3i6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ment begin about the same time, neither inhibits the other. The great 

 mass of experimental data on reconstitution in Tubularia offers no diffi- 

 culty to interpretation in terms of gradients and a dominance associated 

 with them, with physiological and physical isolation possible, both experi- 

 mentally and under natural conditions. 



Corymorpha, also a tubularian hydroid, never gives rise to new hy- 

 dranths by budding. The stolons are holdfasts, threadhke outgrowths, 

 developing in large numbers from longitudinal series of buds in the basal 

 region. Those nearest the basal end develop first, are dominant, and in- 

 hibit others until they have become so long that their dominance is not 

 effective. On removal of the buds nearest the basal end by section, those 

 adjoining the section grow out very rapidly and inhibit others. Each sto- 

 lon is a gradient with high end at the tip (Child, 19286; Child and Wata- 



nabe, 1935^) • 



In the reconstitution of stem pieces Corymorpha resembles Tubularia, 

 except that the proximal fifth, more or less, of the stem, the only part 

 secreting perisarc in mature animals, reconstitutes a basal region from its 

 proximal end more frequently than Tubularia; this is according to expecta- 

 tion, since dominance in Corymorpha is evidently more effective than in 

 Tubularia, as the absence of buds and branches shows. Pieces from the 

 naked four-fifths of the stem reconstitute essentially like Tubularia pieces 

 with hydranths at both ends, the proximal developing more slowly than 

 the distal, except when pieces are so short that gradient difference is prac- 

 tically absent (Child, 19266). 



Watanabe (1935c) has made an experimental analysis of the dominance 

 of the original hydranth and the development of dominance by a develop- 

 ing hydranth. With increasing delay in removal of the original hydranth 

 its effectiveness in inhibiting hydranth development at the proximal end 

 of the piece increases, as is shown by the increase in frequency of unipolar 

 forms with the original hydranth at the distal end and proximal hydranth 

 reconstitution completely inhibited. The experimental procedure and the 

 result are shown in Figure 109. Frequency of unipolar forms increases 

 from practically zero with section at both ends at the same time to 86 

 per cent with 72 hours delay of proximal section; that is, if the original 

 hydranth remains 72 hours after the proximal section, it so completely 

 inhibits hydranth development at the proximal end that, even after its 

 removal, only 14 per cent of the pieces show hydranth development there, 

 and most of the other pieces develop a basal end. If the original hydranth 



