Human embryos were grown in a dish past day thirteen

Researchers at Rockefeller University and the University of Cambridge built a culture system that let embryos, donated for research, keep developing outside the body well past implantation, which normally hides this stage from view entirely.

The embryos organized themselves without any maternal tissue present, forming the bilaminar disc, amniotic cavity, and yolk sac expected at this stage, and kept developing on schedule up to day 13 after fertilization, one day short of the agreed limit for culturing human embryos in the lab.

At that point the team stopped the experiment, in keeping with the 14-day rule that has guided embryo research for decades. The work gave the first direct look at a stage of human development previously known only from rare, incomplete specimens, and it now underpins newer embryo models built from stem cells instead of donated embryos. Extending culture further would require revisiting that limit, a question still being debated.

Development no one had watched, for the first time.