148 



P. B. SIVICKIS. 



Behre, '18; Buchanan, '22). The evidence indicates further that 

 the positive determining factor is the rate of activity of the cells 

 at or near the cut surface (Fig. 31, x) which react to section by 



Fig. 31. Diagrammatic outline of piece after section: x, region directly con- 

 cerned in formation of head; y, region of correlative inhibitory effect on 

 head development. 



dedifferentiation, division, and growth, and are directly concerned 

 in head formation. The inhibiting factor, on the other hand, is 

 apparently the correlative influence on x of other parts of the 

 piece (Fig. 31, y) which tends to retard or inhibit the dedifferenti- 

 ation and growth of the x cells. More or less excitation of the 

 region y occurs temporarily after section, probably largely because 

 of the injury to the nerve cords, and experiments have shown, 

 first, that head frequency decreases as the degree of this excitation 

 increases — -e.g., at the more posterior levels of the first zooid and 

 in shorter pieces — and, second, that inhibition of this excitation 

 increases head frequency. It has been shown further that the 

 differential susceptibility of the regions x and y and the different 

 degrees of excitation of y at different levels of the body provide a 

 physiological basis for altering head frequency experimentally in 

 either direction with the same concentration of a single chemical 

 agent (Child, '16) and with many different agents and conditions 

 (Child, '20a; Behre, '18; Buchanan, '22). In short, the facts 

 indicate that head frequency varies directly with rate of metabolism 

 in x and inversely with rate in y. This relation has been stated in 

 the following brief form : 



ratejr 

 head frequency = — — . 

 rate y 



This formula is perhaps not complete, but serves provisionally to 

 indicate the opposite relation of the two factors to head frequency. 

 If it indicates the relation correctly, it follows that head formation 

 in reconstitution really takes place in spite of the rest of the piece. 



