formed my inmost being;
you knit me in my mother's womb."
Which comes first, the body or the soul? They come together exactly at the same moment, at conception. The soul is an invisible, a unique intelligent entity created by God from nothing. The body's genetic makeup (DNA) is given by man and woman but it comes together as the beginning as "my body", at conception because "I am present".
The soul as the intelligent form of the body is always actual or in act on all levels of its being: the level of nourishment, sensation, local movement and understanding. The manifestations of the soul's physical powers becomes gradually visible in the mother's womb as it forms its body by growing, developing sense organs and the brain, the organ of intelligent thought. The union of soul and body is not external, like a a vessel and its contents, or a ship and its captain but essential. The union between body and soul is so close as to form one person, one unique entity as an ensouled body. The soul is present in the whole body and each part of the body. St. Augustine teaches that "Pain of the body is simply suffering of the soul arising from the body." (Augustine, City of God, XIVI.15) In this, there is a parallel in the biological body. The biological body as coded information (DNA) is present in almost every cell of the body.
The biological sciences are giving an evermore detailed information of what happens in the mother's womb. The fertilization process, the fertilized egg developing into a sphere of cells, the differentiation of cells into organs etc. There is certainly encoded information and wisdom concerning bodily characteristics and development in the genes but the activation of this potential is due to the active presence of the soul.
Finally, reason tells us that the soul is immortal because it is simple, not made of parts which can be separated. Christ confirmed this, "Do not be afraid of those who kill the body but cannot kill the soul ...." (Mt 10:28) Even though the soul is separated from the body at death, it will be reunited with it at the Resurrection to continue forever as an embodied spirit.
3. Wonderfully Made