In order to breed any organism true you have to stabilize the gene pool. Our first worm was a worm (I assume) that was the best choice for experimental reasons. Larger eggs, more hardy, faster reproduction, something like that.
When it worked they needed a worm more suited for commercial production. Like dogs and cats, silkworms have been bread for different uses. Commercial worms would grow fast and large. It makes much more sense to have a larger worm/cocoon if you want to produce masses of silk.
So we breed our monster silk worm with a commercial worm. then we need to breed their offspring with a parent to stabilize the strain. I would guess as the worms are hatching Kim is putting the largest of the worms together and letting them hatch and breed so the strain of worm can be as large as possible.
If the ZFN worked on the first round I assume they would have started with a normal silk worm and created almost the same worm we got the first time but with the genes where we want them.
They may have tried to remove the silkworm dna but I would do it one step at a time if I was Kim. That is the only sure way to know that the modifications you make are the only change in the worm.
This is all IMO I am no expert
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