The Stages of Human Embryonic Development

Stage 7 embryos are in the presomite period and have a well defined embryonic disc that typically has a greatest diameter between 0.3 and 0.7 mm. The embryonic disc can vary in shape from oval to pyriform but most specimens have a circular disc. The disc is symmetrical and slightly convex in the plane of its longitudinal axis. The age of stage 7 embryos was initially believed to be about 16 days postfertilization, however, O'Rahilly and Müller (2001) have reassigned the ages of the Carnegie stages and have placed stage 7 at ca. 18 - 21 days. The stage is characterized by the appearance of the notochordal process and the gastrulation (primitive) node.