EARLY EMBRYONIC-DEVELOPMENT OF THE MAYFLY EPHEMERA-JAPONICA MCLACHLAN(INSECTA, EPHEMEROPTERA, EPHEMERIDAE)

In the newly laid egg of the mayfly Ephemera japonica, an egg nucleus (oocyte nucleus) at metaphase of the first maturation division is in t he polar plasm at the mid-ventral side of the egg, and a male pronucle us lies in the periplasm beneath a micropyle situated just opposite th e polar plasm or at the mid-dorsal side of egg. The maturation divisio ns are typical. An extensive and circuitous migration of the male pron ucleus is involved in the fertilization process: it first moves anteri ad in the periplasm from beneath the micropyle to the anterior pole of the egg and then turns posteriad in the yolk along the egg's long axi s to the site of syngamy, near the center of the egg. Cleavage is supe rficial. The successive eight cleavages, of which the first five are s ynchronized, result in the formation of the blastoderm, and about ten primary yolk cells remain behind in the yolk. Even in the newly formed blastoderm, the thick embryonic posterior half and the thin extraembr yonic anterior half areas are distinguished: the former cells are conc entrated at the posterior pole of the egg to form the germ disc, and t he latter cells become more flattened, forming serosa. Time-lapse VTR observations reveal a yolk stream that is in accord with the migration of the male pronucleus in time and direction. The yolk stream is also generated in activated unfertilized eggs, and it is probable that the migration of the male pronucleus in association with the fertilizatio n may be directed by the yolk stream. J. Morphol. 238:327-335, 1998. ( C) 1998 Wiley-Liss, Inc.