144 



GENERAL PHYSIOLOGY 



frequently succeeds in bringing the cell into such a position and so 

 holding it fast by a delicate viscous secretion that its pseudopodia 

 are able to grasp the alga completely. Then, by flowing more and 

 more about the cell, the protoplasm encloses it gradually on all sides, 

 and the alga finds itself surrounded by a thin covering of water, form- 

 ing the so-called food-vacuole, in the interior of the amoeba, which 

 then creeps on unhindered. Amoeba, therefore, takes in solid food 

 by causing its protoplasm simply to surround the food-mass. 

 But the act does not always go on so smoothly. The difficulties 

 that arise before the food-mass, which yields continually to the 

 pressure of the encroaching protoplasm, is so fixed that the proto- 

 plasm can enclose it upon all sides, are frequently so great that not 

 rarely the amoeba, with its pseudopodia flowing on continually in 



u b c d 



FIG. 43. Amoeba devouring an alga-cell. Four successive stages of the process of food-ingestion. 



other directions, is taken away from its victim, and must creep 

 toward it anew in order to seize it, if it has not been taken entirely 

 out of the sphere of influence of the food-mass. 



The ingestion of food by other lihizopuda takes place exactly as 

 in the case of Amoeba, whether they have pseudopodia that are 

 thick and broad, fine and thread-like, or branched and tree- like. 

 If the food-bodies are motile organisms, e.g., Infusoria, they 

 usually cause the excretion of a viscous substance by stimulation 

 resulting from their swimming against the rhizopod body ; this is in- 

 creased by stimulation arising from their attempts to escape ; hence 

 they stick firmly and can be drawn into the protoplasm. The 

 amoeboid wandering-cells and leucocytes also, like Amoeba, ingest 

 solid substances which exist in the blood or in the interstitial spaces 

 between the cells. As the admirable work of Metschnikoff ('83, '84) 

 has lately shown, they possess very great importance in the protec- 

 tion of the body from infectious diseases by devouring the bacteria 

 that have entered a wound; they thus prevent the increase of the bac- 

 teria and protect the body from further infection (Fig. 44). Finally, 

 the ingestion of microscopic fat- droplets on the part of the intestinal 

 epithelium-cells represents the same mode of food-ingestion. In 

 lower animals e.g., in worms these cells are really amoeboid cells, 

 and by means of their pseudopodia flow around the fat-globules of 



