PHYSIOLOGY OF RECONSTITUTION OF PLANARIA LATA. 137 



Another fact pointing in the same direction is that the original 

 polarity can be more readily obliterated and biaxial heads produced 

 by chemical agents than in P. dorotocephala. In certain concentra- 

 tions of acids, for example, the frequency of biaxial heads is much 

 increased. 



The Physiological Condition of Pieces Following Section. 



Section of the body results in exposure of a cut surface which 

 gradually contracts and within a few hours cell division and 

 growth begin, giving rise to new embryonic tissue. It has been 

 shown for P. dorotocephala that increase in rate of respiration and 

 in susceptibility is slight or inappreciable in fourths or longer 

 pieces, while in sixths and shorter pieces it is marked and increases 

 as length of piece decreases, and also increases from anterior to 

 posterior levels of the first zooid and decreases again in the poste- 

 rior zooids (Child, '14a; Robbins and Child, '20; Buchanan, '22; 

 Hyman, '22). Since these changes show definite relations to the 

 polar gradient and are factors in determining head frequency 

 (Child, '14c?, '16; Buchanan, '22), it is of interest to determine 

 whether similar changes occur and influence head frequency in 

 P. lata. 



Changes in Susceptibility Following Section. — The data are most 

 readily presented as graphs, plotted by the method used in earlier 

 work (Child, '15a, p. 81). This method is briefly as follows: 

 Five stages in the progress of disintegration in KNC or some other 

 agent from intact animals or pieces to completely disintegrated are 

 more or less arbitrarily distinguished and these are given, respec- 

 tively, the numerical values, 40, 30, 20, 10, o. In determinations 

 of susceptibility a certain number — e.g., ten — of animals or pieces 

 is placed in the solution used and the number or individuals or 

 pieces in each stage is recorded at hourly or half-hourly intervals. 

 The ordinate of the susceptibility curve for any time is the sum of 

 the products obtained by multiplying the number for each stage by 

 the numerical value of that stage, divided by the number of ani- 

 mals or pieces in the lot. These ordinates are plotted against 

 times in hours as abscissae. 



In the experiments on pieces the susceptibility of the fourths, 

 sixths, and eighths was determined separately for each level of the 



