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In recent years the bacterial endospore has become a popular object for investigation, and much has been learned about its structure, its chemical composition, the cytological processes and physiological factors involved in its formation and germination, and its resistance to various physical and chemical agents. Unfortunately, fundamental investigations on physiology, composition, and resistance have been handicapped by inadequate information on the state of the spores investigated. Are they all similar? If not, is their dissimilarity inherent or acquired because of environmental influences? Finally, if acquired, what is the nature of the variation, and under what conditions may the spores be kept without immediate danger of a change in their properties. Knaysi (1945a) observed germination of the spores in the agar medium where they were formed. There was evidence that it was due to the availability of a fresh supply of food by diffusion from remote parts of the slants and by autolysis of vegetative cells. Knaysi et al. (1947) observed with the electron microscope two types of spores in slant cultures, one totally opaque to the electron beams at 50 kv, and the other showing transparent spots. These spots were considered an indication of incipient germination. Two types of spores were also later observed with the phase microscope (Knaysi, 1951, unpublished data), one bright and the other dark by dark contrast (figure 1). Dark spores germinated faster than shiny ones and were considered to have undergone incipient germination. Attempts to demonstrate a difference in heat resistance between the two types led to some paradoxical results. The present work was begun with the hope of finding an explanation to these early results. It is evident that a bright spore always turns dark as it germinates, but how early in the germination process does this change take place, and how soon after a bright spore turns dark does its resistance to heat drop to the vegetative level? Furthermore, do all dark spores originate in this manner or are there inherently dark and inherently bright spores? In an attempt to answer these questions it was soon realized that the endospore is far from being the inert cell it is considered to be, and that its high degree of sensitivity to certain environmental factors requires the development of new and exact methods before the correct answers can be found.

OPTICAL DENSITY OF THE ENDOSPORE OF BACILLUS CEREUS AND ITS RELATION TO GERMINATION AND RESISTANCE