72 C. M. CHILD 



the few cells at x, even though rate y per unit of weight or volume 

 is not actually higher than rate x. The development of conduct- 

 ing paths, and in general the differentiation of y, must render it 

 more capable of producing correlative effects upon other parts 

 than are the few cells which are undergoing dedifferentiation 

 at X. Probably the rate of metabolism, cell for cell, is higher 

 in X than in y or becomes so very soon after section, for the cells 

 at X are those most affected by section. Susceptibility experi- 

 ments indicate that the region x usually possesses the highest rate 

 of any part of the piece. But in order to be able to develop in 

 spite of the region y, rate x must in all probability be considerably 

 higher than rate y, and the differences in susceptibility of the 

 regions x and y indicate that this is actually the case. In fact, 

 before the new tissue has developed far enough to permit the 

 distinction between heads and headless forms, we find in general 

 a greater susceptibility in the region x as compared with y, in 

 those pieces which would later show the higher head-frequency. 



But whatever particular relation between rate x and rate y 

 which may prove to be necessary for the inhibition of head- 

 formation or the development of a head, the expression head- 

 rate a: . 

 frequency = ~ still serves to indicate the internal conditions 



^ "^ rate y 



which influence head-formation. And not merely head-frequency 

 in general but the frequency of any of the different types of head, 

 normal, teratophthalmic, teratomorphic, anophthalmic, is deter- 

 mined in the same way. It will be shown that all these different 

 types of head represent simply different degrees of retardation of 

 the process of head-formation, whether by external or internal 

 factors. In very short pieces, where the whole or nearly the 

 whole piece represents region x and region y is absent or very 

 small, head-frequency should become proportional to rate x, 

 and this is actually the case. Any conditions which decrease 

 the rate sufficiently decrease the frequency of head-formation. 

 This point will be considered more fully at another time. If 

 the region x once succeeds in beginning its independent course 

 of development it soon becomes the dominant region of the piece 

 (Child '11 d), develops into a head and determines the establish- 



