- ORIGIN OF PHAGOCYTIC MONONUCLEAR CELLS 29 
particles inside cells is not, therefore, certain evidence of phago- 
cytosis. It appears further that foreign bodies may come to 
lie within the cytoplasm of cells other than by the diffusion of 
substances in molecular or colloidal solution and by phagocytosis. 
The introduction of a foreign body into cells without the activity 
of the latter appears to take place when leprosy bacilli pro- 
liferate in epithelial structures such as the coil glands of the skin 
(Mallory, 714). The writer has found rather large particles of 
carbon in living cells twenty-four hours after the injection of 
lampblack suspensions directly into the liver parenchyma. Here 
as in the growth of leprosy bacilli in epithelial cells it appears 
that the microscopic particles are mechanically forced into the 
cell protoplasm. . 
In considering phagocytosis in human tissue or in the tissue 
of any metazoa the question of the differentiation and the 
specificity of cells arises, because specific activities cannot be 
assigned to cells that are not themselves specific and that may 
change into entirely different cells under a varying environment. 
Proof of any such transition of one type of cell to another, how- 
ever, is lacking except in the case of a limited number of closely 
related cells, and the very function of organs and tissues is 
dependent upon the specificity of the cells which compose them 
and which frequently undergo mitosis to replace dead cells. 
Such unfailing reproduction of the same variety of cells can 
result only from the existence of a living protoplasm specific for 
that kind of cell, and once a definite physical and chemical unit 
characterizing a cell has arisen in embryologic development it 
certainly rarely changes. In the adult body many examples of 
cell differentiation may be observed. The neutrophile dif- 
ferentiates from a younger cell through well-defined changes, 
but once differential characters have been acquired the neutro- 
phile does not change to an eosinophile or a basophile. 
A point of equal importance with the specificity of differen- 
tiated cells is the identification of such cells by morphologic or 
other characters. It is idle talk to speak of phagocytosis by a 
fibroblast when the preparation on which the statement is hased 
reveals none of the ‘earmarks’ which are known to characterize 
