DEVELOPMENT OF THE OPOSSUM 83 
The specimens are unique, too, in that the entoderm is somewhat 
lighter in stain than the ectoderm, as described by Minot. 
The surface mount is nicely fixed and is, histologically, an 
excellent preparation. The light areas are as described by 
Minot, and they are even more striking in the specimen than in 
his figure 2B. It is my Judgment, however, that the vesicle in 
question is not entirely normal, for the reason that among all of 
my numerous specimens, I have never encountered any possess- 
ing such large light spaces. It is true that normally small 
lightly staining areas occur in almost all opossum vesicles of about 
this stage; and they usually mark the presence of cells in mitosis 
(figs. 12 and 12A, pl. 22), or cells that have just divided or are 
preparing to divide, and they are especially prominent in speci- 
mens fixed in bichromate mixtures. But they never attain such 
size as in Doctor Minot’s unusual specimen. I have no explana- 
tion to offer of the phenomenon; I saw no evidence of degenera- 
tion of cells at those points. 
Again, among all of my specimens I have looked in vain for 
entodermal cells coming to the surface in bilaminar blastocysts, - 
either in surface views or sections; and I am certain that 
normally this does not occur. I have convinced myself, how- 
ever, that also in the Harvard specimen the entoderm is nowhere 
at the surface and that Doctor Minot was in error in his inter- 
pretation. In the first place, by careful focusing with the oil- 
immersion lens the (ectodermal) nucleus within the light area is 
in several instance seen to be superimposed over an entodermal 
nucleus. Furthermore, if one plot the entodermal nuclei of 
the embryonic area, it is seen that they are uniformly and 
continuously distributed, entirely without reference to the above- 
mentioned light areas. These areas are without doubt ecto- 
dermal and not entodermal. Hence Minot’s comparison be- 
tween the supposedly superficial entodermal cells in the tropho- 
blastic area of: the opossum with the entoderm mother cells of 
the unilaminar blastocyst of Dasyurus is a futile one. 
The entoderm, therefore, never comes to the surface in the 
bilaminar stage of the opossum egg. Entodermal cells in 
Dasyurus and in the opossum, and doubtless in all marsupials, 
