Comparison of Early Development of Sternum and Clavicle in Striped Dolphin and in Humpback Whale

The morphogenesis of the sternum and the clavicle in whales was studied for the first time. The mode of development of these structures shows considerable differences between the toothed whales (Odontoceti) and the whalebone whales (Mysticeti). The toothed whales are, in this case, similar to all other mammals. Their sternum originates from three different embryonal elements. During development of the embryo a rudimentary clavicle is to be found in the shouldergirdle. The whalebone whales, on the other hand, differ from this mode of development. Their sternum originates from only two embryonal elements and there is no clavicle at all. The third sternum element cannot even be found as a rudimentary element, which is unique among mammals. The significance of these findings are discussed.