FERTILISATION. 



255 



here be touched upon. A cell near the apex of 

 the^ nucellus undergoes division, which is repeated 

 until a line of cells, with thick walls, has been 

 formed. The lowest cell of the number now 

 enlarges greatly, at the expense of the* others, 

 which are absorbed, and an enormous cell (the em- 

 bryo sac, es, A and F) results. During the growth of 

 the latter, its nucleus divides, and the two new 

 nuclei travel to its opposite ends, a large central 

 h L 



Fig. 50.— Embryo Sac, before Fertilisation', in Three Stages of 

 Development. 



A, B, and C—h, Helper Cells ; a, Antipodal Cells, derived from Division of 

 Original Cell Nucleus ; pn, Polar Nuclei ; dn, Definitive Nucleus ; em.v, 

 Embryonal Vesicle. 



sap cavity, called a vacuole,* being formed. These 

 nuclei, now stationed at the upper and lower ex- 

 tremities of the embryo sac, twice divide into two, 



* The life of the cell inheres in its protoplasm. When this separates 

 and gives place to cell sap, the spot occupied by the cell sap is 

 called a vacuole. 



