Human Embryo
Most embryos at stage 23 are approximately 56-57 postovulatory days old and measure 23-32 mm in length. Distinguishing criteria for this stage include fusion of the eyelids at the medial and lateral margins, clear distinction of the subdivisions of the upper and lower limbs, the forearms appear at or above the level of the shoulders, the superficial vascular plexus of the head is very close to the vertex, and the external genitalia are well developed but not always sufficiently to distinguish the embryo's sex.
(NOTE: These specimens are late stage 23.)