streak

A line or stripe, germinal s. See primitive s. medullary s. See medullary groove, under groove, primitive s. Syn.: axial plate, primitive trace. In embryology, a temporary structure consisting of a sickle-shaped opacity (which becomes a strap like thickening) of the germinal disk of the blastoderm, starting from that side of the disk which afterward becomes the narrow end, and gradually extending to or beyond the center of the transparent area; a linear or strap like ma of cells formed by direct proliferation from the lower (inner) cells of the epiblast, constituting the origin of the mesoblast. It is the first indication of the lineaments of the future embryo. Supposed by some writers to be the vestige of a blastopore. [Ang.- Sax., strica, line, from strican, to go.]