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ABSTRACT Eggshell ultrastructure is described in a branchiuran, Argulus japonicus. Growing oocytes of various sizes are found on the outer surface of the ovary. They are protruded outward, covered by a thin basal lamina of the ovarian epithelium which connects them on the ovarian surface. Many microvilli develop on the surface of oocytes from the late previtellogenic stage. These microvilli elongate to raise the basal lamina from the oocyte surfaces. In the early vitellogenic oocytes, large and small electron-dense granules appear and increase in number in the outer and inner regions, respectively, of the perivitelline space between the basal lamina and the oocyte surfaces. In the late vitellogenic oocytes, other large electron-lucent granules appear and increase in number in the innermost region of the perivitelline space. Mature eggs in the oviduct have a two-layered eggshell: the outer layer composed of large electron-dense granules and the inner layer of small electron-dense granules and large electron-lucent granules. Some unique features on the origin and formation of the eggshell in A. japonicus are discussed.


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