The Vortex (Spiral)

This looks like a crossword, but the words are all swirling toward (or away from) a vortex of some sort. Can you determine what's causing this?

How to solve

This puzzle contains two series of words — one reading inward from 1 to 99, the other outward from 99 back to 1. Every space will be used exactly twice. Work from both sets of clues to complete the puzzle.