Vitellogenesis in the lugworm Arenicola marina L. I. Cytological and ultrastructural observations

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Abstract
Proliferative and previtellogenic stages of oogenesis take place on the ovary. Organelles usually associated with autosynthetic yolk production are established prior to ovulation and a small number of multivesicular bodies are found in the ooplasm. Ovulation occurs chiefly between April and mid-June. During this period the eggs continue to grow slowly in the coelom and presumptive cortical granules and lipid droplets appear in the ooplasm as well as yolk granules. Well synchronised, rapid growth with proliferation of oocyte inclusions occurs during June to September. This appears to coincide with a marked decrease in ovulation so that only a few small oocytes, which fail to mature, are found in the coelom in the latter part of the oogenic cycle. Mid-way through the vitellogenic period when the eggs are about 100 μm in diameter, coated pits are found on the oolema. The significance of heterosynthesis in vitellogenesis of solitary oocytes is unknown. The accumulation of electron dense cortical granules to form a cortical layer is described. Their contents are partially extruded when maturation begins. This is associated with ripening and spawning rather than the ‘fertilisation reaction’.