MORPHOLOGY OF CYCLOSTOMATA. 405 



tions of the mesoblast which are called by him the " Epimer," 

 " Mesomer," aiul " Hypomer " respectively. The epimere of 

 VAN Wyhe corresponds solely to the myotome ; his mesomere 

 comprises the Anlage of the mesonephros and the sclerotome ; 

 and the hypomere consists of the Anlage of the pronephros, 

 the genital gland, and the lateral plate. The epimere, mesomere, 

 and the dorsal part of tlie hypomere undergo the metameric 

 segmentation, while the remaining portion of the hypomere 

 remains unsegmentcd. According to van Wyhe, the dorsal seg- 

 mented part of the hypomere is, therefore, the 2:)roduct of the 

 lateral plate (see p. 389). It seems to me that this division of 

 the mesoblast does not agree with the facts observed in Fetromyzon 

 and the other Anamnia above referred to ; for the mesoblast in 

 these groups consists, in early stages, of two portions : (1) the 

 segmented, and (2) the unsegmentcd, and of nothing more, just as 

 RÜCKERT ('88) and Eabl ('88, '96) have remarked. According to 

 RiJCKERT and Rabl, the segmented portion — the somite — com- 

 prises the myotome and the sclerotome ; the pronephros and the 

 mesonephros are derived from its ventral (distal) portion, which is 

 called by Eückert the '' Nephrotom " ('88, p. 272). 



In Petromyzon, these two portions of the mesoblast, the 

 segmented and the unsegmentcd, are, in early stages, clearly dis- 

 tinguished, being histologically different (see p. 31Ö). The meso- 

 blast in such an undifferentiated state is almost entirely occupied by 

 the segmented portion, while the unsegmentcd jiortion is very 

 small, being represented by the loose tissue of a few cells. Such a 

 mesoblastic segment exactly corresponds to the somite of Buckert 

 and Rabl. The proximal half of the segmented portion coincides 

 with the sclero-myotome of those authors. It consists not only 

 of the myotome, but also includes the sclerotome. And it is the 



