The current developmental concept of the thallic mode of conidiogenesis essentially involves the simple conversion and disarticulation of preexisting hyphal elements, in which extension growth has ceased, into conidia which are delimited by transverse septa. Two kinds of thallic development were originally interpreted from light microscopic investigations. In this paper, holothallic conidium formation, which is demonstrated by Geotricluim candidum, and enterothallic development, which is exemplified by Sporendonema purpurascens and Briosia cubispora, are compared and ultrastructural and karyological characteristics of these mechanisms of conidiogenesis are illustrated. Scanning electron microscopic analyses of conidium formation in Oidiodendron trancatum and Trichosporon beigelii and light microscopic investigations of Sympodiella sp. and Amblyosporium spongiosum have revealed that additional variations exist in the thallic mode of conidium ontogeny. On the basis of these new data, and a consideration of other species of imperfect fungi which demonstrate similar mechanisms of conidiogenesis, a revised concept of thallic development is presented.
