THE AGE INCIDENCE OF CANCER 



somatic cells are subject, like reproductive cells, to random 

 mutation, then cancer becomes logically as inevitable as the 

 process of evolution and adaptive radiation that takes place 

 when any type of organism colonizes a new major habitat. 



3. The control of cell relations 



Cancer is commonly, and probably rightly, regarded as the 

 escape of certain cells from the controls normally exercised by 

 the organism as a whole. In one way or another the fertilized 

 human ovum gives rise to an intricately intercoordinated 

 system of cells and supporting material. The maintenance of 

 that system in healthy function obviously demands continuing 

 controls of many sorts. At the macroscopic level experimental 

 physiology has been mainly concerned in elucidating the 

 nature of these homeostatic controls. At the cellular and 

 morphological level, with which we are concerned, there is 

 much less of substantial knowledge. Most of it concerns the 

 hormonal control of general growth or of the growth of 

 particular organs. In the field of cancer research proper, 

 there has emerged the very important concept of con- 

 ditioned and unconditioned neoplasms. A conditioned neo- 

 plasm is one which is capable of growth only in hosts that 

 have been specially conditioned — for example, by thyroidec- 

 tomy in the case of certain pituitary tumours, or by the 

 induction neonatally of immunological tolerance. The neo- 

 plasm will grow on transplantation to such conditioned hosts 

 but not in normal hosts of the same stock. In almost every 

 such instance, however, it is found that after repeated passage 

 in conditioned hosts the tumour becomes capable of transfer 

 to normal animals. It is now an unconditioned transmissible 

 neoplasm. 



Such observations can tell us little more than that in some 

 tissues the level of circulating hormone plays an important 

 part in maintaining control of cellular relationships but that 

 it is never the only factor. 



Tissue cells are obviously influenced by the nature of the 



13 193 BC 



