primitive streak n.
An ectodermal ridge in the midline at the caudal end of the embryonic disk from which the intraembryonic mesoderm arises.
The primitive streak is however the most interesting structure in the area pellucida at this stage.
The median line of the primitive streak becomes marked by a shallow groove, known as the primitive groove.
Running forward from this is a band—the primitive streak—in the centre of which lies a darker line—the primitive groove.
The mesoblast still remains attached to the epiblast along the line of the primitive streak.
Our view as to the elongation of the primitive streak appears to be that adopted by Gerlach.
The primitive streak now ends with an anterior swelling (not represented in the figure), and is usually somewhat unsymmetrical.
Section through the anterior part of the area pellucida in front of the primitive streak.
The anterior is formed from what was originally the apparent posterior part of the primitive streak.
Section through the primitive streak of an eight hours' blastoderm.
The neurenteric passage is shewn at ne, and behind it is the front part of the primitive streak.