THE REPRODUCTIVE DUCTS 



351 



In Ceratodus 



the sperm from the anterior end of the testis and carrying it farther back for 

 passage through the mesonephros. Here the anterior end of the Wolffian duct 

 is purely excretory. A farther concentration of the efferent functions to the 

 last vas efferens would give, with a few other modifications, the conditions of 

 the teleosts (fig. 373, 5). In all of this group there is no coimexion of testes 

 with mesonephroi. The seminiferous tubules are connected by a longitudinal 

 canal (apparently the longitudinal testicular canal of other vertebrates) which 

 runs in the membrane (mesorchium) supporting the testis, back to the external 

 opening, which is either directly to the exterior between the urinary opening 

 and the anus (fig. 383) or into a urogenital sinus (fig. 373, B). 



This view is farther supported by the relation in the dipnoi, 

 there are numerous vasa efferentia which extend 

 from the testis into the mesonephros. In Lepi- 

 dosiren the efferent ductules are fewer in number 

 and they arise from a posterior degenerate portion 

 of the testis, while in Protopterus (fig. 373, D) 

 there is but a single vas efferens on either side 

 and this passes through the posterior end of the 

 Wolffian body. 



Oviducts. — In the elasmobranchs the 

 Miillerian duct, which, as described above, 

 arises by a splitting of the pronephric duct, 

 serves as the oviduct. After separation 

 from the Wolffian duct this opens in front 

 into the ccelom by means of the pronephric 

 tubules and their nephrostomes. Then 

 these flow together, forming a large open- 

 ing, the ostium tubae abdominale, on 

 either side (in elasmobranchs the ostia of 

 the two sides are usually united ventral to the liver) through which 

 the eggs, which pass from the ovary into the coelom are carried into 

 the oviduct. 



In some amphibia (Salamandra) the pronephric tubules and 

 nephrostomes take a part in the formation of the ostium tubae and the 

 beginning of the oviduct, while in Amhly stoma the ostium develops in 

 close connexion with the pronephric nephrostomes. Here, as in 

 all other tetrapoda, the rest of the oviduct arises by the formation of 

 a groove of the peritoneal membrane close beside the Wolffian duct. 

 This becomes rolled into a tube, the Mtillerian duct (fig. 374). In 

 the amniotes the anterior end of the groove does not close, but re- 

 mains open as the ostium tubae (fig. 368, A). 



Usually the condition in the elasmobranchs has been regarded as 

 the primitive one, a supposition which renders it difficult to homolo- 



FiG. 374. — Section through 

 the urogenital region of a four- 

 day chick embryo, after Wald- 

 eyer. g, analage of gonad with 

 primordial ova; m, involution 

 of peritoneum to form the 

 Mtillerian duct; ms, mesentery; 

 s, cells to form stroma of gonad; 

 I, tubules of mesonephros; if, 

 Wolffian duct. 



