222 



THE MESODERMAL DERIVATIVES 



Wolffian 



esonephric) 



duct 



•enal gland 



Urogenital system of the male frog during amplexus, showing vasa efferentia 

 white with spermatozoa. Note the mesorchium and huge posterior vena cava. 



The primary mesonephric vesicle gives rise to a dorsal evagination 

 which becomes the secondary unit of the mesonephric vesicle. Fol- 

 lowing this there is another evagination from the ventro-lateral side 

 of the primary vesicle, toward the segmental duct with which it be- 

 comes continuous. This is known as the inner tubule of the tertiary 

 mesonephric vesicle. It becomes greatly coiled and encroaches upon 

 the developing and adjacent cardinal vein. The inner tubules become 

 the tubular portion of the adult kidney. The segmental duct from this 

 level posteriorly is known as the Wolffian or mesonephric duct, and 

 this will remain as the excretory duct or ureter. 



A third evagination, the outer tubule, develops ventrally from 

 the mesonephric unit. The cavity of this tubule joins the coelom by 

 way of the nephrostome. However, by the 20 mm. stage this unit has 

 broken away from the rest of the mesonephric unit and has changed 

 its connection from the mesonephros to the lateral division of the 

 cardinal vein. This vessel becomes the renal portal vein. This con- 

 nection can be demonstrated very easily in the recently metamor- 

 phosed adult frog where persistent ciliated nephrostomes can be seen 

 on the ventral face of the kidneys and which can be shown to main- 



