FIELDS AND GRADIENTS IN NORMAL ONTOGENY 319 



Susceptibility experiments also demonstrate the existence of a 

 dorso -ventral gradient in general activity, from the region of the 

 grey crescent ventralwards over the egg. In respect of its position 

 at the high end of a gradient, the organiser of the amphibian egg 

 shows a further resemblance to the dominant regions of a Coelen- 

 terate, Planarian, or Annelid. (See p. 68 and fig. 28.) 



As already pointed out in Chap, vi, the action of an apical region 

 such as a Planarian head is extremely similar to that of an organiser 

 in ontogeny. Not only does it exert a morphogenetic effect during 

 regeneration, but also when grafted into an intact worm. But the 

 morphogenetic action of the amphibian organiser is normally 

 exerted in a way somewhat different from that of a regenerating 

 head, for its definitive influence is exerted on those parts which 

 it actually comes to underlie as a result of gastrulation, and appears 

 to be a chemical effect, demanding contact for its realisation. In 

 this respect the dorso-ventral gradient of the amphibian egg reveals 

 itself as a gradient-system of secondary effect, thus differing 

 importantly from the apico-basal gradient system (see Chap, viii, 

 p. 310). However, the labile determination effected before the onset 

 of gastrulation can only be the result of action at a distance, as with 

 the effects of a regenerated Planarian head. 



In Amphibia the end-result, in the shape of the main morpho- 

 logical organisation of the embryo, is dependent on the interaction 

 of the organising capacity of the dorsal lip with the primary apico- 

 basal gradient-system. The most important action of the dorso- 

 ventral gradient, from the point of view of developmental physiology, 

 is the production of a specific inducing substance localised in the 

 dorsal lip region, which acts as a trigger or releasing stimulus for 

 the differentiation of its own and other tissues. On the other hand, 

 the most important action of the apico-basal gradient is the pro- 

 duction of an individuation-field, which sees to it that the develop- 

 ment released by the non-specific action of the organiser is in the 

 first place different in different parts of the field, and in the second 

 place is correlated into an organised whole. 



Either system also appears to have minor effects of the opposite 

 type to its main effect. The organiser, as just mentioned, appears to 

 exert an action at a distance prior to gastrulation, and this may be 

 comparable with that of the individuation-field set up by a Planarian 



