276 INFECTION AND IMMUNITY 



presence of a weakly alkaline 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. Never- 

 theless, 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 "wandering cells" and "fixed cells." The wandering cells 

 are the polymorphonuclear leucocytes, called ." micro phages " 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 quan- 

 tity 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 repel- 

 lent influence which the injected foreign material exerts upon the wan- 

 dering cells. 1 Very soon after this, however, the exudate becomes ex- 

 tremely rich in leucocytes, chiefly of the polymorphonuclear variety, the 

 maximum of the reaction being reached about eighteen to tw T enty-four 



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



