Metameric Segmentation and Homology

Abstract

ABSTRACT In two recently published papers on the development of the fins of fish and on the segmentation of the head of Amphibia (12, 15), I had occasion to discuss incidentally the segmental relations of homologous organs, and to point out that, in the Vertebrates at all events, corresponding parts must be considered as fully homologous although occupying different segments of the body. In this paper I shall not attempt to define the nature of segmentation nor trace its origin, but shall merely try to show that a practical definition of the homology of an organ must not depend on its position in the series of segments. The subject of metameric segmentation has been very clearly dealt with by Sir E. Ray Lankester in articles on the Arthropod and Metamerism in the tenth and eleventh editions of the ‘Encyclopasdia Britannica’ (reprinted in vol. 47 of this journal). While giving a comprehensive review of the whole question of metamerism, he states thirteen “laws,” or general propositions, with most of which what follows will be found in complete agreement. But, in spite of the results of Fiirbringer (9, 10) from anatomical investigations, of Bateson (1) from observations on variation, and of others, there is, I think, a reluctance on the part of many anatomists to give up the idea that true homology depends on segmental correspondence. For instance, when discussing the homology of the occipital condyle in the Amphibia and the Amniota, we are asked on what segments they occur, it being implied, if not expressly stated, that if they are not on the same segment they cannot be homologous.