CHEMIOTAXIS 369 



cells. When a foreign body is introduced into this cavity 

 there is a new formation of mesoblastic cells. These cells 

 are attracted to the foreign body ; they surround it, and if 

 it is of protein nature they digest it. The attraction of the 

 cells towards the foreign body is known as Chemiotaxis. 

 It corresponds to the movement of amoeba towards its 

 food. It will be seen that the gastrula has in this respect 

 advanced beyond amoeba in two ways, in the specialisation 

 of certain cells for the purpose of defence, and in the 

 reproduction (prohferation) of the cellular defending 

 agents. 



The response to invasion is essentially the same in higher 

 animals. Certain cells, the phagocytes, are the defending 

 agents, and they destroy the bacteria by intracellular diges- 

 tion. The greater complexity of the process in the higher 

 animals is due to the more comphcated manner in which the 

 phagocytes are mobihsed in large numbers to the site of 

 infection. There is also a further difference. In the course 

 of evolution host and parasite have, as it were, developed 

 together. Each has become in some degree immune to 

 the other. In some cases they may live in symbiosis, each 

 deriving some benefit from the other. The bacilli which 

 inhabit the large intestine of the horse live upon the 

 cellulose which the horse eats. Owing to the bacterial 

 hquefaction of the cellulose the horse is enabled to absorb 

 nutriment from the grasses. But there is not always this 

 mutual advantage. The host may tolerate the presence 

 within it of certain bacteria. Yet under certain conditions 

 these same bacteria may cause fatal illness. The respira- 

 tory passages of human beings are the normal habitat of 

 the pneumococcus. It is only when the natural resistance 

 to this organism is lowered, as by exposure to cold, that 

 the pneumococcus produces an acute inflammation of the 

 lungs. 



When pathogenic organisms are introduced into the 

 body the phagocytes acquire an adaptation to them. At 

 first the cells are repelled (Negative Chemiotaxis) ; then 

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