274 INFECTION AND RESISTANCE 



algse, etc. (or particles of dead organic matter), into the cell 

 of the protozoon. 



These materials are gradually engulfed by the body of the ameba, 

 which flows about them with its pseudopods, and within the cyto- 

 plasm undergo gradual digestion. The process has been carefully 

 studied by Mouton. 8 In symbiotic cultures of amebaB with colon 

 bacilli on agar plates, the bacteria are taken up in large numbers 

 and about them are formed small vacuoles. That the digestion takes 

 place in a slightly acid medium with the vacuoles can be proved by 

 adding a drop of neutral red to the hang-drop preparation of amebse 

 and observing the brownish-red color taken by the materials in the 

 vacuoles. Mouton was able to obtain a digestive ferment from the 

 amebse, by glycerin extraction, which exerted strong proteolytic action 

 upon various albuminous substances, liquefied gelatin, and digested 

 dead colon bacilli in vitro, acting best in slightly alkaline, but also 

 in slightly acid, reactions. It is plain, therefore, that the most prim- 

 itive form of digestion is an intracellular one carried on by ferments 

 comparable in every way to the secreted digestive enzymes which 

 accomplish the same purpose outside of the cells in higher animals. 

 In essence, however, there is no fundamental difference physiolog- 

 ically between intra- and extracellular digestions, and the intracellu- 

 lar manner of assimilating solid nutritive particles may be retained 

 in forms much higher in the scale of evolution than the rhizopods. 

 It has been studied by Metchnikoff and others in certain of the flat 

 worms (Dendrocelum ladeum) in which typical phagocytosis is car- 

 ried on by the cells of the intestinal mucosa. Many of these plan- 

 aria obtain their nourishment by sucking the blood of higher ani- 

 mals. Placed under a microscope after feeding, it may be seen that 

 the foreign blood cells are rapidly taken up by the intestinal epithe- 

 lial cells, which engulf them by means of pseudopodia not unlike 

 those of the ameba. After ingestion, here, too, the blood cells are 

 surrounded by vacuoles within which their gradual disintegration or 

 digestion is accomplished. Similar intracellular digestion seems to 

 be general among the coelenterates, and has been thoroughly studied 

 by Metchnikoff in the' actinia. Here the food particles are carried 

 by the tentacles into the esophagus, and are taken up by the endo- 

 dermal cells of the so-called "mesenteric filaments," where they are 

 digested by a trypsin-like enzyme. In these animals digestion is 

 entirely intracellular, though the ingesting cells are the parts of a 

 specialized tissue. In other forms, still higher in the scale, although 

 there is persistence of intracellular digestion, the extracellular 

 process begins to be developed. Thus in certain mollusca the solid 

 food is taken into the intestinal canal, where it first undergoes a 

 preliminary digestion by secreted intestinal juices. After it has 



8 Mouton. C. E. de I'Acad. des Sciences, Vol. 133, 1901. 



