The asymmetry of the habenular nuclei of female and male frogs in spring and in winter.

Male and female frog brains stained according to the Nissl method and cut transversely, 15 microns thick, at the level of the habenular nuclei, were investigated in spring and winter. Right and left habenular nuclei were examined. The volume and the standard deviations were calculated in each portion of the habenular nuclei investigated. The frog habenula consists of a single cell group: one on the right and two on the left side of the brain which differ, among themselves, both in the volumes of the neuropil and of the cellular ring. Functional corollaries of this striking asymmetry are still unknown. However, female and male frogs' habenular nuclei are longer and larger in spring--when frogs are sexually active--than in winter. We propose that structural brain asymmetries may be sex linked and may be induced by steroid hormonal effect in the central nervous system.