endospore swelling (noun, ascospore term,spore morphology term, pl. endospore swellings) – a septum-like swelling of the endospore that is characteristic for polarilocular spores, constricting the cell lumen into several locules that remain connected by an isthmus. The endospore swelling gives the cell lumen of a bilocular spore the appearance of an hour-glass. The swelling is strictly speaking not a septum. Even though most authors commonly refer to the swelling as a septum, ultrastructural studies have demonstrated that a thin septum develops inside the isthmus at the very latest stage of spore development of polarilocular spores. The endospore swelling may thus be regarded as an early stage towards septum development but is not necessarily part of the thin septum that finally closes the isthmus and thus segregates one single cell with several locules a into several new cells with separate new cell lumina. Related terms: endospore thickening. This is:part of spore.