Genesis 15:7-21 - What is the meaning of the animals being cut in half?

In Genesis 15:8-10 God tells Abram to bring certain animals which were to be of a certain age.  Abram brings the animals, slaughters them, cuts them in half and lays out the halves facing each other. This was the conclusion of an ancient custom in that part of the world to finalize a covenant.  God had just told Abram that he through his offspring would inherit the land on which he was living as a stranger at that time.  Abram wants to know what assurances God will give him that the promise God made will happen.

God chooses to use the culturally accepted way of sealing a covenant so Abram would have confidence in the promise God had made.  This is where God asks Abram to bring the animals, Abram, knowing the customs of the their culture understands what God is doing and slaughters and cuts the animals in half and lays out the pieces.  The custom of the sealing the covenant was that the two parties making the covenant would walk between the halves of the dead animals and in doing so covenanting that if either party broke the covenant they would become like the dead animals.  You pledged your life in theses covenants.

In this scene there is one difference.  Only God passes through the pieces and not Abram because only God could bring about the fulfilling of the promise of inheritance He had just made to Abram.  Abram had no part to play in the fulfillment of this covenant.