CONTROL OF HEAD FORMATION IN PLANARIA 
9 
levels of the gradient. In fact, the data show that such posterior 
pieces as C are stimulated to a rate of metabolism equal to or 
greater than that of anterior pieces (Hyman, '21 b). The de- 
pressing effect of KNC on Rate Y in such posterior pieces with 
short time of exposure is sufficient to obscure in the end result 
any direct effect on Rate X. Later Hyman showed that the 
Fig. 3 Planaria dorotocephala, showing regions from which A, B, and C 
pieces were taken, E to F, posterior zooid. 
rate of oxygen consumption of Planaria is actually much re- 
duced in such concentrations of KNC and Child showed that 
carbon-dioxide production was similarly reduced (Hyman, '19 a; 
Child, '19 a). 
The data also show that head frequency may be shifted down- 
ward by KNC in pieces posterior to the A region (e.g., B and C 
in fig. 3) by increasing the concentration or extending the time 
of exposure. The decrease is greatest in A pieces, however. 
In pieces from lower levels of the gradient the shift upward in 
