= 
56 CARL G. HARTMAN 
take a much darker stain, as in figure 4, plate 7. This elongated 
type of cell is common in the collection. If the attachment of 
such a cell in the wall continues, it may give rise by cell division 
to columns of three, four, or more cells, as numerous examples 
serve to indicate (figs. 3 and 8, pl. 6, and figs. 20 and 21, pl. 16). 
d. The detachment of entoderm mother cells 
It more commonly happens, however, that the entoderm 
mother cells leave their place in the wall soon after attaining 
their maximum size, and their behavior at this time constitutes 
perhaps the most remarkable phenomenon in the entire de- 
velopment of the opossum egg. Their performance at this 
stage is little short of spectacular. Such partly or wholly 
detached cells are present in nearly every egg of litter No. 88, 
which covers this critical period in the formation of the ento- 
derm by a series of more than two dozen preparations, and 
there are identical cells in numerous other excellent prepara- 
tions from various litters. The cells, moreover, have such a char-. 
acteristic appearance that I should term them the more typical 
entoderm mother cell of the opossum. 
After a period of growth the entodermal cell rounds up on all 
sides. In this way its contour no longer conforms to the 
curvature of the ovum, and as a result, the contact with the 
adjoining cells is broken—the cell seems to roll out of its place, 
as it were, into the blastocyst cavity. But the gap thus formed 
does not long remain, for the vacant spaces are filled at once 
by a flowing in of the surrounding cells. This is clearly seen at 
A, figures 7 to 11, plate 16, which specimens were not selected 
originally with this point primarily in view, but they.illustrate 
the phenomenon without exception. The entoderm mother cells, 
when they leave their place in the wall, do not, therefore; 
leave gaps that may be called ‘blastopores’ (Selenka), and such 
gaps as occur in earlier stages, with or without included free 
cells, are due to a different cause, as was shown above. Some- 
what more advanced stages, moreover, still proliferate cells of the 
‘same type, as will appear below (pl. 17). 
