n 6 THE AMPHIOXUS. 



The cells in contact with the notoohord, which, as 

 we saw earlier during the consideration of the whole 

 embryos, stretch through the whole length of the 

 segment, are intended to provide the lateral muscles 

 of the body. 



The remaining part of the interior side of the meso- 

 blastic somites, which is in contact with the medullary 

 plate, takes no part in the formation of the muscles, 

 bnt its cells become in the later stages a plate-like 

 epithelium. 



In just the same way, too, the cells of the outer 

 mesoblastic somites which are in contact with the 

 outer surface become flattened. 



We can see these differentiations quite plainly 

 expressed in the stage with six mesoblastic somites 

 (Fig. 100). 



In the following stages, with nine mesoblastic 

 somites their extension towards the ventral side may 

 be followed (Fig. 116). We see that this extension 

 is dependent on the outer surface, which is constantly 

 flattening more and more, and on the fibrous 

 layer of the alimentary canal. These layers, while 

 intruding like a wedge between hypoblast and epi- 

 blast, grow towards the ventral middle line. While 



