124 THE AMPHIOXUS. 



front. This is perhaps to be explained through a con- 

 nexion with the mechanical process of the formation 

 of the mesoblast folds, and the diminution of the body 

 form. 



It still remains to explain the condition of the neural 

 plate in the anterior region of the body, where 

 there are conditions before us which differ from those 

 of the typical mesoblastic somite. The neural 

 plate reaches, as we have seen in the sections of the 

 previous stages, forward beyond the region of the 

 mesoblastic somites, and its last prolongation forms 

 there the ground of the channel, which in this place 

 is open outwards (Fig. 86). The closing of this opening 

 still makes slow progress during this period of develop- 

 ment, and, as already mentioned, in a direction from 

 back to front, as may be seen from a comparison of 

 the serial sections, and from carefully comparing the 

 side views of the embryos. 



The front end of the neural foundation is also 

 distinguished by other peculiarities from the parts 

 which follow further back. It can already be seen in 

 the earlier stages that the neural plate in the 

 region of the first mesoblastic somite is formed some- 

 what more thickly than further back. During the 



