IMMUNITY AGAINST MICROBES. 105 



smaller lymphocytes, which are ultimately digested in the 

 interior of the larger cells. 



The writer^ found in the lymphoid structures (tonsils, 

 Peyer^s patches, mesenteric glands) of the alimentary canal of 

 many animals large wandering cells containing two, three, or 

 more lymphocytes in their interior, and described all the stages 

 of the intracellular digestion of the latter. 



More lately, the writer^ has discovered that the so-called epi- 

 thelioid cells of the spleen-pulp have the power of taking into 

 their interior and of digesting smaller amoeboid cells. The same 

 cells also swallow inert substances such as vermilion when this is 

 injected into the blood-stream, this fact showing that they are 

 amoeboid structures — a supposition suggested by their irregular 

 shape. 



Before proceeding further, it is necessary to discuss the 

 origin of these cells — Phagocytes, as Metschnikoff has called 

 them. " The function of Phagocjtes,^^ says MetschnikoflF,^ 

 is usually the property of two kinds of cells. Small cells (mi- 

 grating cells) possessing one or many nuclei — leucocytes in 

 the narrower sense of the term — are scattered through all 

 tissues and concentrated in the lymphatic and blood-system, 

 but emigrate in case of need to any part of the body which is 

 invaded by parasites. I give to these cells the comprehensive 

 name of Microphages. On the other hand, I give the name of 

 Macrophages to the fixed cells of connective tissue, the epithe- 

 lioid cells of pulmonary alveoli, in fact, to all kinds of structure 

 which possess the power of taking solid bodies into their interior, 

 and which are provided with a single large nucleus, less easy 

 to stain than the nuclei of Microphages." This definition, 

 although clear, is, as will be seen later on, not quite adequate. 



Since MetschnikofF's first paper appeared it has become 

 evident that the Macrophages are often, if not always, derived 

 from Microphages. Heidenhein,^ describing the Macrophages 



1 Armand Ruffer, ' Quart Journ. of Micr. Sci.,' February, 1890. 



2 The writer gave a short account of these researches before the British 

 Medical Association, August, 1890. 



2 Metschnikoff, ' Ann. de I'lnstitut Pasteur,' 1887, p. 324. 

 •* Heidenhein, loc. cit. 



