#fetal development

• Allantois — A sac-like, highly vascular embryonic membrane characteristic of reptiles, birds, and mammals, developing as a diverticulum of the fetal hindgut; a structural organ with vital functions in embryonic respiration, excretion, and nutrition, actively participating, in placental mammals, in the formation of the umbilical cord and placenta through its dense network of blood vessels.
• Amnion — The innermost, thin, and tough extraembryonic membrane that delineates the gestational sac in reptiles, birds, and mammals. This cellular structure secretes the amniotic fluid, providing the embryo and later the fetus with a sterile, thermally constant, and protective aquatic environment against mechanical shocks or destructive tissue adhesions.