152 ANIMAL AGGREGATIONS 



slow diffusion of products of metabolism or secretions from one cell 

 to another. He thinks these travel by protoplasmic bridges and are 

 independent of Carrel's "trephones," since fibroblasts that cease to 

 grow in the presence of an abundance of these trephones may be 

 restored to rapid growth by the presence of active healthy cells. 



Burrows and his co-workers (1925, 1926) have put forward an in- 

 teresting and ingenious suggestion which explains many aspects of 

 the interrelations between cells and the fact that they must be pres- 

 ent in numbers before growth will occur and at the same time ex- 

 plains other characteristic activities of cells in tissue cultures. These 

 workers suggest that in the presence of a sufficient amount of oxygen, 

 about one-third of an atmosphere, the cells secrete a hypothetical 

 substance or group of related substances which as stated above are 

 called "archusia," which are supposed to function somewhat like the 

 desmones of Fischer except that the function of archusia is profound- 

 ly modified by their concentration. If present in high concentration, 

 they display an enzyme-like action which causes self-digestion of the 

 tissues; if the concentration is somewhat lower, the presence of ar- 

 chusia allows the cells to digest fats and proteins and to grow, pro- 

 viding the medium is otherwise suitable. In more dilute solutions, 

 tissue growth ceases; but the cells display their characteristic mi- 

 grating ability, which is frequently shown in cultures, or parts of 

 cultures, in which no growth is going forward. In yet more dilute 

 concentrations, the cells lose their power of carrying on their ordi- 

 nary activities, round up, and become dormant. 



Archusia are water soluble, are secreted by cells, and can diffuse 

 through cell membranes to the outside medium. They tend to collect 

 in quantity when many cells are together in a minimum volume 

 under stagnant conditions, which are known to favor growth of 

 tissue cultures. When too great a volume of medium is present in 

 proportion to the number of cells, or if such cells as fibroblasts are 

 isolated, archusia escape into the surrounding medium and growth 

 ceases. Cells isolated into sufficiently small volume should grow, 

 according to the implications of this hypothesis; but the needed 

 volume may be so small that other complicating factors arise. Such 

 a substance would also be carried away by repeated washings, which 



