ABSTRACT
The homology concept was introduced in 1843 by R. Owen to designate the ‘same’ structure in different species. With the advent of cladistics, molecular systematics and modern developmental biology, several sets of definitions of homology have come into common use. Modern molecular developmental biology could reveal congruence in gene expression during ontogeny in lungs and swimbladder. In lungfish, the lungs originate ontogenetically as unpaired ventral swellings which give rise to the paired lung rudiments. In light of variations in lung structure and the lack of plausible ontogenetic transitions between lungs and swimbladder, it appears probable that the air-breathing organ of basal bony fish had both dorsal and ventral components, and that one may have developed at the expense of the other during evolution of the separate groups. The hypothesis based on extant phylogenetic bracketing that Osteichthyes originally possessed paired ventrolateral pouches that served as air-breathing organs is supported by embryological data.
