284 



FIELDS AND GRADIENTS 



extent, a complete animal can only be produced by a remodelling 

 of the organisation of the original fragment (see below). 



In Hydroids such as Tubiilaria and Corymorpha, the new 

 hydranth is produced entirely by reconstitution within the old 



Fig. 134 

 Reconstitution from pieces of stem in Corymorpha. A, Normal unipolar form 

 showing hydranth and base with holdfasts. B-G, Reconstitution of very short 

 pieces to form partial structures, either unipolar (B, C) or bipolar (D, E, F, G). 

 The extreme apical region is always present. In E, the original apical end has 

 formed more than the basal end. H, J, Total and partial twinning of hydranth. 

 K, L, Formation of numerous apical and basal regions in relation to a single 

 hydranth (K) or independently (L). (Redrawn after Child, Biol. Gen. 11, 1926.) 



tissue, not by regeneration from the cut surface, so that here the 

 delimitation of the apical region is less clear-cut. The independence 

 of extreme apical regions is, however, very well shown in these 

 forms (fig. 134). 



Extremely small fragments of their stems do not become recon- 



