DYNAMICS — MORPHOGENESIS AND INHERITANCE 411 



more or less clearly indicated by the differences in head-frequency 

 at different levels (Child, '11 d). In pieces representing the same 

 fraction of body-length the head-frequency is lower in smaller, 

 physiologically younger animals than in larger, older animals 

 and lower in starved than in well-fed animals (Child, '11a, 

 '20 a). In pieces of like size from animals of the same size and in 

 the same physiological condition, the head-frequency is higher 

 in pieces excited to active locomotion during the early stages of 

 reconstitution than in pieces remaining undisturbed (Child, '11a, 

 '20 a). 



On the other hand, it has been possible to control and alter 

 head-frequency by means of various external physicochemical 

 factors. KNC, for example, decreases head-frequency at levels 

 near the head and, except in relatively high concentrations, 

 increases it at levels near the posterior end of the first zooid and 

 produces little change at intermediate levels (Child, '16 a). Ex- 

 tensive work by Mr. J.W. Buchanan, which is not yet published, 

 shows that various anesthetics act in essentially the same way 

 as cyanides on head-frequency. Some work not yet completed, 

 with agents which act chiefly as accelerants rather than inhibi- 

 tors indicates that such agents alter head-frequency in the oppo- 

 site direction from the inhibiting agents. And finally, experi- 

 ments with different temperatures have shown that both high 

 and low temperature are very effective in altering head- fre- 

 quency (Behre, '18). Most of these experiments have been used 

 in laboratory class work and have therefore been repeated by 

 many different persons, with essentially identical results. In 

 general the inhibiting agents decrease head-frequency in pieces 

 from regions near the head, where it is normally highest, and 

 increase it in pieces near the posterior end of the first zooid, 

 where it is normally lowest. It has been shown by increase 

 in susceptibility and in COo production that the pieces are tem- 

 porarily stimulated by the act of section (Child, '14 a. Bobbins 

 and Child, '20), and data on oxygen consumption obtained by 

 Doctor Hyman and soon to be published also demonstrate 

 this stimulation. The data on stimulation show that in pieces 

 .of given length, the stimulation is least at levels nearest the 



