No. 501] EMBRYOLOGY OF MYOSUBUS MINIMUS 587 



ing three, except in one case where fonr were present, 

 become distinctly separated from the rest of the cytoplasm 

 of the sac. After the first division of the egg cell the 

 endosperm nuclei with their cytoplasm move to the 

 periphery of the sac, where they form a complete lining 

 one cell in thickness. The central portion of the sac is 

 filled with a large vacuole (PI. II, Fig. 20). The nuclei 

 continue their divisions till the whole sac is filled with 

 endosperm. This occurs about the eight-celled stage of 

 the embryo. 



There is considerable variability in the time when the 

 walls appear, but it can be safely stated that they gen- 

 erally appear a little after the eight-celled stage of the 

 embryo (PI. II, Fig. 22). However, they may occur when 

 the sac is lined with the single layer of nuclei. The 

 antipodals can still be seen, but are on the road to degen- 

 eration. In one case I found a peculiar condition of the 

 endosperm. At one end of the sac were the antipodals 

 and afr the other the synergids and egg-cell. Scattered 

 in the cytoplasm of the central portion were three large, 

 oval, endosperm nuclei containing several large nucleoli. 

 One of them was dividing amitotically. 



The endosperm nuclei, like the pollen mother cells, 

 always divide simultaneously. There is a rhythmical 

 succession of activities. This rhythm is so closely fol- 

 lowed that every nucleus is found in the same phase of 

 mitosis at the same time. This condition is localized in 

 individual embryo sacs and there must be, therefore, a 

 mutual reflex stimulus among the nuclei of a given sac. 



As the embryo sac proceeds in its development the 

 cells of the nucellus degenerate (PI. II, Fig. 14). By the 

 time fertilization takes place all have disappeared except 

 the original epidermal covering of the nucellus (PI. II, 

 Fig. 18). At this stage the seed coats which possess 

 three or four layers of cells have grown considerably 

 beyond the nucellus to form the micropyle. When the 

 endosperm nuclei take a peripheral position, the original 

 epidermal layer begins to degenerate, except a few cells 



