1. An extremely resistant resting spore formed by certain bacteria, especially those of the Bacillaceae, e.g. Bacillus and Clostridium. Only one endospore is formed in any given cell. Spor-ulating cells are termed sporangia and the size, shape, and position of the endospore is characteristic of the species. Endospores are resistant to heat, desiccation, radiation, and chemicals, including organic solvents. They retain the capacity to germinate for a considerable number of years.
2. A naked cell, many of which are formed by division of the protoplast in certain, usually unicellular, blue-green algae. On release they form a cell wall and develop into new individuals.