“…In chick embryos, the flexed posture of the neck, spine, wings, and legs becomes progressively more extreme in the final week before hatching (plates 1-3, Hamburger & Oppenheim, 1967). Kinematic study of RLMs in ovo indicated that at E20, hip, knee, and ankle excursions averaged 3°, compared to an average of approximately 8°at E18 (Bradley et al, 2014) and almost 50°at E12 (Bradley, Solanki, & Zhao, 2005). This flexed posture may be mechanically imposed by spatial constraints, as when the embryo or fetus begins to outgrow the volume of the prenatal environment and there is no longer room to extend the limbs.…”