“…On the other hand, there is a remarkable consistency throughout the species reviewed with regard to the size of the developing fetuses at this stage. This includes studies of the rabbit (Mitsukuri, 1882;Kohn, 1903), cat (Davies, 1937), guinea pig (Harmon and Derbyshire, 1932), insectivores (Adams, 1958), rat (Pankratz, 1931;Elfvin, 1967;Daikoku et al, 1969), mouse (Miller, 1926;Waring, 1936;Fernholm, 1971;Theiler and Muntener, 1974), pig (Stadnicka and Van Wynsberghe, 19821, and man (Keene et al, 1927;Crowder, 1957;Coupland, 1952Coupland, , 1965Hervonen, 1971). Our data from the opossum agree with these studies; thus, it may be important to note that this critical phase in the development of the mammalian adrenal medulla is initiated in fetuses of a characteristic size (14-20 mm crown-rump length).…”