454, F. G. HEATHCOTE. 
In fig. 17, a, 6, I have shown the protoplasmic network under 
a high power. Early on the third day some of the segmenta- 
tion masses make their appearance on the outside of the ovum 
at different parts, and there undergo rapid division, the re- 
sulting cells spreading out to form the blastoderm in a manner 
very similar to that which takes place in Amphipods (14). In 
figs. 6, 18, I have shown this process taking place. 
The large flat-shaped cells which form the first beginning of 
the blastoderm differ considerably from the segmentation 
masses from which they originate. Their outline is clear and 
distinctly marked; their nucleus is very distinct, of an oval 
shape, with its long axis pointing in the direction of the long 
axis of the cell. A section through an ovum in this stage, 
when seen through a low power, shows the blastoderm cells as 
flat, pavement-like cells, with a long-shaped nucleus. An oil 
immersion lens, however, shows further details. Each cell is 
directly continuous with the neighbouring blastoderm cells, and 
also with the cells which remain in the yolk, by means of fine 
processes of protoplasm. There is also a difference observable 
in the cells within the yolk, which at this stage constitute the 
endoderm. Their outline is far more distinct; their nucleus 
is round, deeply stained, and rather smaller than at an earlier 
stage. 
Fig. 6 shows a single segmentation mass appearing at the 
surface of the ovum, and about to divide to give rise to 
blastoderm cells. 
Fig. 18 is part of a transverse section through an ovum at a 
slightly later stage seen, under a high power; it shows a seg- 
mentation mass which has divided, giving rise to several blas- 
toderm cells, while some of the cells arising from the original 
segmentation mass remain behind in the yolk as endoderm, 
but are still connected with the blastoderm cells by processes. 
At the stage represented in the last-mentioned section the 
blastoderm is present in isolated patches on the: surface of the 
ovum. At the close of the blastoderm formation, then, the 
ovum consists of an external layer of flat cells—the ectoderm— 
with deeply stained nuclei, these cells being continuous on the 
