Niddah 31: Who Creates an Embryo

On Niddah 30, we are told the story that embryos learn the entire Torah and at birth they are slapped by an angel which causes them to forget what they have learned.  We are given several stories that would justify this, including what would make learning easier in the future.  

On today's day, we learn that a child is born of their father's white seed which contributes to bones, sinews, brain, and white in the eyes.  They are born also of their mother's red seed, which contributes blood, flesh, skin and the black of their eyes.  G-d also contributes to the development of a child, including their spirit, soul, countenance, eyesight, hearing, speech, movement, understanding and wisdom.  When the child eventually dies, G-d takes back G-d's portion and leaves the other portions.

The Gemara notes that the father contributes five elements, the mother contributes four elements, and G-d contributes nine elements, which is the same amount as the mother and father's contributions combined.  Rav Aharon Kutna writes in Mishchat Aharon: these eighteen elements equal eighteen, which means "life" according to gemmatria. Once G-d takes G-d's elements, the person no longer has a life force which leads to death.

We learn that Rabbeinu Tam teaches that a mother and father each contribute five elements to an infant and G-d contributes ten elements.  The She'iltot speak of adding the element of blood so that the parents contribute equally.  The Netziv teaches in Ha'amek She'elah that mothers obviously contributed blood, and so it did not need to be noted.