4 C. M. CHILD. 



of undergoing redifferentiation. In other words, the cells which 

 are capable of regulation approach or return to the general type 

 of reaction characteristic of the specific protoplasm. 



II. REJUVENESCENCE IN RECONSTITUTION. 



The new whole which arises from the experimentally isolated 

 part may be much smaller than the whole of which it originally 

 formed a part. But the question at once arises, is it younger? 

 Has the process of dedifferentiation carried it back again toward 

 the beginning of the developmental cycle? I have recently 

 shown (Child, 'n&) that for Planaria this question must be 

 answered in the affirmative and I have further evidence of the 

 same kind for other forms. The new individual resulting from 

 the regulation of a piece is in all respects younger than the animal 

 of which it formed a part. Its rate of metabolism is higher, 

 it is capable of renewed growth and grows at the same rate as a 

 young animal: in every respect it has undergone a process of 

 rejuvenescence. Moreover, the degree of rejuvenescence is in 

 general proportional to the degree of reorganization in the piece, 

 i. e., the degree to which the old structure has disappeared and 

 new structure developed. This process of rejuvenescence 

 involves not only the portions directly concerned in regeneration 

 of lost parts but to a greater or less extent the other portions of 

 the body as well. For example, the removal of the posterior 

 portion of the body of Planaria brings about rejuvenescence 

 not only in the posterior region of the piece remaining but in 

 the whole piece, including the head, which plays no direct part 

 in the formation of the new posterior end (Child, 'lib). 



Rejuvenescence may also be accomplished by starvation 

 followed by -feeding, as I showed in the paper referred to, or by 

 any other means which determines the use as a source of energy 

 or the breaking down in any other way of portions of the struc- 

 tural substance of the organism or part. My experiments led 

 me to the conclusion that senescence in its simplest terms con- 

 sists in a decrease in the rate of metabolism determined by the 

 gradual accumulation of relatively inactive structural obstacles 

 to metabolism, which in turn are the necessary consequence 

 of continued metabolism under constant or relatively constant 



