At first glance, the opening frames of this movie might look like an egg undergoing first cleavage with two distinct (albeit oddly placed) nuclei and the beginning of a cleavage furrow.  This is actually pseudocleavage, an event that precedes first cleavage in C. elegans.  Those are in fact nuclei, but they are the nucleus of the sperm and of the egg – also called the male pronucleus and female pronucleus – that will migrate together and meet near the middle of the fertilized egg to form the zygote nucleus.  

Polarity of the anterior-posterior axis in C. elegans is determined by* the point at which the sperm enters the egg, which then becomes the posterior end (on your right in this movie).  The pseudocleavage furrow and the ruffled appearance of the cell’s surface at the anterior end result from cell surface contractility that sweeps material from the posterior to the anterior end of the zygote.

The material being swept anterior-ward by this contraction includes molecules that will distinguish the two daughters of the zygote.  The anterior one, called the AB cell, is slightly larger, and its descendants become skin and nervous system; descendants of the posterior cell (P1) become muscles, gut, and the germ line.

Although we call it pronuclear “fusion”, you will notice that the male and female pronuclei don’t literally fuse (as they would in many other animals) once they get together.  Indeed, chromosomes from the sperm and the egg don’t share space until the nuclei both break down as the zygote prepares to divide.  The pronuclei usually meet near the middle, and center themselves before mitosis.  During anaphase, dramatic rocking of the mitotic spindle reflects asymmetric pulling forces which shift the mitotic spindle slightly toward the posterior.  Because the cleavage furrow always bisects the spindle, this posterior shift leads to the slightly-unequal cytokinesis at first cleavage.

* There is a subtlety here worth mentioning: what the sperm entry point really determines is the direction of surface contractility.  The oocyte is polarized before fertilization in the sense that meiosis takes place at one end, even though the polarization of the cell surface and A-P axis determinants has not yet begun.  During normal life, it so happens that the first part of the oocyte exposed to sperm, as the oocyte emerges from the ovary into the spermatheca, happens to be opposite the site of meiosis.  However, by accident or experiment, sperm can enter elsewhere, and when they do, cortical flow proceeds away from the sperm entry point, which, as usual, becomes the posterior of the embryo. 

— text by Katie Bennett & George von Dassow