Comparative biology of oogenesis in nemertean worms

In order to supplement previous analyses of oogenesis in nemertean worms, t his study uses light and electron microscopy to compare the ovaries and ooc ytes in 16 species of nemerteans that represent various taxa within the phy lum. Nemertean ovaries comprise serially repeated sacs with an ovarian wall that characteristically includes myofilament-containing cells interspersed among the germinal epithelium. Each oocyte can attach to the germinal epit helium by a vegetally situated stalk and resides in the ovarian lumen witho ut being surrounded by follicle cells. In the ovary, oocytes arrest at prop hase I of meiosis and contain a hypertrophied nucleus ('germinal vesicle') that often possesses multiple nucleoli. Intraovarian growth apparently invo lves an autosynthetic mode of yolk formation in most nemerteans and generat es oocytes that measure similar to 60 mum to 1 mm. When fully developed, oo cytes can be discharged through a short gonoduct and are either spawned fre ely or deposited within egg cases. In most species, oocytes released from t he ovary possess extracellular coats and resume maturation by undergoing ge rminal vesicle breakdown (GVBD). Such post-GVBD specimens also form a punct ate endoplasmic reticulum that may facilitate fertilization and development .