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nated in the order of their separation from the yolk, as A, B, C and D. The yolk is 
represented by stippling. The metentoblast is designated MH, the mesoblast M, the 
entoblast Z. 
Figures 1—5 are surface views drawn with polar body (P) above, and represent 
stages with from two to eight cells. 
Fig. 6. Side view of 16-cell stage; the fourth ectomere D is being formed from 
the yolk cell. 
Fig. 7. Longitudinal section of 16-cell stage. The mesentoblast occupies the cleav- 
age cavity. 
Fig. 8. Transverse section through the posterior end of 32-cell stage at the point 
where a small portion of the primary mesoblast is exposed to the exterior. 
Fig, 9. Optical section of a stage with sixty cells in the blastoderm, with four 
mesoblast-cells (JZ) and two entoblasts (Z). 
The fourth cleavage appears earlier in the ectoblast cells than in 
the yolk-cell. The yolk-cell divides unequally and gives off toward 
the animal pole the last ectomere (Figs. 6, 7). It will be noticed that 
in the formation of the four ectomeres there is a strongly marked 
bilateral symmetry. The first was formed anteriorly, the second right, 
the third left, the fourth in the median plane on the side toward the 
animal pole. In the transition from the 8- to the 16-cell stage 
the yolk becomes crowded into the “cleavage” cavity (Fig. 7). 
Thus the ectoblasts come to almost surround the yolk-cell. This is 
now completely covered on the side toward the animal pole. The 
point where the yolk-cell is still exposed to the exterior is on the 
posterio- ventral side, and marks the position of the blastopore. The 
main mass of the yolk-cell lies in the blastopore, and constitutes the 
blastoporic plug. 
The fifth division again involves all of the cells. The yolk-cell 
(mesentoblast now) divides, cutting off a cell which lies in the blasto- 
pore (Figs. 7—8 M). This is the primary mesoblast. The yolk-cell 
represents the entoblast. The ectoblasts have at this time almost closed 
over the blastopore. In the series of sections from which fig. 8 was 
drawn only a small portion of the surface of the mesoblast-cell is ex- 
posed to the exterior. The ectoblast-cells have grown over the cell, 
so that the main mass lies within the blastoderm. The plastopore 
becomes completely closed between the stages with 30 and the one 
with 60 ectoblast-cells. The primary mesoblast-cell usually begins to 
divide while still in the mouth of the blastopore. As the ectoblasts 
close over the blastopore the products of the primary mesoblast be- 
come crowded into yolk-mass and lie just in front of the closed 
blastopore. Division of the mesoblast-cells takes place much more 
rapidly than that of the ectoblasts, so that in the stage with 60 ecto- 
blasts there are usually four mesoblasts (Fig. 9 M). No evidence of 
another origin of the mesoblast has been seen. The nucleus of the 
ig) 
