28o FIELDS AND GRADIENTS 



ment is originally polarised by virtue of possessing part of the 

 general gradient system of the organiser from which it has been 

 isolated, it is possible to abolish this original field and to substitute 

 another for it. This can be done, for example, with pieces of the 

 stem of the hydroid Corymorpha, by placing them in dilute solutions 

 of various narcotics (see p. 63), In these conditions the pieces 

 round themselves off, and dedifferentiate. If sea-water is now sub- 

 stituted, they redifferentiate, but with a new polarity, at right angles 

 to the substratum.^ This is probably to be explained by the greater 

 oxygen-concentration away from the substratum. Normally, the 

 differential established by this means, at right angles to the original 

 polarity and to the long axis of the piece of the stem, is less powerful 

 than the already existing differential due to the physiological 

 gradient between the two ends of the piece. But when this latter 

 has been abolished by narcotics, the other comes into play, and 

 establishes a new physiological gradient. 



It is to be noted that although this differential is smaller than that 

 constituting the original polarity of the piece, the polyp eventually 

 formed is normal. Once the gradient has been established, it acts 

 as a realisation-factor for the production of an apical region. If the 

 conditions permit of this developing normally, then, as will be seen 

 later, the rest of the reconstituted organism will be normal, provided 

 that the piece is not too small (see p. 285). This is clearly similar to 

 the processes leading to the establishment of the plane of bilateral 

 symmetry and the grey crescent in Amphibia, described in 

 Chap. IV. Numerous differentials, of very varying intensity, can 

 lead to the establishment of bilateral symmetry : if conditions are 

 normal, the bilaterality of the embryo is always normal, whatever 

 the intensity of the trigger action which has released the processes 

 leading to its formation. 



Reversal of polarity has also been obtained by appropriate 

 methods of grafting in Hydra and other forms. The reversal may 

 occur in the small engrafted fragment or in the major "host" 

 portion.^ 



In other cases, the regenerating portion of tissue is not isolated 

 in such a way as to take over a part of the original field-system of 

 the organism from which it is derived, and therefore possesses no 



^ Child, 1925 B, 1927. ^ Goetsch, 1929. 



