Conjugating progametangia of Rhizopus sexualis rapidly enlarge and become flattened at the apical zone of contact. The walls over this zone fuse immediately, lose their separate identity and form a single plate. The fusion wall then thickens by gelatinization and swelling: there is no evidence of deposition of secondary wall material. The plasmalemmas on either side of the fusion wall become wrinkled and enclose pockets of electron transparent material along the sides adjacent to the wall. Vesicles and later lomasome-like masses are seen in a few of these pockets. Numerous small vesicles are present in the adjacent zones of cytoplasm on either side of the wall; some may be seen attached to the plasmalemmas. The wall then thins from the centre of the plate outwards but remains in being until after the gametangia have been delimited by new septa. Cell fusion then takes place by the development at the centre of the fusion wall of a hole which enlarges rapidly until the whole of the plate is dissolved. The evidence from electron micrographs suggests that the dissolution process is entirely chemical and that both matrix and microfibrillar skeleton are dissolved enzymatically while still enclosed in the plasmalemma.
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