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LEWIS R. GARY. 



going on, although over the remainder of the gastrula wall there 

 is no appreciable change in the characteristics of the cells. 



Appellof makes no direct mention, although his Fig. 13, PI. 2, 

 shows, that while the transfer of the nutritive material is taking 

 place the yolk spheres are breaking up rapidly, so that to infer 

 that the spheres pass through the invagination layer in their 

 original condition is unnecessary. In the older gastrulse of A. 

 bermudensis a complete yolk sphere was scarcely ever found in 

 the gastrocoel. While many of the masses of nutritive material 

 still retained their identity and practically their original volume 

 it could be observed in every instance, that the sharp outline 

 was no longer apparent, and usually the granules were separated 

 from one another. It is also noticeable in the section shown in 

 Fig. 6, that nearly all of the yolk material present in the blastoccel 

 is massed about the invaginating cell layer and that the inner 

 ends of these cells are much less sharply defined than they were 

 in the younger stages, Fig. 5, or than they are more laterally in 

 the invaginating area. The central part of the invaginating area 

 is more densely filled with granules than are those parts farther 

 to the sides. The granules in this denser area are also markedly 

 larger than the granules in the cytoplasm of the more laterally 

 placed cells. 



It seems, then, beyond question that in A. bermudensis, just 

 as in Urticina crassicornis, there is an actual passing of the yolk 

 material, in a practically unaltered state, through the layer of 

 invaginating cells to the forming gastroccel. As the cells of the 

 invaginating layer approach those of the outer gastrula wall all 

 of the yolk passes through so that the two layers come into con- 

 tact and the supporting layer is secreted between them. 



In an older embryo, Fig. 7, in which the formation of the 

 stomodeum has begun, the gastroccel gastro-vascular cavity- 

 still contains a considerable amount of the yolk material which 

 now appears as distinct granules. In nearly all instances, how- 

 ever, the granules are arranged in groups which show clearly their 

 origin from an originally more circumscribed mass. 



In this last-mentioned stage the cells making up the endoderm 

 have undergone a considerable change so that tlu-ir histological 

 characteristics are very different from those of the ectoderm cells 



