5 



Mitosis and Meiosis 



When a cell divides the nucleus usually does likewise, and by a process 

 which insures that the chromosomes will divide and separate with the 

 mathematical precision necessary to maintain genie uniformity within 

 required limits. There are two common forms of mitosis associated with 

 cell division, namely, somatic and meiotic. The former is, in general, the 

 process which gives rise to new cells in the growing regions of an organ- 

 ism and maintains continuity of chromosome number and type, while the 

 latter is the modified form which gives rise directly or indirectly to sexual 

 gametes. In meiosis the chromosome number is reduced to half and 

 partner or homologous chromosomes are segregated. A third form of 

 mitotic activity which appears to be fairly common in differentiated 

 tissues is endomitosis, and involves progressive duplication of the chro- 

 mosomes without cell division or any of the usual mitotic events. 



SOMATIC MITOSIS 



Somatic mitosis may be defined as a process whereby the chromo- 

 somes become duphcated longitudinally into two more or less equivalent 

 parts which separate to opposite poles of the cell. The stages of this 

 process and their chief characteristics (Figures 5-1, 5-2, 5-3), are as 

 follows : 



Interphase 



Most nuclei in an organism are in a state of inactivity so far as micro- 

 scopically resolvable changes are concerned. For this reason one usually 



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