In (human) zygote cell differentiation, do we know how the cells coordinating the process know their current position?
There's a protein called Bicoid in fruit flies (see https://en.wikipedia.org/wiki/Bicoid_%28gene%29) that determines the head/tail axis. I remember from biology class that there's a similar protein that determines the up/down axis as well, although I forget its name.

There's almost certainly something similar in humans.