I looked but cannot find any credible report of fully developed hindlimbs in any extant whale. There are some reports of rudimentary limb bones in living whales, but these would be far from fully developed hindlimbs. Snakes such as pythons have true hindlimb buds but these buds cannot develop into fully functional hindlimbs either, even if the embryos are manipulated in the laboratory. Snakes have apparently lost part of the developmental program for fully functional hindlimbs. The same is probably true of whales, which have probably irreversibly lost their hindlimbs. Nevertheless this hypothesis is not as easy to ascertain in whales as someone has pointed out that it is difficult to perform observations and/or experiments on the whale embryos, given the fact that they develop to full term inside large, active, aquatic mammals.
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