The formation of mineral and organic matrix of fowl cortical and medullary bone during shell calcification

Synopsis 45Ca and [3H]‐lysine were injected into laying fowls up to 15 h after the start of shell calcification. The birds were killed 3 h later and the isotopes assayed in extracts of the medullary and cortical fractions of the tibias and femurs. It was found that medullary bone is extensively resorbed and resynthesised during egg shell calcification. It is maximally loaded with calcium about 8 h after the initiation of calcification. Cortical bone, on the other hand, is inert relative to medullary bone but may be resorbed during the early stages of shell calcification, probably providing some calcium for medullary synthesis. The absolute amounts of both types of bone were not found to vary during the egg‐laying cycle.