ACTION OF CYTOST 119 



ture medium is properly adjusted, a number of the 

 cells which have migrated to the periphery of the ex- 

 plant soon show mitotic division. 



As stated above, the presence of blood plasma ap- 

 pears to be of value principally because of its ability to 

 form a solid matrix suitable for the migratory activity 

 of the cells. So far as is known, it will not sustain un- 

 limited growth, and if taken from an old animal may 

 exhibit definitely toxic effects (Carrel and Ebeling, 

 1923), although when taken from young animals 

 heterologous plasma appears to be almost as satisfac- 

 tory as homologous plasma for migratory purposes 

 (Burrows, 1910, 1911). 



While a large number of substances have been ex- 

 amined with reference to their suitability as a medium 

 for cell growth in tissue cultures, but one substance 

 seems to be completely suitable for this purpose: 

 namely, an aqueous extract of young embryos. (Ebe- 

 ling, 1913; Carrel, 1913.) By the use of such an ex- 

 tract Ebeling (1922) has succeeded in keeping a cul- 

 ture of chicken fibroblasts more or less indefinitely — 

 for fifteen years at the time these experiments were 

 reported. In the course of this period the original 

 chick embryo cells had passed through some two thou- 

 sand generations, and no apparent diminution in 

 growth rate was observed. The duration of this ex- 

 periment was well beyond the average life span of 

 the hen and in consequence offers excellent evidence 

 of the potential immortality of somatic cells, to which 

 we have referred previously. 



Just how embryo juice keeps cells in culture alive 

 is unknown. It is obvious that the growing cells de- 

 mand a source of nitrogen for the development of 



