THE EPIGENETICS OF THE EMBRYONIC AXIS Ipp 



Barth (1941) showed that the gastrula ectoderm of the axolotl (A. puncta- 

 turn) developed sometimes into neural tissue when isolated in, as he 

 thought, completely neutral salt solutions. This was a startling claim be- 

 cause until that time it had always been held that ectoderm could only 

 become neural if definitely induced to do so, and Earth's result seemed to 

 be putting this in doubt. The matter was reinvestigated by Holtfreter 

 (1945) who found that the truth of the matter is that A. punctatum ecto- 

 derm is particularly sensitive to abnormal external conditions and reacted 

 by neuralisation to salt solutions wliich have no particular effect on the 

 ectoderm of other amphibian species. If however the salt solution is made 

 to depart considerably from the optimum (by a considerable raising or 

 lowering of pH or by lack of calcium), it can evocate neuralisation even 

 in the more resistant ectoderm of other species. Now in such experiments 

 the evocated neural tissue may develop by self-individuation into a fairly 

 well-defmed nervous organ. This organ always belongs to the anterior 

 end of the brain (the forebrain or archencephalon). hi considerable con- 

 trast to this is the result of another type of unnatural evocating condition. 

 Pasteels (summarised: 1953) has shown that fairly mild centrifugation 

 of early gastrula ectoderm will often cause it to develop into neural 

 tissue and also into notochord, somites and other mesodermal derivatives. 

 The ease with wliich the action is produced varies in different amphibian 

 species. The point to note is that once again definite organs may be 

 produced and, in this case, they never belong to the archencephalon 

 but always to the posterior end of the brain (deuterencephalon) or 

 spinal column. Finally, Yamada (1950) has found that gastrula ecto- 

 derm may be caused to develop into mesoderm by treatment with 

 ammonia. 



These differences in the results of evocation make rather unplausible 

 the suggestion which has sometimes been put forward (e.g. by Barth) 

 that the evocator reaction is like that of the artificial parthenogenesis. 

 Recent results have only added confirmation to the conclusion reached by 

 Waddington (1940^^) that, if the competence of the gastrula ectoderm is 

 set on so fme a hair-trigger that any of a number of stimuh are sufficient 

 to touch it off, we have to admit that the ectoderm can, unlike the egg, 

 shoot in more than one direction. We must in fact be dealing with an 

 orderly system of alternative processes in which the end-result is related 

 in a rather direct way to the nature of the initiating cause. If this were not 

 so, we could expect to get very little profit from an analysis of the evoca- 

 tors, and would have to confme our attention solely to what we can dis- 

 cover as to the processes going on in a reacting ectoderm. As it is we can 

 fmd important clues to further understanditig not only in the ectoderm 



