Origins of Mosaic Development?: Zygote Surface Differentiation During Meiosis in the Polyclad FlatwormHoploplana

Summary Changes in egg shape and surface morphology during maturation may be related to the localization of cytoplasmic determinants in the embryos of organisms with spiral cleavage. The eggs of the polyclad turbellarian Hoploplana inquilino undergo pronounced shape changes during the meiotic divisions which have been examined with the scanning electron microscope. Unfertilized eggs have a smooth surface that becomes covered with microvilli and microblebs within 10 min of fertilization. First polar body extrusion is accompanied by the asymmetric appearance of large blebs (Blebbing Cycle I) primarily in the animal hemisphere with one quadrant characteristically smoother than the others and bearing fewer blebs. Blebbing Cycle II, which is less pronounced than the first but is still characterized by a relatively un blebbed quadrant of the zygote, coincides with second polar body formation. These asymmetric shape changes in the animal hemisphere during meiosis may possibly correlate with a primitive form of morphogenetic segregation and beginning quadrant specialization in polyclads, the most primitive spiralians with mosaic development.