514 J. BRACKET 



Matthews,^^^ who found that 8-azaguanine is incorporated in TMV virus 

 PNA, with a simultaneous loss of infectious power, in agreement with the 

 results with thiouracil obtained by Jeener and Rosseels.^" These findings 

 have been extended by Lesnitzki, Matthews, and Smith^^^ to various animal 

 cells and bacteria. Jeener, Lemoine and Lavand'homme's^" experiments 

 on the production of a nonvirulent protein antigenically related to the 

 virus, have also been confirmed and extended by Commoner et al}"^^ 



Regarding the role of PNA in embryonic development, Osawa and 

 Hayashi'"^ and Takata-^" have confirmed, by direct chemical analysis, the 

 cytochemical observations of Brachet^* on the PNA distribution in oocytes, 

 as well as in gastrulse and in neurulse. Of special interest is the finding 

 by Niu and Twitty^^^ that explanted chordomesoderm produces in the 

 medium a neuralizing agent: they have found that isolated ectoderm 

 differentiates nerve fibers and pigment cells when cultivated in the saline 

 medium in which chordomesoderm has been explanted for a few days. 

 This medium shows a strong ultraviolet absorption typical of nucleic acids. 

 A comparable finding is that of Levi-Montalcini and Hamburger,^^^ who 

 discovered a diffusible agent from mouse sarcoma that produces hyper- 

 plasia of sympathetic ganglia. According to a recent report by Cohen, 

 Levi-Montalcini and Hamburger,-'^^ this agent is accumulated in the 

 microsomes, is precipitated by streptomycin, and shows a typical nucleic 

 acid ultraviolet spectrum. There is little doubt that, in Hamburger's case, 

 the active agent is a PNA. Favorable effects of PNA on the regeneration of 

 Planarians have been reported by Br0nsted and Br0nsted,^^* while nucleo- 

 proteins (but not PNA alone) have been found to stimulate growth in 

 tissue cultures (Kutsky,^^* Maganini et aP^). 



While these findings are all in favor of an important role of PNA in 

 morphogenesis and cell multiplication, Elson and Chargaff^" have, how- 



"" R. E. F. Matthews, J. Gen. Microbiol. 10, 521 (1954). 



"^■^^ I. Lasnitzki, R. E. F. Matthews, and J. D. Smith, Nature VIZ, 346 (1954). 



^" R. Jeener, P. Lemoine, and C. Lavand'homme, Biochim. et Biophys. Acta 14. 



321-334 (1954). 

 22* B. Commoner, Y. Yamada, S. D. Rodenberg, Tung-Yue Wang, and E. Easier, Jr., 



Science 118, 529 (1953). 

 2" S. Osawa and Y. Hayashi, Science 118, 84 (1953). 

 "0 A. Takata, Biol. Bull. 105, 348 (1953). 



231 M. C. Niu and V. C. Twitty, Proc. Natl. Acad. Sci. U. S. 39, 985 (1953). 



232 R. Levi-Montalcini and V. Hamburger, J. Exptl. Zool. 123, 233 (1953). 



2" S. S. Cohen, R. Levi-Montalcini, and V. Hamburger, Proc. Natl. Acad. Sci. U. 



S. 40, 1014 (1954). 

 23" A. Br0ndsted and H. V. Br0ndsted, J. Embrijol. Exptl. Morphol. 1, 49 (1953). 

 236 R. J. Kutsky, Proc. Soc. Exptl. Biol. Med. 83, 390 (1953). 



236 H. Maganini, A. W. Schweitzer, and G. M. Hass, Federation Proc. 12, 453 (1953). 



237 D. Elson, T. Gustafson, and E. Chargaff, J. Biol. Chem. 209, 285 (1954). 



