Seaton: Embryo-sac of Nymphaea advena 287 



Dense cytoplasm surrounds the egg-apparatus, but it is scanty in 

 the rest of the sac, thin streamers extending from the egg-appa- 

 ratus to the large endosperm-nucleus and from that to the antipodal 

 end of the sac, often following the general direction of the sides 

 but not touching them. 



The early embryo-sac appears elliptical in section. The shape 

 of this part of the sac does not change, but in the eight-nucleate 

 stage, at the antipodal end, there is formed a tube-like elongation, 

 which in time reaches to the chalazal end of the ovule. This 

 basal prolongation is always narrower than the older part at the 

 micropylar end of the sac and seldom has a liberal supply of 

 cytoplasm {figure ij). At the juncture of the tube-like lower 

 part with broad elliptical upper part of the embryo-sac is a con- 

 striction. It is in this constricted part that the large fusion-nucleus 

 or endosperm-nucleus lies. Although the sac lies straight in the 

 axis of the ovule, occasionally, in sectioning, cells from the sur- 

 rounding tissue appear to be in the sac. These fragments may 

 ve given rise to the idea that a cross-wall appears in the sac at 

 point at the time of the division of the large endosperm- 

 which, as stated, always lies in this constricted part {fig- 



ha 



this 



nucleus 



ures ij t r 6), l n my material, this endosperm has been observed 

 soon after division, but a cross-wall in the embryo-sac between the 

 two nuclei, such as described by Cook for Castalia odorata, has 

 not been seen. The endosperm-nucleus lying in the constricted 

 part of the sac then divides into two. The upper endosperm-nucleus 

 later divides to form the endosperm tissue while the lower (anti- 

 podal) endosperm-nucleus moves down into the chalazal end of the 

 tube-like portion of the sac, enlarges, and often persists until the 

 embryo is quite advanced {figure 21). Soon after fertilization of 



: e gg-nucleus the perisperm shows an accumulation of starch. 

 This food supply is remarkably abundant in all the older ovules 

 studied {figure 20). 



After fertilization the cytoplasm of the sac always gathers 

 about the fertilized egg in a spherical mass. It is vacuolate, with 

 thread-like dense portions radiating from the nucleus to the sur- 

 ace of the sphere of cytoplasm. The cytoplasm from the pollen- 

 tUbe ,s den -se and takes the stain deeply {figure 14). One syner- 

 S'd persists and is very similar to the fertilized egg in appearance 



the 



