An egg is a single cell.

That's a eukaryote though, whole different kettle of fish. This is bacteria, far simpler and smaller.

Aside: Searching for "Valonia ventricosa" in YouTube gives me absolutely atrocious results. A bunch of clickbait medical things and random gross-out stuff. Bleh. Anyone else?

also https://en.wikipedia.org/wiki/Acetabularia_acetabulum