>> The darker seeds are plump, and look fertile. The lighter seeds are flat, and look infertile. Is this correct?
I don't know about squash, but I've been wrong often enough that when I clean seeds, I save some of anything that might be a fertile seed. I found "chaff" from alyssum that had 100% germination rates.
I also save some big seeds (expecting them to have more food stored and make more viable seedlings. But I also save some small ones, in case the "big seeds" were actually infected with something and swelled up before dieing and drying.
My rule of thumb is that whatever the plant produced in the greatest quantity, all looking similar, is probably what a normal seed should look like.
Before I throw away the "sterile ones" or the "maybe chaff", I set up some small, stacking glass bowls with part of a coffee filter in each one. I write the description on the filter or a scrap of mini-blind slat in the bowl, and drop a bunch of maybe-seeds into each bowl and leave them until they sprout or rot.
Usually the ones that look like seeds, are seeds. But sometimes, the ones that look wonky sprout better.
If you have enough squash seeds, I would germination-test 10 of each kind.
And if you were planning the throw any of them away, like the small ones, why not test-germinate 30-50? If they germinate well, you can save and trade them, and even include a germination test report: "80% germination on 7/23/2014".
