FORMATION OF EGG ENVELOPES IN THE FRESH-WATER CALANOID COPEPOD, HEMIDIAPTOMUS INGENS

In Hemidiaptomus ingens, fertilization occurs at the time of oocyte spawning in the ovisac. Fertilization induces the reactivation of the oocytes, arrested in meiotic prophase I, and above all, the development of two successive membrane envelopes arising before the formation of the eggshells. At once, within 25 min after spawning, a first cortical reaction induces the development of the fertilization envelope. This envelope is composed of a thin amorphous coat delaminated from the plasma membrane and of a material exocytosized from cortical vesicles which coalesces with the coat. This process results in the formation of a perivitelline space between the fertilization envelope and the plasma membrane. Subsequently, a new envelope appears from 25 to 60-90 min after spawning according to the same process, i.e., the association of a coat arising from a second delamination of the plasma membrane and of a material issued from the exocytosis of another type of cortical vesicles. This second envelope divides the perivitelline space into two compartments, an outer compartment or primary shell space, and an inner one or secondary shell space. An outer multi-layered shell begins to develop in the inner compartment within 1 h after spawning from exogenous material which is presumably produced by specialized shell glands located in the last segment of the prosome. This shell reaches its maximal thickness 48 h after spawning. Finally, when the outer shell is completed, an inner shell arises from embryos.