120 THE ACTION OF THE LIVING CELL 



their unique proteins ; yet when amino acids are added 

 to non-dialyzable portions of embryonic extract, the 

 mixture does not exhibit growth-stimulating proper- 

 ties analogous to that of the undialyzed extract 

 (Baker and Carrel, 1926) . On the other hand, Wright 

 (1925) has obtained from extracts of chick embryos 

 an active dialysate which markedly stimulates the 

 growth of cells in cultures. 



It follows, therefore, that the growth-promoting 

 principle must be a substance of relatively low molec- 

 ular weight; otherwise one should not expect it to 

 pass through the collodion membranes used for dial- 

 ysis. Such being the case, this substance cannot be a 

 proteose, as suggested by Baker and Carrel (1928), 

 who found that peptic digests of albumin and fibrin, 

 in contradistinction to the pure proteins, augmented 

 the growth of tissue cultures. Unlike the embryo ex- 

 tracts, such digests will not allow the tissues to live 

 indefinitely; hence their growth-stimulating proper- 

 ties are more apparent than real. 



Presumably such preparations offer the tissues ni- 

 trogen in a readily assimilable form, but do not pos- 

 sess some as yet unknown factor which is essential to 

 the continual growth of the tissue. This factor con- 

 tained in Wright's dialysates may be a stimulant which 

 permits the cells to utilize the foodstuffs available in 

 the media. Carrel (1928) , Fischer ( 1925) , and others 

 have observed that embryo extracts lose their growth- 

 stimulating activity if heated above 56° C. and in con- 

 sequence it has been suggested that the mysterious 

 growth factor is of enzymatic nature, since, as is well 

 known, many enzymes are rapidly inactivated at this 

 temperature. 



