48 EARLY AMPHIBIAN DEVELOPMENT 



note that the existence of this mosaic phase, so different in its total 

 lack of plasticity and of power of regulation from anything known 

 in other stages of development, was only detected through experi- 

 mental analysis. 



c 



Fig. 17 



The development of regions by self-differentiation after the stage of irreversible 

 chemo-differentiation. a, A piece of presumptive eye region from an early 

 neurula of Boyiibiiiator is grafted into the flank of another embryo of similar age, b. 

 c, Transverse section through the resulting embryo showing the eye-cup which 

 has developed by self-differentiation from the graft in its abnormal position; 

 oc. grafted eye-cup ; pron. pronephros. (From Mangold, Ergeb. der Biol, iii, 1928, 

 after Spemann.) 



During the period of plasticity, chemo-differentiation sets in 

 progressively until irrevocable determination of the various regions 

 is achieved. But although the regions may still be plastic before 

 this critical time, in the sense that they can be made to undergo a 



