Why do a chicken embryo and a cow embryo look very similar even though the adults do not? they are not related, but still have similar development. they are distantly related, so their development is similar. the embryos mutate in utero to form different structures. the genetic makeup of the two species is similar.

Correct
"They are distantly related, so their development is similar."

All animals came at some point from a common ancestor accordingly to the evolutionary theory of the organisms. So, this means that both cow and chicken are related, distantly related. They may have had a common ancestor. Although nowadays these two animals may seem quite distant, their embryological development is quite similar because of this common origin of both animals.