The early development of a wild-type C. elegans embryo from fertilization to the two-cell stage. The oocyte pronucleus migrates posteriorly to fuse with the sperm pronucleus and its centrosome pair. Subsequently, the two pronuclei move to the center of the embryo with concomitant rotation. This rotation places the centrosomes longitudinally in the embryo, thus allowing a spindle to develop along the longitudinal axis. Cleavage-furrow formation and ingression result in the generation of two differently sized cells in the two-cell wild-type embryos.

The early development of a nud-1 RNAi C. elegans embryo from fertilization to the two-cell stage. The centrosome-pronucleus complex in nud-1 RNAi embryos migrates toward the center of the embryo but fails to rotate. Consequently, the spindle assembles perpendicular to the longitudinal axis but reorients longitudinally as elongation occurs. Once reoriented, the spindle in nud-1 RNAi embryos elongates as in the wild-type embryos. Although the cleavage furrow forms at the correct time and location, it regresses rapidly following telophase to generate a multinucleate one-celled embryo.