92 CARL G. HARTMAN 
27. The entoderm arises from entoderm mother cells of very 
characteristic appearance. They are cells in the blastocyst wall 
which round up and usually roll out of their place, as it were, 
into the blastocyst cavity, as in certain invertebrates, or they 
may remain attached to the wall for some time, in either case 
multiplying by mitotic division (pls. 7 and 16). 
28. The entoderm mother cells all arise from one-half of the 
egg, the future embryonic area (figs. 15 to 22, pl. 16). 
29. The area that remains free of entoderm mother cells is 
the trophoblastic area; it soon begins to thin and spread out so 
that the growth of the ovum now begins. Growth is, therefore, 
at first due to the spreading of the trophoblastic area (pls. 16 
and 17). 
30. Since the entodermal cells spring from the superficial 
epithelial layer in the embryonic area, this would better be 
termed embryonic entectoderm. 
31. When the blastocyst has attained a diameter of 0.3 to 
0.35 mm., the entoderm is several cells deep, being crowded into 
a mass which superficially somewhat simulates an Eutherian 
inner cell mass in the process of spreading. In the opossum 
only the superficial cells are embryonic ectoderm, all the rest 
are entodermal (figs. 13 to 15, pl. 17). 
32. Such a stage was removed from an animal four days after 
copulation, or about a day and a half after the beginning of 
cleavage. 
33. The superficial layer of cells is never transitory; it is 
embryonic ectoderm and not Rauber’s layer; it is in active 
state of mitosis throughout. Rauber’s layer is homologous with 
the non-embryonie or, as Hill has expressed it, the ‘tropho- 
blastic’ area. 
34. The proliferation of entoderm is at an end when the 
blastocyst has attained a diameter of 0.45 to 0.5 mm., when the 
trophoblastic area has attained its greatest degree of attenuation 
(pl. 18). 
35. The entoderm now spreads by an active migration of the 
flattened, definitive entodermal cells toward the opposite pole of 
the egg (pl. 18). 
