Light microscopy of a fetal rat skeleton. Cartilage is stained blue and newly forming bone is stained red. Tissues unstained are undifferentiated connective tissue. Most of the future adult bones begin their development as cartilage tissue which in fetal and postnatal life are gradually replaced by bone tissue. This process is called endochondral ossification. When bones form directly from connective tissue (several skull bones) this is called intramembranous ossification. Magnification x4 when printed at 10 cm.