94 THE AMPHIOXUS. 



During the formation of the first two mesoblastic 

 somites, when, the arching over the neural plate 

 at once takes place, the mechanical processes are 

 quickest. They lead to an important dorsiventral 

 depression of the embryo, which is very distinctly to 

 be recognised in Fig. 37, and, as the series of sections 

 tell us, is marked most clearly at the posterior end of 

 the body (Fig. 78, 81). From the stages, with two and 

 three mesoblastic somites, whose dorsiventral diameter 

 is in consequence of the depression less than the trans- 

 verse diameter, a lateral compression becomes notice- 

 able. 



I would be inclined in the mechanical part of these 

 processes, to ascribe to the hypoblast the prepoiider- 

 atingly active part. On a superficial consideration of 

 the representations, we shall at once attribute an 

 active part in the changes of form far sooner to the 

 thick hypoblast layer than to the thin epiblast- 

 covering layer. The chief part in the invaginatioii of 

 the dorsal side, and the considerable flattening of the 

 embryo may proceed from the hypoblast. Still the 

 arching over the neural plate, as we shall see later 

 on, must be referred to energies of growth inside the 

 epiblast itself. In the lateral compression too of the 



