A re-investigation of the centres of ossification in the avian skeleton at and after hatching.

The centres of ossification occurring in the skeleton of the domestic fowl from hatching onwards have been re-investigated in groups of birds from the same hatches, reared under standardised conditions and sampled at intervals from hatching to 182 days. Selected areas have been surveyed in adult birds. The numerous centres which are already present at hatching have been identified. Those first appearing around the time of hatching were for the uncinate processes and the phalanx of digit IV. The centres appearing after hatching were those for metacarpal II, four carpal centres, the orbitosphenoid, rostal and caudal basibranchials, epibranchials, entoglossal, proximal tibial centre, patella and tarsal sesamoid. These have been illustrated and the mean time of appearance and range of time of appearance of each calculated. Some evidence of slight variation in sequence of appearance was detected. The proximal tibial centre was concluded to be the only true secondary centre in the long bones and to correspond to the traction epiphysis of this region in the mammal. In adults, the phalangeal formula of the manus was found usually to be 2:2:1, but occasionally it was reduced to 1:2:1. The only sesamoidean centre found, other than the patella and the tarsal sesamoid, was in the carpal region and was termed the dorsal carpal sesamoid. Some of the many controversies existing in the previous literature have been assessed in the light of the findings of this study.