126 ARTHUR ROBINSON, M.A., AND RICHARD ASSHETON, M.A. 



Fig. 5. — A sagittal section of an ovum after the disappearance of the seg- 

 mentation cavity, and when the blastopore ia partially closed, x 23. 



Tig. 6. —A portion of a transverse section of an an ovum after the separa- 

 tion of the chorda from the mesoblast, but before the complete separation of 

 the mesoblast and chorda from the hypoblast, x 138. 



Fig. 7 — A nearly horizontal section through the posterior part of an embryo 

 in which the blastopore had only slightly diminished. The section is taken 

 through the centre of that part of the original blastopore which still remains 

 open. Drawn with camera, x 30. 



Fig. 8. — A nearly horizontal section through the same embryo, but taken 

 ventrally to the existing blastopore through the ventral portion of the original 

 blastopore, the lateral lips of which have coalesced, x 30. 



Fig. 9. — A nearly horizontal section through the same embryo, but taken 

 ventral to both figs. 7 and 8. It was the forty-first section ventral to that of 

 Fig. 7. x 30. 



Fig. 10. — A diagram to illustrate the changes that had taken place in the 

 shape of the blastopore, in the embryo of which Figs. 7, 8, and 9 are sections. 

 The lines numbered 7, 8, 9 are drawn about the levels at which the sections 

 bearing those numbers are taken. The space lettered B B, enclosed by the two 

 small concentric circles, represents the position and extent of the fused layers 

 of the blastoporic rim at the time of the first formation of the circular blastopore 

 or anus of Eusconi ; the shaded space B B' represents the position and extent 

 of the fused layers at the stage of the embryo of which Figs. 7, 8, and 9 

 represent sections. The line lettered x represents the extent of the closure of 

 the ventral portion of the lateral lips up to this stage. The letters Z, Z', ZZ, 

 ZZ', refer to the same spots as those letters do in Figs. 5 and 7. 



Fsg. 11. — A surface view of the posterior end of an embryo, in which the 

 blastopore has diminished considerably, but in which it is still circular, The 

 neural plate, N P, is distinctly visible. The line PG is the primitive groove, at 

 this period only with difficulty made out in surface view. In the figure it is 

 drawn too distinctly. The levels at which the sections Figs. 12 and 13 were 

 taken are indicated by the lines numbered 12 and 13. These sections (Figs. 

 12 and 13) were not taken from the same embryo from which Fig. 11 was 

 drawn. This drawing was made from a living embryo. 



Fig. 12. — A transverse section through the posterior portion of the neural- 

 plate region, showing the commencing separation of the mesoblast from the 

 chorda. X 115. 



Fig. 13. — A transverse section through the posterior part of the primitive 

 streak of an embryo, about the same age as that represented in Fig. 5. The 

 section is taken along the line x in the latter figure, x 50. 



Figs. 14, 15, 16, and 17 are horizontal sections through the posterior end of 

 the same embryo, the stage being precisely the same as that of Fig. 23, on 

 which the levels of the four sections are shown by the lines 14, 15, 16 and 17. 

 Fig. 14 is taken through the centre of the nearly closed "blastopore," and the 

 sections 15, 16, and 17 are the 16th, 32nd, and 44th sections respectively below 

 section 14. 



