SPERMATOPHYTA 43] 
becomes oval in outline (Fig. 485). Its nucleus divides and 
the two daughter nuclei migrate to the opposite ends of the 
sac. By two successive divisions each of these daughter nuclei 
gives rise to a group of four nuclei, the groups being at oppo- 
site ends of the sac (Fig. 485). At each end three of the nuclei 
become surrounded by cell walls. 
The three cells thus formed at the 
end of the sac away from the micro- 
pyle are known as antipodal cells. 
The group of three cells at the mi- 
cropylar end consists of two cells 
— (synergids) and an egg cell. The two 
nuclei which are free in the cyto- 
plasm are polar nuclei, and they move 
to the center of the sac. In this 
condition the female gametophyte 
is mature and the egg is ready for 
fertilization. 
In a considerable number of spe- 
cies the megaspore mother cell, in- 
Seadeot forming a row of separate 16.487. Wertilization stage 
megaspores, becomes the embryo sac. ™ en Oe 
Male gametophyte and fertilization. ?,Pollen tube; s,synergid; ¢,ege 
containing nucleus formed by 
The microspore (pollen grain ) be- the fusion of male and female 

gins to germinate before it is shed, nuclei; m, male nucleus in con- 

and forms a male gametophyte, or tact with nucleus formed by 
“1: . the fusion of two polar nuclei; 
_prothallus, within the microspore ¢, antipodal cells. (Redrawn 
(Fig. 486). This gametophyte 1s pro- after Brown and Sharp) 
duced by the division of the nucleus 
of the microspore to form a tube nucleus and a generative nucleus. 
The growth of the pollen tube appears to be connected with the 
activity of the tube nucleus. The generative nucleus divides to 
form two male nuclei. This division usually takes place in the 
pollen tube, but may occur while the pollen is in the micro- 
sporangium (Fig. 486). After being deposited on the stigma 
the pollen grain sends out a pollen tube into which the nuclei 
