414 THE HEREDITARY FACTORS AND DIFFERENTIATION 



female abdomen, are both brought about developmentally by the 

 substitution of a steep growth-gradient with subterminal high point 

 for a flat growth-gradient with subcentral high point. Any genes 

 controlhng chela size and shape will act first by controlling the 

 general form of the gradient involved, and secondly by influencing 

 its steepness. In addition, there will doubtless be other genes 



Phase 2 

 (Largest newborn 

 to medium-sized) 



0-4 



1 2 



distal — > 



Fig. 198 

 Growth-gradients in the limbs of domestic sheep, from birth to half-grown 

 specimens. Ordinates : growth-coefficients (differential growth-ratios) for weights 

 of parts of limb relative to weight of vertebral column. The horizontal broken line 

 represents isogony (growth-coefficient = i -o) ; values above it signify positive 

 heterogony, values below it negative heterogony. Abscissae: i, limb-girdles; 

 2, humerus or femur ; 3, radius and ulna or tibia and fibula ; 4, carpals or tarsals ; 

 5, metacarpals or metatarsals. Solid line, fore-limbs; dotted Hne, hind-limbs. 

 (From Huxley, Problems of Relative Growth, London, 1932, based on data of 

 Hammond.) 



modifying the growth of local regions of the gradient, and influenc- 

 ing detailed characters such as bristles, ridges, etc. ; but the main 

 factors operative will concern the gradients as a whole. 



The importance of this way of regarding the facts is well shown 

 in sheep. ^ Here, in the first place, the limbs during postnatal 

 development show a marked growth-gradient with terminal or 

 subterminal low point, and high point in the limb-girdles: the 

 growth not only of the bones but also of the muscles is aflFected by 



1 Hammond, 1929; Huxley, 1931. 



