86 GROWTH AND MORPHOGENESIS 



There is, however, a certain amount of evidence for the view that 

 the induction of neural structure can be obtained also in ectoblast 

 cells treated with soluble agents, without direct contact as in the 

 living organizer or killed tissues. We refer now to the above-men- 

 tioned experiments of Niu and Twitty (1953) and Niu (1956), who 

 found that if chordomesoblastic and ectoblastic fragments are cul- 

 tivated side by side but without direct contact of the explants, 

 neuralization of the organizer fragments liberates a diffusible sub- 

 stance which has the spectroscopic properties of a nucleoprotein. 

 As we have already mentioned, the "conditioned medium" in which 

 this nucleoprotein has accumulated is itself neuralizing. In the ab- 

 sence of any chordomesoblastic cell, it produces neural differentia- 

 tion in ectoblastic explants. 



We have seen that in more recent experiments of Niu (1958a, b) 

 the mere addition of ribonucleoprotein or even of RNA from thy- 

 mus can produce the neuralization of ectoderm explants. It is inter- 

 esting to note that in the case of RNA the inducing activity is 

 markedly increased when a protein, which is inactive by itself, is 

 mixed with the nucleic acid. This fact strongly suggests that the up- 

 take and subsequent neuralization of the explants is linked to a 

 pinocytosis mechanism which is induced by the protein. We have 

 observed that in organisms which are capable of pinocytosis {e.g. 

 amoebae), the uptake of RNA is greatly increased if a protein is 

 added to the medium in order to induce pinocytosis. A careful 

 study of pinocytosis in ectoderm explants placed in various experi- 

 mental conditions might be rewarding and might throw some light 

 on the mechanisms of induction. 



Interesting as they are, these experiments do not carry entire con- 

 viction because of the risk of sublethal cytolysis which is always 

 present. Niu and Twitty (1953) and their followers were of course 

 aware of this danger and they thought that they could avoid it. But 

 the fragility of gastrula cells is such that it is extremely difficult to 

 avoid death or injury of a few of the explanted cells even when 

 one works very carefully. Furthermore, results obtained with 

 artificial systems such as those used by Niu and Twitty (1953) and 

 Niu (1956, 1958a, b) can give little information about the processes 



