CHAPTER XXIV 



PHAGOCYTOSIS 



Phagocytes. We owe our knowledge of the most important facts 

 in the new science of immunology to the labor and genius of Metch- 

 nikoff, whose elucidation of biologic processes we will follow in some 

 detail. In many forms of life, from the simplest unicellular to the 

 most complex multicellular, there are certain cells, one of whose 

 functions is to engulf and digest other cells. These eating cells are 

 designated by Metchnikoff as "phagocytes." The fungi which devour 

 fallen wood, dead leaves and other decaying vegetable matter consist 

 of sporangia or clusters filled with innumerable round spores or cells. 

 Under proper conditions these zoospores are set free and develop into 

 naked masses of protoplasm known as plasmodia * which vary in size 

 from small bodies to those several feet in length. In the free state, 

 these plasmodia are motile and they may engulf and digest solid 

 matter. They shun dry, and seek moist, places. Certain substances 

 attract and others repel them. This phenomenon is known among 

 botanists as chemotaxis. Most dead vegetable matter attracts them 

 while solutions of salt and sugar repel them. In other words, they are 

 attracted by substances which supply them with proper food, and are 

 repelled by substances which harm them. The former is known as 

 positive, and the latter as negative, chemotaxis. However, in this 

 respect they are capable within limits of accommodating themselves 

 to their surroundings, and negative chemotaxis may become positive. 



These plasmodia can feed on both soluble and insoluble sub- 

 stances. In the latter instance, they project pseudopodia, surround 

 the solid and digest it. This can be demonstrated by springkling on 

 their food some color such as carmine. These plasmodia absorb and 

 digest not only dead, but also living particles. For instance, they feed 

 upon living algae and the process of digestion within the cell can be 

 studied. Within the cells, green algae can be seen to turn brown, then 



* This has nothing to do with the plasmodia of malaria. 



