The early development of human embryos.

The development of the human embryo from the time of fertilization through the eighth post-fertilization week is described for medical policy purposes. During pre-implantation stages, differentiation occurs between precursors of embryonic and extra-embryonic structures. During implantation formation of a fore-hind axis begins within the inner cell mass. By the end of the eighth week, head, face, hands, and feet are suggestive as to species-recognition but not yet definitive. Data from laboratory studies of non-human mammalian embryos elucidate important aspects of human embryonic development.