540 STEVENS: DIOECISM IN THE TRAILING ARBUTUS 
Instances in which the first division of the primary endosperm 
nucleus is followed by the formation of a transverse wall across 
the embryo sac, are fairly common especially among the Sym- 
petalae (Coulter and Chamberlain, loc. cit. 176); and the for- 
mation of a two-chambered embryo sac has been reported in the 
three subfamilies most closely related to the Ericoideae, that is, 
Fics. 2 and 3. ‘‘Chambered’’ embryo sac of Epigaea repens. X600. 
Fic. 2. Two-celled stage 
Fic Four-celled sae The fertilized egg is partly covered by the remains 
of the solic tube, and the antipodal cells have largely degenerated. 
in the Pyroloideae, Monotropoideae, and Vaccinioideae (Hof- 
meister,’ p. 141). The later development of the endosperm 
differs somewhat in the different groups. In the Vaccinioideae 
the endosperm is reported to develop only in the antipodal chamber 
while in the Pyroloideae and Monotropoideae endosperm is formed 
in both chambers. The only case in which, as in Epigaea, the 
embryo sac becomes divided into four superposed chambers all 
