196 PRINCIPLES OF EMBRYOLOGY 



from the organiser regions (Bautzmann 1929). These facts strongly sug- 

 gested that the whole of the ectoderm contains all the factors necessary to 

 develop into mesoderm and to induce, and that it is some process occur- 

 ring at the blastopore region which activates these factors and enables 

 them to take effect. 



The well-known axial gradient theory of Child (p. 314) would suggest 

 that the activating process might be something to do with the respiratory 

 metabolism. Waddington, Needham and Brachet (1936) therefore tested 

 the effect on pieces of ectoderm of treatment with dyes known to stimulate 

 respiratory processes. The dye used was methylene blue. It was found, 

 as had been surmised, that if small pieces of ectoderm were isolated in a 

 salt solution containing this substance they sometimes developed into 

 neural tissue, and, if implanted into a young gastrula, were able to 

 induce a neuralisation of the competent ectoderm against which they lay 

 (Fig. lo.ii). The dye was, then, acting as an evocator. It would however 

 be ridiculous to suppose that the normal amphibian egg contains methy- 

 lene blue. It was therefore proved that evocation can be performed by 

 substances other than the substance, whatever it is, which produces that 

 reaction in normal development. This fmding put the whole investigation 

 of the nature of inducing action on to a new basis, since, if any substance 

 is grafted into a gastrula and is found to cause neuralisation, that fact 

 cannot be taken as evidence that the substance is the same as, or even 

 necessarily nearly related to, the natural evocator. 



There are several mechanisms by which such unnatural evocators might 

 be supposed to operate. In the first place, we have just seen that Holt- 

 fretcr (1934^) had shown that if non-inducive tissue of the gastrula is 

 killed it thereby acquires the power of induction. There is therefore the 

 possibihty that if, in a piece of ectoderm, a certain number of cells were 

 killed they might release sufficient evocating substance to induce the 

 remainder to develop into neural tissue. There is little doubt that pro- 

 cesses of this kind can occur. For instance Okada (1938) has brought about 

 inductions by mechanical irritants such as siHcious earth and Holtfreter 

 (1945) has done the same thing by killing a certain number of cells with a 

 glass needle. It seems certain however that this is not the only way in 

 wliich unnatural evocators act. There is no sign of excessive mortality of 

 the cells at the concentration of methylene blue utilised by Waddington, 

 Needham and Brachet; and this has also been pointed out by Pasteels 

 (195 1) who confirmed the activity of this substance. Moreover Wadding- 

 ton (1940^) found that the very actively evocating steroid substances tend 

 to stimulate the rate of growth in ectoderm submitted to them rather than 

 to operate as depressants or cytolytic agents. Holtfreter (cf. 1945) held 



