RELATION OF BURSAEIA TO FOOD 29 



properly controlled and accounted for. We shall attempt to 

 determine what the relation of Bursaria is, to specific physical 

 and chemical properties of the food itself. First it will be deter- 

 mined how the external part of the reaction is modified, that is, 

 what is the behavior of the cell in so far as this has to do with 

 the selection of food. 



SELECTION OF FOOD AND THE FACTORS CONCERNED 



The object of the experiments described in the present section 

 is to answer the question: Can Bursaria discriminate quantita- 

 tive or qualitative differences between the yolk grains? 



When fresh hard boiled yolk grains, prepared as described 

 on page 8, are treated with different kinds of water-soluble 

 dyes, the amount of dye which is adsorbed by a grain of yolk 

 varies with the kind of dye used. At first a considerable number 

 of different dyes in aqueous solution were tested in a compara- 

 tively rough way; first, for the relative amount of each dye 

 which would be taken up by the grains of yolk; second, for the 

 r^te at which the dyes were adsorbed and the ease with which 

 they could be washed out (reversibility of the adsorption) ; and 

 third, for the relative toxicity of aqueous solutions of these dyes 

 to the organisms. Among the dyes so tested were fuchsin, lyons 

 blue, methylin blue, eosin, cyanin, gentian violet, saffranin, janus 

 green, congo red, and an aqueous solution of hematoxylin. 



The results of the following experiments on food selection, in 

 so far as they are related to the dye, depend upon the three 

 factors named: (1) The amount of dye adsorbed (2) The rate 

 of the reversible adsorption reaction, and (3) The relative tox- 

 icity to Bursaria, of the dye in aqueous solution. 



It was quickly found that certain dyes were better suited than 

 others, for the particular end in view. Aqueous solutions of 

 saffranin and janus green were found best to fulfil the necessary 

 conditions. Both show a reversible adsorption with yolk, while 

 the velocity of the reversible adsorption is sufficiently low to 

 prevent a too rapid washing out of the stain. By this means 

 one is able to control the amount of adsorbed dye much more 



