STIMULATION OF GROWTH BY CROWDING 151 



from a single cell even when the nutrient conditions are most favor- 

 able. Likewise small groups of cells if isolated do not undergo divi- 

 sion and their growth remains at a standstill. On the other hand 

 certain tumor cells (Rous chicken sarcoma) are capable of multiply- 

 ing and producing colonies when isolated singly." Similarly, we 

 know that in nature single egg cells will grow. The germinal area of 

 the hen's egg is an excellent example of an isolated bit of protoplasm 

 which, under fav^orable conditions, will grow. It is of interest to us 

 to note that Wright (1926) has found by dialysis a growth-stimulant 

 in the incubated yolk of hen eggs which is not shown when such 

 yolk is added directly to tissue-culture medium without dialysis. 



Haberlandt {vide Fischer), in his work with plant cells, could se- 

 cure increase in size from certain isolated cells but did not find cell 

 division in such cultures. He (Haberlandt, 1919-22) reports a direct 

 relation between the size of the piece of plant tissue transplanted, or 

 the number of cells within it, and the number of cell divisions. From 

 these studies this investigator has concluded that the inciting to cell 

 division comes from substance given off by injured cells, which he 

 terms "wound hormones" or "division hormones." 



A dramatic instance of the effect of heterotypic crowding upon 

 growth of tissue cells in vitro is furnished by Carrel and Ebeling 

 (1923). Cultures of leucocytes and of fibroblasts were made together 

 in the same flask of plasma. As usual under these conditions, the 

 fibroblasts did not grow, while the leucocytes grew well. In time they 

 spread until they came in contact with the languishing fibroblasts, 

 when a marked revival and initiation of growth took place in the 

 latter cells. This agrees with the generally known fact that in the 

 tissues of early embryos, when growth is taking place most rapidly, 

 there is a mass of growing tissue tightly packed together which is 

 supplied with a relatively small amount of blood. In tissue cultures 

 growth takes place best when the cells are present in relatively large 

 numbers in a small amount of medium which is stagnant but proper- 

 ly supplied with oxygen. Both kinds of observations suggest that 

 the cells forming metazoan tissues are dependent greatly upon one 

 another for their growth. 



Fischer (1925) has suggested that this dependence is due to the 



