566 



STRUCTURE OF VERTEBRATA 



Amniota, except Crocodiles and Turtles, it is from the first rudimentary 

 and functionless. 



The segmental or pronephric duct is of mesodermic origin, arising 

 by the fusion of the outer ends of the primordia (or early beginnings) 

 of the pronephric tubules. 



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 mesoderm, and, acquiring a con- 

 nection with the segmental duct, constitutes the mesonephros, or mid- 

 kidney. The tubules arise usually, though not invariably, nearer the 



ao 



Atv 



Fig. 321. — Development of excretory system of Vertebrate. — In 



part after Boveri. 



In I. the primitive segments are not separated off from the lateral plate, and the 

 pronephros [pn.) is seen arising from the lower part of the primitive segment. 

 In II. the pronephros is completely separated off from the primitive 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 ; nch., notochord ; pn., pronephros ; g., gut ; p.s., primitive segment ; 

 mes., mesonephric tubule ; pn.d., pronephric duct ; b.c, body cavity ; ao., aorta ; 

 si. v., sub-intestinal vein, with vessel to the aorta. 



posterior end of the body than the pronephros, and are formed from 

 the portion of the mesoderm which connects the muscle segment and 

 the lateral plate (see Fig. 321). 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 permanent 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 — (i) an internal ciliated 

 funnel (nephrostome), which opens into the body cavity, but is only 

 rarely represented ; (2) a small cavity (Malpighian capsule) derived 



