252 MEAD. [Vol. XIII. 



is nearer the prototroch). Thus, the cells which at first occu- 

 pied a large part of the area of the subumbrella have left the 

 surface entirely and sunk into the cavity. But since the gen- 

 eral contour of the egg is but little altered, it is obvious that, 

 meanwhile, other cells must have come to occupy this area. 

 They are those of the somatic plate, and the process in general 

 is simply this : the mesoderm and entoderm constantly sink in 

 and so diminish in surface area; on the other hand, the somatic 

 plate becomes thinner, and extends its surface area until it 

 occupies, not only nearly all its original portion of the sub- 

 umbrella, but also that of the mesoderm and entoderm. 



The manner in which the somatic plate extends into the 

 new area is of especial interest. Upon the invagination of 

 the mesoderm, the posterior border of the somatic plate moves 

 slightly backwards, and meets the entoderm plate at the center 

 of the subumbrella, — the point b, text Fig. VII, A, B. This 

 is the only backward movement which occurs on the border of 

 the somatic plate in the viiddle line. The material at b 

 always remains 90° from the prototroch, and finally becomes 

 the posterior end of the metameric larva. The border of the 

 plate on either side continues to move round this pivotal point : 

 its outline is at first convex, but soon becomes nearly straight, 

 then V-shaped, and finally the edges on either side meet and 

 concresce in the mid-ventral region of the embryo (text Figs. 

 VII and VIII). 



In this movement the material which shifts its position does 

 not change its latitude, i.e., the material of the plate which 

 was at first nearest the prototroch remains always nearest; 

 that nearest the point /, or farthest from the prototroch, 

 remains always in this relation, and forms the posterior end 

 of the embryo. 



So it comes about that the somatic-plate cells form a cap 

 over the whole posterior portion of the subumbrella. The 

 only other ectoderm cells are in the region of the stomodaeum 

 and the few small cells, which, as we have seen, migrated from 

 the upper hemisphere through the interruption in the prototroch. 

 These occupy at most but a limited area just posterior to the 

 prototroch. 



