Check out this website for everything you need to know about growing clematis from seed.

Here is a link that might be useful: clematis from seed

The link has changed - try the one listed below. You should also know that while many different clematis can be grown from seed, only the species will come true to form. Offspring grown from seed of any of the large flowered hybrids will not necessarily resemble the seed parent.

Here is a link that might be useful: clematis from seed

And to answer the other part of your original question - Jackmanii is a cultivar, so it will only come true by vegetative propagation, not from seed.