Professor Max Gottlieb infects a guinea pig with anthrax germs before a nervous bacteriology class, too respectful to stand close. He advises his students to be careful about infecting themselves when handling deadly germs and that the most important part of experimentation is not the experiment itself but taking "accurate, quantitative notes" in ink.
Duer observes to a fraternity brother that Gottlieb might have been a first-rate surgeon and made fifty thousand dollars a year instead of four thousand as a laboratory man. Martin Arrowsmith, however, visualizes himself as doing the same experiment.
The guinea pigs die in two days, and the class reassemble for the necropsy. Smears from the lungs, spleen, liver, and kidneys of the animals are made on glass slides and examined under a microscope. The uneasy class members recall nervous rumors of a former student who had died from infection under such circumstances.
Martin, in imitation of Gottlieb, has begun working in the laboratory at night with an enjoyment little short of ecstasy. One evening, the young student is invited to share coffee and small, foreign sandwiches with his master at midnight.
Gottlieb talks of the laboratories of London and Sweden and the epidemic in Marseilles as well as of his family: a sick wife, a son, and a daughter, Miriam. He recalls events of thirty years ago, such as his expulsion from Germany for refusing to sing Die Wacht Am Rhein. He wonders wryly whether deadly germs should be prevented from killing off at least a part of the human race, thus solving economic questions. The young scientist and the old look at the slides before Martin says goodnight.
The personality of Gottlieb shines throughout these pages, the true scientist in a world of froth and fraud. The effect of Gottlieb's teaching, his experiments, and his confidences is to deepen Martin's admiration of the man and his lifetime devotion to the search for truth in the field of Martin's own choice. The guinea pigs and their experimental scientific treatment foreshadow the use of phage later on to control a plague in humans.