5 io 



STRUCTURE OF VERTEBRATA. 



nek 



At a late period in those types in 

 which the pronephros is a functional 

 larval organ, but much earlier in the 

 higher Vertebrates, another series 

 of tubules is differentiated from the 

 mesoblast, and, acquiring a con- 

 nection with the segmental duct, 

 constitutes the mesonephros, or mid- 

 kidney. The tubules arise usually, 

 though not invariably, nearer the 

 posterior end of the body than the 

 pronephros, and are formed from 

 the portion of the mesoblast which 

 connects the muscle segment and 

 the lateral plate (see Fig. 270). 

 Below the Amniota the mesonephros 

 forms the permanent excretory 

 organ. In higher forms another 

 series of nephridial tubules arises 

 still farther back in the body, and 

 forms the metanephros, or perma- 

 nent kidney. The mesonephric 

 and metanephric tubules resemble 

 each other closely, but the relation 

 of the former to the pronephros 

 is still a debated point. When 

 fully developed, a mesonephric 

 tubule consists of (l) an internal 

 ciliated funnel (nephrostome), which 

 opens into the body cavity, but 

 is only rarely represented ; (2) a 



FlG. 270. Development of excre- 

 tory system of Vertebrate. In 

 part after Boveri. 



In I. the primitive segments are not 

 separated off from the lateral plate, 

 and the pronephros (/>n.) is seen arising 

 from the lower part of the primitive 

 segment. In II. the pronephros is com- 

 pletely separated off from the primi- 

 tive segment and lateral plate. In 

 III. the origin of the mesonephric 

 tubules is seen. They arise from the 

 upper part of the lateral plate, which 

 is now completely separated from the 

 primitive segment, and curving round 

 the pronephric duct come to open into 

 it. 



n.c., nerve cord; ncA., notochord ; /., 

 pronephros ; ., gut ; p.s. t primitive 

 segment; mes., mesonephric tubule; 

 pn.d.) pronephric duct ; b.c., body 

 cavity; 00.^ aorta; szv., sub-intestinal 

 vein, with vessel to the aorta. 



