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

As human gestation progresses, the allantois undergoes a transformation:

: The intra-embryonic portion of the allantois contributes to the development of the urinary bladder. After birth, it regresses into a fibrous cord known as the urachus or the median umbilical ligament. Summary of Key Roles Species Context Excretion Stores uric acid and nitrogenous waste. Birds and Reptiles Respiration Facilitates gas exchange via the chorioallantoic membrane. Birds and Reptiles Circulation Develops into the umbilical blood vessels. Humans and Mammals Organogenesis Contributes to the formation of the urinary bladder. allantois

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

The allantois is one of the four essential membranes of the —an evolutionary milestone that allowed vertebrates to transition from aquatic to terrestrial environments. It originates as a hollow, sac-like diverticulum from the hindgut (posterior region) of the embryo. : In humans, the allantois is largely vestigial

: 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.

: The allantois and yolk sac are incorporated into the connecting stalk to form the umbilical cord. Developmental Fate in Humans As human gestation progresses,

: The allantois is typically small and unattached in most marsupials, though it does fuse with the chorion in specific groups like bandicoots and koalas.