Ultrastructural Study of Oogenesis and Fertilization inSycon ciliatum(Porifera, Calcispongiae)

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Abstract
The oogenesis of the calcareoous sponge Sycon ciliatum has been studied by electron microscopy. In this species, oogonia probably derive from choanocytes through loss of collar and flagellum and formation of phagosome-like inclusions. Oogonia can be occasionally found within flagellated chambers and show a prominent rough endoplasmic reticulum, several mitochondria and polygonal dense granules. The latter are also visible in choanocytes. Oocytes lie in the mesohyl beneath the choanoderm. They contain a nucleolated nucleus, vesiculated granules and phagosome-like inclusions involved in the formation of fibrillar yolk material. Mature eggs are large, irregularly shaped, and filled with fibrillar yolk inclusions. A second ultrastructural confirmation of the carrier-cell mediated fertilization of calcareous sponges is also given.