Morphological changes in the growing and maturing oocytes of Patiria (Asterina) pectinifera were studied by electron microscopy.An extensive system of rough endoplasmic reticulum (ER) and Golgi complex (GC) develops in the ooplasm forming the cortical, yolk and secretory granules in its peripheral regions. The contents of the latter granules are released from the oocyte and form the vitelline membrane. At early stages of oogenesis, extensive multiplication of mitochondria results in formation of a large aggregate of these organelles in the perinuclear cytoplasm ("yolk nucleus"). After maturation of full grown oocytes has been induced by 1-methyladenine, the membranous cell structures are rapidly rearranged : vast aggregates of ER cisternae in the surface cytoplasm layer and single ER cisternae among yolk granules are disintegrated to small vesicles; the GC is reduced. These processes are suggested to be somehow related to changes in hydration of the cytoplasm and in rigidity of its surface layer. In maturing oocytes, the yolk granules form characteristic linear rows, trabeculae, traversing the cytoplasm and their boundary membranes fuse in zones of contact. Some granules are converted to multivesicular bodies, thus suggesting the activation of hydrolytic enzymes that form part of the yolk in echinoderms.Oogenesis is of the solitary type.In 1969, KANATANI first showed (1) that radial nerve extracts of starfish induced synthesis and secretion of 1-methyladenine (1-MA) by the follicle cells. These cells contract and seem to push the oocytes out of the ovary (2). 1-MA acts on the oocyte surface and induces the formation of maturation-promoting factor in the cytoplasm that causes breakdown of the germinal vesicle and oocyte maturation (3).Within 15 years after this discovery by KANATANI a great number of papers were published dealing with the physiological and molecular mechanisms of maturation (for review, see 3-5). But there have been few morphological studies on starfish oocytes at different stages of growth and maturation, apparently due to the absence of appropriate fixatives for echinoderms suitable for electron microscopic studies.The information obtained on structural changes in the oocytes during this period is essentially as follows. Within the first five minutes after treatment of full grown oocytes with 1-MA, the rigidity of the surface layer of the cytoplasm markedly decreases (6, 7). Microvilli