OOGENESIS OF PHASCOLOSOMA ESCULENTA: HISTOLOGICAL STUDY

Oogenesis of Phascolosoma esculenta is studied using histological microtechnique for ovary structure and the morphological changes. Ovary at the base of introvert retractor muscle has a banded or massive structure of many folioles. Oogonium develops into early oocyte in the ovary, and then the oocyte leaves the ovary for the coelomic fluid in the form of single cell or cell mass followed by the rapid separation of the group of oocytes into individual ones. Oocyte enters into the nephridium after its maturation. The rupture of germinal vesicle marks the oocyte maturation. Oocyte in the coelom does not have follicle membrane and vitelline membrane is formed and developed by the oocyte itself. Smaller oocyte (60μm in diameter) is round, and larger ones (≥60μm in diameter) is ovate. The short and long diameters of a morphologically mature oocyte are about 115—120μm and 140—145μm respectively, and the vitelline membrane is 7— 9μm thick. The oogenesis of P. esculenta can be divided into the following phases in morphological characteristics of oogonium and oocyte development: the proliferative phase of oogonium (10μm in diameter), the initial growth phase of oocyte (10—20μm in diameter), the grand growth phase-Ⅰof oocyte (20—60μm in diameter), the grand growth phase-II of oocyte (60μm×70μm to 120μm×145μm in size), the mature phase of oocyte, and the declining phase, showing a dynamic changes in oogenesis of P. esculenta.