212 C. M. CHILD AND L. H. HYMAN. 



of the body is completed before that of the bud. In Figs. 20 to 

 22 the susceptibility of a late bud is about equal to that of the 

 parent and in Figs. 23 and 24 the same is true. In Figs. 25-27 

 both the earlier and later bud are less susceptible than the parent, 

 but here the susceptibility of the earlier bud is increased by irra- 

 diation of the excitation from the parent body. Fig. 28 shows 

 another case where susceptibility of a rather early bud stage is 

 less, even with gradient reversed by irradiation of excitation, 

 than that of the parent. In Figs. 65 and 66, 70 and 71, 73 and 

 74 the susceptibility of advanced buds is seen to be about equal 

 to that of the parent. 



This apparent discrepancy in the results of the susceptibility 

 method finds a very simple explanation in the total or almost 

 total absence of motility in the earlier bud stages and its gradual 

 development. In view of all the evidence presented above to 

 show the influence of muscular activity upon susceptibility, it 

 cannot be doubted that the development of motility in the bud 

 compensates the effect of progressive development as regards 

 susceptibility. The non-motile cells of the early bud, although 

 physiologically younger than the cells of the late bud or the sep- 

 arate animal, are less susceptible than the cells which have de- 

 veloped a functional contractile mechanism,, The susceptibility 

 method does not of course distinguish between the intrinsic high 

 metabolic rate of embryonic cells and the "functional" high rate 

 of differentiated cells resulting from excitation. In these ani- 

 mals, where the muscles are merely portions of the cells and where 

 excitation and muscular activity evidently alter the condition of 

 the cells so greatly, the functional differences in metabolic rate 

 usually overbalance the intrinsic differences associated with dif- 

 ferences in physiological age. 



When the bud has become a separate animal with fully devel- 

 oped excitability and motility, it is fairly comparable with the 

 full-grown animal and at this time susceptibility is greater than 

 that of the latter, i. e., the functional characteristics being de- 

 veloped in both, the age differences now appear in the greater 

 susceptibility of the younger animal. 



This interpretation of the susceptibility relations of buds and 

 parents and earlier and later stages of development is based on 



