THE CHANCES OF DEATH 77 



erating agent. The same experiments were repeated with the serum of 

 a 3 year old and a 9 year old chicken. The medium made of a high 

 concentration of serum had a markedly depressing effect on the growth, 

 and this effect was greater in the serum of the older animal. 



"The results of the experiments showed in a very definite manner that 

 certain changes occurring in the serum during the course of life can be 

 detected by modifications in the rate of growth of pure cultures of fibro- 

 blasts and that these changes are characterized by the increase of an 

 inhibiting factor, and not by the loss of an accelerating one. It appeared, 

 therefore, that the substances which greatly accelerate the multiplication 

 of fibroblasts and are found in the tissues do not exist in the blood serum, 

 or are constantly shielded by more active inhibiting factors. The curve 

 which expresses the variations of the inhibiting factor in function of the 

 age was compared with that showing the variations of the rate of healing 

 of a wound according to the age of the subject. For wounds of equal size, 

 the index of cicatrization, which expresses the rate of healing, varies in 

 inverse ratio to the age. The different values of the index of cicatrization 

 of a wound 40 sq cm. in area, taken from measurements made by du Noiiy, 

 were plotted in ordinates, and the age of the subject in abscissae. The 

 curve showed a decrease in the activity of cicatrization, which resembled 

 the decrease in the rate of growth of fibroblasts in function of the age 

 of the lanimal. This suggested the existence of a relation between the 

 factors determining both phenomena." 



These results suggest that there is produced in some 

 cases by the body or some of its parts, a substance 

 which inhibits the power of cells to multiply or to remain 

 alive. How general such a phenomenon is in occurrence 

 does not yet appear, but, apparently, it must be absent 

 in the case of clonal reproduction in plants already dis- 

 cussed, and in the analogous case of agamic reproduction 

 in lower Metazoa (cf. planarians). It seems possible 

 that the results of Carrel and Ebeling might be open to 

 a slightly different interpretation than that which they 

 give, which hypothecates a specific inhibiting substance 

 in the serum, increasing in either amount or specific 

 potency with age. It seems to me that all of their facts 

 could be interpreted with equal cogency on the supposi- 

 tion that the serum from an old animal is itself senes- 



