REACTION OF CELLS TO CROTON OIL 225 
An hour after the injection, the diapedesis of leucocytes has 
commenced and continues for as long a time as the croton oil 
remains in the fin. The leucocytes come through the walls of 
blood-vessels situated outside the granular area. They make 
their way rapidly through the tissue spaces toward the croton 
oil, moving past many of the vacuolated mesenchyme cells. 
However, these leucocytes do not reach the globule, but instead 
they become stationary at a short distance from the croton oil. 
Here they send out numerous fine processes until they appear to 
be covered with spicules and resemble sea-urchins. New leu- 
cocytes approach, and they, too, become stationary and send 
out processes until a thicket of such cells is formed (figs. 1 and 2). 
The fine processes change shape rapidly at first, but after an 
hour or two they become more stable, larger, and fewer in number, 
and the migrated leucocytes then resemble small connective- 
tissue cells (fig. 6). 
Outside this newly formed wall of sessile leucocytes, the 
connective-tissue cells and endothelial cells, which had possessed 
the opaque granular appearance characteristic of injury, become 
clear again and regain their normal contours and refractility. 
From now on the new leucocytes which approach the area of 
inflammation move in nearer to the globule before becoming 
stationary. 
In the meantime, numbers of the large leucocytes containing 
brown pigment have arrived on the scene, and these cells, ap- 
apparently a more resistant type than the ordinary clear leuco- 
cytes, move in very close to the oil globule, even flattening out 
on its surface. | 
The epidermal cells, directly over the globule, become heaped 
up. This apparent increase in the epidermal cells is due to 
the drawing together of cells of the vicinity rather than to a 
proliferation. 
The clear leucocytes continue to migrate into the region, to 
become stationary, and to send out processes. By this means, 
the area involved in the inflammation becomes more and more 
circumscribed. The globule finally shifts its position toward 
the surface of the fin, probably owing to the accumulation of 
