220 RIOHAED ASSHETON. 



f IG. 7.— Surface view of the embryonal area of a rabbit embryo, in wliicli 

 the primitive streak has become much shallower. Growth in length of the 

 embryo has now taken place. Age about 188 hours, X 18. 



Fig. 8.— Surface view of the embryonal area of the embryo of a rabbit. 

 The primitive groove has entirely disappeared. The ecto-placental region 

 is now firmly attached to the placental lobes of the uterus. Age about 192 

 hours. X 18. 



Fig. 9. — Surface view of embryonal area of the embryo of a rabbit. Age 

 about 196 hours. X 18. 



Fig. 10. — Surface view of the embryonal area of the embryo of a rabbit. 

 The primitive streak has now become compressed to its former dimensions ; 

 the mid. dorsal portions of the embryo have become thrust forward, and the 

 head-fold has commenced to be formed. Age about 200 hours. X 18. 



Fig. 11. — Diagram to illustrate the relative positions of the two areas of 

 cell-production. 



Fig. 12. — Diagram to illustrate the effect produced upon the two areas of 

 cell-production by the increasing hydrostatic pressure within the blastodermic 

 vesicle. 



Fig. 17.— a sagittal section through the specimen of which Fig. 4a is a 

 drawing. It is taken along the line 17. X 100. 



Fig. 18. — A median sagittal section through a specimen intermediate be- 

 tween Figs. 5 and 6. X 72. 



Fig. 28. — A median sagittal section through Fig. 10. X 54. 



Fig. 39. — A horizontal section through a portion of the sheet of mesoblast 

 surrounding the primitive streak. The portion drawn was a lateral portion. 

 X 350. 



PLATE 21. 



Figs. 13, 14, 15, 16. — Transverse sections through specimen Fig. 4, along 

 lines 13—16. X 175. 



Figs. 19, 20, 21, 22, 23. — Transverse sections through specimen Fig. 5, 

 along the lines 19—23. X 175. 



Figs. 24, 25, 26, 27. — Transverse sections through the specimen Fig. 6, 

 along the lines 24—27. X 175. 



Figs. 29, 30, 31. — Transverse sections through a specimen similar to that 

 drawn in Fig. 8, along lines corresponding to those marked 29, 30, 31, 

 in Fig. 8. X 175. 



Figs. 32, 33, 34. — Transverse sections through the anterior end, the 

 middle, and posterior end of the primitive streak of a specimen slightly older 

 than that of which Fig. 10 is a drawing, x 175. 



