390 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



that "there were no indications of a displacement of cells inside the fold". The implant 

 takes the shape of a toadstool, with three perceptible regions. The basal one contains 

 "neural material only", the transitional region consists of neural crest and epidermal 

 material, and the distal region is composed of atypical epidermis. 



When the 1952 embryos were reared a sufficient time to show discernable brain parts 

 and sense organs, the fate of the implant followed a definite rule. In cases where it is 

 finally connected with the fore-brain, most of the grafted tissue takes part in the formation 

 of the prosencephalon or, more frequently, builds acrencephalic parts supplementing the 

 normal ones. When the graft shows final connections with the hind-brain, the proximal 

 part of the implant is of rhombencephalic character, but the distal part sometimes contains 

 parts of the acrencephalon. It should be added that several cases in which the implant 

 combined with the brain show a striking reduction in, or even absence of, the host prosen- 

 cephalon. When using the middle yolk plug- or preneurula stages, implantations in a 

 more trunkal site could be obtained. Sometimes, a reduced acromerit appeared together 

 with deutomeritic structures. The results remained about the same for implantations on 

 the early- or even middle-neurula stages. One or two cases are described in which implants 

 which were connected with the hind part of the spinal cord seemed to have formed a small 

 acro-deutomerit complex. 



Complementary results have just been published (Nieuwkoop, 1958) concerning the 

 same experiment, repeated on axolotl embryos, a species with a specially sensitive ecto- 

 blast. Presumptive cranio-ventral ectoblast at various stages has provided folds always 

 grafted in the deutomerit territory of young neurulae. Planimetry of organs has allowed 

 some quantitative appreciation of the inductive effect that was obtained. As soon as the 

 blastopore of the donnor becomes circular the competence of the ectoblast abruptly falls, 

 with the interesting feature that the acrencephalic structures still appear in the tip of the 

 fold but soon regress into mesenchyme. Even this labile reaction vanished at the next 

 small yolk plug stage (Harrison's stage 12) and only rhombencephalic structures, if any, 

 are then observed. Thus, the diffusion of some inducing factor(s) through the basis of the 

 fold seems granted but, at the critical stage, the apex of the fold is still submitted to 

 "activation" but not stabilized by the "transformation" process. The author invokes a 

 fall of the competence threshold but discards Gallera's idea {cf. p. 423) of a gradual im- 

 permeabilisation of the reacting ectoblast. 



-....' 



.*'■■*"-' 



Fig. 58. Diagrammatic representation of the primary activation and subsequent interaction 

 of transforming mesoblastic influences, in folds of competent ectoblast attached to acro- 

 merit, deutomerit, cormomerit. Hatched, the archenteric roof; plain pointed lines, the 

 activating factor penetrating the folds ; crosses, the transforming factor, most dense near the 

 blastopore. From Nieuwkoop et al., 1952. 



