Blastoderm of a Chick
Vertical section of blastoderm of chick (1st day of incubation). S, epiblast consisting of short columnar cells; D, hypoblast, consisting of a single layer of flattened cells; M, formative cells." They are seen on the right of the figure, passing in between the epiblast and hypoblast to form the mesoblast; A, white yolk granules. Many of the large "formative cells" are seen containing these granules.