URINOGENITAL ORGANS 347 



to a metanephros, or hind-kidney, with which is connected a 

 metanephric duct or ureter. 



The metanephros corresponds to a later developed posterior 

 section of the mesonephros. Each metanephric duct apparently 

 arises as a hollow outgrowth from the posterior end of the meso- 

 nephric duct, where the latter opens into the cloaca. It gradually 

 extends forwards, and comes into connection with a series of 

 tubules developed as buds from the hinder end of the mesone- 

 phros and provided with coelomic Malpighian capsules and with 

 glomeruli, but not with nephrostomes. The posterior end of the 

 ureter soon loses its connection wdth the mesonephric duct, and 

 opens independently either into the cloaca or into a urinary bladder 

 (Figs. 294—297). 



The Male and Female Generative Ducts. 



In the Elasmobranchii, Amphibia, and Amniota, tvio canals are 

 formed in connection with the primary excretory apparatus : one 

 of these is known as the secondary mesonephric or Wolffian duct — 

 which in male Elasmobranchii and Amniota functions as a seminal 

 duct or cas deferens and in male Amphibia as a urinogenital duct, and 

 the other as the Miillerian duct — which opens anteriorly into the 

 ccelome and serves in the female as an oviduct (Figs. 278, 279). 

 The Wolffian duct becomes rudimentary in the female — except in 

 Amphibians, in which it still serves as a urinary duct (Fig. 279) — 

 and the MuUerian duct remains in a more or less rudimentary 

 condition in the male. These two ducts in soma cases (Elasmo- 

 branchs) arise by a splitting of the primary mesonephric duct 

 into two (Fig. 278), but more usually the Miillerian duct arises 

 independently from the coelomic epithelium. All the urinogenital 

 ducts are lined by a mucous membrane, external to which are 

 muscular and connective tissue layers. (For the relations of the 

 urinary and generative ducts in other Fishes and in Dipnoans 

 see pp. 360-363.) 



The Gonads (" Generative Glands "). 



The sexual cells, which give rise to the ova and spermatozoa 

 originate from the germinal epithelium, which corresponds to 

 a differentiation of part of the coelomic or peritoneal epithelium on 

 the dorsal side of the body-cavity on either side of the mesentery, 

 and into which the adjacent mesoblastic stroma penetrates ; thus 

 a pair of gonads or " sexual glands " is formed (Fig. 274). 



Primitively the gonads were arranged segmentally, and extended through- 

 out a greater number of body segments (compare Amphioxus, p. 359). 



The primitive germinal cells are at first entirely undifferen- 

 tiated, but in the course of development a differentiation takes 

 place, resulting in the formation of a male or a female gonad, i.e., 

 a testis or an ovary. 



