THE MITOTIC CYCLE 



its environment' (Weiss^). The cultivation of tissues in vitro has helped 

 to emphasize that 'cells and medium are an indivisible whole. . . To 

 each modification of the medium corresponds a structural and func- 

 tional change of the tissues' (Carrel^ '^^). It is also from tissue culture 

 that most of the evidence has come that many limiting factors affecting 

 the metabolism of the cell can unequivocally be described as nutritional. 

 If nutrition is defective, mitosis is only one of the activities of the cell 

 which is inevitably restrained. Certain other factors, e.g., the hormones, 

 some of the higher peptides and the labile cytoplasmic particulate 

 fractions, could have a specific, overriding stimulating function on cell 

 division. But until more is known about the chemical and enzymatic 

 reactions in which they take part, as well as those which initiate and 

 accompany mitosis, it is difficult even to guess whether the eflfects of 

 these stimuli (if such they are) are just those required to set in motion 

 the train of events which culminates in cell division. It can still be 

 argued that to designate as 'growth-promoting factors' those substances 

 whose precise function in the cell metabolism is at present unclear or 

 unknown is to use the term as a curtain for ignorance of some vital 

 chapters of cell nutrition. But difficulties arise if too rigid a classi- 

 fication is adopted in describing cell metabolites, for 'a given substance, 

 required as a component of one of the essential metabolic processes, 

 might appear in three different roles ... (i) as an "essential nutrient", 

 when its rate of synthesis by the cell was so slow as to be insignificant; 

 (2) as a growth stimulant, when its rate of synthesis was somewhat 

 faster but still slow enough to be a limiting factor; or (3) as a substance 

 not required at all for nutrition, because the cell could synthesize it so 

 fast that it was not a limiting factor in growth. It is the metabolic 

 process which is the essential thing. . .' (Knight^'^). The cell and its 

 environment form a single, dynamic metabolizing system, 'a complex 

 interwoven series of processes' (Knight^'^^) which is the life of the cells. 

 It is suggested by one biologist that 'the nutrient of some animal cells 

 . . . appears to be made up of both chemical as well as biological com- 

 ponents. It may thus become rather difficult to explain nutrition of the 

 animal cell in purely chemical terms' (Liebman^^). It is true that certain 

 nutrients (in a wide sense) are too complex in their structure and 

 organization to be accessible to exact analysis by the present tech- 

 niques of the chemist. But such 'biological components' need not, 

 because of this difficulty, be lifted out of the field of biochemistry. The 

 chemical and enzymatic activities of the complex separable and mor- 

 phological parts of the cell, and their interactions, are being elucidated 

 (Claude ;i^" Schneider and HogeboomI^^). Not only are the activities 

 of the cell and its environment closely integrated ; so are those within 

 the cell itself (Weiss^"^). 'A living cell acts as a whole, and it is difficult 

 to imagine an element, be it a gene or a mitochondrion, that would 



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