The allantois is approximately T-shaped, once it and the amniotic sac are fully formed. The (horizontal) crossbar of the T is very long and runs the length of the __1__. The ends of the crossbar are dilated and fully occupy the extremities of the __1__. The central section of the crossbar is narrow and flattened; it lies between the __2__ and the __3__ on the right side (relative to the position of the embryo/fetus). The (vertical) shaft of the T connects the crossbar to the __4__ of the embryo/fetus. Since the crossbar lies to the right of the __2__, it is clear that the distal part of the shaft must curve dorsally to reach the __4__.