lod 
76 CARL G. HARTMAN 
has reached the shell membrane, which occurs almost com- 
pletely when the egg is 1 mm. in diameter, more perfectly 
at a diameter of 1.5 to 2 mm. It then maintains the spherical 
‘rom until, through crowding of large litters in the pregnant 
uterus, the vesicles are somewhat misshapen through mutual 
pressure. 
The embryonic area of the larger blastocysts, due to its pro- 
toplasmic differentiation, now stands out clearer, so that it is 
plainly visible in living eggs. It is recognizable in photographs 
of living eggs, but much more clearly in photographs o° eggs 
immersed for a few minutes in the fixing fluid (fig. 2, pl. 11). 
The size of the embryonic area varies greatly, even in pro- 
portion to the total surface area of vesicles in the same litter. 
In general its diameter occupies between one-fifth and one- 
fourth of the circumference of the egg, occasionally a little less 
than one-fifth, sometimes nearly one-third. But it never reaches 
the equator as in Dasyurus. An average 1 mm. blastocyst is 
shown in figure 9, plate 21. Various measurements are given 
under the legends of the eggs illustrated. 
Aside from these details, little may be learned from a study 
of the living egg, and we must turn to preparations for more 
intimate details of structure. The progress of development will 
be followed by describing first the youngest stage, then the 1 
mm. blastocyst, and lastly the blastocyst just preceding the 
proliferation of mesoderm. 
THE JUST COMPLETED BILAMINAR BLASTOCYST 
The bilaminar stage may be said to begin when the entoderm 
has migrated to the trophoblastic pole of the egg opposite its 
point of origin, thus forming a closed sac within the ectoderm. 
This stage was attained by most of the eggs in litter No. 299’ 
about four days after the beginning of cleavage. The vesicle 
occupies about one-half of the egg contents. 
a. The embryonic ectoderm 
In the youngest bilaminar blastocysts the embryonic area is 
approximately circular in shape and clearly visible, but not as 
