CHAPTER X 



THE EPIGENETICS OF THE EMBRYONIC AXIS 



I. Amphibia 



The account which has so far been given of the development of 

 amphibian and bird embryos has been concerned almost entirely to 

 describe the events which take place; it is time now to see how far we 

 can go in giving a causal explanation of them. Attempts to analyse 

 experimentally the processes which bring about developmental change 

 have of course been made for many years, but it is only in the last three 

 or four decades that we have begun to understand the causal connections 

 involved. This deepening of our understanding may not too unfairly be 

 dated from Spemann's discovery in 1918 of the 'organiser', which will be 

 discussed in some detail below. There were, of course, foreshadowings in 

 earlier years of Spemann's discoveries, as there always are of important 

 scientific advances ; but in this case they were so slight, and their import- 

 ance so little understood even by their authors, that they serve rather to 

 illuminate the magnitude of Spemann's advance than to dim its lustre. 

 It is therefore of great importance to understand exactly what Spemann 

 discovered and the way in which it is significant (Reviews : Spemann 193 8, 

 Dalcq 1941, Needham 1942, Lchmann 1945). 



Since most eggs are small, the manipulative difficulty of experimenting 

 on them is considerable, and the older experimental embryologists had 

 found themselves restricted almost entirely to the expedient of cutting 

 the egg into fragments, which were then allowed to develop in isolation. 

 The result of such an experiment was normally either that the fragment 

 developed into the fate which would have been in prospect for it if it had 

 remained untouched in the egg, or that it developed into a complete 

 embryo. In the former case, the egg was called a mosaic egg, in the latter 

 a regulation egg; and, if a further step in theoretical analysis were called 

 for, the fragments of the former type might be called 'unipotent' and those 

 of the latter 'totipotent', these words implying that the former had only 

 one potency for development while the latter had all the potencies re- 

 quired to produce a complete organism. Where it was possible to perform 

 experiments on a series of younger and older stages, it was commonly 

 found that while fragments from an early stage might be totipotent, 

 those from a later one had become unipotent. 



There grew up a considerable body of discussion of the way in which the 



173 



