76 BIOLOGY OF DEATH 



cytological methods. On the other hand, by clearing 

 away the unessential elements, it indicates where research 

 into the problem of causation of senescence may be 

 profitable. 



An extremely interesting contribution to the problem 

 of senescence has been made by Carrel and Ebeling in 

 their most recent paper, in which they show that the rate 

 of multiplication of fibroblasts in vitro, and the duration 

 of life of such cultures, is inversely proportional to the 

 age of the animal from which the serum for the culture 

 medium is taken. These results are of such considerable 

 interest that it will be well to quote in full the summary 

 of them given by the authors: 



"Pure cultures of fibroblasts displayed marked differences in their 

 activity in the plasma of young, middle aged, and old chickens. The rate 

 of cell multiplication varied in inverse ratio to the age of the animal from 

 which the plasma was taken. There was a definite relation between the 

 age of the animal and the amount of new tissue produced in its plasma 

 in a given time. The chart obtained by plotting the rate of cell prolifera- 

 tion in ordinates, and the age of the animal in abscissae, showed that the 

 rate of growth decreased more quickly than the age increased. The de- 

 crease in the rate of growth was 50 per cent, during the first 3 years of 

 life, while in the following 6 years it was only 30 per cent. When the 

 duration of the life of the cultures in the four plasmas was compared, a 

 curve was obtained which showed about the same characteristics. The 

 duration of life of the fibroblasts in vitro varied in inverse ratio to the 

 age of the animal, and decreased more quickly than the age increased. 



"As the differences in the amount of new tissue produced in the 

 plasma of young, middle aged, and old chickens were large, the growth 

 of a pure culture of fibroblasts could be employed as a reagent for detect- 

 ing certain changes occurring in the plasma under the influence of age. 



" A comparative study of the growth of fibroblasts in media containing 

 no serum, and serum under low and high concentrations was made, in order 

 to ascertain whether the decreasing rate of cell multiplication was due to 

 the loss of an accelerating factor, or to the increase of an inhibiting one. 

 In high and low concentrations of the serum of young animals, no difference 

 in the rate of multiplication of fibroblasts was observed. This showed 

 that the serum of an actively growing animal did not contain any accel- 



