412 C. M. CHILD 



head and greatest at levels nearest the posterior end of the 

 first zooid, pieces from intermediate levels showing intermediate 

 degrees of stimulation (Child, '14 b) and that in pieces of dif- 

 ferent length with anterior ends at the same level, the shorter 

 pieces are more stimulated than the longer. And finally, it 

 has been shown that it is determined during this period of 

 stimulation following section whether or not a piece shall 

 give rise to a head (Child, '14 b), and that the greater the 

 degree of stimulation, the lower the head-frequency (Child, 

 '14 a, b). According to the experimental data, then, this 

 stimulation of the pieces following section inhibits rather 

 than favors head-development. As pointed out elsewhere, this 

 relation between head-frequency and stimulation of piece 

 enables us to understand how the same concentration of an 

 inhibiting agent such as KNC may alter head-frequency in two 

 opposite directions. First, it may inhibit to a greater or less 

 extent the activity of the cells directly concerned in head-develop- 

 ment and so decrease head-frequency. Second, it may inhibit 

 or decrease the stimulation following section, i.e., it may inhibit 

 the physiological factor which inhibits head-formation and so 

 may actually increase head-frequency under certain conditions 

 (Child, '16 a). In the same paper it was shown that relatively 

 short exposures to the agent, which merely prevent the tem- 

 porary stimulation and permit recovery of the head-forming 

 cells are more effective in increasing head-frequency, while ex- 

 posures continuing through a considerable part of the head- 

 development are more effective in decreasing head-frequency. 

 This is of course to be expected from the facts. 



The data of experiment in general force us to the conclusion 

 that head-development is not determined by other regions of 

 the piece, though it may be inhibited by them. Head-develop- 

 ment is a self-differentiation which occurs if the cells near the 

 cut surface undergo a sufficient degree of dedifferentiation and 

 attain a sufficiently high rate of metabolism. In pieces under- 

 going reconstitution, whatever the level of the pieces the head 

 arises directly from the cut surface, and it is evident that in 

 all cases except at levels directly behind the original head, the 



