70 PROTOPLASM OF PROTOZOA 



able membrane delimiting the food particles from the surrounding q^to- 

 plasm prevents the diffusion of undigested substances, such as proteins 

 and starches, until they have been broken down into diffusable com- 

 pounds of much smaller dimensions. 



The formation of food vacuoles by means of food cups (Kepner and 

 Taliaferro, 1913; Schaeffer, 1916) is of interest in connection with a 

 consideration of their surface structure. In Amoeba the membrane sur- 

 rounding the food particles has been observed to be derived directly 

 from the surface of the organism i.e., the plasmalemma. Schaeffer 

 (1916) reports that A. proteus may form several hundred such food 

 vacuoles a day, a fact which would necessitate considerable replacement 

 of the plasmalemma. If this observation be correct, one need only as- 

 sume a structure for the food vacuole membrane like that of the plasma- 

 lemma. Furthermore, it has been shown that the membrane of a food 

 vacuole which has completed its function in the cell may fuse upon eges- 

 tion with the cell membrane from which it has been derived ( Rowland, 

 1924c). In addition, some vital stains, such as neutral red, readily pene- 

 trate the surface membranes of many Protozoa and stain the food vacu- 

 oles, which suggests similar permeable properties for both the mem- 

 branes surrounding the surface and those surrounding the food vacuoles. 

 In contrast to its penetration of the surface and food-vacuole mem- 

 branes, neutral red does not readily diffuse through the membranes 

 of the nucleus or that of the contractile vacuole. 



Dissection and multilation studies upon a number of Protozoa have 

 shown that the food vacuoles are capable of existing free of the cyto- 

 plasm for relatively long intervals of time. For instance. King and 

 Beams (1937) have observed food vacuoles in water to retain their 

 form for over one-half hour, after which time the vacuolar membrane 

 was observed to wrinkle, followed by a breakdown of the vacuole. 



Dogiel and Issakowa-Keo (1927) immersed Paramenum in various 

 salt solutions and India ink. In solutions of MgSO^, MgCL, and FeSOj 

 the food vacuoles are much elongated. These sausage-like food vacuoles 

 may swell up or may be extruded through the gullet. In BaClg the food 

 vacuoles are small and spindle-shaped. Whether this effect is produced on 

 the membrane of the food vacuole or upon the cytoplasm is not clear. 



Mast (1938b) has recorded for A. proteus that a food vacuole may 



