<P> The nutrient artery enters via the nutrient foramen from a small opening in the diaphysis . It invades the primary center of ossification, bringing osteogenic cells (osteoblasts on the outside, osteoclasts on the inside .) The canal of the nutrient foramen is directed away from more active end of bone when one end grows more than the other . When bone grows at same rate at both ends, the nutrient artery is perpendicular to the bone . </P> <P> Most other bones (e.g. vertebrae) also have primary ossification centers, and bone is laid down in a similar manner . </P> <P> The secondary centers generally appear at the epiphysis . Secondary ossification mostly occurs after birth (except for distal femur and proximal tibia which occurs during 9th month of fetal development). The epiphyseal arteries and osteogenic cells invade the epiphysis, depositing osteoblasts and osteoclasts which erode the cartilage and build bone . This occurs at both ends of long bones but only one end of digits and ribs . </P> <P> Several hypotheses have been proposed for how bone evolved as a structural element in vertebrates . One hypothesis is that bone developed from tissues that evolved to store minerals . Specifically, calcium - based minerals were stored in cartilage and bone was an exaptation development from this calcified cartilage . However, other possibilities include bony tissue evolving as an osmotic barrier, or as a protective structure . </P>

When do secondary ossification centers appear in the epiphysis