Ill 



PRIMARY INDUCTION 



EXPERIMENTAL DATA 



391 



Fig. 59. Ventral implantation of a fold formed of presumptive "activated" neurectoblast 

 of Urodele gastrula. (a-d) The operation procedure; (e-g) sagittal sections of successive 

 stages to show the areas utilizable for this exploration, (a) A lengthy piece is cut from the 

 dorsal part of the gastrula, above the probable limit of invagination, (b) The piece is folded 

 under the weight of a sheet of agar and a glass bridge, (c) The same egg is placed in an 

 appropriate cavity, a slit is made on its medio-ventral side, and the fold caused to adhere 

 by the weight of glass rods, (d) The operated embryo, before healing of the dorsal wound 

 (a variable event), (e) The extent of the zone of an early gastrula which had contact with 

 the immigrating archenteron; only one location can be used for the fold at this stage; (f^ 

 middle gastrula, with curved blastopore, a stage where contact allows cutting the fold at 

 two different levels; (g) advanced gastrula, with a yolk plug, a stage which authorizes 

 cutting the fold at three different levels, with corresponding tissues of "activating" and 

 "modulating" contacts. From Eyal-Giladi, 1954. 



An interpretation of these data, which certainly strike out a new line in this 

 field, has been influenced by results obtained with explantations and abnormal 

 inductors, experiments to be considered later (p. 396 and 419). The induction 

 of acrencephalic structures is attributed to an activation of the ectoblast, i.e., to the 

 "liberation of prosencephalic differentiation tendencies" (1952, his p. 86). This 

 activation is considered as the first step of induction ; a second step is imagined 

 to be a "counteraction", according to suggestions presented by Lehmann (1950) 

 and by Yamada (1950b). The counteraction expresses the inductive action of 

 the chordomesoblast and is responsible for the notoneviron (Fig. 39, p. 368). 

 Such an appHcation of the experimental data to the normal course of morpho- 

 genesis leads Nieuwkoop to represent the differentiation of the implants like in 

 Fig. 58. Thus the two inductive factors are not only assumed during the process 

 of gastrulation, but are also implied in the later influence of the archenteric roof 

 on the implanted folds. The figure shows the activating principle expanding from 

 the neural plate, and probably being transmitted along the ectoblastic layer. 

 However, in 1954, Nieuwkoop (in: Nieuwkoop and Nigtevecht, 1954, their p. 187) 

 seems to see the activating source in the chordomesoblast, with which the foot of 

 the implanted fold necessarily comes in contact. 



Literature p. 481 



