REACTION OF CELLS TO CROTON OIL 2a 
which had previously shown all the signs of injury, cleared up 
and regained their normal appearance as soon as a ring of these 
sessile leucocytes with processes had formed between them 
and the globule of croton oil. Moreover, in contrast with the 
appearance just described, we observed one case of inflammation 
in a very young tadpole in which only a few wandering cells 
were present in the tissue spaces and only an occasional white 
blood-cell in the blood-vessel. In this case the granular area 
was very extensive. Connective-tissue cells and endothelial 
cells became swollen and vacuolated and the circulation in the 
blood capillaries stopped in a region three or four times as large 
as the one involved in the case of a similar injury in an older 
tadpole in which the migration of leucocytes had been abundant. 
In the later stages, after the extrusion of the globule, these 
clear leucocytes were seen to act as phagocytes, picking up 
extruded red blood-cells. 
2, Pigmented leucocytes 
These cells are normally present in the tissue spaces of the 
fin of Amphibian larvae, and they are also found occasionally 
inside blood-vessels and lymphatics. They are relatively large 
cells and contain brown and black pigment granules of varying 
sizes. Their origin and their relation, if any, to the large branch- 
ing chromatophores of tadpoles have not been studied. In 
stained specimens they may be seen to possess a single nucleus, 
usually round or oval in shape. Maximow and Friedsohn 
consider the pigmented leucocytes to be a special class of cells 
without any homologue among mammalian blood-cells. 
The pigmented leucocytes are strongly attracted toward the 
croton oil, and those located near the site of injury wander 
toward it soon after injection. These cells are remarkably 
resistant to injury. They wander in through the ‘granular area,’ 
past the line at which all the non-pigmented leucocytes have 
stopped, and make their way directly into the opaque region 
immediately surrounding the croton oil, and even flatten out 
directly against the oil globule. In some cases pigmented 
