The Development of the Rabbit

Text Book of Biology, Part 1: Vertebrata by H. G. Wells, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. The Development of the Rabbit

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.

Section 37. As the egg develops, however, we are astonished to find an increasing resemblance to that of the fowl. A split occurs at one point between outer layer and inner layer cells, and the space resulting (Y in Figure 2) is filled by an increasing amount of fluid, and rapidly enlarges, so that presently we have the state of affairs shown in 3, in which the inner layer cells are gathered together at one point on the surface of the ovum, and constitute the germinal area. If, with Hubrecht, we regard the outer layer cells as an egg membrane, there is a curious parallelism between this egg and the fowl's the fluid Y representing the yolk; and the inner layer cells the cells of the fowl's germinal area.

At any rate, the subsequent development goes far to justify such a view. The inner cells split into epi-, meso-, and hypo-blast, like the blastoderm in the fowl; there is a primitive streak and no blastopore; an amnion arises; the yolk sac, small and full of serous fluid, is cut off just as the enormous yolk of the fowl is cut off; and an allantois arises in the same way. There is no need to give special diagrams-- Figures 3, 4b, 5, and 6 of the fowl will do in all respects, except proportion, for the development of the rabbit. The differences are such as we may account for, not on the supposition that the rabbit's ovum never had any yolk, but that an abundant yolk has been withdrawn from it. The nutrition of the embryo by yolk has been superseded by some better method. The supposition that the rabbit is descended from ancestors which, like the birds and reptiles, laid eggs with huge quantities of yolk, meets every circumstance of the case.

Section 38. But the allantois and yolk sac of the rabbit, though they correspond in development, differ entirely in function from the similar organs of the fowl. The yolk sac is of the very smallest nutritive value; instead of being the sole source of food, its contents scarcely avail the young rabbit at all as nourishment. Its presence in development is difficult to account for except on the supposition, that it was once of far greater importance. At an early stage, the outgrowing allantois, pushing in front of it the serous membrane, is closely applied to the lining of the mother's uterus. The maternal uterus and the embryonic allantois send out finger-like processes into each other which interlock, and the tissue between the abundant bloodvessels in them thins down to such an extent that nutritive material, peptones and carbohydrates, and oxygen also, diffuse freely through it from mother to foetus,* and carbon dioxide, water, and urea from the foetus to the mother. The structure thus formed by the union of the wall of the maternal uterus, allantois, and the intermediate structures is called the placenta. Through its intermediation, the young rabbit becomes, as it were, rooted and parasitic on the mother, and utilizes her organs for its own alimentation, respiration, and excretion. It gives off CO2, H2O, and urea, by the placenta, and it receives O and elaborated food material through the same organ. This is the better method that has superseded the yolk.

* The embryo.

Section 39. In its later development, the general facts already enunciated with regard to the organs of frog and fowl hold, and where frog and fowl are stated to differ, the rabbit follows the fowl. In the circulation the left fourth vascular arch (second branchial) gives rise to the aortic arch; in the right the corresponding arch disappears, except so much of it as remains as the innominate artery. The azygos vein (Chapter 3) -is a vestige of- [is derived from] the right posterior cardinal sinus. Both pulmonary arteries in the rabbit are derived from the left sixth vascular arch (= fourth branchial). Compare Section 32. The allantois altogether disappears in the adult fowl; in the adult mammal a portion of its hollow stalk remains as the urinary bladder, and the point where it left the body is marked by the umbilicus or navel. The umbilical arteries become the small hypogastric arteries on either side of the urinary bladder. There is no trace of a pronephros at all in the rabbit.

Section 40. We may note here the development of the eye. This is shown in Figure 4, Sheet 24. A hollow cup-shaped vesicle from the brain grows out towards an at first hollow cellular ingrowth from the epidermis. The cavity within the wall of the cup derived from the brain is obliterated, [and the stalk withers,] the cup becomes the retina, and -its stalk- [thence fibres grow back to the brain to form] the optic nerve. The cellular ingrowth is the lens. The remainder of the eye-structures are of mesoblastic origin, except the superficial epithelium of the cornea. The retinal cup is not complete at first along the ventral line, so that the rim of the cup, viewed as in Figure 1, r., is horseshoe shaped. -Hence the optic nerve differs from other nerves in being primitively hollow.- In all other sense organs, as, for instance, the olfactory sacs and the ears, the percipient epithelium is derived, from the epiblast directly, and not indirectly through the nervous system. These remarks apply to all vertebrate types.

Section 41. The supposition, that the general characters of the rabbit's ovum were stamped upon it as an heritage from a period when the ancestors of the mammals were egg-laying reptiles, is strengthened by the fact that the two lowest and most reptile-like of all the mammalia, the duck-billed platypus and the echidna, have been shown to depart from the distinctive mammalian character, and to lay eggs. And, in further confirmation of this supposition, we find, in tracing the mammals and reptiles back through the geological record, that in the Permian and Triassic rocks there occur central forms which combine, in a most remarkable way, reptilian and mammalian characteristics.

Section 42. In conclusion, we would earnestly recommend the student to see more of embryological fact than what is given him here. It is seeing and thinking, much more than reading, which will enable him to clothe the bare terms and phrases of embryology with coherent knowledge. In Howes' Atlas of Biology there is a much fuller series of figures of the frog's development than can be given here, and they are drawn by an abler hand than mine can pretend to be. There is also an Atlas d'Embryologie, by Mathias Duval, that makes the study of the fowl's development entertaining and altogether delightful. Such complete series as these are, from the nature of the case, impossible with the rabbit. Many students who take up the subject of biology do so only as an accessory to more extended work in other departments of science. To such, practical work in embryology is either altogether impossible, or only possibly to a very limited extent. The time it will consume is much greater, and the intellectual result is likely to be far less than the study of such plates as we have named.

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