MITOSIS. 37 



This is the equal mitotic division, but in certain cases 

 a reducing division occurs. In a reducing division the 

 mitotic phenomena are much the same, but the chromo- 

 somes do not divide into ttuo, hence the resulting daughter 

 nuclei have only half the number of chromosomes of the parent 

 nucleus. 



It is difficult to see the full meaning of mitosis, but 

 it has been interpreted as a process for ensuring the 

 equal division of the chromatin. The astrospheres appear 

 to act as centres of attraction for the chromosomes, and 

 there can usually be discerned a nuclear spindle uniting 

 the rays of the two astrospheres, giving the whole the 

 appearance of a magnetic field. 



The reducing division is characteristic of gonogenesis, or 

 the production of the sexual elements. The primitive germ- 

 cell produces sperm-mother cells, or egg-mother cells, which 

 at the moment of division contain twice the number of 

 chromosomes. Two rapid reducing divisions then produce 

 four sperm cells in the male, or the mature ovum and polar 

 bodies in the female. Hence the mature ovum and the 

 spermatozoon have in their nuclei ( $ and ? pronucleus) just 

 half the normal number of chromosomes. When fused 

 they produce a normal nucleus with the full number. 



The student should compare this account carefully with 

 that given in Chapter V., page 42, and it will be clear that 

 the reducing divisions and the enumeration of the chromo- 

 somes lead to the same conclusion, namely, that the male 

 and female elements (spermatozoon and mature ovum) are^ 

 morphologically speaking., merely half cells produced by two 

 rapid divisions at the limit of groivth instead of the normal 

 single division. 



