Genesis 15:10
And he took unto him all these, and divided them in the midst, and laid each piece one against another: but the birds divided he not. And he took unto him all these, and divided them in the midst, and laid each piece one against another: but the birds divided he not.
At God's direction the covenant sacrifice was prepared by taking the appointed animals and dividing them in the midst. The heifer, the she-goat, and the ram were each cut in two down the middle, while the turtledove and the young pigeon were left whole and undivided. The matching halves were then laid apart, each piece set opposite its fellow to form a prepared pathway between them.
That laying out of the divided pieces belonged to the covenant ceremony by which God confirmed His promise. Abraham himself passed between the severed portions, and in so doing made a solemn vow of perpetual obedience.