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MAX LUDWIG HENNING DELBRÃCK 79 demic criteria, this entailed certification of "political maturity" following participation in "free discussion" groups at a Nazi indoctrination camp. His failure to display sufficient "maturity" at two sessions, probably as a result of too much frankness, made it clear that a university career would not be open to him in the foreseeable future. Finally, in 1937 the Rockefeller Foundation offered him an unsolicited Fellowship (Biology) to travel abroad, so he took this opportunity to visit the California Institute of Technology in order to learn Drosophila genetics from Thomas Hunt Morgan and his world-famous group. THE BACTERIOPHAGE EPOCH (1937-53) Early Days at Caltech Max's initial introduction to Caltech was frustrating and disappointing, despite the help of A. H. Sturtevant, and Calvin Bridges with whom he was especially friendly, he found the highly specialized Drosophila jargon too difficult and exacting to grasp, let alone master, in a reasonable time. One day he inadvertently failed to attend a seminar on bacteriophages by Emory Ellis, and went to him to find out what he had missed: "I had vaguely heard about viruses and bacteriophages, and I had read the paper by Wendell M. Stanely on the crystallization of tobacco mosaic virus before I had left Germany. I had sort of the vaguest notions that viruses might be an interesting experimental object for a study of reproduction at a basic level" (1). Ellis showed him the very rudimentary materials and the simple techniques needed for his experiments, and Max saw for the first time the small macroscopic areas of clearing, or plaques, on a lawn of bacterial growth on solid culture medium, each plaque representing the multiplication of a single virus particle. Ellis also demonstrated
MAX LUDWIG HENNING DELBRÃCK 80 some step-growth curves revealing the kinetics of a cycle of phage multiplication in newly infected bacterial populations. According to Ellis, Max's first comment was, "I don't believe it" (2, p. 53); but Max's own recollection was, "This seemed to me just beyond my wildest dreams of doing simple experiments on something like atoms in biology [which perhaps means the same thing!], and I asked him whether I could join him in his work, and he was very kind and invited me to do so" (1). So began what has been called "The Phage Renaissance." Before this, d'HÃ©relle's initial studies demonstrating the particulate and viral nature of phage had been followed by the highly original investigations of F. M. (later Sir Macfarlane) Burnet and Martin Schlesinger, which laid the foundation of modern phage research, but Schlesinger died prematurely in 1936 and Burnet changed his field shortly afterwards; neither left disciples to carry on their pioneering mission. Moreover, after working for a year with Max, Ellis returned to his original work on malignant tumors of mice. So Max was left alone, and alone was responsible for giving continuity to phage research by founding and guiding an expanding, if loosely knit, lineage of phage workers that sowed the seeds that finally blossomed into modern molecular biology (see 5). One of Max's first contributions, in his early days with Ellis, was to bring his analytical approach and mathematical knowledge to bear on study of the phage life cycle. For example, formulae were devised to check the rate of adsorption of free phage to bacteria under various experimental conditions, while the then unknown proportion of free particles able to produce plaques (the plating efficiency) was assessed by the application of Poisson's statistics of random sampling. In their only paper together, Ellis and DelbrÃ¼ck (1939) invented and greatly refined the