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 secondary 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 OF THE EMBRYO OF Hylodcs 

 martinicensis (3 millimetres long). (After E. Selenka. ) 



A, urinary bladder ; c, stalk of the latter which communicates with the intestine ; 

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

 volutions of the segmental duct ; Tg, segmental duct ; W, ciliated regions of 

 the peritoneal epithelium ; zz, urinogenital cord (formative region of the 

 mesonephric vesicles ; 1, 2, 3, the three csecal processes of the right and 

 left pronephros, with their branches ; a, a, apertures of the pronophrie 

 ducts into the bladder ; u, rudiments of the anterior urinary tubules, in the form 

 of solid ccrds ; u l to u*, urinary tubules ; r, r, apertures of the urinary tubules 

 into the segmental duct (Vg l , Vtf\ which thus becomes the duct of the 

 mesonephros. 



rudimentary organs of secondary importance. Nevertheless, it 

 may 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 formed, giving rise to the 



1 The glands formed by the posterior urinary tubules in Elasmobranclis and 

 Urodeles which give rise to the functional excretory organs, and which may be 

 provided with special ureters, are considered by Balfour to be the equivalents of the 

 kidneys proper (metanephros) of Amniota (comp. Fig. 238). 



