ASEXUAL CYCLE OF PLANARIA VELATA. 1 93 



cyanide, decreases, the rate of growth and the degree of activity 

 also decrease, they finally stop feeding, lose their dark color and 

 give rise to cysts again and from these a new generation of young 

 worms emerges. Stocks of animals produced from pieces have 

 passed through this cycle repeatedly in the laboratory. 



The degree of rejuvenescence in this experimental reproduc- 

 tion varies with the size of the piece. The smaller the piece, the 

 more extensive the reorganization and the younger the worm 

 which results. In all respects these results are essentially the 

 same as those obtained with Planaria dorotocephala and described 

 in an earlier paper (Child, 'n&). 



It is evident also that there is no essential difference in this 

 respect between the process of fragmentation in old worms and 

 the reproduction of young worms from the encysted pieces in 

 nature and the process of experimental reproduction of animals 

 from pieces isolated by section. In nature the fragmentation 

 occurs only in old animals by a process characteristic of a certain 

 stage of the life cycle. In experiment the pieces can be isolated 

 at any stage of the life history and may be of any size. In both 

 cases the reorganization, together with the period of starvation 

 which is also a factor as will appear, brings about rejuvenescence 

 and the worms thus produced are capable of repeating the life 

 history from the stage at which they begin again to feed to the 

 stage of fragmentation. 



IV. THE NATURE OF THE PROCESS OF ENCYSTMENT. 



It has been shown that the frequency of encystment of pieces 

 increases w r ith rising temperature, with decreasing size of the 

 piece, with increasing distance of the level of the piece from the 

 head region and with advancing age of the animals. Pieces from 

 any region of the body may encyst if the temperature is suffi- 

 ciently high, if the pieces are sufficiently small or if the animal is 

 sufficiently old. All of these conditions must have something in 

 common as regards their effect upon the pieces since all produce 

 similar results. What is this common factor? 



When a piece is cut from the body it is stimulated and its rate 

 of metabolism increases. This is generally admitted but it can 

 also be demonstrated by the cyanide method. The suscepti- 



