THE EPIGENETICS OF THE EMBRYONIC AXIS 



195 



traced the activity to the fraction of the extract which contained the sterol- 

 hke substances, and Waddington demonstrated a high degree of activity 

 in certain synthetic substances of the same nature (Fig. 10. 11); v^hile Barth 

 suggested that the evocator substance w^as cephalin (Reviews : Needham 

 1942, Waddington 1940^7, Brachet 1944). Obviously not all of these con- 

 clusions could be true; and although some of the claims were mistakes 

 based on the presence of impurities, the situation appeared to be one of 

 complete confusion. It only began to clear up when it was shown that 







B 



FlGUHE lO.II 



Diagrammatic section showing a neural tube {Ind) induced by a graft (Gr) 

 of gastrula ectoderm which had been cultivated for two days in methylene 

 blue. The graft has also formed some neural tissue. B, A neural tube induced 

 by an implant (Itftpl) of coagulated albumen containing oestrone. (After 

 Waddington, Needham and Brachet 1936 and Waddington 1938^). 



evocation could be produced by chemicals which quite certainly are not 

 the evocator which occurs in the naturally developing embryo. 



The idea of looking for such non-natural evocators arose from another 

 line of thought. In 1927 Spemann and Geinitz had shown that if a piece 

 of ectoderm from the early gastrula is grafted into the region of the 

 organiser and left there for some time, it becomes, as they put it, infected 

 with the ability to induce. Again, various authors have shown that if 

 small fragments of ectoderm are isolated in a situation in which they are 

 bathed by the body fluids (e.g. in the abdominal cavity of a tadpole) they 

 frequently develop into mesodermal tissues such as are normally derived 



