86 CARL G. HARTMAN 
c. Modified entodermal cells 
In many of these larger eggs the entoderm undergoes slight 
differentiation at one point. Over the junctional line which 
marks the border of the embryonic area there is often a group 
of entodermal cells which attract attention by virtue of their 
number, the roundness of their nuclei and the volume of the 
cytoplasm (HNT, fig. 9A). They are sometimes found in eggs 
of 1 mm. (ENT, fig. 5, pl. 21), more often in larger blastocysts 
(ENT, figs. 4A, 6 and 7, pl. 22). Similar entodermal cells have 
been described by Van Beneden for the bilaminar blastocyst of 
the rabbit. He states that they mark the anterior end of the 
area and that the future primitive streak appears at the opposite 
side. In the preparations made from litter No. 353’, in which 
there occurs the first anlage of the primitive streak, these cells 
appear in the region where the mesodermal cells are found, 
hence not in the anterior, but in the posterior portion of the 
embryonic area. I shall treat this subject further at a later 
date. 
d. The ectoderm of late bilaminar blastocyst 
As was stated in the preceding section, the entoderm attains 
its maximum thickness in vesicles of about 1.3 mm. diameter 
(figs. 3A and 3B, pl. 22). Larger vesicles may have thinner 
formative areas or they may remain about the same, although 
they are apparently more slender because of their length in 
sections. In some of the eggs, especially in litters Nos. 193’ 
and 343’, the trophoblastic areas are as greatly attenuated as in 
the 0.8-mm. stage (figs. 1 and 9A, pl. 22). Sometimes the 
trophoblastic area becomes gradually thicker towards the lower 
pole (fig. 4, pl. 22), or there may be ectodermal pockets or 
‘blisters’ at this point, as described above in connection with 
the 1-mm. stage. 
As seen in surface view, the distribution of cells is practically 
the same as in the 1-mm. blastocysts (figs. 12A and 12B, pl. 22). 
All of the cells of the embryonic ectoderm are crowded closely 
together and are darker than the trophoblastic cells because 
