72 THE CHANGES FROM STAGE A TO F. 



Stage A, and in the earlier embryos of Stage b, vanishes later 

 in Stage b. I do not know what becomes of it. 



Of the mesoblastic somites during Stage c there is but 

 little to say. They maintain the same relations and struc- 

 ture (Plate VI, fig. 6 a — d) as in the preceding stage, 

 that is to say their walls are everywhere in contact with the 

 adjacent ectoderm or endoderm. The cells of the somatic 

 layer are thicker than those of the splanchnic layer. This 

 difference was observable in the previous stages (Plate VI, 

 fig. 5 a), and becomes, as we shall see, much more marked 

 in the subsequent stages. During Stage d, somites are still 

 being formed at the hind end from the actively growing tissue 

 of the primitive streak. This stage may be said to mark the 

 close of the formation of new somites, i. e. by Stage e the 

 embryo has acquired its full complement, though the posterior 

 are so small and rudimentary that they are not visible from 

 the exterior. 



