Ill EXTRINSIC MORPHOGHORESIS 363 



(Katayama, 1956), The reaction first appears in the anterior node, in which the 

 material for the notochord is incorporated, then in the neural plate and finally 

 in the somites. 



Recourse to isotopic tracer techniques helps further analysis but again implies 

 an experimental procedure which, however, remains, by its aim, strictly obser- 

 vational. The most useful tracer has been so far DL-methionine-^^S. According to 

 Waddington and Mulherkar (1957), this isotope is incorporated in the induced 

 structures, mainly in the nucleoles, the associated chromatine and in the cyto- 

 plasm. The transmission of inductor to reactor seems to be performed together 

 under the form of amino acid and of a SH-protein, which could be the real 

 inducing factor. In the same laboratory, Pantelouris and Mulherkar (1957) have 

 cultivated the embryos in albumen (New's method) added with radioactive 

 methionine until the 10-14 somites stage. They found that "the neural tube and 

 the notochord show a higher energy uptake than any other structure". By grafting 

 a labelled organizer into a normal embryo, it can be ascertained that the inducing 

 notochord and mesoblast proceed from the implant, but the secondary neural 

 plate or tube shows a substantial activity uptake. This phenomenon becomes 

 negligible if unlabelled methionine is added "as a trap" to the culture medium. 

 "This is evidence for diffusion of free molecules of the labelled amino acid. It 

 also proves that the transferred unit is the amino-acid molecule and not the 

 labelled radical only." On the other hand, by grafting more lateral ectophyll, 

 it may happen that the labelled implant develops in neural tissue adjacent to the 

 primary neural tube, and that the latter shows a gradient of activity. This proves 

 that migration of the radioactive molecules may take place irrespective of whether 

 an induction process occurs or not. "The conclusion seems inevitable, therefore, 

 that induction is not correlated to a transfer of methionine-containing macro- 

 molecules from the graft on any large scale. But, as Waddington and Sirlin (1955) 

 pointed out, this does not exclude the transfer from the graft to the inductively 

 activated cells of a few molecules able to trigger off a chain reaction. If the results 

 of these experiments provide no positive answer to the question of the nature of 

 the inducing factor, they exclude at least the possibility of a large-scale transfer 

 of macromolecules, and perhaps make more likely the alternative of the transfer 

 o£ small quantities of the effective agent". This work is typical of the intricacies 

 of the induction process and shows how necessary it is to consider all its aspects 

 before drawing any definite conclusion. 



In the paper quoted above. Rounds and Flickinger report several experiments 

 in which chordomesoblast previously incubated in Naj^'^CO, was wrapped in 

 non-radioactive competent ectoblast (and reciprocally). After 8 h. contact, both 

 parts, and due controls of these, were homogenised and the radioactivity checked 

 in the TCA-insoluble and the nucleic fraction. The induced ectoblastic fraction 

 showed a much higher level of activity. This result suggests a passage of labelled 

 nucleoprotein molecules from the inductor into the reactor. 



A similar conclusion has been reached, by the same authors, by a serological 

 procedure. A Rana p. inductor was combined with Taricha t. ectoblast. After 4-5 

 days culture the Taricha tissue was separated and tested by serum of rabbits 

 prepared by injections of Rana p. embryos. The tests performed with convenient 



Literature p. 483 



