416 C. M. CHILD 



In the pieces stimulated to motor activity, head-frequency is 

 increased as compared with those at rest. The interpretation 

 suggested for this fact is that the cells at the anterior end which 

 are concerned in head-formation (fig. 3, x), are stimulated to a 

 greater degree than the rest of the piece by the forward move- 

 ment and that their independence of the correlative factors in y 

 is therefore greater than in the pieces at rest. In other words, 

 rate x undergoes increase in relation to rate y in this case and the 

 result is increase in head-frequency. 



It is clear, then, first, that the three physiological factors, age 

 as indicated by size, nutrition, and motor activity, influence head- 

 frequency; second, that this influence consists in increasing or 

 decreasing the relative number of pieces in a lot which produce 

 either heads or heads of a particular degree of development. As 

 regards the various degrees of head-development, it is a fact of 

 considerable interest that the different forms of head produced 

 remain the same as regards their structure, whether such factors 

 as size and region of piece (Child, '11 b, '14 b, '14 d), physiologi- 

 cal age, nutrition, and motor activity or an external chemical 

 agent such as KNC (Child, '16) or a physical agent such as tem- 

 perature (Behre, '18) are concerned. It is impossible to escape 

 the conclusion that the characteristics of the series of head forms 

 from normal to acephalic are determined by the specific consti- 

 tution of the protoplasm and that the effects of the various physi- 

 ological conditions in altering head-frequency are essentially 

 quantitative and non-specific. In the light of all these and var- 

 ious other facts, the quantitative interpretation presented above 

 has gradually taken form and thus far no facts have been ob- 

 served which are in conflict with it. 



In conclusion, it may be pointed out that the tabulated data 

 confirm earlier work on head-frequency in relation to length of 

 piece and region of body (Child, '11 b, '16). In table 1 the pieces 

 of series 201 and 277 are longer in relation to total length of body 

 than the pieces of other series of the same table and the head- 

 frequency is higher in these longer pieces. Similarly, the X 

 pieces of series 327 in table 2, being longer than the A pieces of 

 the same series, show a higher head-frequency than these, al- 



