ORIGIN OF PHAGOCYTIC MONONUCLEAR CELLS 31 
not commonly ingest microscopic particles, and it does not seem 
necessary to review the great mass of literature to prove that 
phagocytosis is not a facultative function of most differentiated 
cells of adult human tissue because such cells may readily be 
brought into contact with foreign substances in vitro and in, 
Vivo. 
b. The nature of carbon ingested under experimental conditions 
by phagocytosis 
Although a considerable number of microscopic substances in 
suspension, such as neutral oils, India ink, staphylococcus, 
bacillus subtilis, and bacillus tuberculosis, were employed to 
determine the phagocytic properties of cells in vivo, lampblack 
was chosen as the most suitable for this purpose. All the 
suspensions of this substance used show on examination with the 
high-dry lens great numbers of microscopic particles which cor- 
respond in size and shape to those in the cytoplasm of phago- 
cytic cells placed in the suspensions. Since the particles of carbon 
are small, the question of colloid solution and intravital staining 
is raised. 
Bouchard (’06) was the first to demonstrate the possibility of 
staining living tissue intra vitam by the administration of certain 
benzidine dyes in colloid solution. Recently many dyes, pyrrhol 
blue, trypan blue, Janus green, and carmine solutions, some in 
molecular solution and others in colloid solution, have been 
injected into animals in normal and pathologic conditions to 
determine the behavior of living cells toward them. Many 
conflicting opinions in regard to the results obtained by intra 
vitam staining have been advanced. It was found by H. M. 
Evans (’15) that the endothelial cells lining small blood-vessels 
were uniformly stained by the intra vitam method. His ob- 
servations on the peripheral blood failed to show appreciable 
numbers of cells containing the dye granules, and he says, 
‘‘Macrophages do not occur in the peripheral blood stream except 
when produced in numbers so great that the condition may be 
called. pathological.” He defines a macrophage as a vitally 
staining mononuclear cell and applied the term phagocytosis to 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 25, NO. 1 
