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THE TAIL
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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 |
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| PIG EMBRYO | |||||||||
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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. |
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has long been noted that tail-like structures could occur in human infants
( |
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