THE AXIAL GRADIENTS IN HYDROZOA. 211 



themselves at once in permanganate and always undergo extreme 

 contraction. They show double color gradients, basipetal from 

 the apical end and acropetal from the foot, but modifications of 

 these gradients by contractile activity of stalk or body cannot 

 be observed because extreme contraction occurs in all cases. As 

 far as they go, however, these observations on color gradients 

 agree \vith and confirm the data on susceptibility. 



SUSCEPTIBILITY IN RELATION TO PHYSIOLOGICAL AGE. 



The view has been advanced by one of us that a process of de- 

 differentiation and rejuvenescence is associated with agamic re- 

 production, and that the individuals arising from a part of a pre- 

 existing individual by process of budding, fission, etc., are phys- 

 iologically younger than the individual from which they arose 

 (Child, '156, especially Chaps. VI., X.). In general the rate of 

 the fundamental metabolic processes decreases with advancing 

 senescence, and the agamically produced individuals show in their 

 higher rate of growth, their higher direct susceptibility and greater 

 capacity for acclimation, and in all cases thus far examined their 

 higher rate of CO^ production, that they possess a higher meta- 

 bolic rate than the individuals from which they arose, and their 

 morphological and cytological condition as compared with that 

 of the parent individual also indicates that rejuvenescence has 

 occurred. 



If this view is correct, and if susceptibility to toxic agents is a 

 criterion of physiological or metabolic condition, we might ex- 

 pect to find that the earlier bud stages of hydra are more suscep- 

 tible than the animals from which they arise and that suscepti- 

 bility decreases with advancing development. 



As a matter of fact, however, the early bud is almost invariably 

 less susceptible than the parent animal and the susceptibility of 

 later bud stages is greater than that of earlier stages and is often 

 equal to that of the parent. On the other hand, the "young" 

 separate animal recently developed from a bud shows in general 

 a higher susceptibility than the "old" full grown animal. As 

 regards the early bud, Fig. 14 shows the usual condition, in which 

 disintegration of the parent body occurs before that of the bud. 

 In Figs. 1 8 and 19, a somewhat later bud stage, the disintegration 



