The bone marrow fills the space in the ring of osteoblasts, and eventually fills the bony matrix. Since flat bones are usually thinner than the long bones, they only have red bone marrow, rather than both red and yellow bone marrow (yellow bone marrow being made up of mostly fat). This hardened matrix forms the body of the bone. They form a ring in between the membranes, and begin to expand outwards. The fontanelles contain connective tissue stem cells, which form into osteoblasts, or bone cells which secrete calcium phosphate into a matrix of canals. On a baby, those spots are known as fontanelles. Ossification is started by the formation of two connective tissue membranes that hold the spot where the flat bone is to come. The flat bones are: the occipital, parietal, frontal, nasal, lacrimal, vomer, scapula, os coxæ (hip bone), sternum, and ribs. The intervening cancellous tissue is called the diploë, and this, in certain regions of the skull, becomes absorbed so as to leave spaces filled with air (air-sinuses) between the two tables. In the cranial bones, the layers of compact tissue are familiarly known as the tables of the skull the outer one is thick and tough the inner is thin, dense, and brittle, and hence is termed the vitreous table. In an adult, most red blood cells are formed in flat bones. These bones are composed of two thin layers of compact tissue enclosing between them a variable quantity of cancellous tissue, which is the location of red bone marrow.

Long bone vs flat bone