Area Embryonalis

AREA EMBRYONALIS (from Latin area-field and embryonalis-embryonic), embryonic field (synonyms: embryonic spot, embryonic shield), a white spot appearing on the transparent embryonic vesicle of mammals at the beginning of development; at first it appears as a round shield, later becoming oval or pear-shaped and representing the primary embryonic anlage. The area embryonalis of mammals is three-layered in the early stage and consists of 1) trophoblast or Rauber's exfoliating layer, 2) ectoderm, and 3) entoderm; after the exfoliation of Rauber's layer, the area embryonalis becomes two-layered. In humans, the area embryonalis forms inside the trophoblast, at the point of contact of two vesicles (amnion and yolk, or ectoblastic and entoblastic), and is composed of cells lining the cavities of these vesicles; thus, initially the area embryonalis in humans consists of elements of ecto- and entoderm, to which the middle layer is later added. In later stages of development, the area embryonalis is contrasted with the extra-embryonic (vascular) field.