What is "primitive streak"

Primitive \Prim"i*tive\, a. [L. primitivus, fr. primus the first: cf. F. primitif. See Prime, a.]

1. Of or pertaining to the beginning or origin, or to early times; original; primordial; primeval; first; as, primitive innocence; the primitive church. ``Our primitive great sire.''
   --Milton.

2. Of or pertaining to a former time; old-fashioned; characterized by simplicity; as, a primitive style of dress.

3. Original; primary; radical; not derived; as, primitive verb in grammar.

   Primitive axes of co["o]rdinate (Geom.), that system of axes to which the points of a magnitude are first referred, with reference to a second set or system, to which they are afterward referred.

   Primitive chord (Mus.), that chord, the lowest note of which is of the same literal denomination as the fundamental base of the harmony; -- opposed to derivative.
   --Moore (Encyc. of Music).

   Primitive circle (Spherical Projection), the circle cut from the sphere to be projected, by the primitive plane.

   Primitive colors (Paint.), primary colors. See under Color.

   Primitive Fathers (Eccl.), the acknowledged Christian writers who flourished before the Council of Nice, A. D. 325.
   --Shipley.

   Primitive groove (Anat.), a depression or groove in the epiblast of the primitive streak. It is not connected with the medullary groove, which appears later and in front of it.

   Primitive plane (Spherical Projection), the plane upon which the projections are made, generally coinciding with some principal circle of the sphere, as the equator or a meridian.

   Primitive rocks (Geol.), primary rocks. See under Primary.

   Primitive sheath. (Anat.) See Neurilemma.

   Primitive streak or Primitive trace (Anat.), an opaque and thickened band where the mesoblast first appears in the vertebrate blastoderm.

   Syn: First; original; radical; pristine; ancient; primeval; antiquated; old-fashioned.

n. (''embryology'') A formation that occurs during the initial phases of embryonic development in birds, reptiles and mammals.

The primitive streak is a structure that forms in the blastula during the early stages of avian, reptilian and mammalian embryonic development. It forms on the dorsal (back) face of the developing embryo, toward the caudal or posterior end.

The presence of the primitive streak will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation. To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the second embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation. The primitive streak extends through this midline and creates the left–right and cranial–caudal body axes, and marks the beginning of gastrulation. This process involves the ingression of mesoderm progenitors and their migration to their ultimate position, where they will differentiate into the mesoderm germ layer that, together with endoderm and ectoderm germ layers, will give rise to all the tissues of the adult organism.