ORIGIN OF PHAGOCYTIC MONONUCLEAR CELLS 43 
lymphatic vessels and the second was to determine whether or 
not phagocytic mononuclear cells are present in the lymph 
entering the great veins of the neck. The first technic was 
carried out by injecting a 5 per cent suspension of lampblack 
in 1 per cent gelatin subcutaneously over large areas on the 
lower portion of the abdomen, in the groins, and in the flexor 
surfaces of the thighs. In guinea-pigs 211 and 212 (table 2) the 
lampblack was introduced into the subcutaneous tissue, the 
substance of the kidney, and the intestinal wall. 
After repeated injections (dog 152) a large anthracotic iliac 
node is present on either side of the vertebral column. Exami- 
nation of these shows carbon in the endothelial cells lining the 
lymph sinuses, but most of it is within large mononuclear cells 
in the sinuses and in the extravascular spaces near them 
(fig. 10). In an animal killed twenty-four hours after a single 
injection (dogs 177 and 210) an occasional cell with carbon is 
found in the sinuses, but none elsewhere (fig. 9). In the regional 
nodes in the one-day animals there is no macroscopic blackening, 
but with the lapse of a few days a sufficient number of carbon- 
containing cells have arrived from the site of injection to cause 
a blackening. Dilated lymphatic vessels in the subcutaneous 
tissue into which carbon has been forced are lined with carbon- 
containing cells (fig. 8), and there is little question but what these 
divide to form free phagocytic cells. That mobilization of phago- 
cytic mononuclear cells at the point of injection requires about 
twenty-four hours (dogs 177 and 210) indicates that these cells 
at first at least come from free cells in the blood stream, since 
cells do not arise by mitosis in less than this time. The attrac- 
tion of leucoctyes from the blood stream requires a number of 
hours for very few have appeared after two hours. 
Although the impossibility of phagocytosis by the lymphocytes 
has not been seriously questioned, the resemblance of some 
phagocytic cells to lymphocytes morphologically and the many 
conflicting statements on the subject have led to a close study 
of this phase of the subject. The distinct separation of lymph- 
vessels and lymphoblastic tissue has been referred to in fore- 
going paragraphs. The lymph-vessels like the blood-vessels are 
