WEATHERWAX: GAMETOGENESIS IN ZEA Mays 75 
THE EMBRYO SAC 
The germination of the megaspore is the same as that in most 
plants where an ordinary seven-celled embryo sac is to be formed. 
In the two-nucleate stage (FIG. 9) there appears a large central 
vacuole, accompanied by a smaller one at the chalazal end of the 
cell; these vacuoles persist through subsequent stages until the 
ultimate organization of the embryo sac. The position of the 
spindles following the four-nucleate stage (FIG. 11) and the arrange- 
ment of the following eight nuclei (Fic. 12) substantiates the 
observation made on numerous other plants that one of the polar 
nuclei is the sister of the egg. 
When the membranes are first formed (Fic. 12), dividing the 
gametophyte into cells, the antipodals are much larger than the 
cells of the egg apparatus; but, by the time the embryo sac is 
fully organized (PLATE 7, Fic. 13), the egg and the synergids 
have greatly enlarged, and, with little or no increase in size, the 
antipodal cells have begun to divide, forming the multicellular 
antipodal tissue which seems to characterize the embryo sacs of 
most grasses. Meanwhile, the polar nuclei have come together 
near, or in contact with, the plasma membrane of the egg. 
After its organization, the embryo sac continues to increase in 
size, at the expense of the nucellus, until, at the time of fecundation, 
it has a volume five to ten times as large as it had at the eight- 
nucleate stage. This growth is accompanied by rapid changes 
in other parts of the pistil as well. The nucellus grows rapidly 
and pushes the integuments out against the ovary wall, which, 
growing less rapidly, is kept tightly stretched over the turgid 
ovule. The style, or “silk,” increases from a length of one or 
two centimeters to its full length, which may be forty centimeters 
or more in some varieties, and develops its numerous stigma hairs. 
Guignard (10, p. 44) has given a good description of the embryo 
sac. It occupies relatively a very small part of the nucellus 
(Text FIG. 2). The cells of the egg apparatus are very large 
(Fics. 13, 16). The dense cytoplasm of the pear-shaped synergids 
shows the longitudinal striations characteristic of these cells in 
many other plants. The nuclei are located well toward the micro- 
pylar ends of the cells and react so feebly to stains as often to be 
indistinguishable. The egg is larger and more rounded than a 
