McKenney—Observations on Eimbryo-sacs. 81 
EMBRYOLOGY. 
The following descriptions refer principally to the two 
species of Sci//a which differ but little from each other. the 
ovules of Sci//a are anatropous and are arranged in two tiers, 
in each of the three cells of the superior ovary. 
The ovules make their first appearance as small knob-like 
outgrowths from the placenta. No differentiation is discern- 
ible among the cells of these outgrowths. Soon one of the 
hypodermal cells grows larger than its neighbors, and is further 
distinguished from them by the rather denser consistency of 
its protoplasm and the larger size of its nucleus. This cell is 
the archesporium. Unlike that of the surrounding cells, the 
cytoplasm of the archesporium is usually free from sap vacuoles 
of any appreciable size. During this stage in the life of the 
archesporial cell, two to three large nucleoli are contained in 
its nucleus. By the time the archesporium becomes three to 
four times the size of its neighbor cells, the cells near the free 
end of the young ovule become raised in an annular welt-like 
fashion. This is the beginning of one of the integumentary 
coats, the primine (Plate XI, Fig. 1 p). The young ovule 
meanwhile continues to grow and bends slightly on its axis 
away from its fellow. The archesporium keeps pace in its 
growth with the ovule, its growth, however, being greater in 
the long than in the short diameter. The nucleus of the 
archesporium now divides, and by the deposition of a periclinal 
wall the archesporial cell is divided into two cells (Plate XI, 
Fig. 1). The smaller and outer of these two cells is the 
primary tapetal cell (Fig. 1 T), The formation of the primary 
tapetal cell is soon followed by another division of the arche- 
sporium into two unequal cells (Fig. 2). The smaller and 
outer cell remains passive and at a later period disintegrates. 
The lower and larger remains in an active condition. 
About the time of the second division of the archesporium, 
the cells just within the primine become raised up to forma 
second annular elevation (Fig. 2 s). This is the secundine 
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