A COMPARISON OF PRIMITIVE FOREBRAIN ORGANIZATION IN METATHERIAN AND EUTHERIAN MAMMALS *

Fossil evidence suggests that both the marsupial opossum and the placental insectivore existed in the Cretaceous period about 100 million years ago (Simpson, 1945). The North American opossum (Didelphis marsupialis virginiana) is an omnivorous, nocturnal, semiarboreal animal that exists as one of the least modified mammals. Insectivores are the only known placental mammals which were present during the Cretaceous period. One existing group, the hedgehogs (our results are from the Pakistanian hedgehog, Paruechinus hypornelus) , are omnivorous, nocturnal animals that also retain some of the earliest mammalian features. A comparison of forebrain organization in the opossum and the hedgehog is of special interest because these two animals are as close, perhaps, as any living form to the archetypical mammals that first showed the multilayered cellular pattern typical of mammalian neocortex. Comparison of the detailed forebrain connections of the opossum and hedgehog with reptiles on the one hand and with more specialized mammals on the other may be one useful approach to clarifying the origin and development of neocortical structures. For the past several years, we have been studying the neocortical organization of the opossum in considerable detail with an emphasis on the connections of the thalamus, basal telencephalic nuclei, and neocortex. More recently, we have been comparing the opossum forebrain with similar connectional and cellular patterns in the hedgehog and turtle forebain in an effort to demonstrate connectional homologies among these existing forms of primitive eutherian, metatherian, and reptilian brains. The techniques employed in all of these studies have been the Nissl and GolgiCox stains for cell configurations and the Nauta-Gygax (1954) and Fink-Heimer ( 1967) modification of the Nauta technique for defining fiber connections.