Blood production —The marrow, located within the medullary cavity of long bones and the interstices of cancellous bone, produces blood cells in a process called haematopoiesis.
Ground meristem The central region of the apical meristem from which the ground tissues of pith, cortex, medullary rays, and mesophyll differentiate. In root meristem it is also called periblem.
This ends up causing new bone formation and the medullary cavites of the skull can expand outward, which causes enlarged cheeks and a " hair-on-end" appearance on skull x-ray.
Of course, the technique is not foolproof, as only female dinosaurs that are preparing to lay eggs, not juveniles or ones that just recently laid eggs, show medullary bone.
These flared ends, or epiphyses bookend the bone's shaft, or diaphysis, which — instead of having spongy bone in the center — surrounds a hollow medullary cavity that's full of that yellow marrow.
Another part of this study also showed that the fossils of those brooding dinosaurs didn't have medullary bone, a type of bone tissue found in ovulating female birds that remains for a short time after they lay their eggs.