Blastoderm of an Egg
Vertical section of area pellucida and area opaca (left extremity of figure) of blastoderm of a fresh laid egg (unincubated). Labels: S, superficial layer corresponding to epiblast; D, deeper layer, corresponding to hypoblast; and probably in part to mesoblast; M, large “formative cells,” filled with yolk granules, and lying on the floor of the segmentation cavity; A, the white yolk immediately underlying the segmentation cavity.
Baker, W. Morrant & Harris, Vincent Dormer Kirkes' Hand-book of Physiology, 13th ed. (Philadelphia: P. Blakiston's Son & Co., 1892) 793