54 CARL G. HARTMAN 
more cells within the blastocyst cavity, as well as other enlarged 
and modified cells still within the wall (pls. 7 and 16; compare 
Hartman, ’16, p. 36). I conjectured that the free cells might 
have arisen by accidental inclusion of a blastomere in about the 
16-celled stage (compare fig. 17, pl. 15) or by proliferation from 
the large cells within the blastocyst wall, since frequently a 
number of cells would be united into a column projecting into 
the cavity (figs. 3 and 8, pl. 6). These cells appeared to come 
from various points in the blastocyst wall. 
My next stage consisted of considerably advanced unilaminar 
blastocysts (compare figs. 1 to 4, pl. 18), in which I found ento- 
derm in various stages of differentiation, including certain few 
cells that appeared to come out of the formative area of the 
blastocyst in precisely the same manner described for Dasyurus 
by Hill; and I figured cases in point. 
With these two considerably separated stages before me, I 
concluded that the entoderm arose, as in Dasyurus, after the 
formative area had become well differentiated, and hence I 
considered the included cells of the young stages as of ‘no 
morphological importance.’ 
Since publishing my report on these young blastocysts, I have 
been fortunate enough to collect an unbroken series of tran- 
sitional stages between the just completed unilaminar blas- 
tocyst and the just completed bilaminar stage, ‘and of especial 
interest are litters Nos. 344, 356, 194’, and 349, of which I 
possess numerous preparations (pls. 8, 9, 16, 17). I also have 
more than five dozen additional young unilaminar eggs of the 
stage previously described, so that I now have before me 100 
such preparations, besides a considerable number which I did 
not consider necessary to section. In these blastocysts I again 
find the persistent occurrence of the peculiar included cells such 
as previously described, which my new material now teaches are 
the true entodermal mother cells of the opossum. What I had pre- 
viously described as entoderm formation marks the end and 
not the beginning of this process. The true entoderm formation 
begins in blastocysts containing 50 to 60 cells within the blas- 
tocyst wall; that is, these large modified cells in the blastocyst 
