366 



EMBRYO SAC 



sac results. This affords another instance of the reduction 

 characteristic of parasites (cf. pp. 179, 231). 



Until it has reached a considerable size the embryo sac con- 

 tains but a single nucleus, which usually lies in the middle of a 

 strand of cytoplasm running from end to end of the sac and 

 bridging the large central vacuole. Sooner or later, however, 

 a nuclear division occurs, and the two resulting nuclei wander 

 towards opposite ends of the embryo sac, where each again 



divides twice. Three of the four 

 nuclei thus formed, at the end 

 remote from the micropyle, 

 usually become separated by cell- 

 walls and constitute the antipodal 

 cells (Figs. 206, 214, 215, D, a.), 

 which probably assist in the 

 nourishment of the young em- 

 bryo. They frequently enlarge 

 after fertilisation, and, in some 

 few cases (e.g. Burr-reed Spar- 

 ganium), they may even divide 

 to form an antipodal tissue. 



Of the four nuclei at the 

 micropylar end of the embryo 



sac, three become surrounded by 

 FIG. 214. Longitudinal section of J 



ovule of Marsh Marigold (Cal- an envelope of specialised cyto- 

 tha), showing the structure of plasm, and constitute a group of 

 the mature embryo sac. a., naked cells known as the egg- 

 antipodal cells; e. t egg; m., ,, , /T r ^ ^, 



apparatus (Fig. 206). This con- 

 micropyle ; p., primary endo- f" v 



sperm nucleus. sists of the egg (e.) and two 



synergidce (s.), the latter, which 



usually lie in front of the former, being supposed to aid in 

 the passage of the male nucleus to the female cell. The 

 remaining nuclei (one at each end, one of them the sister- 

 nucleus of the egg) pass back to the middle of the embryo sac, 

 where these two polar nuclei (Figs. 206, p. ; 215, D, p.n.) meet 

 and ultimately fuse to form the primary endosperm nucleus 

 (Fig. 214, p.). This is the stage reached by the embryo sac at 

 the time of fertilisation. 



The sequence of events during this development and the 



