76 THE AMPHIOXUS. 



In the next stages, with four and five mesoblastic 

 somites, the elongation of the embryo advances, and 

 at the same time there takes place an alteration of its 

 transverse section, the dorsiventral diameter being in 

 the meantime enlarged at the expense of the trans- 

 verse diameter. Thus there takes place a lateral 

 compression ; at the same time the dorsal surface, which 

 was originally sunken and afterwards flat, becomes at 

 last convex. 



The most important advances which we are able to 

 observe in the stages with four and five mesoblastic 

 somites are concerned however with the further de- 

 velopment of the latter. Their cavities situated one 

 behind the other, which hitherto stood in open com- 

 munication with one another, are separated so that 

 there is formed a double-layered wall of cells between 

 two contiguous cavities. At the same time however 

 the individual cavities of the mesoblastic somites still 

 stand in open communication with the archenteron. 

 Especially large is the opening of the first mesoblastic 

 somite (Fig. 47). 



A further and important step which is to be re- 

 marked in the mesoblastic somites in the living object 

 is the appearance of a sharp lateral boundary between 



