174 THE AMERICAN NATURALIST. [VoL. XXXIV. 
rapidly and as well as in tap water or the water of the habitat ; 
thus demonstrating that the substances contained in these 
waters were not necessary for regeneration. 
However, the decrease in size already mentioned was so rapid 
and marked as to appear to deserve special study. So I isolated 
a number of active planarians in the redistilled water, measured 
them, and kept them in a thermostat at temperatures that 
ranged from 20° to 27° C. The dishes and water were changed 
regularly and frequently, measurements of each individual being 
made at the same times. The table on the opposite page shows 
the history of these specimens; less than half of the actual 
measurements in this series are given. 
It will be noted that the rate of decrease was not the same 
in all; in 2, for instance, it was much more rapid than in the 
others. This is probably due in part to greater activity of 2. 
Nor is the rate perfectly uniform in any given specimen, 
probably owing to variations in the temperature and in activity 
at different times. 
The smallest specimen obtained (No. 5, after 43 days) was 
certainly less than one-hundredth the bulk of the original 
animal. Its length was one-fifteenth the original length, its 
width one-third the initial width, and if we suppose that its 
dorso-ventral diameter was reduced by only one-half, the bulk 
would be one-ninefieth of the original bulk. But there can be 
little doubt that its dorso-ventral diameter was reduced more 
than one-half. 
Increase by fission was entirely stopped, but, on the other 
hand, the power of regeneration remained. Thus, when No. 5 
had been reduced to less than one-half its original length, it 
was cut in two parts, and both regenerated completely, although 
with constant diminution in bulk. The same experiment suc- 
ceeded in No. 3, after it had been starved to less than one- 
fourth its original length. 
It is thus demonstrated that from a given individual one of 
less than one-hundredth the original bulk may be produced 
by appropriate means. This constitutes a criticism on those 
experiments that have been made to determine the limits of 
regeneration in planarians by direct operation. The possible 
