Structure and Function of the Brain of the Opossum (Didelphis virginiana) at the Time of Birth

Abstract

The developmental stage the mammalian central nervous system may reach at the time of birth is not the same in the various species, the most variable region being the cerebral cortex (Riese, 1944). A mammal can be born at any stage of cortical development, ranging from the still unstratified cortical layer in the new-born bear (Ursus arctos L.) (Riese, 1942) to the practically finished cerebral cortex of the human infant showing all of its regional variations at the time of birth (Conel, 1939, 1941). In spite of the great variation in their cortical development at the time of birth, all of the mammals must meet the requirements of extra-uterine life. Little is known about the developmental stages the constituent elements of the central nervous system (fibers, cells) may reach in the various species at the time of birth. Although there are numerous species in which the morphological (convolutional) pattern of the brain is already finished at the time of birth (ungulates, some carnivores, man), there is no species in which the process of histological maturation would have come to its end at the time of birth at all of the levels of the central nervous system. In the most immature brain hitherto described in a new-born mammal, i.e. that of the newborn bear (Riese, 1942), maturation of nerve cells is obvious in lower levels, but does not reach beyond the midbrain. The results of this investigation are based on the study of four new-born opossums, two of which measured 11 mm, one 10 mm, and one 9 mm, crownrump length. Two specimens were given by the Wistar Institute, Philadelphia. I was further given the opportunity by Dr. A. A. Zimmermann, Professor of Anatomy, University of Illinois, College of Medicine, Chicago, to study his own material. Included in this study are one prenatal stage, about 24 hours before birth, and one pouch young opossum, 22 mm crown-rump length, both being specimens of the collection of Dr. A. A. Zimmermann. I wish to express my gratitude to the Wistar Institute, to Dr. Zimmermann, and to Dr. E. C. L. Miller, secretary of the Virginia Academy of Science.