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Ultrastructure of the blood vessels in the chick allantois and chorioallantois.

The earliest rudiment of the chick allantois is a diverticulum of presumptive gut coated with mesoderm, which projects into the extra-embryonic coelom towards the end of the 2nd day of incubation, at the 20-28 somite stage (Hamilton, 1952; Romanoff, 1960; Leeson & Leeson, 1963). After 72 h, the allantoic vesicle measures only 1 mm in diameter. It fuses with the chorion after 100 h of incubation to form the chorioallantois. This lines the whole of the inner shell membrane by the 12th day of incubation.