THE MITOTIC CYCLE 



well-known discovery that mustard gas both provokes mutations 

 (AuERBACH and Robsonis*"^^^) and causes cytological abnormalities 

 in germ cells (Darlington and Koller^^'^) and also in somatic cells 

 in mitosis (Hughes and Fell^^^). The essential feature common to 

 these effects is a disturbance of the normal pattern of chromosome 

 reproduction, which persists in subsequent cycles of mitosis. This 

 principle has led to researches on the action of chemical substances on 

 the cell nucleus in which effects on the chromosomes of genetical 

 significance have been stressed (Loveless and Revell^^^). 



It seems probable that the structures within the dividing cell differ 

 in their sensitivity towards particular agents, the overall action of which 

 may be due to effects on the cell at many points. Moreover, it is now 

 clear that the end result can vary with different types of cell; for the 

 relative effects on the spindle and on the chromosomes of some agents 

 are not uniformly the same. Other substances have been shown to 

 inhibit dividing cells with an equal facility at any point in the mitotic 

 cycle. The present topic, therefore, in common with most aspects of 

 cell biology has the property of becoming increasingly complex with 

 time. 



Studies on the living cell of the influence of agents offer the obvious 

 advantage of a direct approach to the problems of mitotic phar- 

 macology. Where marine eggs are used as experimental material, the 

 effects of the treatment on other aspects of cell physiology such as 

 respiration and protoplasmic viscosity may be investigated in parallel. 

 Such combined studies have yielded information of much interest. 

 In a preceding chapter (p 70) has been given an account of the normal 

 mitosis of some cells in which fine detail can be observed in life. Further 

 studies of the action of chemical agents have been made on each of 

 these types of cell. The effects of colchicine in Acanthamoeba have been 

 studied by Comandon and de Fonbrune,i^° and on the grasshopper 

 neuroblast by Gaulden and Carlson,^ ^^ though research on this 

 material has been mainly concerned with the effects of radiation. 

 Experimental studies on the staminal hair cells of Tradescantia in division 

 have been made by Wada^^^ ^^^ and on the petal cells of the same plant 

 by Shigenaga.^^* Tissue cultures have been used for this purpose on 

 many occasions. By means of phase-contrast cinemicrography, the 

 detailed events within such cells under experiment can be recorded. 

 The agent used can either be incorporated in the tissue culture medium, 

 as in the study by Hughes and Fell^^^ on the effects of mustard gas, or 

 the cultures can first be grown under normal conditions and the sub- 

 stance dissolved in a suitable saline solution can be added thereto 

 while a cell is under observation (Hughes^ ^^ ^"^). By this means one can 

 study the effects of a rapidly penetrating agent on each point in the 

 mitotic cycle. This is of importance where more than one phase is 



184 



