Three-day-old chick embryos were cultured in Petri dishes until they reached developmental stages 37 or 39 (Hamburger & Hamilton, 1951). The electron microscopical study of the chorio-allantoic membranes showed that, even in the absence of the shell and the corresponding calcium supply, 'capillary-covering' and 'villus-cavity' cells differentiated well. The parathyroid glands from the cultured embryos showed ultrastructural signs indicative of active synthesis and secretion of the parathyroid hormone. This correlates well with the significant hypocalcemia (5.4 mg/100 ml) observed in these embryos. In all cultured embryos mineralization of bones was greatly reduced as shown by alizarin bulk staining and confirmed by histological and electron microscopical analysis. The ultrastructural characteristics of osteoblasts and osteocytes, as well as of the bone matrix, appeared normal. The defect in mineralization appeared thus to be due to a deficiency in the availability of calcium and not to a delay in bone differentiation. This implies that the yolk sac appears to lack calcium regulatory capacity since it cannot compensate for the absence of shell by increasing its own contribution in order to assure an adequate mineralization of the embryonic bones.