Definition of primitive streak in US English:

primitive streak


  • The faint streak which is the earliest trace of the embryo in the fertilized ovum of a higher vertebrate.

    • ‘Some consider life to begin at conception, others see it starting with development of the primitive streak, while others believe life starts with birth.’
    • ‘After fourteen days, there begins to appear the first traces of what will become the central nervous system of the embryo, the primitive streaks (or the two primitive streaks, since at about this time identical twins may be formed).’
    • ‘Some have argued that because an embryo can split before the primitive streak appears and form the bases of identical twins, it does not merit the same protection as the embryo that is the basis for a definite individual person.’
    • ‘Some embryologists and ethicists insist that only after the primitive streak develops can this being be called an embryo.’
    • ‘It is at fourteen days that an embryo first begins to develop the primitive streak, the first indication of a nervous system.’