Seed Saving from Tomatoes
Most modern varieties of tomato are self pollinating, and will not cross. The anthers on tomato flowers (which make the pollen) are fused together to make a tight cone that insects cannot enter. Usually the stigma (the receptive surface for receiving pollen) is very short, and so is located deep inside this cone of anthers. No insects can get to it and the only pollen that can fertilise it comes from the surrounding cone of anthers.
To collect the seed, allow your tomatoes to ripen fully. Then collect a few of each variety that you want to save seed from. Slice them in half across the middle of the fruit, and squeeze the seeds and juice into a jar. You then need to ferment this mixture for a few days – this removes the jelly-like coating on each seed, and also kills off many diseases that can be carried on the seeds. To do this put the jar of seeds and juice in a reasonably warm place for three days, stirring the mixture twice a day. It should develop a coating of mould, and start to smell really nasty!
After three days, add plenty of water to the jar, and stir well. The good seeds should sink to the bottom of the jar. Gently pour off the top layer of mould and any seeds that float. Then empty the good seeds into a sieve and wash them thoroughly under running water. Shake off as much water as possible, and tip the sieve out onto a china or glass plate (the seeds tend to stick to anything else). Dry somewhere warm but not too hot, and out of direct sunlight.
Once they are completely dry, rub them off the plate and store in a cool dry place, where they should keep well for at least four years.