1 Introduction.- 1.1 Endoderm Formation.- 1.2 State of Visceral and Other Structures at Birth.- 1.3 The Scope of This Review.- 2 Oral Cavity.- 2.1 Prenatal Development.- 2.2 Postnatal Development.- 2.3 Adult Teeth.- 2.4 Adult Tongue.- 2.5 Jaws and Associated Musculature.- 2.6 Adult Salivary Glands.- 3 Esophagus.- 3.1 Prenatal Development.- 3.2 Postnatal Development.- 3.3 Structure of Adult Esophagus.- 4 Stomach.- 4.1 Prenatal Development.- 4.2 Postnatal Development.- 5 Small Intestine and Colon.- 5.1 Prenatal Development of Small Intestine and Colon.- 5.2 Postnatal Development of Small Intestine.- 5.3 Postnatal Development of Colon.- 5.4 Caecum.- 5.5 Comparison of Enteroendocrine Cells in the Gastrointestinal Tract of the Adult.- 5.6 Interaction Between Enteroendocrine and Exocrine Components of the Gut.- 6 Pancreas.- 6.1 Prenatal Development.- 6.2 Postnatal Development.- 7 Liver.- 7.1 Prenatal Development.- 7.2 Postnatal Development.- 7.3 Gallbladder and Common Bile Duct of the Adult Opossum.- 7.4 Opossum Bile.- 8 Factors That Influence the Differentiation and Growth of Glands Associated with the Gastrointestinal Mucosa.- 9 Concluding Remarks.- References.

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 have concentrated on early cleavage stages and the formation of the three primary germ layers (Hartman 1916, 1919) and fetal membranes (Selenka 1887; McCrady 1938). The ova of Didelphis remain in the oviduct only for about 24 h before entering the uterus. A corona radiata is absent and each oocyte is surrounded only by a perivitel line space and a zona pellucida (Talbot and DiCarlantonio 1984). During the short transit period, the egg is fertilized by a single spermatozoon (Rodger and Bedford 1982a,b).