CONTROL OF MORPHOGENESIS IN PLANARIA. 315 



formation. But such animals must grow to a much greater length 

 than young norrrtal animals before the second zooid appears, 

 because in them the dominance of the head region is effective 

 over a greater distance than in young aninals, and a sufficient 

 degree of physiological isolation (Child, 'iob, 'na) of the pos- 

 terior region does not occur until a greater length is attained 



On the other hand, in pieces of Planaria which remain head- 

 less second, third and further zooids arise at once and if the pieces 

 can be induced by stimulation to move about sufficiently, fission 

 often occurs. The headless piece, unless very small, always 

 produces a very large amount of new tissue at its posterior end 

 (Fig. 7). This new tissue consists of new zooids; if we cut it 

 off at any of the levels b, c, d (Fig. 7) we find that it always pro- 

 duces a normal whole very rapidly. If we remove the anterior 

 end of the headless piece at the level a in Fig. 7 after the second 

 zooid has formed at its posterior end we find that it is now capable 

 of forming a normal head, but this is possible only when the 

 second zooid is present, for if we remove both the anterior end and 

 the second zooid, the remaining piece is incapable of forming a 

 head. This last experiment as well as others to be described 

 elsewhere shows that the second zooid influences regions anterior 

 as well as those posterior to its anterior end. 



In general any condition which retards or inhibits the de- 

 velopment of the head or the dynamic processes in the developed 

 head favors the development of a second zooid at the posterior 

 end and any conditions which accelerate the dynamic processes 

 in the anterior region retard the development of the second zooid. 

 As noted above, the length which the animal attains before it 

 gives rise to a second zooid may also be varied experimentally. 

 In these experiments we have a demonstration of the spatial 

 limit of effectiveness of physiological correlation and of the re- 

 lation between this limit and the rate or intensity of dynamic 

 processes in the region of origin. 



One other point may be added as indicating the dominance 

 of the head region. In animals which decrease in size in con- 

 sequence of starvation the head region decreases less rapidly 

 than other parts and so becomes relatively larger the further the 

 reduction proceeds. Manifestly the head region is able to live 



