CHAPTEE XIX. 



PHAGOCYTOSIS. 



As one may learn from the writings of Metchni- 



koff, phagocytosis, in its broad sense, exercises 



three distinct functions: nutritional, resorptive 



and protective. 



Phagocytosis Phagocytosis, for purposes of nutrition, is most 



for Purposes ,. ,, to ., J , ' . . ,, , , ' 



of Nutrition, highly developed in unicellular ameboid organ- 

 isms, but is found also in animals of considerable 

 organic differentiation. It is, perhaps, nowhere 

 more striking than among certain myxomycetes, 

 which are large, naked, multinucleated, protoplas- 

 mic masses belonging to the plant kingdom, and 

 which possess a peculiar, slow, undulating motility. 

 Ingestion is accomplished through protoplasmic 

 arms (pseudopodia) which are thrown out to en- 

 velop the object. Minute plant and animal cells, 

 living or dead, are ingested in this manner by the 

 myxomycetes, amebaa and other unicellular organ- 

 isms and are subsequently digested by means of 

 intracellular ferments. The ferments which have 

 been extracted from such cells are proteohlic since 

 they digest gelatin and fibrin, usually in an acid 

 but sometimes in an alkaline medium; that from 

 amebae has been called amibodiastase. In the proc- 

 ess of digestion a "vacuole," acid in reaction and 

 containing the ferment, forms around the in- 

 gested particle. In certain phagocytic unicellular 

 organisms the protoplasm shows a degree of differ- 

 entiation, a mouth and an anus being simulated at 

 points where the food is most readily taken in and 

 discharged. Instances are cited in which ameboid 



