102 SENESCENCE AND REJUVENESCENCE 



age, i.e., as growth and development proceed. Here the earliest 

 stages tested were young of about five millimeters in diameter. 

 Their susceptibility is greater than that of later stages and very 

 much greater than that of full-grown animals. In the course of 

 investigations not yet published on several species of oligochete 

 annelids, Miss Hyman has found that the young animals show a 

 greater susceptibility to cyanide than the old. The young in these 

 cases arose by the asexual process of fission and not from fertilized 

 eggs. Various species of entcmostracean Crustacea which have 

 been examined show in every case a greater susceptibility in the 

 young than in the old animals, but it is possible that differences in 

 size may be a factor in the result in these forms. In the larvae cf 

 amphibia the susceptibility is greater in newly hatched animals 

 than in later stages. 



CONCLUSION 



* 



The uniform results obtained from widely different groups show 

 very clearly that age differences in susceptibility to cyanides and 

 other narcotics are of general occurrence. Moreover, in all cases 

 the young animals, at least beyond a certain stage, show the 

 highest susceptibility, and susceptibility decreases with advancing 

 development. In other words, the rate of metabolism is highest 

 in the young animals and decreases with advancing age. This 

 conclusion is in full agreement with what we know of the physio- 

 logical aspects of senescence in the higher animals, and it forces 

 us to the further conclusion that a decrease in rate of metabolism is 

 at least very generally associated with growth and differentiation. 



REFERENCES 



CHILD, C. M. 



1913. "The Asexual Cycle in Planar ia velata in Relation to Senescence 

 and Rejuvenescence," Biol. Bull., XXV. 



MINOT, C. S. 



1908. The Problem of Age, Growth and Death. New York. 



