370 GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



however, the growth-gradient on the one side of the mantle 

 aperture is concave upwards, on the other side concave downwards, 

 the result is what is known as a turbinate spiral — i.e. a form such 

 as that of a whelk- or snail-shell, characteristic of most Gastropods. 

 The corkscrew horns of sheep, goats, etc. are due to similar asym- 

 metrical growth-fields. 



Experiments on fowls have shown that here (fig. 178), too, 

 growth-gradients exist throughout life which affect the rate of re- 

 generative growth of feathers.^ These gradients diflFer in different 

 regions, but within a given region are simple in form. Similarly, 

 there is a gradient in regeneration-rate of anuran larval tail skin.^ 



Perhaps the most interesting evidence that growth-potencies are 

 regulated in relation to some form of field-gradient system is de- 

 rived from a study of the effects of a localised region of high growth- 

 rate on the growth of neighbouring parts. In general, these are 

 slightly enlarged, the effect gradually grading away with distance. 

 This is seen in the increased size of the walking legs on the side of 

 the large claw in male fiddler-crabs, where the enormous male- 

 type chela is confined to one side of the body.^ A similar effect on 

 the walking legs behind the large claw is seen in other Crustacea, 

 such as Maia and Palaemon, but here, as the large claws are sym- 

 metrical, it is found on both sides of the body.^ In male stag-beetles 

 the disproportionate increase in relative size of mandibles with 

 increase in total absolute size is correlated with a slight increase in 

 relative size of antennae, and of first as against third legs^. 



As regards Crustacean limbs, this effect of a localised region of 

 intensive growth appears only to be exerted posteriorly, while 

 anteriorly the result is partly or wholly reversed. In some cases the 

 induced increase of growth is less in limbs immediately anterior 

 to the region of intense growth-rate than in those immediately 

 posterior; in other cases, their growth is even slightly inhibited. 

 Examples of this positional eflFect are seen in the second and third 

 maxillipeds of male spider crabs {Maia and Inachus), and in the 

 first pereiopod of male prawns (Palaemon) in which the second 

 pereiopod is enlarged as a large claw.^ Such a differential action 



^ Juhn, Faulkner and Gustavsen, 193 1 ; Lillie and Juhn, 1932. In this case, 

 an additional point of great interest is the correlation found between regeneration- 

 rate and susceptibility to hormones. 



3 Clausen, 1932. ^ Huxley, 1932. 



