If instead Doob went "down" in the nadir direction, he entered a similar three-dimensional maze of modules. Most of these were devoted to storage of genetic material: seeds, sperm samples, eggs, and embryos. All of these needed to be kept cold, which in space wasn’t too difficult; it was primarily a matter of shielding the storage containers from sunlight, which could be done with a featherweight piece of metallized Mylar film, and secondarily a matter of just preventing the warmth of surrounding objects from seeping into the samples. Doob always paused when passing by that hatch. He was not a spiritual man, but he could not humanly ignore the fact that his potential fourth child, the embryo he and Amelia had created, was in there somewhere. Along with tens of thousands of other fertilized embryos waiting to be thawed out and implanted in human wombs.