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 view 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 

 arc 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- 



