Tissue densities in developing avian embryos.
Changes in the weight:mass ratio provide the physical basis for the biological responses of terrestrial organisms to alteration in the ambient acceleration field. Where organisms such as aquatic animals occupy dense media, changes in the gravitational environment produce compensating changes in the weight:mass ratios of organism and medium, such that little net load is imposed upon the organism. This relationship also applies to organs of terrestrial animals. Changes in the ambient acceleration field produce compensatory changes in surrounding tissues so that the organ may not develop a significant net load. This relationship has been investigated in the case of the vertebrate brain. However, density gradients within the organ/organism will produce a local gravitational loading, which may lead to biological responses. In fact, a significant density gradient would be an essential character for a gravity receptor. Prenatal development, both in mammals and birds, occurs characteristically in a buoyant condition. In both cases a volume of amniotic fluid develops and surrounds the embryo while it is still of microscopic size. This situation prevails until the latest stages of prenatal development. In mammals the amniotic fluid is lost immediately prior to parturition through rupture of the sac. In chick embryos the amniotic fluid is ingested, beginning on the 13th day with the process being completed by the 18th day of development, just prior to the pre-hatch reorientation of the embryo. Consequently, a net load upon the embryo/fetus is not considered to be a major factor in gravitational experiments of prenatal development. Prenatal development includes marked changes in chemical composition as well as changes in size. This is readily apparent from extensive and detailed examination of the chemical growth for the chick embryo. These chemical materials vary in density, as well as in distribution among the tissues of the developing organism. Consequently, the existence of density gradients, and changes in them may be anticipated during prenatal development.