CHAP, xm VRIXOGEXITAL ORGANS 603 



al. bl. allantoic bladder ; an. anus ; d. cloaca : gon. gonad ; int. intestine ; me. 

 Malpighian capsule ; msn.d. mesonephric (\Vol?fian) duct : ms.npk. mesonephros ; 

 mt.n. d. metanephric duct ; mt.nph. metanephros ; nst. nephrostomes ; or. 

 ovary ; p. n. d. Miillerian duct ; p.nph. pronephros ; sg. d. pronephric duct 

 /. spermary ; v.e. efferent ducts. (From Parker and Haswell's Zoology.) 



164, ms. d, ms. i] . In Fishes and Amphibians the mesone- 

 phros remains as the functional kidney throughout life, 

 though the posterior part alone may retain the renal 

 function and develop special ducts (dogfish, Fig. 127), 

 thus foreshadowing the metanephros of higher 

 Vertebrates. 



The metanephros is characteristic of Reptiles, Birds, 

 and Mammals. Its tubules arise later than those of 

 the mesonephros and open into a special ureter or 

 metanephric duct, which arises as an outgrowth from the 

 posterior end of the mesonephric duct : they resemble 

 the mesonephric tubules except that they possess no 

 nephrostomes and usually show no signs of a metametric 

 arrangement. 



In the majority of Vertebrates the generative organs 

 take on a close connection with certain of the urinary 

 tubules and their ducts (pp. 193 and 466), which may thus 

 undergo a change of function. In the dogfish the pro- 

 nephric duct early undergoes longitudinal subdivision 

 into two, one of which is retained as a mesonephric duct 

 (now often spoken of as the Wolffian duct] ; while the 

 other (Miillerian duct], retaining its connection anteriorly 

 with the coelome by a persistent pronephric nephro- 

 stome, assumes the function of an oviduct in the 

 female, and atrophies in the male, though traces of it 

 may persist in the adult (Fig. 127, m.d). Owing to 

 the connection which is formed between the spermary 

 and certain of the mesonephric tubules by means of 

 efferent ducts, the Wolffian duct may now serve as a 

 urinogenital duct (frog, p. 193) ; or, when an epididymis 

 is formed from the mesonephros, and special ureters arise 



p p 2 



