Ill 



PRIMARY INDUCTION 



EXPERIMENTAL DATA 



401 



cultivated in vitro for some hours began to elongate and, if left for sufficient time, became 

 club shaped, and eventually formed a pigmented streak. In this state, they were again 

 wrapped in ectoblast and reared. Their ability to form notochord considerably decreased 

 and they often induced forebrain. Thus, they had been modified as if they had performed 

 invagination. 



A similar inquiry has now been realized in axolotl gastrulae (Hoessels, 1957). The area 

 of the blastopore lip, representing the prechordal plate, was dissected at successive stages 

 and reared in ectoblast sandwiches. At the youngest stages of gastrulation, the induced 

 structures were of notogenic character, but, as soon as the material had invaginated, the 

 character became either acrencephalic or neuroid. It was also found that the roof of the 

 incipient archenteron, i.e. presumptive pharyngeal endoblast, induced acrencephalon. 

 On the other hand, it seemed impossible, by this technique to isolate from invaginated 

 tissue a purely notochordal explant ; prechordal material always appeared simultaneously. 



Fig. 64. In vivo aspects of sandwiches formed of the anterior part of the neurula notochord, 



between two pieces of ectoblast. The species used were newt and axolotl. (a) 21 h., (b) 70 h., 



(c) 96 h., (d) 1 1 days after the operation. From Nieuwkoop and Nigtevecht, 1954. 



Thus, there is good agreement between implantations and explantations in 

 showing that the organizing and inducing abihties of the dorso-marginal zone 

 change in Urodeles during the transit through the blastoporal lip. The prechordal 

 potency does not preexist, but effectively appears during the first steps of gastru- 

 lation. In my opinion, this change is not simply brought about by an aging 

 process, but by the more special process of the accomplishment of kinematic 

 performances. These are not suppressed during cultivation in vitro, although they 

 may be appreciably reduced. 



But an other relation has been recently disclosed. The fate of this material 

 does not only depend on its age and location, but also on the tissue in which it 

 is wrapped. If one uses, in place of gastrular ectoblast, neurular epiblast, no 

 chorda is formed by the uninvaginated blastoporal lip, but simply endoblast 

 (Kato and T. S. Okada, 1957.) Would it be because it could not release its 

 inducing substances? 



Literature p. 483 



