 Limblessness in cetaceans does not represent a regression of fully formed limbs nor the absence of limb bud initiation , but rather arrest of limb bud development . Limb buds develop normally in cetacean embryos . Limb buds progress to the condensation phase of early skeletogenesis , where nerves grow into the limb bud and the apical ectodermal ridge ( AER ) , a structure that ensures proper limb development , appears functional . Occasionally , the genes that code for longer extremities cause a modern whale to develop miniature legs ( atavism ) . 
