Just prior to first cleavage in C. elegans, the mitotic spindle is aligned with the anterior-posterior axis of the egg.  The agitated rocking of the mitotic spindle at first cleavage is associated with the subtle shift of the spindle toward the posterior end.  Because the cleavage plane bisects the spindle, and the spindle is farther toward the posterior end, first cleavage in C. elegans gives rise to two cells of unequal size.  The larger daughter is called AB, and the smaller one is called P1 because it occupies the future posterior end of the embryo.

The anterior lineage founded by AB forms skin and nervous system, largely through equal divisions.  Some of the subsequent divisions in the posterior lineage founded by P1 are unequal, creating sub-lineages defined by notable founders in turn: MS makes body muscles; E makes intestine (the “E” is for “endoderm”); and P4 is the founder of the germ line.

Of particular note in this movie is gastrulation, which in C. elegans involves the migration or engulfment of just two cells – the two daughters of E, which are the posterior grand-daughters of EMS, and two of eight great-grand-daughters of P1, found the intestine.  Gastrulation in C. elegans differs from most other animals primarily in that so few cells ingress through the blastopore.  The blastopore itself is notable in that it is extremely small and almost indiscernible once the E cells get inside.

— text by Katie Bennett & George von Dassow