The allantois evaginates on day (1) and is initially restricted to the central region of the ovoid chorionic sac. As it expands into the exocoelom, the allantois pushes (2), between the chorion and amnion, on the (3) side of the embryo (which lies parallel to the longitudinal axis of the chorionic sac). Once it has extended over the amniotic sac, it continues (4) on the (5) side and pushes between the chorion and detached yolk sac. Ventral to the yolk sac it meets that portion of the allantois, which expanded (4) on the (3) side (apparently, the left and right portions of the allantois do not fuse when they meet - see Figs. 36 and 37). The allantois later also grows into the cranial and caudal ends of the chorionic sac.