I50 ANIMAL AGGREGATIONS 



fibroblasts or epithelial cells. Extracts of sarcomas are superficially 

 similar to extracts of embryos in their growth-producing qualities. 

 Leucocytes (macrophages) behave in reverse fashion, growing per- 

 manently in pure serum and being inhibited by the presence of em- 

 bryonic extracts. 



Inorganic substances, oxygen excepted, apparently do not affect 

 growth-rate of cells in vitro when present in approximately the same 

 concentrations as in the blood of the animal furnishing the tissues 

 under cultivation. Any departure from such concentrations yields 

 adverse results. Only approximately isotonic media allow indefinite 

 survival. The exact nature of the growth-promoting substance found 

 in embryonic extracts is still unknown. It appears to be associated 

 with the protein fraction and is particularly associated with pro- 

 teoses which result from a brief digestion of the protein with peptone 

 (Carrel and Baker, 1926). Prolonged digestion destroys the effec- 

 tiveness of this material. Willmer (1928) has been unable to confirm 

 this work, but concludes from the evidence furnished by Carrel, 

 Baker, and others that tissues can get some energy from amino- 

 acids but that their nitrogen supply is chiefly obtained from pro- 

 teoses (embryo extract contains both elements). These are heat- 

 stable substances; but most workers find that there is present in 

 embryonic juice a thermolabile growth-promoting substance which 

 is easily destroyed by heat or is adsorbed when heated, which does 

 not pass through a Chamberlain filter, and which is destroyed by 

 prolonged shakings. Carrel (1924) has called such substances "tre- 

 phones"; Fischer (1925a) calls supposedly similar substances "des- 

 mones"; and Burrows and Johnson (1925) named them the 

 "archusia." 



Tissue-culture workers appear to be agreed upon the necessity of 

 keeping the cells from normal tissues in numbers, for successful cul- 

 tivation in vitro. Harrison (1928) says in this connection: 'Tt is a 

 very interesting and at present inexplicable fact that single somatic 

 cells isolated in culture media do not proliferate. Experiments to 

 this end made in my own laboratory some years ago but not pub- 

 lished did not succeed and other workers have reported similar ex- 

 perience. As Fischer puts it, a colony of fibroblasts cannot arise 



