80 GROWTH AND MORPHOGENESIS 



versene itself is inactive), the first visible result is that the ectoderm 

 cells dissociate, lose their basophilia and finally cytolyze (Brachet, 

 1959). The consequence is the formation of "ectodermless embryos", 

 which have well-differentiated chorda and somites, but no nervous 

 system (Fig. 31, p. 76) or a very reduced one. It was also found in 

 further experiments that cells of the organizer exhibit a marked 

 differential susceptibility towards the ribonuclease-versene mix- 

 ture. An explanted organizer cytolyzes much more quickly in this 

 medium than explanted ventral mesoblast. 



The fact that it is possible, after gastrulae have been treated with 

 ribonuclease and versene, to obtain embryos which have no nerv- 

 ous system should not be taken as a proof that RNA is necessary 

 for inductive processes in the normal organizer. Of course, even a 

 fully active organizer cannot produce its inductive effects unless the 

 ectoderm cells of the reacting system are present. In the experiments 

 just described (Brachet, 1959), the absence ofinduction is obviously 

 due to the pealing off of the ectoderm cells. Nevertheless, the ex- 

 periments have some interest in suggesting that the cement or ma- 

 trix which holds the ectoderm cells together might be of a ribonu- 

 cleoprotein nature. We shall return to this point a little later in this 

 chapter. 



Since it is clear that it is not an easy task to establish, with ribonu- 

 clease as a specific agent, whether RNA is directly involved in in- 

 duction or not, more indirect methods of attack must be employed 

 in the future. Studies of the effects exerted by specific analogues 

 (which inhibit RNA or protein synthesis) on the inducing activity 

 of explanted or implanted organizers might be rewarding for that 

 purpose. 



5. PHYSICAL PROPERTIES OF THE INDUCING AGENT: 

 THE POSSIBLE ROLE OF MICROSOMES IN INDUCTION 



We owe to Raven (1938) the demonstration of the important fact 

 that the inducing principle can diffuse from cell to cell. If a non- 

 inducing fragment of presumptive ectoderm is left for some hours in 

 contact with the living organizer, it acquires inducing capacities. 



