CONTROL OF HEAD FORMATION IN PLANARIA 
Direct measurement of the oxygen consumed in such head- 
frequency experiments is not practicable; first, because fifty 
pieces do not afford sufficient bulk of living material to produce 
in a convenient period of time marked changes in the oxygen 
content of 500 cc. of liquid, which is the smallest amount from 
which it is practicable to obtain a fair sample of 125 cc. for analy- 
sis; second, the operation of weighing, necessary in calculating 
the rate of oxygen consumption for purposes of comparison, 
undoubtedly injures the pieces to some extent, particularly the 
delicate tissues of the new head, and may result in the loss of a 
few pieces. Either of these events vitiates the results of a head- 
frequency experiment. It was necessary, therefore, to measure 
the oxygen consumption in a separate series of experiments that 
are comparable to those on head frequency. The conditions of 
an oxygen-consumption experiment were of necessity some- 
what different from those of a head-frequency experiment. It 
was not always practicable to use the same concentrations of 
the anesthetics employed in head-frequency experiments nor 
animals of the same size. More than fifty worms were used 
in most cases, and all three pieces, A, B, and C, were dropped 
into the same flask. Thus after the pieces were cut there were 
two or more flasks, one, the control, containing A, B, and C 
pieces in water and One or more others containing A, B, and C 
pieces in the proper solution of the anesthetic. The solution 
and water were then poured off and fresh introduced at the 
bottom of the flasks by means of inlet tubes passing through the 
rubber stoppers. The stoppers also held outlet siphons and 
holes to allow for overflow during the process of filling. The. 
solution and water were allowed to overflow through the stoppers 
for some time, until all the liquid that had been exposed to air had 
been replaced. All air bubbles were thus forced out of the flasks. 
The holes were then plugged, the siphons and inlet tubes clamped, 
and the flasks thus sealed were placed in a water-bath in which 
the temperature did not vary one degree during the time of the 
experiment. Fair samples were taken from the original stock 
solution and water and after a certain time interval fair samples 
were taken from the flasks by means of the outlet siphons. After 
the Withdrawal of the samples from the flasks, the remaining 
