STEVENS: DIOECcISM IN THE TRAILING ARBUTUS 539 
accordingly been traced. The mature embryo sac (FIG. 1) pre- 
sents no unusual features except that it is somewhat curved and 
is prolonged, toward its micropylar end, into a narrow funnel- 
shaped region which contains the greater part of the egg and 
synergids. The nucellus either does not persist or else it coalesces 
so completely with the integument that they appear continuous. 
the epidermis, a middle storage region, and an inner tapetal region 
surrounding the embryo sac. The cells of the epidermis are large, 
regular, and rather thick-walled. The storage region shows little 
differentiation except that the cells in the funicular region are 
considerably elongated. The “ tapetum”’ is composed of a 
single layer of cells except at the antipodal end, where it is usually 
two or more cells in thickness. Its cells have the large nuclei 
and dense granular cytoplasm characteristic of nutritive cells. 
It is noteworthy that the tapetum does not surround the narrow 
micropylar portion of the embryo sac, a fact which suggests that 
the micropylar portion may be a secondary development due to 
an encroachment of the growing egg and synergids upon the tissues 
of the integument. 
As noted above, at the time of maturity of the embryo sac 
the polar nuclei lie close together in the middle of the sac. They 
remain entirely distinct, however, until fertilization occurs and 
the primary endosperm nucleus is formed by the simultaneous 
~ fusion of the polar nuclei with the male nucleus. The division 
of the primary endosperm nucleus is accompanied by the for- 
mation of a transverse wall dividing the embryo sac into two ap- 
proximately equal chambers (FIG. 2). Each of these chambers 
is further divided by a cross wall resulting in a “ four-chambered ” 
embryo sac (FIG. 3). No division of the fertilized egg has occurred 
at this stage. 
In the further development of the endosperm, cell divisions 
occur rapidly in all four of these “‘ chambers.’’ The cell divisions 
take place in various planes, and by the time the proembryo 
contains five or six cells it is impossible to distinguish the original 
cross walls which separated the four chambers of the embryo sac. 
There is thus no period of free nuclear division in the development 
of the endosperm in Epigaea. 
