MITOTIC POISONS I23 



it seems probable that they can do so in the absence 

 of the cell nucleus. The characteristic required for cell 

 division appears to be a process involving an activity of 

 the cortical gel layer of the egg and also the formation of 

 two asters in the cytoplasm. The reagents also have in 

 common the fact that if used somewhat excessively they 

 cause not two asters but many asters to appear in the 

 cytoplasm. It seems probable, therefore, that the various 

 agents act at different stages in a chain of processes which 

 leads up to gelation of the endoplasm and a concomitant 

 activity of the cortical gel. Beyond this it is not possible 

 to proceed at present. If the mechanism of partheno- 

 genesis is to be understood it will probably be necessary 

 to work backwards from the biological response of aster 

 formation to the details of the mechanism of activation 

 involved in each particular type of reagent. 



Mitotic Poisons 



A considerable number of drugs have been classed as 

 mitotic poisons, and in the chemotherapy of cancer in- 

 creasing attention is being paid to the action of drugs 

 on mitosis. Mitosis is a complex process and can be 

 regarded as consisting normally of at least eight steps. 

 These are: 



1. Division or duplication of chromosomes (involving 

 duplication of each gene). 



2. Spiralisation of the chromosome accompanied by con- 



