74 PROTOPLASM 



question. Toxic secretions are formed which stop growth. 

 Unless these are removed, aging results. We grow old because 

 we poison ourselves. If the requisite environment is produced, 

 an old cell may regain its youth; its latent potentialities for 

 unlimited growth are revived. This fact plays an important 

 part in life, in those extraordinary cases of rejuvenation of parts 

 in lower organisms, in wound healing, and in tumor growths. 

 In each of these instances, old cells carrying on their usual func- 

 tions, suddenly begin to grow. They are stimulated into active 

 growth when adjoining tissue is wounded, as, for example, in the 

 loss of the claw of a crab or the tissue otherwise irritated. The 



remaining cells become young 

 again and produce a new part, 

 heal a wound, or form a tumor. 

 It seems, therefore, that cells are 

 young or old or, let us say, 

 behave as though young or old, 

 not because of time but because 

 of environment, of body fluids. 

 Poisonous waste products pro- 

 duce age. Freedom from these 

 Fig. 58.— Living culture of chick and f resh solutions in their place 



heart showing migration of cells g-ryQ vouth 

 (taken with dark-field). ,r 



Yet another important feature 

 of the technique of tissue culturing remains to be referred to. It 

 is subculturing. Where continued and active growth over a long 

 period of time is desired, subculturing must be done. This 

 involves the making of new cultures, by dividing the old one and 

 transplanting a bit of it on a new substratum. This is possible 

 only when the cultures are grown on coagulated organic material 

 such as blood plasma. In this manner has Carrel kept his 

 cultures going many years, though cultures may live months by 

 simply washing and adding fresh nutrient solution without 

 subculturing. 



It was formerly thought that blood plasma was relatively 

 devoid of nutrition for cells, that serum (plasma is serum plus 

 fibrin) even when obtained from a very young animal, not only 

 fails to supply fibroblasts with the nitrogen necessary for multi- 

 plication but actually inhibits their proliferative activity and 

 shortens their life in vitro. But more recent work has shown — • 



