INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 43 I 



digits will be produced. It is a remarkable fact that in the homozygotes, 

 which of course show the more extreme expressions of the gene, skeletal 

 elements are frequently missing (not only pre-axial digits but also the 

 tibia). Thus the same genetic influence, which at fairly low intensity 

 produces polydactyly, in higher intensity has an effect of a rather opposite 

 character. Carter tentatively suggests that a field responsible for producing 

 the five-toed pattern, and perhaps inherent in the apical ectoderm (see 

 p. 276) is in the luxate heterozygotes shifted anteriorly in relation to the 

 underlying competent mesenchyme, Tliis draws into the process of limb 

 formation material which would normally lie beyond the range of the 

 limb-inducing influence, and tliis may account for the increased size of 

 the pre-axial region. To explain the reduction in the limb skeleton in the 

 extreme cases he supposes that the pattern-inducting field is shifted so 

 far anteriorly that part of it overlies mesenchyme which is not competent 

 to respond. 



This hypothesis provides a formal explanation of the facts, but still 

 leaves one quite in the dark as to the nature of the processes which cause 

 the limb mesenchyme to form a certain number of condensations. It is 

 probably significant that in polydactylous limbs it is normally on the pre- 

 axial side that the extra digits appear. Gabriel (1946) found that if the 

 opposite (post-axial) side of the limb-bud of a polydactylous strain of 

 chicks was inhibited in growth by treatment with colchicine the Poly- 

 dactyly was exaggerated. This suggests that one aspect at least of the 

 normal process of pattern formation involves a control of the pre-axial 

 side by the post-axial, this control being weakened when the post-axial 

 side is inhibited. It is perhaps simplest to imagine the control involving 

 the diffusion of substances, but there is still no direct evidence of this. 

 Indeed, once again one has to admit that we have no clear-cut indication 

 even of the general category of process to which we ought to look to 

 find an explanation of the apparently simple fact that an originally homo- 

 geneous mass of mesenchyme begins to draw itself together into a num- 

 ber of separate regions of condensation. Phenomena of this kind obviously 

 lie at the basis of the whole of animal morphology and our almost total 

 ignorance as to how they are brought about offers a challenge which it is 

 to be hoped experimentalists will soon take up successfully. 



In the last few paragraphs we have considered the original initiation 

 of the pattern of the limb skeleton. It must of course be remembered that 

 this may undergo modifications of quite a fundamental character after its 

 first formation. For instance, Tschumi (1953) showed that if the young 

 limb-buds of the toad Xenopus are treated with colchicine there is a 

 considerable inhibition of growth which leads to a reduction in the size 



