Ok, so since gastrulation occurs 15 days after conception, prior to implantation/pregnancy, certainly before a woman would even know she was pregnant, you are essentially against abortion.  
I am certainly arguing justice as you stated.  If we can't respect the right to, and sanctity of, life, then there is simply no solid foundation for any kind of justice.  Again, at conception a genetically whole human being is formed.  Eye color, gender, and too many other things to mention are determined from that point onward.  Gastrulation is but a stage of development, it does not bequeath life nor humanity any more than birth.  Human life of a genetically distinct human being exists from conception onward.  "Personhood" is not a scientific argument, it is a philosophic one.  There is no difference between a human being and a human person.  The newly conceived might not be fully developed, but neither is a new born, or even a 12 year-old for that matter.