20 DEAN. [Vol. XL 



Blastttla. 



A very late stage of segmentation is shown in surface view 

 in PI. I, Fig. 12, and in vertical section in PI. II, Fig. 29, 

 Contrasting it with the stage last figured, the following differ- 

 ences are apparent : the summit of the cell-cap is larger and 

 rounder ; division has caused the remaining cells of its margin 

 to become indistinguishable ; its outline has become sharply 

 defined, and its margin, if anything, slightly projects above the 

 marginal groove : of the latter the outer boundary cells have 

 almost disappeared, and their nuclei are now found dividing 

 and migrating into the underlying yolk. The cell-cap has now 

 a thickness of seven or eight cells ; its segmentation cavity has 

 become larger and more distinct ; its line of separation from 

 the yolk is even clearer than in the former stage. At certain 

 points, as before, it receives increments of cells from the un- 

 derlying yolk. The yolk nuclei are now exceedingly numerous 

 in both central and peripheral regions, and of them as many as 

 four or five tiers may be counted. The entire periphery of the 

 cell-cap is now clearly distinct and separate from the yolk. 



Gastrula. 



From these conditions the transition to early gastrulation is 

 readily followed. In PI. I, Fig. 13, the cell-cap of the former 

 figure has flattened out as a disk whose margins bend around 

 and enclose the animal pole of the egg. The boundary be- 

 tween blastoderm and yolk is clearly drawn, although at one 

 side, the future tail region of the embryo, its demarcation is' 

 more sharply drawn. Pigment is now noticeable in the yolk 

 region, and becomes still more evident as the blastoderm con- 

 tinues to enclose the yolk ; a light olive color is acquired which 

 gives a sharper outline to the adjacent blastopore. A sagittal 

 section of this stage permits the following growth changes to 

 be noted : the cell-cap of Fig. 1 2 has resolved itself into a well 

 defined roof for the segmentation cavity, forming a homogene- 

 ous layer of small cells, whose thickened margins are continuous 



