Posted: 14/08/2015 at 06:36
Joe, they should be a distinct plum shape as per Bob's link. The smaller ones on your plant look to be heading in the right direction. The bigger ones aren't looking plum-shaped, they're more heart-shaped.
That split will gradually open and expose the fruit to insects, infection, etc. It's all right for the moment but take it inside as soon as you see it opening. You'll need to keep an eye on it inside too.
Commercial hybrid varieties have been well and truly stabilised (the gene pool set to produce identically) before they're released to the public. It's very rare, though not impossible, to have glitches.
In short, you cross your selected varieties with some sort of goal in mind. The first generation of saved seed will produce an assortment of versions of the parents. You save seed only from the fruits that seem to be heading towards your goal - colour, shape, size, whatever. With each successive generation you, again, only save seeds from fruit heading towards your goal. Eventually, you end up with the fruit you want.
Depending how any parent varieties are involved, it can take a number of years to stabilise them, although commercial operations develop them on a large scale so they can hasten the process. It can be a slow, painstaking process for the home gardener with limited growing space and restricted to one season a year.
In fact, when I lived in Sydney, tomato-growing friends and I were in touch with several growers in the US who were hybridising. They used to send us seeds to grow out in our SH season, thus giving them two seasons a year, theirs and ours. They would tell us their goal, we would save seeds from the resulting fruit (that matched their goal) and send the seeds back for them to use for the next generation.
Perversely, some of the US growers, fanatics, were even trying to dehybridise varieties, getting back to the original parent varieties. An almighty challenge because you have no idea where you're going beyond suspicions and hunches. Made all the harder by the commercial operations, the original hybridisers, guarding the names of their parent varieties fiercely.