(eNotes educators only answer one question per response. If you need further help, please resubmit questions separately.)
As Professor Herbert attempts to explain to Pa some of the activities of his students, he mentions "protozoa", which Pa obviously doesn't know. When the Professor asks Pa if he knows what germs are, Pa replies, "I don't believe in germs. I'm sixty-five years old and I ain't seen one yet!" Professor Herbert promises to show Pa a germ before he leaves the school that day.
That afternoon, the Professor has Pa "take his knife and scrape tartar from one of his teeth." Professor Herbert prepares a slide and places it in the microscope for Pa to examine. When Pa looks through the eyepiece of the microscope, he is amazed by what he sees in the tartar he had removed from his tooth.
I see 'im," says Pa. 'Who'd a ever thought that? Right on a body's teeth! Right in a body's mouth. You're right certain they ain't no fake to this, Professor Herbert?" "No, Luster," says Professor Herbert. "It's there. That's the germ.