444 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



with a factor which it lacks to become chordomesoblast. Since it has been estab- 

 lished by several observations {cf. p. 430, Holtfreter's data and also Grobstein's, 

 p. 463) that induction does not require intimate contact, one may assume, in 

 satisfactory agreement with the studies using isotopes and antigens, that a trans- 

 port of organic macromolecules occurs. Before penetrating into the offered ecto- 

 blast, these molecules are integrated, at least for a while, in the cortex of its cells. 

 Regardless of resulting intimate changes in metabolism, visible early manifestations 

 of induction are: synthesis of new cytoplasm, cell multiplication, and special 

 arrangement of the young cell units. The initial difference between (chordo-) 

 mesoblast formation and pure neuralization apparently concerns the rate of 

 mitosis and the reciprocal arrangement of the resulting cells. Urodele ectoblast, 

 the usual reactor, is a practically one-layered cylindrical epithelium (Fig. 34, 

 p. 359). The first step in induction is a local initiation of numerous mitoses 

 and the formation of a pluri-layered epithelium with more cuboidal inner cells. 

 If both reactions are to take place, the deeper elements stop their proliferation 

 earlier; then, their cells arrange themselves in a rather loose pavimentous epithe- 

 lium. The upper cells, on the contrary, divide further, mainly from the surface 

 downward, and remain closely adherent, their elongated nuclei suggesting a 

 columnar arrangement inside a nearly continuous matrix. Clearly, the surface 

 properties have much to do with these differences. As often mentioned {cf. 

 Fujii, 1944), the same reactor cells may easily differentiate in one direction or 

 another. 



Acrogenesis takes place, at least in some instances, as a purely neural induction, 

 limited to prosencephalon structures. In such cases, the xeno-inductor, or its 

 specific isolated fraction, is unable to induce the ectoblast to form the equiva- 

 lent of the prechordal mesoblast, it cannot cause a sufficient rate of inward 

 proliferation nor adequate surface properties in the reacting epithelium. In this, 

 its primary activity is like the neuralizing agent present in notogenesis, but its 

 effects are more powerful. Given a sufficient concentration, it causes the maximum 

 possible thickening of the ectoblast. This mode of reaction directly attains the 

 structural level of the primary optic vesicle, which is normally the earliest definite 

 primordium to appear in the whole neural organ. Its prodviction expresses the 

 most intense action of the neighboring inductor, as apparent in Fig. 80 (p. 418), 

 as already observable in Toivonen's illustrations (1940), as admitted by Gallera 

 and by myself, also by Yamada (1950b) and by Hayashi (1956). The complement 

 of the neuralized epithelium, i.e. the part which does not bulge out as an optic 

 vesicle, forms the prosencephalon, with its moderate power of elongation and its 

 ability to induce olfactory placodes. Sometimes, acrogenesis elicits prosence- 

 phalic structures without optic vesicle(s). No result, however, suggests the existence 

 of distinct inducing agents; thus, the difference seems to be related to quantitative 

 conditions of the one acrogenic agent. 



In notogenesis as in acrogenesis, relations of symmetry are especially important, 

 for they intervene largely in the striking impression of ressemblance with the cor- 

 responding regions of the normal embryo. In the whole notomerit, bilateral 

 symmetry results from the differentiation of the central part of the new formed 

 middle-layer into notochord, and from the position of the neural tube. In an 



