation?
17. Give a schedule of distinctive mammalian features.
18. Describe the rabbit's brain (with diagrams).
19. Give a list of the cranial nerves of the rabbit, and note their origin
in the brain.
20. Give a list of the nerve apertures of the dog's skull.
21. What are the chief anatomical differences between a typical
cranial, a spinal, and a sympathetic nerve?
22. Describe and figure the distribution of nerves V., VII., IX., and X.
23. Describe the muscles, glands, and nerves of the orbit of the
rabbit.
24. Describe, with figures, the eye of the rabbit.
25. Give a diagram of the rabbit's internal ear.
26. Draw and describe the ear ossicles. What is their function?
27. Draw and state the precise position of the hyoid bone, the
clavicle, the calcaneum, and the olecranon process.
28. Describe, as accurately as possible, the position of palatine
bones, pterygoids, the ethmoid bone, the pre- and basi-sphenoids,
in the dog's skull.
29. What is membrane bone? What is cartilage bone? Discuss their
mutual relationship.
30. What is an excretion? What are the chief excretory products of an
animal? How are they removed?
31. Describe the minute anatomy of the liver. Give a general account
of its functions.
32. Describe the minute anatomy of the kidney, and the functions of
the several parts.
33. What is ciliated epithelium? Where does it occur in the rabbit?
34. Describe the mechanism of respiration. What is the relation of
respiration to the general life of the animal?
35. What are the functions of the skin? Describe its structure.
36. What is a secretion? Tabulate and classify secretary organs.
What is a goblet cell?
37. Draw, from memory, the dorsal and ventral aspects of, and a
median section through, a dog's skull.
38. Name any structures that appear to you to be vestiges or
rudiments, i.e., structures without adequate physiological reason, in
the rabbit's anatomy.
39. How are such structures interpreted?
40. Describe the structure of striated muscular fibre. Describe its
functions, and the various means by which they may be called into
activity.
41. Describe the characters and structure of the blood of the rabbit.
What is the lymphatic system? Describe its relation to the blood
system in a mammal.
42. Describe the structure of (a) blood, (b) hyaline cartilage, (c)
bone, in the rabbit; (d) point out the most important resemblances and
differences between these tissues; (e) state what you know of the
development of the same tissues.
43. Draw diagrams, with the parts named, of the male and female
generative organs of the rabbit.
44. In the rabbit provided dissect on one side and demonstrate by
means of flag-labels the main trunk of the vagus nerve, the phrenic
nerve, and the recurrent laryngeal nerve.
45. Dissect the rabbit provided so as to expose the abdominal
viscera. Mark with flag-labels the duct of the pancreas, the ureters,
and the oviducts or the sperm ducts (as the case may be).
[Many of the above questions were actually set at London
University Examinations in Biology.] {In Both Editions.}
-The Frog_
1. _General Anatomy_
Section 1. We will now study the adult anatomy of the frog, and
throughout we shall make constant comparisons with that of the
rabbit. In the rabbit we have a distinctly land-loving, burrowing animal;
it eats purely vegetable food, and drinks but little. In the frog we have
a mainly insectivorous type, living much in the water. This involves the
moister skin, the shorter alimentary canal, and the abbreviated neck
(Rabbit, Section 2) of the frog; the tail is absent-- in a fish it would do
the work the frog accomplishes with his hind legs-- and the apertures
which are posterior in the rabbit, run together into one dorsal opening,
the cloaca. There is, of course (Rabbit, Section 4), no hair the skin is
smooth, and an external ear is also absent. The remarkable
looseness of the frog's skin is due to great lymph spaces between it
and the body wall.
Section 2. If we now compare the general anatomy of the frog (vide
Sheet 11) with that of the rabbit, we notice that the diaphragm is
absent (Rabbit, Section 4), and the body cavity, or coelom, is, with
the exception of the small bag of the pericardium round the heart, one
continuous space. The forked tongue is attached in front of the lower
jaw, and can be flicked out and back with great rapidity in the capture
of the small insects upon which the frog lives. The posterior nares
open into the front of the mouth-- there is no long nasal chamber, and
no palate, and there is no long trachea between the epiglottis and the
lungs. The oesophagus is less distinct, and passes gradually, so far
as external appearances go, into the bag-like stomach, which is
much less inflated and transverse than that of the rabbit. The
duodenum is not a U-shaped loop, but makes one together with the
stomach; the pancreas lies between it and the stomach, and is more
compact than the rabbit's. There is no separate pancreatic duct, but
the bile duct runs through the pancreas, and receives a series of
ducts from that gland as it does so. The ileum is shorter, there is no
sacculus rotundus, and the large intestine has no caecum, none of
the characteristic sacculations of the rabbit's colon, and does not loop
back to the stomach before the rectum section commences. The
anus opens not upon the exterior, but into a cloacal chamber. The
urinary and genital ducts open separately into this cloaca, and
dorsally and posteriorly to the anus. The so-called urinary bladder is
ventral to the intestine, in a position answering to that of the rabbit,
but it has no connection with the ureters, and it is two-horned.
Section 3. The spleen is a small, round body, not so intimately
bound to the stomach as in the rabbit, but in essentially the same
position.
Section 4. Much that we knew of the physiology of the frog is arrived
at mainly by inferences from our mammalian knowledge. Its histology
is essentially similar. Ciliated epithelium is commoner and occurs
more abundantly than in the rabbit, in the roof of the mouth for
instance, and its red blood corpuscles are much larger, oval, and
nucleated.
Section 5. The lungs of the frog are bag-like; shelves and spongy
partitions project into their cavities, but this structure is much simpler
than that of the rabbit's lung, in which the branching bronchi, the
imperfect cartilaginous rings supporting them, alveoli, arteries and
veins, form together a quasi-solid mass.
Section 6. The mechanism of respiration is fundamentally different
from that of the mammal. The method is as follows:-- The frog opens
its anterior nares, and depresses the floor of the mouth, which
therefore fills with air. The anterior nares are then closed, and the floor
mouth rises and forces the air into the lungs-- the frog, therefore,
swallows its air rather than inhales it. The respiratory instrument of
the rabbit is a suction pump, while that of the frog is a "buccal force
pump."
Section 7. The heart is not quadrilocular (i.e., of four chambers), but
trilocular (of three), and two structures, not seen in Lepus, the
truncus arteriosus and the sinus venosus, into the latter of which
the venous blood runs before entering the right auricle, are to be
noted. The single ventricle is blocked with bars of tissue that render
its interior, not an open cavity, but a spongy mass. Figure 2, Sheet
11, shows the heart opened; l.au. and r.au. are the left and right
auricles respectively; the truncus arteriosus is seen to be imperfectly
divided by a great longitudino-spiral valve (l.s.v.); p.c. is the
pulmo-cutaneous artery -going to the lungs- [supplying skin and
lungs]; d.ao., the dorsal aorta [furnishing the supply of the body and
limbs]; and c.a. the carotid artery going to the head; all of which
vessels (compare Figure 1) are paired.
Section 8. It might be inferred from this that pure and impure blood
mix in the ventricle, and that a blood of uniform quality flows to lungs,
head, and extremities; but this is not so. The spongy nature of the
ventricle sufficiently retards this mixing. It will be noted that the
opening of pulmonary arteries lies nearest to the heart, next come the
aortic and carotid arches, which have a common opening at A.
Furthermore, at c.g.l. [the carotid artery, repeatedly divides to form a
close meshwork of arterioles, the carotid gland, forming a sponge-like
plug in this vessel.] is a spongy mass of matter, the carotid gland
inserted upon the carotid. Hence the pulmonary arteries yawn nearest
for the blood, and, being short, wide vessels, present the least
resistance to the first rush of blood-- mainly venous blood for the right
auricle. As they fill up, the back resistance in them becomes equal,
and then greater, than the resistance at A, and the rush of blood, now
of a mixed quality passes through that aperture. It selects the dorsal
aorta, because the carotid arch, plugged by the carotid gland, offers
the greater resistance. Presently, however, the back resistance of the
filled dorsal aorta rises above this, and the last flow of blood, from the
ventricular systole-- almost purely oxygenated blood for the left
auricle-- goes on towards the head.
Section 9. At the carotid gland the carotid artery splits into -an- [a]
-external carotid- [lingual] (e.c.), and a deeper internal carotid. The
dorsal aorta passes round on each side of the oesophagus, as
indicated by the dotted lines in Figure 2, Sheet 11, and meets its
fellow dorsal to the liver. Each arch gives off subclavian arteries to the
limbs, and the left, immediately before meeting the right, gives off the
coeliaco-mesenteric artery [to the alimentary canal]. This origin of the
coeliaco-mesenteric artery a little to the left, is the only asymmetry
(want of balance) in the arterial system of the frog, as contrasted with
the very extensive asymmetry of the great vessels near the heart of
the rabbit. [Posteriorly the dorsal aorta forks into two common iliac
arteries (right and left) supplying the hind limbs.]
Section 10. Figure 3 gives a side view of the frog, to display the
circulation.
{Lines from Second Edition only.}
[The venous return to the heart, as in the rabbit, is by paired venae
cavae anteriores and by a single vena cava inferior. The factors of the
anterior cava on either side are an external jugular (ex.j.) an
innominate vein (in.v.) and subclavian (scl.v.). The last receives not
only the brachial vein (b.v.) from the fore limb, but also a large vein
bringing blood for the skin, the cutaneous (p.v.). The innominate vein
has also two chief factors, the internal jugular (l.i.j.v.) and the
subscapular (s.s.v.). The blood returns from each hind limb by a
sciatic (l.sc.) or femoral (f.m.) vein, and either passes to a renal portal
vein (l.r.p.), which breaks into capillaries in the kidney, or by a paired
pelvic vein (l.p.v. in Figures 1 and 3) which meets its fellow in the
middle line to form the anterior abdominal vein (a.ab.v.) going forward
and uniting with the (median) portal vein (p.v.) to enter the liver.]
-The vessels are named in the references to the figure, which should
be carefully copied and mastered. Here we need only- [Comparing
with the rabbit, we would especially] call attention to the fact that the
vena cava inferior extends posteriorly only to the kidney, and that
there is a renal portal system. The blood from the hind limbs either
flows by the anterior abdominal vein to the portal vein and liver, or it
passes by the renal portal vein to the kidney. There the vein breaks
up, and we find in the frog's kidney, just as we find in the frog's and
rabbit's liver, a triple system of (a) nutritive arterial, (b) afferent*
venous and (c) efferent** venous vessels.
* a, ad = to;
** e, ex = out of.
{This Section missing from Second Edition.}
-Section 11. It is not very improbable that the kidney of the frog
shares, or performs, some of the functions of the rabbit's liver, or
parallel duties, in addition to the simply excretory function. Since
specialization of cells must be mainly the relatively excessive
exaggeration of some one of the general properties of the
undifferentiated cell, it is not a difficult thing to imagine a gradual
transition, as we move from one organism to another, of the functions
of glands and other cellular organs. It is probable that the mammalian
kidney is, physiologically, a much less important (though still quite
essential) organ than the structures which correspond to it in position
and development in the lower vertebrate types.-
Section 12. The lymphatic system is extensively developed in the
frog, but, in the place of a complete system of distinctly organized
vessels, there are great lymph sinuses (compare Section 1). In Figure
5, Sheet 12, the position of two lymph hearts (l.h., l.h.) which pump
lymph into the adjacent veins, is shown.
Section 13. The skull of the frog will repay a full treatment, and will
be dealt with by itself later. The vertebral column (Sheet 12)
consists of nine vertebrae, the centra of which have faces, not flat, but
hollow in front (pro-coelous), and evidently without epiphyses
(compare the Rabbit). The anterior is sometimes called the atlas, but
it is evidently not the homologue of the atlas of the rabbit, since the
first spinal nerve has a corresponding distribution to the twelfth cranial
of the mammal, and since, therefore, it is probable that the
mammalian skull = the frog's skull + one (or more) vertebrae
incorporated with it. Posteriorly the vertebral column terminates in the
urostyle, a calcified unsegmented rod. The vertebrae have transverse
processes, but no ribs.
Section 14. The fore-limb (Figure 6, Sheet 12) consists of an upper
segment of one bone, the humerus, as in the rabbit; a middle section,
the radius and ulna, fused here into one bone, and not, as in the
mammalian type, separable; of a carpus, and of five digits, of which
the fourth is the longest. The shoulder girdle is more important and
complete than that of the higher type. There is a scapula (sc.) with an
unossified cartilaginous supra-scapula (s.sc.); the anterior border of
the scapula answers to the acromion. On the ventral side a
cartilaginous rod, embraced by the clavicle (cl.) (a membrane bone in
this type), runs to the sternum, and answers to the clavicle of the
rabbit. In the place of the rabbit's coracoid process, is a coracoid
bone (co.), which reaches from the glenoid cavity to the sternum; it is
hidden on the right side of Figure 6, which is a dorsal view of the
shoulder girdle. There is a pre-omosternum (o.st.) and a
post-omosternum, sometimes termed a xiphisternum (x.).
Section 15. Figure 7 shows the pelvic girdle and limb of the frog.
There is a femur (f.); tibia and fibula (t. and f.) are completely fused;
the proximal bones of the tarsus, the astragalus (as.), and calcaneum
(cal.) are elongated, there are five long digits, and in the calcar (c.) an
indication of a sixth. With considerable modifications of form, the
three leading constituents of the rabbit's pelvic girdle occur in
relatively identical positions. The greatly elongated ilium (il.)
articulates with the single (compare Rabbit) sacral vertebra (s.v. in
Figure 5). The ischium (is.) is relatively smaller than in the rabbit, and
the pubis (pu.) is a ventral wedge of unossified cartilage. The shape of
the pelvic girdle of the frog is a wide departure from that found among
related forms. In connection with the leaping habit, the ilia are greatly
elongated, and the pubes and ischia much reduced. Generally
throughout the air-frequenting vertebrata, we find the same
arrangement of these three bones, usually in the form of an inverted.
Y-- the ilium above, the ischium and pubis below, and the acetabulum
at the junction of the three.
Section 16. The uro-genital organs of the frog, and especially those
of the male, correspond with embryonic stages of the rabbit. In this
sex the testes (T., Sheet 13) lie in the body cavity, and are white
bodies usually dappled with black pigment. Vasa efferentia (v.e.) run
to the internal border of the anterior part of the kidney, which answers,
therefore, to the rabbit's epididymis. The hinder part of the kidney is
the predominant renal organ. There is a common uro-genital duct, into
which a seminal vesicle, which is especially large in early spring,
opens. This is the permanent condition of the frog. In the rabbit, for
urogenital duct, we have ureter and vas deferens; the testes and that
anterior part of the primitive kidney, the epididymis, shift back into the
scrotal sacs, and the ureters shift round the rectum and establish a
direct connection with the bladder, carrying the genital ducts looped
over them. The oviducts of the female do not fuse distally to form a
median vagina as they do in the rabbit. In front of the genital organ in
both sexes is a corpus adiposum (c.ad.), which acts as a fat store,
and is peculiar to the frogs and toads. The distal end of the oviduct of
the female is in the breeding season (early March) enormously
distended with ova, and the ovaries become then the mere vestiges of
their former selves. The distal end of the oviduct is, therefore, not
unfrequently styled the uterus. There is no penis in the male,
fertilisation of the ova occurring as they are squeezed out of the
female by the embracing fore limbs of the male. The male has a pad,
black in winter, shown in Figure 1, which is closely pressed against
the ventral surface of the female in copulation, and which serves as a
ready means of distinguishing the sex.
Section 17. The spinal cord has a general similarity to that of the
rabbit; the ratio of its size to that of the brain is larger, and the
nerves number ten pairs altogether. The first of these (sp. 1, in
Figure 2, Sheet -12- ) {First Edition error.} [13] corresponds in
distribution with the rabbit's hypoglossal nerve, a point we shall refer to
again when we speak of the skull. The second and third constitute the
brachial plexus. The last three form the sciatic plexus going to the
hind limb.
Section 18. The same essential parts are to be found in the brain of
both frog and rabbit, but in the former the adult is not so widely
modified from the primitive condition as in the latter. The fore-brain
consists of a thalamencephalon (th.c. and 1), which is exposed in the
dorsal view of the brain, and which has no middle commissure. The
cerebral hemispheres (c.h.) are not convoluted, do not extend back to
cover parts behind them, as they do in the rabbit, and are not
connected above the roof of the thalamencephalon by a corpus
callosum. Moreover, the parts usually regarded, as the olfactory lobes
(rh.) fuse in the middle line. The mid-brain gives rise to the third
nerve, and has the optic lobes on its dorsal side, but these are hollow,
and they are not subdivided by a transverse groove into corpora
quadrigemina, as in the rabbit. In the hind-brain the cerebellum is a
mere band of tissue without lateral lobes or flocculi, and the medulla
gives origin only to nerves four to ten; there is no eleventh nerve, and
the hypoglossal is the first spinal-- from which it has been assumed
that the rabbit's medulla equals that of the frog, plus a portion of the
spinal cord incorporated with it. The hypoglossal is very distinctly
seen on opening the skin beneath the hyoid plate.
Section 19. The first, second, third, and fourth cranial nerves of the
frog correspond with those of the rabbit in origin and distribution. So
do five, six and eight. The seventh nerve forks over the ear-drum-- the
larger branch emerging behind it and running superficially, as shown
in Figure 4. There is also a deeper palatine branch of VII. (P.) running
under V2 and V3 below the orbit, and to be seen together with V1 and
V2 after removal of the eyeball. The ninth nerve similarly forks over the
first branchial slit of the tadpole, and evidence of the fork remains in
the frog. It is seen curving round anterior to the hypoglossal nerve, and
lying rather deeper in dissection. The vagus (tenth) nerve is distributed
to heart, lungs, and viscera-- in the tadpole it also sends for forking
branches over the second, third, and fourth branchial slits. It lies
deeper than IX., and internal to the veins, and runs close beside the
cutaneous artery. Most of these nerves are easily dissected and no
student should rest satisfied until he has actually seen them.
Section 20. The sympathetic chain is closely connected with the
aorta. It is, of course, paired, and is easily found in dissection by
lifting the dorsal aorta and looking at its mesentery. In the presence of
ganglia corresponding to the spinal nerves, and of rami
communicantes, it resembles that of the rabbit.
Section 21. The whole of this chapter is simply a concise
comparison, of frog and rabbit. In addition to reading it, the student
should very carefully follow the annotations to the figures, and should
copy and recopy these side by side with the corresponding diagrams
of the other types.
2. _The Skull of the Frog (and the vertebrate skull
generally)_
Section 22. We have already given a description of the mammalian
skull, and we have stated where the origin of the several bones was in
membrane, and where in cartilage; but a more complete
comprehension of the mammalian skull becomes possible with the
handling of a lower type. We propose now, first to give some short
account of the development and structure of the skull of the frog,
and then to show briefly how its development and adult arrangement
demonstrate the mammalian skull to be a fundamentally similar
structure, complicated and disguised by further development and
re-adjustment.
Section 23. Figure 1,I. Sheet 14, shows a dorsal view of a young
tadpole cranium; the brain has been removed, and it is seen that it
was supported simply upon two cartilaginous rods, the trabeculae
cranii (tr.c.). Behind these trabeculae comes the notochord (n.c.), and
around its anterior extremity is a paired tract of cartilage, the
parachordals (p.c.). These structures, underlying the skull, are all that
appear[s] at first of the brain box. In front, and separate from the
cranium, are the nasal organs (n.c.); the eyes lie laterally to the
trabeculae, and laterally to the parachordals are two tracts of cartilage
enclosing the internal ear, the otic capsules.
Section 24. Figure 1, II., is a more advanced, phase of the same
structures. The trabeculae have met in front and sent forward a
median (c.t.) and lateral parts (a.o.) to support the nasal organs. They
have also flattened, out very considerably, and have sent up walls on
either side of the brain to meet above it and form an incomplete roof
(t.) over it. The parachordals have similarly grown up round, the
hind-brain and formed a complete ring, the roof of which
is indicated, by b. Further, the otic capsules are fusing with the
brain-case. With certain differences of form these elements-- the
trabeculae, the parachordals, and the otic capsules, are also the first
formed structures of the mammalian cranium.
Section 25. In Figures 1,I. and II., there appears beneath the eye a
bar of cartilage (p.p.), the palato-pterygoid cartilage, which is also to
be seen from the side in Figures 8,I. and III. It will be learnt from these
latter that this bar is joined in front to the cranium behind the nasal
organ, and behind to the otic capsule. The cartilaginous bar from the
palato-pterygoid to the otic capsule is called the quadrate, and at the
point of junction, at the postero-ventral angle of the palato-pterygoid,
articulates with the cartilaginous bar which is destined to form the
substratum of the lower jaw-- Meckel's cartilage (M.c. in Figure 8,I.).
Section 26. Figure 2 shows a dorsal view of these structures in a
young frog. The parts corresponding to these in 1,II. will be easily
made out, but now ossification has set in at various points of this
cartilaginous cranium. In front of the otic capsule is the paired
pro-otic bone (p.o.); behind it at the sides of the parachordal ring is
the paired ex-occipital (e.o.); in front of the cranium box, and behind
the nasal capsules, is a ring of bone, the (median, but originally
paired) sphenethmoid (s.e.). -A paired ossification appears in the
palato-pterygoid cartilage the pterygoid bone (pt.), while- A splint of
bone, the quadrato-jugal, appears at the angle of articulation with the
lower jaw. These are all the cartilage bones that appear in the cranium
and upper jaw of the frog.
Section 27. But another series of bones, developed first chiefly in
dermal connective tissue, and coming to plate over the cranium of
cartilage, are not shown in Figure 2. They are, however, in Figure 3.
These membrane bones are: along the dorsal middle line, the
parieto-frontals (p.f.), originally two pairs of bones which fuse in
development, and the nasals (na.). Round the edge of the jaw, and
bearing the teeth, are pre-maxillae (p.m.), and maxillae (mx.), and
overlying the quadrate cartilage and lateral to the otic capsules are
the T-shaped squamosal bones (sq.). In the ventral view of the skull
(Figure 4) we see a pair of vomers (vo.) bearing teeth, a pair of
palatines (pal.), [and a pair of pterygoids (pt.)] (which [palatines and
pterygoids, we may note,] unlike those of the rabbit, are -stated to be-
membrane bones), and a great median dagger-shaped para-sphenoid
(p.sp.). These two Figures, and 5, which shows the same bones in
side view, should be carefully mastered before the student proceeds
with this chapter. The cartilage bones are distinguished from
membrane bones by cross-shading.
Section 28. Turning now to Figure 8,I., we have a side view of a
tadpole's skull. On the ventral side of the head is a series of vertical
cartilaginous bars, the visceral arches supporting the walls of the
tadpole's gill slits. The first of these is called the hyoid arch (c.h.),
and the four following this, the first (br.1), second, third, and fourth
(br.4), branchial arches. Altogether there are four gill slits and between
the hyoid arch and the jaw arch, as it is called (= Meckel's cartilage +
the palato-pterygoid), is "an imperforate slit," which becomes the
ear-drum.* The frog no longer breathes by gills, but by lungs, and the
gills are lost, the gill slits closed, and the branchial arches
consequently much reduced. Figures 8, II., and 8, III., show stages in
this reduction. The hyoid arch becomes attached, to the otic capsule,
and its median ventral plate, including also the vestiges of the first,
second, and fourth branchial arches, is called the hyoid apparatus. In
Figure 5, the apparatus is seen from the side; c.h. is called the (right)
anterior cornu** of the hyoid. The function of the hyoid apparatus in
the frog is to furnish, a basis of attachment to the tongue muscles; it
remains cartilaginous, with the exception of the relic of one branchial
arch, which ossifies as the thyro-hyal (Figure 7 th.h.). It will be noted
that, as development proceeds, the angle of the jaw swings backward,
and the hyoid apparatus, shifts relatively forward. These changes of
position are indicated in Figure 8, III., by little arrow-heads.
* We may note here that, comparing the ear of the frog with that of the
rabbit, there is no external ear. There is, moreover, no bulla supporting
the middle ear, and the tympanic membrane stretches between the
squamosal in front and the anterior cornu of the hyoid behind. A
rod-like columella auris replaces the chain of ear ossicles, and may,
or may not, answer to the stapes alone, or even possibly to the entire
series. In the internal ear there is no cochlea, and the otic mass is
largely cartilaginous instead of entirely bony.
** Plural cornua.
Section 29. Before proceeding to the comparison of the mammalian
skull with this, we would strongly recommend the student thoroughly
to master this portion of the work, and in no way can he do this more
thoroughly and quickly than by taking a parboiled frog, picking off the
skin, muscle, and connective tissue from its skull, and making out the
various bones with the help of our diagrams.
Section 30. Figure 9 represents, in the most diagrammatic way, the
main changes in form of the essential constituents of the cranio-facial
apparatus, as we pass from the amphibian to the mammalian skull. F.
is the frog from the side and behind; b.c. is the brain-case, o.c. the
otic capsule, e. the eye, n.c. the nasal capsule, p.p. the
palato-pterygoid cartilage, mx. the maxillary membrane bones, sq.
the squamosal, and mb. the mandible. The student should compare
with Figure 5, and convince himself that he appreciates the
diagrammatic rendering of these parts. Now all the distinctive
differences in form, from this of the dog's skull (D.), are reducible to
two primary causes--
(1) The brain is enormously larger, and the brain-case is vastly
inflated, so that--
(a) the otic capsule becomes embedded in the brain-case wall;
(b) the palato-pterygoid rod lies completely underneath the
brain-case instead of laterally to it;
(c) the squamosal tilts down and in, instead of down and out,
and the lower jaw articulates with its outer surface instead
of below its inner, and, moreover, with the enormous distention
of the brain-case it comes about that the squamosal is
incorporated with its wall.
(2) The maxilla anteriorly and the palatine posteriorly send down
palatine plates that grow in to form the bony palate, cutting off
a nasal passage (n.p.) from the mouth cavity (m.p.), and carrying the
posterior nares from the front part of the mouth, as they are in the
frog, to the pharynx. Hence the vomers of the dog lie, not in the
ceiling of the mouth, but in the floor of this nasal passage.
Section 31. The quadrate cartilage of the frog is superseded by the
squamosal as the suspensorium of the lower jaw. It is greatly
reduced, therefore; but it is not entirely absent. In the young mammal,
a quadrate cartilage can be traced, connected with the
palato-pterygoid cartilage, and articulating with Meckel's cartilage. Its
position is, of course, beneath the squamosal, and just outside the
otic capsule. As development proceeds, the increase in size of the
quadrate, does not keep pace with that of the skull structures. It loses
its connection with the palato-pterygoid, and apparently ossifies as a
small ossicle-- the incus of the middle ear. A small nodule of
cartilage, cut off from the proximal end of Meckel's cartilage, becomes
the malleus. The stapes would appear to be derived from the hyoid
arch. Hence these small bones seem to be the relics of the discarded
jaw suspensorium of the frog utilized in a new function. Considerable
doubt, however, attaches to this interpretation-- doubt that, if anything,
is gaining ground.
Section 32. The tympanic bulla of the dog is not indicated in Diagram
9, and it would appear to be a new structure (neomorph), not
represented in the frog.
Section 33. Besides these great differences in form, there are
important differences in the amount and distribution of centres of
ossification of the skull of frog and mammal. There is no
parasphenoid in the mammal*; and, instead, a complete series of
ossifications, the median-, basi-, and pre-sphenoids, and the lateral
ali- and orbito-sphenoids occur. The points can be rendered much
more luminously in a diagram than in the text, and we would counsel
the student to compare this very carefully with that of the Rabbit.
* Faint vestigeal indications occur in the developing skulls of some
insectivora.
Section 34.
-Cranium_
-Nasal_ (paired), -Vomer_ (paired)
-Fronto-Parietal_, Sphenethmoid Bone (median), Eye, Pro-otic Bone,
Otic Cartilage, Ex-occipital (paired)
-Para-sphenoid Bone_
-Upper Jaw_
-Pre-Maxilla_ (paired), -Palatine_ (paired), Pterygoid (paired),
-Squamosal_, Quadrate Cartilage {To 1.}
-Maxilla_
1. Quadrato-Jugal
-Lower Jaw_
Mento-meckelian, -Dentary_, -Articulare- [-Angulo Splenial_]
Section 35. -Points especially- [Additional points] to be noticed are:
(1) The otic capsule (= periotic bone) of the dog ossifies from a
number of centres, one of which is equivalent to the frog's prootic.
(2) The several constituents of the lower jaw are not to be
distinguished in the adult mammal.
(3) The frog has no lachrymal bone.
Section 36. We are now in a position to notice, without any danger of
misconception, what is called the segmental theory of the skull. Older
anatomists, working from adult structure only, conceived the idea that
the brain-case of the mammal represented three inflated vertebrae.
The most anterior had the pre-sphenoid for its body, the
orbito-sphenoids for its neural processes, and the arch was
completed above by the frontals (frontal segment). Similarly, the
basi-sphenoids, ali-sphenoids, and parietals formed a second arch
(parietal segment), and the ex-, basi-, and supra-occipitals a third
(occipital segment). If this were correct, in the frog, which is a more
primitive rendering of the vertebrate plan, we should find the vertebral
characters more distinct. But, as a matter of fact, as the student will
perceive, frontal segment, parietal segment, and occipital segment,
can no longer be traced; and the mode of origin from trabeculae and
para-chordals show very clearly the falsity of this view. The vertebrate
cranium is entirely different in nature from vertebrae. The origin of the
parietals and frontals as paired bones in membrane reinforces this
conclusion.
Section 37. But as certainly as we have no such metameric
segmentation, as this older view implies, in the brain-case of the frog,
so quite as certainly is metameric segmentation evident in its
branchial arches. We have the four gill slits of the tadpole and their
bars repeating one another; the hyoid bar in front of these is evidently
of a similar nature; and that the ear drum is derived from an
imperforate gill slit is enforced by the presence of an open slit (the
spiracle) in the rays and dog-fish in an entirely equivalent position.
Does the mouth answer to a further pair of gill slits, and is the jaw
arch (palato-pterygoid + Meckel's cartilage) equivalent to the arches
that come behind it? This question has been asked, and answered in
the affirmative, by many morphologists, but not by any means by all.
The cranial nerves have a curious similarity of arrangement with regard
to the gill slits and the mouth; the fifth nerve forks over the mouth, the
seventh forks over the ear drum, the ninth, in the tadpole and fish,
forks over the first branchial slit, and the tenth is, as it were, a leash
of nerves, each forking over one of the remaining gill slits. But this
matter will be more intelligible when the student has worked over a
fish type, and need not detain us any further now.
Section 38. See also Section 13 again, in which is the suggestion
that the occipital part of the skull is possibly a fusion of vertebrae, a
new view with much in its favour, and obviously an entirely different
one from the old "segmental" view of the entire skull, discussed in
Section 36.
_Questions on the Frog_
[All these questions were actually set at London University
Examinations.] {In Both Editions.}
1. Give an account, with illustrative sketches, of the digestive organs
of the common frog, specifying particularly the different forms of
epithelium met with in the several regions thereof.
2. Describe the heart of a frog, and compare it with that of a fish and
of a mammal, mentioning in each case the great vessels which open
into each cavity.
3. Compare with one another the breathing organs and the
mechanism of respiration in a frog and in a rabbit. Give figures
showing the condition of the heart and great arteries in these animals,
and indicate in each case the nature of the blood in the several
cavities of the heart.
4. Draw diagrams, with the parts named, illustrating the arrangement
of the chief arteries of (a) the frog, (b) the rabbit. (c) Compare briefly
the arrangements thus described. (d) In what important respects does
the vascular mechanism of the frog differ from that of the fish, in
correlation with the presence of lungs?
5. In the frog provided, free the heart, both aortic arches, dorsal aorta
as far as its terminal bifurcation, and both chains of sympathetic
ganglia from surrounding structures; and remove them, in their
natural connection, from the animal into a watch-glass.
6. Describe the male and female reproductive organs of the common
frog, and give some account of their development.
7. Describe, with figures, the bones of the limbs and limb-girdles of a
frog.
8. Remove the brain from the frog provided, and place it in spirit. Make
a lettered drawing of its ventral and dorsal surfaces.
9. Point out the corresponding regions in the brain of a frog and a
mammal, and state what are the relations of the three primary
brain-vesicles to these regions.
10. (a) Give an account, with diagrams, of the brain of the frog; (b)
point out the most important differences between it and the brain of
the rabbit. (c) Describe the superficial origin and the distribution of the
third, (d) of the fifth, (e) of the seventh., (f) of the ninth, and
(g) of the tenth cranial nerves of the frog.
11. Describe, with figures, the brain of a frog, and compare it with that
of a rabbit. What do you know concerning the functions of the several
parts of the brain in the frog?
12. Describe briefly the fundamental properties of the spinal cord in
the frog. By what means would you determine whether a given nerve
is motor or sensory?
13. Prepare the skull of the frog provided. Remove from it and place in
glycerine on a glass slip the fronto-parietal and parasphenoid bones.
Label them. Mark on the skull with long needles and flag-labels the
sphenethmoid and the pro-otic bones.
14. Compare the skull of the rabbit and the frog; especially in regard
to the attachment of the jaw apparatus to the cranium, and other
points which distinctly characterize the higher as contrasted with the
lower vertebrata.
15. Describe the skeleton of the upper and lower jaw (a) in the frog,
(b) in the rabbit. Point out exactly what parts correspond with one
another in the two animals compared. (c) What bone in the rabbit is
generally regarded as corresponding to the quadrate cartilage of the
frog?
-The Dog-Fish_
1. _General Anatomy_
Section 1. In the dog-fish we have a far more antique type of
structure than in any of the forms we have hitherto considered.
Forms closely related to it occur among the earliest remains of
vertebrata that are to be found in the geological record. Since the
immeasurably remote Silurian period, sharks and dog-fish have
probably remained without any essential changes of condition, and
consequently without any essential changes of structure, down to
the present day. Then, as now, they dominated the seas. They
probably branched off from the other vertebrata before bone had
become abundant in the inner skeleton, which is consequently in their
case cartilaginous, with occasional "calcification" and no distinct
bones at all. Unlike the majority of fish, they possess no swimming
bladder-- the precursor of the lungs; but in many other respects,
notably in the uro-genital organs, they have, in common with the
higher vertebrata, preserved features which may have been disguised
or lost in the perfecting of such modern and specialized fish as, for
instance, the cod, salmon, or herring.
Section 2. Comparing the general build, of a dog-fish with that of a
rabbit, we notice the absence of a distinct neck, and the general
conical form; the presence of a large tail, as considerable, at first, in
diameter as the hind portion of the body, and of the first importance in
progression, in which function the four paddle-shaped limbs, the
lateral fins, simply co-operate with the median fin along the back for
the purpose of steering; and, as a consequence of the size of the
tail, we note also the ventral position of the apertures of the body. The
anus, and urinary and genital ducts unite in one common chamber,
the cloaca. Behind the head, and in front of the fore fin (pectoral fin),
are five gill slits (g.s.) leading from the pharynx to the exterior. Just
behind the eye is a smaller and more dorsal opening of the same
kind, the spiracle (sp.). On the under side of the head, in front of the
mouth, is the nasal aperture (olf.), the opening of the nasal sac,
which, unlike the corresponding organ of the air-frequenting vertebrata,
has no internal narial opening. There is, however, a groove running
from olf. to the corner of the mouth, and this, closing, in the vertebrate
types that live in air and are exposed to incessant evaporation of their
lubricating secretions, constitutes the primitive nasal passage. The
limbs are undifferentiated into upper, lower, and digital portions, and
are simply jointed, flattened expansions.
Section 3. The skin of the dog-fish is closely set with pointed
tooth-like scales, the placoid scales, and these are continued over
the lips into the mouth as teeth. Each scale consists of a base of true
bone, with a little tubercle of a harder substance, dentine, capped by
a still denser covering, the enamel. The enamel is derived from the
outer layer of the embryonic dog-fish, the epiblast, which also gives
rise to the epidermis; while the dentine and bony base arise in the
underlying mesoblast, the dermis. A mammalian tooth has
essentially the same structure: an outer coat of enamel, derived from
epiblast, overlies a mass of dentine, resting on bone, but the dentine
is excavated internally, to form a pulpcavity containing blood-vessels
and nerves. Most land animals, however, have teeth only in their
mouths, and have lost altogether the external teeth which constitute
the armour of the dog-fish. Besides the teeth there perhaps remain
relics of the placoid scales in the anatomy of the higher vertebrata, in
the membrane bones. How placoid scales may have given rise to
these structures will be understood by considering such a bone as
the vomer of the frog. This bone lies on the roof of the frog's mouth,
and bears a number of denticles, and altogether there is a very strong
resemblance in it to a number of placoid scales the bony bases of
which have become confluent. In the salamander, behind the
teeth-bearing vomers comes a similar toothed parasphenoid bone.
The same bone occurs in a corresponding position in the frog, but
without teeth. In some tailed amphibians the vomers and splenials are
known to arise by the fusion of small denticles. These facts seem to
point to stages in the fusion of placoid bases, and their withdrawal
from the surface to become incorporated with the cranial apparatus as
membrane bones, a process entirely completed in the mammalian
type.
Section 4. The alimentary canal of the dog-fish, is a simple tube
thrown into a Z shape. The mouth is rough with denticles, and has a
fleshy immovable tongue on its floor. In the position of the
Eustachian tube there is a passage, the spiracle (sp.), running out to
the exterior just external to the cartilage containing the ear. The
pharynx communicates with the exterior through five gill slits (g.s.),
and has, of course, no glottis or other lung opening. There is a wide
oesophagus passing into a U-shaped stomach (st.), having, like the
rabbit's, the spleen (sp.) on its outer curvature. There is no coiling
small intestine, but the short portion, receiving the bile duct (b.d.)
and duct of the pancreas (pan.), is called the duodenum (d'dum.). The
liver has large left (L.lv.) and right lobes, and a median lobe (M.lv.), in
which the gall bladder (g.bl.) is embedded. The next segment of the
intestine is fusiform, containing a spiral valve (Figure 4), the shelf of
which points steeply forward; it is sometimes called the colon (co.). It
is absorptive in function and probably represents morphologically, as
it does physiologically, the greater portion of the small intestine. A
rectal gland (r.g.) opens from the dorsal side into the final portion of
the canal (rectum).
Section 5. The circulation presents, in many respects, an
approximation to the state of affairs in the developing embryos of the
higher types. The heart (Figure 3, Sheet -14- {Error in First Edition}
[16]) is roughly, Z shape, and transmits only venous blood. It lies in a
cavity, the pericardial cavity (P.c.c.), cut off by a partition from the
general coelome. At one point this partition is imperfect, and the two
spaces communicate through a pericardio-peritoneal canal (p.p.c.),
which is also indicated by an arrow (p.p.) in the position and direction
in which the student, when dissecting, should thrust his "seeker," in
Figure 1 Sheet 15. A sinus venosus (s.v. in Figure 3, Sheet 16)
receives the venous trunks, and carries the blood through a valve into
the baggy and transversely extended -auricle- [atrium] (au.), whence it
passes into the muscular ventricle (Vn.), and thence into the truncus
arteriosus. This truncus consists of two parts: the first, the conus or
pylangium (c.a.), muscular, contractile, and containing a series of
valves; the second, the bulbus or synangium (b.a.), without valves and
pulsatile. In the rabbit both sinus and truncus are absent, or merged in
the adjacent parts of the heart.
Section 6. From the bulbus there branch, on either side, four arterial
trunks, the first of which forks, so that altogether there are five
afferent branchials (a.br.) taking blood to be aerated in the gills, here
highly vascular filamentary outgrowths of the internal walls of the gill
slits.
{Lines from Second Edition only.}
[There are altogether nine vascular outgrowths (demi-branchs), one
on each wall of each gill slit except the last, on the hind wall of
which there is none. (In the spiracle is a miniature demibranch, the
pseudo-branch. This suggests that the spiracle is really a somewhat
modified gill slit.)]
Four efferent branchials (e.br.) carry the aerated blood on to the
dorsal aorta (d.ao.). A carotid artery runs forward to the head, and a
hypo-branchial artery supplies the ventral side of the pharyngeal
region. There are sub-clavian, coeliac, mesenteric, and pelvic
arteries, and the dorsal aorta is continued through the length of the
tail as the caudal artery (Cd.A.).
Section 7. A caudal vein (Cd.V.), bringing blood back from the tail,
splits behind the kidneys (K.), and forms the paired renal portal
veins (r.p.v.), breaking up into a capillary system in the renal organ. A
portal vein brings blood from the intestines to the liver.
Section 8. Instead of being tubular vessels, the chief veins of the
dog-fish are, in many cases, irregular baggy sinuses. Three main
venous trunks flow into the sinus venosus. In the median line from
behind comes the hepatic sinus (H.S.); and laterally, from a dorsal
direction, the Cuvierian sinuses (C.S.) enter it. These, as the student
will presently perceive, are the equivalents of the rabbit's superior
cavae. They receive, near their confluence with the sinus venosus, the
inferior jugular vein (I.J.V.). At their dorsal origin, they are formed by
the meeting of the anterior (A.C.S.) and posterior (P.C.S.) cardinal
sinuses. The anterior cardinal sinus -is, roughly, the equivalent of the
internal jugular vein-, lies along dorsal to the gill slits (g.s.), and
receives an orbital sinus from the eye. The posterior cardinal sinus
receives a sub-clavian vein (s.c.v.) and a lateral vein (L.V.), and fuses
posteriorly with its fellow in the middle line. This median fusion is a
departure from the normal fish type. It must not be confused with the
inferior cava, which is not found in the dog-fish, the [right] posterior
cardinals representing the rabbit's azygos vein. A simplified diagram of
the circulation of a fish is given in Figure 2, Sheet 16, and this should
be carefully compared with the corresponding small figure given of the
vascular system of our other types.
{Lines from Second Edition only.}
[The blood of the dog-fish resembles that of the frog.]
Section 9. The internal skeleton, as we have said, is entirely
cartilaginous, and only those parts which are pre-formed in cartilage in
the skeletons of the higher types are represented here. The spinal
column consists of two types of vertebrae, the trunk, bearing short,
distinct, horizontally-projecting ribs (r.), and the caudal. The
diagrams of Figure 5 [(Sheet 18)] are to illustrate the structure of the
centrum of a dog-fish vertebra; C is a side view, D a horizontal median
section, A and B are transverse sections at the points indicated by -B
and A- [A and B] respectively in Figure C. -(By an unfortunate slip of
the pen in the figure, A was substituted for B; section A corresponds
to line B, and vice versa.)- The vertebrae are hollowed out both
anteriorly and posteriorly (amphi-coelous), and a jelly-like notochord
runs through the entire length of the vertebral column, being
constricted at the centres of the centra, and dilated between them.
The neural arch above the centrum, and containing the spinal cord, is
made up of neural plates (n.p.), and interneural plates (i.n.p.),
completed above by a median neural spine (n.s.). In the caudal region,
instead of ribs projecting outwardly, there are haemal processes,
inclined downwards and meeting below, forming an arch, the haemal
arch, containing the caudal artery and vein-- the vein ventral to the
artery-- and resembling the neural arch, which contains the spinal
cord above, in shape and size.
Section 10. The pectoral limb and girdle (Figure 4, Sheet 16) have
only a very vague resemblance to the corresponding structures in the
rabbit. The girdle (g.) is a transverse bar lying ventral to the
pericardial wall, and sending up a portion (sc.), dorsal to the attachment
of the limb, which answers to the scapula and supra-scapula of the forms
above the fish. Three main cartilages, named respectively the
pro- (p.p.), meso- (m.p.), and meta-pterygium, form the base of the
limb. With these, smaller cartilaginous plates, rods, and nodules
articulate, and form a flattened skeletal support for the fin.
Section 11. The pelvic girdle and limb (Figure 2, Sheet 15) are
similar in structure, but the pro-pterygium and meso-pterygium are
absent, and the cartilage answering to the meta-pterygium goes by
the name of the basi-pterygium. In the male, but not in the female, the
pelvic fins are united behind the cloaca, and there are two stiff grooved
copulatory organs, the claspers (cl. in Figure 1), which have a
cartilaginous support (cl.c.). These claspers form the readiest means
of determining the sex of a specimen before dissection.
Section 12. The skull consists of a cartilaginous cranium, and of jaw
and visceral arches. The cranium persists throughout life, in what
closely resembles a transitory embryonic condition of the higher
types. There is a nasal capsule (na.c.), a brain case proper, and
lateral otic (auditory) capsules (ot.c.) containing the internal ear.
(This should be compared with the frog's embryonic skull.) The upper jaw
has a great bar of cartilage, the palato-pterygoid, as its sole support;
the arch of premaxilla, maxilla, jugal, and squamosal-- all membrane
bones-- is, of course, not represented. In the frog this bar of cartilage
is joined directly to the otic capsule by a quadrate portion, but this is
only doubtfully represented in the dog-fish by a nodule of cartilage in
the pre-spiracular ligament (p.s.). The lower jaw is supported, by
Meckel's cartilage (M.C.). The hyoid arch consists of two main
masses of cartilage, the hyomandibular (h.m.), and the ceratohyal
(c.h.); the former of these is tilted slightly forward, so that the gill
slit between it and the jaw arch is obliterated below, and the cartilage
comes to serve as the intermediary in the suspension of the jaw from
the otic mass. There are five branchia[l] arches, made up pharyngo-,
epi- and cerato-branchials, and the ventral elements fuse in the
middle line to form a common plate of cartilage. Outside these arches
are certain small cartilages, the extra branchials (ex.b.) which,
together with certain small labials by the nostrils and at the sides of
the gape, probably represent structures of considerably greater
importance in that still more primitive fish, the lamprey. The deep
groove figured lateral to the otic capsule is the connecting line of the
orbital and anterior cardinal sinuses; the outline of the anterior
cardinal sinus in this figure and in Figure 1 is roughly indicated by a
dotted line.
Section 13. Figure 3a is a rough diagram of the internal ear-- the only
auditory structure of our type (compare Rabbit, Sheet 7). To dissect
out the auditory labyrinth without injury is a difficult performance, but
its structure may be made out very satisfactorily by paring away
successive slices of the otic mass. Such a section is shown by
Figure 3b; through the translucent hyaline cartilage the utriculus and
horizontal canal can be darkly seen. The ductus endolymphaticus
(vide Rabbit) is indicated by a dotted line in our figure. It is situated
internal to the right-angle between the two vertical canals, and
reaches to the surface of the otic capsule.
Section 14. The brain shows the three primary vesicles much more
distinctly than do our higher types. The fore-brain has large laterally
separated olfactory lobes (rh.), there are relatively small
"hemispheres" (pr.c.), the stalk of the pineal gland tilts forward, and
the gland itself is much nearer the surface, being embedded in the
cartilage of the brain case, and the pituitary body is relatively very
large, and has lateral vascular lobes on either side. Following the
usual interpretation of the parts, we find optic lobes (op.l.) as the roof
of the mid-brain, and behind a very large, median, hollow,
tongue-shaped cerebellum (c.b.). The medulla is large, and certain
lateral restiform tracts (r.t.) therein, which also occur in the higher
types, are here exceptionally conspicuous.
Section 15. The dog-fish has ten pairs of cranial nerves,
corresponding to the anterior ten of the rabbit very closely, when we
allow for the modification the latter has suffered through the
conversion of some part of the spiracular cleft to an eardrum, and the
obliteration of the post-hyoid branchial slits.
The first and second nerves are really brain lobes, and nerves of the
special senses of smell and sight respectively.
The third (oculomotor), the fourth (patheticus), and the sixth
(abducens) are distributed to exactly the same muscles of the eyeball
as they are in the rabbit.
The fifth nerve, has, in the dog-fish, as in the rabbit, three chief
branches. V.2 and V.3 fork over the mouth just as they do in the
mammal; V.1 passes out of the cranium by a separate and more
dorsal opening, and runs along a groove along the dorsal internal wall
of the orbit, immediately beneath a similar branch of VII., which is not
distinct in the rabbit. The grooves are shown in the figure of the
cranium, Sheet 18; the joint nerve thus compounded of V. and VII. is
called the ophthalmic (oph.). It is distributed to the skin above the
nose and orbit. When the student commences to dissect the head of
a dog-fish he notices over the dorsal surface of the snout an exudation
of a yellowish jelly-like substance, and on removing the tough skin
over this region and over the centre of the skull he finds, lying beneath
it, a quantity of coiling simple tubuli full of such yellowish matter.
These tubuli open on the surface by small pores, and the nerves
terminate in hair-like extremities in their lining. These sense tubes
are peculiar to aquatic forms; allied structures are found over the head
and along a lateral line (see below) in the tadpole, but when the frog
emerges from the water they are lost. They, doubtless, indicate some
unknown sense entirely beyond our experience, and either only
possible or only necessary when the animal is submerged.
In addition to the ophthalmic moiety mentioned above, the seventh
nerve has a vidian branch (vid.) running over the roof of the mouth, and
besides this its main branches fork over the spiracle, just as V. forks
over the mouth, and as IX. and X. fork over gill clefts. This nerve in the
rabbit is evidently considerably modified from this more primitive
condition.
The eighth is the auditory nerve, as in the rabbit.
The ninth nerve forks over the first branchial cleft.
The tenth nerve is easily exposed by cutting down through the body
wall muscles over the gill clefts, into the anterior cardinal sinus
(A.C.S.). It gives off (a) branches forking over the posterior four gill
slits, (b) a great lateral nerve running inward, and back through the
body-wall muscle, and connected with a line of sense organs similar
to those in the head, the lateral line, and (c) a visceral nerve curving
round to the oesophagus and stomach. In dissection it becomes very
evident that the tenth nerve is really a leash of nerves, each one
equivalent to the ninth.
We may here call the attention of the reader to the fact of the singular
resemblance of V., VII., IX., and the factors of X. That each has a
ventral fork, we have already noticed. Each also (?IX.) has a dorsal
constituent connected with the sense organs of the skin. The vidian
branch of VII., however, is not evidently represented in the others.
Section 16. The coelom of the dog-fish is peculiar-- among the types
we treat of-- in the possession of two direct communications with the
exterior, in addition to the customary indirect way through the oviduct.
These are the abdominal pores (a.p.) on either side of the cloaca in
either sex. They can always be readily demonstrated by probing out
from the body cavity, in the direction indicated by the arrow (a.p.) in
Figure 1, Sheet 15. They probably serve to equalize the internal and
external pressure of the fish as it changes its depth in the water, just
as the Eustachian tubes equalize the pressure on either side of the
mammal's tympanic membrane.
Section 17. The musculature of the dog-fish body is cut into
V-shaped segments, the point of the V being directed forward. The
segments alternate with the vertebrae, and are called myomeres.
Such a segmentation is evident, though less marked, in the body wall
muscles of the frog, and in the abdominal musculature of the rabbit
and other mammals it is still to be traced.
Section 18. The uro-genital organs of the female dog-fish (Figure 1,
Sheet 17) consist of an unpaired ovary (ov.), paired oviducts (o.d.),
enlarged at one point to form an oviducal gland (o.d.g.), kidneys (k.),
with ureters (ur.) uniting to form a urinary sinus (u.s.) opening into the
cloaca by a median urinary papilla separate from the oviducal
openings. The eggs contain much yolk, and, like those of the fowl, are
very large; like the fowl, too, one of the ovaries is suppressed, and it
is the right ovary that alone remains. The two oviducts meet in front of
the liver ventral to the oesophagus, and have there a common opening
by which the ova are received after being shed into the body cavity.
The eggs receive an oblong horny case in the oviduct; in the figure
such a case is figured as distending the duct at e. The testes of the
male (T. in Figure 2) are partially confluent in the middle line. They
communicate through vasa efferentia (v.e.) with the modified anterior
part of the kidney, the epididymis (ep.), from which the vas deferens
(v.) runs to the median uro-genital sinus (u.g.s.), into which the
ureters (ur.) also open. The silvery peritoneum (lining of the body
cavity) covers over the reddish kidneys, and hides them in
dissection.
Section 19. Figure 3, Sheet 17, is a generalized diagram of the
uro-genital organs in the vertebrata; M.L. is the middle line of the
body, G. is the genital organ, Pr. is the pronephros, or fore kidney,
a structure which is never developed in the dog-fish, but which has
functional importance in the tadpole and cod, and appears as a
transitory rudiment in the chick. A duct, which is often spoken of as
the pronephric duct (p.d.), and which we have figured under that
name, is always developed. Anteriorly it opens into the body cavity. It
is also called the Mullerian duct, and in the great majority of
vertebrata it becomes the oviduct, uniting with its fellow, in the case of
the dog-fish, ventral to the oesophagus. In the male it usually
disappears; the uterus masculinus of the rabbit is still very generally
regarded as a vestige of it. Kolliker has shown, however, that this
interpretation is improbable. Ms. is the mesonephros, some or all of
which becomes the epididymis in the male of types possessing that
organ, and is connected with G. by the vasa efferentia. Mt., the
metanephros, is, in -actual fact- [the frog], indistinguishably
continuous with Ms., and is the functional kidney, its duct
(metanephric duct) being either undifferentiated from the mesonephric
(as is the case with the frog) or largely split off from it, as in the
dog-fish, to form the ureter.
Section 20. The correspondence of the male organs of the dog-fish
with those of the rabbit, will be more evident if the student imagine--
(a) the testes, vasa efferentia, and epididymis of each side to
shift posteriorly until they reach a position on either side of
the cloaca; and
(b) The uro-genital apertures, instead of meeting dorsally and
posteriorly to the anus, to shift round that opening and meet
anteriorly and ventrically to it.
Section 21. This completes our survey of this type. Except where we
have specified differences, the general plan of its anatomy follows the
lines of the other vertebrate types described.
_Questions on the Dog-Fish_
1. Describe the alimentary canal of the dog-fish, and compare it with
that of the rabbit in detail.
2. Compare the coelom of the dog-fish and rabbit.
3. Draw diagrams to illustrate the course of the circulation in the
dog-fish.
4. (a) Describe fully the heart of a dog-fish. (b) Compare it with that of
a rabbit.
5. Give an account of the respiratory apparatus of the dog-fish.
6. Draw diagrams of a dog-fish vertebra, and compare the centrum
with that of a rabbit.
7. Compare the vertebral column of the dog-fish and rabbit.
8. Draw diagrams of the limbs and limb-girdles of the dog-fish.
Compare the pectoral with the pelvic fin.
9. Draw diagrams of (a) the male and (b) the female urogenital organs
of the dog-fish. (c) Compare them carefully with those of the rabbit.
10. Compare the circulation in the kidney of dog-fish and rabbit.
11. Give an account of the cranio-facial apparatus of the dog-fish.
State clearly what representation of this occurs in the frog and in the
rabbit.
12. Give drawing (a) from above, (b) from the side, of the dog-fish
brain.
13. State the origin and the distribution of the fifth, seventh, ninth, and
tenth cranial nerves in the dog-fish.
14. Compare, one by one, the cranial nerves of the dog-fish with those
of any higher vertebrate, as regards their origin and their distribution.
15. Describe the auditory organ of the dog-fish. What parts are added
to this in the higher type?
16. Draw the cloaca (a) of a male, (b) a female dog-fish.
17. (Practical.) Demonstrate in a dog-fish the pathetic nerve, the
opening between pericardium and coelom. the abdominal pores, and
the ureter.
-Amphioxus_
1. _Anatomy_
Section 1. We find in Amphioxus the essential vertebrate features
reduced to their simplest expression and, in addition, somewhat
distorted. There are wide differences from that vertebrate plan with
which the reader may now be considered familiar. There are no
limbs. There is an unbroken fin along the median dorsal line and
coming round along the ventral middle line for about half the animal's
length. But two lowly vertebrates, the hag-fish and lamprey, have no
limbs and a continuous fin. There is, as we shall see more clearly, a
structure, the respiratory atrium, not apparently represented in the
true vertebrate types, at least in their adult stages. There is no
distinct heart, only a debateable brain, quite without the typical
division into three primary vesicles, no skull, no structures whatever of
cartilage or bone, no genital ducts, no kidneys at all resembling those
of the vertebrata, no pancreas, no spleen; apparently no sympathetic
chain, no paired sense organs, eyes, ears, or nasal sacs, in all of
which points we have striking differences from all true vertebrata; and
such a characteristic vertebrate peculiarity as the pineal gland we can
only say is represented very doubtfully by the eye spot.
Section 2. The vertebral column is devoid of vertebrae; it is
throughout life a rod of gelatinous tissue, the notochord (Figure 1,
n.c.), surrounded by a cellular sheath. Such a rod is precursor to the
vertebral column in the true vertebrates, but, except in such lowly
forms as the lamprey, is usually replaced, partially (e.g., dog-fish)
or wholly (as in the rabbit) by at first cartilaginous vertebrae whose
bodies are derived from its sheath. Further, while in all true vertebrata
the notochord of the developing young reaches anteriorly at most to
the mid-brain, and is there at its termination enclosed by the middle
portion of the skull, in Amphioxus it reaches far in front of the anterior
extremity of the nervous system, to the end of the animal's body.*
On this account the following classification is sometimes made of
those animals which have a notochord:--
-Chordata_ (= Vertebrata, as used by Lankester).
1. Having the notochord reaching in front of the brain.
Cephalochorda = Amphioxus.
2. Having the notochord reaching anteriorly to the mid-brain, a
brain of three primary vesicles and a skull.
Craniata = all "true vertebrata": fishes, amphibia, reptiles,
birds, and mammals (Vertebrata of Balfour).
3. Having the notochord confined to the tail.
Urochorda = the ascidians, or sea-squirts, certain forms of life
only recently recognised as relatives of the vertebrata.
* The anterior end of the notochord in the developing rabbit or dog lies
where the middle of the basisphenoid bone is destined to be.
Section 3. Figure 1, Sheet 19, shows the general anatomy of
Amphioxus. We recognise four important points of resemblance to the
earlier phases of the higher and the permanent structure of the lower
members of the vertebrata, and it is these that justify the inclusion of
amphioxus in this volume. In the first place there is the--
-Notochord_.
In the next, just above it (at s.c.) we find--
-A Dorsal Tubular Nervous Axis_,
the spinal cord. Thirdly, the pharynx (ph.) is perforated by--
-Respiratory Slits_,
though these, instead of being straight slashes, are modified from
a U-shape [slant very much forward and are much more numerous than
in any true vertebrate.]. -And-, Fourthly, there is, as we shall
see, a--
-Vertebrate Type of Circulation_.
[And finally the body-wall muscles are divided into--]
[-Myomers_.]
Section 4. The alimentary canal of Amphioxus commences with an
"oral cavity," not represented in our vertebrata, surrounded by a
number of cirri, or tentacles, supported by a horny substance which
seems to be chitin, a common skeletal material among invertebrates.
A velum (v.) forms a curtain, perforated by the mouth and by two
smaller hyoidean apertures, between the oral cavity and the pharynx
(ph.). "Pharyux" is here used in a wider sense than in the true
vertebrata; it reaches back close to the liver, and is therefore
equivalent to pharynx + oesophagus + a portion or all of the stomach.
The [so-called] hyoidean apertures are not equivalent to the
similarly-named parts of the vertebrata. Behind the pharynx the
intestine (int.) runs straight out to the anus (an.), which opens not in
the middle line, as one might expect, but in the left side! The liver lies
usually on the creature's right, and instead of being a compact gland,
is simply bag-like.
Section 5. The circulation is peculiarly reduced (Figure 2). The
cardiac aorta (c.ao.) lies along the ventral side of the pharynx, and
sends branches up along the complete bars between the gill slits.
There is no -distinct- heart, but the whole of the cardiac aorta is
contractile, and at the bases of the aortic arches that run up the bars
there are contractile dilatations that assist in the propulsion of the
blood. Dorsal to the pharynx, as in fishes, there is a pair of dorsal
aorta (d.ao.) that unite above the liver (compare the frog, for instance),
and thence run backward as a median dorsal aorta (d.ao.'). A portal
vein (p.v.) bring blood back from the intestine (and apparently from the
whole posterior portion of the animal) to the liver. Thence hepatic veins
(hep.) take it to the cardiac aorta.
{Lines from First Edition only.}
-When we remember that in the embryonic vertebrate the heart is at
first a straight tube, this circulation appears even more strikingly
vertebrate in its character than before.-
Section 6. The coelom, or body cavity, of Amphioxus lies, of course,
as in the vertebrata, between the intestinal wall and the body walls,
and, just as in the vertebrata, it is largely reduced where gill slits
occur. But matters are rather complicated by the presence of an
atrial cavity round the pharynx, which is not certainly represented in
the vertebrata, and which the student is at first apt to call the body
cavity, although it is entirely distinct and different from that space. The
mutual relation of the two will become apparent after a study of
Figures 10, 11, 12 (Sheet 21). Figure 10 gives diagrammatically a
section of a very young stage of Amphioxus; P is the pharynx portion
of the alimentary canal, coe. is the coelom surrounding it at this stage
here as elsewhere; mt.c. are certain lymph spaces, the metapleural
canals, between which a small invagination (i.e., a pushing-in), at., of
the outer epidermis occurs; n.c. is the notochord, and s.c. the spinal
cord. The gill slits, by which P. communicates with the exterior, are
not shown. Next Figure 11 shows the invagination (at.) pushing its
way in, and cut off from the exterior by a meeting of the body wall
below. Note that at. is a portion of the animal's exterior thus
embraced by its body, and that its lining is therefore of the same
material as the external integument. In Figure 12, at. is developing
upward, so that the true body hangs into it. Now imagine the gill slits
perforated, as shown by the double-headed arrow in Figure 12. Figure
3, on Sheet 20, is a less diagrammatic representation of a
cross-section of the pharyngeal region (vide Figure 1, Sheet 19). The
student should compare Figure 3, Sheet 20, and Figure 12, Sheet 21.
The atrium and metapleural canals are easily recognised in both. In
Figure 3 the coelom is much cut up by the gill slits, and we have
remaining of it (a) the dorsal coelomic canals (d.c.c.) and (b) the
branchial canals (br.c.) in the bars between the slits. The atrial cavity
remains open to the exterior at one point, the atrial pore (at.p.).
Section 7. The method of examining cross-sections is an extremely
convenient one in the study of such a type as Amphioxus. The
student should very carefully go over and copy the six sections on
Sheet 20, comparing Figure 1 as he goes. He should do this before
reading what follows. One little matter must be borne in mind. These
figures are merely intended to convey the great structural ideas, and
they are considerably simplified; they must not be regarded as a
substitute for the examination of microscopic sections. [He will notice
a number of rounded masses from the body wall. The] -For instance,
the body-wall- muscles of Amphioxus are arranged in bundles bent
sharply in an arrow shape, the point forward. -A number of these
bundles are cut in any one section, and so the even shading of our
diagrams, if they professed to be anything more than diagrams,
should be broken up into masses.- These -bundles, we may mention-,
are called myomeres, and they are indicated in Figure 1 by lines
pointing acutely forward. [Several are consequently cut in any
transverse section (Sheet 20), and these are the rounded masses he
sees.] Similar myomeres, similarly situated, are found in fish, behind
the head, and, less obviously, they occur with diminishing importance
as the scale of the vertebrata is ascended.
Section 8. If we compare the nervous system of amphioxus with that
of any vertebrate, we find at once a number of striking differences. In
the first place, the skeletal covering of it, the cranium and the neural
arches of vertebrae, are represented only by a greatly simplified
connective tissue. In the next, a simple and slight anterior dilatation
alone represents the brain. A patch of black pigment anterior to this
(e.s.) may or may not be what its name implies an eye-spot. There is
a ciliated funnel, c.f. (Figure 1, Sheet 19), opening on the left side,
which has been assumed to be olfactory in its functions, and in the
mouth chamber a ciliated pit (c.p.), which may, or may not, be an
organ of taste. The ventral fissure of the spinal cord is absent. The
dorsal nerves are without ganglia, and do not come off in pairs, but
alternately, one to the left, then one to the right, one to the left, one
to the right, and so on. The ventral nerves are very short, more numerous
than the dorsal, and never unite with these latter to form mixed
nerves.
The student will observe that here, just as in the case of the ciliated
funnel and anus, the Amphioxus is not strictly symmetrical, but
twisted, as it were, and so departs from the general rule of at least
external bilateral symmetry obtaining among the vertebrates. It
habitually lies on one side in the mud of the sea bottom, and it is
probable that this external asymmetry is due to this habit, so that
too much classificatory importance must not be attached to it. The
soles and other related fish, for instance, are twisted and
asymmetrical, through a similar specific habit, to such an extent that
both eyes lie on one side of the animal.
Section 9. No kidney on the vertebrate pattern is found, but the
following structures have, among others, been suggested as renal
organs:--
(a) Certain canals, the brown tubes of Lankester (b.t.L., Figure 2,
Sheet 19), a pair of pigmented tubes opening into the atrium at the
hind end of the pharynx, lying forward along by the dorsal coelomic
canals, and having an internal opening also.
(b) Certain tubuli described by Weiss as situated in a series along
the upper corners of the atrial cavity, and communicating, after the
fashion, of the "nephridia" of the earthworm, with the coelom and with
the exterior (or, rather, with that portion of the animal's exterior
enclosed in by the atrial wall; compare Section 6).
(c) The general epithelial lining of the atrium.
The reproductive organs (Figure 4, Sheet 20, g.) are masses of
cells situated in an isolated part of the coelom in the atrial folds, and,
having no ducts, their contents must escape into the atrium by
rupture of the body-wall. Thence they escape either by gill-slits,
pharynx and mouth, or, more generally, through the atrial pore. The
animals, like all the vertebrata, are dioecious, i.e., male or female.
Section 10. The endostyle (end.), in Figures 3 and 4, is a ciliated
path or groove on the under side of the pharynx, which is generally
supposed to represent the thyroid gland of vertebrates. The vertebrate
thyroid, early in development, is certainly an open and long narrow
groove in the ventral side of the pharynx. The hyper-pharyngeal
groove (h.p.) has been in the past compared to the pituitary body,
but there is little doubt now that this structure is represented by the
ciliated pit.
Section 11. The student is advised to revise this chapter before
proceeding, and to schedule carefully the anatomical features under
the headings of (1.) distinctly vertebrate characters, (2.) characters
contrasting with the normal vertebrate structure, (3.) facts of doubtful
import, with the suggestions given in the text written against them.
2. _The Development of Amphioxus_
Section 12. The development of amphioxus, studied completely, is at
once one of the most alluring and difficult tasks in the way of the
zoologist; but certain of its earlier and most obvious fasts may very
conveniently be taken into consideration now.
Section 13. The phenomena of the extrusion of polar bodies and
fertilization are treated of later, and will, therefore, not be considered
now. We will start our description with an egg-cell, which has
escaped, of course, since there are no genital ducts, by rupture of the
parent, has been fertilized by the male element, and is about to
develop into a young amphioxus. It is simply a single cell, with some
power of amoeboid motion, a single nucleus and nucleolus; and in
amphioxus its protoplasm is clear and transparent. Frequently ova are
loaded with granules of food store (yolk), which enable the young
animal to go far with its development before it is hatched and has to
begin fending for itself. Such an ovum as that of our present type,
however being devoid of such yolk (alecithal = without yolk),
necessitates a very early start in life, and, for reasons too
complicated to state fully here, the development in such a case is
considered particularly instructive and primitive by zoologists.
Section 14. The first thing to be seen in the developing cell is a
deepening circular groove (Figure 1, Sheet 21), which divides the
ovum into two parts. Another groove then cuts at right angles to this
subdividing the two into four (Figure 2). Another groove, at right angles
to both the former, follows, making the four eight (Figure 3). And so
subdivision goes on. The whole process is called segmentation or
cleavage.
Section 15. At the end of segmentation we get a hollow sphere of
small cells, the cells separating from one another centrally and
enclosing a cavity as the process proceeds. This is the
blastosphere, shown diagrammatically in Figure 4, and of which an
internal view, rather truer to the facts of the case as regards shape,
is given as Figure 5. The central cavity is the segmentation cavity
(s.c.).
Section 16. Invagination follows (Figure 6). In this process a portion
of the blastosphere wall is the tucked into the rest, as indicated by
the arrow, so that a two-layered sack is formed. The space ar. is the
archenteron, the primordial intestine, and its mouth is called, the
blastopore (bp.). The outer layer of this double-walled sac is called
the epiblast. For the present we will give the inner lining no special
term. The young amphioxus has, at this stage, which is called the
gastrula stage, a curious parallelism with such a lowly form as the
Hydra of our ditches. This latter creature, like the gastrula, consists
essentially of two layers of cells, an outer protective and sensory
layer, and an inner digestive one; it has a primordial intestine, or
archenteron, and its mouth is sometimes regarded as being a
blastopore. All animals that have little yolk, and start early in life for
themselves, pass through a gastrula stage, substantially the same
as this of amphioxus.
Section 17. The anus is perforated later near the region occupied at
this stage by the blastopore. Hence the anterior end of the future
amphioxus, the head end, is pointing towards the Figure 6, and the
letters ep. are marked on the side which will be dorsal.
Section 18. Figure 7 i. is a dorsal view of the gastrula at a somewhat
later stage, and here indications of distinctly vertebrate relationships
already appear. Figure 7 ii. is a cross-section, its position, being
shown by cross-lines in 7 i. and 6. Note first that the epiblast along
the mid-dorsal line is sinking in to form what is called the neural
plate (n.p.), and simultaneously on either side of it rise the neural
folds (n.f.). Now, at Figure 8, a slightly later stage is represented, and
at 9 i. the inturned part is separated from the general external epiblast
as the spinal cord. The remainder of the epiblast constitutes the
epidermis.
Section 19. Reverting to Figure 7 ii., along the dorsal side of the
archenteron a thickening of its wall appears, and is gradually pinched
off from it to form a cellular rod, lying along under the nervous axis and
above the intestine. This is the notochord (compare Figures 8 and 9).
Section 20. Finally, we note two series of buds of cells, one on either
side of the archenteron in Figure 7 ii. In 8 these buds have become
hollow vesicles, growing out from it, the coelomic pouches. They are
further developed in 9; and in 9 ii., which is a diagrammatic figure,
they are indicated by dotted lines. They finally appear to (? entirely)
obliterate the segmentation cavity-- they certainly do so throughout
the body; and their cavities are in time cut off from the mesenteron, by
the gradual constriction of their openings. In this way the coelom
(body cavity) arises as a series of hollow "archenteric" outgrowths,
and ms. becomes the alimentary canal. mt.c., the metapleural
canals, probably arise subsequently to, and independently of, the
general coelomic space, by a splitting in the body-wall substance.
Section 21. Hence, in considering the structure of amphioxus, we
have three series of cells from which its tissues are developed:--
1. The epiblast.
2. Walls of the coelomic pouches, which form (a) an inner lining
to the epiblast, (b) an outer coating to the hypoblast, and (c) the
mesentery (m.), by which the intestine is supported. This is the
mesoblast.
3. The lining of the mesenteron, or hypoblast.
From the epiblast the epidermis (not the dermis), the nervous system
(including the nerves), and the sensory part of all sense organs are
derived. From the mesoblast the muscles, the dermis genital and
excretory organs, circulatory fluid and apparatus, any skeletal
structures; and all connective tissue are derived. The mass of the
body is thus evidently made of mesoblast. The hypoblast is the lining
of the intestine and of the glands which open into it; and
the material of the notochord is also regarded, as hypoblast.
Section 22. Figure 9 ii. shows all the essential points of the structure
of amphioxus. Epiblast is indicated by a line of dashes, mesoblast
by dots, and hypoblast, dark or black. The true mouth is formed late
by a tucking-in of epiblast, the stomodaeum (s.d.), which meets and
fuses with the hypoblast, and is then perforated. The position of this
mouth is at the velum. The formation of the atrium has been
described. The metapleural folds run forward in front of the velum, as
the epipleurs (ep. in Sections 1 and 2), and form an oral hood (b.c.),
around which the tentacles appear, and which is evidently not
equivalent to the vertebrate mouth cavity, but in front of and outside
it. The anus is formed by a tucking in, the proctodaeum, similar to the
stomodaeum.
Section 23. The formation of the respiratory slits is complicated, and
difficult to describe, but, since investigators have still to render its
meaning apparent, it need not detain the elementary student.*
* See Balfour's Embryology, Volume 2, and Quarterly Journal of
Microscopical Science March, 1891.
_Questions on Amphioxus_
1. Draw diagrams, with the parts named, of the alimentary canal of (a)
amphioxus, (b) any craniate; (c) indicate very shortly the principal
structural differences between the two.
2. Describe, with a diagram, the circulation of amphioxus. Compare it
with that of the craniata.
3. Draw from memory transverse sections, of amphioxus (a) in the
oral region, (b) through the pharynx, (c) just anterior, and (d) just
posterior to atrial pore.
4. Describe fully the coelom of amphioxus, and compare it with that of
the frog in regard to (a) development, (b) its relation to other organs in
the adult.
5. Compare the atrial cavity and coelom of amphioxus. To what series
of cavities in the frog are the metapleural canals to be compared?
6. Describe the notochord of amphioxus, and point out its differences
from the vertebrate notochord.
7. Describe, with diagrams, the nervous system of amphioxus, and
compare its nervous axis, in detail, with that of a vertebrate.
8. Compare the genital organs of amphioxus with those of a higher
vertebrate.
9. What structures have been regarded, as renal organs in
amphioxus?
10. What is a gastrula? With what lower type has the gastrula been
compared? Discuss the comparison.
-Development_
_The Development of the Frog_
Section 1. We have now to consider how the body of the frog is built
up out of the egg cell, but previously to doing so we must revert to the
reproductive organs of our type.
Section 2. In the testes of the male is found an intricate network of
tubuli, the lining of which is, of course, an epithelium. The cells of this
epithelium have their internal borders differentiated into spermatozoa,
which, at a subsequent stage, are liberated. A spermatozoon (Figure
3, Sheet 13, sp.) is a rod-shaped cell containing a nucleus; in fact,
consisting chiefly of nucleus, with a tail, the flagellum, which is
vibratile, and forces the spermatozoon, forward by its lashing. The
spermatozoa float in a fluid which is the joint product of the testes,
anterior part of the kidney, and perhaps the prostate glands.
Section 3. In the ovary, the ova are formed, and grow to a
considerable size. They are nucleated cells, the nucleus going by
the special name of the germinal vesicle and the nucleolus the
germinal spot. The ova prey upon the adjacent cells as they develop.
The protoplasm of the ovum, except at that part of the surface where
the germinal vesicle lies, is packed with a great amount of food
material, the yolk granules. This yolk is non-living inert matter. An
ovum such as this, in which the protoplasm is concentrated towards
one pole, is called telolecithal.
Section 4. After the ovum has finished its growth, and elaborated the
yolk within itself, a peculiar change occurs in the small area free from
yolk-- the animal pole, in which the germinal vesicle lies. This
germinal vesicle divides, and one moiety is budded off from the ovum.
The ovum has, in fact, undergone cell division into a very large cell
containing most of its substance, and a small protoplasmic pimple
surrounding half of its nucleus. The disproportion is so great between
the two cells, that the phenomenon does not at first suggest the idea
of cell division, and it is usually described as the extrusion of the
first polar body. There follows a second and similar small cell,
behind the first, the second polar body. Since the nucleus of the
ovum has divided twice, it is evident that the nucleus remaining now in
the ovum is a quarter of the original nucleus. Very little protoplasm is
given off with the polar bodies; they play no further part in
development, but simply drop off and disappear. Not only in the frog's
ovum, but in all vertebrata, two polar bodies are given off in this way
before the sexual process occurs. Their exact meaning has been
widely discussed. It is fairly evident that some material is removed
from the nucleus, which would be detrimental to further developments,
and the point debated is what is the precise nature of this excreted
material. This burning question we can scarcely deal with here.
Section 5. But here we may point out that in all cells the function of
the nucleus appears to be to determine growth and division. It is the
centre of directive energy in the cell.
Section 6. Fertilization is effected by a spermatozoon meeting with
the ovum. It fuses with it, its nucleus becoming the male pro-nucleus.
This and the female pro-nucleus, left after the extrusion of the polar
cells, move towards each other, and unite to form the first
segmentation nucleus.
Section 7. The ovum next begins to divide. A furrow cutting deeper
and deeper divides it into two; another follows at right angles to this,
making the two four, and another equatorial furrow cuts off the animal
pole from the yolk or vegetative pole. (See Sheet 22, Figures 1, 2,
and 3.) And so segmentation (= cleavage) proceeds, and, at last, a
hollow sphere, the blastosphere (Figure 4) is formed, with a
segmentation cavity (s.c.). But, because of the presence of the yolk
at the vegetative pole of ovum, and of the mechanical resistance it
offers to the force of segmentation, the protoplasm there is not nearly
so finely divided-- the cells, that is to say, are much larger than at
the animal pole. The blastosphere of the frog is like what the
blastosphere of amphioxus would be, if the future hypoblast cells
were enormously larger through their protoplasm being diluted with
yolk.
Section 8. The next phase of development has an equally curious
resemblance to and difference from what occurs in the case of the ova
of animals which do not contain yolk. In such types (e.g., amphioxus)
a part of the blastosphere wall is tucked into the rest, and a gastrula
formed by this process of invagination. In the frog (Figure 5) there is a
tucking-in, but the part that should lie within the gastrula, the
yolk-containing cells, are far larger than the epiblast (ep.) which
should, form the outer layer of cells. Hence the epiblast can only by
continual growth accommodate what it must embrace, and the
process of tucking-in is accompanied by one of growth of the epiblast,
as shown by the unbarbed arrow, over the yolk. This stage is called
the gastrula stage; ar. is the cavity of the gastrula, the archenteron;
b.p. is its opening or blastopore. Such a gastrula, formed mainly by
overgrowth of the epiblast, is called an epibolic gastrula, as
distinguished from the invaginate gastrula of amphioxus. The
difference is evidently entirely due to the presence of yolk, and the
consequent modification of invagination in the former case.
Section 9. Comparing the two gastrulas, it is not difficult to see that if
we imagine the ventral wall of the archenteron of amphioxus to have
its cells enormously enlarged through the mixing of yolk with their
protoplasm, we should have a gastrula essentially like that of the frog.
Section 10. Figure 6 shows a slightly later ovum than Figure 5, seen
from the dorsal side. b.p. is the blastopore. In front of that appears a
groove, the neural groove, bordered on either side by a ridge, the
neural fold (n.f.). This is seen in section in Figure 7; s.c. is the neural
groove; n.f., as before, the neural fold. The neural folds ultimately bend
over and meet above, so that s.c. becomes a canal, and is finally
separated from the epiblast to form the spinal cord. Below the neural
groove a thickening of the dorsal wall of the archenteron appears, and
is pinched off to form a longitudinal rod, the precursor of the vertebral
column, the notochord, shown in Figure 7 (n.c.), as imperfectly
pinched off.
Section 11. Simultaneously, on either side of the notochord appear a
series of solid masses of cells, derived mainly by cell division from the
cells of the wall of the archenteron, and filling up and obliterating the
segmentation cavity. These masses increase in number by the
addition of fresh ones behind, during development, and are visible in
the dorsal view as brick-like masses, the mesoblastic somites or
proto-vertebrae (Figure 6, i., ii., iii.). In Figure 7, these masses are
indicated by dotting. In such a primitive type as amphioxus these
mesoblastic -somites- [masses] contain a cavity, destined to be the
future body cavity, from the first. In the frog, the cavity is not at first
apparent; the mesoblast at first seems quite solid, but subsequently
what is called the splitting of the mesoblast occurs, and the body
cavity (b.c. in Figure 7) appears. The outer mesoblast, lying
immediately under the epiblast, constitutes the substance of the
somatopleur, and from it will be formed the dermis, the muscles of the
body wall, almost all the cartilage and bone of the skeleton, the
substance of the limbs, the kidneys, genital organs, heart and
bloodvessels, and, in short, everything between the dermis and the
coelom, except the nervous system and nerves, and the notochord.
The inner mesoblast, the mass of the splanchnopleur, will form the
muscle and connective tissue of the wall of the alimentary canal, and
the binding substance of the liver and other glands that open into the
canal.
Section 12. Figure 8 is one which we reproduce, with the necessary
changes in each plate of embryological figures given in this book, so
that the student will find it a convenient, one for the purpose of
comparison. The lines of dashes, in all cases, signify -epiblast-
[hypoblast] , the unbroken black line is -hypoblast-, [epiblast] dotting
shows mesoblast, and the shaded rod (n.c.) is the notochord. c.s. is
the spinal cord; br.1, br.2, br.3 are the three primary vesicles which
constitute the brain, and which form fore, mid, and hind brain
respectively. I. is the intestine and Y. the yolk cells that at this early
stage constitute its ventral wall.
Section 13. Figure 9 gives a similar diagram of a later stage, but here
the blastopore is closed. An epiblastic tucking-in at st., the
stomodaeum pre-figures the mouth; pr., the proctodaeum, is a
similar posterior invagination which will become the anus. Y., the
yolk, is evidently much absorbed. Figure 10 is a young tadpole, seen
from the side. The still unabsorbed yolk in the ventral wall of the
mesentery gives the creature a big belly. Its mouth is suctorial at this
stage, and behind it is a sucker (s.) by which the larvae attach
themselves to floating reeds and wood, as shown in the three black
figures below.
Section 14. We may now consider the development of the different
organs slightly more in detail, though much of this has already been
approached. The nervous system, before the closure of the neural
groove, has three anterior dilatations, the fore-, mid-, and hind-brains,
the first of which gives rise by hollow outgrowths to two pairs of lateral
structures, the hemispheres and the optic vesicles. The latter give rise
to the retina and optic nerve as described in {Development} Section
40.
Section 15. The hypoblastic notochord is early embraced by a
mesoblastic sheath derived from the protovertebrae. This becomes
truly cartilaginous, and at regular intervals is alternately thicker and
thinner, compressing the notochord at the thicker parts. Hence the
notochord has a beaded form within this, at first, continuous
cartilaginous sheath. This sheath is soon cut into a series of
vertebral bodies by jointings appearing through the points where the
cartilage is thickest and the notochord most constricted. Hence what
remains of the notochord lies within the vertebral bodies in the frog;
while in a cartilaginous fish, such as the dog-fish, or in the embryonic
rabbit, the lines of separation appear where the notochord is thickest,
and it comes to lie between hollow-faced vertebrae. Cartilaginous
neural arches and spines, formed outside the notochordal sheath,
enclose the spinal cord in an arcade. The final phase is ossification.
As the tadpole approaches the frog stage the vertebral column in the
tail is rapidly absorbed, and its vestiges appear in the adult as the
urostyle.
Section 16. The development of the skull is entirely dissimilar to that
of the vertebral column. It is shown on Figures 1 and 8, Sheet 14; and
in the section devoted to the frog's skull a very complete account of
the process is given. The process of ossification is described under
the histology of the Rabbit.
Section 17. The origin of the circulatory and respiratory organs is
of especial interest in the frog. In the tadpole we have essentially the
necessities and organization of the fish; in the adult frog we have a
clear exposition of the structure of pigeon and rabbit. The tadpole has,
at first, a straight tubular heart, burrowed out in somatic mesoblast,
and produced forward into a truncus arteriosus. From this arise four
afferent branchial arteries, running up along the sides of the four
branchial arches, and supplying gills. They unite above on either side
in paired hyper-branchial arteries, which meet behind dorsal to the
liver, to form a median dorsal aorta. Internal and external carotid
arteries supply the head. These four afferent branchial arches are
equivalent to the first four of the five vessels of the dog-fish. At first,
the paired gills are three in number, external, and tree-like, covered by
epiblast (Figures 10 and 11, e.g.), and not to be compared to fish gills
in structure, or in fact -with- [to] any other gills within the limits of
the vertebrata. Subsequently (hypoblastic) internal gills (int.g., Figure
12), strictly homologous with the gills of a fish, appear. Then
a flap of skin outside the hyoid arch grows back to cover over the gills;
this is the operculum (op. in Figures 11 and 12, Sheet 22), and it
finally encloses them in a gill chamber, open only by a pore on the
left, which resembles in structure and physiological meaning, but
differs evidently very widely in development, from the amphioxus
atrium. At this time, the lungs are developing as paired hollow
outgrowths on the ventral side of the throat (Figure 12, L.). As the
limbs develop, and the tail dwindles, the gill chamber is obliterated.
The capillary interruptions of the gills on the branchial arches (aortic
arches) are also obliterated. The carotid gland occupies the position
of the first of these in the adult. The front branchial arch here, as in
all higher vertebrata, becomes the carotid arch; the lingual represents
the base of a pre-branchial vessel; the second branchial becomes the
aortic arch. The fourth loses its connection with the dorsal aorta, and
sends a branch to the developing lung, which becomes the pulmonary
artery. The third disappears. A somewhat different account to this is
still found in some text-books of the fate of this third branchial arch.
Balfour would appear to have been of opinion that it gave rise to the
cutaneous artery, and that the third and fourth vessels coalesced to
form the pulmocutaneous, the fourth arch moving forward so as to
arise from the base of the third; and most elementary works follow
him. This opinion was strengthened by the fact that in the higher
types (reptiles, birds, and mammals) no fourth branchial arch was
observed, and the apparent third, becomes the pulmonary. But it has
since been shown that a transitory third arch appears and disappears
in these types.
Section 18. The origin of the renal organ and duct has very
considerable controversial interest.* In Figure 13, Sheet 22, a
diagrammatic cross-section, of an embryo is shown. I. is the
intestine, coe. the coelom, s.c. the spinal cord; n.c. the notochord,
surrounded by n.s., the notochordal sheath, ao. is the dorsal aorta.
In the masses of somatic mesoblast on either side, a longitudinal
canal appears, which, in the torpedo, a fish related to the dog-fish,
and in the rabbit, and possibly in all other cases, is epiblastic in
origin. This is the segmental duct, which persists, apparently, as the
Wolffian duct (W.D.). Ventral to this appears a parallel canal, the
Mullerian duct (M.D.), which is often described as being split off from
the segmental duct, but which is, very probably, an independent
structure in the frog. A number of tubuli, at first metamerically
arranged, now appear, each opening, on the one hand, into the
coelom by a ciliated mouth, the nephrostome (n.s.), and on the other
into the segmental duct. These tubuli are the segmental tubes or
nephridia. There grows out from the aorta, towards each, a bunch, of
bloodvessels, the glomerulus (compare Section 62, Rabbit). These
tubuli ultimately become, in part, the renal tubuli, so that the primitive
kidney stretches, at first, along the length of the body cavity from the
region, of the gill-slits backward. The anterior part of the kidney, called
the pronephros, disappears in the later larval stages. Internal to the
kidney on either side there has appeared a longitudinal ridge, the
genital ridge (g.r.), which gives rise to testes or ovary, as the case
may be.
* In the discussion whether the vertebrata have arisen from some
ancestral type, like the earthworm, metamerically segmented, and of
fairly high organization, or from a much lower form, possibly even from
a coelenterate. Such a discussion is entirely outside the scope of the
book, though its mention is necessary to explain the importance given
to these organs.
Section 19. The student should now compare the figures on Sheet 17.
In the male, tubular connections are established between the testes
and the middle part of the primitive kidney (mesonephros). These
connections are the vasa efferentia (v.e.), and the mesonephros is
now equivalent to the epididymis of the rabbit. The Wolffian duct is the
urogenital duct of the adult, and the Mullerian duct is entirely
absorbed, or remains, more or less, in exceptional cases.
In the female, the Mullerian duct increases greatly in length-- so that
at sexual maturity its white coils appear thicker and longer than
the intestine-- and becomes the oviduct; the Wolffian duct is the
ureter, and the mesonephros is not perverted in function from its
primary renal duty.
Section 20. Tabulating these facts--
In the adult male:
Pronephros disappears.
The Mullerian duct (? = pronephric duct) disappears.
Mesonephros = Epididymis; its duct, the urogenital.
Metanephros and duct, not clearly marked off from
Mesonephros.
(Compare Dog-fish, Section 19.)
In the adult female:
Pronephros disappears.
The Mullerian duct, the oviduct.
Mesonephros and Metanephros, the kidney, and their unseparated
ducts, the ureters.
Section 21. Hermaphrodism (i.e., cases of common sex) is
occasionally found among frogs; the testis produces ova in places,
and the Mullerian duct is retained and functional. The ciliated
nephrostomata remain open to a late stage of development in the frog,
and in many amphibia throughout life. Their connection with the renal
tubuli is, however, lost.
Section 22. The alimentary canal is, at first, a straight tube. Its
disproportionate increase in length throws it into a spiral in the tadpole
(int. Figure 11), and accounts for its coiling in the frog. The liver and
other digestive glands are first formed, like the lungs, as hollow
outgrowths, and their lining is therefore hypoblastic. The greatest
relative length of intestine is found in the tadpole, which, being a
purely vegetable feeder, must needs effect the maximum amount of
preparatory change in its food.
_The Development of the Fowl_
Section 23. The frog has an ovum with a moderate allowance of yolk,
but the quantity is only sufficient to start the little animal a part
of its way towards the adult state. The fowl, on the contrary, has an
enormous ovum, gorged excessively, with yolk, and as a
consequence the chick is almost perfected when it is hatched. The
so-called yolk, the yellow of an egg, is the ovum proper; around that is
a coating of white albumen, in a shell membrane and a shell. At either
end of the yolk (Figure 1, y.) twisted strands of albuminous matter,
the chalazae (ch.) keep the yolk in place. The animal pole is a small
grey protoplasmic area, the germinal area (g.a.), on the yolk.
Section 24. We pointed out that the presence of the yolk in the frog's
egg led to a difference in the size of the cells at the animal and
vegetable poles. The late F.M. Balfour, borrowing a mathematical
technicality, suggested that the rate of segmentation in any part of an
ovum varies inversely with the amount of yolk. In the fowl's egg,
except just at the germinal area, the active protoplasm is at a
minimum, the inert yolk at a maximum; the ratio of yolk to protoplasm
is practically infinity, and the yolk therefore does not segment at all.
The yolk has diluted the active protoplasm so much as to render its
influence inappreciable. The germinal area segments, and lies upon
the yolk which has defeated the efforts of its small mingling of
protoplasm to divide. Such a type of segmentation in which only part
of the ovum segments is called meroblastic. If we compare this with
the typical blastosphere of the lower type, we see that it is, as it
were, flattened out on the yolk. This stage is shown in section in the
lower figure of Figure 1. b.d., the blastoderm, is from this point of
view, a part of the ripped and flattened blastosphere, spread out on
the yolk; s.c. is the segmentation cavity, and y. the yolk.
Section 25. There is no open invagination of an archenteron in the
fowl, as in the frog--, the gastrula, like the blastosphere, stage is
also masked. But, in the hinder region of the germinal area, a thick
mass of cells, grows inward and forward, and, appearing in the
dorsal view of the egg as a white streak, is called the primitive
streak (p.s.). By a comparison of the figures of frog and fowl the
student will easily perceive the complete correspondence of the
position of this with the blastopore of the frog. The relation of the two
will be easily understood if we compare the fowl's archenteron to a
glove-finger under pressure-- its cavity is obliterated-- and the frog's
to the glove-finger blown out. The tension of the protoplasm,
straining over the enormous yolk, answers to the pressure. The
gastrula in the fowl is solid. The primitive streak is, in fact, the scar
of a closed blastopore. As we should expect from this view of its
homology, at the primitive streak, the three embryonic layers are
continuous and indistinguishable (Figure 2). Elsewhere in the
blastoderm they are distinctly separate. Just as the yolk cells of the
frog form the ventral wall of the intestine, so nuclei appear along the
upper side of the yolk of the fowl, where some protoplasm still
exists, and give rise to the ventral hypoblastic cells. By conceiving a
gradually increasing amount of yolk in the hypoblastic cells in the
ventral side of the archenteron, the substantial identity of the
gastrula stage in the three types, which at first appear so strikingly
different, will be perceived. Carry Figures 4 and 5 of the frog one
step further by increasing the size of the shaded yolk and leaving it
unsegmented, and instead of ar. in 5 show a solid mass of cells,
and the condition of things in the fowl would at once be rendered.
Section 26. Figure 3a of the fowl will conveniently serve for
comparison with Figure 7 of the frog. The inturning of the medullary
groove is entirely similar in the two cases. The mesoblast appears as
solid mesoblastic somites. In the section above Figure 4 this layer is
shown as having split into somatopleur (so.) and splanchnopleur
(spch.). Figure 3 answers to Figure 6 of the frog, and Figure 4 is a
later stage, in which the medullary groove is beginning to close at its
middle part. The clear club-shaped area around the embryo (a.p.) is
the area pellucida; the larger area without this is the area opaca
(a.o.), in which the first bloodvessels arise by a running together and
a specialization of cells. The entire germinal area grows steadily at its
edges to creep over and enclose the yolk.
Section 27. So far, the essential differences between the development
of fowl and frog, the meroblastic segmentation, absence of a typical
gastrula, and the primitive streak, seem comprehensible on the theory
that such differences are due to the presence of an enormous amount
of yolk. Another difference that appears later is that, while the tadpole
has an efficient pronephros, the fowl, which has no larval (free
imperfect) stages in its life history, has the merest indication of such
a structure.
Section 28. Another striking contrast, due to, or connected with, this
plethora of yolk, is the differentiation of a yolk sac (= umbilical
vesicle) and the development of two new structures, the amnion and
allantois, in the fowl. If the student will compare Figure 10 of the frog,
he will see that the developing tadpole encloses in its abdomen all the
yolk provided for it. This is a physical impossibility in the fowl. In the
fowl (Figure 2, Sheet 24) the enormous yolk (Y.) lies outside of the
embryo, and, as the cells of the germinal area grow slowly over
it, umbilical bloodvessels are developed to absorb and carry the
material to the embryo. In the case of an embryo sinking in upon, as
it absorbs, this mass of nutritive material, a necessity for some
respiratory structure is evident. From the hinder end of the fowl's
intestine, in a position corresponding to the so-called, urinary bladder
of the frog, a solid outgrowth, the allantois, which speedily becomes
hollow, appears. Early stages are shown in Figures 1 and 2, Sheet
24 (al.); while the same thing is shown more diagrammatically on
Sheet 23, Figure 6 (all.). This becomes at last a great hollow sac,
which is applied closely to the porous shell, and the extent of which
will be appreciated by looking at Figure 5, Sheet 24, where the
allantois is shaded. Allantoic bloodvessels ramify thickly over its
walls, and aeration occurs through the permeable shell.
Section 29. The nature of the amnion will be understood by following
Figures 4b, 5, and 6 on Sheet 23. The three embryonic layers are
indicated by broken lines, dots, and black lines, just as they are in
the frog diagrams. Not only is the embryo slowly pinched off from the
yolk sac (y.s.), but, as the yolk is absorbed beneath it, and it grows
in size, it sinks into the space thus made, the extra-embryonic
somatopleur and epiblast rise up round it as two folds, which are
seen closing in 5, and closed in 6, over the dorsal side of the young
chick. In this way a cavity, a., lined by epiblast, and called the
amniotic cavity, is formed. Dorsal to this, in 6, comes a space lined
by somatic mesoblast, and continuous with p.p., the
pleuro-peritoneal cavity, or body cavity of the embryo. Outside this,
again, is a layer, of somatopleur internally and epiblast externally, the
false amnion (f.a.), which is continuous with the serous membrane
(s.m.) enclosing the rest of the egg. The student should, carefully
copy these diagrams, with coloured pencils or inks for the different
layers, and should compare them with the more realistic renderings of
Figures 2, 5, and 8, Sheet 24.
Section 30. The heart in the fowl appears first as a pair of vessels,
which unite to form a straight trunk in the median line, as the
flattened-out embryo closes in from the yolk. The way in which this
straight trunk is thrown, first of all, into the S shape of the fish heart,
and then gradually assumes the adult form, is indicated roughly by
Figure 3. In one respect the development of the heart does not follow
the lines one would expect. Since, between the fish and the higher
form comes the condition of such an animal as the frog, in which the
auricles are divided, while there is only one ventricle, we might expect
a stage in which the developing chick's heart would have one ventricle,
and a septum between the auricles. But, as a matter of fact, the
ventricles in fowl and rabbit are separated first, and the separation of
the auricles follows, and is barely complete at birth.
Section 31. Two vitelline veins from the yolk sac (v.v.) flow into the
heart from behind, as shown in Figure 1. A later more complete and
more diagrammatic figure of the circulation is seen in Figure 7. At first
there are two anterior cardinal (a.c.), and two posterior cardinal veins
(p.c.) uniting to form Cuvierian sinuses (c.s.) that open into the heart
just as in the dog-fish. But later the inferior cava is developed and
extends backward, the posterior cardinals atrophy, the Cuvierian
sinuses become the superior cavae, and the anterior cardinals the
internal jugular veins. The vitelline veins (v.v.) flow, at first,
uninterruptedly through the liver to the inferior cava, but, as
development proceeds, a capillary system is established in the liver,
and the through communication, the ductus venosus, is reduced-- at
last-- completely. Bearing in mind that the yolk is outside the body in
the fowl and inside it in the frog, the vitelline veins of the former
have a considerable resemblance in position, and in their relation to the
portal vein, to a portion of the single anterior abdominal vein. Blood is
taken out to the allantois, however, by the arteries of the latter type.
Section 32. Five aortic arches are generally stated to appear
altogether in the fowl, but not simultaneously. The first two, the
mandibular and the hyoid vascular arches, early disappear, and are
not comparable to any in the frog. The third is the first branchial arch,
and, like the corresponding arch in the frog, forms the carotid artery;
the second branchial is the aortic arch; and what has hitherto been
regarded as the third (the fifth arch, i.e.) the pulmonary artery. A
transitory arch, it is now known, however, appears between the
second branchial and the last, and it is therefore the fourth branchial
arch which is the pulmonary, just as it is in the frog.
Section 33. Blood, it may be mentioned, first appears in the area
vasculosa, the outer portion of the area opaca. Embryonic cells send
out processes, and so become multipolar; the processes of adjacent
cells coalesce. The nucleus divides, and empty spaces appear in the
substance of each of the cells.
In this way, the cavities of the smaller vessels and capillaries are
formed, and the products of the internal divisions of the cells become
the corpuscles within the vessels. The red blood corpuscles of the
rabbit, it may be added, are nucleated for a considerable portion of
embryonic life. Larger vessels and the heart are burrowed, as it were,
out of masses of mesoblast cells. The course of the blood in the
embryo is by the veins to the right auricle, thence through the
imperfection of the auricular septum already alluded to, into the left
auricle. Then the left ventricle, aortic arches (for the future pulmonary
artery is in communication by a part presently blocked, the ductus
arterious, with the systemic aorta), arteries, capillaries, veins. The
liver capillary system and the pulmonary system only become
inserted upon the circulation at a comparatively late stage.
Section 34. With the exception of the reduction of the pronephros,
what has been said of the development of the frog's nervous system,
renal and reproductive organs, and skeleton, applies sufficiently to the
fowl for our present purposes. The entire separation of Wolffian and
Mullerian ducts from the very beginning of development is here beyond
all question (vide Section 18). But the notochord in the fowl is not so
distinctly connected with the hypoblast, and so distinct from the
mesoblast, as it is in the lower type, and no gills, internal or external,
are ever developed. The gill slits occur with a modification due to the
slitting and flattening out of the embryo, already insisted upon; for,
whereas in the tadpole they may be described as perforations, in the
fowl they appear as four notches between ingrowing processes that
are endeavouring to meet in the middle line.
_The Development of the Rabbit_
Section 35. The early development of the rabbit is apt to puzzle
students a little at first. We have an ovum practically free from yolk
(alecithal), and, therefore, we find it dividing completely and almost
equally. We naturally assume, from what we have learnt, that the next
stages will be the formation of a hollow blastosphere, invagination, a
gastrula forming mesoblast by hollow outgrowths from the
archenteron, and so on. There is no yolk here to substitute epiboly
(Section 9) for invagination, nor to obliterate the archenteron and the
blastopore through its pressure.
Yet none of these things we have anticipated occur!
We find solid mesoblastic somites, we find primitive streak, allantois
and amnion, features we have just been explaining as the
consequence of an excess of yolk in the egg. We even find a yolk sac
with no yolk in it.
Section 36. A solid mass of cells is formed at the beginning, called a
morula, Figure 1. In this we are able to distinguish rather smaller
outer layer cells (o.l.c.), and rather larger inner layer cells (i.l.c.),
but these cells, in their later development, do not answer at all to the
two primitive layers of the gastrula, and the name of Van Beneden's
blastopore (V.B.b.), for a point where the outer layer of cells is
incomplete over the inner, only commemorates the authorship of a
misnomer. The uniformity, or agreement, in the development of our
other vertebrate types is apparently departed from here.
{Illustration: Development Section 36.}