466 ON INCREASE IN SIZE [pt. iii 



instance the number of mitotic figures was: saline 2, extract 24, 

 diffusate 26. These results were interpreted by Carrel & Baker as 

 being due to a stimulatory action of amino-acids on migration, and 

 they stated that they observed similar phenomena themselves with 

 pure amino-acids, which, however, gave nothing approaching that 

 large extension of area seen in extracts with the protein fraction. 

 All amino-acids, according to Carrel & Baker, stimulate cell- 

 multiplication and migration, without increasing the mass of the 

 tissue, i.e. without causing growth. 



In a succeeding paper Carrel & Baker put to the test the suggestion 

 that it was not the proteins as such in the embryonic tissue juice 

 which carried the growth-promoting factor, but rather their larger 

 split-products such as proteoses, broken off from them by the cells 

 of the tissue culture themselves. They found that if tissue juice from 

 embryos was digested with pepsin for 16 or 32 hours, the hydrolysate 

 was rather toxic for the fibroblast explants, but if the digestion was 

 only carried on for 3I hours, the presence of the growth-promoting 

 factor readily revealed itself, growth proceeding seven times as fast as 

 in Ringer solution. The large protein split-products which were 

 evidently responsible could be obtained just as well from peptic 

 hydrolyses of egg-albumen, and commercial fibrin, the latter especially 

 proving a rich source of growth-promoting activity. Chemical ex- 

 amination of the different fractions of the digests, compared with 

 tissue culture examination, showed that the substances responsible 

 were undoubtedly proteoses. Pepsin digests of rabbit brain were 

 found to be active. Commercial "peptones" were found to vary 

 in activity, according to the proportion of higher split-products 

 contained in them. Tryptic hydrolysis was not so satisfactory. 

 Carrel & Baker suggested that the function of the proteose is to 

 furnish a higher concentration of amino-acids to the cells than 

 could be obtained even from their saturated solutions, and to 

 supply them to the cells in this tightly packed, yet soluble and 

 diffusible form. Carrel & Baker's identification of proteoses as the 

 important substances was confirmed by Fischer & Demuth and by 

 Willmer. 



All these facts led Carrel & Baker to conclude that there was no 

 justification for speaking of a "growth-promoting hormone". The 

 growth-promoting factor is probably no more than a right con- 

 junction of nutrient materials and the appropriate capacities for 



