**Abstract**

Bahia grass (*Paspalum notatum* Flugge) is an important tropical forage grass and sets seed by apospory. I) To clarify the mechanisms of aposporous embryo sac initial cell (AIC) appearance and apomictic embryo sac formation, and II) to make it clear the mechanism of multiple embryo seed set a development in polyembryonic ovules, several apomictic and sexual varieties of bahia grass were studied cytologically and quantitatively by Nomarski differential interference-contrast microscopy. The results were I) there was no difference between sexual and apomicts to megasporogenesis; and then, the megaspore degenerated in apomicts; at the same time, AIC originated from nucellar tissue appeared and its numbers increased as the ovary grew before anthesis; II) at anthesis, the sac derived from AIC located in the micropylar end (first sac) were 92.5 to 100%, and those in the chalazal ends (other sacs) were 40.4 to 86.0% among the apomicts; the first sac divided dominantly and were 56 to 87% comparable to 0 to 1% of the other sacs at 4 days after anthesis; however, 4 to 17% of the other sacs also showed embryo formations but endosperm. In final, the first sac occupied the whole space of the ovule, in which the embryos in the other sacs coexisted.

**Keywords:** apospory, aposporous embryo sac initial cell appearance (AIC), differential interference-contrast microscopy, *Paspalum notatum* Flugge, polyembryonic seed set
