What is the purpose of the portholes on the plates of plate heat exchangers?

You can't just assume that everybody knows what you are talking about. You should post a link to a picture or an explanation. Here are some pictures, but I can't pick out anything that might be called a porthole.
www.google.com/search?site=&tbm=isch&source=hp&biw=1024&bih=739&q=plate+heat+exchanger