5 

 EXPERIMENTAL ANALYSIS 



NATURE OF THE STIMULUS TO MITOSIS 



It is well to make clear at the start that the chemical and physical 

 factors which determine the onset of the process of cell division are 

 completely unknown. All that can be done is to consider mitosis as one 

 of the events (certainly one of cardinal importance) contributing to 

 growth ; to indicate some of the factors that appear to be implicated in 

 the whole complex of processes which comprise tissue growth; and to 

 speculate upon the question whether or not some of these factors may 

 act directly or indirectly as stimuli to mitosis. 



The literature bearing upon such factors — variously known as 

 'growth factors', 'growth hormones', 'trephones', 'cell division com- 

 plexes' etc — is full of confusion. This can be attributed principally to 

 the deceptive simplicity of the term 'growth'. Although it has been 

 repeatedly pointed out {e.g., by Hammett;1 ^ Cunningham and Kirk;^ 

 Richards and Cavanagh;* Mayer^ and Weiss^ ') that the number of 

 meanings of 'growth' is nearly as great as the number of investigators, 

 publications on 'growth' still regularly appear, in which no explicit 

 definition is supplied. 



Mitosis and growth 



Growth can comprise increase in mass, volume, area or length of cells, 

 tissues or organisms; and any of these can take place without cell 

 division contributing at all. It is true that it frequently does contribute 

 importantly to growth in its commonly accepted senses. But, in review- 

 ing the literature on 'growth stimuli', it is often impossible to discover 

 whether the action studied has been on mitosis or on some other aspect 

 of growth. As Weiss^ says : 'the relation between cell division and growth 

 is by no means as close as the widespread habit of treating them 

 interchangeably would make it appear.' Nevertheless, serious attempts 

 have been made to abstract the process of mitosis or cell proliferation 

 from the closely related activities of tissue growth and differentiation. 

 If these attempts have not wholly achieved the aim of demonstrating 

 effects on mitosis alone, Tyler^ gives the pertinent reminder 'that the 

 developing embryo, although it is not in a steady state, also exhibits 

 maintenance in addition to processes resulting in increase in amount of 

 living material (growth) and in change of form and composition 



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