Scientists found that as a chicken embryo grows inside the egg, the proteins in the yolk change a lot—13 of them become more or less common—showing the yolk is being used up and transformed to feed the growing chick.

We analyzed the available evidence and found that proteins in hen egg yolk change during incubation as the embryo develops. Specifically, 10 studies or assertions support the idea that 13 proteins in the yolk increase or decrease in amount over time, suggesting the yolk is being broken down and repurposed to support the growing chick [1]. These changes are not random—they appear to follow a pattern tied to the embryo’s needs. As the chick grows, certain proteins become less common, likely because they’re being used as building blocks or energy. Others become more common, possibly to help with development, protection, or metabolism. This process shows the yolk isn’t just a passive food source—it’s actively being reshaped to meet the demands of the developing life inside. We don’t know exactly how each protein functions in this process, or why these 13 in particular change. The evidence doesn’t explain the mechanisms behind the shifts, nor does it say whether all eggs follow the same pattern. But what we’ve found so far points to a dynamic transformation, not a static storage of nutrients. For anyone curious about how eggs support life, this suggests the yolk is more like a carefully timed supply kit than a simple bag of protein. The changes happen naturally, without outside help, and are part of how a single cell becomes a chick.