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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.

Blastoderm of a Chick

Vertical section of blastoderm of chick (1st day of incubation). S, epiblast consisting of short columnar…

"Gastrulation of an Ascidian. I. Vesicular Morula of an Ascidian flattened and about to undergo gastrulation. II. Gastrulation effected. fh, cavity of the morula, or blasto-invagination; ch, dd, large blastomeres of the hypoblast, inclosed in small blastomeres of the epiblast." -Whitney, 1911

Sea Squirt Gastrulation

"Gastrulation of an Ascidian. I. Vesicular Morula of an Ascidian flattened and about to undergo gastrulation.…