STIMULATION OF GROWTH BY CROWDING 157 



HETEROTYPIC CROWDING IN TISSUE CULTURES 



We have noted above the case of fibroblasts growing in plasma 

 only when under the close influence of leucocytes, as an instance of 

 the direct effect of different kinds of tissues grown together upon the 

 ability of the one to grow at all ; there is also evidence that differen- 

 tiation is stimulated or accelerated by the presence of two sorts of 

 cells in close association. Thus Ebeling and Fischer (1922) combined 

 a ten-year-old strain of fibroblasts grown in pure culture with a two 

 months' strain of epithelium which had been similarly grown in pure 

 culture. After the two had been grown together for some time, the 

 epithelium became rounded into a sort of epithelial glandular tissue 

 lying within a supporting network of fibroblast cells. Champy (191 4) 

 and Drew (1923) have reported somewhat similar results from com- 

 bining these two kinds of tissue cells into one culture. 



GROWTH-PROMOTING SUBSTANCES 



The possibility of growth being promoted by small amounts of 

 obscure chemical substances is indicated by the well-known work 

 upon vitamins in connection with the growth and well-being of man 

 and certain other mammals and of birds. The exact application of 

 the facts developed in connection with work on vitamins with ani- 

 mals at the level of group life with which we have been dealing is at 

 present unknown, since practically no work has been done upon the 

 vitamin relations of the invertebrates and little upon those of the 

 lower vertebrates. From the work upon the higher vertebrates we 

 know that vitamins A and B are both growth-promoting substances 

 whose absence from the diet leads to serious disturbances and finally 

 to death, and whose presence even in minute amounts promotes the 

 normal metabolic processes which result in growth. 



The possibility of growth-promoting substances being concerned 

 with the physiological effects of groups of animals upon the individ- 

 uals composing the group is further indicated by the work on "bios." 

 The literature on this subject is voluminous and confused. Tanner 

 (1925) presents an exhaustive review and bibliography of the re- 

 searches from i860 to 1924. "Bios" is the name provisionally given 



