THE EARLY DEVELOPMENT OF THE MARINE PROSOBRANCH NASSARIUS (HINIA) RETICULATUS (LINNAEUS)

The early development up to the end of gastrulation of the marineprosobranch Nassarius (Hinia) reticula-tus (Linnaeus) has beenexamined by scanning and transmission electron microscopy. Thesynchrony of the cleavage divisions ends with the formationof the second quartet. The somatoblast 2d and the mesen-toblast4d appear earlier than the other micromeres of their quartets.From the 16- to the 32-cell stage the turret cells have an outgrowth.At the end of cleavage, Nassarius shows a typical sterroblastula.During gastrulation, the epibolic encirclement of the yolk macromeresby the micromeres is produced by an active flattening of themicromere cap and at the end of gastrulation the blastoporeis completely closed. The nuclei of the four yolk macromeresmigrate from the animal cell regions towards the vegetativepole. The development of Nassarius reticulatus is very similarto that of the related species llyanassa obsoleta (Say). (Received 19 December 1990; accepted 18 August 1991)