INDIVIDUATION— FORMATION OF PATTERN AND SHAPE 429 



condensations within the mass of loose mesenchyme which makes up 

 the body of the limb-bud (Fig. 20.5). 



The first point we may notice is that the pattern is not at all closely 

 dependent on the mass of material available. If a limb-bud is halved at an 

 early stage, the complete pattern may appear within the half-sized part. 

 There is indeed a considerable tendency in limb-buds which have been 



Figure 20.5 

 On the left, the structures in the right hindHmb-bud of a normal laj-day 

 mouse embryo, projected on to the plane of the footplate. The mesenchy- 

 mal thickenings for the femur {FE), fibula (FI), tibia {TI) and digits are 

 visible. AER is the apical ectodermal ridge. On the right is the disturbed 

 pattern found in a luxate homozygote. The pre-axial side of the limb-bud 

 IS enlarged, but the blastema of the tibia is absent. (From Carter 1954.) 



disturbed in some way, for instance by transplantation to other sites, for a 

 spontaneous subdivision to occur, so that dupHcate limbs are formed. 

 These are nearly always mirror-images of one another. If the division 

 takes place at a somewhat later stage and is incomplete, partial duphcation 

 may occur, giving rise to structures with more than the normal number of 

 toes. 



Such polydactylous limbs are also produced by a number of genetic 

 factors, hi some of the extreme forms, particularly in birds (Reviewed: 

 Waddington 1952^7, Landauer 1948) the genetically caused Polydactyly 



