12 E. J. LUND 



food vacules formed at the same time, of the same material, and 

 in the same individual, may behave diversely as to the resorption 

 of liquid, and also in rate at which digestion takes place. But 

 this individuality of behavior in different vacuoles does not 

 prevent the conception of equilibrium from being applied to the 

 phenomena of resorption. Essentially the same observable 

 physical changes take place in vacuoles containing fresh yolk as 

 has been described for vitellin. Further evidence for this view 

 will be given on page 25. 



2. Chemical changes in the food vacuole 



Some of the chemical conditions in the food vacuole of Bur- 

 saria during digestion of vitellin can be shown by the use of 

 sensitive indicators adsorbed by the grains. 



Vitellin or fresh yolk grains stained with neutral red remain 

 bright red during the whole process of digestion. Grains stained 

 in an alkaline alcoholic solution of alizarin, quickly lose the 

 blue color and remain colorless throughout the digestive process. 

 Similarly, grains stained in an aqueous blue litmus solution 

 change to red and remain red until nothing remains of the grains. 

 These three indicators agree therefore in showing that the whole 

 process of digestion of vitellin takes place in an acid medium, 

 and that during no part of the process does alkalinity appear, 

 as it does in food vacuoles of Parmecium and some other ciliates. 



Grains of vitellin or yolk stained with congo red become dark 

 brown after one, two or more hours, and continue to remain 

 dark brown throughout digestion. In some grains no change in 

 color from red to brown takes place, but what differences in 

 conditions account for this has not been determined. Two 

 other indicators were used, Tropaeolin 00 and diethylaminoazo- 

 benzene. Vitellin or fresh yolk stained with Tropaeolin 00 

 showed Uttle or no change in color. Diethylaminoazobenzene 

 likewise showed no change. This result is due to the fact that 

 these two indicators are not sensitive to very weak acids or 

 strong acids in high dilution. 



