In the SPF Sprague-Dawley(SD) rats, the order and time of ossification was investigated from day 16 to day 21 post conceptionem(p.c.). In addition. the rate of ossification of some skeletal districts on day 19, 20 and 21 p.c. was compared. The following results were obtained.
1. Day 17-fetuses showed the ossification of the clavicula, mandibula, humerus and ribs. In day 18-fetuses the skull(os frontale, as parietale and as occipitale), scapula, radius, ulna, femur. tibia, fibula, as ilium and the vertebral arch(cervical, thoracic and lumbar) were ossified.
2. The rate of ossification in day 2.1-fetuses seems to be the most reliable index for evaluating retarded ossification in teratogenic studies in rats, because the ossification of sternebrae, metacarpi, metatarsi, and phalanges distales of hand and foot in day 21-fetuses was sufficiently advanced.
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