THE FLOWER 229 
vegetative parts of the organism, and so the same number of 
chromosomes are distributed to every cell in these parts. 
Reduction of the number of chromosomes. If the same number 
of chromosomes as are found in the vegetative nuclei went into 
the egg and sperm nuclei, the fertilized egg of each generation 
would contain twice as many chromosomes as the nuclei of the 
preceding generation. This result is prevented by the presence 
of one division in the life cycle in which the number of chromo- 
somes is reduced to half that found in the ordinary vegetative 
nuclei. In flowering plants this reduction takes place before the 
formation of pollen grains, and also before the development of 
the embryo sac. 
Kach anther produces a number of cells known as pollen mother 
cells. By two successive divisions each pollen mother cell 
gives rise to four pollen grains. The reduction in the number of 
chromosomes takes place in the first division of the pollen mother 
cell, so that the nuclei of the pollen grains, and consequently 
the male nuclei, contain half the number of chromosomes found 
in ordinary vegetative nuclei. 
The formation of the embryo sac is preceded by the develop- 
ment of a single, rather large cell called the megaspore mother 
cell. This gives rise, by two successive divisions, to four cells 
which are known as megaspores. The reduction in the number . 
of chromosomes takes place during the first division of the mega- 
-. spore mother cell. Typically, three of the megaspores degen- 
erate and disappear, while the other develops and produces the 
embryo sac. The nuclei of the embryo sac, including the nucleus 
of the egg, thus resemble the male nuclei in containing half the 
number of chromosomes found in the ordinary vegetative nuclei. 
The union of the male nucleus with the female nucleus gives the 
fertilized egg the same number of chromosomes as were in the 
fertilized egg of the preceding generation. Thus the same num- 
ber of chromosomes is preserved from generation to generation. 
Heterotypic mitosis. The division in which the number of 
chromosomes is reduced is known as the heterotypic division. 
In the prophase of this division there is a stage which is 
