GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 369 



the centres for horn-formation (which are doubtless specifically 

 chemo-differentiated regions) are present. In many other mam- 

 mals, presumably, similar gradients are present, but we are ignorant 

 of their existence, as no horn-centres exist by which they can mani- 

 fest themselves. 



In all these cases, if growth-potency is evenly graded along the 

 growing zone, the resultant hard structure assumes the form known 



mm. 

 14- 



mid 

 vrentraJ 



dorsal 



Ant 

 Q 



■3^ 



Fig. 178 

 Persistent gradient-fields affecting feather-growth in adult birds. In all, the 

 ordinates represent growth of regenerating feathers per day. The abscissae 

 represent distances within the breast-region, in A and C antero-posteriorly, in 

 B and D ventro-dorsally. A, In a capon (® single feathers, (^ means). B, In 

 a cock, in two regions of the breast. C, In a capon. D, In a cock and a hen. 

 (Based on data of Juhn, Faulkner and Gustavsen, from Huxley, Problems of 

 Relative Grozoth, London, 1932.) 



mathematically as the logarithmic spiral. Slight departures from 

 a straight-line growth-gradient give rise to departures from strict 

 logarithmic-spiral form. 



The most important of such departures is seen in Molluscs. The 

 growing edge of the mantle here makes a more or less circular 

 aperture. If growth is equally graded on the two sides of this 

 aperture between high and low point of growth-activity, the shell 

 produced is a plane spiral, as in Ammonites or Scaphopods. If, 



HEE 24 



