Watching Norris in there... gave me the idea that maybe every part of you bastards is a whole.  Every piece of you is self-sufficient an animal unto itself.  When a man bleeds it's just tissue.  But blood from one of you Things won't obey. It's a newly formed individual with a built-in desire to protect its own life.  When attacked your blood will try and survive -- and crawl away from a hot needle say.