Maybe you need to expand you thinking beyond just the biological? Into the social maybe? I'd say the difference between an egg and an adult is a major metamorphosis.Except, we Human beings do not morph.
At least not in the biological sense, we don't.
Human beings grow, mature and develop continually through a cell division. Not metamorphosis.
My personal thinking is irrelevant.
The constitution says that ALL persons are entitled to the equal protections of our laws.
The most basic (inclusive) definition of a person is "a human being" and as you have acknowledged already, a human being, even in the zygote stage of their life, IS one.