84 CARL G. HARTMAN 
occupy the superficial position only as undifferentiated entoderm 
mother cells from which all of the entoderm is destined to be 
formed. ‘The formative area is, from the beginning, potentially 
ectoderm, entoderm, and mesoderm, giving rise first to the 
entoderm and later in quite a similar manner to the mesoderm, 
the residue becoming definitive ectoderm. ‘The trophoblastic 
area consists of a single layer, the ectoderm, until lined with the 
entoderm arising from the embryonic area. 
THE LATE BILAMINAR BLASTOCYST 
a. General description 
Passing now to the later stages, we note that superficially the 
blastocyst appears to have changed but little, except in. size 
(compare figs. 3, 5, and 6, pl. 2). The embryonic area remains 
prominent at the upper pole and less and less albumen remains 
at the lower pole. Important changes in the blastocyst wall 
_ are, however, to be discovered from a study of the sections. 
In blastocysts of 1.2 to 1.5 mm. the ectoderm, both embryonic 
and trophoblastic, has attained its maximum thickness for the 
bilaminar stages. Two eggs shown in plate 22 illustrate this 
point. Egg No. 353 (4), shown in figures 3, 3A, 3B, 3C measured 
1.22 mm. in alcohol. The embryonic area consists of tall cells, 
for the most part of the columnar type; the trophoblastic area 
is the same as in smaller blastocysts above described. Practically 
the same holds true for egg No. 360 (4) (fig. 6), which measured 
about 1.3 mm. in diameter (compare stereogram fig. 8, pl. 10). 
The wall of a somewhat smaller egg, No. 347’ (1), 1.1 mm. in 
diameter in alcohol, has a somewhat thinner embryonic area 
(fig. 5, pl. 22). There is, however, considerable variation in this 
respect, even within the same litter of eggs of equal size. 
In all of the larger blastocysts the entoderm can be followed 
as a continuous layer completely lining the vesicle. Except 
where pulled away in the preparations, the entoderm fits 
closely against the ectoderm and is always distinctly recognizable 
at all points (fig. 6, pl. 10). 
