122 CM. CHILD 



In general the higher the concentration of the KCN the more 

 uniform its effect in shifting head-frequency downward. This 

 feature of the action of KCN is indicated only in table 1 where 

 the higher concentration, m/50000, produces a marked shift 

 downward in the anterior pieces and no change in the posterior 

 pieces. Still higher concentrations produce a shift downward 

 in all pieces. In all such cases the effect is of course produced 

 by the direct action of the cyanide on the x-cells, the cells con- 

 cerned in head-formation. The higher concentrations inhibit 

 these cells to a greater degree then ;hc lower and there is much 

 less acclimation or recovery, consequently with such concentra- 

 tions this direct effect on the head-forming region overbalances 

 any indirect effect in inhibiting the stimulation of the ^/-region 

 and the head-f requeue}'' shifts downward in all cases. Only 

 where the concentration is so low that a high degree of acclima- 

 tion or recovery of the .T-region occurs is it possible to shift 

 head-frequenc3^ upward by means of cyanide. 



As i*egards the region of the posterior zooids, I have found 

 that pieces from this region are in general much less stimulated 

 by section than pieces of the same length from the posterior 

 region of the first zooid. This difference is due first, to the 

 partial physiological isolation of the posterior zooids from more 

 anterior regions (Child, 'lid) and second, to the slight develop- 

 ment of physiological dominance (Child, 'lie, '15b) within 

 each zooid. In consequence of the lower degree of stimulation 

 after section, the head-frequency of pieces from the posterior 

 zooids is much higher than that of the c-pieces of the anterior 

 zooids as shown in tables 1 and 2 above. But the region of 

 the posterior zooids is not entirely independent of more anterior 

 regions and the more posterior levels of any zooid are to some 

 extent subordinate to the anterior region of that zooid. Con- 

 sequently, as I have found, some degree of stimulation follows 

 section even in these pieces, except perhaps in the most posterior 

 region of the body which is the most completely isolated physio- 

 logically of all. In accordance with this fact we find that short 

 periods in cyanide shift head-frequency upward to some extent 

 in all except the extreme posterior pieces from this region (tables 



