114 C. M. CHILD 



sixteen hours (III c) produces a marked shift upward, and forty- 

 eight hours (IV c) a still greater shift upward. 



For the sake of direct comparison the effects of a long period 

 in the same concentration of KCN are given in V a-c. The 

 long period is very much more effective than any of the short 

 periods in shifting head-frequency downward in the a-pieces. 

 In the ^-pieces there is also a shift downward but much less 

 marked than in the a-pieces. In the c-pieces, however, the long 

 period (V c) shows a marked shift upward from the headless 

 opthalmic and teratomoiphic to the teratophthalmic but as 

 in the long periods of preceding series, there is no increase, in 

 this case a decrease, in normal head-frequency. Here then, 

 as in preceding series, the characteristic difference in the effect 

 of short and long periods appears. 



IV. DISCUSSION 



The most important results obtained from the data of the 

 preceding section are briefly as follows: first, in the controls 

 the head-frequency shifts downward as the level of the piece 

 becomes more posterior within the first zooid, but shifts upward 

 again in the posterior zooids and is verj^ high at the extreme 

 posterior end of the body. Second, KCN in low concentrations 

 if it acts for a sufficiently long time shifts the head-frequency 

 downward in the anterior region of the first zooid, produces 

 comparatively little change in the middle region and shifts 

 head-frequency upward except in the extreme posterior region 

 of the body. Third, short periods, up to a day or two in KCN 

 are much less effective than long periods in shifting head-fre- 

 quency downward in the anterior regions of the first zooid (a- 

 pieces) but are more effective than long periods in shifting head- 

 frequency upward in the posterior regions of the first zooid 

 (c-pieces). 



These rather remarkable results demand analysis. Consider- 

 ing for the present only the first zooid, since the same reagent 

 shifts head-frequency in opposite directions in different regions 

 of the same body, it is evident first, that the factors concerned 



