316 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
pronephros and its derivatives, and in the female forms the tube 
(oviduct) by which the eggs are carried to the exterior. In other 
amphibia and in the amniotes the pronephric duct does not divide, 
but remains solely in the service of the mesonephros and -forms the 
Wolffian duct, while the oviduct arises in another manner, to be de- 
scribed in connexion with the reproductive organs. In the teleosts 
also there is no division of the pronephric duct. 
Fic. 321.—Diagrams of urogenital structures in (A) indifferent and in female elas- 
mobranchs and amphibians; (B) male elasmobranchs and amphibians; (C) male amniote 
(mammal); (D) female amniote (mammal). 5, urinary bladder; ¢, cloaca; e, epididymis; 
k, kidney (metanephros); f, Fallopian tube; g, gonad; h, ‘stalked hydatid’ ; /, longitudinal 
tubule; m, Miillerian duct (oviduct), rudimentary in B andC 3 mn, mesonephros; 0, ovary; 
ot, ostium tube abdominale; pd, paradidymis; po, paroéphoron; pu, parovarium; r, rectum; 
#, testis; u, uterus; wa, urethra; ur, ureter; va, vas aberrans; vd, vas deferens; ve, vasa effer- 
entia; w, Wolffian duct, urinary in A, urogenital in B, genital in C and rudimentary in D. 
Metanephros.—The mesonephros is functional in the embryos of 
all vertebrates and throughout life in the ichthyopsida. It also func- 
tions for a short time after birth in certain reptiles (lizards) and in the 
lowest mammals (Echidna, opossum). It becomes replaced in the 
adults of all amniotes by the mesonephroi, the only structures to which 
the name kidneys is strictly applicable. Each metanephros arises 
behind the mesonephros of the same side. From the dorsal hinder 
end of the Wolffian duct, near its entrance into the cloaca, a tube, the 
