I was reading The Canon by Natalie Angier last night, and this fact boggled my imagination. It had never occured to me before, which further boggled.
We all know that a sperm is a single cell, and an ovum is a single cell, but I'd never translated that into other types of ova.
So when she mentions, almost casually, that a chicken egg is a single cell, well, my jaw dropped.
"Yes, believe it or not, an unfertilized chicken egg of the kind you buy at the grocery store is a single cell, although strictly speaking, it's the cheery, marmalade-colored yolk of the egg that is bounded by the plasma membrane and thus qualifies as the cell proper."
She adds that the egg white and the calcium chloride shell are added later, as the yolk descends down the cloaca. The largest egg in the world, and thus the largest cell in the world, she goes on to tell us, is the ostrich egg, which weighs abut three pounds with it's shell.
Elaborating, she goes on to say that for all its size, the ostrich egg is the smallest relative to the size of the mother. Kiwis and hummingbirds, on the other hand, lay eggs which are 25% as big as they are.