Researchers have created a new batch of ersatz embryos that may help them delve deeper into early development. Earlier this month, a team reported that by mixing three types of mouse embryonic stem cells, it had produced so-called embryoids that replicate the first 8.5 days of the animals’ development. Using a similar recipe, another group has now generated mouse embryoids (bottom) that closely resemble genuine embryos (top). The embryoids featured rudimentary muscles, a burgeoning brain, and “the beginnings of … a beating heart,” New Scientist reveals. The new study, reported this week in Nature, added a twist by including stem cells that lack a key gene, suggesting genetically tweaked embryoids could help researchers probe how certain genes shape development.