542 Brown: EMBRYO-SAC AND EMBRYO IN PHASEOLUS 
neous, for in several instances both resting nuclei and nuclei in the 
various stages of division are to be seen distributed from one end 
_ of the sac to the other (Fic. 28); but the order of their arrange 
ment was not identical in the different sacs observed. 
The endosperm remains in the form of a peripheral layer in 
the embryo-sac until its final absorption by the growing embryo; 
it is typically thicker around the embryo than in other regions of 
the sac. After the differentiation of the dermatogen, cell division 
occurs in that part of the endosperm which immediately surrounds 
the embryo itself. Each nucleus in this part of the endosperm is 
surrounded by a very definite cell wall; the cells so formed, how- 
ever, do not form a compact mass but lie isolated in the undivided 
cytoplasm (Fic. 29). These cells do not persist but are soon ab- 
sorbed, and all traces of the endosperm have entirely disappeared 
by the time of the maturity of the seed. 
SUMMARY 
1. A large hypodermal cell is early differentiated in the ovule; 
this either functions as the macrospore mother cell or possibly ° 
divides once, one of-its daughter cells being the macrospore mother 
cell. 
2. The fully grown macrospore mother cell lies in either the 
third or fourth layer from the micropylar end of the nucellus. 
3. An axial row of three macrospores is formed, the innermost 
of which develops into the embryo-sac. 
4. The nucellus is entirely destroyed at the micropylar end 
and along the sides by the development of the embryo-sac; the 
nucellar tissue at the base of the sac takes on a peculiar structure 
and persists for some time, but finally is itself gradually absorbed 
by the embryo-sac. 
5. The polar nuclei begin to approach each other soon after 
the eight-nucleate stage of the embryo-sac is reached and remain 
close together for some time just below the egg; then their fusion 
takes place. 
6. The three antipodal cells disappear at about the time of 
fertilization. 
7. The synergids form a conspicuous filiform apparatus. 
8. The pro-embryo consists of a filament of three cells; the 
