104 CICATRIZATION OF WOUNDS 



is still alive to-day. The cultures are as young as they ever 

 were and show no signs of ageing as measured by their growth 

 activity, which has not varied since the beginning. There is 

 actually no reason why those fragments of living flesh should 

 ever die, except accidentally. If it were not necessary, as we 

 shall soon see, to cut these cultures in half every two days, if 

 they could have been left to grow unrestrictedly, they would 

 now occupy a literally inconceivable volume. For, as their 

 size doubles in forty-eight hours, they would already have 

 attained 6, 1 30,000,000,000,000,000,000,000,000,000,000,000, 

 000,000,000 cubic metres at the end of the first year (2^^^ cubic 

 milHmetres or 613 X lo'*^). At the end of twenty-four years the 

 calculated volume would be 2^-^^^ or 5,862 followed by one 

 thousand three hundred and three zeros, in cubic metres. At 

 the end of one year they would have been more than thirteen 

 quatrillion times bigger than the sun. I hasten to add that 

 these calculations are absurd and devoid of significance as, if 

 the culture is not periodically divided in two, the growth of 

 volume brings about its death in a few days. Owing to the 

 fact that there is no circulating system, the interior cells are 

 not nourished and cannot ehminate the toxic products result- 

 ing from the reactions which they engender. They therefore 

 die, and in doing so liberate poisons which accelerate the death 

 of the other cells. The culture can persist only in very thin 

 layers or in very small fragments where all the exchanges can 

 take place on the surface. 



Nevertheless it is possible to experiment indefinitely on the 

 same family of cells proceeding from the same strain. A much 

 greater precision is possible than when operating in vivo 

 because of tlie elimination of innumerable causes of error due 

 to the individual characteristics of animals of different origin. 

 We have already mentioned these advantages in the first 

 chapter, but they are not the only ones inherent to this method. 



To preserve these tissues in a state of activity they must be 

 divided at short intervals of time, viz. every other day. Two 

 cultures rigorously identical from a biological point of view 

 are therefore available instead of one. Consequently, for each 



