8 E. J. LUND 



well as for growth can not be answered at present. But for the 

 questions dealt with in the present paper the important fact to 

 establish is that the yolk or vitellin can be drawn upon for the 

 energy requirements and growth of Bursaria. 



PROTEIN DIGESTION 



1 . The food vacuole; formation and physical changes 



The passage of the grain into the body is brought about par- 

 tially by ciliary action at the base of the buccal pouch ; but large 

 grains or masses of food are pushed through the base of the gullet 

 and into the endoplosm, by what seem to be contractions of the 

 wall of the base of the gullet, and also apparently by activity of 

 the endoplasm about the gullet behind the food. During this 

 process of swallowing, a liquid comes to be included about the 

 food so that when the vacuole separates from the base of the 

 gullet liquid surrounds the yolk grain. 



Where does the liquid enclosed with the grain come from? 

 Part of it is derived from the external medium, as is readily 

 determined by direct observation of the process of swallowing. 

 But the liquid in the vacuole, when the latter is separated from 

 the base of the gullet, is likewise partially made up of an acid 

 secretion from the cytoplasm of the lower portion of the gullet, 

 as will be demonstrated below. When the vacuole has formed 

 it is usually carried toward the middle of the cell and may remain 

 practically stationary, especially if it is large. If movement 

 takes place, which is nearly always the case when the vacuole 

 is small, then the vacuole may traverse any part of the cyto- 

 plasm, and in any direction. In Bursaria there is no such regu- 

 larity in the course of the food vacuole as has been described for 

 Paramecium by Nirenstein ('05) ; and for Carchesium by Green- 

 wood ('94). Often digestion and resorption begin and are 

 completed in one and the same place, without any circulation of 

 the vacuole. Residues are extruded from a small area on the 

 dorsal side of the body. 



The first visible change which takes place is the absorption of 

 the liquid which has been enclosed with the grain during the 



