PHYSICAL PROPERTIES OF THE INDUCING AGENT 83 



This question has been taken up more recently by Rounds and 

 Flickinger (1958) and Flickinger et ah (1959), who worked with 

 chemical and immunological methods. Their experiments indicate 

 that in explantations there is a definite (but quantitatively small) 

 transfer of nucleoproteins from mesoderm to ectoderm. Of special in- 

 terest are experiments in which Taricha ectoderm was cultivated in 

 contact with Rana mesoderm. Serological tests showed the pre- 

 sence of Rana antigens in Taricha ectoderm, which again indicates 

 a passage of nucleoproteins from mesoderm to ectoderm. It cer- 

 tainly would be very interesting to follow cytochemically this pro- 

 cess with methods involving labeled antibodies. 



These autoradiography and serological experiments are in good 

 agreement with our earlier results with vital dyes, since staining was 

 only visible in cells which were adjacent to those of the stained ex- 

 plant. In this case also, there was no large scale transfer of micros- 

 copically visible inclusions. The available results thus strongly 

 suggest that exchanges of substances, even of a macromolecular 

 size, occur at the interface separating the inductor from the reacting 

 cells at the late gastrula stage. All these experiments emphasize the 

 importance in induction of either direct contact between cells (Weiss, 

 1947) or of the intracellular matrix (Grobstein, 1956, 1958). These 

 questions will now be discussed in more detail. 



In a very stimulating paper published in 1947, Weiss suggested 

 that an intimate contact between the organizer and the presumptive 

 ectoderm is required for successful induction. The cell membrane 

 would be the main site of the inductive processes. Such a conclusion 

 is, of course, in agreement with all that has just been said about the 

 diffusion of vital dyes and labeled ribonucleoproteins during in- 

 duction. 



There is no doubt that neural induction can be completely sup- 

 pressed when a cellophane membrane is placed between explants of 

 organizer and ectoblast (Brachet, 1950). Induction stops abruptly 

 when the membrane has interrupted direct contact between the 

 two fragments. Since the membrane used is easily permeable to 

 mononucleotides and slowly permeable to RNA of a molecular 

 weight of about 10,000, it can be concluded that the induction is not 



References p. 90/93 



