FOEMATION AND PATE OP THE PRIMITIVE STREAK. 501 



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 stU. 

 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 BB, en- 

 closed 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 Rusconi ; the shaded space BR' 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. 



Fig. 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 P ^ 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 " blasto- 



