Abstract
The development of vitelline cells in the fresh-water Triclad Dugesia lugubris has been studied by means of electron microscopic and cytochemical techniques. We observed a single cell type in different stages of development more or less evenly distributed from the periphery towards the centre of the follicles. Young vitelline cells have the characteristic morphology of cells in early stages of maturation: they show a large nucleus, a nucleolus with fibrillar and granular components, and many ribosomes in an undifferentiated cytoplasm. Subsequently, large amounts of parallel arrays of granular endoplasmic reticulum and some Golgi complexes appear and the vitelline cells assume the typical appearance of secretory cells. Reserve substances progressively accumulate in the vitelline cells, which when mature appear very electron dense. We have recognized the following types of reserve substances: 1) Neutral lipid droplets which are the first to appear and can reach a diameter of 4–5 μm or more. 2) Yolk globules of 2–3 μm in diameter which appear simultaneously with cocoon-shell globules but are clearly different from them in structure and chemical composition. The yolk globules probably arise from the cooperation between endoplasmic reticulum and Golgi complexes and have a very complex proteinaceous nature. 3) Large amounts of α-glycogen granules which occupy cytoplasmic sacs.
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