ORGAN DEVELOPMENT IN VERTEBRATES 273 



almost molecular dimensions and perhaps comparable to the oriented 

 arrangements found in liquid crystals, but x-ray microscopy was not able 

 to detect any such structure (Harrison, Anthony and Rudall 1940). Takaya, 

 on the other hand, suggests that the fundamental factor is capacity for 

 growth, which is highest in the antero-dorsal part of the early limb-disc. 

 In his view it is the fixing of gradients in growth capacity which deter- 

 mines the polarity which the limb will exhibit. 



If an early limb-disc is excised and grafted into a new position in such 

 a way that its polarity is opposed to that of the immediately surrounding 

 area, a frequent result is the development of a pair of duplicated limbs. 

 These are nearly always mirror images of one another, one having the 

 symmetry of a right limb and the other that of a left. The frequency with 

 which this mirror-imaging occurs suggests that two limbs developing 

 close to one another influence each other's polarity. This is confirmed by 

 the results of grafting two limb-buds into each other's neighbourhood. 

 It is found that even if they are grafted so that their polarities are con- 

 cordant, nevertheless the two limbs which form often turn out to be 

 mirror images, one of the two having had its original polarity reversed 

 by its neighbour (Fig. 12.11). 



It is a remarkable fact that the polarity of a normal embryo does not 

 run in a constant direction throughout the whole flank of the animal. 

 Opposite the anterior somites, from somite i to about somite 7, the 

 polarity is such that it tends to cause the pre-axial side of the limb to 

 develop on the side nearest the head of the embryo. The same is true of 

 posterior regions where the hindlimb forms, from about somite 14 back- 

 wards, but in the midflank region, from the level of somite 7 to that of 

 somite 14, the polarity of the flank is in the opposite direction (Takaya 

 1941) (see Fig. 12.10). 



This reversal of polarity in the flank is clearly expressed in the experi- 

 ments which have been made on the induction of limbs. Tliis was first 

 successfully accomplished by Balinsky (1925). He found that if an ear 

 vesicle is transplanted into the flank of a young tail-bud embryo it induces 

 the formation of a supernumerary limb. Later work (Balinsky 1933) 

 showed that the same effect could be produced even more regularly by 

 the implantation of a nasal placode. The organs used in these grafts can 

 not, of course, be the normal inducer of the limb. It is not at all clear what 

 organ or tissue fulfils tliis function in normal development. Attempts to 

 induce limbs by the implantation into the flank of the pronephros, which 

 lies near the site of the normal forehmb, have so far been unsuccessful. 



The induction of limbs by such 'foreign' tissues as the auditory and nasal 

 vesicles, raises in an acute form the problem of the specificity of the 



