Darlene Casey, age l2, of Fredericton, N.B., Canada, for her question:

What is bone marrow?

The outside of a big bone  is stiff and strong, but its inside is soft and spongy. The outside is made of calcium and phosphorus and combinations of other sturdy minerals. The inside is stuffed with softer spongy material and bone marrow. This red or yellow marrow is a mass of living celis filled with networks of nerves and blood vessels.

The rope of red marrow inside a big bone is a factory where red blood cells are manufactured. One by one these vital cells seep out into the blood stream to replace tired old red cells on their way to be broken up by the liver. Down the center of a long Ieg bone there is a core of soft yellow marrow. Its soft cells contain fatty materials and as in the red marrow, its tissues are supplied with nerves, lymph and blood.