The same gene that causes the crest feathers on the Silkie, Polish, etc. also causes what they call a vaulted skull. The skull is arched upward, and there actually are gaps in it. Birds that have one copy of the crested gene don't have much of either a vault or a crest; birds with two copies have a lot more. If you breed two birds with small crests together, you can get some offspring that have bigger crests, but they also have the vaults, too. Because there are gaps in the bone of the skull, these birds are more vulnerable to brain injury from the pecks of other birds (this is something I learned the hard way).
I did not know that but one of my Silkie babies was born either without the back of it's skull or maybe the others pecked the skin off while it was hatching. IDK The baby didn't make it but before it died it looked like the swelling or whatever was getting better. I had put it in with the others to see how they would do and before I could stop it, one grabbed it by the head and pulled. I don't know if the baby would have made it before that happened or not but I did read that this does happen because the heat in the incubator gets too high in the first 6 days. I didn't think it got high enough to do that but all the others were/are fine.