The cell physiology of early development • The time-graded regeneration field in planarians 



Discussion 



OF PAPERS BY (i) C. H. WADDINGTON AND (2) H. V. BR0NDSTED 



Chairman: E. ^euthen 



C. D. Darlington. With regard to the classification of genetic particles, it is important 

 to remember that the biological distinctions between virus and plasmagene (gene- 

 initiated or otherwise) break down at the chemical level. The most recent evidence 

 is especially significant. Kenneth Smith (1952, Biol. Rev. 27, 347-357) has found 

 that cell constituents of the sugar-beet at a certain stage of its development propagate 

 themselves without limit in the cow-pea. They behave like a virus, or provirus I 

 should say, as infection is artificial. Thus a particle or a chemical entity determined 

 by heredity can in the course of development become the means of infection. 



E. W. Temm. The sugar-beet and cow-pea virus described by Professor Darlington 

 seems to imply a transmission of autonomous particles, capable of protein synthesis, 

 from one species to another. In this connexion it is of interest that cellular proteins 

 from leaves of different species are very similar with regard to their constituent 

 amino-acids; we have recently shown that the similarity extends to the cytoplasmic 

 and chloroplastic proteins of the same species. It follows that the amino-acids re- 

 quired for synthesis of cytoplasmic proteins may not vary very much from species 

 to species, and this may in part account for the possibility of interspecific transmis- 

 sion of the type discussed. It is possible that variation of infectivity depends on the 

 activity of the leaf in protein synthesis at different stages of development. 



C. H. Waddington. The example of the cow-pea virus shows that certain embryonic 

 proteins, when transplanted so as to become associated with a foreign nucleus, can 

 multiply in a more or less uncontrolled manner. I agree with Darlington that this 

 implies that the problems of normal cellular differentiation and of pathological 

 virus-like infections belong to the same general area of developmental-genetical dis- 

 course. But it is to my mind much less clear that there is any very close parallel be- 

 tween the processes which bring about the orderly development of locally differen- 

 tiated regions in an embryo and those which underlie the usually pervasive and un- 

 controlled alterations of cell type caused by a virus infection. 



L. Rinaldini. The narrowing down of potentialities that accompanies differentiation 

 suggests a progressive loss of synthetic properties coupled with the appearance of 

 new specialized functions. This loss, if genetically determined, may mean that some 

 genes are systematically inhibited during embryogeny. In Professor Waddington's 

 scheme of cyclic reactions the reaction products (P) would on mass active considera- 

 tions be expected to depress the formation of their precursors, but if P were removed 

 from the cell the reaction would be accelerated, and a type of cell would be obtained 

 which would produce more and more P. By cell division more cells with identical 

 metabolism could be produced, but the excess P shed into the surroundings would 

 inhibit cells from developing in the same direction. In this way we have two parallel 



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