THIRD PERIOD OF DEVELOPMENT. 119 



We can thus indicate in the anterior prolongation 

 of the mesoblast, a cavity which is connected with 

 the cavity of the first mesoblastic somite. Further, 

 as in the region of the body, so here there are lamellse, 

 corresponding to the muscular layer in contact with 

 the notochord, and to the muscular layers of the body- 

 wall and alimentary canal. 



Some important conditions are still to be observed 

 in the side view of the embryo on a closer considera- 

 tion of the boundaries of the mesoblastic somite. The 

 latter run at first in a straight line from the dorsal 

 surface to the ventral side (Fig. 48). Gradually they 

 begin in their ventral section gently to bend back- 

 wards (Fig. 50). Thereby is introduced the later 

 characteristic curvature of the segment boundaries. 



Another important process, too, may be indicated on 

 a closer observation of the mesoblastic somite boun- 

 daries. In the early stages, indeed, with eight meso- 

 blastic somites the beginning of a process may already 

 be seen which profoundly influences the construction 

 of the Amphioxus, and explains to us many a hitherto 

 unintelligible peculiarity of it. 



This is an unsymmetrical movement of the meso- 

 blastic somites. This gains a continually sharper 



