The Epiphyseal Complex in Fish and Reptiles

From a phylogenetic point of view, few organs of vertebrateshave shown such a plasticity of form and presumed function ashas the epiphyseal complex. From paired, probably sensory, evaginationsthrough the skulls of primitive vertebrates, the single remnantremaining in the higher vertebrates has evolved into a denselycellular, vascular organ with presumably secretory functions.Too few studies are available from throughout the vertebrateseries to allow conclusions as to function. Demonstrated effectson reproductive mechanisms, on phototaxis and pigment migration,on behavior, activity, and a variety of other mechanisms aretoo inconsistent among and between the various groups to permitgeneralizations other than the likelihood that the epiphysealcomplex directly or indirectly is responsive to some manifestationof light or to its absence. Working backward from this assumption,it seems likely that underlying the activity of these complexstructures will be a modulation of some basic, homeokineticmechanism having much to do with the seasonal adjustment ofthe organism to its environment.