ASEXUAL BREEDING AND PREVENTION OF SENESCENCE IN PLANARIA VELATA

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Abstract
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 apparently balance each other, at least during thirteen generations and probably will continue to do so indefinitely.

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