358 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



the result of the fusion of the pronephric nephrostomes. The eggs, escaping 

 from the ovaries, are received by the ostium and are carried backward by the 

 oviducts to a shell or nidamental gland, where the shell (slight in the viviparous 

 species, stronger in the others) is secreted. The rest of the tube is enlarged and 

 serves, especially in the viviparous species, as a uterus, the inner surface bearing 

 vascular villi by which nourishment and oxygen are brought to the embryo. 

 The eggs are very large, those of some species exceeding those of the ostrich in 





Fig. 378. — Urogenital organs of (^4) 

 immature female and {B) male Torpedo 

 marmorata, after Borcea. e, epididymis; 

 tn, mesonephros; md, degenerate anterior 

 end of mesonephros of female; 0, ostium 

 tubse; od, oviduct (Miillerian); ov, ovary; r, 

 rectum; rg, rectal gland; u, definitive ureter; 

 up, urogenital papilla; v, vas deferens; w, 

 Wolfl&an duct. 



Fig. 379. — Testis and anterior end 

 of mesonephros of Chimotra, after 

 Parker and Burland. bv, blood- 

 vessel; cvl, longitudinal tubule; m, 

 Miillerian duct; ms, anterior end of 

 mesonephros (Leydig's gland); spd, 

 sperm duct; ve, vet, vasa efferentia; v$, 

 seminal vesicle. 



size. The usual form of the egg shell is quadrangular with tendril-like pro- 

 longations at each corner, but this is varied, some {Acanthias) being simple sacs; 

 others, like Heterodontus, having complicated cases. Some species of both sharks 

 and skates are viviparous. 



The relations of the male ducts to the testes are typical (p. 349), the anterior 

 end of the mesonephros forming an epididymis. The vasa deferentia of the 

 two sides unite just before the entrance to the cloaca to form a urogenital sinus, 



