AXIAL SUSCEPTIBILITY GRADIENTS. 403 



DISCUSSION AND SUMMARY. 



The existence in the apico-basal axis of the blastula, gastrula 

 and later stages of a definite and conspicuous gradient in suscepti- 

 bility which is the same for cyanide, alcohol, acid and alkali is 

 a significant fact, particularly in the light of the relations between 

 susceptibility to cyanides and various narcotics and general 

 metabolic rate. The results with HC1 and NH 4 OH show that 

 this relation is the same for these substances. The further 

 evidence for the existence of this gradient in the earlier develop - 

 mental stages, together with the difficulty of conceiving how an 

 apico-basal gradient could arise de novo during the earlier develop- 

 ment, leave little doubt that this axial gradient persists from the 

 unfertilized egg to the mature pluteus. This conclusion will be 

 confirmed by the indirect evidence presented elsewhere. 



As regards gradients in other axes the evidence is less con- 

 clusive. It is probable that the asymmetry in the progress of 

 death frequently observed in the blastula (Figs. 13, 14) is an 

 indication of a difference between anterior and posterior regions 

 and this probability is strengthened by the appearance of a 

 similar asymmetry in the gastrula and prepluteus stages (Figs. 

 17 and 19). 



It must be remembered, however, that this method of deter- 

 mining susceptibility is far from being a perfect method for the 

 demonstration of differences in metabolic rate. It can be ex- 

 pected to show only the grosser differences, for the reagents used 

 tend to decrease the differences which they are used to demon- 

 strate, and if the differences are originally slight they may disap- 

 pear so rapidly in the reagent that no appreciable or constant 

 differences in the time of death appear. 



It is evident from the course of development that the apico- 

 basal gradient is the most strongly marked and in the early 

 stages even this is not very clearly defined by differences in 

 susceptibility as determined by time of death, though it becomes 

 more distinct later. It is not to be expected that the minor 

 differences along the axes of symmetry should appear as clearly 

 by this method as the differences along the major axis. The 

 important facts are that the major axis appears so distinctly as 



