On the development of Cetacean extremities: I. Hind limb rudimentation in the Spotted dolphin (Stenella attenuata).

The Cetacea are group of animals which have completely lost their hind limbs during the course of evolution as a result of their entirely aquatic mode of life. It is known, however, that during their embryonal period, the hind limb buds are temporarily present. The control mechanisms of this regression are not yet understood, and vestigial limbs can sometimes be found in adults. The aim of the present study is to describe the course of hind limb rudimentation during prenatal development of Stenella attenuata (Spotted dolphin) at tissue and cell levels and compare the results with other natural or experimentally induced amelias. Hind limb buds of dolphin embryos, CRL 10-30 mm, were examined histologically. Before total disappearance, they show histodifferentiation comparable with other mammals. Initially, they form the apical ectodermal ridge, which soon regresses. The mesenchyme undergoes the process of condensation to form anlagens of prospective skeletal elements. These condensations are surrounded by vascular plexuses. During the course of rudimentation, some mesenchymal cells die, while the others are incorporated into the body wall. Nerve ingrowth into rudimentary limb buds was also detected. The temporary presence of hind limb rudiments in cetacean embryos can be regarded as a good example of recapitulation of phylogenesis in ontogenesis.