74 CARL G. HARTMAN 
All of the eggs still lie free in the lumen of the uterus (fig. 10, 
pl. 1; fig. 8, pl. 2) and are distributed as in the preceding stages, 
often grouped near the os uteri; hence one should not speak of 
the ‘implantation’ of the eggs even at the 2-mm. stage. The 
shell membrane has attained considerable thickness (Hartman, 
16) and throughout the stage in question maintains the shape 
of a perfect sphere. 
Before the entodermal spreading is well under way the blas- 
tocyst occupies approximately the center of the egg (fig. 1, pl. 9). 
Before the entoderm has reached the opposite pole of the blas- 
tocyst the embryonic area has almost or quite come into con- 
tact with the shell membrane (compare figs. 3 and 4, pl. 19), 
giving the blastocyst a decidedly eccentric position, It now 
fills one-half or less of the egg and has the shape of a bi-convex 
lens (fig. 5, pl. 10). This migration of the blastocyst may be 
due to the increased metabolism of the more voluminous cells 
of the embryonic area, as a result of which the albumen is here 
more rapidly digested and absorbed. This position is maintained 
in the subsequent stages (compare eggs No. 299’, fig. 1, pl. 6). 
The size of the entire egg containing the youngest bilaminar 
blastocysts with just closed entodermal sae is very little greater 
than the youngest uterine eggs, although the albumen has 
become denser and the vesicle wall has become considerably 
differentiated. Thus, for example, the diameter of the eggs in 
figures 4 to 6, plate 9 (all bilaminar blastocysts), is only a little 
greater than that of eggs in cleavage stages (figs. 1, 3, and 5, 
pl. 1). Again, the two litters shown in figures 3 and 4, plate 1, 
exhibit an evident, but not striking growth in volume, although 
they have developed from the 4-celled stage in the former to 
young bilaminar blastocysts in the latter in a period of four 
days, or 40 per cent of the entire period of gestation! 
In all of the young bilaminar blastocysts the embryonic area 
is plainly outlined and distinctly marked off at the junctional 
line from the trophoblastic region. This differentiation increases 
with the growth and development of the egg. 
From the beginning, the bilaminar stage is acentalie the 
period of growth; this period thus follows the formation of the 
