How the snake lost its legs

mystery of how the snake lost its legs might make a good 'Just so' story but it's also a tantalizing developmental question. Snakes have hundreds of similar vertebrae, as shown by the stained skeleton of a python embryo, Antaresia maculosa, (24 days after egg-laying) at top left. Snakes have no forelimbs but pythons have a pair of hindlimb rudiments (removed in picture at top right) at the junction between the thoracic vertebrae and the vertebrae with short, forked ribs. The pelvis and rudimentary femur of a hindlimb are shown in the picture at middle right. The hindlimb rudiments of an adult Python regius are visible in the images at lower right. Expression of the developmental genes Hoxc8 and Hoxb5, detected in python embryos by immunohistochemistry (lower left and middle), is not confined to specific regions of the trunk as it is in four-legged vertebrates. This expansion of Hox expression domains might underlie the expansion of rib-bearing vertebrae and the loss of forelimbs in snake evolution. For more details, see Cohn M. How the snake lost its legs