GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 367 



heterogony) ; if equal to unity, it will stay constant (isogony). The 

 external conditions, such as temperature^ and, notably, nutritive 

 level, will modify the partition of material between various parts 

 of the body ; but in every case a total equilibrium is concerned in 

 the process. 



Such an equilibrium does not constitute a field-system. How- 

 ever, it is further found that the growth-potencies of various 

 regions are frequently graded in a quantitative way, so that the 

 body appears to be permeated by a field-system of interconnected 

 growth -gradien ts . 



The most clear-cut examples of such growth-gradients are de- 

 rived from the study of the large chela of Crustacea. When, as in 

 the males of many species and both sexes of others, these show 

 marked positive heterogony, they always exhibit a growth- 

 gradient with subterminal high point. When they are not dispro- 

 portionate in their growth (approximately isogonic), all their joints 

 are growing at approximately the same rate — i.e. their growth- 

 gradient is almost flat. The same is true of the abdomen of female 

 Brachyura, which shows marked positive heterogony, and has a 

 well-defined growth-gradient with subterminal or terminal high 

 point, whereas the male abdomen is almost isogonic and has a very 

 slight growth-gradient, with central rise^ (fig. 177). 



In limbs which show negative heterogony, the sign of the growth- 

 gradients is reversed. For instance, the limbs of sheep decrease in 

 relative size after birth; here the girdle is the high point of the 

 growth-gradient, the foot the low point ^ (see fig. 198, p. 414). In 

 other cases, growth within an organ is regulated in a more complex 

 way, though still in a graded pattern. A good example of this is seen 

 in the antennae of Copepods. (Forfurther details, see Huxley, 1932.) 



These gradients may not only act within an appendage, or a 

 region of the body, but may permeate the body as a whole. Ex- 

 amples are seen in the relative growth of the appendages along the 

 axis of the body in hermit-crabs, or in the growth-profiles of male 

 and female stag-beetles. It is probable that the growth of the diflFer- 

 ent regions of the body in PlanariaHs also occurs in relation to a 

 simple gradient (with posterior high point) ; but the available data 

 only concern themselves with the proportions of the head and of the 



^ Przibram, 1917, 1925. ^ Huxley, 1932, p. 83. ^ Huxley, 1932, p. 88. 



