Embryos at 4 to 8 Weeks
Rapid differentiation of cells and an astounding rate of growth characterize the first weeks of embryonic development. At 4 weeks, the embryo is the size of a grain of rice. Its heart has already begun to beat, and the early divisions of what will be the heart's four chambers are apparent. At 6 weeks, the embryo may be half an inch (10-14 mm) long and is starting to acquire a human face, although it is impossible to differentiate male from female embryos at this stage. An 8-week-old embryo may measure over an inch (28-30 mm) in length, and all of the body's parts-cells, tissues, organs, systems-have been differentiated.