Figure 1.

Carnegie images depicting stage 4, 5a, and 5c human embryos. These histological sections from the original Carnegie collection represent approximately days 6–12 postfertilization. At stage 4 the polar trophectoderm attaches and adheres to the decidualized endometrium. By stage 5, the CTB has given rise to a multinucleated primitive syncytium that has begun to invade and form lacunae. Carnegie stage images are from the Virtual Human Embryo Project at Louisiana State University with permission to reprint (http://virtualhumanembryo.lsuhsc.edu).