Chapter X 



GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



Chap. VIII was concerned with phenomena which could only be 

 explained by postulating the existence in adult Hydroids, Plan- 

 arians and Annelids of gradient-fields concerned with morpho- 

 genesis and reproduction. In higher animals, such as Arthropods 

 and Vertebrates, in which asexual reproduction does not occur and 

 in which total regeneration is no longer possible, the existence of 

 gradient-fields in adult life is not easy to detect. In such forms, the 

 presence of total axial gradient-fields is especially noticeable during 

 the earliest stage of development when they constitute the only or 

 at least the major organisation of the developing embryo. Similarly 

 the presence of partial (regional) fields is especially noticeable during 

 the immediately succeeding phase, when the organism consists 

 essentially of a patchwork of chemo-diflFerentiated regions, each 

 with its own field but as yet not differentiated into organs. 



It might be reasonably supposed that these gradient-systems 

 were only operative during the stage when they are most noticeable, 

 and that the organisation of one stage does not persist, but is wholly 

 supplanted by that of the next stage. This, however, does not in 

 point of fact appear to be the case, and there is considerable 

 evidence for the persistence of the gradient-fields of the embryo 

 throughout life, even in the highest animals. 



There is the natural presumption that the gradient-field in 

 Hydroids and worms is directly derived from the primary gradient- 

 field of the egg which has persisted into the adult phase. But even 

 if this be so, in less plastic and more complex types the gradient- 

 fields might be imagined to fade out at a certain stage of develop- 

 ment. In what follows, various lines of evidence to the contrary 

 will be presented. 



Examples of the persistence of the main axial gradient of the 

 organism, as evidenced by its influence upon the polarity of the 

 later developed regional fields, are to be found in the differentiation 



