Early in development, all vertebrate embryos are remarkably similar. The similarities among the embryos imply that the development of all vertebrates (having a backbone) evolved as new genetic instructions were placed on Top of older ones.
The chick embryo is on the right. It is easier to compare a newly hatched chick and a newborn baby. After birth, a backbone or vertebrae are clearly visible, as are legs, a mouth, a pair of eyes, etc. The chick’s Skeletal system, in general, has multiple similarities to ours, including having one bone in the upper leg and upper wing and two bones in the lower leg and lower wing. This mimics our arm and leg skeletal structures.
Mammals, birds, reptiles, and fish, look very similar at the earliest stages of growth inside the womb of the mother animal.Vertebrate embryos begin developing similar features due to related ancestors. As the embryo grows, it may drop some of these ancestral traits as the genetic code becomes more specific for that species.
By the process of cellular differentiation which consists of physical and functional changes of the cell used by processes at the genetic level. The expression of these in each type of cell is different. That is to say that the type of cell follows at a time of development there is differential activation of some genes and repression of others
