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This week, a pair of research teams published the results of an amazing feat: for the first time ever, they managed to keep human embryos alive in petri dishes for two weeks—nearly doubling the life expectancy of embryos in the lab.

This breakthrough in science, though, is sure to roll some heads.

International laws and guidelines for embryonic research advise that scientists end their experiments when embryos are 14 days old. The rule has to do, in part, with how we value human life. On day 15 of an embryo's lifecycle, it develops what's known as the "primitive streak," beginning to assemble itself into a complex organism and becoming, in some sense, an individual.

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Until now, the question of whether it is ethical to experiment on embryos beyond that was mute—most labs struggled to keep embryos alive for longer than a week. But the researchers behind studies published Wednesday in the journals Nature and Nature Cell Biology were forced to end their experiments to avoid crossing the Rubicon. And now some scientists are pushing to rethink that 14-day rule.

Predictably, this has already begun to spark moral outrage.

"If this research does not stop at 14 days, where does it stop?” David Prentice, a former life sciences professor at Indiana State University and founder of the organization Do No Harm: The Coalition of Americans for Research Ethics told Life News, the anti-abortion news website. “Is there any bright line which should not be crossed? This is a risky step which could encourage further eugenic attitudes and actions. Congress ought to have a full and open debate on the issue of human embryo research before the research community moves further without oversight.”

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Using techniques previously used to mimic the process of womb implantation within a petri dish for the embryos of mice, researchers at Rockefeller University and the University of Cambridge were able to closely observe the development of human embryos up to days 12 and 13.

"This it the most enigmatic and mysterious stage of human development," Cambridge University's Magdalena Zernicka-Goetz, a co-author, told Reuters. "It is a time when the basic body shape is determined."

In an op-ed in Nature, scientists argued that pushing past that 14-day limit could lead to "scientists being able to study all aspects of early human development with unprecedented precision."

Allowing for scientists to culture embryos in a lab could provide valuable information about the basics of human biology and the early stages of human life—we might, for example, gain insight into the causes of autism or why some environmental factors affect a developing fetus while others do not.

The op-ed writers, though, do not argue for abolishing limits altogether.

The 14-day limit was first proposed in 1979 by the Ethics Advisory Board of the US Department of Health, Education, and Welfare and eventually became law in at least 12 countries. It is, they suggest, to be thought of as a useful public policy tool to allow the advancement of science rather than a moral judgement on when an embryo becomes an individual life.

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"Views differ on the moment in development at which a human embryo obtains sufficient moral status that research on it should be prohibited," they write. "The 14-day rule was never intended to be a bright line denoting the onset of moral status in human embryos. Rather, it is a public-policy tool designed to carve out a space for scientific inquiry and simultaneously show respect for the diverse views on human-embryo research."

All this, they conclude, leaves room for those limits to be "legitimately re-calibrated."

But even scientists without such strong opposition to embryonic research have called into question the value of pushing those limits.

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"I do not see a politically, or, for most people, morally acceptable line after 14 days," Stanford's Center for Law and the Biosciences Director Henry Greely said in a statement given to the nonprofit Genetic News Expert Service. "Given the questionable scientific value of the research, no case has been made for even revisiting the line, let alone changing it."

In a pluralistic society that must balance belief with science, the path forward is unclear.