26 SCIENCE PROGRESS 



affected with cancer. This increase in the amount of physio- 

 logically active hydrochloric acid is to be regarded as a com- 

 pensatory response to the needs of a normal mouse plus a 

 tumour. No assumption is made in inferring that the increase 

 in hydrochloric acid is required for the digestion of proteids. 

 The increase in hydrochloric acid is in all probability accom- 

 panied by increased secretion of pepsin and trypsin, the ferments 

 concerned in proteid digestion, although this is difficult to 

 determine in mice. The increased activity of proteid digestion 

 is easily comprehended. Whereas, in a general way, the com- 

 bustion of carbohydrates suffices for the output of energy in 

 an animal organism, proteids are essential to the building up 

 of protoplasm, and the building up of protoplasm is proceeding 

 apace in the continual production of new tumour cells. 



On a preceding page I have pointed out the specific characters 

 of the proteids which even nearly allied animals reconstruct from 

 identical pabulum, and how the mouse alone supplies the nutritive 

 environment or pabulum suited to the continued growth of 

 mouse cancer. These facts are to be remembered in the present 

 connection, for there is no need to assume that the increase in 

 proteid digestion involves any other divergence from the normal 

 process. No doubt proteid digestion prepares the food-stuffs 

 for absorption into the fluids of the body in the usual way, 

 and for their utilisation by its constituent cells and by the cells 

 of the tumour. So fine an adjustment can only mean that the 

 cancer cells of the mouse and normal mouse cells compete for 

 the products of proteid digestion. In view of what we know 

 of the specificity of these processes of nutrition, on which so 

 much light has been thrown by Starling and Bayliss, this 

 implies that the cells of mouse cancer are mouse cells able to 

 impress their needs for food on the mouse as a whole by their 

 higher assimilative energy. The facts reveal, at any rate, how 

 great are the demands for food which growing tumours make 

 on healthy and young adult animals, and at the same time 

 show that the normal animal responds to these demands at 

 least by increased proteid digestion. These observations are 

 the mere beginnings of the experimental study of the problems 

 of metabolism in cancer, and which are not easy to attack in 

 so small an animal as a mouse, and will be attacked still more 

 profitably in the case of larger animals, e.g. the dog. The 

 problems of excretion which may have practical importance 



