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THE TAIL

One of the major characteristics of chordates is a postanal tail.  In mammals this has been reduced and it apes an external tail is absent, although a tail region develops in embryonic development similar to that of other organisms.

 

Amphioxus

AMPHIOXUS
FROG EMBRYO
FROG EMBRYO
PIG EMBRYO
PIG EMBRYO
At the beginning of the 4th week the human embryonic tail is long and curled; by the end of the 4th week it is shorter.   By the 8th week the tail is shorter still and by the end of the 8th week it is gone (Moore, p. 91, 99).  Some of the coccygeal vertebrae fuse to form the coccyx in all apes, including humans. As the ancestors of apes became larger and spent at least some time erect (either walking or hanging), the pull of gravity on the urinary and reproductive organs became problematic: there was originally no reinforcement of the body wall in this region.  Apes solved the problem by tucking their tail between their legs (refer to the following photo of the human pelvis).  The tail was reduced to form a bone a fused vertebrae called the coccyx (in humans, the tail is much more significant earlier in development) and positioned to help support these pelvic organs.  The muscles which used to move the tail are still present but they now reinforce the body wall, forming the pelvic diaphragm.
HIP
It has long been noted that tail-like structures could occur in human infants (Harrison, 1901).  Vestigial tails in human infants are derived from portions of the embryonic tail, although they lack bone, cartilage, notochord, and spinal cord.   These vestigial tails include adipose, muscle, nervous tissue, and connective tissue (Belzberg, 1991).
DRawing of infant