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. 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
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. Most other bones (e.g. vertebrae) also have primary ossification centers, and bone is laid down in a similar manner. Secondary centers The secondary centers generally appear at the epiphysis. Secondary ossification mostly occurs after birth (except for
the epiphyses. Ossification center The first step in ossification of the cartilage is that the cartilage cells, at the point where ossification is commencing and which is termed as an ossification center, enlarge and arrange themselves in rows. The matrix in which they are imbedded increases in quantity, so that the cells become further separated from each other. A deposit of calcareous material now takes place in this matrix, between the rows of cells, so that they become separated from each other by longitudinal columns of calcified matrix, presenting a granular and opaque appearance. Here and there the matrix between
Ossification center The first step in ossification of the cartilage is that the cartilage cells, at the point where ossification is commencing and which is termed as an ossification center, enlarge and arrange themselves in rows. The matrix in which they are imbedded increases in quantity, so that the cells become further separated from each other. A deposit of calcareous material now takes place in this matrix, between the rows of cells, so that they become separated from each other by longitudinal columns of calcified matrix, presenting a granular and opaque appearance. Here and there the matrix between two cells
growth. The process of appositional growth occurs when the cartilage model also grows in thickness due to the addition of more extracellular matrix on the peripheral cartilage surface, which is accompanied by new chondroblasts that develop from the perichondrium. The first site of ossification occurs in the primary center of ossification, which is in the middle of diaphysis (shaft). Then: About the time of birth in mammals, a secondary ossification center appears in each end (epiphysis) of long bones. Periosteal buds carry mesenchyme and blood vessels in and the process is similar to that occurring in a primary ossification center.
When do primary ossification centers appear in an embryo?

Reference answer:
prenatal development