INTRODUCTION 5 



Wildiers advanced the hypothesis that a substance 

 which he termed bios was necessary for the growth 

 of the yeast cell. Fulmer, Nelson, and Sherwood 

 found that yeast will grow in a synthetic medium pre- 

 pared from inorganic salts and pure sugar without the 

 addition of bios from another source. It follows 

 therefore that the yeast is itself able to produce the 

 growth-stimulating substance. In order to explain 

 the difference in growth rates as a function of the 

 amount of inoculum used, it is assumed that the 

 growth of yeast is accelerated by the presence of a 

 sufficient amount of bios of which each cell is capable 

 of supplying a certain amount; if the amount of the 

 inoculum is insufficient to contribute the necessary 

 quantity of bios then the growth of the yeast is re- 

 tarded. 



Alternatively, a pure culture of cells may, by their 

 metabolic activities, give rise to toxic substances 

 which hinder and even completely repress the normal 

 activities of the cells. As an illustration we may cite 

 the fact that dextrose broth cultures of B. typhosus 

 become sterile after a few days, due to the lethal ef- 

 fects of the end products of the bacterial metabolism. 

 It is worthy of note, however, that the medium may 

 still be utilized as a suitable environment for the 

 growth of bacteria of another species. 



It is well known that tissue cultures in vitro suffer 

 death unless supplied with fresh medium from time 

 to time. This fact is open to two interpretations: that 

 death may be due to the exhaustion of some essential 

 food factor; or that the cells, analogously to B. ty- 

 phosus, produce a toxic product. That the latter 

 hypothesis is the correct one is shown by the observa- 



