Some hold with the theory that the development of an embryo shows the stages of evolution. In other words, what first develops is fishlike, and then like a small mammal, and then like a lemur or ape, and then something we would recognize as human. Very early embryos have what look like little gill slits in the beginning of their development. At about four weeks, embryos have a little tail. At around six to twelve weeks, the white blood cells dissolve the tail, and the fetus develops into an average, tail-less baby... most of the time, at least. Every now and again, we get a little extra bit of baby, in the form of a vestigial tail.
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