518 I. IKED A : 



Caldwell. Their small size and the total al)seiice of imelear 

 substance make it easy to tlistingnish them from the trne meso- 

 blast cells. 



The corpuscles are still frefjuently discovered in the l)]asto- 

 cœlic cavity at the beginnini^' of gastrulation, together with a few 

 mesoblast cells. But in an advanced gastrula they have whollv 

 disappeared, })Ossibly having been absorbed \)y the Ijlastoderm cells. 

 I think the temporary co-existence of the plasmic corpuscles and 

 of the true mesoblast cells in the blastocœle of the gastrula, has 

 led some previous authors to confound the two elements. As to 

 the signihcance of the corpuscles, I can at present offer no opinion 

 '^ uule-^s they be merely an excess of supply of nourishment analo- 

 gous to food yolk " as has been suggested by Caldwell ('82, I.e., 

 p. 18). 



C. GaöTKULATIÜN and m esq BL AST-Foil m ATION. 



In this section, I shall iirst describe what I conceive to be 

 the tj'ue history of gastrulation and mesoblast-formation, and then 

 pass on to a discussion of the views of other writers. The two 

 developmental processes are so intimately related to each other, 

 that it seems best to treat lliem together. 



First as lo external changes. The hilalei'nl synnnelry of the 

 |>lano-convex Mastula becomes more clearly marked than hefore 

 when the gastral invagination begins on the ventral or the flat- 

 tened side. The initial depression occurs over the whole ^'entral 

 wall, so that a saucer-shaped emljryo is produced. At first it is 

 so shallow as to be perceived with difficulty in the surface view. 

 Soon it deei)ens, becoming deepest at a point somewhat nearer to 

 the 1)roader end than to the narrow qi\<^i of the embryo. The 



