Santiago Calatrava said the roof would open.

And evidently it will.

On Friday morning, a 5,700-pound glass panel was hoisted into place as a 355-foot-long operable skylight took final form in the Oculus pavilion of the World Trade Center Transportation Hub, designed by Mr. Calatrava. Another panel went up in the afternoon.

Those are among the last of 996 pieces of blast-resistant glass to have been installed at the Oculus since March 15. The glazing should be finished on Monday, said Steven Plate, director of World Trade Center construction for the Port Authority of New York and New Jersey, which is building the $3.9 billion rail, subway and shopping hub.

The winged Oculus is by far the most conspicuous element in the hub, which is nearing completion after 10 years.

Each Sept. 11, the skylight will be opened to the elements for 102 minutes, Erica Dumas, a spokeswoman for the authority, said. That is how long the 2001 terrorist attack lasted, from the time the first jetliner hit the trade center at 8:46 a.m. until the collapse of the second tower at 10:28 a.m. In the towers, on the ground and in the hijacked planes, 2,753 people were killed.