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SUMMARY:The life cycle of a Streptomyces sp. was studied by the Robinow HC1-Giemsa method of nuclear staining, and is described in the following manner:(1) initial nuclear division phase ; (2) primary mycelium ; (3) secondary mycelium (including aerial); (4) the formation of spores. It was observed that the method of nuclear division in the ungerminated spores closely resembled mitosis of certain higher organisms and lower plants, by the appearance of two terminal bodies whose behaviour was similar to mitotic centrosomes. The primary mycelium developed after the initial nuclear division phase and produced side branches, then later gave rise to single swellings in the hyphae. These sweIlings grew to form large round cells, each of which contained many nuclei. It was from these cells that the secondary mycelium developed, a part of which became aerial and terminated in the form of chains of uninucleate spores.Recent developments in the cytology of bacteria have brought forth the publication of many photomicrographs in which nuclear configurations have been interpreted as phases in mitosis (Delamater, 1951 ; Delamater & Woodburn, 1952; Beutner, 1953). But the demonstration of a mitotic spindle in bacteria has been seriously questioned on the grounds that it is possible to confuse artifacts with mitotic spindles (Bisset, 1953). It is the purpose of this paper to present observations of a nuclear division that closely resembles mitosis in higher organisms and of the life cycle of a Streptomyces sp. whose taxonomic position links the bacteria with the moulds (Henrici, 1947).In the course of my work on the genetic effects of radiations on a Streptomyces sp., a cytological study was undertaken to determine the time of nuclear division in the germinating spores, and to observe whether the secondary mycelium arose from distinctive ' initial ' cells similar to those described in actinomyces by Klieneberger-Nobel (1947). In the streptomyces examined it was observed that the first nuclear division was preceded by the formation of two terminal bodies arranged similarly to those already demonstrated in certain bacteria by Delamater (1951) and who interpreted his observations as a mitotic division mechanism. But in the streptomyces spores the terminal bodies were more readily distinguishable. At a certain stage of growth in the primary mycelium the initial cells were seen to develop from swellings in the hyphae, and not from ' nests ' of tangled filaments as was observed by Klieneberger-Nobel (1947) in actinomyces. MATERIAL AND METHODSAn organism, a Streptomyces sp. (laboratory strain T12) isolated from the soil by Dr J. W. Rouatt was used. It produces colonies consisting of a primary mycelium with deep orange colour, and a fluffy layer of paler orange secondary or aerial mycelium.

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