REACTION OF CELLS TO CROTON OIL 233 
transformation of polymorphonuclear leucocytes into connective- 
tissue cells. His description of this process and his illustrations 
show clearly that he was dealing with the same phenomenon 
described here, although croton oil is apparently a much more 
powerful irritant than silver nitrate, since the leucocytes 
migrated in much greater numbers in our experiments. In our 
first observations of the reaction to croton oil, we studied the 
region intensively for six or eight hours after injection of the 
irritant, returning the specimen to fresh water overnight. And, 
in this case, we followed many individual leucocytes from the 
blood-vessels during their wanderings through the tissue and 
through the stages illlustrated in figure 6, and we were practi- 
cally convinced that leucocytes may become transformed into 
connective-tissue cells. However, in subsequent studies in 
which certain of these cells were followed without interruption 
for twenty-four to thirty hours, we found that, at the time of 
the extrusion of the croton oil, these same stationary cells all 
withdraw their processes and resume their normal amoeboid 
form (fig. 7). 
An hour or two later, the same cells again become stationary, 
but this time they have a perfectly spherical form and remain in 
this rounded condition for several hours—some of them for 
several days, in spite of the fact that the pigmented leucocytes 
and even the stellate connective-tissue cells of the same region 
are moving and changing their shape constantly. This rounding 
up of the leucocytes takes place regardless of whether the tail 
is edematous or not (determined by measurement with the fine- 
adjustment screw). Moreover, simultaneous observations and 
records showed that, at the very moment when all of these 
leucocytes near the injured area were motionless and rounded, 
the wandering cells of the uninjured region were moving around 
through the tissue in anormal manner. Figure 7 shows in detail 
the changes which an individual leucocyte underwent shortly 
before and for some time after the extrusion of a globule of croton 
oil. 
The wandering cells present in the tissue before the injection 
reacted in exactly the same manner as the leucocytes from the 
