PLATE 3 



EXPLANATION OF FIGURES 



47 Portion of a section from blastocyst No. 311. It shows the well-organ- 

 ized layer of entoderm, which takes a deeper stain than does the overlying ecto- 

 derm. X 417. 



48 Similar section from specimen No. 340. This shows the first step in the 

 rounding up of the ectoderm to form a ball-like mass. X 416. 



49 Median section of blastocyst No. 332. The ectodermal mass has become 

 included within the entoderm. X 149. 



50 This section is taken four sections to the right of the preceding, and 

 shows with especial clearness the vacuolization of the ectodermal mass to form a 

 vesicle. X 149. 



666 



