A human embryo kept alive in the lab for 12 days begins to show signs of early development. The green cells seen here in the center would go on to form the body. This embryo is in the process of twinning, forming two small spheres out of one. Courtesy of Gist Croft, Cecilia Pellegrini, Ali Brivanlou/Rockefeller University hide caption

Iraqis walk at the Hamam al-Alil camp for displaced people, south of Mosul, on Wednesday, during an offensive by security forces to retake the western parts of the city from ISIS fighters. Aris Messinis/AFP/Getty Images hide caption

"The job of the media is the accountability of government," says CNN President Jeff Zucker. "And I think we are uniquely positioned to do that with our resources and our reach." Mike Coppola/Getty Images for Turner hide caption

Tyler Cowen argues, in his new book The Complacent Class, that Americans are in a period of stagnation because we are doing less and less of what made us successful in the past: embracing change, moving to different parts of the country and associating with different kinds of people. mathisworks/Getty Images/iStockphoto hide caption

Tubes of hematite, an iron-rich mineral, might be evidence of microbial life that lived around underwater vents billions of years ago. Matthew Dodd/University College London hide caption

The Capitol of Puerto Rico, Capitolio de Puerto Rico, in San Juan. Puerto Rico is under federal control, but isn't allowed voting representation in Congress, and residents can't vote for president. Greg Allen/NPR hide caption