It's just a jump to the left

Pharyngula: Generating right-left asymmetries

At your time of gastrulation,
Feeling flattish, so to speak,
You've a groovy indentation:
Dare I say, "your primitive streak"?
And a darling dainty dimple
All would call your Henson's node;
Still your symmetry is simple
Till your epiblast implodes.
On your surface cells are skating,
Drifting, diving down your streak
While your cilia jut, gyrating,
Bending backwardly oblique.

Here an asymmetric shock lies
On biology's vast frontier:
Since these cilia circle clockwise
As they're reaching to the rear,
Right and left force they administer,
Yet the net result is
Sinister.