CONTROL OF HEAD FORMATION IN PLANARIA 45 



same types developed in the controls, and only those types 

 which are developed in the controls are found in the series treated 

 with the anesthetics; no forms are found which are characteris- 

 tic of the effects of anesthetics. Second, all the anesthetics used 

 when employed under comparable conditions of concentration 

 and period of exposure produce similar changes in head frequency. 

 Third, with a single anesthetic increases or decreases in head 

 frequency can be produced almost at will by employing differ- 

 ent concentrations and periods of exposure. 



SUMMARY 



Evidence is presented w^hich shows very plainly that the fac- 

 tors controlling head formation in pieces of Planaria are non- 

 specific and strongly supports Child's conclusions regarding the 

 nature of these factors, viz., that head formation is determined 

 chiefly by the relative activities of tw^o antagonistic factors: 

 1) the tendency of the cells near the anterior cut surface of the 

 piece to dedifferentiate and develop into the head of a new indi- 

 vidual; 2) the tendency of the whole of the piece, exclusive of 

 the cells directly concerned in the development of the new head, 

 to maintain the differentiation of the old individual. This region 

 exerts a certain degree of control over the cells near the anterior 

 cut surface and consequently tends to prevent the development 

 of a new head. 



The new evidence presented includes mass experiments in 

 which it is shown, 1) that head frequency in pieces of Planaria 

 can be controlled in either direction by subjecting the pieces 

 for short periods after section to appropriate concentrations of 

 the following anesthetics: chloretone, chloroform, chloral hy- 

 drate, ether, and ethyl alcohol; increases or decreases can be 

 produced with a single anesthetic by employing different con- 

 centrations and periods of exposure; 2) that in such concentra- 

 tions of anesthetics the increase of oxygen consumption following 

 section does not occur. An exception occurs in the case of ethyl 

 alcohol, and evidence is presented which indicates that the in- 

 crease in oxygen utilization is alcohol solutions is due to oxida- 

 tion of the alcohol and does not result from the stimulation of 

 section. 



