412 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



The time factor was also considered by Johnen (in Nieuwkoop et al., 1952) when she 

 repeated the insertion of anterior notochordal material in a sandwich and extirpated the 

 implant after various delays. A prolonged contact produces "rather big inductions with 

 central rhombencephalic and peripheral prosencephalic structures". With short exposures, 

 it is possible to obtain an exclusively acrencephalic induction. The necessary time of 

 contact is much shorter in the axolotl (even 5 min.) than in the newt (at least 4 h.). 

 Longer exposures cause deutencephalic structures to appear. In both cases, neural crest 

 derivations are often present and sometiines are the sole sign of induction (Johnen, 1956). 

 A variant of Damas' experiment (p. 371) has been recently executed in his laboratory on 

 Pleurodeles embryos (Denis, 1957). Pieces of (non-induced) gastrular neurectoblast were 

 implanted in the flat neural plate just behind the cerebral region. After 3 h., 6 h., . . . 24 h., 

 they were again isolated and either reared in sandwiches, or implanted in the abdomen 

 of advanced gastrulae. In both cases, the sole signs of induction have been either neural 

 crest derivates or a piece of neural tissue of the spinal cord type (never cerebral) or often 

 both. The differentiation is much better in the grafts than in the explants. With short 

 exposures, a gradation clearly related to time does not appear in these conditions. After 

 3 h. of induction, in some cases neuralization can be as manifest as when the sojourn in 

 the neural tube has been longer. 



Ultraviolet irradiation of the dorso-marginal zone, with a variety of locaUzations, 

 has been actively used from 1933 to 1941 (Brandes, 1940, 1941) with appreciable 

 results. Brandes, particularly, has made an extensive study on several species and 

 at various stages. An effect on morphochoresis is evident, but it varies according 

 to the irradiated territory and to the duration of the treatment. The most 



Fig. 73. Effect of U. V. irradiation of the organizer on the conformation and differentiation 



of the neural organ, (a ) Reconstruction of a normal axolotl embryo, the neural organ seen in 



median sagittal section; (b) the neural organ of the irradiated specimen, with the irregular 



lumen and dorsal thickening. From Brandes, 1941. 



