4o6 



COMPARATIVE ANATOMY 



nephric tubules also in the fact that they acquire connexions with glomeruli 

 and thus are enabled to eliminate liquid wastes directly from the blood 

 vessels and not indirectly by way of the body cavity. 



In the male the anterior portion of the mesonephros changes its 

 function from that of an excretory organ to that of a reproductive organ. 

 In other words, the tubules of the anterior part of the mesonephros 



Fig. 335.- — Diagrams of urogenital structures in (.4) indifferent and in female 

 elasmobranchs and amphibians; {B) male elasmobranchs and amphibians; (C) male 

 amniote (mammal); (Z?) femal'e amniote (mammal), b, urinary bladder; c, cloaca; 

 e, epididymis;/, Fallopian tube ; g, gonad; /?, " stalked hydatid "; ^, kidney (metanephros) ; 

 I, longitudinal tubule; m, MuUerian duct (oviduct), rudimentary in B and C; mn, 

 mesonephros; o, ovary; ot, ostium tubae abdominale; pd, paradidymis; po, paroophoron; 

 pv, parovarium; r, rectum; ti, testis; u, uterus; iia, urethra; nr, ureter; va, vas aberrans; 

 vd, vas (ductus) deferens; ve, vasa efferentia; w. Wolffian duct, urinary in A, urogenital 

 in B, genital in C and rudimentary in D. (From Kingsley's " Comparative Anatomy of 

 Vertebrates.") 



return to the original function of the coelomoducts from which, assuming 

 an annelid ancestry, they are derived. (Fig. 335) 



Metanephros. The third kidney or metanephros appears not to be an 

 amniote novelty, since transitional conditions between mesonephros and 

 metanephros occur in some amphibia in the form of a combined meso- 

 nephros-metanephros, the opisthonephros. In the amniote embryo, the 

 nephrogenic tissue from which the excretory tubules of the metanephric 

 kidney arise resembles that which forms the mesonephros. The tubules 

 . of the metanephros, like those of the posterior portion of the mesonephros, 

 arise from the nephrotome and are non-metameric in origin. 



