During the first 9 d of development in Didelphis, the 3 germ layers are established and the initial formation of their derivatives occurs. During this time the embryos float within uterine secretions, each separated from the uterine mucosa by a shell membrane. Organogenesis begins during the last 3 1/2 d of the 12 1/2 d gestation period and corresponds to the time when the yolk sac placenta is associated with the uterine epithelium. During this period development proceeds at an explosive rate, culminating in a fetus-like creature capable of surviving in the pouch. Early in d 10, the forelimbs are present as limb buds. Late in the 10th d the embryo descends into and becomes enveloped by its own yolk sac. Mandibular and hyoid arches, and prominent digital ridges on the forefeet are characteristic external features. By d 11, the hindlimb is represented by a bud the eyes and external auditory meatus are established and an open mouth with protruding tongue is characteristic of this stage. Digits are present on the forefeet but lack claws. During the 12th d the oral shield develops, curved claws are prominent features of the forefeet and digital ridges are seen on the hindfeet. Prior to birth the periderm covers the eyes, ear pinnae and sides of the mouth.