PREFACE Vii 



theme which was repeated throughout the symposium: the relation- 

 ships between nucleic acids and proteins. Assuming natural selection 

 as a fundamental evolutionary mechanism, Crick postulated in his 

 paper some essential characteristics for any chemical system upon 

 which evolutionary selection has acted. He demonstrated that nucleic 

 acids satisfy the need of a living system to replicate geometrically to 

 some extent and to mutate to stable forms that can be copied, while 

 proteins provide the versatility needed for the performance of many 

 chemical tasks. Hoagland carried this relationship between nucleic 

 acids and proteins forward in his outline of recent progress in elucidat- 

 ing the detailed mechanisms of protein synthesis. 



The symposium moved to the consideration of growth at the cellu- 

 lar level with the papers of Mazia and Puck. Mazia considered two pos- 

 sible schemes for cellular reproduction: (1) fission, wherein every cell 

 element has the power of self-replication, and (2) generative repro- 

 duction, wherein only a few elements are capable of self-reproduction 

 but contain the information necessary for generation of many other 

 non-self -replicating elements. He presented evidence which suggests 

 that the second scheme is the one used by growing plant and animal 

 cells. 



In his paper. Puck showed the exteme sensitivity of the mamma- 

 lian cell to radiation. He reported experiments showing survival curves 

 of mammalian cells in tissue culture which demonstrated that the 

 lethality was due to action on the chromosomes. He also pointed out 

 that all cell functions studied thus far other than those which involve 

 the function of the chromosomes (mitosis, DNA synthesis) are from 

 tens to hundreds of times more resistant to X-rays than is cell repro- 

 duction itself. 



Puck discussed a medium for cell growth which contains only two 

 purified macromolecular fractions. One of these (fetuin) is present in 

 calf fetal serum and seems necessary for morphological changes which 

 are a prelude to reproduction. This substance may be present in the 

 alpha globulin fraction of adult mammalian serum and may be involved 

 in wound healing and in whole-body response to ionizing radiation. 



The cellular level of growth continued to be the focus of attention 

 during the second day of the s>'mposium, when the following papers 

 were read: "Tissue Reconstruction from Dissociated Cells," by A. A. 

 Moscona, University of Chicago; "Cellular Differentiation in the Slime 

 Mold," by M. Sussman, Brandeis University; "The Role of Ribonucleic 

 Acid and Sulfhydryl Groups in Morphogenesis," by J. Brachet, Uni- 

 versite Libre de Bruxelles; "Crowth and Development of the Inflores- 

 cence and Flower," by C. W. Wardlaw, Manchester University; "Origin 



