A L. variegatus late gastrula, processed via scanning electron microscopy (SEM) and fractured open to reveal the archenteron making contact with the animal pole. Image courtesy of John Morrill, Univ. of South Florida.

Secondary mesenchyme cells (SMCs) reliably attach the tip of the archenteron to a region that will become the mouth of the later larva. This oral (ventral) attachment site presumably expresses specific adhesion and.or guidance molecules to which SMCs respond. SMCs that make contact with this region change their behavior dramatically, making extremely long-lived filopodial connections to this site, as this classic footage from my days as a postdoctoral fellow shows.

Secondary mesenchyme cells make stable contacts with a specific region near the animal pole. Frames from a time-lapse sequence of gastrulation in L. variegatus. Time is shown in minutes in the upper right. Three filopodia are followed during the course of the movie. Note that filopodium #3 remains connected where it makes its attachment for more than 45 minutes. Images by Jeff Hardin, Univ. of Wisconsin.