72 THE AMPHIOXUS. 



sion of the dorsal surface, which even in the living 

 object may be plainly enough seen, especially in 

 optical transverse sections. This depression extends 

 from the anterior fourth of the body as far as the 

 gastrula mouth situated at the posterior end (Figs. 35, 

 36, 39). The larva shows upon the optical transverse 

 sections a now almost triangular outline. The two 

 edges which constitute the boundary of the dorsal 

 surface require the formation of two longitudinal 

 folds of the hypoblast. 



These folds, which are in no way more sharply 

 separated from the rest of the hypoblast, form the 

 material which becomes the mesoblast. We will 

 from now on characterize these longitudinal folds of 

 the hypoblast as mesoblast folds. 



In these mesoblast folds the anterior part begins 

 to separate itself from the posterior and greater part 

 by a sharper contour. On this anterior section the 

 folding is at once more sharply marked, and it appears 

 therefore as a distinct and prominent fold which 

 represents the anterior mesoblastic somite (Figs. 36, 

 38). 



The most anterior mesoblastic somite appears 

 accordingly marked off by a sharp boundary from. 



