144 INDUCTION AND ORGANISATION 



Interesting results were also obtained in successful attempts 

 to induce limbs in abnormal places by grafting material into 

 the side of the body. Balinsky (1925-27) discovered that in 

 Triton taeniatus grafting of an ear-vesicle under the ectoderm 

 of the flank often results in the outgrowth of a limb in this 

 place. That this is not a specific induction is shown clearly by 

 the fact that Balinsky obtained the same result by grafting 

 an olfactory pit, or even a piece of celloidin, under the flank 

 ectoderm. He made an extensive analysis of this phenomenon 

 in a series of further experiments (1929-37). He found that a 

 limb can be induced anywhere in the flank. The percentage of 

 successful inductions decreases from the fore-limb region in 

 a caudal direction, but increases again in the vicinity of the 

 hind-limb. In the anterior regions of the body, the induced 

 limbs have the character of fore-limbs, farther caudally more 

 that of hind-limbs. The more caudal the induced limb, the later 

 it will develop, irrespective of the moment of implantation. 

 At a certain stage, however, the capacity to react to this in- 

 duction disappears. Probably the whole of the flank of a urodele 

 has a latent limb-forming potency in such a way that there is 

 some sort of an equilibrium between the tendencies to form 

 "body-wall" and "limb". Presumably, a non-specific agent may 

 lead to preponderance of the latter tendency by causing a local 

 increase in the intensity of tissue metabolism. Where this 

 intensity is low, however, the tendency to form body-wall 

 predominates. 



A large number of experiments might be added to those 

 mentioned above. These, however, will suffice to give an im- 

 pression of the very complicated interplay of inductions that 

 is at work in the embryo during this phase of development. 

 It is hardly an overstatement to say that all parts of the 

 embryo which are in contact, or which in the course of topo- 

 genesis come into contact, influence each other by contact 

 induction. Each part, therefore, is at the same time an inductor, 

 emitting certain influences, and a reaction system, reacting 

 to the influences received. The physicochemical constitution, 

 and therefore the potency, of each cell or cell group is modified 

 according to the inductions it encounters, either simultaneously 

 or successively. 



