180 Cook : EMBRYOLOGY OF RHYTIDOPHYLLUM 
was this cell divided into megaspores and then only two cells were 
formed (figure 3). The single archesporial cell elongates without 
division, in the antipolar direction, the new part being smaller in 
diameter than the older part (figures 16, 4). This cell then elon- 
gates very rapidly, and forms the two- and four-nucleate stages 
of the embryo-sac (figures 1c, 5, 6). At this time the sac is 
usually about twice as long as wide. Without further enlarge- 
ment of the sac the nuclei now divide, thus forming the eight- 
nucleate stage. The egg is large and the synergids very small 
( figure 7), but they stain very deeply. The antipodals ( fgwres 8, 
9) are about the same size as the synergids and disintegrate very 
quickly. The polar nuclei (figure zo) may unite in any part of 
the sac, but usually in the antipodal end. The micropyle and the 
pollen-tube are very conspicuous at this time, and the entrance of 
the latter obliterates the synergids (figures rz, 12). Immediately 
after the completion of the eight-nucleate stage of the sac the 
antipodals disintegrate and a very long tube is formed extending to 
the opposite end of the ovule (figures rd, 7, 8, 9,72). The 
formation of this tube is the result of the disintegration of a single 
row of cells. In avery short time the nucellar cells which sur- 
round this tube disintegrate, producing a sac uniform in diameter, 
extending almost the entire length of the ovule and enclosed by | 
only a few layers of nucellar cells, usually not more than six. 
This is followed by fertilization of the egg and union of the polar 
nuclei. The endosperm undergoes its primary division (figures 
12, 73) before the division of the egg. One of the daughter-nuclei 
remains in the micropylar part of the sac, while the other enters 
the tube and the two are separated by a wall (figure 13) which per- 
sists fora very short time. Similar walls, dividing the two daughter- 
nuclei of the primary division of the endosperm-nuclei have been 
described in a number of plants by Hofmeister (4), Johnson (5, 6), 
Schaffner (10), Cook (1), Strasburger (11), and others. Both 
nuclei now undergo repeated divisions, forming a cellular endo- 
sperm. The endosperm in the micropylar part of the sac disinte- 
grates very quickly, but the other persists for some time ( figures 
14, 15). It is very evident that the disintegration of the endo- 
sperm begins at the micropylar end of the sac and progresses 
rapidly towards the opposite end. In this end of the sac there 
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