ORIGIN OF PHAGOCYTIC MONONUCLEAR CELLS oe 
of much coarser carbon particles, the carbon taken up by the 
phagocytes is invisible or only a few particles (those visible 
microscopically in the sol) are seen in the cells. There is no 
absolute limit on the maximum size of colloid particles, but 
masses or lumps which settle out in a few minutes would hardly 
be called colloid particles. Many at least of the dyes used for 
staining intra vitam appear to fall in the class of colloid sols, 
but the term colloid in this connection has been rather loosely 
applied because, although there is no definite minimum size for 
colloids and no clear separation between colloid and molecular 
sol, no one should call a readily diffusible substance such as an 
alkaline carmine solution a colloid. The point of greatest 
interest is an explanation for the failure of those working with 
intra vitam stains to demonstrate the presence in the blood of 
numerous cells containing the dye granules, since cells which 
have ingested microscopic particles of lampblack are readily 
-demonstrated here in vivo and in vitro. In the animals receiving 
intravenous injection of lampblack-citrate suspensions carbon- 
containing cells appear in the peripheral blood within thirty 
minutes, but at the end of five hours few remain in the blood 
obtained in the usual way and after twenty-four hours no 
carbon-containing cells are demonstrable. Examination of all 
the tissues shows that owing to some change brought about in 
the leucocytes by the presence of carbon particles in them they 
are held in the capillaries of the liver, spleen, lungs, and other 
organs, and that after twenty-four hours many are migratng 
though the walls of the capillaries into the extravascular tissue. 
There is every reason to think that vitally stained cells may 
behave in the same way, and since the process of intra vitam 
staining requires twenty-four hours or more granule-containing 
cells do not appear in the peripheral blood. In the capillaries 
even in the thinnest and most perfectly stained paraffin sections 
it is not easy to differentiate between the cells present in the 
capillaries and the tissue about them, and it would seem that 
little accurate information could be obtained by the study of 
frozen sections of such tissue. 
