The female gonad of Tapes decussatus has no follicular cells, a common feature shared by all bivalve molluscs. Oocytes are thus in direct contact with the extracellular environment. The ooleme, abounding in microvilli, is covered by the vitelline membrane, an specialization of the extracellular matrix which is characteristic of oocytes. Through this membrane, the cell interns the extracellular material necessary for the synthesis of yolk and incorporates contaminants into its cytoplasm. These contaminants will, to a higher or lesser degree, alter the normal development of the cell. The cytoplasm abounds in yolk platelets with different degrees of formation. Their origin might either be the above mentioned endocytosis or the synthesis and subsequent modifications of some cytoplasmic organelles: endoplasmic reticulum, mitochondria, and annulate lamellae. Another feature worth noting of the endomembranous system of the oocyte we are dealing with is its nuclear envelope, as it shows remarkable modifications geared towards the formation of yolk material.