The North American opossum (Didelphis virginiana) generally is regarded as an important animal, phylogenetically. It is considered to represent a prototype marsupial and closely resembles fossil didelphids (Tyndale-Biscoe 1973). Numerous studies concerning the reproductive biology, embryology, and neurobiology of the opossum have been published. More recently, Didelphis has become popular as an animal model for gastroenterological studies because of the remarkable anatomical and physiological similarities of the esophagus as compared to that of man. Most of the studies of early development...