Diagramatic representation of the zootype concept presented by Slack, J. M. W.; Holland P. W. H. & Graham, C. F. [Nature 361, 490-492 (1993).]

A. Chromosomal organization and anterior limits of expression of six HOX genes presumed to be present in the last common ancestor of all metazoans. These genes would code for relative position within the developing organisms.

B. Proposed origin of the zootype within the Metazoa.