14 



E. J. LUND 



It has so far not been possible to determine the relation between 

 the number of grains eaten and the time of acidification of each, 

 because litmus is slightly toxic to Bursaria and hence stained 

 grains of even fresh yolk are not eaten very readily. 



The arrow at a in figure 4 shows the point at which the acid 

 first begins to be secreted into the forming vacuole. This is 

 indicated by the sudden appearance of a faint pink color about 

 the edge of the grain which gradually increases. Figure A in 

 plate 1 shows the degree of acidity reached before the grain 

 leaves the gullet. 



Fig. 4 Outline drawing of Bursaria showing steps in the formation of food 

 vacuoles and resorption of the liquid enclosed; a, place at which acid secretion 

 begins. Grains of relative size shown will not as a rule be rejected after they 

 have passed the point indicated by b. 



As the grain passes through the mouth surrounded by the 

 mixture of the acid and the external medium, the change toward 

 red progresses rapidly until the grain's interior has been reached 

 by the acid (plate 1, figs. A, B and C). When the acid had 

 reached the center of the grain the times noted in table 2 were 

 taken. There is no way of telling whether acid is continuously 

 poured into the vacuole after it has left the end of the gullet. 

 No evidence was obtained as to the nature of the acid secreted, 

 nor as to whether it is in combination or in the free condition. 



