Our Rabbis taught: There are three partners in man, the Holy One, blessed be He, his father and his mother. His father supplies the semen of the white substance out of which are formed the child’s bones, sinews, nails, the brain in his head and the white in his eye; his mother supplies the semen of the red substance out of which is formed his skin, flesh, hair, blood and the black of his eye; and the Holy One, blessed be He, gives him the spirit and the breath, beauty of features, eyesight, the power of hearing and the ability to speak and to walk, understanding and discernment. When his time to depart from the world approaches the Holy One, blessed be He, takes away his share and leaves the shares of his father and his mother with them.
According to the Gemara, the father supplies five elements, and the mother supplies four elements, while God supplies nine elements – equal to that of father and mother combined. In his Mishhat Aharon, Rav Aharon Kutna points out that these eighteen elements together make up the “life” (hai, the numerical equivalent of 18) of the person. Once God removes the elements that He contributed, the life-force is removed and the person dies.
In truth, there are numerous traditions regarding these issues. According to Rabbenu Tam, both mother and father contribute five elements to the newborn and God contributes ten elements. The She’iltot adds one further element – blood – to the contribution of the mother, so the number of elements contributed by each of the parents is equal. In his Ha’amek She’elah, the Netziv explains that this was not included in our Gemara since it is obvious that the entire development of the fetus is dependent on the blood of its mother.