20 THE BIOLOGICAL PROBLEM 



gears, etc. Second, the breaking up and analysis of the 

 workings of each of these isolated elements. The knowledge 

 of the whole is thus naturally enlarged. The comparison is, 

 however, very superficial, inasmuch as the characteristic of 

 Dr. Carrel's method consists in maintaining the physiological 

 activity of the cellular elements, when they are detached from 

 the organ. This renders possible a profound analysis of the 

 mechanisms. 



Fragments of tissue can be kept alive out of the organism. 

 These fragments, preferably explanted^ from an embryo, 

 grow actively in an appropriate medium and possess the 

 faculty of living indefinitely. The descendance of a small 

 piece of chicken heart extracted from the egg in 1912 is still 

 alive to-day after twenty-four years and shows no signs of 

 ageing, that is to say that its growth-activity is the same as 

 at the beginning. Barring ever-possible accidents — for the 

 care it requires is considerable, and in spite of the precautions 

 taken the cessation of its development can always be feared — 

 there is no reason why it should ever die. It is superfluous 

 to add that, had the chicken from which it was extracted 

 lived its normal life, it would hSve been dead long ago. Ten 

 years constitutes about the extreme limit of duration of a 

 chicken's existence. This method, on which we will have 

 occasion to dwell in detail later on, presents tremendous 

 advantages over the old cytology. 



'The task of the new cytology,' says Carrel, 'is to discover 

 the physiological properties which characterize each type 

 of cell. It is impossible to attempt the study of these 

 properties by any other method than that of pure cultures' 

 (namely, constituted by a single type of cell). 'It is the only 

 method which enables one to introduce precise modifica- 

 tions in the conditions of life of the colonies and to show up 

 potentiahties which often remain hidden during the normal 



^ Explanted, technical expression used in tissue-culture work, 

 meaning 'removed from', 'cut off'. The term 'explant' is often used 

 as a noun to define the fragment explanted. 



