GERMINATION OF THE MEGASPORE 229 



Germination of the Megaspore.— The megaspore how 

 germinates, its nucleus dividing as shown in Fig. 131, I; the 

 daughter nuclei mov6 to opposite poles and divide, and the 

 youngest nuclei in turn divide giving rise to eight nuclei sus- 

 pended in the cytoplasm of the megaspore, which we may now 

 call the embryo-sac. Two nuclei, called the polar nuclei, one 

 from each of the opposite ends of the embryo sac, move to 

 the center and fuse, forming the primary endosperm nucleus 

 (Fig. 131, J, pV). This now moves into close proximity to the 

 three nuclei remaining at the micropylar end, one of which is 

 shown by its subsequent history to be the egg cell, and the other 

 two seeming to assist in fertilization are called synergids (Fig. 

 13 1) J) ^i ^^^ ^^)- All three are termed the egg apparatus. 

 The three nuclei at the other end of the embryo sac, each asso- 

 ciated with a part of the general cytoplasm, become surrounded 

 by cell-walls, and thus are formed the antipodal cells, J and L, an. 



Fertilization and Germination of the Egg. — The egg cell 

 is now ready for fertilization, and we shall see the relation of 

 the microspore to this process. When the microspores are first 

 formed each is essentially a protoplast with one nucleus. In 

 the germination of the microspore the nucleus divides once, and 

 one of the daughter nuclei divides once again, giving two that 

 are known as the male or sperm nuclei (Fig. 127, m in I and J). 

 The other daughter nucleus which did not divide is called the 

 vegetative nucleus (Fig. 127, 1, J). These divisions may take place 

 before or after the microspore has become lodged on the stigma. 



The two male nuclei descend within the pollen tube as 

 it makes its way down the style and into the ovular cavity, 

 and are discharged into the embryo sac, where one fuses with 

 the egg, cell and the other with the primary endosperm nucleus 

 (Fig. 131, K, mn^, and mn^). 



megaspore cavity, two polar nuclei, pi, in process of fusion, egg cell, eg, two synergids, 

 s, s; K, Showing only the lower half of the megaspore or embryo sac cavity, with pollen 

 tube, pt, and male nucleus, wm,, fusing with egg cell, eg, and second male nucleus, mn^, 

 uniting with piimary endosperm nucleus, Pe' s, synergid; L, later stage in the embryo sac, 

 an, antipodal cells, em, beginning of the embryo by division of fertilized egg cell; five 

 dividing nuclei descended from primary endosperm nucleus. (A' after W. J. G. Land, all 

 others after W. D. Merrell.) 



