What makes an egg an egg? Of the two reproductive cells, it’s the larger, less mobile one. It receives the sperm cell, accommodates the joining of the two gene sets, and then divides and differentiates into the embryonic organism. It also provides food for at least the initial stages of this growth. This is why eggs are so nutritious: Like milk and like plant seeds, they are actually designed to be foods, to support new creatures until they are able to fend for themselves.