... It was for this reason that man was first created as one person [Adam], to teach you that anyone who destroys a life is considered by Scripture to have destroyed an entire world; and anyone who saves a life is as if he saved an entire world ...

You shall not murder

He who kills his sons. It is told about and embryo that was killed in its mother's womb that is as destructing the building of Hashem and his Omanut (art, creation or faith)

If anyone kills any human being, he shall be put to death.

"And if a man strikes any soul of a man": I might think that an eight-month birth, (who is destined to die,) is also included. It is, therefore, written "If one strikes a man" — whereby we are apprised that he is not liable unless he kills one who is destined to live.

Rabbi Yirmeya raised yet another dilemma: Does a dead fetus in its dead mother’s womb form a mixture with regard to her, so that the bodies are considered like two corpses buried together, or not? The Gemara explains the two sides of the dilemma: Do we say that since the Master said that a fetus is considered as the thigh of its mother, it is therefore like her body and it does not form a mixture with it? Or perhaps one should maintain: Since in most cases a fetus will ultimately emerge from the womb at birth, it is already considered separated from her, and it is like any other corpse buried with the woman.

באדם דמו ישפך, a reference to a human tribunal.

R. Yaakov bar Acha found it written in the scholars' Book of Aggada A heathen is executed on the ruling of one judge, on the testimony of one witness, without a formal warning, on the evidence of a man, but not of a woman, even if he [the witness] be a relation. On the authority of R. Yishmael it was said: [He is executed] even for the murder of an embryo. Whence do we know all this? — Rav Yehuda answered: The Torah saith, And surely your blood of your lives will I require this shows that even one judge [may try a heathen]. At the hand of every living thing will I require it: even without an admonition having been given; And at the hand of man: even on the testimony of one witness; at the hand of man: but not at the hand [i.e., on the testimony] of a woman; his brother: teaching that even a relation may testify. On the authority of R. Yishmael it was said: [He is executed] even for the murder of an embryo. What is R. Yishmael's reason? Because it is written, Whoso sheddeth the blood of man within [another] man, shall his blood be shed. What is a man within another man? — An embryo in his mother's womb.

At what point is the soul given to a human? Is it from the moment of decree [that such a child will exist], or from the moment of formation?

The moment of decree - when his flesh, tendons and bones come together

The moment of formation -when the angel comes to the drop and brings is before Hashem to see what will happen to it, as we say in (Daf 16 B) immediately a soul and life is thrown into it

For R. Chanina b. Papa made the following exposition: The name of the angel who is in charge of conception is 'Night', and he takes up a drop and places it in the presence of Hashem saying, 'Sovereign of the universe, what shall be the fate of this drop? Shall it produce a strong man or a weak man, a wise man or a fool, a rich man or a poor man?' Whereas 'wicked man' or 'righteous one' he does not mention, in agreement with the view of R. Chanina. For R. Chanina stated: Everything is in the hands of heaven except the fear of G-d, as it is said, And now, Israel, what doth does Hashem require of thee, but to fear etc.

R. Simlai delivered the following discourse: What does an embryo resemble when it is in the bowels of its mother? Folded writing tablets. Its hands rest on its two temples respectively, its two elbows on its two legs and its two heels against its buttocks. Its head lies between its knees, its mouth is closed and its navel is open, and it eats what its mother eats and drinks what its mother drinks, but produces no excrements because otherwise it might kill its mother. As soon, however, as it sees the light the closed organ opens and the open one closes, for if that had not happened the embryo could not live even one single hour. A light burns above its head and it looks and sees from one end of the world to the other, as it is said, then his lamp shined above my head, and by His light I walked through darkness. And do not be astonished at this, for a person sleeping here might see a dream in Spain. And there is no time in which a man enjoys greater happiness than in those days, for it is said, O that I were as the months of old, as in the days when God watched over me; now which are the days' that make up 'months' and do not make up years? The months of pregnancy of course. It is also taught all the Torah from beginning to end, for it is said, And he taught me, and said unto me: 'Let thy heart hold fast my words, keep my commandments and live', and it is also said, When the converse of G-d was upon my tent. Why the addition of 'and it is also said'? — In case you might say that it was only the prophet who said that, come and hear 'when the converse of God was upon my tent. As soon as it, sees the light an angel approaches, slaps it on its mouth and causes it to forget all the Torah completely, as it is said, Sin coucheth at the door. It does not emerge from there before it is made to take an oath, as it is said, That unto Me every knee shall bow, every tongue shall swear; 'That unto Me every knee shall bow' refers to the day of dying of which it is said All they that go down to the dust shall kneel before Him; 'Every tongue shall swear' refers to the day of birth of which it is said, He that hath clean hands, and a pure heart, who hath not taken My name in vain, and hath not sworn deceitfully. What is the nature of the oath that it is made to take? Be righteous, and be never wicked; and even if all the world tells you, You are righteous', consider yourself wicked. Always bear in mind that the Holy One, blessed be He, is pure, that his ministers are pure and that the soul which He gave you is pure; if you preserve it in purity, well and good, but if not, I will take it away from you.

At what stage is the embryo discernible? Symmachus citing R. Meir replied: Three months after conception

A woman who was having trouble giving birth, they cut up the fetus inside her and take it out limb by limb, because her life comes before its life. If most of it had come out already they do not touch it because we do not push off one life for another.