Beroe is a ctenophore – or comb jelly – with a well-earned reputation for voracious predation on other ctenophores.  When it swims (which is always), using coordinated movements of its eight rows of cilia, it orients mouth forward, the better to engulf whole prey in its gaping maw. 

First of all, there are a few things to know about this video.  The Beroe egg is about 1 mm in diameter and as clear as a glass bead.  We’ve labeled the oral and aboral poles, but those designations are approximate because the embryo is tilted (actors can be so difficult to work with).  Later on, the embryo comes to look a little squashed or abnormal, but this too is a result of the focal plane; as far as we can tell, all the developmental events we point out proceeded normally and resulted in a normal larva, or pseudo-larva, or ... let’s leave that discussion for another time. 

Cleavage in Beroe, as in other ctenophores, is unipolar – the cleavage furrow ingresses from only one side of the cell.  The first two cleavages create four equal-sized cells.  Third cleavage is slightly unequal, and results in four large central blastomeres and four smaller peripheral blastomeres.  Subsequent divisions are highly asymmetric, creating a crown of many small cells atop the few large ones.  (For a better view of cleavage pattern in ctenophores, see the video of Pleurobrachia).

The first phase of gastrulation in ctenophores is epiboly, in which the micromeres at the aboral pole (conveniently labeled here so you know where to look) move as a sheet around the embryo toward the oral pole, engulfing the vegetal cells and resulting in two germ layers.  Subsequently, the oral micromeres lead the charge of cells invaginating at the oral (=animal) pole; the opening left behind becomes the mouth.

— text by Katie Bennett