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SENESCENCE AND REJUVENESCENCE 



kept in water often emerge as young worms within two or three 

 weeks after encystment. There is therefore no difficulty in ob- 

 taining small animals which are known to have developed from 

 encysted pieces for comparison with the larger animals at various 

 stages of the Ufe cycle. 



Fig. 36 shows the susceptibility of ten animals about two milli- 

 meters in length newly emerged from cysts (curve ab) compared 

 with that of ten full-grown animals raised from cysts in the labora- 



Hours 1234567 



Fig. 36. — Susceptibility of Planaria velata to KCN o.ooi mol.: ab, animals 

 newly emerged from cysts; cd, full-grown animals. 



tory (curve cd). The susceptibility of the small, newly emerged 

 animals is very much greater than that of the full-grown animals. 

 In other words, the newly emerged worms are young as regards 

 rate of metabolism, as they appear to be in every other respect, and 

 the full-grown animals which are about to undergo fragmentation 

 are old. In this species, as in P. dorotocephala, agamic reproduction 

 is simply a separation and reconstitution of pieces, and rejuvenes- 

 cence is associated with the reconstitutional changes in the piece. 



