EXCRETORY SYSTEM. 



459 



The segmental duct has possibly an epiblastic origin, 

 it grows gradually backwards from its place of origin, and 

 it seems probable that the original excretory opening was 

 far forwards. At a late period in those types in which the 

 pronephros is a functional larval organ, but much earlier in 

 the higher Vertebrates, another set of tubules are differenti- 

 ated from the mesoblast, nearer the posterior end of the 

 body, and acquiring a connection with the segmental or 

 pronephric duct, constitute the mesonephros or mid kidney. 

 Below the Amniota this forms the permanent excretory 

 organ. 



In higher forms another series of nephridial tubules 





Fig. 147. — Transverse section through a Vertebrate 

 Embryo. (After Semon.) 



^c, Spinal cord ; My.^ Myotomes; A^. , notochord ; a., aorta, with 

 vessels to two glomeruli; ,^. , rudiment of genital organ; fi.n.d., 

 pronephric or segmental duct; N.. nephrostome. The letters 

 a.r.b. lie in the top part of the gut. 



arises still further back in the body, and forms the meta- 

 nephros, or permanent kidney. 



In each case, when typically developed, the tubules 

 consist {a) of an internal ciliated funnel (nephrostoma) 

 opening into the body cavity, but only rarely persistent ; 

 (b) of a dilatation (Malpighian body), into which blood 

 vessels project ; and {c) of a coiled tube in part excretory, in 

 part a conducting canal for the waste filtered from the blood. 



