(AP) – Scientists are using human stem cells to create a structure that mimics a pre-embryo and can serve as a research alternative to a real one. They’re called “blastoids.” Researchers say they provide an efficient, ethical way to study human development and pursue biomedical discoveries in fertility and contraception. The latest effort was detailed Thursday in the journal Nature. The structures aren’t embryos, but scientists nevertheless didn’t let them grow past two weeks in deference to longstanding ethical guidelines. Nicolas Rivron of the Austrian Academy of Sciences is one of the authors of the study. He says blastoids would complement human embryos used in research.