The pattern of gemma/gemmaling ontogeny in the liverwort Blasia pusilla L. is described from its origin as a primordium within the gemma receptacle to the formation of a juvenile gametophyte. Data were obtained from herbarium specimens, field-grown and axenically cultured plants, using a combination of light and scanning electron microscopy techniques. A consistent, uniform, fundamental developmental pattern was elucidated that is characterized by the production, within the receptacle, of a discoid to ellipsoid, stalked gemma composed of distinct tiers of thick-walled cells and margined by lateral columns of thin-walled, potentially active cells. After dispersal from the receptacle, lateral growth ensues from a gemmaling initial that is produced via quadrant formation from one lateral margin cell. Ultimately, an adult cuneate apical cell is honed that generates segments the same as in the adult plant to produce the juvenile gametophyte typical of Blasia. Both the gemmaling and sporeling ontogenies of Blasia incorporate a quadrant system that is fundamental to generative cell formation. However, sporeling ontogeny is reduced as compared with that of gemmaling ontogeny in its lack of a cell comparable to the gemmaling initial, and in its less extensive, less variable basal quadrant segmentation. The existence of similar quadrant stages in the primary ontogenetic patterns of Blasia and the frequency of quadrant production in sporelings of numerous complex thalloid liverwort taxa suggest the need to precisely elucidate the primary ontogenies of these key taxa to allow comparative analyses that will aid in more accurately deducing the phylogenetic affinities of the Blasiales.