INDUCTION 

 1400 



1200 



GENERAL REMARKS 



477 



1000 



■g 800 



O600 



400 



200 



230 



250 



270 



290 



310 



Fig. loi. The same document as in Fig. loo, but concerning the posterior part of the neural 

 plate, in T. torosus. From Niu, 1956. 



be submitted to analysis. It could easily be ascertained that the induction was 

 not due to cytolysis caused in the reactor by a change of pH, for this remains 

 7.8 i: 0.15. The main interest lies in the spectrophotometric results. Two sets of 

 measurements, made on Triturus tigrinum and T. torosus (Fig. 100, 10 1) both show 

 a minimal absorption at 2400 or 2450A, a maximal at 2580 or 2650A. In both 

 cases, there is a difference between the ist and 2d order media, the maximal 

 absorption shifting from around 2580A to around 2650A. Niu comments on the 

 intrinsic differences between the two cultures, in relation to normal development, 

 and suggests that the first medium contains mainly the neural inductor, the second 

 one mainly an homoeogenetic inductor, causing myoblast diflferentiation. Accord- 

 ingly, there would be some difference between these two inductors. This is con- 

 tradictory to our suggestions concerning retention versus liberation of the inductive 

 agent. Its storage in the middle layer cells may well imply some further trans- 

 formations of the molecular structure. Assuming this step to be reversible, it 

 could explain the chordal islets sometimes included in the neuroepithelium (p. 413). 

 Whether or not the inducing agent is simple or double, spectrophotometric study 

 points to a nucleoprotein. This diagnosis agrees with the results of dialysis, which 

 show that the inducting substances are non-dyalizable, macromolecular compounds. 

 They are also precipitable by ethanol and, according to color tests, contain DNA 

 and RNA, in the ratio i /6. The presence of DNA can be attributed to a certain 

 degree of cytolysis occurring in the deep well cultures. Finally, the effect of enzymes 

 on the conditioned media was tested, but with the difficultly that they could also 



Literalure p. 4% 



