ASEXUAL BREEDING AND PREVENTION OF SENESCENCE. 293 



for a long period of time by preventing growth and the later 

 stages of differentiation. 



SUMMARY. 



1. Planaria velata has been bred asexually through thirteen 

 generations in less than three years without any indications of 

 progressive senescence in the stock. In each generation the 

 animals have passed through the following cycle: reconstitution 

 and rejuvenescence in the cysts, emergence as physiologically 

 young, small animals, growth and senescence with feeding, 

 cessation of feeding, fragmentation and encystment. During 

 the period of breeding none of the animals have ever become 

 sexually mature. 



2. Starvation and reduction being about an increase in rate 

 of metabolism and the reduced animals after renewed feeding 

 are in the same physiological condition as young growing animals 

 and are again capable of growth and senescence. 



3. Senescence has been inhibited or so far retarded as to be 

 inappreciable in a stock of Planaria velata during more than two 

 years by partial starvation. During this time the animals have 

 been kept at approximately the same size and in approximately 

 the same physiological condition, viz., that of half grown animals, 

 and no reproduction has occurred. During the same period 

 another stock of animals collected at the same time and originally 

 in the same generation has been fed and bred asexually and has 

 passed through twelve generations. 



4. In this species neither sexual nor asexual reproduction is 

 necessary for the production of young individuals from old. 

 Senescence is associated with growth and differentiation and 

 rejuvenescence with reduction and reconstitution. In the 

 asexual cycle senescence and rejuvenescence alternate and ap- 



t 



parently balance each other, at least during thirteen generations 

 and probably will continue to do so indefinitely. 



HULL ZOOLOGICAL LABORATORY, 

 UNIVERSITY OF CHICAGO. 



