McAllister—Cytology and Embryology. 621 
paratus and lacking any cell which might he regarded as an egg 
cell. 
At about the time of the formation of the cell membranes 
about the nuclei in the outer end of the embryo-sac, the two 
polar nuclei have come together in the central part of the sac. 
They remain in contact, but entirely distinct from one another, 
until about the time of the beginning of embryo formation, 
when they are seen to be fused. 
During the rapid growth of the embryo-sac the nucellus has 
been enlarged, to accommodate the growing cell, by a consider¬ 
able increase in the size of the cells of the outer layer of the 
nucellar wall and by anticlinal division of some of these cells. 
Disintegration and absorption of the inner cell’ layers of the 
nucellus follow until at the upper end of the nucellus there re¬ 
mains but the outer layer of cells. In the immediate vicinity 
of the micropyle however, certain cells of the inner layers of 
the nucellus remain plump and dense and even increase in size. 
Ultimately, during the increase in the size of th© embryo-sac 
and the disintegration of the neighboring cells these persistent 
cells become more or less isolated, round up and become approxi¬ 
mately spherical. (Figs. 44, 45, and 46.) The number of 
such cells is variable and seems to range from one to four or 
five. Frequently small rounded cells which have earlier re¬ 
sisted absorption succumb and are absorbed thus diminishing 
the number of these cells. 
As suggested above, the cells derived from the nucellus tend 
to become separated from the nucellar wall and thus come to lie 
within the outlines of the space occupied by the embryo-sac. In 
many cases such cells could still be distinguished from those of 
the embryo-sac (Figs. 47, 48 and 49) but in other cases, especi¬ 
ally in the older stages it was impossible to distinguish between 
these rounded nucellar cells and those of the gametophyte. Al¬ 
though there was considerable variation in the size and content 
of the nuclei of the embryo-sac in this region, still no sharp 
distinction in appearance could be made between those of the 
sporophvte and the gametophyte. 
