DEVELOPMENT OF THE OPOSSUM 93 
36. When the spreading is well under way, the blastocyst, 
previously spherical and centrally placed, is usually flattened 
like a thick biconvex, lens at one pole of the egg, with the em- 
bryonic area in contact with the shell membrane (figs. 1 and 2, 
pl. 6; fig. 4, pl. 10; pls:.12 and 19). 
37. Eggs in which the entoderm has just become closed at 
the lower pole, and which are thus in the beginning of the 
bilaminar stage, are still about the same size as in the cleavage 
stages, but the albumen has become more dense and the shell 
membrane thicker and more resistant (figs. 14 and 17, pl. 13). 
The albumen disappears with the growth of the blastocyst and 
egg (pl. 13). 
38. The bilaminar blastocyst is simply a double-walled sac 
consisting of ectoderm without and entoderm within. The two 
layers are closely applied to each other and to the shell membrane 
and albumen, and any variation from this condition is due to 
shrinkage or to abnormality of the egg. There is no ‘perivitel- 
line’ space in the normal opossum egg, but frequently occurs in 
abnormal material (pls. 19 and 20). 
39. The bilaminar stage is the period of growth, little dif- 
ferentiation occurring until near the first appearance of mesoderm 
in vesicles 1.5 to 1.8 mm. in diameter (pls. 19 to 22). 
40. The 1-mm. stage was once found seven and one-half days 
after copulation (litter. No. 343, fig. 5, pl. 2); the mesoderm first 
appears about eight hours later (compare litters 343’, 346, 346’, 
353, 353’); the 0.8-mm. stage was once removed about five days 
after the beginning of cleavage (litter No. 306’, fig. 17, pl. 13; 
figs. 1 and 1A, pl. 21), and 1.4-mm. blastocysts were found at 
about four and a half days after the beginning of cleavage 
(compare litters Nos. 191 and 198, figs. 1 and 9, pl. 22). 
41. The embryonic area grows in extent with the growth of 
the egg, so that in the later bilaminar stage its diameter is about 
one-fifth to one-fourth of the circumference of the egg (compare 
pl. 18 and figs. 1 to 3, pl. 21, with figs. 1 to 4, pl. 22). 
42. As the egg develops, the embryonic area becomes increas- 
ingly more sharply set off from the trophoblastic -area (fig. 9, 
pl. 21). The embryonic ectoderm becomes thicker, the cells 
