In this video we are looking straight down at the vegetal pole of an ascidian embryo that has already undergone multiple cleavages.  The focal plane is near the surface, so the chorion and most of the test cells are out of focus.  This is a good thing because the major point of interest, gastrulation, takes place at the vegetal pole and is best visualized in this focal plane.  The anterior is to the left in this movie.

Gastrulation commences as ten endoderm precursor cells at the vegetal pole begin to change shape.  The tops of eight of these cells emerge in sharp focus in this video as they invaginate.  Watch as the outer faces of these cells seem to widen, then go out of focus as they invaginate, migrating inside the embryo and leaving behind a deep, broad valley that constitutes the blastopore.  What they’re each doing, all together, is changing from a column fatter on the outside to a column fatter on the inside.  At the beginning they’re arranged roughly like kernels on a corn cob, and at the end... well, they’re like, um, carrots that someone buried upside down?  Once they’re done changing chape, it’s as if we are looking into a cup with a wide rim.  That rim is made up of mesoderm cells  – precursors of the notochord and muscles – that will ultimately roll over the edge of the blastopore, leaving only ectoderm – presumptive epidermis and neural plate – outside.

— text by Katie Bennett & George von Dassow