366 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



earlier investigations, such as those of J. Brachet (1944) and Osawa (1951) on 

 amphibians embryos, and of Moog (1944) on the bird embryo. Indeed a positive 

 reaction of neural structures has been described there, but not especially strong 

 when compared with the underlying material. It seems that, at least in amphibians, 

 the bkirring of the difference depends on the presence of the yolk, which is more 

 abundant in the mesoblastic cells than in the external layer. The yolk platelets 

 and the cytoplasm which surround them seem also to possess this enzyme. In 

 chicken embryos, an improved technique entirely confirms, in spite of a more 

 positive mesenchyme, the aspects observed in mammals (Milaire, impubl. 1959). 

 It seems advisable to correlate this richness in phosphatase of the induced 

 material with the positivity of the apical cap which develops on the tip of limb 

 buds of mammals. Immediately after being formed, this cap acquires (Fig. 38) 

 a positive reaction (McAlpine, 1956; Milaire, 1956). This is also true for chicken 

 limb bvids (Milaire, tmpubl.). I have suggested that this fact means that the apical 

 cap has also been induced, an idea which seems to throw some more light on the 

 differentiation of the limb buds (see p. 456). 



B. Experimental data relative to priinary induction 



To put this intricate field into some order, it seems advisable to deal first with 

 the primordial event which invokes the formation of the neural organ, and to 

 classify the experiments in order of complexity. The most simple attack is 

 certainly the purely surgical one, performed aseptically on the whole egg in a 

 well-balanced physiological saline solution. Isolation of fragments, and their 

 eventual combinations, are farther from normality, and not necessarily easier to 

 interpret. The conditions of environment are less normal, and the territory which 

 is being investigated does not benefit from the instalment of circulation as it 

 would in the complete embryo. The objection has been raised that uncontrolled 

 factors present in the whole embryo could viciate the experiment. Yet this 

 inconvenience must not be exaggerated, and can be avoided by appropriate 

 controls. 



{a) Operations on the whole embryo 



From the numerous experiments performed on amphibian eggs by Spemann 

 and his followers, the type of result obtained by grafting, exchanging or trans- 

 posing the various territories of the egg can be formulated in general terms. 

 The maximum plasticity is encountered in the blastula and the young gastrula, 

 and a restriction gradually occurs during gastrulation. 



In the young, more malleable stages, many operations remain without ap- 

 preciable consequence for the embryo. The perturbation is neutralized because 

 the extraneous material becomes assimilated into its new surrounding [of. p. 351). 

 Only territories of the dorso-marginal region, that is presumptive pharyngeal 

 endoblast, prechordal, chordal and somitic mesoblast, show a distinct tendency 

 to overcome the uniformizing action of the surrounding and to keep their own 

 individuality. 



In order to follow the fate of a graft or of any transposed territory, it has 



