292 FIELDS AND GRADIENTS 



The relation between the effect of a dominant region and that of 

 general physiological stimulation is clearly brought out by this 

 experiment, and lends additional weight to the view that the 

 dominant region owes its inducing capacities in part at least to 

 its high physiological activity. 



(vi) The sixth rule is that one at least of the influences exerted by 

 the more apical regions on regions at lower levels is that of inhibition. 

 There appears to exist both inhibition of general activity (as 

 evidenced chiefly by susceptibility experiments), and also of differ- 

 entiation. The inhibition of differentiation is well shown by the 

 following experiment. If a polyp of Haliclystus be cut across trans- 

 versely, it will regenerate new tentacles over the whole cut surface. 

 If, however, an oblique cut be made, reaching down as far as the 

 transverse cut in the previous experiment, and continuing upwards 

 so as to leave intact a small portion of the original distal rim, no 

 regeneration will occur on the less apical part of the cut surface. 

 The presence of the apical region inhibits lower levels from re- 

 generating.^ 



This rule is really another way of putting certain consequences 

 of our third and fourth rules. Within the region of the body capable 

 of regenerating a new apical region at all (which may include the 

 whole organism, or may be restricted to its more apical portion: 

 see p. 297), any piece of tissue, if by reason of an operation it finds 

 itself at the front cut surface of a fragment, can develop into an 

 apical region. That it does not do so in the intact animal is due to 

 the presence of the apical region. The control exerted by the apical 

 region is thus twofold : it inhibits the appearance of other apical 

 regions within the limit of its field, and it influences the tissues to 

 develop into subordinate organs in relation to the morphogenetic 

 gradients which it sets up within its field. 



The inhibition set up is not merely morphogenetic; it is also 

 trophic. In portions of Hydroid colonies kept in suboptimal con- 

 ditions, the stolons that are formed frequently detach themselves 

 from the stock and move slowly across the substratum, their original 

 tip leading the way. This appears to be due to the tip being the 

 dominant region within the subsidiary gradient-field of the stolon : 

 it is able to grow by abstracting material from the proximal, sub- 

 1 Child, Sci. Rep. Tohoku hnp. Univ. 4th Ser. Biol, vin, 1933, p. 75. 



