12 THE PROBLEM OF DIFFERENTIATION 



"Entwicklungsmechanik" is hardly translatable, and, now that its 

 birth has been described, may best be avoided in English writings) 

 is that it is enabling biologists to discover the complex components 

 of development, and so to explore new aspects of the biological order. 

 The dorsal lip of the amphibian blastopore (the so-called "organ- 

 iser") has been shown (see fig. i) to be capable of inducing 

 neighbouring tissues to give rise to all the essential structures of an 

 embryo ^ (brain, spinal cord, eyes, ears, muscles, kidney tubes, etc.). 

 The result of grafting an organiser into a suitable environment is 

 just as definitely causally determined and predictable as the result 

 of mixing two known reagents in a test-tube, although the pheno- 

 mena are in the one case of the biological and in the other of the 

 physico-chemical order. It may be confidently expected that in 

 time the physiological basis of the organiser's action will be dis- 

 covered and accurately analysed in physico-chemical terms. ^ Until 

 then, however, it is both desirable and necessary to push the analysis 

 as far as possible on the biological level. 



It is as a contribution to the analysis of early development on the 

 biological level that the following pages have been written. 



^ Spemann and Mangold, 1924. 



- Already it is known that the organising action is due to a substance which 

 is almost certainly lipoidal and probably a sterol (Waddington, Needham and 

 Needham, 1933). See pp. 154 and 497. 



