186 



ESSENTIALS OF ZOOLOGY 



The nephrotome remains connected with the splanchnoeoel, 

 and its parietal wall expands into a swelling. In the Elasmo- 

 branch fishes the anterior part of the kidney does not become 

 functional, and the nephrotome and the swelling are at first 

 without a cavity. The same changes take place in the 

 succeeding four to seven segments, and the outgrowths of the 

 nephrotomes fuse to form a single mass, the pronephros. The 

 outer part of the mass forms a duct and it extends posteriorly 

 and fuses with the terminal end of the intestine. This is the 



Nerve cord 



Sclerotome 



Notochord 



Myocoel 



Aorta 



Pronephric duct 



Vit. a. 

 Pectoral fin 



Intestine 

 Vit. 



FIG. 93. Transverse section through region of anterior end of 

 pronephric duct of embryo of Acanthias. 



pronephric duct. In typical conditions the pronephric tubules 

 possess a cavity which opens in each case into the dorsal 

 angle of the splanchnoeoel, and this opening is called a nephro- 

 stome ; this state of the- pronephros is retained in the 

 lowliest Craniates, the lampreys and hags. The blood, more- 

 over, in such comes into close association with expansions of 

 the tubules forming a glomerulus. 



The pronephros is a larval kidney and it will be seen that 

 it is altogether different in origin and structure from that of 

 Amphioxus. It is derived from the coelom and is discharged 

 by a coelomiduct. 



The pronephros is succeeded by another series of tubules 

 formed in precisely the same fashion, but, as is plain, the 



