: The allantois is typically small and unattached in most marsupials, though it does fuse with the chorion in specific groups like bandicoots and koalas.
: 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. allantois
myesr.org/poster/esr/ecr2020/C-10164/background">urachal cysts or other developmental remnants? : The allantois is typically small and unattached
: It is present in all amniotes, including reptiles, birds, and mammals. allantois