ABSTRACT
The problem of homology becomes particularly sticky when structures in, for instance, the urochordates and the cephalochordates are compared with those in the vertebrates. The nonvertebrate branchial apparatus is composed of cartilage-like tissue, whereas that of the vertebrates always at least begins ontogenetically as true cartilage. Although the philosophical aspects of homology are being reworked, consideration of the morphogenetic aspect has not kept pace. For assessing the homology of many characters used by systematists, topographic relationship is the main criterion available. It seems obvious that the branchial skeleton of Amphioxus is homologous with that of vertebrates but to be meaningful a hypothesis about this homology should include a statement about the difference in morphogenesis. The ways in which enamel and enameloid are homologous can be stated with some precision because there is a testable hypothesis regarding the morphogenetic pathway for each.
