PHYSIOLOGY OF RECONSTITUTION OF PLANARIA LATA. 147 



or standard external conditions can be altered to those character- 

 istic of other levels by changes in external conditions which are 

 primarily non-specific and quantitative in physiological effect. 

 This has been shown for P. dorotocephala in many ways and is 

 also true for P. lata, though only a part of the evidence appears 

 in the present paper. It has also been shown for P. dorotocephala 

 that the susceptibility gradient is an indicator of a corresponding 

 gradient in rate of respiration (Robbins and Child, '20; Hyman, 

 '22), and the data on the changes in susceptibility and rate of 

 respiration in pieces following section leave no doubt that a similar 

 relation between susceptibility and respiratory rate exists in P. 

 lata. If this is true, the inference is justified that the reconstitu- 

 tional differences at different levels are in some way associated 

 with the differences in rate of metabolic reactions, as indicated by 

 rate of respiration. 



The relations of reconstitutional processes to length of piece also 

 appear to be non-specific and quantitative in character, for they, 

 too, can be altered by the quantitative action of external factors. 

 And, finally, the relation of head frequency to physiological age 

 is apparently non-specific and quantitative and can be altered ex- 

 perimentally by changes in condition which affect primarily rate, 

 rather than kind of metabolic reactions. 



Nowhere do we find any evidence for the existence of specific 

 formative substances. Given the specific protoplasm of a plana- 

 rian species, the differences in the reconstitutional processes and 

 results are apparently dependent primarily upon quantitative dy- 

 namic differences rather than upon specific qualitative factors. 



Physiological Analysis of Head Frequency. — It has been shown 

 that the head forms differing from the normal which appear in 

 the reconstitution of P. dorotocephala represent various degrees of 

 differential inhibition of head development, and that they can all 

 be produced by chemical and physical agents, as well as by physi- 

 ological factors (Child, '16, '20a, '21; Behre, '18; Buchanan, '22). 

 All the experimental evidence supports the conclusion that two 

 antagonistic factors are concerned in the reconstitution of a head — 

 the one positive or determining, the other negative or inhibitory — 

 and that head frequency in any particular case depends on the 

 relation between these two factors (Child, '14a, 'i^d, '16, '20a, '21 ; 



