Enterocoely in Chaetognaths

Marley Jarvis did a plankton tow at the F dock last week, and among the contents were many chaetognath embryos, almost certainly Sagitta elegans (because adults of these are common here, and a few small ones were in the same plankton sample). A couple were just finishing gastrulation. I made a time-lapse film of one of them overnight, and it so happened that it was a transverse view at about the middle of the embryo. This means that the formation of coeloms, as pouches that are pinched off of the archenteron, is easily visible in this movie.

At the beginning of the movie, the archenteron has folded all the way in. Primordial germ cells (four of them) emerge from the roof of the archenteron while the blastopore closes. The embryo begins to elongate along the anterior-posterior axis (not visible yet) as folds grow down from the roof of the archenteron. These drag the primordial germ cells with them. The coelomic compartments are the dorso-lateral chambers thus carved out by ingression of these folds. I am not clear on what becomes of the actual archenteron, because it's not visible in this film, and in embryos after this stage, the apparent archenteron is quite small. It may form by re-opening the space between the two ingressing folds.