REACTION OF CELLS TO CROTON OIL 231 
LEUCOCYTES 
1. Clear leucocytes 
This is a large group of cells, which includes wandering cells 
of the tissues and the leucocytes which migrate from the blood- 
vessels and lymphatics, without regard to their staining reac- 
tions. Such cells were included in one group in the present study 
for the reason that, in the living animal, their morphological 
characteristics are the same, as is also their behavior toward the 
croton oil. 
All wandering cells in the vicinity of the injected globule 
make their way toward the croton oil soon after the injection. 
At the end of an hour, the leucocytes in the blood-vessels just 
outside the ‘granular zone’ become adherent to the walls of the 
vessels and move directly toward the site of injury. This 
movement and the change in shape of these migrating leucocytes 
take place so rapidly that it is necessary to follow individual 
cells continuously. In some cases, leucocytes which had already 
migrated from blood-vessels were observed to crawl from the 
tissue into a lymphatic capillary and then to proceed rapidly 
down toward the tip by amoeboid movement. On reaching 
the tip of the lymphatic capillary, the leucocytes were seen 
to crawl out through the wall again. This observation was 
repeated several times in different larvae and in some cases 
several leucocytes in succession were observed to follow each 
other into a lymphatic, down to the tip and out again. By this 
means, the leucocytes reached the site of injury by a much more 
direct and speedy route, since they were not forced to make their 
way in and out among the fine processes of the connective tissue 
cells (fig. 14). 
Figures 1 and 2 give the general picture of the migration of 
the leucocytes and the formation of a ring of sessile leucocytes 
with processes at a short distance from the globule. In some 
specimens individual cells were watched consecutively for a 
number of hours. The series of sketches in figure 6 shows the 
changes which occur in a leucocyte which has migrated from a 
blood-vessel and has become stationary after approaching within 
