122 THE ACTION OF THE LIVING CELL 



Again, when cytost extracts such as those frequently 

 referred to in our previous discussion are injected into 

 animals, the cells at the site of the injection are to a 

 certain extent stimulated to multiplication, as shown 

 by the presence of mitotic figures. This was observed 

 in the following way: A homologous cytost prepara- 

 tion was injected intramuscularly into one limb of a 

 cat, and a like quantity of sterile water was injected 

 similarly into the parallel leg. After the lapse of six 

 hours the tissues surrounding the sites of the two in- 

 jections were excised, fixed, and sectioned. Upon mi- 

 croscopic examination the tissues surrounding the cy- 

 tost injections showed distinct evidence of mitosis, 

 such as is seen in a healing wound, while the tissues 

 about the water injection showed no such evidence of 

 proliferation. 



Similarly when the heterologous cytost prepared by 

 a rat or rabbit was injected in this fashion, localized 

 mitosis did not take place. Hence these simple ex- 

 periments offer further evidence of the specificity of 

 cytost. It is interesting to compare these results with 

 the growth-stimulating factor present in embryo ex- 

 tracts where such marked specificity is not shown, for 

 Fischer (1925) has found that duck fibroblasts grow 

 well in chicken embryo extract, while Carrel and 

 Ebeling (1922) report that the tissues of the rat may 

 likewise be grown in the same medium. 



In view of these results, it became of interest to 

 examine the effects of cytost upon cells in tissue cul- 

 ture. Quite unexpectedly, cytost added to hanging- 

 drop cultures of various tissues was found to exhibit 

 two effects : in very low concentration it stimulates the 

 cells to growth in a manner similar to that of chick 



