GENERAL PHYSIOLOGY 187 



cellular digestion. This advance has been due mainly to the appli- 

 cation of the method first devised by Gleichen (1778) of introducing 

 into the body with food substances, inorganic, usually colored par- 

 ticles, which clearly outline the limits of the digestive cavities. These 

 cavities, early termed gastric vacuoles, were recognized as digesting 

 centers of the organisms, and Gleichen's method, employed by 

 Ehrenberg (1833-1838) led to his elaborate and at first widely 

 accepted, but erroneous, conception of the Polygastrica. INIodern 

 applications of this method consist in the introduction with the 

 food of delicate chemical substances, or indicators, which change 

 in color according to the acid or alkaline nature of the fluids in 

 which they lie. The observations of le Dantec (1890), Fabre- 

 Domergue\l888), ISIetschnikoff' (1889), Greenwood (1887-1894), 

 Nirenstein (1905), Khainsky (1910), and Metalnikoff (1903, 1912), 

 together with the study of extracti^•es by ^Vlesnil (1903), Mouton 

 (1902), Metschnikofl' 0893), Krukenberg (1886), Hartog and 

 Dixon (1893), etc.. have given a fairly comprehensive idea of the 

 processes of intracellular protein digestion in Protozoa. Another 

 group of observers including Meissner, Greenwood and Saunders, 

 Stol? (1900), Wortmann (1884), Celakowski (1892), Nirenstein, 

 etc., have shown the digestive possibilities in relation to carbo- 

 hydrates and fats. 



The majorit^• of Protozoa which ingest "solid" food take in at 

 the same time more or less water, which forms the gastric vacuole. 

 Thus in trichostomatous ciliates a vacuole is formed at the base of 

 the cytopharynx which varies in size according to the abundance 

 of food particles present. In Paramecium caiidaUnn the vacuole, 

 when formed, becomes spindle-shape as though pulled away from 

 the gullet by endoplasmic force, but it soon becomes spherical as 

 it moves about in the fluid endoplasm (Nirenstein, 1905). With the 

 ingestion of larger food bodies such as infusoria, flagellates of larger 

 size, diatoms, rotifers, etc., comparatively little water accompanies 

 the prey. Parameciwn caudatum when eaten by Didinium nasu- 

 tum, for example, lies in close contact with the protoplasm of its 

 captor and no water at all can be made out (Fig. 89). In such cases 

 the ingested organism is paralyzed and therefore motionless when 

 swallowed, but it very often happens that resistant food bodies 

 continue to struggle after they have been taken into the protoplasm; 

 rotifers, for example, are usually not motionless w^hen engulfed b}^ 

 Amoeba proteus. In such cases a considerable volume of water 

 gives the prey ample room to move without danger to the make-up 

 of the captor. In other cases in which water does not appear to 

 be taken in with the food, the latter becomes surrounded by fluids 

 secreted by the protoplasm. 



With many types of Protozoa the process of digestion begins 

 before the living prey is taken into the protoplasm of the captor. 



