Ill 



PRIMARY INDUCTION EXPERIMENTAL DATA 



403 



Fig. 66. Schematic representation of the processes in the sandwich considered in Figs. 

 64-65; (a) the arrows suggest "activation" by the implant (center) and by the external 

 medium acting on the exposed inner layer of the ectoblast; (b) spreading of these activating 

 processes; (c) formation of areas with different differentiation tendencies; central arrows, the 

 "transforming" action; oblique hatching, central neural area; dots, mesectoblast and 

 activated epiblast; (d) neuralization phase, with the full complex of results, with rhom- 

 bencephalon inside, acrencephalon outside, olfactory placodes (circles) surrounding the 

 organizer center; outside the vindifferentiated epiblast an olfactory placode (wheel, right) 

 takes its origin from the independent activation mentioned in (a). From the same authors 



as Figs. 64-65. 



Fig. 67. Conventional representation of the '"activating" and "transforming" effects exerted 



by explants of the successive medial parts of the archenteric (lower row) roof. Plain line, 



"activation"; broken line, "transformation". From Sala, 1954. 



Lileralure p. 483 



