Diagrammatic representation of the developing mouse embryo from the neural plate (allantoic bud) stage to the 22-26 somite (s) stage highlighting the development of the allantois and its vasculature. The age in embryonic days (E), and the stages of development are indicated across the top, as are the developmental processes taking place in the allantois. The allantois first appears as a mesodermal bud emerging from the posterior end of the primitive streak. The bud grows and expands across the exocoelomic cavity and fuses with the chorion to form the chorioallantoic placenta. The processes in allantois vessel development at each stage are indicated across the bottom of the figure. ECs differentiate and migrate to form a primary vascular plexus. The plexus connects with the dorsal aortae of the embryo before chorio-allantoic fusion. The primary plexus remodels to form the central vessel; later, the umbilical artery and the umbilical vein form at E9.5. After chorio-allantoic fusion, the allantoic vessels invade the chroion and form the fetal vascular component of the labyrinthine layer of the placenta. C indicates allantois core; CAP, chorio-allantoic plate; DA, dorsal aorta; epc, ectoplacental cone; exc, exocoelomic cavity; lab, placental labyrinth; m, mesothelium; NP, neural plate; UA, umbilical artery; UV, umbilical vein; and ys, yolk sac.