GENITAL DUCTS 89 



the mesonephros and its duct may exactly resemble the pronephros 

 both in structure and development. 



We may conclude, then, that all the kidney tubules and their 

 ducts are derived from one source, the nephrotome or intermediate 

 cell -mass, that the multiplication of tubules takes place by a 

 budding process from this rudiment, the earlier being more ventral 

 and the later more dorsal ; that the pro-, meso-, and metanephros 

 are not so many regions of one single continuous series, but that, in 

 the Petromyzontia and Gnathostomata, the mesonephros is formed 

 of one primary and several generations of secondary tubules, the 

 hindmost of which become further specialised as the metanephros. 



The archinephric longitudinal duct (provisional pronephric) 

 becomes bodily converted into the mesonephric duct, except in 

 the Elasmobranchs, where a splitting takes place resulting in the 

 formation of a Miillerian duct and a mesonephric duct. The 

 significance of this fact is discussed below. 



Some very important differences between the systems of 

 excretory and genital ducts in the Cyclostomes and in the 

 Gnathostomes must now be considered. - As already mentioned 

 (p. 45), in the former the excretory duct on each side remains 

 single, and the generative cells escape to the exterior through 

 paired short funnel-like openings at the hinder end of the abdominal 

 coelom. The coelomostomes, in the Gnathostomata, retain to some 

 extent their original function as genital ducts, and the single 

 longitudinal archinephric duct is always replaced by two ducts. 

 Where a metanephros occurs it also acquires its own special duct, 

 probably by the gradual separation of the distal end of its tubules 

 from those of the mesonephros, and their union to a common canal 

 opening separately into the cloaca. Of the two ducts mentioned 

 above the first is the Miillerian duct, which functions in -the adult 

 female as the oviduct ; the second is the Wolffian duct, which, in the 

 male sex, acts as a sperm-duct or vas deferens in all Gnathostomes, 

 and also as a urinary duct in those Gnathostomes in which the 

 mesonephros (Wolffian body) represents the adult kidney (Pisces 

 and Amphibia). 



In the male sex of all Gnathostomes the testis is shut off from 

 the coelom, and (except in some Teleostomes in which the conditions 

 are highly specialised, p. 364) its products are poured by means of 

 fine canals, the vasa efferentia, into the tubules of the mesonephros, 

 through these into the mesonephric duct, and so to the exterior 

 (Figs. 55, D; 56, D). Originally the vasa efferentia probably 

 extended along the whole length of the gonad (Ceratodus, Lepi- 

 dosteus) ; later they became restricted to the anterior (Elasmo- 

 branch), or to the posterior region (Lepidosiren, etc.). 



As in the case of the ovary, so in that of the testis, the primary 



