160 C. M. CHILD 



or even months. Moreover, the direct method affords no oppor- 

 tunity for the complication of the results by various factors which 

 may play a part in the indirect method, e.g., starvation. 



In my earlier experiments the indirect method with alcohol was 

 used because I desired first of all to determine the effect of this 

 and other substances on morphogenesis : the existence of the rela- 

 tion between length of life and rate of reaction was discovered 

 by this method (Child '11a, pp. 568 to 571). Later, as I became 

 more clearly aware of the general significance of this relation, the 

 effects of different concentrations were compared and the inver- 

 sion of the relation was discovered. Later still it was found that 

 more exact results could be obtained with KCN than with alcohol 

 and this reagent has since been used to a large extent. 



For Planaria dorotocephala the following concentrations have 

 been found to be most satisfactory. For the direct method KCN 

 0.001 m. serves, although concentrations considerably lower than 

 this may be used without altering anything but the time factor. 

 For the indirect method very low concentrations of KCN must be 

 used, 0.00004 m. or lower, i.e., acclimatization to KCN occurs 

 only in very low concentrations. 



In the case of alcohol a 4 per cent solution of absolute alcohol 

 is commonly used for the direct method though higher concen- 

 trations may of course be used.' For the indirect method a 1 

 per cent solution is sufficiently high when loss is prevented. In 

 my earfier experiments, where there was some loss 1.5 per cent 

 was used: in tightly closed flasks with very small air space this 

 concentration is too high. 



For ethyl ether 2 per cent or higher serves for the direct method 

 and a 0.3 per cent or lower for the indirect. Chloretone has been 

 used only for the indirect method thus far with concentrations 

 of 0.0014 771. or lower. 



This resistance method in general is applicable not only to 

 Planaria but to any forms in which the skeleton or .the connec- 

 tive tissue are not sufficiently developed or too closely coherent 

 to permit the occurrence of disintegration very soon after death. 

 I have obtained results of great interest by this method with 

 various planarians, with coelenterates and with a number of 



