Cleavage in ctenophores, like this Pleurobrachia, is unipolar: the cell division furrow ingresses from only one side of the cell.  The first two divisions result in four cells of roughly the same size.  Third cleavage is slightly unequal, and yields four large cells – named the ‘M’ macromeres for middle – and four slightly smaller cells – named ‘E’ for edge.  Subsequent highly-unequal divisions of ‘M’ and ‘E’ cells create the aboral micromeres, which will steal the show during epiboly, the first stage of ctenophore gastrulation. 

Stated in the most general way, gastrulation is the process which turns an embryo with a single layer of cells into an embryo with multiple layers.  It's called gastrulation because it generally involves making the embryonic gut.  Among animals, there are three or four stereotypical ways in which embryos gastrulate: either one set of cells crawls over and swallows up some others (epiboly), or one set of cells dives into the inside (invagination if they go together, ingression if they go as individuals), or oriented cell division places some cells inside (delamination). 

Usually, one way is enough for each embryo, and old textbooks often make it seem as if these "types" of gastrulation characterize major groups of animals – i.e., invagination for deuterostomes, epiboly for protostomes, or something like that.  Ctenophores, however, do it all: in this one organism (and in this one video), you can see gastrulation by epiboly, followed by delamination, then invagination to convert a pile of cells into a multi-layered embryo with a tube running through it and a mouth at one end.

So let’s go back to the formation of the aboral micromeres, a series of small cells on the (you guessed it) aboral pole of the embryo.  Creation of successive new micromere contingents, and of course divisions of the pre-existing ones, builds up a crown comprising about 50 small cells.  This crown of cells is the presumptive ectoderm.  During epiboly, the small micromere-derived cells spread around to the oral (animal) pole of the embryo, engulfing the macromeres (presumptive endoderm).  To visualize this, imagine trying to force your head through the collar (the aboral micromeres) of a too-small sweater.  The point of this analogy is to picture the crown of cells moving from one pole down all sides of the embryo until they almost converge at the opposite pole. 

The formation of oral micromeres constitutes gastrulation-by-delamination: in which the axis of cell division is perpendicular to the surface of the embryo, and thus creates inner and outer cells.  In this case, small daughters on the oral surface complete the enclosure of large yolk-laden daughters inside. 

The oral micromeres then lead the invagination of the archenteron, the embryonic gut.  The blastopore – the site where the invagination started – becomes the mouth... although the so-called "oral" micromeres, since they are at the tip of the invaginating tube, end up clear on the other side of the animal.

— text by Katie Bennett & George von Dassow