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).