B22 ELIOT R. CLARK AND ELEANOR LINTON CLARK 
connective-tissue cells are fewer in number than in the larvae of 
frogs and toads. They are therefore wonderfully transparent 
and the individual cells may be followed readily. As was 
previously mentioned (E. R. Clark, ’12), this form is much easier 
to anaesthetize than the larger larvae of the Anura, it responds 
more quickly to chloretone and remains motionless for longer 
periods without deleterious effects on the circulation. 
The irritating substance selected for these experiments was 
croton oil. The undiluted oil was injected in a few larvae, but, 
in the majority of cases, 10 per cent croton oil (made of one part 
croton oil to nine parts paraffin oil) was employed. The mixture 
was sterilized before injection. 
The tadpoles were anaesthetized in chloretone 1 to 3000, 
made up in pond water. Under the binocular -microscope, 
small globules of croton oil were injected into the fin by means 
of small glass cannulae. The larvae were then transferred 
immediately to the observation chamber, previously described 
(E. R. Clark, ’12) and examined under the compound microscope 
in 1 to 4000 chloretone. With the microscope tube horizontal 
and a drawing-board placed beneath, at an angle of 45 degrees, 
and the use of a Leitz drawing eye-piece (no. 112), it was possible 
to make consecutive sketches of individual cells and to record 
the changes which occurred. In some cases, the region was 
watched and frequent records made over periods of six to twelve 
hours, after which the larvae were returned to fresh water over- 
night, and the region again observed on the following day. In 
other cases, especially where it was desirable to follow individual 
rapidly moving and changing cells, the region was kept under 
continuous observation for twenty-four hours or more. 
The cells and tissues present in the transparent fin, before 
he production of an injury, are the following: 
1. Stellate connective-tissue cells. 
2. Wandering cells: a) pigmented; b) non-pigmented. 
3. Blood capillaries lined with a single layer of endothelium. 
4. Lymphatic capillaries, also lined with endothelium. 
5. Blood-cells—red_ blood-cells and leucocytes inside the 
vessels. 
