THE EPIGENETICS OF THE EMBRYONIC AXIS 219 



between these sub-alternatives persists longer than that between the 

 main ones ; thus pieces of tissue which have already become neural plate 

 (and thus entered on one of the main alternative paths) may still for a 

 short time be responsive to influences tending to change the region of 

 the neural system which they will form. There is a considerably longer 

 interval between the time at which the non-neural ectoderm is delimited 

 and that when it is decided whether it shall become lens or ear. We have 

 then a succession of competences in time. 



The first study devoted to the causal relations, if any, between successive 

 competences was made in the chick (Waddington 1934^). In that form, as 

 we have seen (p. 182), the embryonic endoderm induces the formation 

 of the primitive streak; does it also provoke the arising of neural com- 

 petence in the ectoderm ? If so, this competence should be absent in the 

 area opaca, outside the region occupied by embryonic endoderm. It was 

 found, however, that the area opaca reacts to organiser grafts just as well 

 as does the area pelhicida. The neural competence therefore seems to arise 

 independently of the endoderm, 



A similar investigation was made on the Amphibia (Waddington 1936). 

 Pieces of ectoderm were removed from the gastrula before the organiser 

 had acted on them, and were cultivated in isolation in salt solution until 

 control embryos of the same age had reached the neural plate stage (i.e. 

 had completed the primary organiser action) ; fragments of anterior neural 

 plate were then implanted into them. The first fact that emerged was that 

 the neural competence had been to a large extent, though not completely, 

 lost. There is therefore an autonomous lapse of competence, although 

 this is not as rapid as it would be in a complete embryo in which organiser 

 action had taken place. Secondly, it was found that when the implanted 

 anterior organisers developed into eyes, they were often able to induce 

 the formation of lenses from the isolated ectoderm, in which lens com- 

 petence must therefore have arisen independently of the action of the 

 primary organiser. In more extensive experiments, Holtfreter (1938/') 

 obtained similar results on the loss of competence, and showed that this is 

 a gradual, not a sudden occurrence; as the ectoderm ages, the magnitude of 

 the neural inductions diminishes, and one obtains more of the 'weaker' 

 reactions such as the derivatives of the neural crest. The observation of 

 Pasteels (1953) that centrifugation in the blastula stage often causes the 

 appearance of mesodermal as well as neural tissues, while later only 

 neural, and finally neural crest derivatives appear, is probably also to be 

 explained by the waning of neural competence, but in this case the equiva- 

 lence of the treatments appHed at the different stages is not so certain, 

 since the cells may change in their internal viscosity. 



