392 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



More should be said concerning the theoretical implications of this inter- 

 pretation, but these comments will be reserved for the final part of this section. 

 The factual contribution of these experiments is evident, and acromerits have 

 been obtained in conditions which could not have been predicted beforehand. 



Another aspect of the same phenomenon is found in the work of Eyal-Giladi 

 (1954). Here, similar folds were used, but were implanted on the ventral territory 

 of the same embryo (Fig. 59, a, b, c). The latter shows a variable deficit in its 

 dorsal zone (Fig. 59, d), and the author has attempted to compare the structure 

 appearing in the folds with those lacking in the main embryo. The analysis is 

 however complicated by the fact that the operation on the dorsal side interferes 

 with morphochoresis. 



We may immediately dispose of the folds obtained on neurulae, on gastrulae with a slit 

 blastopore, and even in the majority of cases concerning gastrvdae with middle to small 

 yolk plug. In these the author asserts a satisfactory agreement between the deficiencies 

 observed in the main embryo and the structures obtained in the folds. The absent brain 

 parts, eyes, nasal pits, ear vesicles seem to be re-found in the ventral protuberance derived 

 from the implant. For stages following the blastopore closure, this balancing only means 

 that induction was already well on the way, and that the structures could be formed, at 

 least qualitatively, in the physiologically-isolated neurectoblast. Their development was 

 probably favored by the fact that, since the lengthy symmetrically-cut piece was exactly 

 folded, each potency could find its equivalent vis-d-vis, without undue interference. 



As a second group, we shall consider the operations performed on relatively "early" 

 gastrulae (Fig. 59, e) . Taking into account table I of Mrs. Giladi's paper and the description, 

 the events can be pictured in the following way. In "some" cases, most of which are not 

 tabulated, the fold became atypical epidermis. In two Pleiirodeles embryos of this category, 

 which were not discarded becavise they were considered as controls, the brain of the main 

 axis was entirely missing. The author considers that all these negative results were due to 

 strips "lying in front of the cranial border of the archenteron roof" (Fig. 60, a). This, 

 however, does not sound so convincing; it may well be that strips of this stage often remain 

 negative in spite of some contact with the archenteron roof. This suspicion is confirmed, 

 in my opinion, by an analysis of her Table I. Among g of the Pleurodeles embryos, 4 show 

 only "mesectoderm", while others yield: (/) dubious telencephalon and paraphysis; (2) tel- 

 and diencephalon, 2 olfactory placodes, epiphysis, ganglion; (j) telencephalon, rudimentary 



Fig. 60. Further possibilities lor cutting the strip for a fold in a rather young axolotl gastrula. 

 (a) Anterior to the archenteron pouch; (b) above the anterior part of the same pouch; (c) 

 entirely above the pouch; (d) in the zone of plain contact with the roof. Modified from 



Eyal-Giladi, 1954. 



