KEGEMEKATION 



311 



The rate of formation of the hydranths falls off from the distal to the 

 proximal fragments, and this gives a measure of the gradient in intrinsic 

 regeneration rate, independent of dominance (Fig. 14.2). 



Li order to estimate dominance, a fragment of medium length is taken 

 and allowed to regenerate at the proximal end; and its rate of regeneration 

 is compared with that in a similar fragment in which the influence of the 

 original hydranth has been suppressed by a ligature just behind it. Again, 

 if a piece of coenosarc is isolated, the original hydranth being discarded, 

 the distal end will regenerate more rapidly than the proximal, and the 



i'- i! 



PROXIMAL NJ 4 2 



u 



FIGURE'14.2 



Figure a, rates of regeneration (in /x per hour) at various levels along a stem 

 of Tnhularia; this is the inherent rate, any interference by dominance being 

 avoided by ligaturing ; b, c, d, e, showing bipolar regeneration in the shortest 

 lengths, then complete dominance of the distal end, then partial dominance, 

 which weakens in the longest lengths;/, regeneration at the proximal end 

 is rapid when a ligature prevents the distal end exerting dominance, but 

 slower [g) when dominance is possible. (From Barth 1940.) 



distal regenerating hydranth will exert some dominance over the proxi- 

 mal one. The degree of dominance can be measured in the same sort of 

 way as before, by comparing the rate of proximal regeneration when 

 the distal end is left free or is ligatured. 



These experiments reveal the existence of two gradients, one of intrinsic 

 regeneration capacity and one of dominance. Since the hydroid stem is a 

 linear structure, we are only confronted with gradients in one dimension, 

 along it. We can take them as being the simplest possible instances of fields, 

 which in other cases will usually extend in two or three dimensions. The 

 two gradients obviously correspond to the two types of fields men- 

 tioned on p. 25 ; the gradient of intrinsic regeneration rate is a field' of 



