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 s.e.xual 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 inv.a.g.i.n.ation. 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 inv.a.g.i.n.ate gastrula of amphioxus. The difference is evidently entirely due to the presence of yolk, and the consequent modification of inv.a.g.i.n.ation 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 ca.n.a.l, 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 ma.s.ses 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 ma.s.ses increase in number by the addition of fresh ones behind, during development, and are visible in the dorsal view as brick-like ma.s.ses, the mesoblastic somites or proto-vertebrae (Figure 6, i., ii., iii.). In Figure 7, these ma.s.ses are indicated by dotting. In such a primitive type as amphioxus these mesoblastic -somites- [ma.s.ses] 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, const.i.tutes 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 ma.s.s of the splanchnopleur, will form the muscle and connective tissue of the wall of the alimentary ca.n.a.l, and the binding substance of the liver and other glands that open into the ca.n.a.l.
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 const.i.tute 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 const.i.tute 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 inv.a.g.i.n.ation which will become the a.n.u.s. 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 h.o.m.ologous 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 ma.s.ses of somatic mesoblast on either side, a longitudinal ca.n.a.l 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 ca.n.a.l, 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 p.r.o.nephros, 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 s.e.xual 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: p.r.o.nephros disappears.
The Mullerian duct (? = p.r.o.nephric 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: p.r.o.nephros 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 s.e.x) 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 ca.n.a.l 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 quant.i.ty 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 alb.u.men, in a sh.e.l.l membrane and a sh.e.l.l. At either end of the yolk (Figure 1, y.) twisted strands of alb.u.minous 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 inv.a.g.i.n.ation 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 ma.s.s 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 h.o.m.ology, 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 ident.i.ty 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 ma.s.s 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 p.r.o.nephros, 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 ma.s.s 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 sh.e.l.l, 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 sh.e.l.l.