144 p - B - SIVICKIS - 



mining head frequency are to some extent established during this 

 time. Since this is the period of greatest stimulation of pieces, 

 since that stimulation is greater in shorter than in longer pieces 

 and in posterior than in anterior pieces of an individual or zooid, 

 and since decrease of that stimulation during the first few hours 

 after section increases head frequency, it appears highly probable 

 that the differential stimulation of pieces after section is a factor 

 in determining that under ordinary conditions head frequency is 

 lower in shorter than in longer and in posterior than in anterior 

 pieces of individual or zooid. 



Rate of Growth and Differentiation and Regional Differ- 

 entials in Development at Anterior Cut Surfaces. 



Only a brief statement of general results along these lines is 

 given at the present time. The growth reaction at anterior cut 

 surfaces is much more rapid in P. lata than in P. dorotocephala. 

 In the former the strong contraction following section has very 

 largely disappeared and a distinct outgrowth of new tissue is 

 present over the whole surface twenty-four hours after section 

 (Fig. 26), while in the latter the cut surface is still strongly con- 

 tracted and there is less than half as much new tissue (Fig. 27). 

 On posterior surfaces the differences between the two species are 

 less marked (Figs. 26, 27). Two days after section the differ- 



n 



26 27 



Figs. 26, 2.7. New tissue twenty-four hours after section: Fig. 26, P. lata; 

 Fig. 27, P. dorotocephala. 



ences between the species remain about the same, but during the 

 third day the rate of growth in P. dorotocephala becomes more 

 rapid, as compared with that in P. lata, and on the fourth day the 

 amounts of new tissue are apparently about the same. Evidently 

 the initiation and acceleration of growth at the cut surface occurs 



