FOOD 31 



forming any pseudopodia. After ingestion a 

 vacuole is rapidly formed round the bacterium, 

 and in this the latter enters into violent agitation, 

 spinning round with the greatest rapidity, and 

 eventually coming to rest and assuming a typical 

 shaking Brownian movement, slowly becoming 

 digested and disappearing the whole process 

 occupying less than five minutes. Large bodies 

 are taken in by the process of " invagination," 

 and are usually soon expelled, but small yeasts 

 and spores may be retained for some time. Stained 

 by the jelly method, using polychrome methylene 

 blue, the contents of the food vacuoles become 

 an intense blue colour, while with pyronin they 

 turn red in contrast with the yellow endoplasm. 

 Bacteria boiled with litmus until blue, on ingestion 

 turn red. These reactions are suggestive of 

 acidity. Food vacuoles as a rule disappear during 

 encystment, but on the other hand there may be 

 an accumulation of spherical granules in vacuoles 

 within the living amceba (fig. 9) which can be 

 distinguished in the cyst even if unstained (the 

 so-called " corpuscles chromatoides.") The con- 

 tractile vacuole is a prominent colourless sac 

 (regarded by some as a rudimentary kidney) which 

 rhythmically contracts and expands, and at intervals 

 bursts and expels its contents to the exterior. It 

 occasionally breaks up into several smaller vacuoles, 

 and these may persist for a time, or may quickly 

 re-form into a single one. During life the con- 

 tents of the vacuole are invisible, but after death 

 a number of highly refractile granules appear, 

 possibly being a precipitate due to some change 

 in the character of the fluid in the vacuole. The 



