The embryo in the film featured on this page was caught in a plankton tow taken by Marley Jarvis in the Charleston boat basin.  It almost certainly belongs to the chaetognath Sagitta elegans.  This is the most common chaetognath in our plankton, and some small adults were found in the same plankton sample.  It is not uncommon to find Sagitta embryos in the plankton, and there is little to confuse them with; their size, clarity, close-fitting smooth eggshell, and characteristic embryonic stages distinguish them from practically all other animals.

This film illustrates one of the most characteristic features of chaetognath embryogenesis: formation of coelomic compartments by partitioning of the archenteron.  This mode of coelom formation – enterocoely – is classically considered to be a central aspect of the deuterostome stereotype.  Enterocoely contrasts with schizocoely – a part of the protostome stereotype – in which coelomic compartments form by the hollowing-out of mesodermal tissue masses that are disconnected from the primordial endoderm.  But recent phylogenetic studies agree that wherever Chaetognaths belong on the tree of life, it’s not among the deuterostomes.  Since the Chaetognaths, ironically, epitomize enterocoely better than do many bona fide deuterostomes, in doing so they highlight the dubiousness of many developmental characters for phylogenetic inference.  Convergence in traits like these are probably more common than people once thought.

But enough about that.  Chaetognaths are their own creatures.  What happens in the movie?  It begins with the archenteron completely invaginated, obliterating what little blastocoel the embryo once had.  At this point, the embryo is radially symmetrical.  As the blastopore closes, four cells emerge from the roof of the archenteron.  These happen to be the primordial germ cells.  About the time the blastopore closes, it becomes apparent that the focal plane is a transverse section.  The archenteron develops a rhomboidal profile, then the dorsal surfaces begin to fold inward.  These folds elongate, and incidentally drag the germ cell precursors with them.

The folds eventually partition the archenteron into at least three tubes that run most of the length of the anterior-posterior axis.  At this stage, none of these tubes has much space inside.  One of the ones in the middle must be the archenteron, but it is not clear to me from this movie whether it is the space between the folds, or the region below them (the archenteron in a hatchling chaetognath is very small).  The two dorsal-lateral pouches are the coeloms.

As this is taking place, the embryo elongates dramatically within the eggshell, eventually coiling upon itself by half a turn or so.  In this movie the head ends up more or less in focus.  Chaetognath hatchlings are very long and thin, almost perfectly transparent, and very twitchy.  They hang motionless in the water, and occasionally dart forward in an invisibly-rapid movement.

— text by George von Dassow