1. During the course of growth and sporulation of Bacillus subtilis in chemically defined media, measurements were made of 16 different parameters, including the specific activities of nine intracellular enzymes. 2. Towards the end of exponential growth, proteolytic activity increased and reached a maximum soon after growth ceased. 3. In the presence of an excess of phosphate the specific activity of alkaline phosphatase increased fivefold at the end of exponential growth. 4. The specific activity of malate dehydrogenase remained at a high constant level throughout sporulation, but the specific activity of fumarase showed a two- to three-fold increase 5–9hr. after the end of exponential growth. 5. Aconitase activity was barely detectable during exponential growth in a glucose–glutamate medium, but increased rapidly when glutamate was replaced by citrate or when the glucose in the medium was exhausted. 6. The specific activity of alanine dehydrogenase increased threefold 1–5hr. after the end of exponential growth. 7. The specific activity of soluble NADH oxidase doubled 4–6hr. after the end of exponential growth. 8. Glucose dehydrogenase was undetectable until 4hr. after the end of exponential growth, but its specific activity increased 20-fold over the next 3–4hr. 9. The onset of refractility, the synthesis of 2,6-dipicolinic acid and the appearance of heat-resistance occurred in this order some 6–12hr. after the end of exponential growth. 10. The significance of these changes is discussed in relation to the morphological development of the spore.

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