SEQUENTIAL ORDER OF APPEARANCE OF OSSIFICATION CENTERS IN THE OPOSSUMDIDELPHIS-ALBIVENTRIS (DIDELPHIDAE) SKELETON DURING DEVELOPMENT IN THE MARSUPIUM

Pouch-young of the opossum Didelphis albiventris were studied. They we re removed from the marsupium at a crown-rump length (CRL) of 9.0 to 1 00.0 mm and submitted to differential staining of the bone and cartila ge. 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 thor acic limbs, structures necessary to ensure the survival and maintenanc e of the newborn in the marsupium. Ossification of most of the skull b egins in 15.5 to 16.0 mm CRL pouch-young. The ossification in the pelv is 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, tar sus 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 omo sternum haemal arches, prepollex, prehallux and parafibula, are presen t. These are inconstant or absent elements in eutherian mammals.