274 PRINCIPLES OF EMBRYOLOGY 



inducing stimulus. Do the inducers give off some specific substance which 

 IS essential for limb formation, or do they only activate potentialities 

 already fully present in the flank? Needham (1942) argues that the former 

 alternative is the more likely on general grounds and is the better guide to 

 future experimental w^ork. Balinsky (1937), on the other hand, suggests 

 that the inductors owe their power to a rather ill-defmed quality which he 

 speaks of as a 'high physiological activity', and he beheves that it is this 

 which sets in motion the inherent capacity of the flank mesenchyme to 

 develop into a limb. The question needs much further study; no serious 

 attempt has yet been made to investigate the phenomenon at a biochemi- 

 cal level. It seems likely that it will turn out to be very similar to the situa- 

 tion with which we are confronted in the induction of the neural plate : 



Mirror imaging when limb-buds develop near one another. In a the ap axes 

 are concordant; one of the limbs frequently has its polarity reversed. In b 

 the ap axes point away from each other, and both limbs retain their polarity. 

 In c the ap axes point towards one another, and a supernumerary limb, of 

 ill-defmed polarity, often appears. (From Takaya 1941.) 



that is to say, that the induction can be performed by relatively unspecific 

 implants, but that these may act in a secondary way, their first effect being 

 the release of specific substances within the reacting tissues. One single 

 case has been described [Bahnsky 1927] of the induction of a hmb follow- 

 ing the implantation of a fragment of celloidin into the flank. This may 

 probably be compared with the induction of neural plate by treatments 

 which produce 'sub-lethal cytolysis' (see p. 196). 



Whatever the position as regards the specificity of the inducing stimu- 

 lus, there is no doubt that the competence of the reacting material plays 

 a large part in the production of the limb. One evidence of this is the asym- 

 metry of the induced limb in the various regions of the flank. Back to the 

 level of somite 7 the pre-axial side develops anteriorly, but between 

 somites 7 and 14 the asymmetry is reversed (Takaya 1941). Again Balinsky 

 (1933) showed that the frequency with which limbs are induced falls oft 

 fairly steadily from the region of the forehmb- to that of the hindlimb-bud 



