THE PROBLEM AND METHODS OF INVESTIGATION 75 



determined in cases where such methods are not available by 

 determining the limits of recovery, i.e., at stated intervals a certain 

 number of the organisms are removed from the narcotic solution to 

 water: the length of time in the narcotic at which recovery ceases 

 to occur is at least approximately the survival time. 



With the flatworms and other simple naked forms the suscepti- 

 bility method can usually be employed independently of differences 

 in size, for in such cases the death changes at the surface of the body 

 may be used as a basis for comparison. Moreover, in such elon- 

 gated flattened forms as the flatworms, surface increases almost 

 as rapidly as volume. But in forms where the permeable surfaces 

 are limited to certain regions of the body or are internal, as in air- 

 breathing forms, or where the body is covered by an exoskeleton, 

 the certain elimination of the factor of size often presents a difficult 

 problem. 



While most of my determinations of susceptibility have been 

 made upon the lower invertebrates, some experiments with the 

 higher invertebrates and the lower vertebrates have demonstrated 

 that the relation between susceptibility to cyanide and general 

 metabolic rate is the same in these as in the lower forms. But at 

 least as regards the vertebrates this is not true for all narcotics. 

 Vernon ('13) has found, for example, that the susceptibility of tad- 

 poles to some narcotics increases and to others decreases with ad- 

 vancing age, and suggests that these differences are due to changes 

 in the constitution of the cell lipoids. This is probably not the only 

 factor concerned : differences in the lipoid solubility of the different 

 narcotics and differences in the amount as well as the constitution 

 of lipoids in the nervous system and still other factors are probably 

 also involved, but further investigation is necessary before the sub- 

 ject is cleared up. In the lower invertebrates I have as yet found 

 no indication of such differences in the action of different narcotics 

 as Vernon describes. With some narcotics the age changes in sus- 

 ceptibility are greater than with others, but in all cases thus far 

 the changes during a given developmental period, as determined by 

 different narcotics, proceed in the same direction. It seems prob- 

 able that the differences in the direction of change in susceptibility 

 observed by Vernon result, at least in part, from differences in the 



