Localization of cytoplasmic determinants responsible for primary mesenchyme formation and gastrulation in the unfertilized egg of the sea urchin <i>Hemicentrotus pulcherrimus</i>

Abstract

Abstract The jelly canal of the unfertilized egg of the sea urchin H. pulcherrimus was found to serve as a stable landmark of the egg axis. The eggs were bisected with a micro glass needle into two egg‐halves either equatorially or meridionally in relation to the egg axis defined by the jelly canal. The development of both egg‐halves after fertilization was observed 1 or 2 days later until the control embryos reached the gastrula stage. The embryo derived from the vegetal half formed primary mesenchyme and gastrulated while the animal half remained as the permanent blastula forming neither primary mesenchyme nor archenteron. Both embryos obtained by the meridional bisection formed primary mesenchyme and gastrulated. The presence of the female pronucleus or its adjacent cytoplasm was not a prerequisite for the occurrence of primary mesenchyme formation and gastrulation. These results indicate that unfertilized eggs already localize the cytoplasmic determinant responsible for primary mesenchyme formation and gastrulation. Localization of the cytoplasmic determinant occurs independently of formation of the cortical pigmentation polarity.