- 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. K 149. ; 
666 
