81 



phenomenon which could be suppressed by freeing Ihe 

 cells i'rom the caUibolic substances in the exhausted culture 

 medium and transfer them to a fresh medium. 



Later Carrel'"^) found that certain tissue extracts ac- 

 celerated the growth markedly and that especially the ad- 

 dition of embryonic tissue juice of the same species as the 

 tissue cells to tlu' plasma medium had an enormously 

 growth-promoting effect. In this way it was possible to keep 

 the tissues living in vitro indefinitely. The importance of 

 having pure strains for experiments has already been men- 

 tioned. 



In contrast to this technique, other investigators have 

 been employing totally diffei'ent techniques which are sim- 

 pler in a way but give a field of investigation which seems 

 to be far more limited. Lewis and Lewis who have 

 developed this technique are not interested in having strains 

 of tissue, but make their observations on one-generation 

 experiments. This technique is limited to morphological stu- 

 dies of the tissue cells and is an excellent method for 

 histological and embryoiogical investigation. For this purpose 

 much simpler culture medi.i can be applied. Experiments 

 of this kind will not give us much physiological informa- 

 tion; because it litei'ally is not a culture, but a mere survival 

 of tissues under much loo complex conditions. If one wishes 

 to study the influences of certain substances, more or less 

 harmless in character, it often takes time before they affect 

 the tissues, just as w'ell as the fresh, in saline solution 

 explanted. tissues do not immediatel}' die, on account of 

 the juices which are brought to the culture medium by 

 the tissue itself. There is a certain period of latency before 

 the tissue becomes adapted to new environments. 



The technique used for the cultivation of tissue cells 

 has in the meantime, been simplified. 



It is known that during a certain stage of the early investi- 

 gations in tissue cultivation it was found that a slight hypo- 

 tonic plasma-medium gave the best results. This does not 



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