A correlative study was made of some structural and functional features of a reasonably synchronized culture of Bacillus cereus strain terminalis during spore formation and maturation. The successive stages of development could be recognized by phase contrast microscopy and in electron micrographs of ultra-thin-sectioned cells. Attempts were made to correlate these changes with the acquisition of heat resistance and the synthesis of dipicolinic acid. The outer coat of the spores was observed to be formed first around the forespore; the exosporium, cortex, and inner coat then appeared sequentially and independently of existing sporangial membranes. Dipicolinic acid synthesis began in the early transitional stage, just after forespore formation, and reached one third of the maximum level before an increase of heat resistance in the population was detectable, indicating the possibility of a correlation only above a threshold level of the compound.

Spores of Bacillz~s ccrezLs strain terminalis formed "endotrophically" by transferring-granular vegetative cells to distilled water were found to be relatively susceptible t o heat and deficient in dipicolinic acid. Calciuin ions alone, added in low conceiltratio~l shortly after the cells were placed in water, could completely relieve these abnormalities. Altho~rg-11 the water-formed spores were sensitive to heat, they were as fully resistant as normal spores to gamma radiation or phenol.

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The cytoplasmic fibrils of Treponema refringens were studied in situ by electron microscopy of thin sectioned and negatively stained cells. From 5 to 21 parallel fibrils ran through the cell in a band adjacent to the inner side of the cytoplasmic membrane, on the inner sides of the curves of the spirochete. The nuclear areas of cells were adjacent to the fibrils. Cross sections of fibrils isolated from cells which had been lysed were polygonal and not uniformly electron dense. Polyacrylamide gel electrophoresis of partially purified fibril preparations indicated their main component to be a protein with a molecular weight of 97,000. Fibrils were solubilized by 1% trypsin, 1% pronase, 6 M urea, 1 N HCl, 0.005 N NaOH or 1.3% sodium dodecyl sulfate. By electron microscopy of negatively stained isolated fibrils, each fibril was found to be a complex arrangement of strands rather than a single tubule.

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