CHAPTER X 

 PHAGOCYTOSIS 



METCHNIKOFF'S researches on phagocytosis in the lowly organized 

 animals formed a starting-point for an entirely new series of 

 researches on the subject of immunity, and his treatise on the 

 <' Comparative Pathology of Inflammation" must ever remain a 

 great medical classic, as well as a most fascinating work. 

 Metchnikoff was primarily a biologist, and his attention was 

 attracted to the spectacle of amoebae and other unicellular 

 organisms containing bacteria. In these lowly constituted 

 animals the process by which the cell takes in foreign particles 

 can be watched with ease, and the steps of the process traced. 

 The amoeba throws out arm-like processes, which surround the 

 bacterium, close on it, and in a few minutes an organism 

 which was previously lying free is deeply embedded in the 

 animal's protoplasm. Metchnikoff watched its fate, and found 

 it lost its sharp outline and clear appearance, became granular, 

 and in a little while disappeared altogether. He found also that, 

 in many cases at least, a minute vacuole was formed round the 

 ingested organism, and further research showed that this vacuole 

 contained a fluid which was acid in reaction and held in solution 

 a digestive ferment allied to pepsin. Considering this observation 

 more closely, it is obvious, firstly, that the amoeba must be regarded 

 as being immune to the organisms which it ingests and digests, 

 and, secondly, that in this case the processes concerned in im- 

 munity are those concerned in nutrition : the amoeba is immune 

 to the bacterium because it can make use of its protoplasm as 

 nourishment. 



But this process, as Metchnikoff soon found, is not confined 

 to the unicellular organisms. His most beautiful and classical 

 series of researches deal with the properties and action of the 

 leucocytes in a small fresh-water crustacean (daphnia), which, 

 from its transparency and small size, is a very suitable object for 



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