Sequential order of appearance of ossification centers in the opossum Didelphis albiventris (Didelphidae) skeleton during development in the marsupium.

Pouch-young of the opossum Didelphis albiventris were studied. They were removed from the marsupium at a crown-rump length (CRL) of 9.0 to 100.0 mm and submitted to differential staining of the bone and cartilage. Newborn D. albiventris of 9.0 to 10.0 mm CRL have a cartilaginous skeleton, with no macroscopic evidence of ossification. In 10.5 to 13.0 mm CRL pouch-young, ossification occurs in the bones surrounding the oral cavity, clavicle, ribs, cervical and thoracic vertebrae and thoracic limbs, structures necessary to ensure the survival and maintenance of the newborn in the marsupium. Ossification of most of the skull begins in 15.5 to 16.0 mm CRL pouch-young. The ossification in the pelvis and the pelvic limbs is observed at 18.0 mm CRL and in the sternum and the epipubic bone at 25.0 to 28.0 mm CRL. Most of the skeleton is ossified in 45.0 to 58.0 mm CRL pouch-young, except in the carpus, tarsus and epiphyses of the long bones where ossification occurs in 60.0 to 100.0 mm CRL pouch-young. In the skeleton of D. albiventris the omosternum, haemal arches, prepollex, prehallux and parafibula, are present. These are inconstant or absent elements in eutherian mammals.