PLATE 12 
EXPLANATION OF FIGURES 
Résumé of stages in the development of the opossum eggs from cleavage to 
the completed bilaminar blastocysts; drawn from actual specimens cleared in 
oil of wintergreen, the measurements being made from photographs of the living 
egg. X 50. 
1 The unsegmented uterine egg. 
2 The 4-celled ovum. 
3 The just completed unilaminar blastocyst of about 32 cells. 
4 Blastocyst with polar differentiation well under way; primitive entoderm 
present; drawn after No. 356 (6) (compare fig. 2, pl. 6). 
5 Blastocyst with attenuated unilaminar trophoblastic area, bilaminar only 
in the embryonic region (after litters Nos. 194’, 175’, and 352). 
6 More advanced blastocyst with spreading entoderm; after No. 290 (4), 
photographed in fig. 2, pl. 6; the flattened shape of the vesicle is the usual one at 
this stage. 
7 Similar stage with more unusual spherical blastocyst, drawn after No. 299’ 
(2), shown photographically in fig. 2, pl. 6. 
8 Completed bilaminar blastocyst, drawn after No. 299’ (1), shown photo- 
graphically in fig. 2, pl. 6. 
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