TRILLIUM SP. 153 



4. The embryo-sac and female gametophyte. From the 

 megaspore that is formed within the nucellus of the ovule 

 (megasporangium) the embryo-sac with the contained 

 female gametophyte develops. When the spore first 

 begins to germinate its nucleus divides and its wall 

 enlarges. Nourishment is absorbed from adjacent tis- 

 sues. Of the two nuclei formed by the first division, 

 one passes to each end of the embryo-sac. The two nuclei 

 divide into four, the four into eight, four of these being 

 placed at each end of the embryo-sac. One from each 

 end then passes to the central part of the sac, and these 

 two unite, thus forming the primary endosperm nucleus. 

 Of the three cells remaining nearest to the micropyle one 

 is the egg or oosphere, and the other two are synergids 

 (helpers). At the opposite end of the embryo-sac are the 

 three antipodal cells. By the time these structures are 

 developed the embryo-sac has enlarged sufficiently to 

 occupy a considerable part of the ovule. 

 By means of sections of ovules within ovaries of various 

 ages, locate and study the various stages in the development 

 of the female gametophyte. The following stages should be 

 observed: 



a. The relatively large cell, the megaspore. Observe its 

 prominent nucleus. 



b. The embryo-sac, with two nuclei. Observe the change 

 in form in the old megaspore wall, now the embryo-sac, 

 and the positions of the nuclei. (It is possible in many 

 such preparations to see the nuclei in process of division.) 



c. The embryo-sac with four nuclei. 



d. The embryo-sac with eight nuclei. 



e. Nuclei from each end (polar nuclei) uniting to form 

 primary endosperm nucleus. 



/. The completed female gametophyte consisting of seven 

 nuclei, one of which is the egg ready for fertilization. 



