The Ammon's Horns of over twenty inbred strains of house mice are being examined histologically for differences in neuron number and arrangement and for differences in the neuropil laminae. General aims are to study how these variations are inherited, what behavioral differences may be associated with them, and what developmental events may be involved in producing them. We hope to establish the value of genetically associated morphological variations as precise experimental tools for the study of the functional involvements of brain structures in behavior, and for the identification and description of some major events in neural ontogeny.