MORPHOLOGY OF CYCLOSTOMATA. 413 



clifferentiated, so to speak, in sUu from the mesoblast in its whole 

 length, and as recent anthors agree, is composed of a series of 

 the abortive tubules formed in each nephrotome. This is not 

 the case in Sehichia ; here it is brought about, as Rabl states, by 

 the posterior growth of the collecting duct wdiicli is formed by 

 the confluence of the lateral extremities of the pronephric Anlagen. 

 It is not easy to bring these tw^o ^videly divergent modes of 

 formation into harmony with each other. 



A few morphological considerations, however, would, I believe, 

 enable one to derive one type of the system from the other. I 

 may be permitted to state here some of these considerations. 



I will start with the question : Is the segmental duct of 

 Selachia the morphological equivalent of that of Peirom.yzon, 

 Teleostei, and Amphibia ? I believe the question can be answered 

 safely in the negative, if we consider (1) the position of the 

 pronephros first formed, and (2) the origin of the duct. 



In the first place, the pronephros in Selachia appears, as 

 we learn fiom Rabl, in the mesoblastic somites lying 'posterior 

 to the Anlage of the liver ; thus the Anlage of the liver lies 

 under the fourth and fifth somites, and that of the pancreas 

 under the sixth, while the pronephros covers the seventh to 

 tenth somites ('96, p. 667). On the contrary, in Petromyzon^^ the 

 pronephros originates in the mesoblastic somites anterior to the 

 hepato-pancreatic Anlage, only the posterior one or two nephro- 

 tomes covering the liver. Such being the case, the pronephric 

 segments in Selachia correspond to the same number of the 

 abortive tubules in Petromyzon and the other Craniota above 



1) As we learn from Goette ('75) and Oellaclier ('78), the anterior section of the 

 pronephric system in Amphibia and Teleostei, is also found in the mesoblast opposite to the 

 posterior section of the fore-gut. 



