When they out the calf in twain, etc. This clause should be translated differently, and placed, for clearness, in a parenthesis (the calf which they cut in twain, and between the parts of which they passed).
The division of the calf might, in fact, be called in Hebrew either "the covenant" or "the token of the covenant" (comp. Genesis 17:10, Genesis 17:11). It was a solemn assurance that he who should transgress God's Law should share the same fate as the victim.
The same idea seems to have dictated the Hebrew phrase, "to cut a covenant," and the Greek and Latin equivalents ( ὅρκια τέμνειν: foedus icere); comp. the parallel narrative in Genesis 15:10.