TISSUE CULTURE 189 



addition, they may lack certain products of differentiation which might serve 

 as an effective point of attack for injurious substances present in the 

 circulating blood ; this is suggested by the fact that in actively growing cells, 

 whether they are embryonal, regenerating adult, or tumor cells, there is less 

 tendency to differentiation and a full development of the tissue or organ 

 differentials is lacking — a condition noted apparently also in plant cells 

 growing in vitro, as the experiments of White indicate. On the other hand, 

 if factors capable of inflicting a limited degree of injury, act on these stimu- 

 lated, actively-growing cells, either normal or abnormal processes of dif- 

 ferentiation may occur, which, as an endstage, may lead to cell death. It 

 seems that with this diminution in the development of tissue differentials and 

 in tissue differentiation, as well as with the increase in growth momentum, 

 there is perhaps associated also a diminution in the sensitiveness to not quite 

 adequate individuality differentials. These factors, taken together, might then 

 explain why tissues growing in tissue cultures are less affected by not quite 

 harmonious individuality differentials than normal adult, relatively resting, 

 differentiated tissues. 



