How to fill a square area with tiles, but in a spiral order?

I know how to fill a space with a loop, filling row by row until I complete a square area field. But how can I fill an area, starting from the edges to the center?

http://www.gameus.de/c2/test.capx

http://www.gameus.de/c2/test.capx

thank you! I was able to solve the problem using a very similar logic

https://orig00.deviantart.net/b3f1/f/2017/195/5/6/spiral_by_vubidugil-dbga57r.png