It contains a massive amount of information about gene activity at a crucial time in development — just as the cerebral cortex is developing. The scientists believe it holds important clues about the biological origins of disorders like autism, as well as insights into what makes the human brain unique.

Halfway through gestation, a human brain could fit in the palm of your hand. But it’s around this time that the cortex, which is responsible for many of our cognitive capabilities, is starting to take shape, says neuroscientist Ed Lein of the Allen Institute for Brain Science in Seattle, who led the new study.

To build the new atlas, Lein and colleagues constructed the map from four fetal brains obtained through a tissue bank. They sliced each one into about 3,000 ultra-thin sections. They used dyes and genetic markers on some of those slices to create a reference atlas. On other slices, they used microscopes equipped with laser beams to snip out tiny bits of tissue for genetic tests. The team looked at the activity level of about 20,000 genes, and they report their findings today in Nature.