Atrophy, Burial, Suppression or Total Loss in Evolution

Abstract

AN examination of a series of sheep embryos has indicated that the ungulates are not removed from the general mammalian plan to the extent usually indicated by the specialization of their fore- and hind-limbs in the earliest Tertiary period. In 1890¹ Wincza described a transient rudiment of a bony clavicle in the embryo of a sheep at the site of the fibrous raphe of Leisering in the muscle sheet spreading from the head to the trunk and forelimb. The study of a group of fossil forms by Cuvier, Darwin, Owen, Cope, Matthew and others has brought to light a number of extinct suborders, of which the Typotheria, alone amongst the adult ungulates, possess definite bony clavicles.