Ref ID : 2091
Harald Netzel and Barbara Grunewald; Morphogenesis in the Shelled Rhizopod Arcella dentata. Protistologica XIII(2):299-319, 1977
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Morphogenesis of the test in the thecamoeba Arcella dentata has been studied by live observation, by scanning electron microscopy using frozen-dried preparations of succeeding reproductive stages, and by transmission electron microscopy of thin sections and freeze fracture replicas. Reproduction takes about 50 minutes to complete; thecagenesis is accomplished during the first twenty minutes of "budding division", karyokinesis and cytokinesis occur during the following half on an hour. The only materials for the construction of the shell are "thecagenous granules". These can be seen in the central part of a cytoplasmic bud, the test anlage, protruding from the opening of the old theca. The anlage goes through a series of complex changes in volume and in outline. During the first phase of thecagenesis the anlage is surrounded by a circular pseudopodium forming a cupola. The thecagenous granules are extruded almost simultaneously underneath the cupola. By mutual flattening and adhesion the granules form a hexagonal pattern. They are transformed into a single layer of alveoli, which compose the thecal wall. Secretion of the granules is followed by the actual morphogenesis, constituting the second phase of thecagenesis. During its first period the roof of the shell is formed. A crown of spines, first appearing as a ring of hillocks, then as series of incisions in the outline of the anlage, subsequently as protuberances in the same radial positions, is brought to final shape and size. In the finishing period the funnel-shaped impression of the ventral side of the theca is formed. Morphogenesis in this shelled rhizopod must be considered to be a complex sequence of controlled cytoplasmic motions. Further investigations should take into account experiments of Hegner, concerning effects of sodium silicate on reproduction in Arcella dentata.