DEVELOPMENT OF THE OPOSSUM 61 
g. Included cells which may not be entodermal 
It sometimes happens that a blastomere in early cleavage 
becomes displaced, fails to attain its proper pos tion at the 
periphery and thus comes to be surrounded by its fellows. I 
have several 16-celled eggs with one such misplaced blastomere 
(fig. 17, pl. 15). Another egg, No. 314 (2), shown in figure 2, 
plate 5, has two included cells. One of these in the section 
figured is near a gap in the blastocyst wall, and it might be 
supposed that it had migrated from the unoccupied space. But 
this egg is made up of only 28 cells, a stage at which the blas- 
tocyst would hardly be expected to be completed. The cells 
were probably accidentally included at a somewhat earlier 
stage and are probably not typical entoderm mother cells. The 
included cell in ovum No. 838 (5) shown in figure 12, plate 16, 
has every characteristic of an entoderm mother cell except that 
it is unusually large. Some other included cells, as, for example, 
those in figures 10, plate 21, and 13, plate 22, are clearly undi- 
vided blastomeres of an early cleavage stage, as previously 
pointed out. The true entoderm mother cells are quite dis- 
tinctive and are not readily mistaken for abnormal cells. 
h. Further polar differentiation 
After the proliferation of entoderm mother cells is well under 
way, the differences between the embryonic and the non- 
embryonic areas of the blastocyst become more and more pro- 
nounced. ‘The former becomes marked by the large size of its 
cells as well as by the presence at that pole of entoderm mother 
cells; the non-embryonic portion becomes progressively more and 
more attenuated. These changes are readily understood from 
plates 16 and 17. The increase in size of the blastocyst is 
largely due to the spreading of the non-embryonic or tropho- 
blastic ectoderm, and as a result the embryonic area comes to 
occupy a more and more restricted proportion of the surface 
of the vesicle, and this process continues until the formation 
of the bilaminar stage has been completed. The change in 
