The amniotic sac largely occupies the central portion of the _____1______. However, a narrow portion of the __2__ lies ventro-laterally and to the right of the amniotic sac. The amnion thus lies against the __3__ dorsally, laterally on the left, dorso-laterally on the right, and ventrally (except for a narrow central area occupied by the __2__); the amnion and __3__ fuse to form __4__ in these areas.