This movie starts just before third cleavage in the green sea urchin, Strongylocentrotus droebachiensis...

Huh?  Anyone can see there's only one cell there. 

...Yes, it's a single cell, but it isn't the zygote.  This is one cell out of what would have been a four-cell embryo; the extracellular layers – hyaline layer and the fertilization envelope – were removed shortly after fertilization, and the cells cultured in artificial seawater lacking Calcium ions, without which they can't adhere to one another. 

So here we are at third cleavage which means there's only one cell cycle to go before the main attraction in early echinoid development: that’s right, it’s nearly time to make MICROMERES!!!

Micromeres, which organize the initial steps of cell fate specification in the urchin embryo, are always in the same place (the vegetal pole) and always form at the same time (fourth cleavage).  The asymmetric cell division that creates micromeres results from capture of the mitotic apparatus by the cell cortex at the vegetal pole. 

As you you watch the movie, keep these questions in mind: Is there an animal and a vegetal pole in this blastomere?  The urchin embryo as a whole has a well-defined polarity, but what about the individual cells within the embryo?  By dissociating cells from the very beginning, we can test whether cell polarity or spindle orientation depends on the normal arrangement of cell-cell contacts, or if these traits are autonomously retained by each cell.

**SPOILER ALERT: OUTCOME OF THE MOVIE EXPLAINED**

This blastomere does everything it would in the intact embryo! *  After third cleavage, the animal-hemisphere cell is on the left, and the vegetal one is on the right; notice there's a  slight asymmetry at this point too, which is usually apparent in intact embryos as well.  At fourth cleavage the vegetal cell divides unequally, creating a micromere of the same size and even at the same angle (relative to the axis of division at third cleavage).   And, although the next division takes place out of focus, the micromere undergoes a further unequal division – also at an oblique angle to the animal-vegetal axis – resulting in a large and a small micromere. 

— text by Katie Bennett

* actually, it does very slightly more: notice the twist as the cells divide, especially the micromere.  This probably doesn't happen in the intact embryo because cells would be bound to the hyaline layer.  Often, cells seem to twist apart from each other during division, if they are not otherwise constrained by adherence to a substrate or extracellular matrix.