STUDIES ON THE DYNAMICS OF MORPHOGENESIS 313 



And finally, I wish to empnasize the point that polarity may 

 conceivably consist eitier in a single gradient in one direction 

 or in two gradients in opposite directions. At present we know 

 nothing concerning the nature of the differences between the 

 metabolic processes concerned in the formation of head and tail 

 in Planaria, but certainly the two parts are, to a considerable 

 extent the result of differences in arrangement and distribution 

 of similar components. We find one of these capacities decreas- 

 ing in one direction, the other in the opposite direction: mor- 

 phologically the results at the two poles are qualitatively dif- 

 ferent organs, but what the differences in the dynamic processes 

 which form these organs may be does not appear from the experi- 

 ments described. 



According to the above view then, a certain minimal fraction 

 of the axial gradient or gradients is necessary in every case for 

 the formation of a whole. But it is necessary to call attention 

 not merely to the existence of the axial gradient, but to the cor- 

 relative factor in polarity. Morgan's hypothesis of the gradation 

 of substances possesses certain features in common with my idea 

 of a metabolic gradient, out the assumed gradation of substances 

 is more stable and requires the assumption of migration; the most 

 unsatisfactory feature of this hypothesis, however, is its failure 

 to recognize the correlative factor in polarity. The gradation of 

 substances alone cannot account for the fact that the same cells 

 give rise under certain conditions to a head, under certain others 

 to a tail, but when we consider that a gradation of any kind 

 along the axis may give rise to a variety of correlative conditions, 

 the phenomena become less puzzling, and we see that the directive 

 or apparently directive feature of organic polarity is in reality 

 a matter of physiologic correlation. In many cases and in various 

 decrees we see the processes at the different levels of the body, 

 or at the two ends of a piece exerting an effect upon each other, 

 or one upon the other. For example, in pieces from the extreme 

 anterior region just behind the head of the first zooid, the for- 

 mation of a head occurs, even though no pharyngeal or post- 

 pharyngeal region is formed, i.e., the constitution of these pieces 

 is such that they can produce a head without correlative influence 



THE JOURNAL OP EXPERIMENT A', ZOOLOGY, VOL. 10, NO. 3 



