236 ELIOT R. CLARK AND ELEANOR LINTON CLARK 
leucocytes were found containing small refractile globules of oil 
and, in one instance, such a pigmented cell was followed as it 
approached a minute droplet of oil and proceeded to ingest it. 
These cells continue to surround the globule of croton oil 
during its sojourn in the fin, and they occupy the site of injection 
until after the globule has been extruded. ‘Two or three hours 
after the extrusion, they begin to scatter in all directions and to 
wander away from the scene of the injury. The clearing up 
of the opaque area which occurs at this time is chiefly due to 
the dispersal of these pigmented leucocytes. Most of them 
wander away through the tissues, while some of them were 
observed to crawl through the walls of the neighboring lymphatic 
vessels. A number of pigmented leucocytes remain at the site 
of injury during the succeeding days and appear to be instru- 
mental in the disposal of the small mass of debris which remains 
in the tail after the extrusion of the globule. This opaque 
material gradually diminishes in amount during the next few 
days and, at the end of a week, the place of injury is marked 
only by one or two of these pigmented leucocytes. 
Like other leucocytes, these pigmented cells were also seen to 
act as phagocytes of extravasated red blood-cells. 
3. The connective-tissue cells 
The general picture of the reaction of these cells toward the 
croton oil has already been given. In one specimen this class 
of cells was followed with particular care. In this case, a record 
was made, before injection, of a selected portion of the tail and 
every mesenchyme cell in the region was drawn. By using two 
pigment cells as markers, it was possible to insert the globule 
of croton oil in the exact position desired. On examining this 
region immediately afterward in the observation chamber it 
was found that the globule had been inserted without disturb- 
ing any of the cells of the tail. The connective-tissue cells were 
numbered and observed continuously for twelve hours. On 
account of the mildness of the inflammation produced in this 
specimen, it was possible to follow individual cells throughout 
the process. 
