400 s. H ATT A : 



distal lialf of tins segmented portion which folds out in each 

 segment to give rise to the Anlage of the pronephric tnbule on 

 one hand and to the cœlomic projection on the other, and, 

 therefore, corresponds to the " Nephrotom " of Rückert. 



The nephrotome, therefore, constitutes, in both Petromyzon 

 and Selachia, precisely the same part of the mesoblast, viz. the 

 distal (ventral) portion of the somite, through which the sclero- 

 myotome is connected with the lateral plate. 



The above early stage in the differentiation of the mesoblast, 

 in Petromyzon corresponds also to the " Ursegment " of ^m- 

 pliioxus in Hatschek's sense ('88). By further development of 

 it the unsegmented mesoblast is brought into light, and we can 

 then distinguish the *' Urwirbel " and the " fSeitenplatte " of 

 Hatschek ('88). And the ventral half of the Urwirbel constitutest 

 in Petromyzon, the connecting canal between the unsegmented 

 cœlomic cavity and the sclero-myotome, that is to say, the 

 nephrotome. Let us now examine what part of the Ursegment of 

 Ämphioxim represents the nephrotome of the Craniota. 



In his excellent work on '' Die Nierencanälchen des Am- 

 phioxns," BovERi ventures to solve this important question. 

 After a discussion he comes to the conclusions : 



(1) That the " Gononephrotom " in Craniota must correspond 

 to a part of the " Urwirbel " of Hatschek ; 



(2) That the " Gononephrotom " of Craniota is homologous 

 with the genital chambers of the adult Amjjhioxus. 



But these chambers *' sind ursprünglich die segmentale 

 Yerbindungscanäle zwischen der unsegmentirten Leibeshöhle und 

 der Sclero-Myotom gwesen " ('92, p. 493). I can, therefore, 

 ascribe no other significance to the ventral half of the segmented 



