PHAGOCYTOSIS 331 



reaction. This has been shown by the actual extraction, from amebae, 

 of a trypsin-like ferment. 



As we proceed higher in the scale 'of the animal kingdom, we find 

 that this power of intracellular digestion, while not uniformly an 

 attribute of all the body cells, is still well developed and a necessary 

 physiological function of certain cells which have retained primitive 

 characters. In animals like the coelenterata, in which there are two 

 cell layers, an entoderm and an ectoderm, the ectodermal cells have lost 

 the power of intracellular digestion, while the entodermal cells are 

 still able to ingest and digest suitable foreign particles. It is only as 

 we proceed to animals of a much higher organization that the function 

 of cell ingestion of crude food is entirely removed from the process of 

 general nutrition. Nevertheless,- in these animals also, the actual cell 

 ingestion of foreign particles occurs, but it is now limited entirely to 

 a definite group of cells. In the higher animals and in man, this 

 function of phagocytosis is limited to the white blood cells of the 

 circulation, or leucocytes, to certain large endothelial cells lining the 

 serous cavities and blood-vessels, and to cells of a rather obscure 

 origin which contribute to the formation of giant cells within the 

 tissues. A convenient division of these phagocytic cells is that into 

 11 wandering cells" and " fixed cells." The wandering cells are the 

 polymorphonuclear leucocytes, called "microphages" by Metchnikoff, 

 and certain large mononuclear elements or ' ' macrophages. " Fixed 

 cells, also called macrophages by Metchnikoff and possessing the power 

 of ameboid motion, include the cells lining the serous cavities, and the 

 blood and lymph spaces. The small lymphocytes, so far as we know, 

 have no phagocytic functions. 



In studying the cellular activities which come into play whenever 

 foreign material of any description gains entrance into the animal 

 body, a definite reaction on the part of the phagocytic cells may be 

 observed. When we inject into the peritoneal cavity of a guinea-pig 

 a small quantity of nutrient broth, and examine the exudate within the 

 cavity from time to time, we can observe at first a diminution from the 

 normal of the cells present in the peritoneal fluid. This may be due 

 either to an injury of the leucocytes by the injected substance, or to 

 an actual repellent influence which the injected foreign material exerts 

 upon the wandering cells. 3 Very soon after this, however, the exudate 

 becomes extremely rich in leucocytes, chiefly of the polymorphonuclear 



3 Pierrallini, Ann. de 1'inst. Pasteur, 1897. 



