23 July 1976
Vol 193, Issue 4250

About The Cover

Female opossum (Didelphis virginiana) and young. At birth (after a 12¾-day gestation), the opossum is partially equivalent in development to a 4- to 6-week-old human embryo. Once nursing begins, voluntary detachment from the teat is not possible for 2½ months. The young pictured are in characteristic association with the mother just prior to weaning at 3 months of age. The opossum is the only marsupial indigenous to North America. See page 328. [Robert L. Blake, Duke University Medical Center]