THE MITOTIC CYCLE 



expressed by Little,*'' who agrees that 'it has been a well established 

 habit to speak and think of the differentiated cell in mammals as 

 having largely lost its power of cell division', but argues that 'the fact 

 that a cell does not use its power to divide is no indication that it does 

 not possess that power.' In Little's opinion, cell division is the 'natural 

 objective and climax of activity of the healthy, fully-equipped animal 

 cell', and vast numbers of cells in the differentiated tissues of an organ- 

 ism tail to achieve this end because there is a 'battle between chemical 

 and mechanical products of an organized community of cells on the 

 one hand, and on the other the tendency of those cells to return to the 

 function of cell division and independent metabolism inherent in all 

 complete animal cells.' The influence of one tissue or its secretions on 

 another in the organism, and of one cell on another in the tissues, is 

 clearly of very great significance in regulating the conditions which can 

 permit cell division or not. That cell multiplication and cell differentia- 

 tion appear to be mutually exclusive, so far from indicating, as Bloom^' 

 thought, that they are 'distinct processes', may show that they form 

 two aspects of a single kind of cellular activity, differently conditioned 

 by the environment. The animal organism as a whole may be regarded 

 as being in a state of maintenance, striking a balance between con- 

 tinuous disintegration of protoplasm and its continuous repair. Growth, 

 in the sense of an increase in amount of living tissue, can be looked 

 upon as an extension of normal maintenance which results when the 

 building up processes are in excess of breakdown (Tyler^ *^). 



Effects of hormones 



The study of the effects of hormones on growth of parts of the whole 

 organism provides one approach to the problem of growth stimuli. 

 PuLLiNGER*^ observed a quantitative relation between the develop- 

 mental growth of the mammary glands of castrated mice and dosage 

 of oestrogen. Mammary tumours also require oestrogen for the initiation 

 of growth but such tumours, grafted into ovariectomized females or 

 into males, will continue to grow without further oestrogen. Since cell 

 division and functional differentiation in these tumours are thus 

 independent of an external source of ovarian hormone, Pullinger 

 suggests that the simplest assumption to account for the autonomy of 

 the tumours is that a stimulant to mitosis, functional differentiation and 

 secretion, with oestrogen-like properties, 'must be elaborated intra- 

 cellularly' and transmitted heritably by the tumour cells. Assuming for 

 the purpose of hypothesis that cell division in multicellular organisms 

 does require some positive stimulus other than adequate nutrition, 

 though agreeing that 'our ignorance of stimuli to division of somatic 

 cells is very great', she suggests that 'the response of the histospecific 

 hormones' of which oestrogen is an example, are 'pointers to the 



i68 



