RELATIONS OF BURSARIA TO FOOD 15 



The physical and chemical changes taking place in the vacuole 

 containing vitellin or fresh yolk grains, which have been de- 

 scribed above, are briefly summarized in plate 1. Here the 

 series of figures from A to L, inclusive (series 1 and 2), show the 

 usual course of the process of digestion and resorption of fresh 

 yolk when the latter is retained throughout the process. Series 

 1 and 3 show the usual course of digestion and resorption of vitel- 

 lin, when the rate of resorption is less than or equal to the rate 

 of digestion. Series 1 and 4 show the same when the rate or 

 power of resorption is equal to or greater than the rate of digestion 

 of the vitellin grain. The successive figures do not represent the 

 condition at equal intervals of time, but are typical stages in the 

 process. Frequently there occurs an alternate presence and 

 absence of liquid or smaller variations in the quantity of liquid 

 around the grain during the stages from H to L, inclusive, and 

 from H' or H" to L' or L", inclusive. Thus the figures from H, 

 H' and H" to L, L' and L" respectively, represent this process 

 only as it proceeds typically in the majority of cases. 



3. Effect of quantity of vitellin eaten upon the average velocity of 



digestion 



Since each cell could be fed just the desired number of grains 

 of practically uniform size, the effect of the quantity of protein 

 upon the average rate of digestion, and upon certain other 

 processes could be determined. 



Experiments arranged to find out what the relation is between 

 rate of digestion and the quantity eaten, were tried with both 

 fresh yolk and vitellin. Fresh yolk was found unsatisfactory 

 for this purpose, since the grains were generally not retained but 

 extruded before digestion was completed. This was especially 

 true when a large quantity of fresh yolk had been eaten (cf. 

 Experiment VIII, p. 34). Under usual conditions vitellin fed to 

 the animals was not extruded, and this was especially true if 

 care was taken not to let the animals eat too many grains. The 

 average limit in the number of grains that could be eaten and 

 retained was determined from a large number of observations 



