1. The structure of chorion undergoes several changes when it is formed during oocyte development. A trilaminar appearance is produced successively. For convenience of description, the portions are referred to Zone 1, 2 and 3 respectively. Zone 1 first appears as a homogeneous electron-lucent layer between the rnicrovilli of oocyte and rnacrovi li of granulosa cells. Zone 2 is immediately beneath Zone 1, and consists of an extremely electron apaque materials. Zone 3 is subjacent to Zone 2. It is the largest and most complex of the three zones, and consists of amorphous materials organized in a reticular-like network. It has been preliminaryly confirmed that Zone 1 is formed by granulosa cells, Zone 2 and 3 are secreted by oocyte itself with the dense core vesicles.

2. During the late period of oocyte growth, the chorion compacts. As the compaction occurs, Zone 1 turn into a lot of small pieces, Zone 2 becomes a thin dense layer, and Zone 3 becomes a thick multiple lamellae. The main part of the chorion is composed of the Zone 3.

3. Thickness of the chorion in O. mossambicus is 4.5 μm and it is thinner than those of other fresh water teleosts which have been studied. The reduction of thickness in the chorion is closely related to the special reproductive manner, that is, oral hatching embryos.