So much work and a lot of improvement! I notice you started using smaller ovals for the sides of the heads and they start to look more 3D towards the end, nice.
For critique I would say work more on getting the angles the same for the line in the middle of the face and side ovals. Also, with some of the ones on the bottom you wouldn't be able to see the cheek line. It's looks like you're forcing two different angles on one head.
Can't wait to see more, your lines are looking nice!