REGENERATION 3I5 



sought are those of the echinoderm animal-vegetative gradients and the 

 amphibian organiser; in neither of them is a gradient in respiratory rate 

 of demonstrable importance (see pp. 88, 200). 



In the hydroids, the situation is somewhat more favourable to Child's 

 ideas, since there is fairly good evidence that a gradient in rate of respira- 

 tion actually exists, hi the early measurements of the oxygen uptake of 

 fragments taken at successive levels behind the hydranth, the experiments 

 lasted so long that an important amount of regeneration would have 

 occurred, and the gradient of rate found, with the hydranth end respirhig 

 fastest, may have been mainly an expression of the faster rate of regenera- 

 tion of this end. But Barth (see 1940) has made accurate measurements 

 over short periods, and finds that there is almost certainly a gradient in 

 this sense in the hydroid immediately after cutting. This, however, does 

 not by any means necessarily imply that the respiratory gradient is a 

 cause, and not a mere concomitant of the gradient in regeneration rate. 



Turnijig now to a type of 'substance' quite other than oxygen, it seems 

 that considerable importance should be attached to the fmding of Tardent 

 (1954) that there is a gradient in the concentration of neoblasts or regenera- 

 tion cells (known also as interstitial cells). This was found both in Hydra and 

 Tubular ia (Fig. 14.3). These cells increase in number near the cut surface, 

 but it is not yet clear to what extent this is due simply to multiplication 

 or how far migration from other parts of the animal is involved. If the 

 latter process is extensive, it may be that they are the most important 

 'substance' (if one can call them that) for which the two ends of an isolated 

 length of hydroid are competing. It would be very interesting to know 

 whether their rate of respiration is liigher than that of the other tissue cells, 

 in which case the respiratory gradient described by Child and Barth 

 might be a reflection of the gradient in neoblasts ; but there is as yet no 

 defmite evidence on this point. 



It seems probable that when a hydranth is removed from a hydroid, 

 the gradient from the new distal end is established in two stages. First 

 there must be an accumulation of neoblasts at the wound surface, pre- 

 sumably as a response to the increased availability of oxygen. But after 

 the initiation of the hypostome or mouth region of the new hydranth, 

 this may be itself responsible for building up the gradient. Several authors 

 have shown that this region, when transplanted into the side of another 

 polyp, is particularly efficient at causing the production of a new hydranth 

 in its neighbourhood, much of which is formed out of tissues of the host 

 (Beadle and Booth 1938, Yao 1945). This is a typical example of an 

 'assimilative induction', in which the transplant acts as the carrier of a 

 powerful individuation field. In Hydra, whose powers of regeneration 



