40 



E. J. LUND 



TABLE 23 



Experiment XXI 



In this case we have a comparatively low concentration gradient 

 of the dye, together with a low toxicity and hence the compara- 

 tively small difference in the readiness with which Bursaria eats 

 the stained and unstained yolk. 



e. Sudan III. To show that an adsorbed substance which is 

 insoluble in the medium has no determining effect upon the 

 feeding and food selection, Sudan III was employed. This sub- 

 stance is insoluble in water but soluble in ethyl alcohol and fats. 



Experiment XXII. Fresh yolk was stained in an 80 per cent alco- 

 holic solution of Sudan III for a short time. It was then dried in an 

 oven at 27°C. for twenty-four hours. A control of fresh yolk was also 

 kept. The organisms were fed twenty minutes. The yolk takes on 

 a ver}^ deep color ^vith this stain. 



TABLE 24 



Experiment XXII 



Sudan III, stained 



yolk 



Control, unstained. 



NUMBER OP GRAINS EATEN BY EACH INDIVIDUAL 



19 

 4il4 



1825 

 628 



151 5 

 20 6 



7 

 9 3 



1812 

 1310 



26 



824 



243 



274 



It is evident that the insoluble Sudan III had no appreciable 

 effect upon the food reaction. Mixtures of these showed no 

 difference in the amounts of the two kinds of yolk eaten. 



/. Stale yolk. 



Experiment XXIII. ^Mixtures of fresh and stale yolk could not be 

 used since the grains of the two kinds of yolk were visibly indistinguish- 

 able. One experiment is given. The stale yolk was four weeks old 

 while the control was freshlj^ prepared; both were of the same con- 

 centration (table 25). 



