ONTOGENESIS 179 



lively) long pollen-tube. The pollen-tube is the male 

 gametophyte, and it grows directly through the 

 tissues of the stigma and style, to the megasporan- 

 gium, which it penetrates. One of its cells (or 

 nuclei) is the male gamete. 



The Germination of the Megaspore. The first 

 step in the development of the megaspore consists 

 in the division of its single nucleus into two, which 

 move apart into either end of the cell. Each nucleus 

 then divides twice, so that there are produced two 

 groups of four nuclei each, in either end of the spore- 

 cell. The latter, meanwhile, grows rapidly at the 

 expense of the surrounding tissues. Now, one 

 nucleus from each group (the polar nuclei) leaves 

 the others and moves toward the center of the cell, 

 where the two meet and fuse. The cell now contains 

 three nuclei at each end and one double one at the 

 center. The latter forms the endosperm, a mass of 

 food-storing cells that fills the embryo-sac. The 

 three nuclei at the bottom of the cell (the antipodal 

 nuclei) disintegrate. Sometimes they form cell- 

 walls and even build tissue, but have no further fate. 

 Of the other three nuclei at the distal end of the sac, 

 one is the egg or gamete, the other two (known as 

 synergids or " helping cells ") are sacrificed for the 

 nourishment of the gamete, much as are the two 

 polar bodies of the animal-egg. 



The Germination of the Microspore. Each of 

 the microspores is functional. In germination the 

 nucleus divides, forming two cells, a large one called 



