THE REDUCING DIVISION 263 



takes place at the formation of the spores, which arise 

 in sets of four, each set corresponding to a group 

 of four spermatozoa, or to the ovum and the three 

 polar bodies of an animal. In the case of flowering 

 plants the nuclei contained in the spores make a few 

 further divisions, at each of which the reduced number 

 of chromosomes is to be observed, and one or more of 

 the cells thus finally produced take on the character of 

 germ-cells. The spores are of two kinds, large and 

 small, the latter being the pollen grains. The larger 

 spores give rise to female gametes and the smaller to male, 

 and fertilization takes place in the ordinary manner by 

 a fusion between the nuclei of these germ-cells. 



We have seen so far that the number of chromosomes 

 contained in the somatic nuclei of a given species is 

 always the same, and is always even.* We have also 

 seen that this number is made up of two separate sets 

 derived respectively from the two parents, and that 

 the members of the two sets preserve their separate 

 individuality right through the long series of nuclear 

 divisions which take place during the development of 

 the individual zygote. A fusion of chromosomes of 

 paternal and maternal origin respectively takes place 

 only in the direct line of ancestry of the germ-cells 

 which are destined to give rise to new members of the 

 species. This process of fusion takes place in animals 

 immediately before the formation of the actual germ- 

 cells, but in plants a larger or smaller number of cell 

 generations earlier. After fusion the paternal and 

 maternal chromosomes apparently separate, and the 



* Except in cases such as are described on pp. 270-276. 



