394 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



some cavities, was found (Fig. 61, e). From this picture, it can be admitted that these 

 reactions might represent a young prosencephalon, but it can also be remarked that it 

 is not formed as a small neural plate which closes by a converging movement of its borders, 

 but by a process of delamination and neuralization of the inner layer. From this point of 

 view, the two to three days delay is certainly of importance. In my opinion, there may be 

 something more than the simple induction exerted by the passing archenteric roof. 



Leaving this intimate aspect of the process, we may come to the third group, 

 in which the same meaningful manifestations will again be met. This group 

 concerns the "middle*" gastrulae, and probably some of the yolk plug stages. 

 Here the region from which one or two strips were cut (Fig. 59, f ) is the one 

 which, if the embryo had remained intact, would have formed notoneural parts. 

 This situation is not always reflected by considerable deficiencies observed in. the 

 main netiral system, becatise high ctits interfere with the forward migration of 

 the archenteric roof and, the gap having a tendency to expand, no anterior 

 brain has been formed. Giladi believes that there is a closer correspondance between 

 implant and defect because, I think, she did not realize that the presumptive 

 value of the strip could not be deduced from the deficiencies due to its ablation. 

 This slight rectification, however, only makes the morphogenetic activities of the 

 graft more significant, as appears from the somewhat simplified presentation in 

 Table i of the results concerning Pleurodeles middle gastrulae. It is patent that in 

 8 of the 1 1 cases, acrencephalic structures have been found in the fold. 



They are, however, not of a uniform type, and the author has noted these variations, 

 in this series and in others, without proposing an explanation. The three non-prosence- 

 phalic results merit a brief comment. I is simply negative or unreadable; F has only 

 produced a rhombencephalic neural mass, which seems to fit exactly with its origin; 

 K has formed a segment of spinal cord, while the main axis has lost only the hind part of 

 its medulla and the anterior of its spinal cord. One may be surprized that in such cases, 

 where according to the author's and Nieuwkoop's ideas the "transforming" action has just 

 replaced the "activation", no prosencephalic structure is apparent. However, Giladi states 

 that in embryo L, and also in one of her axolotl embryos, "the graft behaved very strangely, 

 forming a "neural mass which showed developmental tendencies in both prosencephalic 

 and rhombo-spinal directions at the same time". This bare assertion is however not too 

 precise, for in L no rhombencephalon, spinal cord or ear vesicle is recorded present, and, 

 in the axolotl, the only result mentioned, is "mixed brain structure". 



In her discussion, this atithor has raised many aspects of the kinematic and dy- 

 namic activities of amphibian morphochoresis. These points have either already been 

 considered in this chapter or will be discussed later on. But we cannot leave Giladi's 

 important paper without quoting her clearly-expressed view of Nieuwkoop's theory : 



"The invaginating anterior part of the archenteron roof in which the inducing factor plays 

 the chief role, on establishing contact with the neuro-ectoderm, causes therein a wave of 

 activation (Nieuwkoop et al., 1952) which, when exceeding a certain treshold, enables 

 the neural competence of this area to realize itself. This wave of activation right from the 

 beginning takes the form of a neural field with a very definite spatial configuration, and 

 leads to the formation of a prosencephalon. This field has a very dynamic character, and 

 is constantly moving in an anterior direction during the period of invagination. Conse- 

 quently, every cell in the presumptive neural area constantly changes its relative position in 

 the field, and at the same time its potency is altered. As invagination advances, the most 

 posterior part of the presumptive neural plate comes to lie above more caudal parts of 



