Sexual Reproduction in Flowering Plants 249 



At an early stage the center of the ovule is occupied by a single 

 large cell. This cell continues to enlarge as the ovule grows, and 

 its nucleus divides, forming two nuclei, each of which divides a 

 second and third time, forming all together eight nuclei. The 

 large cell we will now call the embryo sac. There are four nuclei 

 near each end of it. Three nuclei out of each group move nearer 

 the ends of the sac and form cells. The two remaining move to 

 the center of the sac and fuse, forming one large fusion nucleus. 

 One of the cells at the outer end of the embryo sac is the egg. 

 This completes the development of the female gamete. 



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Figs. 157 and 158. Cross-sections of ovules. At the left, the seven-celled stage of 

 the embryo sac just before fertilization ; note the large fusion nucleus and just above it 

 the egg. At the ight, the embryo sac after enlargement through the development of the 

 embryo and the surrounding endosperm. 



The series of events inside the ovule preceding fertilization 

 begins with (i) the formation of a large cell, (2) three successive 

 divisions of the nucleus, forming the eight-celled embryo sac, 

 (3) the formation of three cells in each end of the embryo 

 sac, (4) the fusion of two of the nuclei at the center of the sac, 

 and (5) the changes connected with the development of one of 

 the three cells at the outer end of the sac into the egg. 



Fertilization. At the beginning of fertilization the pojlen tube 

 grows into the embryo sac and the two sperms are liberated. One 



