286 Seaton : Embryo-sac of Nymphaea advena 



ularly, so that the megaspore mother-cell is soon seen to be buried 

 four cells deep (figure 6). It expands somewhat and is marked 

 by an abundance of cytoplasm (figure 5). At the time of the di- 

 vision of the hypodermal cell, the beginning integuments can be 

 seen, in section, as rounded protuberances from the base of the 

 ovule (figure 2). The megaspore mother-cell soon divides trans- 

 versely into two cells. The succeeding divisions are somewhat 

 irregular. Usually the micropylar daughter-cell next undergoes 

 division (figure 7) and after that the chalazal cell, but sometimes 

 the chalazal cell divides first. At other times after the micropylar 

 cell has divided so that a row of three cells has been formed, the 

 middle cell next divides instead of the lower one. Whatever be 

 the succession of divisions, they result in the normal production of 

 an axial row of four cells, or u megaspores," of which the lower 

 one is functional. I have an abundance of material showing the 

 above steps. By irregular divisions of the parietal tissue and of 

 the epidermal tissue at the tip of the ovule simultaneously with the 

 formation of the four megaspores, the functional " megaspore M or 

 embryo-sac mother-cell is buried to the depth of from six to ten 

 cells below the micropylar end of the ovule. The functionless 



(figure 



(A 



b 



The embryo-sac mother-cell enlarges in the direction of the 

 tudinal axis of the ovule in the two-nucleate stage {figure //), 

 and broadens in the four-nucleate stage {figure 12). The mature 

 embryo-sac further expands toward the micropyle until it is within 

 eight, six, or even four cells of the micropylar end of the ovule 

 {figures 1 7, /<?), but never extends to the superficial row of cells 

 as Cook found in Castalia odorata. The nucellar tissue between 

 the upper end of the" embryo-sac and the micropyle at this time 

 assumes a characteristic appearance ; the cells seem crowded, as 

 they are smaller and more compact than at any previous time. 

 They are also arranged in very regular rows. The cytoplasm is so 

 abundant as to leave no vacuoles in these cells and they appear to 

 be stored with food {figure if). At the time the embryo-sac has 

 reached the eight-nucleate stage it occupies one half the length o( 

 the ovule {figure i 3 ). The polar nuclei fuse and the antipodals 

 soon disappear. The fusion-nucleus is very large {figures 15, if)- 



