ACTION OF CYTOST 125 



human fetus, were made to human plasma containing 

 a trace of chicken cytost, and after 48 hours such 

 preparations were compared with control cultures 

 containing human cytost. In each instance growth 

 w^as apparent, but the cultures made with chick cytost 

 did not show so much growth as the controls, from 

 which it may be concluded that heterologous cytost is 

 not so efifective a growth stimulant as a similar prep- 

 aration from homologous tissues. 



This conclusion was substantiated by experiments 

 made with chick tissues in a medium of chicken 

 plasma plus traces of human cytost. These differences 

 between homologous and heterologous cytost are much 

 more clearly shown by the fact that when chick tis- 

 sues were transplanted to a medium consisting of two 

 drops of chicken plasma and one drop of human cy- 

 tost, the resulting cultures underwent growth, al- 

 though the latter was slight compared to growth ob- 

 tained in control cultures made simultaneously. This 

 result is in marked contrast with that obtained with 

 chicken cytost in equivalent concentration. In the 

 latter case, as stated above, the cultures were killed. 



Thus we arrive at the interesting conclusion that 

 whereas heterologous cytost has not the growth- 

 stimulating potentialities of homologous cytost, it also 

 does not exert the toxic effects characteristic of the 

 latter in high concentrations. 



The stimulating effects of tissue extracts upon tis- 

 sue cultures have been in part confirmed by Drew 

 (1922, 1923), who found autolyzed tissues to possess 

 a growth-stimulating principle analogous to that 

 found in embryo extracts. It will be noted, however, 

 that the growth-stimulating substances present in the 



