When Nettie is tending to the ill Corrine one day, Corrine asks when exactly Nettie met Samuel. It is revealed that Corrine believes Nettie to be the biological mother of the children Adam and Olivia—and that Samuel is the father. Corrine has been living with this anger for years, hence her ill-treatment of Nettie.
The source of Corrine's jealousy is finally made plain. For some reason, the many years in Africa, and perhaps the isolation and difficulty of the life there, have caused Corrine to fixate on this one particular idea, even though there is no evidence to support a relationship between Samuel and Nettie. Though Corrine's belief also suggests that the children may resemble, which further suggests that they likely resemble their true mother: Celie.
Corrine forces Nettie to swear on a Bible that Nettie met Samuel the day she met Corrine. Corrine examines Nettie's stomach to see if there are signs Nettie has ever given birth (she has not). Samuel apologizes to Nettie, in private, for Corrine's behavior. In the meantime, the village lands keep being destroyed by the rubber company, and the men of the village must travel great distances to find wild game to hunt.
The examination of Nettie's stomach is a particularly graphic example of the extent to which Corrine has become fixated, the depth of her jealousy.