FIELDS AND GRADIENTS 285 



stituted into miniature whole polyps; they produce only apical 

 portions of polyps, but these are of normal size. Such small frag- 

 ments frequently form an apical region at both ends, for reasons 

 to be discussed later : in such cases, two sets of apical structures are 

 produced, without any basal portion. Similar phenomena occur 

 in the regeneration of very short pieces of Planaria. In Corymorpha, 

 reconstitution-masses produced from the aggregation of dissociated 

 cells may produce only apical portions of hydranths^ (see p. 65). 



In all cases, what is determined in the first instance is, in fact, 

 the formation of an extreme apical region of a certain standard size, 

 this varying with the size of the piece and also with external con- 

 ditions. Once this extreme apical region is determined, the region 

 next more basal is determined, and so on, until all the available 

 material is used up. This process may be initiated either at one or 

 at both ends of the piece. 



Abnormal external conditions influence the size of the apical 

 region produced. In Planarians, for instance, cold and narcotics 

 reduce its size, while heat up to a certain degree increases it. Beyond 

 a certain degree of cold or concentration of narcotics, no apical 

 region will be formed at all (fig. 135; see also p. 301). 



In the most general terms, it appears that the relative size and 

 the degree of differentiation of the apical region depend in some 

 way upon the physiological activity of the regenerated tissue. If this 

 is depressed by cold or narcotics, the development of the apical 

 region is subnormal. 



(iv) Our fourth rule is that, once an apical, region is produced, it 

 then exerts an influence on other organs and regions within the old 

 tissues of the fragment: this influence is, however, limited in 

 extent. Accordingly, the apical region has been called by Child 

 the "dominant" region. In terms of the field-concept, the apical 

 region establishes a field of a certain extent, which it dominates so 

 as to control the morphogenetic processes of the other regions of 

 the field. The control is exerted in such a way that the various 

 morphogenetic processes occur in harmonious relation with each 

 other : this is because it exerts its control through the establishment 

 of a field. 



If the range of dominance is artificially reduced, as by removal of 



1 Child, 1928 B. 



