298 



COMPARATIVE ANATOMY. 



Thus the primitive urinary system, besides its main function of 

 excreting waste products by means of the epithelial cells, serves 

 also to conduct the peritoneal fluid from the body. 



This sr<-( Hilary urinary system, or mesonephros, is of greatest 

 importance in the Anamnia : in most Fishes it serves exclusively 

 as a urinary organ, but in others (most Elasmobranchs) it also 

 takes on certain relations to the generative apparatus, giving 

 rise to the parorchis, parovarium, and to other more or less 



FIG. 234c. THE ENTIRE EXCRETORY SYSTEM OK THE EMBRYO OF Hylwlcs 

 martinicenais'(3 millimetres long). (After E. Selenka. ) 



urinary Mulder ; r, stalk of the latter which communicates with the intestine ; 

 '//, glomerulus of the pronephros ; P, peritoneal epithelium; S, S-shaped con- 

 volutions of the segmental duct ; Vg, segments! duct : W, ciliated regions of 

 the peritoneal epithelium ; z~, urinogenital cord (formative region of the 

 inesonephrie vesicles ; 1, 2, 3, the three crecal processes of the right ami 

 left pronephros, with their branches ; n, n, apertures of the pronophiic 

 ducta into the 1 (ladder : ?/, rudiments of the anterior urinary till. tiles, in the form 

 of solid cords ; K 1 to if 4 , urinary tubules : /. /. apertures ol' (lie urinary tubule- 

 into the segmental duet < J'i/\ /"</M. \vhieh thus becomes the duel ,,f the 

 mesonephros. 



rudimentary organs of secondary importance. Nevertheless, it, 

 limy remain as the permanent urinary organ (Elasmobranchs. 

 Amphibians 1 ), or may entirely disappear as such (Amniota) ; in the 

 latter case, a third series of tubules are f"nm'd, giving rise to the 



1 The glands I'nrnied by the posterior urinary tubules in Elasmobranchs and 

 "rodeles whieh give rise to the functional exert lory organs, and uhi.'h may be 

 m(\ ided with special ureters, are considered by Bnlfour to be the equivalents of tin: 

 vidneys proper i inetaih']>hros\ of Amniota (c.oiup. Fig. 238). 



