12 



And it has hccii sliowii l)y C a r i* c 1 and Iv I) c 1 i n j^ os^ di^ 

 that till' inliil)ilin^ iiiriiicncc ol' homo or hclcM'o<»('nic ])las- 

 ma xarit'd in inverse ratio to llie aj^e ol the animal from 

 whieli the phisma was taken. Tlie (U-crease in tlie i"atc 

 of growth was ,")() pel" cent during llie first three yeai's of 

 life, whereas in llie l'oih)wing six years it was only .{() per 

 cent. Also the duration of life of the fihrohiasts in vitro 

 varied in invei'se ratio to the age of the animal and (U-ereased 

 more ra])i(lly than the age increased, h'rom tiiose ex|)eri- 

 ments it was also eoneluded llial during the course of life 

 an increase of the inhihitory factor takes place, and nol 

 a loss of an accelerating one. It can therefore be emphasized 

 Ihal tlu> substances which greatly accelerate the nuilli plica- 

 lion of fibroblasts are found in the tissue cells themselves, 

 and do nol exist in the blood serum. 



Carrel li) 12"^ ■''•') has shown that extracts from various 

 tissues increased the rale of growth of fibroblasts very 

 markedly. It was shown that extracts of embryos, of adult 

 spleen and of R o u s chicken sarcoma*) had a marked in- 

 creasing effect on the rate of multiplication of fibroblasts. Se- 

 veral years later Carrel and Ebeling |)rove(l that the 

 embryonic tissue juice was able to cari-y on an indefinite 

 multiplication of fibroblasts in vitro. In his early publica- 

 tion in l!)l.'l ..Ai'tificial acli\ation of growth of connective 

 tissue", Carrel ^«) showed that the activating power of 

 the embryonic juice was reduced when heated to :")() C. 

 and removed when heated at 70 C. By filtration through 

 a Berkefeld filtei* the activation power was decreased 

 markedly and completely sui)pressed by filtration through 

 a Chamberlands filter. These simple treatments of the 

 activating substances (tissue-juice) shows tliat the nature 

 of those substances is very unstable. 



As indicated by Carrel •'^'^^ in his pa|)er on the arlifi('ial 



*) In a recent publication Carrel ■?'^) claims that tfie growtfi-promo- 

 ting effect of tlie extract of Rous sarcoma is only apparent, and it is not 

 able to promote tlie growth of fibroblasts indefinitely. 



