I see a lot of improvement throughout your work from the beginning to the end. You are getting tone down well. If I may advise, you should try increasing the contrast in your drawings by having the darkest portion of the drawing reach a dark black, as well as a the lighest reach white. I don't know if you are using various leads but if you aren't you should consider investing in some pencils like an HB, 2B, & 4B. I think it would make your work improve even more and make your drawings stand out much more. Nevertheless, it is some nice work, keep it up.
Strange coincidence that every man whose skull has been opened had a brain.