That's kind of what I was thinking too! And the 2 that didn't develop I think we're either unfertilized or I had them on the shelf too long before incubation, I cracked them into the sink and it was just a yolk
I have a 35$ cheapo incubator for my first hatch, 10 out of 12 eggs have embryos and I'm on day 18. I've seen reviews of this one with fluffy chicks running around inside that also had ducks in the same batch so I think I'm good but there's no obvious ventilation. Unless the lip around the dome...