22O C. M. CHILD AND L. H. HYMAN. 



special irritating or paralyzing effects of those agents are con- 

 cerned, and the modifications in susceptibility only serve to 

 show all the more clearly the close relation between susceptibility 

 and metabolic condition. 



The dyes were used in these experiments simply as a means of 

 further analysis of the action of external agents. Since they 

 possess color, it is possible to observe directly their penetration 

 into the cells and to see that they enter all cells readily. It is 

 certain that neutral red and methylene blue kill primarily from 

 within the cell rather than by destroying or altering its surface 

 first as perhaps some other agents do. They are adsorbed or 

 otherwise accumulated by certain constituents of the cell, and 

 it is of great interest to find that the general susceptibility rela- 

 tions with these dyes are essentially similar to these with other 

 agents, except where special irritating or other effects are con- 

 cerned. The dyes enter all cells, but they kill certain cells ear- 

 lier than others because the cells differ in some way in physio- 

 logical condition. Judging from their irritating effect, they prob- 

 ably increase metabolism to a very marked degree, at least at 

 first, yet where the irritation is not too extreme, the death 

 gradient is the same as in cyanide and other agents. No assump- 

 tion concerning the specific action of cyanide is necessary as a 

 basis for the susceptibility method, though it is quite true that 

 the conception of the action of cyanide current among physiolo- 

 gists has been adopted and applied in connection with the method. 

 The value of the susceptibility method rests not upon assump- 

 tion but upon certain facts of observation and experiment : 

 first, that in the susceptibility of protoplasm to many, perhaps 

 to all, external agents a certain uniformity of relation appears as 

 regards region of body, stage of life cycle, nutritive condition, 

 etc.; and second, that this uniformity is in some way associated 

 with and dependent upon the fundamental metabolic processes 

 in the protoplasm and may be altered by changes in the rate of 

 these processes. 



Both E. J. Lund in his paper on starvation in Paramecium 

 ('i8c) and B. L. Lund in a paper on the susceptibility of Para- 

 mecium and Didinium to KNC ('18), which confirms earlier 

 work of one of us (Child, '146), maintain that susceptibility is 



