178 HUBERT DANA GOODALE 



determining the distribution of the egg material. The yolk 

 granules undergo little change during the early part of develop- 

 ment. 



A brief account of the later history of the granules is given 

 here in order to afford a connected account of the distribution 

 of egg materials into some of the embryonic organs. In fig. 14, 

 a vertical section of an eight-celled stage, the general distribution 

 of yolk granules has been represented; / indicates the region 

 occupied mostly by fine granules, i the region of intermediate 

 granules and y the region of large granules. The granules of 

 intermediate size are being moved or are moving towards the 

 outer equatorial regions of the egg. The same process at a later 

 stage of cleavage is shown in fig. 16. Some large cells with large 

 yolk granules are present in the center of the egg formerly occu- 

 pied by granules of intermediate size. 



By the time a well-developed blastocoele has formed the gran- 

 ules have become distributed as follows. In the roof are cells 

 whose granules for the most part were derived from the upper 

 zone, although some cells containing larger granules occur. At 

 the sides of the blastula where the roof thickens, most of the cells 

 have granules from the intermediate zone. The granules in the 

 floor cells of the blastocoele are large ones which have main- 

 tained their original position. 



At the close of gastrulation, when the neural plate has become 

 distinct, we find that the upper zone of granules has maintained 

 its position and now occupies the cells of the neural plate, except 

 in its posterior part, which, together with the posterior part of 

 the archenteric roof, the ectoderm exclusive of the neural plate 

 and a small area immediately anterior to the last, have granules 

 from the intermediate zone. Possibly parts of the mesoderm 

 have a similar origin. From the lower zone is derived most of 

 the roof of the archenteron including the mesoderm and noto- 

 chord, besides the large ventral mass of yolk cells. 



Even at a comparatively late stage, i. e., when the somites 

 have begun to develop, the source of the granules of some parts 

 of the embryo can still be determined. For example all of the 

 outer layer of cells have granules of intermediate size. 



