ii 8 THE AMPHIOXUS. 



94). In these sections it may be recognised that the 

 first mesoblastic somite has at once fully attained 

 its separation and given up its continuity with the 

 wall of the archenteron. It may be gathered from 

 this that the continuation which grows out does 

 not arise through a new folding of the hypoblast 

 as was the case with the notochord, but is developed 

 from the already existing mesoblastic somite. Further, 

 if the series of sections of the different stages be 

 followed, it may be seen how the hollow continuation 

 penetrates continually further forward in regions 

 where there was formerly no mesoblast formation to 

 be seen. 



The mesoblast continuation shows, then, like the 

 mesoblastic somite, in general a triangular form (Figs. 

 95, 96), and its parts differentiate later on in a similar 

 way. The extension of the individual parts, espe- 

 cially of that part which is in contact with the noto- 

 chord, is, however, here much slighter. Later on 

 its cells change here, just as in the body, into the 

 muscular band lying by the side of the notochord. 

 These muscle cells, in the region of the first somite, 

 and especially in its anterior prolongation, are already 

 much smaller than in the other somites. 



