120 HARRIET L. ROBBINS AND C. M. CHILD. 



This is all that is meant when susceptibility is interpreted in 

 terms of metabolism or oxidation and the exact nature, degree 

 and extent of this relation of course remains to be determined. 

 This interpretation does not involve the assumption that sus- 

 ceptibility must always be parallel or even proportional to total 

 oxidation or even to total oxygen consumption or COo production. 



The relation between susceptibility and oxidation is undoubt- 

 edly indirect in at least most cases and it is conceivable that 

 susceptibility may be related only or primarily to certain oxi- 

 dative reactions or to conditions associated with them. More- 

 over, it is certain that in many cases susceptibility as determined 

 by death and disintegration is dependent primarily upon con- 

 ditions or reactions in particular regions of the body, e.g., in 

 ciliate infusoria the ectoplasm, in Planaria the ectoderm and 

 body-wall. Moreover, as many investigators have pointed 

 out, it is by no means certain that oxygen consumption and CO 2 

 production are exact quantitative measures of oxidation at 

 any given time. It is to be expected that susceptibility will not 

 always be proportional to total oxygen consumption or CO 2 

 production, but even then susceptibility may prove in the long 

 run to be a better indicator or comparative measure of physiologi- 

 cal condition than the respiratory data. 



From what has been said above and in earlier papers (e.g., 

 Child, 'IQC) it is evident that the criticisms of the susceptibility 

 method recently advanced by Lund ('i8a, &) and Allen ('18, '19) 

 need no discussion here, since they are largely beside the point 

 and result from failure to grasp the conception of susceptibility, 

 which has developed from many different lines of investigation, 

 not from one alone. Even if we grant the correctness of certain 

 of their conclusions from experimental data which are or appear 

 at present to be in conflict with conclusions reached in this 

 laboratory (Child, 'iqa, b, c, Hyman, '190, b) on the basis of 

 more extensive investigation, with more satisfactory technique 

 and several different methods instead of one they do not con- 

 stitute adequate grounds for denying the physiological signifi- 

 cance of susceptibility, but rather merely a starting point for 

 the further analysis of the particular cases in question. 



As regards the real significance of susceptibility, it makes 



