Answer: allantois

AnswerCrossword Clue
allantoisa saclike fetal membrane developing into umbilical cord and forming part of the placenta also ALLANTOID
allantoisThe fetal membrane lying below the chorion in many vertebrates, formed as an outgrowth of the embryo's gut. In birds and reptiles it grows to surround the embryo; in eutherian mammals it forms part of the placenta