The combined membrane formed when the allantois and chorion fuse together during fetal development in mammals.
From allantois (embryonic membrane) + chorion (another embryonic membrane) + -on (neuter ending). These Greek-derived terms describe structures medieval anatomists identified in embryos.
The allantochorion is nature's biological engineering feat—it's how mammals create a blood-delivering interface between mother and baby, and without it, human pregnancy couldn't happen at all.
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