I thought it was multiple cilia, wafting fluid in a current around a cavity, a bit later than the single cell stage but still very early? Either way, the reaso

I thought it was multiple cilia, wafting fluid in a current around a cavity, a bit later than the single cell stage but still very early? Either way, the reason a cilium spins in one direction rather than another is that the motor is a molecular machine built out of amino acids, and amino acids are themselves chiral. If you made a person out of amino acids with the opposite chirality, their cilia would go the other way, and their body would be flipped. It's a lovely example of magnifying something up from the molecular to the macroscopic.