George Papanicolaou's first research subjects were actual guinea pigs. Possibly drunk guinea pigs—they were part of a study on the effects of alcohol on them and their offspring.
The alcohol study required the harvesting of the guinea pigs' eggs, pre-ovulation. But it was difficult to know when, exactly, that was: The guinea pigs weren't obviously menstruating. But, Papanicolaou figured, there must be some changes in their vaginal discharge that would clue him and his colleagues into their cycle. He located a pediatric nasal speculum, and he started extracting samples from the guinea pigs' vaginas.
Papanicolaou had trained as a doctor in Greece, but when he started working on the guinea pig study, he was a recent immigrant to America. In 1912, after he and his wife, Mary, had lived in Germany and in Monaco and after he had served, twice, in the Greek army, the couple moved across the Atlantic. Within a few month, Papanicolaou had found work as an assistant at Cornell's medical college, on the guinea-pig study.
Those samples of cells did allow Papanicolaou to track guinea pigs' sex cycles, and though he wanted to pursue the study further, on humans, without being a full clinician, he didn't have access to patients as potential research subjects. Instead, he started working with the only volunteer he had—his wife Mary.