552 ZOOLOGY SECT. 



fringed. In the Monotremes the two oviducts are distinct through- 

 out their length, and open separately into a urinogenital sinus. 

 In nearly all the Theria more or less coalescence takes place. In 

 the Marsupials this coalescence is confined to the anterior part of 

 the vagina. In the Opossums (Fig. 1142, A) the two oviducts 

 are merely in close apposition at one point behind the uteri, 

 and there is no actual coalescence. In the rest of the Marsu- 

 pials (B, C) the anterior portions of the oviduct in the region 

 (vagina) behind the uteri unite to form a median chamber 

 which may send backwards a median diverticulum (median 

 vagina, Vg. B), and in this way communicate behind with the 

 urinogenital passage. In the Eutheria there is a single median 

 vagina (Fig. 1143, Vg.} formed by the union of the posterior parts 

 of the two oviducts. In some cases the two uteri (A, ut.) remain 

 distinct; in others their posterior portions coalesce (B, C), the 

 anterior parts remaining separate so that there is formed a median 

 corpus uteri with two horns or cornua. In Primates and some 

 Edentates the coalescence goes still further, there being an un- 

 divided uterus (D) in addition to an undivided vagina, the only 

 parts of the oviducts which remain distinct from one another being 

 the narrow anterior parts or Fallopian tubes. In all Mammals 

 there is, in the vestibule or passage through which the vagina 

 communicates with the exterior by the aperture of the vulva, a 

 small body the clitoris the homologue of the penis and some- 

 times perforated by the urethral canal. 



Development. The ova of Mammals (Fig. 1144), like those 

 of Vertebrates in general, are developed from certain cells of 

 the germinal epithelium, the primitive ova (pr. ov.). Each of 

 these, surrounded by smaller unmodified cells of the epithelium, 

 sinks into the stroma of the ovary, in which it becomes imbedded, 

 the small cells forming a Graafian follicle (foil) which encloses it. 

 Soon spaces filled with fluid appear among the follicle cells (Fig. 

 1145, A, sp.), and these eventually coalesce to form a single cavity. 

 This cavity, which in some Mammals is crossed by strings of 

 cells, separates an outer layer of the follicle cells the membrana 

 granulosa (mem.) from the mass cumulus proligerus (disc.) sur- 

 rounding the ovum, except on one side where they coalesce. A 

 basement membrane becomes formed externally to the follicle cells, 

 and the stroma, immediately surrounding, becomes vascular, and 

 forms a two-layered investment for the follicle. The cells imme- 

 diately surrounding the ovum become arranged as a definite layer 

 of cylindrical cells the corona radiata. A thick membrane the 

 zona radiata perforated by numerous radially arranged pores, into 

 which project processes from the cells of the corona, invests the 

 ovum ; and in many, if not in all, there is beneath this a delicate 

 vitelline membrane. As the ovum increases in size, granules of 

 yolk become distinguishable among the protoplasm. 



