Il6 CICATRIZATION OF WOUNDS 



of senescence are the expression of profound physico- 

 chemical and chemical changes occurring in the organism 

 through the influence of time. When the war was over and 

 he had come back to New York he decided to prove this 

 point. 



The first experiments simply consisted in cultivating the 

 conjunctive tissue of chick embryo in plasma taken from 

 chickens of different ages. He ascertained that growth was 

 more active in the plasma of young animals than in that of 

 old ones. This fact demonstrated the possibility of employing 

 fibroblast cultures to put in evidence the modifications 

 introduced into the serum by age. The technique was then 

 improved so as to eliminate the most palpable causes of error 

 and to obtain quantitative values.^ Experiments were per- 

 formed in the following way: an equal quantity of serum 

 taken from six-week to nine-year-old chickens was added to 

 the culture medium. It was observed that the growth of the 

 cultures was hardly or not at all affected by the serum of a 

 chick six weeks old. Growth was retarded, however, by the 

 addition of serum removed from older animals, and all the 

 more so the older the animal was. Furthermore, under well- 

 determined conditions, the span of life of a culture was 

 affected in the same way. That is to say that, if a culture 

 lived four or six days in the plasma of a nine-year-old cock, 

 it would live forty-six days or more in that of a chick six 

 weeks old. The rate of growth is modified in a similar 

 fashion, and decreases as a function of the age of the animal 

 from which the serum proceeds. 



Through further experiments Carrel convinced himself 

 that this retardation was not due to the neutralization of 

 accelerating substances but to the accumulation of toxic 

 products. The curves shown in Figs. 24 and 25 were ob- 

 tained by taking arbitrarily as base (100 per cent) the rate 

 attained when employing the plasma of three-month-old 

 chickens. This was the age of the animals which furnished 



^ A. Carrel and A. H, Ebeling, Journ. of Exp. Medicine, vol. 34, 

 p. 599 (1921). 



