224 F. G. HBATHCOTB. 



In fig. 17, a, b, 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 difler 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 

 fiat, 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. 



Eig. 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 



