Ill 



PRIMARY INDUCTION 



EXPERIMENTAL DATA 



405 



overriding the usual limits. In the first case, the notochord must have been 

 present without exerting its transforming effect. In the five others, it must be 

 accepted that the activated territory was integrally transformed, without any 

 evidence of an activation phase. 



I do not wish to suggest that the idea of two factors acting in these complexes 

 is without foundation. The criticism bears more on the assumption of the two 

 successive influences acting from the notochordal explant by a kind of impregnation 

 into the surrounding ectoblast. It is worthwhile to recall that a rather long delay 

 here preceeds the appearance of neuralization. Do not some unsuspected events 

 take place inside the sandwich during the early period? 



Fig. 68. Influence of the neighboring tissue on the differentiation of the neuro-epithelium 

 in Urodeles and Anurans. (a) the explanted area of a neurula with elevated neural folds; 

 (b) the neuro-epithelium in the absence of niesoblast ; (c) accompanied by myoblasts and 

 fibers (white matter) at the contact zone; (d) surrounding a notochord piece, also with 

 fibers near this tissue; (e, f, g) in simple mesenchyme resulting in three degrees of epithelial 

 differentiation and distension. From Takaya, 1956b. 



The influence of neighboring tissue on the morphology of the neural axis was 

 recognized by Holtfreter (1934a), but has now been explored further by Takaya 

 (1955a, 1956a, b, c). This author has isolated from urodele and anuran neurulae 

 (with a distinct neural fold) the region of ectoblast including the brachial^ part 

 of the neural plate (Fig. 68, a). If the chordomesoblastic substratum is included, 

 typical normogenesis takes place, as expected. Considerable differences occur, 

 however, if the mesoblastic lining is wholly or partly removed. At first, Takaya 

 was struck by the brain-like appearance of certain explants, but he also observed 

 considerable difference in the growth of the neural tissue, in the differentiation 

 of fibers (white matter), and in the degree of swelling, according to the nature 

 of the adjacent material. By expanding the views expressed by Holtfreter (1934a, 

 p. 294), he has now attained a satisfactory interpretation. 



' i.e. the level which will furnish the innervation of the upper limb. 



Literature p. 483 



