PROBLEM OF DIFFERENTIATION 



II 



§5 



We are not concerned, here, with the construction of a philosophical 

 system, nor do we wish to prejudge the question of the relationship 

 to one another of phenomena of the physico-chemical and of the 

 biological order ; the reader to whom these matters are of interest 



Normal 

 Nerve Tube" 



Induced 

 --'' Nerve -Tube 



Normal 

 Ear-rudiment 



Normal 

 Nerve -Tube ^"-^.^ 



Induced 

 Ear-rudimont 



Normal ^^"^ 



Muscle-Seements W^- 



B wm^.. 



Eye - rudiment 



I nduced 

 Nerve-Tube 



I nduced 

 -* Muscle-Segments 



Induced ''-'^^^^#8^ 

 Tall-bud -^^^2^- 



Fig. I 



Normal Tail-bud 



Induction of secondary embryo by grafted organiser in Triton. A, 3 days after 

 operation. B, Some days later. (From Wells, Huxley and Wells, The Scietice of 

 Life, London, 1929; after Bautzmann.) 



may mcst profitably be referred to the recent work of Drs von 

 BertalanfTy and Woodger: Modem Theories of Development. But as 

 biologists we do believe that the phenomena which we study in 

 living organisms conform to a biological order, in which the causal 

 postulate is strictly applicable. The great value of the new science 

 of experimental embryology or developmental physiology (the term 



