An ultrastructural study of soybean embryo sac development was performed. Prior to the final mitotic division and cellularization a nuclear rearrangement occurs that involves the chalazal movement of one of the two micropylar nuclei. During cellularization this nucleus divides to form the egg and micropylar polar nuclei and produces the wall that separates the central ceil from title space occupied by the egg apparatus. Within this space the other nucleus divides to form the two synergid nuclei and one of the two walls that separate the egg and synergid cells from one another. Egg apparatus cells are initially densely cytoplasmic, each is enclosed by thick, highly dissected walls, and they are all similar with respect to distribution of organelles except that synergid nuclei are micropylar to the egg nucleus. There is a progressive thinning and segmentation of egg apparatus walls during cellular expansion until they resemble the beaded chain structure seen in the mature egg and synergid cell walls. Taken as a whole these observations suggest that the chalazal movement of one of the two micropylar nuclei during the 4-nucleate stage is pivotal in determining future patterns of egg apparatus development.
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