ORIGIN OF PHAGOCYTIC MONONUCLEAR CELLS 45 
This observation scarcely proves that phagocytic cells do not 
enter the venous circulation by way of thoracic duct, but ceér- 
tainly large numbers do not travel this route for the reaction in 
the lymph glands of this animal is marked. It seems more likely 
that a certain number of phagocytic cells arising from the endo- 
thelial lining of lymph-vessels pass directly into the blood 
stream by migration. 
C. CONCLUSIONS 
For studying the phagocytic activity of cells lampblack has an 
advantage over intra vitam stains, since the reaction is almost 
immediate. By showing that cells which have ingested carbon 
(3 per cent mononuclear ones) completely disappear from 
the peripheral circulation within twenty-four hours, the fail- 
ure of those employing intra vitam stains to demonstrate 
any considerable number of phagocytic cells in the blood is 
explained. Another advantage of lampblack over intra vitam 
stains is that thin paraffin sections may be made of the tissue 
fixed in a way best calculated to bring out the histologic struc- 
ture. ‘In such sections the limitation of the ingested carbon to 
the endothelial cells and mononuclear leucocytes assumes the 
character of a specific reaction for cells of endothelial origin. 
The demonstration of mitoses in endothelial carbon-containing 
cells lining capillaries which are not lengthening is proof of the 
origin of phagocytic mononuclear blood-cells from the endo- 
thelial cells of vessels. 
That the five per cent of phagocytic mononuclear cells present 
in human blood consists entirely of endothelial leucoctyes seems 
likely in view of the fact that there is positive proof of this 
origin and negative evidence showing that they are not of 
myeloblastic or lymphoblastic origin and that few phagocytic 
cells of any variety are present in normal extravascular tissue. 
For the incentive to undertake these experiments, it affords 
me great pleasure to acknowledge my indebtedness to Prof. 
F. B. Mallory, who has for a great many years taught his 
students the important rédle of the endothelial leucocytes. I 
am indebted to Miss Alice Charlton for valuable technical 
assistance and to Mr. L. Massopust for the illustrations. 
