What is "primitive groove"

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.

A shallow groove, the primitive groove, appears on the surface of the primitive streak, and the anterior end of this groove communicates by means of an aperture, the blastophore, with the yolk-sac.