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MAX LUDWIG HENNING DELBRÃCK 96 pisms. In addition, other mutants affecting the pathway of carotene biosynthesis have been obtained. Classification of these mutants according to their functional and sequential relationships is clarifying the organization of their underlying sensory pathways (review 15). Although it is true that no major breakthrough has been made in understanding the basic mechanism of sensory transduction, this is also the case for other systems. It has been suggested that progress might have been quicker if more effort had been directed toward developing the basic genetics and biochemistry of Phycomyces during the early period. Only the physiological aspects were then energetically pursued, resulting in a lot of models unsupported by strong experimental evidence (A.P.E.). Although Max remained dedicated to Phycomyces from 1953 onwards, he did not lose touch with phage research. Thus, with N. Visconti he developed a mathematical model of phage recombination based on multiple rounds of mating during the eclipse period (1953) while, a little later, he became interested in theoretical problems of DNA replication (1945b, 1957) and the genetic code (1958b). The Cologne Interlude (1961-63) After the war Max returned to Germany on several occasions, first in 1947, and then in 1954 when he visited GÃ¶ttingen for 3 months. In 1956 he was invited to spend 3 months at Cologne by Josef Straub, who was Professor of Botany at the University and wanted Max to bring molecular genetics to his new institute which was among the first being built at that time. Max gave a phage course in the unfinished new building, still without electric light or concrete floors, "which was quite a tour de force" (1). It was during this course, at which Peter Starlinger (now Director of the Institute) came from Hamburg to give a seminar, that the
MAX LUDWIG HENNING DELBRÃCK 97 idea took root of a Genetics Institute embracing several independent, integrated groups headed by professors, but having many facilities in common and an emphasis on research. This was a very novel concept for Germany and it was hoped that Max would agree to become the first director so that his reputation could be used in negotiations with the Government; but Max agreed for 2 years only, on leave of absence from Caltech, in the unlikely event of the project materializing. A first step was the appointment of Carsten Bresch to a Chair of Microbiology in the Botany building, where he was joined by Rudi Haussmann, Peter Starlinger, Thomas Trautner, and A. H. Doermann who spent a sabbatical 1957 with them (P.S.). Then in 1959, thanks to the extraordinary negotiating ability of Josef Straub, the Institute was finally approved. During the developmental stages, Max visited Cologne about once a year to discuss plans for the future, and succeeded in obtaining funds from a semiprivate organization for two additional senior staff appointments which the university could not afford. The Institute of Genetics building was eventually completed, and the staff moved during the summer of 1961. The Institute was formally dedicated in June 1962, with Niels Bohr as the principal speaker. His lecture, entitled ''Light and lifeârevisited," commented on the original one of 1933, which had been the starting point of Max's interest in biology. It was to be Bohr's last formal lecture. He died before completing preparation of the manuscript of this lecture for publication (but see DelbrÃ¼ck 1976; also 10). Max organized four groups of workers, under Carsten Bresch, Walter Harm (radiobiology), Peter Starlinger and Hans Zachau. In addition, he formed a group of his own which, surprisingly, he devoted to the study of the photochemical effects of ultraviolet light on DNA which had in