After the first part the elevator brings you the section were you have to find a way further up (dont want to spoiler her) and after you mangaged that you get to the open with the blue jump gel. After getting over the gap and open the door that seems to lead to nowhere, you can see from there a white portalable surface through the metal floor. If you set a portal there and make a long jump, you are getting back to the end of the first section, however, the elevator is gone and if you jump in the hole you are stuck...