PHYSIOLOGY OF RECONSTITUTION OF PLANARIA LATA. 151 



above that of whole animals. Later a gradual increase coincides 

 with the progress of reconstitution. 



5. The increase in susceptibility after section in relation to body- 

 level and length of piece varies inversely as the susceptibility 

 gradient of whole animals and the head frequency in reconsti- 

 tution. 



6. The factors determining whether a piece shall or shall not 

 develop a head become to some extent effective within a few hours 

 after section. 



7. The experimental data all support the conclusion that in P. 

 lata, as in P. dorotocephala, two factors are concerned in deter- 

 mining head frequency : one the rate of metabolism in the cells at 

 the cut surface, being positive or determining; the other the rate 

 of metabolism in other regions of the piece, being negative or 

 inhibitory. Head frequency in any particular case is determined 

 by the relation between these two factors. Apparently the in- 

 hibitory factor is less effective in the shorter pieces and at more 

 posterior levels in P. lata than in P. dorotocephala. 



8. The physiological analysis of reconstitution agrees with ob- 

 servations on behavior in indicating that P. lata is a less highly 

 specialized form than P. dorotocephala. 



REFERENCES. 

 Behre, Elinor H. 



'18 An Experimental Study of Acclimation to Temperature in Planaria 

 dorotocephala. Biol. Bull., XXXV. 

 Buchanan, J. W. 



'22 The Control of Head Formation in Planaria by Means of Anesthetics. 

 Jour. Exp. Zool, XXXVI. 

 Child, C. M. 



'10 Physiological Isolation of Parts and Fission in Planaria. Arch. f. Ent- 

 wicklungsmech., XXX., II. Teil. 



'11a Experimental Control of Morphogenesis in the Regulation of Planaria. 

 Biol. Bull., XX. 



'11b Studies on the Dynamics of Morphogenesis and Inheritance in Ex- 

 perimental Reproduction. I. Jour. Exp. Zool., X. 



'lie Studies, etc., III. Jour. Exp. Zool., XI. 



'13a Studies, etc., V. Jour. Exp. Zool., XIV. 



'13b Studies, etc., VI. Arch. f. Entwickelungsmech., XXXVII. 



'14 Studies, etc., VII. Jour. Exp. Zool., XVI. 



'14b Asexual Breeding and the Prevention of Senescence in Planaria 

 velata. Biol. Bull., XXVI. 



'14c Starvation, Rejuvenescence and Acclimation in Planaria. 



