CHAPTER II 

 THE GENERAL OUTLINE OF MITOSIS 



WE have seen in the previous chapter that the 

 stimuli which can cause the onset of mitosis 

 are extremely diverse ; they may be regarded as 

 external agencies releasing an inherent chain of bio- 

 chemical and biophysical events in the nucleus. If 

 the cell is almost ready to divide in any case, stimula- 

 tion by a mitogenetic agent or puncturing the nuclear 

 membrane will produce a more or less normal 

 mitosis ; if the mitotic mechanism is not ' wound 

 up ' then pycnosis results. A study of the gross 

 disturbances which lead to mitosis does not help us 

 much to understand the internal causes normally 

 responsible for the initiation of the whole cycle of 

 events ; neither does it explain how in some cases 

 that cycle may be interrupted at certain stages and 

 then subsequently resumed. 



It is usual to divide mitosis into four stages, pro- 

 phase, metaphase, anaphase and telophase. For con- 

 venience in description it seems best to subdivide 

 anaphase into two parts and to insert a stage which 

 can be called prometaphase between prophase and 

 metaphase. 



1. Prophase 



At the beginning of prophase the chromosomes 

 become ' fixable ' — that is to say their appearance 

 in fixed material approximates closely to that seen 

 in living cells by the most reliable methods of observa- 

 tion. This ' fixability ' increases throughout pro- 

 phase until at prometaphase and metaphase there is 



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