The structure that makes nutrients most directly available to a human embryo is the placenta.

Here's why:

* The placenta is a specialized organ that develops during pregnancy. It's formed from tissues of both the mother and the embryo.

* It acts as a bridge between the mother's bloodstream and the embryo's bloodstream.

* Nutrients and oxygen from the mother's blood pass through the placenta to the embryo.

* Waste products from the embryo are also passed back to the mother's blood through the placenta.

Essentially, the placenta is a highly efficient system for transferring essential resources to the developing embryo while removing waste products.