DOMINANCE AND PHYSIOLOGICAL ISOLATION 319 



development and inhibition. In the case of reversal of dominance and 

 polarity (Fig. iii) the reduction gradient was found to be completely re- 

 versed in 86 per cent of pieces from which the distal hydranth primordium 

 was removed 48 hours after isolation of the pieces, the dye-reduction test 

 being made several hours later. In general the experiments along this line 

 indicate that a little more time is required for attainment of complete 

 dominance and reversal of the gradient by a hydranth developing at the 

 proximal end than for attainment of complete dominance by a hydranth 

 at the distal end. 



A very similar increase in unipolar frequency appears in pieces of the 

 stalk of the sessile scyphozoan Haliclystus with delay of proximal or 

 distal section for different periods. With simultaneous distal and proximal 

 section the stalk pieces show a high bipolar frequency (Fig. 113, J-M, 

 p. 334), but this can be reduced to zero with a certain period of delay of 

 proximal section and reversal of polarity in the whole piece can be brought 

 about with increasing frequency in the same way as in Corymorpha pieces 

 (Fig. III). Moreover, gradients in the indophenol blue reaction a few 

 hours after section show almost complete correspondence to the bipolarity 

 or unipolarity developing later (Watanabe, 1937). 



The branching hydroids show spatial and chronological orders very 

 similar to those of multiaxiate plants. Many tubularian (gymnoblast) 

 hydroids are monopodial, that is, the primary axis persists and the first 

 hydranth retains more or less dominance, later buds giving rise to lateral 

 branches in radial or spiral order about the main axis, or opposite or 

 alternate in a single plane. At a sufficient distance from the dominant 

 region or in its absence lateral branches may transform into main axes. 

 In many species of campanularian (calyptoblast) hydroids the axial pat- 

 tern is sympodial, that is, each new bud on each axis becomes temporarily 

 dominant, is later subordinated to the next bud, and becomes a lateral 

 branch of similar sympodial character. In some other hydroid species the 

 axes are more or less equivalent, and the group resulting from budding 

 consists of similar zooids (e.g., Clava). The spatial relations between dom- 

 inant hydranths and new hydranth buds indicate a more or less definite 

 range of effectiveness of dominance but varying with physiological and 

 external conditions. In various hydromedusae other medusae develop 

 from buds on the manubrium. Dominance and physiological isolation are 

 evidently concerned in determining spatial and chronological order identi- 

 cal with that of phyllotaxis in certain plants. According to Wood- Jones 

 (19 1 2), the apical zooid is dominant and radial in form in certain branch- 



