The Common Pond Snail Lymnaea stagnalis

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The common pond snail is a freshwater species widely used in embryological studies. Oogenesis and egg maturation of the pond snail involve complex events of ooplasmic segregation, including polar as well as mosaic differentiation of the cortical layer [141,144,174]. The role of this layer as a morphogenetic system has been clarified to a large extent in studies employing the pond snail [140, 141]. Typical spiral cleavage enables the fate of individual cell descendants to be traced [175, 176]. Cytoplasmic segregation increases in the course of cleavage and individual cells undergo early differentiation. The divisions rapidly become asynchronous, while the regular rhythm of cleavage in different cell lineages becomes an important prerequisite for normal morphogenesis [23, 31]. These events can be conveniently studied using the pond snail because of the changing structure of the mitotic cycles [18, 19, 24]. Early blastulation in the pond snail correlates with the early appearance of specialized cell junctions at the stage of two blastomeres; junctions play an important part in the physiology of the early embryo [72, 171] and during the critical periods of embryogenesis such as the emergence of the body plan and the dorsoventral organization [25, 137, 141, 143]. The structure of these junctions has been studied intensively at the submicroscopic level using various procedures, including freeze-fracturing techniques [56]. Methods have been developed which enable a three-dimensional reconstruction of the whole embryo from its thin sections to be obtained; these techniques allow a specific determination of the types and number of contacts in individual blastomeres [181]. The cell surface of early embryos has been studied using a scanning electron microscope [126].