182 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



words, the head frequency at any level decreases with decrease in length 

 of piece below a certain limit, but this limit differs widely at different 

 body-levels. The length of piece at which differential inhibition of head 

 development begins to appear increases posteriorly in the anterior zooid 



V8-8 



Fig. 66. — Head frequencies of 1/3, 1/4, 1/6, and 1/8 pieces of postcephalic regions of 

 Dugesia dorotocephala. Head-frequency indices plotted as ordinates against body-levels as 

 abscissae; unbroken and broken lines represent frequencies obtained by different groups of 

 students in different years. 



and decreases again in the posterior-zooid region. In these lots of pieces 

 only head forms of the inhibition series appear, except for occasional sec- 

 ondary modifications indicating differential tolerance, conditioning to, or 

 recovery from, the inhibiting factor, and slight diiTerences in size, position, 

 and time of appearance of the two eyes, evidently due to incidental differ- 

 ences on the two sides of the cut end. 



