"hissing like a wild savage..."
See in text (Chapter I)
Note the irony of these words: Gregor's father, still in his human body, begins to act even more like an animal than Gregor, hissing at his son and forcing him back into his little room. Gregor has (quite admirably) attempted to act just like a human and to get up, head to work, catch the train, and even walk as if nothing has happened. This line proves that humans can nevertheless still be inhumane.