primitive streak

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streak

 [strēk]
a line, stria, or stripe.
angioid s's red to black irregular bands in the ocular fundus running outward from the optic disk.
primitive streak a faint white trace at the caudal end of the embryonic disk, formed by movement of cells at the onset of mesoderm formation, providing the first evidence of the embryonic axis. It eventually undergoes degenerative changes and disappears.

prim·i·tive streak

ridge of epiblast in the midline at the caudal end of the embryonic disc from which arises the intraembryonic mesoderm and definitive endoderm; achieved by inward and then lateral migration of cells; in human embryos, it appears on day 15 and provides visual evidence of the cephalocaudal axis.

prim·i·tive streak

(prim'i-tiv strēk)
An ectodermal ridge in the midline at the caudal end of the embryonic disc from which arises the intraembryonic mesoderm; achieved by inward and then lateral migration of cells; in human embryos, it appears on day 15 and gives a cephalocaudal axis to the developing embryo.

primitive streak

a line of cells in the centre of the embryonic disc of reptiles, birds and mammals that forms the future axis of the embryo.

prim·i·tive streak

(prim'i-tiv strēk)
An ectodermal ridge in the midline at the caudal end of the embryonic disc from which arises the intraembryonic mesoderm; achieved by inward and then lateral migration of cells.