The faint streak which is the earliest trace of the embryo in the fertilized ovum of a higher vertebrate.
- 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 embryologists and ethicists insist that only after the primitive streak develops can this being be called an embryo.
- 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.
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