I think the tone definitely takes a dark foreboding tone in chapter 19. Jonas watches as his father takes the smaller one and inserts a filled syringe into the newchild's forehead, injecting it while he apologizes to the newborn for having to use his forehead instead of a smaller vein. Jonas waits for his father to clean the child up but instead sees the baby die, and he realizes that his father has killed the twin. Stunned, he sees his father place the body in a carton and send it into a trash chute. This is really a turning point in terms of the mood and tone of the book.