[Ossification of the sheep skeleton]

The ossification of sheep skeleton was investigated from 4th to 10th week of gravidity by means of cryostat and paraffin serial sections, cleared specimens and grenz ray radiographs. Significant decalcification occurred in paraffin sections and cleared specimens. These artifacts were not observed in cryostat sections and grenz ray radiographs of silver impregnated specimens. The Clavicle being the oldest bone of sheep embryo contains calcium deposits for the first time at a crown rump length (CRL) of 20.5 mm. During the further development of limbs Radius (at 28 mm CRL) and Tibia (at 30 mm CRL) ossify slightly earlier than Humerus and Ulna or Femur and Fibula respectively. The ossification of the diaphyses of all parallel pairs of long bones starts always at the median sides of these bones which are iuxtaposed to each other. The ossification of the vertebral column starts at C2 (vertebral arch, 45 mm CRL) and at Th1 (vertebral body, 49 mm CRL). During the course of course of development (up to 68 mm CRL) arches and bodies ossify at the same time descending and ascending from a cervical and a lumbar starting point in the vertebral column. The ossification of ribs starts at one point and spreads evenly in all directions over the cartilagineous primordium of the thoracic wall. Mandible and Maxilla are the first bones of skull to appear. They ossify with particular regularity at a CRL of 26.5 mm or 27 mm respectively. During the further development of skull the sequence of ossification of Parietal bone and Temporal bone and of Nasal bone and Zygomatic bone varies. The hypothesis is suggested that the observed variations in the development of sheep skeleton are caused by racial differences of the material investigated.