What does the analogy of fencing your garden show about what gets through a barrier?
The analogy shows that a barrier can keep out larger animals, such as rabbits and dogs, while insects and smaller mice still pass through. In the same way, the placenta blocks the major components of the mother’s blood from reaching the fetus, but small chromosomes and genes carrying inherited characteristics get through.
Les explains that the garden-fencing analogy came from a physiology book. A netting around a garden keeps out rabbits, dogs, and the larger animals, but insects and smaller mice can still get through. That is exactly how the placenta in the mother’s womb works: the major components of the mother’s blood never go through the placenta to the fetus, but some small chromosomes and genes that carry inherited characteristics do go through. He uses this to clarify that when Mary was the physical, fleshly mother of the Lord Jesus, none of Mary’s blood coursed through the baby Jesus, because that is not how physiology works. Jesus’ blood system came from His Father, just as any other child’s blood system comes from the father. Les also ties this back to the promise of the seed of the woman in Genesis 3:15 and notes that Matthew chapter 1 introduces the New Testament with similar language.