THE EMBRYO -SAC. 403 



value ; the two upper are the assisting-cells, or synergidae (Fig. 

 146, B) ; that more deeply inserted is the oosphere (o) (germinal 

 vesicle or embryonic vesicle). The synergidae, as can be easily 

 seen, have in their lower part a vacuole, are filled above with 

 protoplasm, and here contain the nucleus, The oosphere, on the 

 other hand, has its cavity above, and the main mass of its cell- 

 protoplasm, with the nucleus, below. Both synergidae are not 

 always visible, as one can cover the other (Fig. 146, C). At the 

 base of the embryo-sac the antipodal cells can usually be recog- 

 nised without difficulty, and three can usually be counted. In the 

 interior of the embryo-sac is usually found a nucleus, with a 

 nucleolus (Fig. 146, A) ; but in other cases there are two nuclei 

 (B) or a nucleus with two nucleoli (C) ; and we judge from this 

 that the one nucleus which we always ultimately find arises from 

 the union of two. Ovules, the fertilisation of which has already 

 commenced, can be recognised by the changes which the syner- 

 gidae have undergone. These appear strongly refractive, both or 

 only one being thus modified. It is then certain that a pollen- 

 tube has penetrated to the embryo-sac, and while it is not easy to 

 see it within the micropyle, it is still not difficult to recognise, 

 projecting beyond the micropyle, the piece torn off from it in 

 preparation. The apex of the pollen-tube, however, has pene- 

 trated to the synergidae, and a reproductive cell of the pollen-tube 

 has passed from it to the oosphere. With careful examination we 

 may happen, in oospheres which border on synergidae that are 

 thus changed, to find two nuclei (D), one larger, the original 

 nucleus of the oosphere, the oo-nucleus, and close by it also a 

 smaller, the spermo-nucleus, which has penetrated from the 

 pollen -tube. This latter quickly increases in size. We can find 

 stages of conjugation between the oonucleus and this spermo- 

 nucleus, afterwards see only one embryo-nucleus, with two 

 unequal nucleoli, of -which the smaller arose from the spermo- 

 nucleus (E), and ultimately an embryo-nucleus with only one 

 nucleolus. While the oosphere is being fertilised, the highly 

 refractive masses of substance in one or both synergidae diminish ; 

 they are apparently used for the nourishment of the oospore, 



At the same time with these changes in the egg-apparatus the 

 formation of endosperm has commenced in the cavity of the 

 embryo-sac ; i.e., we see the embryo-sac divided by walls. The 

 endosperm-formation here, therefore, takes place by cell-division ; 

 while in othev equally frequent, even more frequent, cases, the 



