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Fig. 34. Structure of the ectoblast and neurectoblast in normal Pleurodeles embryos (A-C) 

 and in embryos submitted to urea (D). A. Thin section of the dorsal ectoblast in a young 

 gastrula, stained with pyronin-methyl green. Phase contrast. High columnar cells filled 

 with pigment and yolk. B. Neuro-epithelium of a young neurula. Thin unstained section, 

 polarized light. The nucleus is elongated, the platelets are less abundant, are elongated 

 horizontally and are concentrated at the base of the cells; large "empty" areas of orientated 

 cytoplasm (see schema, left). C. The ventral epiblast at the same stage, under the same 

 conditions. Similar but less marked differentiation in more cubic cells. D. The neuro-epithe- 

 lium of an embryo treated by urea. Same conditions as in A. The epithelium is thicker than 

 in a normal neurula (70(j. rather than 40[jl) and is undifferentiated. The polarization 

 microscope does not reveal a structure comparable to B and C. The enlargement of A and 

 D is about half of B and C. From Gallera and Baud, 1954. 



conspicuous (Fig. 34, B, C) under polarizing microscope (Gallera and Baud, 

 1954). Our information concerning the ultrastructural changes occurring during 

 neurogenic induction is quite recent. We are endebted to Eakin and Lehmann 

 (1957) for an electron-microscopic study of the aspects observable in the amphibian 

 ectoderm, mainly in Xenopus, at various stages from the early gastrula on. The 

 effect of induction on the presumptive neurectoblast is described as a densification 

 of the cytoplasmic reticulum, a reduction of size in the various inclusions and a 

 definite numeric increase of the mitochondria. The intimacy and complexity of 

 the limit between the archenteric roof and the external layer suggest easy transfers 

 of macromolecules. At later stages, a further differentiation of the mitochondria 



Lileralure p. 483 



