THE MECHANISM OF REPRODUCTION 41 



pleting in this manner the peculiar double fertilization 

 characteristic of the angiosperms. 



In the meantime, the egg and the endosperm nucleus 

 have been prepared by the 

 necessary cell divisions of the 

 female gametophyte. The 

 reduction division occurs in 

 the usual manner, and as in 

 animals three of the cells are 

 absorbed, leaving a single 

 one to provide for the 

 hereditary succession. Its 

 container enlarges and be- 

 comes the embryo sac, while 

 the cell itself typically goes 

 through three cell divisions 

 resulting in the formation of 

 eight nuclei. Any of these 

 nuclei may become the egg, 

 but generally the egg can be 

 recognized by its position 

 (Fig. 12). Two others from 

 among these nuclei fuse to- 

 gether and become the endo- 

 sperm nucleus, which in turn 

 fuses with the second male 

 nucleus and by succeeding 

 cell divisions forms the en- 

 dosperm of the seeds, the function of which is to furnish 

 food for the young plant, the embryo. Thus, if we 

 represent the chromosome complex of the gametes by x, 

 the embryo is 2x, and the endosperm 3x. 



FIG. 12. Fertilization in the embryo sac 

 of the lily, e, egg; /s, first sperm; pp, 

 fused polar nuclei ; ss, second sperm. 

 (After Bergen and Davis.) 



