STUDIES ON THE DYNAMICS OF MORPHOGENESIS 



293 



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Fig. 40 Graphic representation of the percentages of head-formation in pieces 

 of different length and from different regions of the body. The ordinate repre- 

 sents percentages, the abscissa the length of the pieces and the region of the body. 

 Beneath the abscissa is drawn the longitudinal axis of a planarian body; with the 

 positions of pharynx and fission plane approximately indicated in dotted lines and 

 divided like the abscissa into twelfths (below) and sixteenths (above). This dia- 

 gram is drawn to the same scale as the curves, so that these can be directly 

 referred to it. The ourves thus show directly the percentage of heads in 

 pieces of different length, for each level of the body where regulation occurred. 

 The curve ah (unbroken line) shows head-formation in the quarter pieces .> the 

 curve ac (long dashes), in the sixth pieces: the curve ad (medium length dashes), 

 in the eighth pieces : the curve ae (short dashes), in the twelfth pieces : the curve a/ 

 (alternate long and short dashes), in the sixteenth pieces. In all except the six- 

 teenth pieces the course of the curves of head formation is identical with that of 

 the curves of whole formation. The curve of whole formation in the sixteenth 

 pieces is .r/: its course differs from that of the curve a/ in the portion shown in 

 dotted line. The data for these curves are obtained from table 1. 



original anterior end to the middle of the postpharyngeal region 

 and then increases rapidly. 



Examination of table 1 shows that in all except the sixteenth 

 pieces the frequency of head formation is the same as the 

 frequency of whole formation: all the curves of fig. 40 except 

 aj may therefore be regarded as curves of whole formation. For 

 the sixteenth pieces the curve of whole formation begins at x 



