Sources of calcium and phosphorus during embryogenesis in bullsnakes (Pituophis melanoleucus)

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Abstract
The yolk of eggs of bullsnakes (Pituophis melanoleucus) contains an average of 88 mg of calcium and 23 mg of phosphorus at oviposition. Calcium and phosphorus in yolk decline dramatically during the growth phase of development, and residual yolk removed from hatchlings contains only 3 mg of calcium and 1 mg of phosphorus. Thus, bullsnake embryos deplete the yolk of virtually all of its calcium and phosphorus, and residual yolk does not contain sufficient quantities of these ions to support significant posthatching growth. Calcium and phosphorus in embryos increase during incubation, and carcasses of hatchlings contain an average of 113 mg of calcium and 21 mg of phosphorus. Total calcium content (calcium in yolk plus calcium in carcass) of eggs increases significantly from 88 mg at oviposition to 116 mg at the end of incubation, but total phosphorus content of eggs does not vary significantly. Embryonic bullsnakes obtain about 24% of their calcium from the eggshell but rely on the yolk for all of their phosphorus. Bullsnake embryos seemingly mobilize considerably more calcium than is required for skeletogenesis. This excess calcium may be stored in endolymphatic sacs, but the functional significance of such deposits is unknown.