STUDIES ON THE DYNAMICS OF MORPHOGENESIS 



AND INHERITANCE JN EXPERIMENTAL 



REPRODUCTION 



VIII. DYNAMIC FACTORS IN HEAD-DETERMINATION IN PLANARIA 



C. M. CHILD 



From the Hull Zoological Laboratory, University of Chicago 



TWO FIGURES 



In earlier papers (Child '11 b, '11 c, '11 e) attention has been 

 called to the fact that the frequency of head-formation in iso- 

 lated pieces of Planaria dorotocephala varies with size of piece, 

 region of body and various external conditions which can be 

 controlled experimentally. This difference in capacity, together 

 with the possibility of comparing rates of metabolism in different 

 pieces by the susceptibility method (Child '13 a), affords a means 

 of determining why some pieces produce a head and others do 

 not and since the formation of the head is the first step in the 

 development of the new individual we have an answer to the 

 question why some pieces give rise to new wholes and others do 

 not. 



I. THE TIME OF HEAD-DETERMINATION 



Since it can be determined experimentally within wide limits 

 in various ways whether a piece of Planaria dorotocephala shall 

 give rise to a head or not (Child 'lib), it is evident that the fate 

 of the piece as regards head formation is not fixedly and finally 

 determined at the time of isolation of the piece from the parent 

 body. The first step in the analysis of the factors concerned in 

 head determination is to find when the determination of the 

 head occurs. This can be readily and very simply done by the 

 following method. 



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