DYNAMICS — MORPHOGENESIS AND INHERITANCE 427 



changes which are non-specific; that is, quantitative in charac- 

 ter. The same is true of the differential inhibition and acclima- 

 tion forms in sea-urchins and polychetes. Moreover, even in 

 the vertebrates similar developmental modifications may be 

 similarly produced. In the frog, for example, Bellamy has pro- 

 duced experimentally a series of differential modifications of 

 development essentially similar to those in Planaria as regards 

 axial relations. The differential inhibitions of head-develop- 

 ment range from normal to almost acephalic, through a series 

 of forms closely comparable to those in Planaria in that they all 

 represent a greater degree of inhibition of median than of lateral 

 regions. The differential acclimations and recoveries, on the 

 other hand, represent changes in the opposite direction. Only 

 some of the data on the earlier stages have as yet been published 

 (Bellamy, '19). It is also evident that the cases of cyclopia 

 experimentally produced by Stockard ('07, '09, '10, '11) in 

 fishes are cases of differential inhibition similar to those in 

 Planaria and other forms. In these cyclopias the median region 

 of the head is more inhibited in early developmental stages 

 than lateral regions. The intermediate forms between the com- 

 plete cyclopias and the normal heads are closely comparable to 

 the various degrees of differential inhibition in the planarian 

 head. The occurrence of essentially similar developmental 

 modifications in such widely different organisms and under so 

 wide a range of experimental conditions indicates the existence 

 in all these forms of certain non-specific or quantitative physio- 

 logical factors concerned with the localization, growth, and 

 differentiation of parts. These factors are, as so often pointed 

 out, the physiological gradients. The median region of the head 

 is, on the one hand, more inhibited than lateral regions and, 

 on the other, acclimates or recovers more rapidly or more com- 

 pletely, because it represents the 'high' region of a physiological 

 gradient; i.e., a region differing in its physiological state from 

 other levels of the gradient in such manner that the rate of 

 the fundamental metabolic reactions is higher in it than at other 

 levels. Similar gradients of this sort may exist in protoplasms 

 which are specifically very different, and their existence determines 



