1 Introduction.- 2 Materials and Methods.- 2.1 Optics.- 2.2 Axonal Degeneration.- 3 Observations.- 3.1 The Neuronal Architecture.- 3.2 Topographical Relationships of the Main Subdivisions.- 3.3 Ventral Division.- 3.4 Dorsal Division.- 3.5 Medial Division.- 3.6 Golgi Type II Neurons.- 3.7 The Axonal Architecture.- 3.8 Ascending Connections.- 3.8.1 Ascending Connections in the Opossum.- 3.8.2 Ascending Connections in the Cat.- 4 Discussion.- 4.1 Comparative Anatomy.- 4.2 Classification of Neurons and Axons.- 4.3 Homologous Nuclei and Nuclear Groups.- 4.4 Comparison of the Opossum and the Cat.- 4.5 Functional Organization and Physiological Implications.- 4.6 Ascending Afferent Connections of the Medial Geniculate Body.- 4.6.1 Ventral Division.- 4.6.2 Medial Division.- 4.6.3 Dorsal Division.- 4.7 Other Studies of the Medial Geniculate Body.- 4.8 Intralaminar Nuclei.- 4.9 The Problem of Homology.- 4.9.1 Topological Considerations.- 4.9.2 Developmental Considerations.- 4.9.3 Connectional Considerations.- 4.9.4 Functional Considerations.- 4.9.5 Comparative Neurology and Homology.- 5 Summary.- 6 Acknowledgments.- 7 References.