REACTION OF CELLS TO CROTON OIL 223 
6. Stationary pigment cells. 
7. Nerve cells. 
8. Epidermal cells, covering the surface of the fin. 
Of these cells, the connective-tissue cells, the wandering cells 
(including migrated leucocytes), and the endothelial cells lining 
the blood-vessels and lymphatics were the ones observed partic- 
ularly. These are the cells which had formerly been studied in 
the normal larvae and in the experiments in which various 
foreign substances had been injected into the fin. The character, 
behavior, and growth of the blood-vessels and lymphatics in the 
normal tail have been described in detail in earlier papers. The 
normal growth of connective-tissue cells has also been studied 
intensively (EH. R. Clark, 712). During the course of the present 
study records were also made of the wandering cells of the tissues 
in normal uninjured larvae. Consecutive records of these cells 
showed that they are constantly moving through the tissues, 
wandering around between the connective-tissue cells and coming 
back again to their starting-point. Each cell seems to have a 
' more or less definite route of its own. 
The blood-cells of Amphibia have been described by Maximow 
(10) and by Friedsohn (’10). The latter divides the leucocytes 
into the following classes: 
a. Leucocytes without granules. 
1. Small leucocytes. 
2. Large leucocytes. 
3. Polymorphonuclear leucocytes. 
4, Small oval cells with oval nuclei. 
(spindle cells). 
6. Leucocytes with granules. 
1. Eosinophile leucocytes. 
2. Mast leucocytes (basophile). 
3. Pigment leucocytes (not present in mammals). 
