294 FIELDS AND GRADIENTS 



ordinate region of the stolon. When, however, the hydranths are 

 healthy and vigorous, they dominate the stolon and maintain them- 

 selves at the expense of any attached stolons, which are gradually 

 resorbed. A similar state of affairs is seen in the Ascidian Pero- 

 phora (see fig. 139 and p. 425). 



The most striking case of trophic dominance is found in the 

 flatworm Stenostomiim.^ Here it can be conclusively shown that 

 the dominance depends on the degree of development of the apical 

 region. Stenostomiim possesses asexual reproduction and forms 

 chains of attached zooids (up to eleven in number), separated by 

 fission-planes. These fission-planes are formed in a regular order, 

 and the relative age of the zooids can thus be determined, as well 

 as by inspection of the degree of development of the head. If a 

 fragment of the chain be isolated by cutting, the zooid possessing 

 the oldest head left in the fragment normally resorbs all younger 

 zooids and any headless portions of zooids which are anterior to it. 

 This is shown in fig. 140. If the posterior cut had been made a 

 little farther back, a still older head would have been included in 

 the fragment, and would have resorbed all regions anterior to itself. 

 If the fragment is made so short as not to contain a head, regenera- 

 tion occurs at the anterior cut surface, and there is no resorption. 

 A similar relation occurs between the earlier- and later-formed 

 holdfasts of Corymorpha? 



These facts show that it is not merely the presence of a cut 

 surface which leads to regeneration: the cut surface must be in a 

 certain relation to the gradient-system of the fragment as a unit.^ 



(vii) This leads on to a seventh rule, which is a corollary of the 

 fourth. This concerns the origin of new apical regions as a result of 

 what Child has called physiological isolation. If a portion of tissue 

 comes to lie outside the field dominated by an existing apical region, 

 a new apical region will arise in this portion, even though it is still 

 in physical continuity with the rest of the organism. The common- 

 est way in which this state of things is brought about is by con- 

 tinuous growth. For instance, in Stenostomiim the first appearance 

 of a new head only occurs at a certain distance from the old, and 



^ van Cleave, 1929; Child, 1929 a. 2 Child, 1928 a. 



^ It is possible that the phenomena of the graded distribution of growth- 

 potency in the animal body (see p. 366) is correlated with this trophic effect 

 of one part of a morphogenetic system upon another. 



