Ill PRIMARY INDUCTION EXPERIMENTAL DATA 375 



of the roof is clearly responsil^le for the fate of the overlying neurectoblast^ 

 Parts of the neuroepitheliiim isolated or transplanted at this moment show an 

 ability to form brain, eyes, spinal cord, etc., autonomously, according to their 

 origin. On the other hand, the normal duration of this process of primary in- 

 duction can be somewhat shortened. If, in a late gastrula, the neurectoblast 

 is completely replaced by ectoblast of the young gastrula, a normal embryo may 

 be obtained (Waddington and Deuchar, 1952): "There was no question but that 

 the presumptive ectoderm was able to develop into a regionally differentiated 

 neural axis, even though it had never experienced archenteron roof actually 

 invaginating beneath it" {ibid., their p. 502). 



For several springs, Gallera continued his analysis, performing operations that 

 were suggested to him by the investigations of Nieuwkoop, which will soon be 

 extensively considered (p. 389, 390). The attention of Gallera was now focussed 

 on the legitimacy of Nieuwkoop's conceptions of the induction process as con- 

 sisting of two successive phases, activation followed by transformation. These 

 new experiments, combining several territories of three embryos, sometimes of 

 different age, and using in vitro cultivation together with grafting on the ventral 

 region of hosts, are rather complex, but indicate the following two points. On 

 one hand, Gallera (1958a) considered whether ectoblast having undergone 

 induction by the anterior chordal part of the archenteron roof (and conse- 

 quently quite able to proceed by itself along the ways of deutogenesis by yielding 

 a rhombencephalon) can, if temporarily grafted over a prechordal plate, still be 

 induced into an acrencephalon; such was the case, which would mean, in 

 Nieuwkoop's interpretation, that activation could still prevail on transformation. 

 On the other hand, Gallera (1958b) sought for the incipient inducing effect of this 

 anterior chordal part of the archenteron roof on the corresponding presumptive 

 ectoblast, but considered at successive ages from middle aged gastrula to young 

 neurula; the author found that acrencephalic structures are only obtained when 

 an age discrepancy exists between the inductor and the reacting ectoblast, a 

 conclusion important for appreciating Nieuwkoop's position. 



With the last group of experiments we have begun to consider operations made 

 on the gastrula. Several other results relative to this crucial stage must also be 

 examined. All of them bear more or less directly on our main problem: what is 

 the mechanism responsible when an acromerit rather than a notomerit (Fig. 39) 

 is obtained? 



In 1942, Okada and Takaya, also working on newt embryos {Triturus pyrrho- 

 gaster), discovered that the same presumptive material did not have an identical 

 inductive capacity when dissected before or after its invagination. The method, 

 borrowed from Schechtman (1938) (Fig. 45), consisted of isolating a piece of 

 tissue from just above the blastoporal groove of a young gastrula and dissecting 

 a fragment of the same size from the wall of the blastocoele. Both pieces were 

 for a short period intimately superimposed on the agar bottom of the dish; then 

 the complex was used to close the gap created in the blastocoele wall. A variant 



^ For a recent examination of the regulative aptitude of the archenteron roof after regional 

 rotations and exchanges, with deviation or reversal of the axes, cf. Sladecek, 1953, 1955. 



Literature p. 483 



