Text Book of Biology, Part 1: Vertebrata

by H. G. Wells

Section 138. The ovum (ov.), is now large, and its nucleus and nucleolus (the germinal vesicle and spot) are very distinct. The wall of the follicle consists, in the mammal, of several layers of cells, the membrana granulosa (or "granulosa" simply); the ovum lies on its outer side embedded in a mass of cells, discus proligerus, separated from actual contact with the ovum by a zona pellucida. The ripening follicle moves to the surface of the ovary and bursts, the ovum falls into the body cavity. In [Figure 2], a ripe Graafian follicle (G.F.), projects upon the ovary.

Section 139. The liberated ovum is caught up by the funnel-shaped opening of the Fallopian tube, which passes without any very conspicuous demarcation into the cornu uteri (c.ut.) of its side; the two uterine cornua meeting together in the middle line form the vagina (V.), which runs out into a vestibule (vb.) opening between tumid lips to the exterior. The urinary bladder (ur.b.) also opens into the vestibule, and receives the two ureters from the kidney.

Section 140. In the male we find, in the position of the female uterus, a uterus masculinus (u.m.). The essential sexual organ is the testis (T.), a compact mass of coiling tubuli, which opens by a number of ducts, the vasa efferentia, into a looser and softer epididymis (ep.), which sends the sexual product onward through a vas deferens (v.d.), to open at the base of the uterus masculinus. The urinary bladder and ureters correspond with those of the female, and the common urogenital duct (= vestibule), the urethra, is prolonged into an erectile penis (P.) surrounded by a fold of skin, the prepuce. A prostate gland (pr.), contributes to the male sexual fluid. The character of the essential male element, the spermatozoon, the general nature of the reproductive process, will be conveniently deferred until the chapters upon development are reached.

9. _Classificatory Points_

Section 141. The following facts of classificatory importance may now be considered, but their full force will be better appreciated after the study of other vertebrate types. They are such as come prominently forward in the comparison of the rabbit with other organisms.

Section 142. In the first place, the rabbit is a metazoon, one of the metazoa, i.e., a multicellular organism, as compared with the amoeba, which belongs to the protozoa or one-cell animals ([Section 55]). In the next place, it is externally bilaterally symmetrical, its parts balance, and where, in its internal anatomy, it departs from this symmetry (as in the case of the aorta, the stomach and intestines, and the kidneys), the departure has an appearance of being the results of partial reductions and distortions of an originally quite symmetrical plan. And the facts of development strengthen this idea; in the very earliest stages we have paired aortic arches, of which, the left only remains, a straight alimentary canal, and less asymmetrical kidneys. In the vast majority of animals the same bilateral symmetry is to be seen, but in the star-fish and sea-urchins, and in the jelly-fish, corals, sea anemones, and hydra, the general form of the animal is, instead, arranged round a centre, like a star and its rays, and the symmetry is called radial.

Section 143. We also see in various organs of the rabbit, and especially in the case of the limbs and vertebral column, what is called metameric segmentation, that is, a repetition of parts, one behind the other, along the axis of the body. Thus the bodies and arches of the vertebrae repeat each other, and so do the spinal nerves. The renal organ of the rabbit, some time before birth, displays a metameric arrangement of its parts; but this disappears, as development proceeds, into the compact kidney of the adult. But the metameric segmentation in the rabbit's organism is not nearly so marked as that of an earthworm, for instance, which is visibly a chain of rings. If the student wants a perfect figure of metameric segmentation he should think of a train of precisely similar carriages, or a string of beads. One bead, one carriage, one vertebra, would be a metamere.

Section 144. In contrast to metameric segmentation is the antimeric repetition of radial symmetry ([Section 142]), in which each ray of the star is called an antimere. It is possible to have bilateral symmetry without a metameric arrangement of parts, as in the mussel and the cuttle-fish; but metameric segmentation without complete or reduced bilateral symmetry does not occur.