Within the nucellus, a single (normally hypodermal) cell becomes a primary sporogenous cell (archesporial cell, or archespore). The archespore rarely consists of more than one cell, though it can be multicellular in a few species (e.g. Brassica campestris86), in which one cell produces the megagametophyte. In turn, the archesporial cell either gives rise directly to the megaspore mother cell (megasporocyte) or undergoes mitosis to form a primary parietal cell and a megasporocyte. The megasporocyte then under-goes two meiotic divisions (megasporogenesis) to form a tetrad of four megaspores, which are usually either in a linear or T-shaped arrangement. In the majority of angiosperm flowers, one megaspore (most commonly the chalazal one) gives rise to the mature embryo sac by further mitotic divisions, and the other three megaspores degenerate (Fig. 5.13). This type of development is termed monosporic. However, in relatively few angiosperms, two or four megaspores play a role in embryosac formation; these types are termed bisporic or tetrasporic respectively69,125,126. Degenerated megaspores are often sur-rounded by persistent callose.
In most angiosperms the mature embryo sac (megagameto-phyte) possesses eight nuclei arranged in seven cells (Fig. 5.15), though types with four and sixteen or more nuclei have also been recorded. The most common type is monosporic and eight-nucleate; this is sometimes termed the Polygonum type of embryo sac development. At the binucleate stage, the two nuclei migrate to the micropylar and chalazal poles and subsequently divide. Of the two micropylar nuclei, the one closest to the micropyle divides to form the synergids, and the other divides to form the egg cell and one of the polar nuclei. The two chalazal nuclei each divide so that one forms two antipodal cells and the other forms an antipodal and a polar nucleus. The two polar nuclei migrate to the centre and fuse to form a diploid fusion nucleus. Cellularization follows, so that the mature megagametophyte consists of three antipodal cells at the chalazal end, a central cell with a fusion nucleus, and two synergids plus an egg cell at the micropylar end.
The synergids and the egg cell are so tightly pressed together that they are collectively termed the egg apparatus. The syner-gids play a role in directing the pollen tube into the embryo sac; they are calcium-rich and normally possess a series of wall thickenings, the filiform apparatus, which extends into the micro-pyle (Fig. 5.15). In many species the antipodals degenerate at an early stage, but in others they persist, and sometimes undergo cell division (e.g. in many grasses) or endoreduplication.