MORPHOLOGY OF CYCLOSTOMATA. 407 



mesoblast in Pelromyzon than that it is the morj3hological 

 equivalent of the " segmentale Yerbindungscanäle." 



It thus follows that the distal half of the segmented mesoblast 

 in Petromyzon undergoes exactly the same fate as that in Am- 

 phioxus : it is transformed into the pronephros and the cœlomic 

 projection or the " dorsal segmental cœlome," which latter gives 

 rise, just as Boyeri suggests in Amphioxus, to the mesonephric 

 tnl)ules and, in the hinder region, to the genital gland. 



As has been pointed out in the historical revie\Y, the rela- 

 tion of the Anlage of the pronephric tubule to the mesoblastic 

 somite is the same for Teleostei and Amphibia, as in Petromyzon. 



It iiiny, tlicrcforCy safely he stated, that the segmented portion 

 of the mesohlad constitutes in these groups a single integral 

 structure until the separation of the nephrotome in cwitinuo with 

 the lateral plate from the sclero-myotome. This separation is, as 

 ahove stated, not the separation of the somite from the lateral plate , 

 hut the differentiation of the somite into the .^clero-myotome and 

 the nephrotome, preparatory to the development of the urogenital 

 system. The reason ichy the separation take>< plaee earlier in some 

 groups than in others, rests only on physiological grounds. 



B. — The Whole System of the Pronejihros of Cyclostomata, 



Teleostei, and Amphihia is Homologous with the 



Nierencan'dlchen of Ampthioxus (Boveri) and 



not perfectly Homologous loith the Selachian 



Pronephric System. 



I have already stated above (pp. 386 and 387) that Boveri 

 has brought the pronephric system of Craniota in harmony with 

 the system of the " Nierencanälchen " of Amphioxu-^, basing his 

 arguments on the structure, the position, the myomeric arrange- 



