FOURTH PERIOD OF DEVELOPMENT. 133 



The bending of the mesoblastic somite boundaries, 

 which at first showed itself only in a slight back- 

 wardly turned curvature of its ventral part, becomes 

 continually more pronounced, till at last it becomes 

 an angular bending of these lines. The apex of the 

 angle is in the region of the notochord (Figs. 54, 61), 

 and gradually moves a little further dorsally. The 

 two sides, both the shorter and dorsal, as well as the 

 longer and ventral, point in the posterior direction. 

 The angle becomes, in the course of development, 

 continually more acute, and more especially the ven- 

 tral side, which, after retroformation of the ventral 

 part of the partitions no longer reaches so far, 

 and is turned more sharply towards the back (Fig. 

 61 A). 



We will now review the histological differentiations 

 of the mesoblast. As we have previously seen, all the 

 parts of the mesoblastic somite as well as the outer 

 surface, and also the archenteron and the part in con- 

 tact with the neural canal, are composed of cells which 

 experience a considerable flattening. Only the cells 

 in contact with the notochord, which are to form the 

 lateral body muscles and which on each side compose a 

 small band running the whole length of the body, 



