That's the interesting thing about all the pictures...although I think the first one is later the other two aren't. One is from 1500 and the other is 1520 (I think)
But yeah on the neckline. Those silly Venetians and their immodest gowns! There were also some very low cut gowns around the 1520's. In some paintings it looks like they were a jacket or bodice to cover the bust area but on others there is only the chemise there.