Hi, This can be a problem when saving your own seeds. When saving seeds you need to choose a tomato with pleanty of seeds in. It looks to me like you have slowly bred these tomatoes to be seedless by saving seeds from tomatoes which have small amount of seeds inside.
Anyway this is a new one on me seedless tomatoes shaped like bums.

Do you know what variety they were to start off with ?? maybe you could start from some fresh seeds, unless you can find some tomatoes with seeds in. However there is another problem your seeds taken from these tomatoes will grow to be simular to the parent plants and in your case bum shaped and seedless

This guide might help you when saving your seeds (click here)

Thanks