SECT. 2] AND WEIGHT 467 



making use of them. The particular position taken by unhydrolysed 

 embryo tissue juice is of course very important from the embryo- 

 logical point of view, and merits much further research. 



Still later, Carrel & Baker went on to investigate the growth- 

 promoting activity of digests of pure proteins, fibrin, egg-albumen 

 and edestin. The proteins themselves, of course, had, from the earliest 

 days of tissue culture, been known to be more or less inert (Smyth; 

 Swezy). They now found that pure fibrin, when taken to pieces by 

 pepsin into its proteoses, had as powerful an effect as commercial 

 fibrin, so that the effect was not due to impurities such as blood 

 corpuscles. The split-products of pure egg-albumen, however, did 

 not show so large a growth-promoting action, and Carrel & Baker 

 found that it could be increased by the addition of pure glycine and 

 of pure nucleic acid. It was interesting that digests of vegetable 

 proteins such as edestin and gluten, showed a marked growth- 

 promoting activity. Carrel & Baker also showed that a medium con- 

 taining a maximum growth-promoting activity could be prepared 

 by digesting calf liver or pituitary with pepsin to the right extent, 

 and that a- and ^-proteoses were equally effective. 



If now we go back some years we find that in 1921 Carrel & 

 Ebeling found that adult serum strongly inhibited the multiplication 

 of fibroblasts and epithelial cells in tissue culture. As already 

 mentioned, it was not long before they found that this property 

 clearly increased with the age of the animal providing the serum 

 (Carrel & Ebeling). Then in 1927 Carrel & Baker showed that this 

 action was due to changes with age in both the lipoid and protein 

 fractions of the adult serum, and that in all probability it was related 

 to the increase of antitrypsin : a significant finding if the hydrolysis of 

 the proteose by the explanted cells is the most fundamental mechanism 

 involved. Carrel & Baker showed that the growth of fibroblasts was 

 1-56 times as good in the young as in the old serum. 



Brody, who was interested in this property of serum as an index 

 of senescence, constructed a chart showing the decline of its growth- 

 promoting power with age, and this is reproduced as Fig. 66. 

 Duration of life of fibroblasts grown in adult serum, plotted against 

 age of fowl giving the serum, declines with a k (instantaneous per 

 cent, growth-rate) of o-i8 and halves itself in 3-9 years. The 

 data from which this graph is derived were given to Brody by 

 Carrel. 



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