Early History and Mechanics of the Chick Blastoderm: A Review

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Abstract
The two themes of research herein reviewed are those of embryology generally. The first deals with the description of normal development for the purpose of localization of organ-forming germinal areas; the 2d with the properties of these areas for growth and differentiation. The two-layered condition of the blastoderm at the time of laying is probably derived by delamination from the original cleavage disc as an upper epiblast and a lower hypoblast. In the formation of the primitive streak, cell proliferation, lateromedian cell movements in the epiblast, invagination, and intussesceptive growth, all play a part. The concrescence theory of streak formation is seen as "having been one of the most fruitful errors of embryology." The formation of chorda from primitive knot, somitic and lateral mesoderm from knot and anterior portion of the streak, respectively, are discussed. Other subjects critically reviewed are primary (anterior) and secondary (posterior) body development; localization of prospective areas and their interactions; methods employed in investigation. Methods used for study of inductions are still inadequate while histochemical methods used for observing internal changes leave something to be desired. In conclusion 2 maps are given, the first showing distribution of potencies of the definitive primitive streak as tested under various exptl. conditions and the 2d showing prospective areas of the definitive streak blastoderm.

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