CO 2 IN RELATION TO REGENERATION IN PLANARIA. IO7 



The pH at the beginning of the experiment was of course the 

 same in all lots to be compared, but the starting point differed 

 somewhat in different experiments, the extreme range being 7.8 

 to 7.95. Observations were made at least every half hour with 

 few exceptions, but the tables, instead of recording all the pH 

 readings, give the times required to reach an arbitrary end point, 

 Hp 7.3 being selected as this end point. As each lot approached 

 this point, observation was more or less continuous. A daylight 

 lamp was used for all color comparisons. 



A serious difficulty in the earlier experiments, particularly 

 with the pieces after section, was the occurrence of motor activity, 

 which of course increased COs production and introduced a 

 source of error. In these experiments as in earlier work, it was 

 observed that pieces from regions near the head are much more 

 likely to show apparently spontaneous motor activity during 

 the first few hours after section, than pieces from the more pos- 

 terior levels of the first zooid. After attempting in various ways 

 to eliminate motor activity, it was found that decrease of a few 

 degrees in temperature was usually effective for the length of 

 time necessary. In all the experiments tabulated below the 

 animals were kept at 21 to 22 C in the stocks and during prepar- 

 ation, but as soon as the tubes were sealed they were placed in 

 water at a constant temperature of 18 in very dim diffuse 

 daylight. Under these conditions motor activity occurred only 

 rarely, but its occurrence was always noted in the record of the 

 experiment. The procedure adopted in these experiments has 

 been described at some length because in experimental work with 

 animals of such small size it is extremely easy to go astray, if 

 the various sources of error are not carefully controlled as far as 

 possible. 



THE STIMULATION OF PIECES FOLLOWING SECTION. 



The temporary increase in susceptibility following section 

 and its characteristic relation, both to size of piece and region of 

 body (Child, 14, also p. 104 above) are so clearly shown by the 

 susceptibility method that they are often used as laboratory 

 experiments. Because they are temporary and -apparently 

 excitatory in character and so definitely related to size of piece 



