The study reported here is the result of a detailed investigation of the changes in shape of the femur with growth and torsion, the type, rate and character of ossification, and the onset, subsequent course, and general features of remodeling.

Forty pairs of femurs, from a series of embryos and fetuses ranging from 26 to 342 mm in crown-rump length, were measured, radiographed, and sectioned for microscopic study.

A primary bony collar was present before the end of the embryonic period, and in a 27 mm embryo it extended for about one-fifth of the length of the femur. Erosion of the collar was evident at 34 mm. and invasion and destruction of calcified cartilage were occurring by 37 mm. Cartilage canals first appeared in the proximal epiphysis at 57 mm and in the distal epiphysis at 61 mm.

Along with the progression of endochondral ossification proximally and distally and the establishment of growth zones, periosteal bone formation also proceeded in both directions, and, until 275 mm, extended about 1 mm beyond the zones of cartilage destruction. After 275 mm, the extents of periosteal and endochondral ossification were the same and at term occupied almost four-fifths of the length of the femur.

Trabeculation of the bony collar was first noted at 37 mm. Fusion of endochondral trabeculae with the inner aspect of the periosteal shell began by 61 mm. A central marrow cavity free of trabeculae was present at 86 mm and thereafter. Evidence of reconstruction appeared in both proximal and distal ends by 92 mm, and was consistently present in both ends in all specimens of 111 mm and larger.