CONTROL OF HEAD FORMATION IN PLANARIA 43 
in producing increases in the C pieces. In the changes in head 
frequency brought about by KNC and chloretone as compared 
with those produced by chloroform and ether, just these differ- 
ences appear. KNC and chloretone are very effective in pro- 
ducing decreases in head frequency in the A pieces and will 
produce decreases in pieces from the B and C regions with suffi- 
cient time of exposure or concentration, while chloroform and 
ether produce marked increases in the C pieces, but little decrease 
in the A pieces even when the time of exposure or concentration 
is very nearly lethal. In other words, KNC and chloretone 
inhibit to a considerable extent the intracellular changes in 
the X region of the pieces as well as the nervous stimulation of 
the Y region, while chloroform and ether in the concentrations 
used inhibit the nervous activity to a relatively greater degree 
than the general protoplasmic activity. KNC and chloretone 
are, then, more effective as direct inhibitors of head formation, 
and chloroform and ether are more effective as inhibitors of the 
apparently nervous stimulation of the Y region which tends to 
inhibit head formation. Further data on the nature of the stimu- 
lation of the X and Y regions and the differences in effect between 
several anesthetics will be presented in another paper. 
The criticism may be advanced that when we find certain 
decreases in head frequency in the A pieces and increases in the 
C pieces on examination after two weeks of regeneration, we 
are not dealing with factors determining whether or not a head 
will form at X, but that the results merely indicate retardation 
of the heads in A pieces and acceleration of development in the 
C pieces. This would be only another way of stating a claim 
for specific action on pieces from the two regions, and that point 
of View has been shown to be untenable. Furthermore, ex- 
periments have been performed which show that in such series 
of regenerated pieces there are no further changes in head fre- 
quency^ after two weeks. 
Criticism of the general nature of the data may also be ad- 
vanced: that controls cut from worms of the same length and 
nutritive condition at different times do not show the same head 
frequency; that in many experiments the C pieces in the con- 
