The Stages of Human Embryonic Development

Stage 6 embryos are characterized by the first appearance of chorionic villi. The amniotic cavity and secondary umbilical vesicle (yolk sac) are well defined by this stage. The embryo is in the presomite, disc period and axial differentiation is present for the first time. During this stage the embryonic endoderm and the gastrulation (primitive) streak first appear. The embryonic disc varies from 0.15 to 0.5 mm in maximum diameter. Initially, stage 6 embryos were believed to be about 13 days postfertilization, however, more recently, O'Rahilly & Muller (1996) have reexamined the ages assigned to the Carnegie stages and have estimated that stage 6 is ca. 17 days postfertilization.