314 PRINCIPLES OF EMBRYOLOGY 



measuring the relevant efficiencies or discovering what substances are 

 being competed for, etc. 



As to the 'substances' involved (we have to use the word in a broad 

 sense), we have two main pieces of information, which have not yet been 

 fully brought into relation with each other. The first to be discovered was 

 that oxygen is highly important. It probably operates in two ways, firstly 

 as a component of the stimulus which sets the regeneration going, and 

 secondly as one of the reactants while the process is proceeding (Barth 

 1940). Its importance as a stimulus can be demonstrated if a hydranth is 

 cut off and the perisarc pulled forward and ligatured in front of the cut 

 surface, so as to shut it off from the surrounding sea water; no regenera- 

 tion takes place. The same result can be obtained by covering the cut 

 surface with a piece of glass tube. Moreover, if the perisarc is cut open so 

 as to expose a region of the coenosarc, regeneration may occur, particu- 

 larly in oxygen-rich water, even if no wound has been infhcted; and the 

 injection of a bubble of oxygen between the perisarc and coenosarc may 

 have the same effect. The continuing importance of oxygen during the 

 whole process of regeneration is shown by the fact that the rate of for- 

 mation of new hydranth is highly dependent on the amount of oxygen 

 in solution in the water. 



Child, and many workers following his lead, have been emphasising 

 for some time the importance of gradients of respiratory rate which they 

 claimed to demonstrate in hydroids and many other regenerating animals, 

 and in eggs in which field phenomena play a leading role (see Child 1941). 

 It was claimed that field processes always depend on gradients in metaboHc 

 activity, and that the metabolic activity which is most crucial is that of 

 respiration. The expression 'metabolic activity' is, of course, so general 

 that, in those terms, the hypothesis is little more than a truism; a field 

 must obviously have graded differences between its parts, and the port- 

 manteau phrase 'metabohc activity' could cover these whatever their 

 nature. The part of the theory which it is important to discuss is, therefore, 

 the notion that it is respiration which is basic. Extended discussions will 

 be found in Child's own book (pro) and in Needham 193 1 (pp. 582 ff.) 

 and Brachet 1945 (contra). The general sense of the situation would seem 

 to be that, whereas various indirect methods (e.g. susceptibiHty to poisons, 

 reaction with vital dyes, etc.) often give evidence for the existence of 

 some sort of gradient, it is by no means clear in most cases that the 

 gradients primarily affect respiration; and moreover it remains obscure 

 whether the gradient of respiration, if there is one, is the causative basis 

 of the observed field or rather merely another expression of it. The two 

 most crucial embryological cases in which critical evidence might be 



