ACTION OF CYTOST 131 



ever, the exact nature of these substances remains un- 

 known, these various names are of no descriptive value. 

 Therefore the writer prefers to retain the name "cy- 

 tost" simply because it was the first one coined to de- 

 scribe this mysterious substance. 



In a large mass of cells such as the transplants used 

 in tissue culture it is inevitable that some suffer injury 

 from one cause or another. In consequence, by the 

 mechanism previously discussed cytost will be liber- 

 ated from such cells. It seems likely, therefore, that 

 the growth observed in cultures made in pure Locke's 

 solution may be due to the stimulating action of the 

 cytost so derived. 



In agreement with this concept is the fact pre- 

 viously mentioned on page 130, that transplants of 

 single cells, even in apparently suitable media, do not 

 undergo division and growth. Further, an enormous 

 number of papers have been published which present 

 evidence that the crowding of tissues in cultures ex- 

 erts a decidedly beneficial effect upon their out- 

 growth. (See Fischer, 1923.) This again may be at- 

 tributed to the action of cytost derived from some 

 cells upon other members of the colony. 



In view of the experimental results obtained with 

 vertebrate tissues, it became of interest to investigate 

 the effects of cytost upon unicellular organisms such 

 as protozoans and bacteria. A survey of the literature 

 concerning cultures of such organisms discloses a num- 

 ber of facts which may be interpreted in a manner 

 similar to that used to explain the empirical observa- 

 tions on tissue cultures. Thus, Robertson (1921), 

 working with Enchelys farcimen, found that the di- 

 vision rate of this organism in a hanging drop is sig- 



