PATTERNS OF MOBILIZATION AND DEPOSITION OF CALCIUM IN EMBRYOS OF OVIPAROUS, AMNIOTIC VERTEBRATES

Embryos of oviparous, amniotic vertebrates obtain calcium for skeletog enesis from both the yolk and the eggshell, but the importance of thes e sources varies among major groups, as does the pattern of deposition of calcium during development. Snake eggs have a rich supply of calci um in the yolk, and snake embryos rely more on the yolk than on the li ghtly calcified eggshell for the calcium needed for skeletogenesis. Em bryonic lizards obtain a larger proportion of their calcium from the e ggshell, but even so, more than half of the calcium required for devel opment is provided by the yolk. Chelonian and archosaurian embryos, on the other hand, rely on the eggshell as the principal source of calci um. Calcium obtained from the eggshell by embryonic chelonians is depo sited primarily, if not exclusively, in the carcass of the developing embryo. Some archosaurian embryos deposit a small proportion of shell calcium in the yolk, whereas others deposit shell calcium almost exclu sively in the yolk. Control of shell calcium mobilization and control of calcium transport by the yolk sac and chorioallantoic membranes hav e the potential to contribute importantly to calcium homeostasis durin g embryogenesis. Unfortunately, our knowledge of the mechanism and con trol of these processes is rudimentary even for embryos of well-studie d species like the domestic fowl, and the subject has not been address ed at all in other species of oviparous, amniotic vertebrates.