CONTROL OF HEAD FORMATION IN PLANARIA 35 
volved in the dedifferentiation and redifferentiation of any of 
these tissues must be much the same in both A and C pieces. 
The direct effect of an external agent on these processes should 
therefore be much the same, at least in the same direction, in 
both. There is, therefore, no reason to suppose that the anes- 
thetics specifically prevent the dedifferentiation and redifferen- 
tiation of any one tissue in one region and facilitate these pro- 
cesses in another, either by chemical or physical action. And 
that any anesthetic could alter the relative amounts of the dif- 
ferent tissues present in the two regions is impossible except 
by differential destruction of the tissues, and there is no evidence 
of such action. 
Can the differential effect of the anesthetics in decreasing the 
head frequency in A pieces and increasing it in C pieces be due 
to their specific action on the different cell constituents present 
in different amounts in the cells of the two regions? It is true 
that the effects of certain of the anesthetics employed may differ 
specifically in the two regions because of possible differences in 
the amount and quality and distribution of certain cell substances, 
lipoids, proteins, water content, etc. But in view of the width 
of the range of conditions and agents which bring about similar 
changes in head frequency, such intracellular effects of certain 
of these agents cannot be considered the effect directly concerned 
in bringing about the changes in head frequency, but as their 
characteristic physical or chemical action which brings about 
certain quantitative metabolic relations. These relations are 
similar for all the agents and conditions that influence head fre- 
quency in the same directions. 
A brief examination of the physical and chemical properties 
of the anesthetics used in this work emphasizes the fact that 
their effect in altering head frequency is non-specific. Ether 
and chloroform are highly fat soluble. Chloroform dissolves 
but very slightly in water; ether dissolves much more readily. 
Chloretone is soluble in both fats and water. Chloral hydrate 
is freely soluble in water, but much less so in fats. Chloroform 
and ether are relatively inert chemically; Chloretone and chloral 
hydrate are rather easily broken up. Thus their widely different 
