Endosperm development prior to cell wall formation was observed in two species ofHibiscussectionFurcaria: H. costatus, a diploid, andH. acetosella, an allotetraploid. During early development of the endosperm, divisions are free-nuclear, highly synchronous, and most nuclei contain only one nucleolus each. However, just prior to initiation of wall formation, the nuclei lose their division synchrony and the number of nucleoli per nucleus increases. The endosperm is organized into an interior and an exterior region. To the exterior there are highly developed proliferations of the embryo sac wall. Adjacent to the wall there is a zone of large, elongated mitochondria. The interior of the young endosperm is filled with an intricate labyrinth of endoplasmic reticulum, interspersed with smaller, more spherical mitochondria and both free ribosomes and polysomes. Organization, development and fine structure of the endosperm is discussed in relation to its historically assumed functions.