Doc Vickerson, in the first chapter of the novel, gives Martin a gift to "start his training." This gift is a magnifying glass. This is important because it represents the keen eye and curiosity that both the physician and the scientist must have. It represents the careful and deep observation that Gottlieb lectures to Martin about over and over again. Although Doc Vickerson is, himself, a kind of failure in his alcoholism, his gift is nevertheless laden with significance, elevating Vickerson to the status of man who has had "influence" on the reader's "hero."
Terry is the symbol of what Martin could be, and he represents the kind of man that Martin is, in fact, by the end of the novel, the man that Martin follows. He represents the careful scientist who is willing to give everything up for his work. He is Martin without the temptations that have led him astray and, thus, is the true kin of Gottlieb.
Gladys the centrifuge at the McGurk institute is Holabird's pride and joy. She is an expensive piece of machinery and the best of her kind and, thus, represents the commercialism and competition present in American medicine.