"IN VITRO" VERSUS "IN vivo" 65 



response to one element, and it will die in the act 

 of mitosis from the effects of the poisonous one. 

 This experiment, which will be described at length 

 later, proves these two points, about which I wish 

 to be emphatic; viz. that a cell cannot control the 

 diffusion of substances into itself, nor can it choose 

 from its surroundings any one substance and leave 

 another. Even at the expense of its life, a cell is 

 bound to absorb from its surroundings any substance 

 which may be present; and this absorption depends 

 entirely upon certain chemical and physical factors. 



Before proceeding to describe these laws and 

 factors, other points must be mentioned. We are 

 dealing with in-vitro experimentation; and we have 

 no proof that the diffusion of substances into cells 

 in vitro is identical with this diffusion into cells 

 in vivo. There is, however, strong presumptive evi- 

 dence that similar conditions prevail. As a matter 

 of fact, apart from the mere phenomenon of diffusion, 

 this possible distinction between the facts learnt from 

 in-vitro experimentation and what actually occurs 

 in vivo must always be borne in mind in researches 

 of this nature. The force of this point will become 

 apparent later on when we come to deal with induced 

 cell-division; for although one can induce the diffusion 

 of substances into cells or cell-division at will on a 

 microscope slide, it will be seen that these phenomena 

 in the body occur under very different conditions, 

 which must be taken into consideration in forming 

 deductions from in-vitro experiments. In the final 

 chapters, however, it will be shown that the results 



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