222 PRINCIPLES OF EMBRYOLOGY 



cell. We are not dealing with a simple interaction between a single stimu- 

 lating substance and a definite and delineated reactant. On the contrary, a 

 choice as fundamental as that between epidermis and neural tissue involves 

 the whole biochemical system of the cell; the protein synthesis, the ribo- 

 nucleo-protein microsomes, the respiration and the genes. It is perhaps 

 disappointing that the complexity of the system makes it impossible for 

 us to reach a quick and definite identification of 'the evocator'. But in 

 compensation we are acquiring a much more profound insight into the 

 whole economy of epigenetic processes. Before we can advance any 

 further, it will be necessary to discuss more fully the body of knowledge 

 which genetics can bring to bear on the problem, which will be done in 

 Part II of this book. 



SUGGESTED READING 



The classical papers on organisers are Spemann 1918, Spemann and Mangold 1924. 

 Spemann's owia summary of his work is in his 1938 book, Chapters 6-14. 



For recent accounts of the experimental facts: Needham 1942, pp. 148-205 and 271- 

 89: Lehmaim 1945, pp. 203-350; Waddington 1952a, pp. 51-139. For biochemical 

 aspects, Brachet 1944, Chapters 9 and 10, Boell 1948. Other valuable discussions, Brachet 

 1952a, b, Holtfretcr 19486, 195 1, Toivonen 1950. 



