96 Inside the Living Cell 



normal components of dna. They become incorporated in dna, but 

 when this has occurred, they do not function properly so that cell 

 division is inhibited. 



The cancer cell may be vulnerable while in the act of cell division, 

 but once formed, it is rather tough. It is less differentiated than the 

 ordinary cells and lacks their special abilities. It seems that many of 

 the special abilities of the cells from which the cancer originated are 

 lost, and all that is left is a high ability to grow and multiply. The 

 cancer cell is in fact usually an aggressive cell which is able to push 

 its way into other tissues. The cancer cell is often less responsive to 

 the immunity mechanism than normal cells — it is often possible to 

 graft a cancerous growth from one animal to another of the same 

 species. This has enormously assisted cancer research because 

 tumours of a standard kind can be multiplied at will by grafting 

 small pieces into healthy animals. 



(2) Virus Theory of Cancer. Many attempts have been made to 

 extract a non-living agent from tumours which is capable to giving 

 rise to tumours when injected into animals. In most cases, if the 

 extracts are free from living cells, no successful transmission of the 

 tumour has been achieved. However, Murphy and Peyton Rous in 

 1910 found that it was possible to transmit certain avian tumours, by 

 injecting cell-free extracts of the tumour cells into healthy birds. 

 These extracts contained the submicroscopic particles of the cyto- 

 plasm but they certainly did not contain living cells. It was concluded 

 that these extracts contain an agent or virus which causes the disease. 



Another example was the Shope papilloma virus of rabbits. These 

 are non-malignant growths, which occasionally become malignant, 

 found in the rabbits of New Jersey and from them Shope succeeded 

 in extracting a virus, which was capable of producing growths in 

 healthy rabbits. 



However, although many attempts have been made to do so, it 

 has not been possible to produce cell-free agents, which will cause the 

 disease, in more than a few types of cancer. 



Nevertheless, some workers adhere to the theory that cancer is 

 essentially a virus disease. It is suggested, for example, that apparently 

 healthy cells harbour the virus but it is only when the cell becomes 

 weakened or diseased that the virus can act effectively. Or it is 

 suggested that the carcinogenic chemicals act by reducing the resist- 

 ance of the cell, so that the virus already present becomes effective. 

 Another version of the same theory is that the change from a normal 

 to a cancer cell is due to the modification of some of the cytoplasmic 

 particles which are normally present. It might be that the carcinogenic 

 substances give rise to a change in the nature of some of these par- 



