CELLULAR RESISTANCE 153 



accustomed to the presence of the sugar a positive chemotaxis appears. 

 The lower animal and vegetable cells exhibit a certain amount of selec- 

 tion in the material which they take up, and the leucocytes of higher 

 organisms may in a similar manner exhibit selectiveness. According 

 to our present-day physical conception of the activity of living proto- 

 plasm, this selectiveness would depend in all probability upon varying 

 sensibility to chemotactic influences or variation in the intensity of the 

 chemotactic stimuli. 



Ingestion of Foreign Body. The actual ingestion of the foreign 

 body depends upon the motility of the cell protoplasm, and this motility, 

 of course, is a function of the irritability of the protoplasm. Such 

 motility determines the ameboid movement of the cell to the material 

 to be ingested. Having approximated itself to the foreign material, 

 the cell throws out pseudopodia in such a fashion as to encircle the 

 foreign body ; as opposite pseudopodia meet the cell resumes in so far 

 as possible its normal form and the material is enclosed in the cell 

 protoplasm. These two stages in the process of phagocytosis have 

 been reduplicated in experiments with non-living material. 



Digestion. The third stage of phagocytosis is the digestion of the 

 foreign material. Such digestion is accomplished by secretions which 

 are poured out by the cell protoplasm so as to constitute a small area 

 of fluid about the ingested particle. By staining with dyes which show 

 the acid reaction it has been found that although the cell protoplasm 

 does not show acidity the fluid within the digestive vacuole is definitely 

 acid in character. Attempts to extract this digestive fluid from protozoa 

 have not been highly successful, but Mouton was able to extract from 

 a symbiotic culture of amebae and colon bacilli an enzyme which is 

 feebly proteolytic. This enzyme is capable of digesting colon bacilli 

 which have been killed but does not act upon living colon bacilli. The 

 intracellular digestion of these particles depends upon their solubility 

 by the digestive fluids. The cell may take up insoluble particles, in 

 which case the particles remain within the cells or are extruded with 

 the excreta of the cells. 



Types of Phagocytic Cells. It is incorrect to think of leucocytes 

 as identical with phagocytes, for numerous other body cells show this 

 capacity, including the eosinophiles, the endothelial cells, the pulp cells 

 of spleen and lymph-nodes, connective tissue cells, including bone cells, 

 striated muscle cells and giant cells. It is probable that the lymphocytes 

 and the mast cells exhibit no phagocytic activity. Metchnikoff divided 

 phagocytes into microphages and macrophages. The microphages 

 include particularly the neutrophilic leucocytes and the eosinophilic 

 leucocytes, the important phagocytes of the circulating blood. The 

 macrophages include the other cells mentioned above, the most important 

 being the endothelial cells. It is perfectly true that the endothelial cell 

 circulates in the blood, but apparently its most important activity is in 

 the tissues and body spaces. The microphages are the more sensitive 

 of the two groups and react not only to chemical stimuli but also to 

 tactile and physical influences. 



