204 COMPARATIVE PHYSIOLOGY 



in metabolic activity in small organisms and embryos. Two 

 of these, neither of which are wholly satisfactory, may be 

 mentioned. One is the use of potassium permanganate as 

 a colorimetric indicator of oxidative processes. Potassium 

 permanganate is readily reduced by protoplasm with the pro- 

 duction of a brown coloration, the intensity of which may be 

 taken as a measure of oxidation in a particular region, but 

 depends on other things besides. Child and Hyman (19 19) 

 studied the effect of placing small organisms and embryos 

 in very dilute solutions (M/ 10,000), and described in all cases 

 a gradient along the oral-aboral axis with maximum activity 

 at the anterior end. 



The other method is the so-called susceptibility method. 

 The results obtained with this, though more striking still, 

 provide evidence of a somewhat indirect nature. In this 

 method Child has concentrated on defining the effects of 

 reagents like the cyanides which are known to reduce oxidative 

 activity. The method of interpretation is elaborate and 

 requires further investigation before it can be applied indis- 

 criminately, and one would feel more assured if Child had 

 confined his observations to the action of the cyanides alone. 

 Child and his co-workers have carried out experiments on 

 tissues at different temperatures and in different states of 

 activity which point to a quantitative relation between suscepti- 

 bility to the toxic action of cyanides on the one hand and to 

 metabolic rate, or, at least, to some form of physiological 

 activity, on the other. This relation is according to these 

 observations a complex one ; in lethal doses which are not 

 sufficiently concentrated to produce death within a short 

 period of exposure, the regions of higher activity are always 

 affected first, so that above a critical concentration suscepti- 

 bility varies directly as the physiological activity, w^hile below 

 this concentration the reverse relation is seen, in that regions 

 of higher activity recover and adjust themselves to the reagent 

 more successfully than regions of lower activity. In applying 

 the susceptibility method to embryonic development, lethal 

 concentrations may be used but not allowed to act long 

 enough to produce death in the embryo, and in such cases 



