EXPERIMENTAL ANALYSIS 



existence of actual positive stimuli'. On this view the tumour cell is one 

 which has acquired the ability to elaborate its own stimulus to cell 

 division and to transmit this ability in perpetuity. The nature of the 

 stimuli can only be a matter for conjecture, both in the case of the 

 postulated self-perpetuating stimulus in tumours and that operating in 

 normal cell division. Pullinger considers the possibility (which would 

 make identification very difficult) that the hypothetical substances 

 might not be extractable because they form an integral part of the cell 

 structure. 



Based on the perspective, already quoted, that cell division is the 

 normal and natural consummation of cell activity, Little's belief^^ is 

 that in metazoan organisms, subordination of parts to the whole 

 introduces an intricate pattern of controlling and inhibiting influences 

 on cells and tissues. Hormone interactions have been studied in relation 

 to this conception. Castration of mice one to three days after birth 

 produces, in adult life, different degrees of compensatory endocrine 

 activity according to the strain of mouse (Woolley et alii^^). In C57 

 black mice, no unusual growth or hormone activity was observed. 

 After about six months, dba mice of both sexes showed some oestrogenic 

 effects and hypertrophy of the adrenal cortex (Woolley, et alii'^^ ** *^). 

 Mice of strain ce developed adrenal cortical carcinoma in 100 per cent 

 of the animals (Woolley et alii^^ *'), and the sex hormone activity dis- 

 played at six months in both sexes was male in character. Little*^ 

 believes that, in normal ce mice, 'the gonads act as inhibitors to the 

 uncontrolled and cancerous growth of the adrenals. When the gonads 

 are removed such growth always occurs. When gonads are present it 

 never occurs', and suggests that this may be a particular example of a 

 quite general phenomenon. 



Tissue culture provides systems in which cells are released from the 

 controls exerted by the whole organism. It has therefore been possible 

 to study, in vitro, what conditions are sufficient for growth by cell 

 proliferation. It is in general found that the limits of permissible varia- 

 tion of the physical environment, for most of the animal cells in- 

 vestigated, are rather narrow. That is, the temperature, pYi, osmotic 

 pressure, etc, must fall within a range close to those of the cells' natural 

 environment. The chemical variations which can be tolerated by differ- 

 ent types of cell are much less fully known — but it is certain that 

 different types of cell have different nutritional requirements for optimal 

 prohferation (Strangeways;^^ Parker*^). Thus while we know with 

 certainty that continued cultivation of a particular strain of cells 

 can be achieved in an empirically selected medium containing, say, 

 chicken plasma, horse serum and chicken embryo extract, we still 

 know little about which chemical components of such a mixture supply 

 the nutrients to maintain the cells alive and healthy, and which may 



M 169 



