Auxospore formation and development inNeidium(Bacillariophyta)

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Abstract
Neidium affine is allogamous and exhibits a form of physiological anisogamy in which the behaviour but not the morphology of the two gametes produced by each gametangium differ: one gamete is active, the other passive. The active gamete moves across to the other gametangium via a narrow fusion canal to effect fertilization. Both active gametes move almost simultaneously and appear to facilitate each other's movement. Each ellipsoid zygote (auxospore) produces a partly silicified, bipartite wall, which persists during auxospore expansion, controlling the size and shape of the transverse perizonial bands, and hence the initial cell. A longitudinal perizonium is also produced. The initial epivalve forms after a contraction of the protoplast, and is produced on the opposite side from the longitudinal perizonium. The method of auxospore formation suggests that Neidium is closely related to Amphipleura and Frustulia. The siliceous component of the zygote wall is probably homologous with the scaly coverings of some centric and araphid pennate diatoms.