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.
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.
Not all things that look like tails - protrusions from
the tailbone - actually are what doctors consider
"true" tails. There are a number of growths or cysts
that can form right on the tip of the tailbone. Some
of the more unpleasant options are large tumors,
elongation of the existing vertebrae, and even parasitic
(A parasitic twin is not a fully-formed twin,
but the product of another fertilized egg that
somehow became fused with the embryo and never
developed into a full human being.) True tails form
when the white blood cells, for whatever reason, don't
absorb all the tissue that formed during embryonic
development. These babies carry the marks of humans
Because there are only between 20 and 30 cases of
"true" vestigial tails since the late 1800s, there is some
controversy about what such a tail contains. Some
early accounts say that there are sometimes extra
vertebrae in such tails. No modern tails have been
found to have any bone tissue. They're mostly skin
with fat, connective tissue, nerves, and muscle tissue.
They can be just a stub, but some babies can be born
with tails 13 centimeters long. The tails aren't strictly
useless inert structures. Because they have muscle
tissue inside, they can actually be twitched back and
forth, or even contracted into curves.
babies don't have their tails long enough to gain a lot
of muscle control over them. Removing them is a simple
operation, usually done not long after birth.