SECT. 2] AND WEIGHT 465 



and a glucoprotein rather like mucin, but when these were prepared 

 in a pure state, they showed no growth-promoting action in tissue 

 cultures. Carrel & Baker tried a great variety of other substances, 

 especially proteins, such as pure egg-albumen, but always without 

 result. Ether extraction, they found, did not remove the growth- 

 promoting factor from the embryo juice proteins, so that it seemed 

 unlikely that it could be of lipoid al nature. 



Some earlier experiments of Carrel & Ebeling had shown that 

 although pure amino-acids produced a slight stimulation of growth, 

 causing greater cell-migration, they did not produce an increase in 

 the mass of the tissue, as the protein fraction of embryo juice cer- 

 tainly did. This was confirmed by Carrel & Baker, who dialysed the 

 embryo juice in collodion bags of high permeability, or else ultra- 

 filtered it. They found that the dialysed juice did lose a certain 

 amount of its growth-promoting power, but they put this down to 

 denaturation of the proteins, in view of the fact that the amino-acids 

 separated in this way were practically without action on the growth 

 of fibroblasts in culture. Nor was there an enzyme in the juice which 

 produced amino-acids in the dialysing mixture more rapidly than 

 they could diffuse away. It is true that the liquid surrounding the 

 collodion bag and containing the diffusible amino-acids had a slight 

 effect on the increase in area of the explants, the rate being 1-28 

 times faster than in Tyrode solution, as against 10 times or upward 

 for the protein fraction. This small effect was put down by Carrel & 

 Baker to migration stimulus, and not to increase of mass of the explant. 

 When Carrel & Baker tried to restore the loss of activity of the protein 

 part by adding the amino-acids which had dialysed away, they 

 found it was impossible to do so — a fact which supported the view 

 that the proteins had been denatured by the dialysis. Thinking that 

 the concentration of amino-acids in the untreated juice might be too 

 small to show the presence of the growth-promoting factor, they 

 hydrolysed the juice protein with acids and with trypsin, but the 

 large concentrations of amino-acids so produced proved to be toxic 

 for the cells of the cultures. This toxicity had already been noticed 

 by Burrows & Neymann. Wright also worked with diffusates of 

 embryonic tissue juice and observed a perfectly equal number of 

 mitoses in explants of chick embryo heart, whether the medium used 

 contained embryonic tissue juice itself or the diffusate from it, e.g. 

 35 ± 5 in the former case and 32 ± 5 in the latter. In another 

 N E I , 30 



