Electron microscopic studies on the development of the chorion of the viviparous teleost Dermogenys pusillus (Hemirhamphidae).

Eight stages in the oogenesis of Dermogenys pusillus were selected in order to demonstrate the formation of the egg membrane. In young oocytes (stages 1 and 2) the contact between oocyte and follicular cells is rather close. During stage 3 microvilli arise from the oocyte, and the follicular cells protrude lobopodia-like cell processes. When the microvilli have become arranged more regularly, the homogeneous material of the zona radiata externa is deposited between them (stage 4). During the subsequent stage (5) the inhomogeneous zona radiata interna appears. It attains its greatest thickness and its characteristic fine structure during stage 6. It then consists of cross-banded fibrillae and lamellae. Later on (stage 7) the egg membrane flattens, and finally it is a compact chorion consisting perhaps of very fine fibrils. Because of this development and of its comlex fine structure the egg membrane of Dermogenys like that of Zoarces resembles that of oviparous teleosts but remains consideribly thinner. The eight stages of oogenesis in Dermogenys are compared with those described by Arndt (1956) in several limnetic teleosts.