Chapter IX 



FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



§ I . Polarity in ontogeny 



As already mentioned, the conclusions reached in the preceding 

 chapter are derived from experiments on regeneration and grafting 

 in adult animals. They are also, however, relevant in the normal 

 ontogeny of higher forms, though the conditions here are often more 

 complex and more specialised. In the present chapter it is pro- 

 posed to illustrate the various principles, so far as possible, from 

 early development. 



(i) Polarity and the main axis of the resultant organism 



The first rule mentioned in Chap, viii was that the inherent 

 polarity of a fragment normally determined the polarity of the 

 organism which arose from it. This obviously holds good in normal 

 ontogeny. The egg is a fragment of the mother, in which a well- 

 marked polarity has been set up before it is detached. In the great 

 majority of cases, the main animal -vegetative axis of the Qgg gives 

 rise to the definitive antero-posterior axis of the resulting organism, 

 with the head or apical region arising at the animal end. In various 

 Echinoderms the main axis of the tgg persists as that of the larva, 

 but later a new axis in a different direction is established in the 

 rudiment of the adult. 



(ii) Polarity determined by external agencies 



We next come to the point that the polarity of an organised 

 portion of living matter has in the long run been determined by 

 agencies external to it ; and that in certain cases the existing polarity 

 can be overridden and a new polarity imposed by external con- 

 ditions. Examples have already been given of how the polarity of 

 the developing oocyte or egg may be determined by factors external 

 to itself, either by conditions within the ovary, or external agencies 

 acting after fertilisation (Fucus). Cases have also been adduced in 

 which the axis of bilateral symmetry is determined from without 



