88 WEATHERWAX: GAMETOGENESIS IN ZEA MAys 
In the frequency with which these bilaterally differentiated 
endosperms occur—about one in 10,000—East (6, p. 220) has 
likened them to ‘‘bud sports.’” This explanation is not materially 
different from the one suggested above, although the cytologica 
phenomena accompanying the formation of “bud sports’’ are 
not well understood at present. In the endosperm, where chro- 
matin behavior seems much less regular than in ordinary embryonic 
tissue, the chances for such occurrence seem exceptionally good; 
but, neither in the division of the primary endosperm nucleus nor 
in the arrangement of the free nuclei in subsequent stages, is there 
anything to account for the perfect symmetry and fine line of 
demarcation characterizing these seeds. 
SUMMARY 
Of the four potential megaspores in maize, only one persists 
and becomes functional. 
The embryo sac is a modified form of the ordinary seven-celled 
type. The antipodals undergo division, forming a large mass of 
tissue before the time of fecundation. 
The sperms, which are exceedingly small, are matured inside 
the pollen grain before the dehiscence of the anther. 
External conditions and the length of the silk determine the 
time required for the growth of the pollen tube, but fecundation 
is known to have taken place in some instances within a little 
more than twenty-four hours after pollination. 
Guignard’s work on double fecundation in maize is verified. 
Both sperms are functional, one uniting with the egg to give rise 
to the embryo, and the other entering into the constitution of the 
primary endosperm nucleus. This is the cytological basis of 
xenia and the attendant hereditary phenomena. Immediate effects 
of pollination made manifest outside the embryo, if such really 
occur, should not be called xenia. 
The triple fusion takes place almost simultaneously with 
fecundation of the egg, but the endosperm develops much more 
rapidly than the embryo. 
In connection with genetic studies, the assumption that the 
endosperm is sporophytic in its genetic behavior seems consistent. 
The cytological data substantiate practically all the assumptions 
