IX NEPHRIDIAL ORGANS 311 



ductSj changes associated with the need of separating off the path by 

 which the reproductive cells reach the exterior from that taken by the 

 highly poisonous excretory products. These changes are seen with 

 particular clearness during the individual development of the Dogfish 

 and its allies. The archinephric duct undergoes a longitudinal splitting 

 from before backwards into a Wolffian duct (Fig. 128, C and D, W.d), 

 into which open the tubules of the opisthonephros, and a Miillerian duct 

 {M.d), into which opens what remains of the pronephros — a single greatly 

 dilated tubule. The Wolffian duct serves for the conveyance (i) of the 

 urine or excretory fluid from the kidney and (2) also, owing to the presence 

 of a connexion with the testis to be described presently, of the micro- 

 gametes or spermatozoa. The Miillerian duct is functional only in the 

 female : it functions as the oviduct, the greatly enlarged pronephric 

 nephrostome at its anterior end allowing the eggs to pass into it from 

 the splanchnocoele. 



The gonads of the vertebrate are originally, like those of the Earth- 

 worm, simple thickenings of the coelomic (peritoneal) epithelium, although 

 here in correlation with their great increase in size there necessarily comes 

 about also a great increase in complexity of detail. 



What has been said so far refers to the urino-genital organs of the 

 Vertebrata in general, and it is now necessary to describe the arrangements 

 as they occur in the Dogfish in particular. 



Both ovaries and testes are originally two in number, but in the case 

 of the former only one (the right) becomes functional, the other dis- 

 appearing. The functional ovary (Fig. 130, B, 0) is a large organ 

 attached to the dorsal wall of the peritoneal cavity by a thin fold of 

 peritoneal lining (mesovarium). Its appearance is very characteristic 

 owing to the eggs, as they approach maturity, storing up in their cyto- 

 plasm enormous quantities of reserve food-material or yolk, so that each 

 forms a large yellowish sphere bulging out from the surface of the ovary. 

 The egg is eventually shed into the peritoneal cavity and is carried for- 

 wards, in a way not yet fully worked out, into the funnel-like opening 

 of the Mullerian duct (Fig. 130, B, M.d). 



In the male the gametes, instead of being shed freely into the peritoneal 

 cavity, pass into a cavity in the interior of the testis (Fig. 130, A, T) — 

 a paired body slung up to the dorsal wall of the peritoneal cavity by the 

 thin membranous mesorchium — and thence by a group of fine tubular 

 vasa eflerentia (v.e), situated at the front end of the testis, into the front 

 end of the opisthonephros. 



The kidney (Fig. 130, op) is an opisthonephros — ^the pronephros 



