. The elements of experimental embryology. Embryology, Experimental; Embryology. 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-
. The elements of experimental embryology. Embryology, Experimental; Embryology. 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, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the origina
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