Allantois -

: In humans, the allantois is largely vestigial as a sac but remains vital as a structural precursor. Its blood vessels become the umbilical arteries and veins , connecting the fetus to the placenta for nutrient and gas exchange. 3. Developmental Fate in Humans

While its fundamental purpose is life support, the specific role of the allantois varies significantly between species: allantois

The is a critical extra-embryonic membrane found in the embryos of reptiles, birds, and mammals that primarily functions in waste storage and gas exchange . In egg-laying species, it acts as a repository for nitrogenous waste and works with the chorion to facilitate respiration, while in most mammals, it evolves to form the structure of the umbilical cord. 1. Evolutionary Significance and Structure : In humans, the allantois is largely vestigial

: The allantois is typically small and unattached in most marsupials, though it does fuse with the chorion in specific groups like bandicoots and koalas. Developmental Fate in Humans While its fundamental purpose

: In these egg-layers, the allantois expands until it fuses with the chorion to form the chorioallantoic membrane . This membrane serves as a "lung," allowing oxygen to enter the egg and carbon dioxide to leave. It also acts as a storage tank for uric acid, a non-toxic form of nitrogenous waste.

: It is derived from the splanchnopleure, which consists of endoderm and splanchnic mesoderm. 2. Functional Diversity Across Species

myesr.org/poster/esr/ecr2020/C-10164/background">urachal cysts or other developmental remnants?