Abstract
The ddN and CF#1 mouse and Wistar rat were examined for the time and order of chondrification during their fetal lives. In the present paper the strain- and species-differences were mentioned with special reference to the development of vertebral bones. The fetuses were removed surgically from the mothers at gestational ages of 12 to 18 days in the mouse and 13 to 20 days in the rat at one day intervals. Their skeletons were examined in methylene-blue stained cleared specimens and alizarin red S stained ones. The chondrification at the initial stage of skeletal development in the ddN mouse started a little later than the CF#1 mouse. But the features of skeletons in the late gestational days showed no difference between the strains. The skeletal development in the rat fetuses was generally one day later at the initial stage of chondrification and was two days later in the late gestational days than both in the ddN and CF#1 mouse. The ossification center in the anterior arch of atlas appeared on the 16th day of gestation in the mouse and on the 20th day in the rat. This difference between the species was the most prominent in the skeletal development.
