CELL DIVISION 131 



In reproduction the chromosomes of the male nucleus enter 

 the nucleus of the egg. Therefore, the fertilized egg contains 

 chromatin from both parents. This union of chromatin is be- 

 lieved to be the basis of heredity. 



REDUCTION DIVISION 



Let us now return to the discussion of fertilization and apply 

 the principles of cell division to fertilization. It has been said 

 that after mitotic cell division each new cell has the same num- 

 ber of chromosomes as the parent cell possessed. There is one 

 important exception to this rule: it can readily be seen that if 

 an egg and a sperm each possessed the full number of chromo- 

 somes, the cell formed by their union Would have twice the 

 regular number. If this fertilized cell grew into a plant and 

 reproduced in the same way and the same procedure continued 

 for a few generations, the number of chromosomes in a single 

 cell would soon become impossibly great. This result is avoided 

 by a special kind of cell division called the " reduction division " 

 in which the number of chromosomes is reduced by one-half. 

 In animals the reduction division takes place immediately pre- 

 ceding the formation of the eggs and sperms, so that these have 

 the single or haploid number of chromosomes and the new organ- 

 ism formed by their union has the double or diploid number. 

 In plants exhibiting the phenomenon of alternation of genera- 

 tions the reduction division immediately precedes the formation 

 of the spores. The spores thus possess the haploid number of 

 chromosomes as do the gametophytes to which the spores give 

 rise, hence, in the formation of the gametes, the eggs and sperms, 

 no further reduction is necessary. The sporophyte, resulting 

 from the union of an egg and a sperm, has, of course, the diploid 



