Os penis of the rat. III. Formation and growth of the bone.

In a series of studies of morphology and morphogenesis of os penis in the rat, the present study deals with initial formation and growth of the bone. Os penis in the rat develops at 1 day p.n. as a membrane bone. It becomes associated with a proximally situated growth cartilage at 3-4 days. Until 10 days it increases in width by periosteal apposition on all surfaces. After 10 days the growth in width takes place by differential processes of apposition and resorption. Results from vital staining with alizarin red S of bones at different ages indicate that os penis during its growth becomes transformed in a ventral direction in the period between 10 and 35 days, while it after that age is transformed in a dorsal direction. No reasonable explanation on this phenomenon is offered. However, the observed periods of time of uniform growth pattern are comparable to time periods of uniform growth patterns of a number of cranial bones.