URINOGENITAL ORGANS. j'j'j 



kidney proper (metanephros 1 ), together with its special duct, 

 the ureter. 2 



The generative cells, that is, the ova and seminal cells, 

 have a similar origin throughout Vertebrates. They become dif- 

 ferentiated from the peritoneal epithelium.v This germinal 

 epithelium arises on the dorsal side of the body-cavity, on either 





TO 



FIG. 235. DIAGRAMMATIC TRANSVERSE SECTIONS OF THE BODY OF A LOAVEI; 

 VERTEBRATE, TO SHOW THE RELATIONS OF THE SEGMENTAL ORGANS. (After 

 Hensen.) The right side of the figure represents a later stage than the left. 



?, spinal cord ; Sp, ganglion of a spinal nerve ; MP, muscle-plate ; JD, wall of intes- 

 tine ; Ch, notochord ; WG, Wolffian duct ; A, aorta ; K, germinal epithelium ; 

 above K on the left side is seen the peritoneal aperture of a segmental tubule : 

 Gl, Malpighian capsule, shown on the left side arising as an expansion of a 

 urinary tubule, and on the right in a fully-formed condition, containing a 

 glomerulus, and communicating with the segmental duct by means of the convo- 

 luted tubule ; M y Miillerian duct in process of formation ; X, cellular trabecuke 

 growing out from the Malpighian capsule into the generative gland (K). 



side of the mesentery, and the adjacent mesoblastic stroma pene- 

 trates into it (comp. Fig. 235, K). The primitive germinal cells 

 are at first entirely undifTerentiated, so that it is impossible to 



1 According to the researches of Mikalovics the mesonephros does not disappear, 

 as such, suddenly, but its greater part remains functional together with the 

 metanephros for some time : in Lizards, for instance, it only becomes reduced after 

 the first winter's sleep, that is, in the second year. Thus, to a certain extent, inter- 

 mediate stages exist between the condition of things seen in the Anamnia and 

 Amniota respectively. At one time Amniota must have existed in which the meso- 

 nephros served as the main excretory organ throughout life, but on the appearance 

 of the metanephros it became no longer needed, and was gradually reduced. 



2 The metanephric duct arises in the form of a paired canal, which grows out frpm 

 the "Wolffian duct at the point where the latter opens into the cloaca. Its anterior 

 end then comes into relation with a series of segmental tubules, provided with 

 glomeruli, and the posterior end loses its connection with the Wolffian duct, and 

 come's to communicate with the allantois (urinary bladder). 



