Webster's English Dictionary

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allantois

al.lan.to.is \*-'lant-*-w*s\ \.al-*n-'to--*-.de-z\ n or al.lan.to.ides [NL, 
   deriv. of Gk allant-, allas sausage] pl  : a vascular fetal membrane of 
   reptiles, birds, or mammals formed as a pou ch from the hindgut and in 
   placental mammals intimately associated with the chorion in formation of 
   the placenta