418 s. HATTA : 



actually preseiii^^* in other groups, the Anlagen are mere thicken- 

 ings. 



As the pronephric tubules are derived, in each segment, from 

 the distal (ventral) half of each mesoblastic somite, the prone- 

 phros is, from the first, of a segmental arrangement, being 

 strictly myomeric. In fact, the separation of the sclero-myotome 

 from the lateral plate is eöected on account of the differentiation 

 of the Anlage of the pronephros or of the nephrotome. 



In Petroiiiyzon, the pronephric tubules which constitute the 

 glandular part of the system and the anterior section of the 

 segmental duct, are formed in the region of the fore-gut and some 

 of them are detected in the region where the gill-pouches are 

 afterwards formed ; these latter disappear entirely before the gills 

 come into view. 



The segmental Aidagen of the pronephric tubules are 

 secondarily connected Ijy the duct formed out of tw^o adjacent 

 pronephric Anlagen and put in connnunication with one another. 



Tlie degeneration of the pronephric tubules takes place from 

 both the cranial and caudal extremities of the system. In the 

 cranial part, the tubules disap]3ear without leaving any trace ; 

 while in the caudal, they are converted into the anterior section 

 of the segmental ikict. The remaining part of the system func- 

 tions for some time as the excretory organ. 



The pronephric Anlagen in the hinder region do not develop 

 beyond a certain point, but are em]3loyed solely to give rise to 

 the segmental duct just as in the somites having degenerated tubules. 



From whit has been said, it is, I venture to think, no rash 

 conclusion to regard the pj-onephric iuhules in Peivomyzon as having 

 once extended over the hody -segments from the branchial region 



]) According to ISwAtx and Buaciikt, tlic same fact is seen in Teleuslei. 



