12 PREFACE 



tumours by transplanting them through various 

 animals under different conditions, especially with 

 a view to the discovery of what will prevent their 

 growth. Work based on the second theory has 

 for its object the endeavour to isolate the para- 

 site, to produce experimentally a malignant 

 growth with it, and then to find some method of 

 preventing and curing its action. 



Apart from many speculations, in addition to 

 these two main theories, there are, of course, 

 several others ; notably one suggesting that the 

 parasite is an ultramicroscopic one, another that 

 it is intracellular, and, lastly, there are those 

 which indicate that cancer cells are normal ones 

 which have reverted to the "reproductive" type, 

 or, again, are inclusions of undeveloped or foetal 

 tissues. Pew of the theories, however, seem to 

 attack the problem of the cause of normal and 

 malignant cell-division directly. The explanation 

 of how a parasite, even if it be ultramicroscopic 

 or intracellular, can cause a living animal cell 

 to reproduce itself too rapidly is not forthcoming. 

 On the other ihand, it has been suggested that 

 cell-division is due to a combination of physical 

 and chemical factors which so far have not been 

 specified ; and, again, there is the theory that all 

 cells have the function of reproducing themselves 

 automatically unless they are restrained by a 

 chemical " anti " substance, although how the 

 latter acts is not yet explained. 



The line of work which we have adopted differs 

 from the above in that we believe that normal 

 cell-reproduction is not merely an intrinsic vital 

 function or duty on the part of the cell, but is 



