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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 2047

Harald Netzel; [Test Formation in Difflugia oviformis (Rhizopoda, Testacea)]. Arch.Protistenk.Bd. 119:1-30, 1977

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During thecagenesis Difflugia oviformis turns its long axis parallel to the substratum. A portion of cytoplasm protrudes from the thecal aperture. This cytoplasmic bud or anlage enlarges allometrically. At first broader than long, it becomes spheroidal, subsequently longer than broad, attending its definitive length (first phase of morphogenesis). The bud is brought to definitive diameter in the second morphogenetic phase. In the short final phase, taking between one and five minutes only, the profile characteristic to the species is given to the anlage. The new test is the cast of this cytoplasmic mould. The thecal wall consists of one layer of polymorphous particles (named idiosomes), juxtaposed alternately with transformed thecagenous granules. The variety of forms of the idiosomes in old and new test may differ greatly. The surface of the smallest bud observed is already covered by idiosomes, laying staggered or one upon the other in several layers. Oblong idiosomes may project from the anlage in vertical position for minutes, until they are gradually layed down parallel to the surface. As the bud enlarges, the layers of idiosomes are spread. They are brought into one plane most likely not until in the final morphogenetic phase. Prior to this no thecagenous granules have been observed among the siliceous particles. Moulding the shape of the test is accompanied by a reversible enlargement of the amoeba's cellular volume. At the final stage of morphogenesis this increase in volume amounts around 28% of the test capacity of about 300,000 µm3 (= maximum size of the interphase cell). Thecagenesis being completed, the anlage cytoplasm separates from its cast. Nuclear division takes place, and starting with the migration of one daughter nucleus into the bud cytoplasm, the whole cell streams four to five times hither and thither in the shells, that have their apertures closely apposed. During this shuttle streaming the cell endings alternately assume droplet-, club-, mushroom-, and cylinder-like forms. These different but regular forms are assumed presumably by mechanisms of amoeboid motion. Some of these forms are known from other thecamoebean shells. After separation of the daughter cells the amoeba in the new test removes the detritus adhering on the outer surface by means of pseudopodia. A naked cell, experimentally deprived of its theca, was able to produce a shell within the normal range of size exhibiting only minor malformations. An amoeba in a giant theca bearing two apertures carried out thecagenesis at either aperture simultaneously, giving rise to two thecae of normal shape and size. Such "double monsters" most likely result from fusion by chance during morphogenesis of two originally separate anlagen. The observations on double monsters are related to cell polarity in thecamoebae. The sequence of morphogenetic phases may differ according to the species of Difflugia, as well as the sequence of moulding the shape and arranging the building materials. Thus forming the new shell and structuring the wall appear to be largely independent processes. Mechanisms of wall formation and mechanics of morphogenesis are discussed in context of model experiments and interpretations of Rhumbler (1898) and physics of interfaces (Wolf 1968).