That guy should not have recommended cutting the egg! 35 days is quite early (they can incubate for 60+ days) but it depends on your incubation temperatures.
The placenta like thing, is most likely the yolk.
I've never dealt with this so only going by what I've read previously. I've heard people have success with patching up an egg with damp tissue. Just enough so that it's covering the cut.
You may want to wait until someone experienced with this comes along though, sorry I can't help much!