WALTER P. THOMPSON 
occurred in all the compartments. There results a mass of uninucleate 
cells with a more or less deep depression in the center (in the position 
of the vacuole). No more compartments are formed in this way. 
Indeed, even while fusion is occurring in the uppermost compart- 
ments division is taking place in the lowermost. Figure 53 shows a 
whole sac at this stage. At the bottom is the mass of uninucleate 
cells; above is the parietal layer of protoplasm with its free nuclei; 
and in the upper expanded part of the sac is the fertilized egg sur- 
rounded by a dense mass of protoplasm and nuclei. 
Some stages in the fusion of the nuclei are represented in figures 
63 to 69. Figure 63 shows the small nuclei grouped together. In 
figure 64 the membrane of the central nucleus is intact while those 
separating the other nuclei are disappearing and are represented by 
dotted lines. In figure 65 the outlines of the individual nuclei are 
still plain though the membranes between them are breaking down. 
In figure 66 the whole mass has lost its scalloped edge and has as- 
sumed more of the appearance of a single nucleus, although the bound- 
ary of each constituent nucleus is still visible. In figure 67 the in- 
dividual nuclei are no longer distinguishable, although the nucleoli 
derived from them remain distinct. The presence of a large number 
of nucleoli in these fusion nuclei is characteristic for a considerable 
time. Sometimes all the nuclei in a compartment do not fuse into a 
single mass at once, but first form two or more masses which later 
fuse to one (fig. 68). Sometimes one small nucleus may remain 
distinct until all the others have fused and then be absorbed into the 
general mass. After a number of divisions of the fusion nucleus the 
numerous nucleoli (each derived from a constituent nucleus) disappear 
and a single larger one with the characteristic cavity in the center is 
found (see fig. 69). The number of nuclei which fuse appears to 
vary widely. I have never observed fewer than three nor more than 
ten. 
The further growth of the endosperm takes place entirely in the 
cellular mass at the bottom. All the cells divide but the growth is 
most rapid at the extreme lower edge of the sac. Both cytoplasm and 
nuclei are much denser here than in the cells at the top of the endo- 
sperm (fig. 57). Owing to its more rapid growth the lower part of 
the endosperm at first becomes wider than the upper part. Gradually 
the tissue grows up the narrow part of the flask but always leaves 
the expanded part empty. The cavity can be seen with the naked 
