40-1 s. HAÏTA : 



step ill the diÖeren dation of the mesoblastic somite, and because 

 the distal (ventral) portion of the hitter happens for a time to be 

 continuous witli the lateral plate, we are not justified in concluding 

 that it is derived from the lateral plate, which, as we know, never 

 undergoes seo-mentation. 



It is a significant fact that in Selachia and Amniota, in 

 which the pronephros does not function as the actual excretory 

 organ, this separation of the mesobhist into the sclero-myotome 

 and the nephrotome is not effected so earl}^ as in the above 

 groups, but takes place only at later stages, with the first dif- 

 ferentiation of the mesonephros. This consideration makes it 

 reasonable to conclude that the early separation of the mesoblastic 

 somite into the proximal and distal portions is caused by 

 physiological necessity and has no morphological significance^^ 



The case of Lacerla ((gilis is very instrnctive. According to 

 Hoffmann" ('80), the Anlagen of the pronephros in this animal 

 are, in the most anterior segment, cut off from the myotome 

 (sclero-myotome) and remain connected with the lateral plate 

 just as in Petromyzon, Teleostei, and Amphibia ; whilst in all the 

 following portion, they are the actual diverticula formed segment- 

 ally in the parietal layer of the lower part of the somite, as in 

 other Keptilia (pp. 2G4 and I'oö). We thus see the two modes 

 of separation in one and the same animal. 



All recent authors airree in thinkinir that the Anlage of the 

 pronephros is expressed in itself segmen tally and is strictly 

 myomeric. Now the question arises : How many parts are to be 

 distinguished in the mesoblast, and to what part of it does the 

 Anlage of the pronephros belong? 



Van Wyhe ('89) has distinguished, in Selachia, three por- 



l)Tliis view is grounded upon the suggestion of 1'eof. Mixsukuri. 



