In order to remove remaining unwanted cells on layers that were peeled apart, one can perform a shearing operation. For example, when making sandwich explants from older gastrula-stage embryos, mesodermal cells will often adhere to the inner surface of the blastocoel wall. The following method can be used to remove them. The explant is positioned with its deep surface uppermost. The eyebrow hair is turned such that a considerable length of its tip lies parallel to the surface and just above the explant, to the right of the area of contamination (Fig. 4C). The tip of the eyebrow hair should be inclined at an angle of about 20-30° off a line running vertically from the bottom to the top of the field of view. The hairloop is then placed against the near edge of the explant, just to the left of the eyebrow hair; this will prevent the explant from moving toward the experimenter. The eyebrow hair is repeatedly and rapidly brought down until it just touches the surface of the explant, and back towards the experimenter, always remaining in its original plane. The restraining hairloop is moved slowly to the left and followed with the eyebrow hair, shearing off any recalcitrant, adherent cells. The eyebrow hair should be oriented at a low angle of attack, since a low attack angle does not exert large forces on the explant (Fig. 4C). The eyebrow hair and hairloop can be switched right to left, and the process begun at the other edge of the explant, if one prefers.
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