278 FIELDS AND GRADIENTS 



was this aspect of biological field-systems which first attracted 

 attention, and led Child to formulate his theory of "axial gradi- 

 ents".^ It is preferable to combine the two ideas in a single phrase 

 by speaking of field-gradient systems (figs. 130, 131). 



In other cases (Annelids), a double gradient is found, with a 

 high point at both ends. As we shall see later (p. 309), the two 

 gradients are probably of qualitatively different nature. The gradi- 

 ent-system of the amphibian early gastrula also appears to be of 

 this type (pp. 310, 318). In other cases, as in the localised areas 

 of the embryo which after the phase of chemo-differentiation are 

 predetermined to give rise to particular organs, we appear to have 

 gradient-fields with a central or subcentral high-point, the gradient 

 apparently being concerned with the concentration of a particular 

 chemical substance. Cases where this form of gradient have been 

 definitely demonstrated are the limb-disc of Urodele embryos 

 (p. 222), the neural plate (p. 243), the rudiments of the auditory 

 vesicle (p. 232), the gills (p. 233), the heart (p. 233), and probably 

 that of the lens (p. 238). 



Although the precise mechanisms underlying these systems are 

 still to seek, various important aspects of morphogenesis cannot 

 be understood or rationally interpreted without postulating their 

 existence. Further, from the large body of empirical data available 

 it is possible to deduce certain general rules which are perfectly 

 valid on their own biological level, in spite of having as yet received 

 no adequate interpretation on the physiological or physico-chemical 

 level. In what follows, an attempt will be made to give some account 

 of the general properties and behaviour of these biological field- 

 gradient systems, and to show how the field-gradient conception 

 illuminates certain processes of morphogenesis. 



§3 



It will be best to base our treatment upon the phenomena of re- 

 generation, since here the field-gradient systems are for the most 

 part less specialised and less restricted than in early embryonic 

 development. From such a study a number of rules emerge. 



(i) Our first general rule is that where complete regeneration is 

 possible from a fragment of the body, the type of regenerate pro- 



1 Child, 1915A. 



