70 Inside the Living Cell 



and proteins required. The virus particles do not have this ability, 

 but they can make use of the apparatus which is provided by their 

 host cells. They must therefore be regarded as parasites and, like 

 parasitic animals, they are intimately related to their host species. 



It is possible that the virus particles have arisen as a mutation of a 

 normal self-reproducing constituent of the cells in which they grow. 

 Suppose that a self-reproducing particle in a cell makes an error of 

 reproduction and produces particles which, although they can 'live' 

 and multiply in the cell, do not function correctly. They will be 

 *rogue' particles which interfere to a greater or less degree with the 

 normal working of the cells. If they are introduced into healthy cells, 

 they will continue to reproduce themselves there. But if they go on 

 multiplying, they may cause such disturbances of the normal working 

 of the cells that the organism eventually dies. This might mean the 

 destruction of the virus as well, but the natural viruses have often 

 developed efficient means of transferring themselves from infected 

 to fresh organisms. They make use of all sorts of agents such as 

 insects and birds to help them to disperse themselves. This implies 

 a long period of adaptation. 



The common viruses must have gone through innumerable genera- 

 tions since they first originated. They are in fact very variable. Even 

 in short periods of observation, changes in their behaviour are often 

 noticed and new strains appear. In fact, their successful maintenance 

 may depend on this ability to change and so keep one step ahead of 

 defensive mechanisms which the hosts may also develop. 



Not only large organisms but also bacteria have their virus enemies. 

 It was noticed by Twort in 1915 that bacterial cultures sometimes 

 cleared suddenly and the bacteria appeared to break up and dissolve. 

 A few drops from this liquid added to another culture caused it to 

 behave in the same way. Two years later d'Herelle made similar 

 observations. The agent which destroyed the bacteria could not only 

 be transferred from one culture to another, but its activity increased 

 with each transfer. This phenomenon is now recognized as being due 

 to bacterial viruses, which are known as bacteriophages. Their form 

 has been definitely demonstrated by electron micrographs of the 

 particles. A number of different types have been found which differ 

 considerably in size and shape. Plates 1 1 and 12 show some of the fine 

 pictures of Dr R. W. G. Wyckoff of successive stages of the attack 

 of Bacterium coli. The 'phage particles are easily recognized by their 

 tadpole-like shapes, with short rod-shaped tails. The attack begins 

 when one or two of these particles attach themselves to a bacterium. 

 The material inside the 'phage particle then passes through the wall 

 and multiplies within the cell until, finally, the whole of the interior 



