354 • ELIOT E. CLAKK 



For making records the camera lucida (E. Leitz drawing eye- 

 piece no. 112), with which the plane of the drawing-board makes 

 an angle of forty-five degrees with the tube of the microscope, 

 was used. A board, set at this angle to the horizontal, is placed 

 directly below the microscope, and is held firmly between clamps. 

 The outline sketches, made with the aid of the camera lucida 

 were usually corrected with a higher power ocular. 



Chloretone (acetone-chloroform), the anaesthetic properties 

 of which were discovered by Abel, and which is rapidly replacing 

 other substances as an anesthetic for small organisms, because 

 of its efficiency and harmlessness, is a well-nigh perfect anesthetic 

 for tadpoles. At the stages on which these studies were made 

 it inhibits bodily movements and respiratory movements; while 

 the movements of the alimentary canal are unaffected, and, if the 

 proper strength is found, the force of the heart beat remains 

 undiminished. The strength of chloretone necessary to produce 

 the proper depth of anesthesia varies somewhat with the species 

 of tadpole. Even for the same species, individual differences 

 are met with, so that a set rule cannot be laid down. In general, 

 a solution made by dissolving 1 gram of chloretone in 5,000 parts 

 of tap water suffices. With larvae of hyla pickeringii, 1:3,000 

 may be used safely. For larvae of rana sylvatica, r. palustris, 

 and r. catesbiana, the necessary strength varies between 1:4,500 

 and 1 :6,000. It was found most convenient to make up a series of 

 dilutions containing 1 :3,000 to 1 ^OOO parts of chloretone. With 

 these on hand the strength may be varied according to indica- 

 tions. Since chloretone is somewhat volatile, fresh chloretone 

 must be added from time to time during the observations. The 

 'indications' which controlled the strength of chloretone to be 

 used, were, on the one hand, the return of muscular movement, on 

 the other, the weakening of the heart action. Evidence of the 

 latter is to be seen in the condition of circulation in the blood 

 vessels of the tail, which are in view throughout the observations. 



The anesthetization of hyla pickeringii larvae is much easier 

 than that of the other three species mentioned. With a solution 

 of between 1 :3,000 and 1 :4,000 one of these larvae has been kept 

 under anesthesia five to twenty hours a day for more than four 



