A chicken embryo is a very young chicken at the start of its life.

I mean seriously use your eyes and look at the picture of the embryo I posted immediately above. You can call it a "fertilized egg" all you want. Its obviously very young chicken about to come out of its shell.

You can go find pictures of chicken embryo younger then 14 days old that are clearly not ready to leave their shells. They will look less and less recognizable the younger they are until you get back to the single cell stage once more. That does not change anything those are still chickens. They are chickens in their very earliest stages of development and unrecognizable visually for what they are but they are chickens nevertheless.