POLYEMBRYONIC DEVELOPMENT IN TATUSIA 577 
this eutherian mammal. Before taking up this account, it should 
be pointed out that the blastocyst of Dasyurus presents an unusu- 
ally favorable opportunity for the study of the origin of the ento- 
derm. The large size of the vesicle, together with the unilaminar 
condition of its wall, makes possible the preparation of the embry- 
onic region as whole mounts, by the means of which detailed ob- 
servations can easily be made. The eutherian blastocyst, on the 
contrary, does not admit of satisfactory whole mount preparations, 
owing to the fact that the embryonic region is more than a cell 
thick. In the blastocyst of the armadillo this is particularly true, 
as the wall of the formative or embryonal region is about four cells 
thick. Nevertheless, the evidence gathered from the study of 
sections, which we shall now present, is convincingly in favor 
of the tiew that the entoderm arises by the means of migrating 
cells, and does not favor the commonly accepted idea that it arises 
by a delamination, that is, by the splitting off of the lower layer of 
cells from the embryonic knob. 
Blastocyst No. 244 is next to the smallest blastocyst secured, 
measuring but 0.263 mm. in diameter, and, when taken, was on the 
point of entering the uterine cavity from the left fallopian tube. 
At this time it was distinctly spherical in outline, with an em- 
bryonic spot, having a slightly irregular margin and covering an 
are 52° 58’. The specimen is described here, not only because it 
shows a continuation of the changes observed above, but also for 
the reason that it is remarkably well preserved, and consequently 
will give more than ordinary confidence to any interpretations 
which may be based upon its structures. 
The embryonic spot, which measured 0.025 mm. deep by 0.100 
mm. in diameter, was too thick to permit the determination of the 
details of structure from a study of the living specimen; but in the 
sections the details stand out brilliantly, and prove beyond a per- 
adventure that there are two types of cells composing the embry- 
onic mass. The entodermal cells, as in the case of specimen No. 
335, are apparently quite generally distributed throughout the 
inner cell mass. The section passing through the center of the 
mass is typical in that it shows all of the essential features, and 
especially the relation which exists between the two elements con- 
