ASEXUAL CYCLE OF PLANARIA VELATA. 



I8 7 



The results obtained in this way are further confirmed by the 

 much greater activity of the small recently emerged animals. 

 They move much more rapidly, are much more irritable and show 

 a much higher rate of growth than the large animals. And 

 finally the small worms from the cysts are capable, as noted in 

 the preceding section, of repeating the life cycle. There can I 

 think be no doubt that the worms emerging from the cysts are 

 physiologically young and that they undergo a process of senes- 

 cence as they grow in size. Evidently a process of rejuvenescence 

 is associated in some way with the asexual reproduction which 

 follows growth and development. 



III. EXPERIMENTAL REPRODUCTION. 

 i. The Course of Experimental Reproduction. 



The process of reproduction of whole animals from pieces 

 isolated by section is very similar to that in other planarians. 

 Pieces from any region of the body and above a certain limit of 

 size, w r hich varies somewhat with the region, are capable of giving 

 rise to whole animals. 



As in other species of Planaria, the process consists in part of 

 the outgrowth and differentiation of embryonic tissue from the 

 cut surface and in part of redifferentiation of other tissues to a 

 greater or less distance from the cut surface. In pieces of equal 

 length the amount of anterior new tissue is greater and of 

 posterior new tissue less in those from the anterior region of the 

 body, while with increasing distance of the end of the piece from 

 the head region the amount of anterior new tissue increases and 



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