136 organisers: inducers of differentiation 



it does by contact. All of the gastrular ectoderm underlain by noto- 



chord and axial mesoderm will become neural plate (see p. 155). 



But these facts, however, do not preclude the possibility of the 



organiser exerting some effect in earlier stages also. And, as a 



C 



Fig. 62 

 Labile determination of neural folds in Urodeles. A, Early gastrula of Pleurodeles, 

 from which, B, the entire dorsal lip region is extirpated. C, Resulting embryo 

 showing spina bifida and neural folds prevented from reaching mid-dorsal line. 

 (From Goerttler, Zeitschr.f. Anat. 11. Entzuick. lxxx, 1926.) 



matter of fact, other experiments have shown that during the period 

 before the irrevocable deterniination of the presumptive neural 

 fold material, it is nevertheless not wholly indifferent, and possesses 

 a labile determination ^ to develop into neural folds. This can be 

 tested in situ in an embryo by removing small portions of the 



^ "Bahnung", Vogt, 1928 a; "competence", Waddington, 1932. 



