176 



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rf"^. 



Fig. 85 



The formation or non-formation of a given structure depends not only on the 

 presence of an inductive or formative stimulus (organiser), but also on local 

 specific factors, intrinsic to the fields. Triton taeniatiis normally develops a 

 balancer, the axolotl does not. Nevertheless, a piece of gut-roof of axolotl 

 grafted into an embryo of Triton can induce the balancer field of the latter to 

 develop supernumerary balancers, A. Conversely, B, a piece of trunk epidermis 

 of Triton grafted on to the head of an axolotl embryo, gives rise to a balancer (left 

 side of photograph), while no balancer is formed from the normal axolotl 

 epidermis on the other side. The axolotl therefore possesses the necessary 

 formative stimulus for balancer formation, but its epidermis fails to react to it. 

 g. grafted axolotl tissue; w. normal balancer; s. supernumerary balancer. (From 

 Mangold, Natunuiss. xix, 193 1.) 



