44 J. AVILLIAM BUCHANAN 



trols produce more normal heads than the B pieces. There are 

 elements, both subjective and objective, which play a part in 

 the results over which I have no control, although their nature 

 may be recognized. I can cut in immediate succession two series 

 of controls from the same selection of worms and be assured of a 

 very slight difference in their head frequencies. But I cannot 

 cut two series on successive days and expect such a close simi- 

 larity in results. Diversion of attention in the interim alters 

 one's judgment of the planes of cutting on the second day. That 

 the physiological condition of the animals varies from day to 

 day has already been pointed out. Therefore, one may not 

 compare in detail the head frequency of similar controls cut on 

 different days nor compare a series subjected to an anesthetic 

 with a control cut at another time. But we are justified in com- 

 paring the head frequency of a series treated with an anesthetic 

 with its own control and in comparing in a general way the effects 

 of the same anesthetic in other experiments. 



As regards the number of normal heads produced in the C 

 pieces in the controls, it may be pointed out that the plane of 

 fission between the anterior and posterior zooids is not a fixed 

 anatomical structure, but a physiological condition which shifts 

 anteriorly or posteriorly with the physiological condition of the 

 animal. If one cuts a C piece so as to include a portion of the 

 posterior zooid, the conditions established in the piece by the 

 act of section will be very like the conditions in an A piece, for 

 the anterior end of the posterior zooid is the apical region of a 

 second metabolic gradient. Consequently, such pieces will 

 show a high head frequency. Since one depends solely on judg- 

 ment and experience in locating the plane of fission, it is not 

 surprising that some of the pieces are cut to include a portion 

 of the anterior region of the posterior zooid and hence develop 

 normal heads. 



In conclusion it may be stated that there are three lines of 

 evidence presented here that demonstrate the non-specific nature 

 of the factors concerned in head determination in pieces of Plan- 

 aria. First, the forms of anterior ends developed in series treated 

 with the various solutions of anesthetics are identical with the 



