Louise, I have heard this said previously about seeds but don't know if it is true or not, and especially don't know if it is true about clematis seed. Maybe someone else can help answer your question. Generally speaking, clematis seeds that are fertilized and therefore more than likely viable will be big and fat. As they dry out, they shrink somewhat in size but it is still very obvious what the fertilized ones look like versus the unfertilized ones. That being said, the size of fertilized seeds can vary from cultivar to cultivar.

Here are a few pictures I posted previously that might help you.

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Thank you so much for your explanation nckvilledudes. A picture is worth a thousand words.

You are very welcome Louise. I agree that a picture says things much more clearly than words in a lot of cases! Good luck with your seeds!