Mosaic and Regulation Phenomena during the Early Formation of the Chick Blastoderm
(a): Ventral view of living symmetrical halves of an unincubated chicken blastoderm, after hemisectioning through the middle part of the Rauber’s sickle (RS). AS, anti-sickle region where no Rauber’s sickle material is present; SH, sickle horn. Bar = 1 mm. (b): Schematic representation of a symmetrically hemisectioned unincubated blastoderm as seen in Figure 1(a), to indicate its distinct components; AS, anti-sickle region; RS, half Rauber’s sickle; SH, sickle horns. Tops are colored in green since they are often fragmentary and not always distinctly visible. Although containing Rauber’s sickle material, they behave differently from the median part of Rauber’s sickle. R, right hemi-blastoderm; L, left hemi-blastoderm; E, endophyll. The thick, red half-arrows represent the formation, after culture of hemi-primitive streaks at the cut edges of the blastoderm. CMZ, caudal marginal zone; CGW, caudal germ wall. The blue interrupted arrows indicate the localization of hypothetical symmetrical primitive streaks with reference to the half Rauber’s sickles, if only regulation phenomena would occur, according to the median line, going through the three elementary tissues in one half blastoderm.
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