78 THE AMPHIOXUS. 



median boundary of the mesoblast folds ; we shall see 

 the same in the later stages coming into continually 

 sharper prominence. 



We will now take into consideration the separation 

 into mesoblastic somites which begins at once in the 

 mesoblast folds. In the most anterior region of the 

 embryo, into which the mesoblast folds do not extend, 

 the hypoblast becomes smaller (Figs. 35, 36). Just 

 behind this flat hypoblast lies dorsal-wards the first 

 mesoblastic somite which originates from a slight 

 cross-fold of the mesoblast. In the optical longi- 

 tudinal section this fold makes itself noticeable 

 through a stronger concavity turned towards the 

 inside in the direction of the archenteron (Fig. 36). 

 The anterior and posterior edge of the first mesoblastic 

 somite is marked by smaller transverse indentations, 

 which especially concern its outer surface. Just 

 behind the lower hypoblast of the anterior extremity 

 lies dorsal-wards the transverse shallow indentation 

 which marks the anterior end of the first mesoblastic 

 somite. A sharper separation inside the cells is how- 

 ever not to be observed here. On the other hand, 

 at the posterior edge of the mesoblastic somite a well- 

 defined separation of the cells is to be observed. This 



