296 



CHORDATE ANATOMY 



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

 annelid ancestry, they are derived. (Fig. 266) 



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

 amniote novelty, since transitional conditions between mesonephros and 

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

 nephros-metanephros, the opisthonephros. The present ureter has been 

 regarded also as an elongated renal tubule which secondarily acquires 

 functional connexions with the numerous tubules of the metanephros. 



Fig. 266. — Diagrams of urogenital structures in (A) indifferent and female 

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

 amniote (mammal); (D) female amniote (mammal), b, urinary bladder; c, cloaca; 

 e, epididymis; /. Fallopian tube; g, gonad; h, "stalked hydatid"; k, kidney (meta- 

 nephros); I, longitudinal tubule; m, Miillerian duct (oviduct), rudimetary in B and C; 

 mn, mesonephros; o, ovary; ot, ostium tubae abdominale; pd, paradidymis; po, paro- 

 ophoron; pv, parovarium; r, rectum; ti, testis; u, uterus; iia, urethra; tir, 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.") 



In the female amniote, the mesonephros disappears except in the form 

 of functionless rudiments, the epoophoron, the paroophoron, and the duct 

 of Gartner. In the male, the primitive mesonephric duct is utilized as a 

 ductus deferens. The anterior part of the mesonephros becomes the 

 epididymis. Such relations are inherited from those of Anamnia. Rem- 

 nants of the posterior portion of the mesonephros may persist in the adult 

 as the rudimentary paradidymis and ductus aberrans. See Fig. 266. 



In vertebrates except most mammals, the excretions are poured into 

 the cloaca. This is also true of monotremes. A bladder comparable to 



