HEAD-FREQUENCY IN PLAN ARIA 415 



With this brief statement of the general conception of head- 

 formation in pieces, we may turn to the interpretation of the 

 data recorded above. As regards the relation between head- 

 frequency and size, it was pointed out above that the rate of oxi- 

 dation is unquestionably higher in the small than in the large 

 animals. In isolated pieces of a smaller animal the tissue 

 throughout is physiologically younger and more active, conse- 

 quently the region x in its reaction to the absence of the parts in 

 front and to the wound does not undergo so great an increase 

 in rate in relation to ?/ as in a piece of a larger older animal, and 

 is therefore less independent of y in its development; i.e., shows 

 a lower head-frequency than in the pieces from larger, older 

 animals. From this point of view the lower head-frequency in 

 the younger animals is essentially a consequence of their higher 

 rate of metabolism or oxidation. 



As regards the starved animals, it has been pointed out that 

 lack of available nutritive material cannot be the primary factor 

 in determining the lower head-frequency. The facts noted above 

 concerning changes in susceptibility, CO2 production and oxygen 

 consumption during starvation indicate that the decrease in CO2 

 production and oxygen consumption in the earlier stages are 

 merely the result of the decrease in activity of the alimentary 

 tract in the absence of food and that the rate of oxidation in 

 ectoderm and body wall — probably also in the parenchyma — 

 increases from the beginning of starvation. These are the re- 

 gions chiefly concerned in the determination of head-frequency 

 in pieces, and if the conclusions concerning rate of oxidation are 

 correct, it is evident that the lower head-frequency in starving 

 as compared with fed animals is due to difference in the relation 



rate x 



— ; — - of the same sort as in young animals as compared with 



rate y 



old. In other words, the rate of energy-liberating metabolism 



being higher in the piece as a whole (fig. 3, y), the changes in 



the region x do not increase its rate over that of y so far as in w^ell- 



fed or old animals. Consequently, in starving animals dedif- 



ferentiation and new development of x is less independent of 



correlative factors in y than in fed animals, and head-frequency 



is therefore lower. 



