149 



only 19 per cent of llial in Ihe phisni;i of Ihe yoiiiiij ani- 

 mal. The result of these ex|)eriinents was Ihal (lie rale 

 of multiplication of f i h r o h 1 a s t s and the < 1 ii r a- 

 t i o n of their life in \' i I r o varied in i n \- e r s e 

 ratio to the age of Ihe animals from w h i c h 

 the plasm a o r the s e r u m was taken. 



Experiments were undertaken by Carrel and 1'^ b e- 

 ling'-^-^ to examine if the aclion on the growth of fibro- 

 blasts of serum, taken from animals of advanced age 

 could be explained by the disappearance from the blood 

 of an accelerating factor, or the development in the blood 

 of an inhibiting factor. The working hy})othesis of the 

 investigators was, that if youth w-ere supposed to be char- 

 acterized by a factor present in the blood and activating 

 the multiplication of fibroblasts, and senescence by a de- 

 crease in the power of this factor, connective tissue cells 

 should proliferate more actively in a medium containing 

 serum under a high concentration than in a medium with 

 a lower concentration of the same serum. On the con- 

 trary, if the same phenomena were caused by the pro- 

 gressive increase in an inhibiting factor, the fibroblasts 

 should multiply more actively in a low than in a high serum 

 concentration. 



It was found that the difference in the rate of growth 

 in the low and high concentration of serum or plasma 

 from young and old animals, were practically the same. 

 This demonstrates that the blood serum in growing animals 

 does not contain any accelerating factor for the proliferation 

 of the fibroblasts. Age does not therefore bring about the 

 disappearance from the blood serum of an accelerating 

 factor, but produces the increase of an inhibiting factor for 

 the growth of fibroblasts. 



A studj/ of the influence of heterogenic sera, taken from 

 animals of different ages, on the rate of growth of fibro- 

 blasts has also been made by Carrel and E b e 1 i n g »^). 

 It was found earlier by Carrel, that the amount of growth 



