Why are plane windows round in shape?

Back in 1950's, the plane used to have the windows squared in shape. Later on, the plane crashed due to the shape of the windows.

So, as a safety, the round shaped windows were innovated. Actually, going high in the sky the outside pressure decreases with the increase in altitude whereas the cabin air pressure remains constant and so the stress is provided to the windows by the air  as I have already mentioned in my previous post 

"Reason behind a small hole in the window pane".  

In case of round windows, the stress is dispersed in the same amount in all directions that prevents the windows to burst out whereas the stress is generally concentrated at the edges and at 90 degrees the windows may splash out.