PHYSIOLOGY OF RECONSTITUTION OF PLANARIA LATA. 149 



In other words, in so far as the x cells become independent of y, 

 they dedifferentiate and begin the development of a new individ- 

 ual, the head arising first as in embryonic development. The vari- 

 ous differentially inhibited head forms result from different de- 

 grees of inhibition of x by y or by some external factor. 



All the experimental evidence at hand indicates that this inter- 

 pretation holds for P. lata as well as for P. dorotocephala, the 

 chief difference being that in P. lata the inhibiting action of y 

 shows less increase with decrease in length of piece and at more 

 posterior levels of the first zooid, and that head frequency is there- 

 fore normally higher in the shorter pieces and at more posterior 

 levels of the first zooid and is less increased by inhibition of 3; in 

 P. lata than in P. dorotocephala. This difference between the 

 species is what we should expect if the inhibiting action of y on 

 head formation is nervous in character, as the facts lead us to 

 believe. The differences in excito-motor behavior and in develop- 

 ment of sense organs certainly indicate a lower degree of nervous 

 organization in P. lata than in P. dorotocephala. Moreover, the 

 differences in degree of excitation of y following section and in 

 head frequency in relation to level of body and length of piece are 

 less in P. lata than in P. dorotocephala, and this again suggests a 

 lesser degree of specialization of different body levels in relation 

 to the axial gradient. 



The fact that in P. lata head frequency is almost as high in 

 young as in old animals, while in P. dorotocephala it is much lower 

 in young than in old, also indicates that the region y is less effective 

 in inhibiting head formation at x in P. lata. Rate y is higher in 

 relation to rate x in young than in old animals because the tissues 

 of young animals have in general a higher rate of metabolism than 

 those of old, but this difference has much less effect on head fre- 

 quency in P. lata than in P. dorotocephala. The more rapid 

 growth reaction in P. lata also indicates that y is less effective in 

 this species in inhibiting dedifferentiation and growth of x. The 

 physiological analysis of reconstitution leads, in fact, to the same 

 conclusion as observations on the behavior of the two species, viz., 

 that P. lata is a more primitive, a less highly specialized form than 

 P. dorotocephala. 



Tailless Forms and Biaxial Heads. — Tailless forms develop 



