Section 137. The female organs of reproduction are shown in Sheet 10. The essential organ is the ovary (ov.), in which the ova (eggs) are formed. Figure 3 gives an enlarged and still more diagrammatic rendering of the ovary. There is a supporting ground mass, or stroma, into which numerous bloodvessels and nerves enter and break up. The ova appear first as small cells in the external substance of the ovary (as at 1), and move inward (2 and 3), surrounded by a number of sister cells, which afford them nourishment. At (4) an ovum with its surrounding group of cells is more distinct and near the centre of the ovary; a fluid is appearing within the ovisac as the development proceeds. (5) is a much more mature ovisac or Graafian follicle. 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.