THE EPIGENETICS OF THE EMBRYONIC AXIS 



221 



see Baltzer 1950, 1952a), This is not so clear for the neural tissue, where 

 there are no very obvious differences between the available species of 

 Amphibia. But as we shall see, there are secondary organisers which act 

 later to induce structures where such differences may be more marked. 

 For instance, the mouth of an early frog embryo has proper teeth, whereas 

 that of a newt does not; and if a newt mouth-organiser acts on frog 

 ectoderm, the result is a frog mouth, complete with teeth (Spemann and 

 Schotte 1932, Rotmann 1935). The inducing stimulus gives the order 

 'Mouth', and the reacting ectoderm carries out the order in accordance 

 with its own book of procedure. The particular substances formed during 

 the differentiation are determined, in the main, by the genetic nature of 

 the competence. Another example of this is illustrated in Fig. 10.18. 



It is worth noting that, as might perhaps be expected, Briggs, Green 

 and KJng (195 1) found no sign of any competence in non-nucleated 

 amphibian cells, of the kind mentioned on p. 64. 



It will be apparent from this discussion that the investigation of evoca- 

 tion has led us into the very heart of the complex metabolic hfe of the 



Mouth 



Sucker 



Figure 10.18 



A graft of newt ectoderm (pale) was made on to a toad embryo. Where the 

 implant comes into the mouth region, it has developed a balancer, an organ 

 which a newt embryo would possess in this region but which is quite 

 foreign to the toad. Thus the implant has reacted in its own characteristic 

 manner to the stimulus of the region. (After Rotmann 1941.) 



