DYNAMICS OF MORPHOGENESIS 67 



is fixedly determined in most pieces within 8 hours after section 

 and in practically all, within 12 hours after section. The de- 

 termination of the character of the head occurs later. In this 

 way it is possible to discover approximately the time of determi- 

 nation, not only of the head as a whole, but of the cephalic lobes, 

 eyes and preocular region and of the various types of head. For 

 comparable results animals of similar size and condition and 

 similar external conditions are necessary, because the time of 

 head-determination varies both with internal and external 

 factors and in fact can be altered experimentally. 



The times obtained in this way, however, represent the times 

 when the structures concerned have become so fixedly determined 

 that they cannot be altered even by extreme changes in con- 

 ditions. There is every reason to believe that in a given piece 

 it is determined whether a head shall form or not some time 

 before that determination becomes so firmly fixed as to be un- 

 changeable by altered conditions. Tables 1 and 2 show that in 

 a considerable percentage of pieces, head-determination has be- 

 come fixed within 3 hours after section. In short, there is no 

 question that under the usual conditions head-determination in 

 pieces must occur, or at least begin, almost immediately after 

 section and the conditions existing in the piece during the first 

 two or three horn's after section must constitute the most im- 

 portant factors in the process of head-determination. 



II. HEAD-FREQUENCY AND DEGREE OF STIMULATION IN PIECES 



In the first paper of this series (Child '11 c) it was shown that 

 the frequency of head-formation in pieces under the usual con- 

 ditions of temperature, etc., decreases with decrease in length 

 and also with increasingly posterior level of the piece within the 

 limits of a single zooid (Child 'lie). In other words, the shorter 

 or the more posterior a piece is, the less likely it is to give rise to 

 a head. In the preceding paper (Child '14 b) it was found 

 that the amount of temporary increase in the rate of metabolism, 

 i.e., of stimulation of pieces resulting from section, increases with 

 decreasing length and increasingly posterior level of the piece 

 within the limits of a single zooid. 



