430 



PRINCIPLES OF EMBRYOLOGY 



may also be fundamentally a duplication. In other cases, however, the 

 condition represents an alteration to the basic pattern by the addition of 

 elements to it, rather than a dupHcation of a pattern which remains 

 essentially unchanged. For instance, in the guinea-pig the forefoot nor- 

 mally has four digits, and the hindfoot three. Wright (1935) has described 

 a dominant gene which when heterozygous produces some tendency 

 towards the formation of extra digits. By selection Wright built up a 

 race in which the general genetic background was such that the hetero- 

 zygote rather regularly had five toes on the forefeet and four on the 

 hindfeet. There is no evidence that this represented a partial duplication of 

 the normal pattern ; it seems rather to be a straightforward modification 

 of it. Animals homozygous for the gene had feet with very large numbers 

 of toes which again showed no evidence of representing multipHcations of 

 the basic four- or three-toed patterns. It is noteworthy, as an indication 

 of the complexity of the reactions whose equilibrium is represented by the 

 normal pattern, that in order to obtain the regular addition of a single toe 

 it was necessary not merely to have a single dose of the main gene, but 

 also to select a large number of other appropriate genes in the genetic 

 background. 



The development of low-grade polydactylous limbs in mammals has 

 recently been carefully studied by Carter (1954) in the luxate strain of 

 mice. Polydactyls occur both among the heterozygotes and the homo- 

 zygotes for this gene (Fig. 20.6). The first effect noticeable during their 

 development is an overgrowth of the anterior (pre-axial) side of the 

 limb-bud. In this region the condensations of mesenchyme are irregular, 

 and extra condensed regions may appear from which the supernumerary 



Figure 20.6 



The hindfeet of two luxate heterozygote mice, ilKistrating various grades 

 of Polydactyly. The left foot of the second animal (on the right of the draw- 

 ing) is normal. (From Carter 1954.) 



