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 bya 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 
