LONG-LASTING EXOCYTOSIS AND MASSIVE STRUCTURAL REORGANIZATION IN THE EGG PERIPHERY DURING CORTICAL REACTION IN PLATYNEREIS-DUMERILII (ANNELIDA, POLYCHAETA)

The course of the cortical reaction in the Platynereis dumerilii egg i s described from live observation and from sectioned fixed material an d is found to differ in several aspects from the course of cortical re actions in better-known systems. Cortical granules are unusually numer ous. They are discharged by exocytosis during a period of about 25 min following fertilisation (18 degrees C). Most of the surplus membrane material brought to the egg surface by exocytosis is set free into the perivitelline space. Swelling of egg jelly precursor secreted by cort ical granule exocytosis may be causal for the detachment of the vitell ine envelope from the egg cell surface which, however, remains attache d punctately to the vitelline envelope by about 30 000 microvilli. Und er the strain of the distending vitelline envelope, the bases of the m icrovilli move and line up, pulling the cell surface into a network of ridges. The grooves in between the ridges are the sites of exocytoses . Cytochalasin B, generally destabilising actin filaments, induces rup ture of the microvilli and exaggerated distension of the vitelline env elope during the cortical reaction. In a final phase of the cortical r eaction the vitelline envelope wrinkles and falls back onto the egg ce ll surface, the microvilli shorten and the egg cell transiently become s deformed by local contractions. The cortical reaction in the nereid egg is discussed as a process of distortion and reorganisation of the egg cortex and plasmalemma. The abundance of cortical granules accommo dating egg jelly precursor in the Platynereis oocyte is attributed to the mode of so-called diffuse oogenesis characteristic of nereids, i.e . of differentiation of oocytes freely suspended in the coelomic fluid . In nereids, egg jelly therefore forms after fertilisation as opposed to ovulation.