368 GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



rest of the body. It is of interest to note that during the reduction 

 in size that accompanies starvation in these animals, the trend in 

 change of proportions is reversed, the trunk becoming relatively 

 more reduced in size than the head, so that the proportions of an 

 animal of a given size are the same whether it has been growing 

 larger or becoming smaller.^ 



Special cases of great interest resuhing from graded growth are 

 those of the shells of Molluscs and Brachiopods. In these cases 



2.5 



"35 2.0 



CO C 

 C " 



.2 <u 

 o 



12 3 4 5 5 7 



Segments of abdomen: distal — > 



Fig. 177 

 Growth-gradients in the abdomen of crabs. The abscissae represent growth- 

 coefficients (differential growth-ratios) of hnear dimensions of abdominal 

 segments relative to carapace length. The ordinates refer to the abdominal 

 segments; 7, telson. Solid line, breadth of segments: ® Teltnessus, (S; x the 

 same, ?; + Pinnotheres (pea-crab), ?. The dotted line refers to segment-length 

 in Pinnotheres, ?. (From Huxley, Problems of Relative Growth, London, 1932.) 



growth takes place at a definite growing edge, and the new material 

 laid down solidifies and takes no further part in growth. A similar 

 type of growth is found in other hard structures such as the horns 

 of mammals, teeth, etc. 



D'Arcy Thompson^ first pointed out that the form and size of 

 the horns of two-horned rhinoceroses could only be understood on 

 the assumption of a growth-gradient, decreasing posteriorly, in the 

 head region, affecting the proliferation of epidermal structures. 

 This is of some general interest, as it can only manifest itself where 



Abeloos, 1928. 



^ Growth and Form, 1917, p. 612. 



