The image of the frog, lying helplessly on her back with her arms and legs pinned (literally) brings the memory of a horribly traumatic experience to Melinda's mind. The frog is completely vulnerable; forceably spread-eagled, she is about to have her belly split open with a knife, a sharp, pointed object. Although Melinda does not specifically mention being raped at this point, it is clear that subconsciously, she remembers being similarly victimized. She "can feel the cut, smell the dirt, leaves in (her) hair," and the "scream that starts in (her) gut" gives evidence of the terror she felt then and is reminded of now.
Later in the book, the author reveals that Melinda was indeed raped, at the party, on the ground, outdoors. Like the frog, she was almost senseless at the time, totally inebriated. Her confusion was such that she did not remember how she came to be lying on the ground; she only knew that, like the frog, she was prone, and her attacker was on top of her. The violation of her body happened quickly; she remembers being unable to move and being hurt and hurt and hurt.
Melinda notes that the class was "scheduled to do the frog unit in April, but the frog company delivered (their) victims on January 14." It is significant that the frog Melinda is assigned to dissect is female, and that it is described as a "victim." The frog was "delivered" too soon, like Melinda, a young teenage girl, who was forced to have sexual relations long before she was ready.