ASEXUAL BREEDING AND PREVENTION OF SENESCENCE. 289 



due to the fact that the animal uses up its own tissues as a source 

 of energy and since it contains no skeleton which takes little or 

 no part in reduction a very great decrease in size may occur 

 before death. 



In a recent paper 1 it was shown that in Planaria dorotocephala 

 the susceptibility of the animals and also the rate of CO 2 

 production increases as the animals undergo reduction from star- 

 vation. These changes certainly indicate an increase in rate of 

 metabolism during starvation and reduction. The only differ- 

 ence between the animals reduced in size by a long period of 

 starvation and young growing animals is that the starved animals 

 possess almost no capacity for acclimation to low concentrations 

 of KCN, alcohol, etc., while the young growing animals possess 

 a high capacity for acclimation. When the starved animals are 

 again fed this difference disappears almost at once and they are 

 in all respects physiologically young and are capable of renewed 

 growth and of repeating the life cycle. 



In Planaria velata the same increase in susceptibility occurs 

 during starvation as in P. dorotocephala, the reduced animals show 

 almost as high a susceptibility as young growing animals of the 

 same size and when feeding is resumed this species is also capable 

 of renewed growth and of once more going through the life cycle. 

 The susceptibility determinations give essentially the same results 

 as in starved individuals of P. dorotocephala. Estimations of 

 CO 2 production and determinations of capacity for acclimation 

 have not been made for starved individuals of P. velata because 

 it seemed unnecessary in view of the other facts. 



There is then no doubt that starvation and reduction bring 

 about rejuvenescence in P. velata as in P. dorotocephala. The 

 large old worms with low susceptibility before starvation are 

 in essentially the same physiological condition after starvation 

 and reduction followed by renewed feeding as young growing 

 animals of the same size and are capable of repeating the life 

 history from the stage at which feeding is resumed. 



1 Child, C. M., "Starvation Rejuvenescence and Acclimation in Planaria doroto- 

 cephala," Arch. f. Entwickelungsmech., XXXVIII, 3, 1914- 



