This is a fish.
Well, a fish embryo. Zebrafish (Danio rerio), a model of vertebrate developmental biology, to be exact.
This video shows the intricate migration of all 16,000 cells in an 18-hour-old fish embryo, and I promise you, it is the coolest thing you will see all day.
From nose to toes to brain to bones, the development of complex multicellular creatures from the humble beginnings of a single cell is the result of nature’s most delicate and gorgeous chemical ballet. There are so many waves of signals, so many interacting gradients, in order to define up from down and left from right and front from back, using only proteins and RNA. It’s a wonder that it works at all.
A wonder, indeed.
Bonus: If this tickles your mitotic fancy, you’re going to want to check out these 3D fruit fly development GIFs.