physiology of reconstitution of planaria lata. 123 



The Axial Susceptibility Gradients. 



The occurrence of definite axial susceptibility gradients in both 

 animals and plants and their relation to metabolic rate has been 

 discussed in various publications {e.g., Child, '20&). Various 

 lines of evidence show that susceptibility to a wide range of chemi- 

 cal agents in concentrations or intensities above the limit of toler- 

 ance or acclimation and below the limit at which death occurs 

 immediately varies in general directly with metabolic rate, or more 

 specifically with rate of respiration. Susceptibility gradients are 

 characteristic features of physiological axes in both plants and 

 animals and many facts indicate that such axes are primarily 

 quantitative physiological gradients. 



It has been found that in P. dorotocephala the head frequencies 

 at different levels of the body show a definite relation to the polar 

 susceptibility gradient, regions of high susceptibility being regions 

 of high head frequency and vice versa. The observations on sus- 

 ceptibility in P. lata show that a similar relation exists in this 

 species. In these observations KNC was used in most cases as 

 agent, because it is known to be a powerful inhibitor of oxidations 

 and has been extensively employed in the study of susceptibility, 

 but many other agents of very different constitution — e.g., HgCl 2 , 

 CuSo 4 , acetic acid, various anesthetics, etc. — in proper concentra- 

 tion give essentially the same results. The procedure consists, 

 first, in determining a concentration which kills slowly enough to 

 permit the differences in susceptibility to appear clearly and which 

 is not low enough to permit acclimation, and, second, in observing 

 the progress of disintegration of the tissues of animals placed in 

 the solution, in closed containers if necessary to avoid loss from 

 volatilization. As death of any part occurs or approaches struc- 

 tural disintegration of the tissues takes place, and such disintegra- 

 tion appears first in certain regions and follows a definite course, 

 so that certain body regions are completely disintegrated while 

 others are still intact and moving. 



The general course of disintegration in P. lata is shown in Figs. 

 7-10. The head and the posterior zooid are most susceptible and 

 disintegration progresses posteriorly from the head and at the 

 same time involves a region anterior to the posterior zooid and 



