104 PROTOZOOLOGY 



half of the body. Later, the vacuoles show a decreased acidity, finally 

 reaching pH 7.0. In Vorticella sp. and Stylonychia pustulata, the 

 range of pH observed in the food vacuoles was said to be 4.5- 

 7.0 and 4.8-7.0 respectively. The food vacuoles of Actinosphaer- 

 ium, according to Howland (1928), possess at the beginning pH 

 6.0-7.0 for 5 to 10 minutes, but this soon changes to acid (pH 4.3) 

 in which digestion appears to be carried on. In older food vacuoles 

 which are of less acid (pH 5.4-5.6), the digestion appears to be at 

 an end. In the species of Bresslaua, Claff, Dewey and Kidder (1941) 

 noted that a Colpoda taken into the food vacuole is instantly killed 

 with a sudden release of an acid which shows pH 3.0-4.2. During 

 digestion the protoplasm of the prey becomes alkaline and the un- 

 digested residue becomes acid before extrusion. 



Mast's observations (1942) on the food vacuoles in Amoeba pro- 

 teus and A. dubia containing Chilomonas or Colpidium, indicate: 



(1) the fluid in the vacuoles becomes first acid and then alkaline; 



(2) the increase in the acidity of the fluid in the vacuole is not due to 

 cytoplasmic secretion, but is probably due to respiration in the in- 

 gested organisms, chemical changes associated with their death, 

 etc.; and (3) the death of the organisms taken in the food vacuoles is 

 probably caused by the decrease in oxygen in the vacuoles, owing to 

 the respiration of the organisms in them. De La Arena (1941, 1942) 

 found the maximum acidity of the fluid of food vacuoles in Pelomyxa 

 carolinensis containing Colpidium striatum was pH 5.8 and was not 

 fatal for the ciliate, but considered the possibility of the existence in 

 the food vacuole of "some lethal agent" which kills the prey. 



Just exactly what processes take place in the food vacuole have 

 been observed only in a few cases. Nirenstein (1925) noticed the ap- 

 pearance of numerous neutral red-stainable granules around the food 

 vacuole which pass into the interior of the vacuole, and regarded 

 them as carriers of a tryptic ferment, while Roskin and Levinsohn 

 (1926) demonstrated the oxidase reaction in these granules. Hopkins 

 and Warner (1946) believe that the digestion of food in Entamoeba 

 histolytica is brought about by enzymes carried to the food vacuoles 

 by "digestive spherules" which arise at the periphery of the nucleus, 

 apparently due to the action of the substances diffusing from the nu- 

 cleus into the cytoplasm. 



As to the localization or distribution of enzymes within protozoan 

 body, definite information is not yet available. In centrifuged 

 Amoeba proteus, Holter and Kopac (1937) found the peptidase ac- 

 tivity independent of all cytoplasmic inclusions that were stratified 

 by centrifugal forces. Holter and L0vtrup (1949) found peptidase in 



