88 CARL G. HARTMAN 
with large or small masses, about which the nucleus lies as if 
about to engulf it. In figure 11, plate 22, are several typical 
cases drawn with the aid of the camera lucida. At A two bodies 
are found in connection with a single nucleus; and a similar case 
is seen in section of a somewhat younger blastocyst at A, 
figure 10. At B, figure 11, the entodermal cell behaves toward 
a vacuole as toward a solid mass, a phenomenon by no means rare. 
The large cell at C seems to have completely ingested a mass, 
the body in the center being not a nucleolus, but a typical 
inclusion like those marked Y in the other figures. 
I have looked through the whole series of stages from the first 
appearance of entoderm to the largest bilaminar blastocyst and 
find that the foreign bodies just described are present in nearly 
all cases, whether in total preparations or in serial sections. I 
am convinced that they are only remnants of undigested yolk. 
If one recall the young blastocyst containing 40 or 50 entodermal 
cells (litter No. 356) at a stage when the trophoblast has become 
considerably attenuated (pl. 17) one notes that the included yolk 
is almost entirely confined to the embryonic area. So in suc- 
ceeding stages, numerous granules of yolk are found among the 
embryonic, seldom within the trophoblastic, cells. More such 
granules are, with some exceptions, found in the younger than 
in the older blastocysts. Thus, while the albumen melts away 
before the embryonic area of the young bilaminar blastocysts, 
the yolk granules maintain their identity in small rounded 
masses for’a longer time. 
f. Mesoderm formation initiated 
With the appearance of the first mesodermal cells about six 
days have elapsed since ovulation, about five days since the 
beginning of cleavage, or about one-half of the period of gesta- 
tion, which I am tentatively stating to be ten days in the 
opossum. The formation of the mesoderm will be treated in the 
next number of these studies. 
