472 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



mation. The author of this fascinating theory insists more and more on the dynamic charac- 

 ter of this two-step process. 



Toivonen, on his side, takes an intermediate position, (1958) well represented 

 by Fig. 99. Not convinced of the validity of Nieiiwkoop's theory, he simply 

 advocates the double action of the normal inductor. One action is neuralizing, 

 and the other is mesodermalizing. These gradients are in opposition to one another. 



Fig. 99. Scheme illustrating the gradients of the two inducing principles in normogenesis. 



N. - neuralizing principle; M. - mesodermalizing principle; A. - archencephalic region; D. - 



deuterencephalic region; S. - spinocaudal region; Arch. - archencephalon; Oc. - eye; PI. 



placodes; B - balancer (Toivonen and Saxen, ig55b). 



About seven years ago, in order to cope with the results which have brought to light the 

 existence in gastrula stages, of the two distinct agents of induction, I have modified (1952) 

 the proposal primarily formulated in collaboration with Pasteels. In my opinion, Nieuw- 

 koop's activation is an experimental result which does not reveal the true nature of the 

 agents initially operating in the normal germinal system. To conceive of the egg as a mor- 

 phogenetic unity seems to me a logically cogent condition of a coherent representation of 

 these events. Also, without adhering too closely to a principle of embryonic recapitulation, 

 I think it preferable to adopt a view which can easily agree with evolution, whose pro- 

 gression has in any case taken place through the fertilized eggs; it is unsatisfactory to imagine 

 that the primitive equipment of vertebrate eggs would have directly aimed at building up 

 a fore-brain with its highly specialized sense organs. On the contrary, the primary purpose 

 must have been alimentary and motoric, still evident in the predominantly muscular or- 

 ganization of inferior groups. Therefore, I think it plausible to attribute more value to the 

 results of operations and centrifugations, which reveal the primary tendency to notogenesis. 

 This tendency exists inherently in the dorso-marginal zone, which is also the source of the 

 normal inductive agents; in order to stress its primitive, archaic character, I prefer to call 

 it protochordal (P). It is not totally restricted to the "organizer" but extends to the whole 

 ectoblast, where normally it remains latent. In the normal course of events, the protochordal 

 tendency implies that in the area where these agents reach their maximum concentration, 

 either a spontaneous "maturation" occurs (anurans), or morphogenetic kinematics comes 

 to the rescue (urodeles), thus causing the acrogenic conditions f^J to evolve. This acquisition 



