That the change in his voice was nothing more than the first sign of a bad cold, an occupational ailment of the traveling salesman, he had no doubt in the least. (1.8)
A bad cold? A bad cold? At this point Gregor's trivialization of his, um, symptom is outrageous. But it is also a little understandable, at least in the sense that many of us also try to grapple with a new or alien experience by either reducing it to something familiar (like a bad cold) or trying to deny that it exists altogether.
Pitilessly his father came on, hissing like a wild man […] If only his father did not keep making this intolerable hissing sound! It made Gregor lose his head completely (1.30)
It seems that every character who comes into contact with Gregor is also transformed, most notably in their behavior toward Gregor. (When you're having lunch with your friends, try throwing a huge cockroach in the middle of the table and see what happens. Suddenly, dignity will be a very scarce commodity.) Ironically, even as the characters are repulsed by Gregor's insect form, their own behavior renders them equally grotesque. Gregor's transformation in this passage brings out his father's domineering side in a particularly savage way.
When Gregor Samsa woke up one morning from unsettling dreams, he found himself changed in his bed into a monstrous vermin. (1.1)
For an opening, the first sentence of the novella is pretty hard to beat for sheer absurdity. The idea of waking up as an insect is so extraordinary that you might find yourself re-reading the sentence, trying to figure out if there's anything in those "unsettling dreams" that precipitated the change. That's part of the game the story plays with you – can people just change overnight? Does there have to be a cause?
Of course it was not only childish defiance and the self-confidence she had recently acquired so unexpectedly and at such a cost that led her to make this demand; she had in fact noticed that Gregor needed plenty of room to crawl around in; and on the other hand, as best she could tell, he never used the furniture at all. (2.22)
Even though Gregor can no longer support the family financially, the family's condition actually seems to improve. In this passage, Grete becomes more independent and assertive by taking on the responsibility of caring for Gregor.
[…] after such excursions, tired to death and sad, he did not budge again for hours. (3.9)
In the beginning of Part 3, references to Gregor as almost dead or dying recur as if to signal that yet another, more final change is in store for Gregor (see Quote #8 below, for example). The frequency of these references are kind of ironic because, when we get to Gregor's actual death, we're not told exactly how he dies (See Quote #10 below).
Sometimes he thought that the next time the door opened he would take charge of the family's affairs again, just as he had done in the old days […] At other times he was in no mood to worry about his family, he was completely filled with rage at his miserable treatment, and although he could not imagine anything that would pique his appetite, he still made plans for getting into the pantry to take what was coming to him (3.7)
This passage suggests that Gregor's transformation isn't just a physical one, but operating at a level that he himself is barely conscious of. Gregor seems to be in touch with his pre-vermin life, the responsible Gregor who supported the family and directed household affairs. But the insect side of him, that just wants to stuff his face regardless of what the others think, is just as much a part of his consciousness. If it is the insect side of him, that is – we have no way of knowing whether he was like this before.
As a matter of fact, Gregor's body was completely flat and dry; this was obvious now for the first time, really, since the body was no longer raised up by his little legs and nothing else distracted the eye. (3.31)
With death, Gregor's transformations come to an end. Of course, we're not told exactly how he dies, just as we're not told exactly how he became a bug. Curiously, Gregor's body in death does not disgust in the same way that his body did in life. The mother doesn't faint in horror, and the sister can evaluate him calmly, without running for cover behind her father. This is the opposite of what we would expect with a human corpse.
At first he thought that his grief at the state of his room kept him off food, but it was the very changes in his room to which he quickly became adjusted. (3.9)
As the passage suggests, Gregor's transformation didn't end when he woke up in his bed as a vermin, but continues as his environment changes. In contrast to Part 2, where he resisted the changes to his room because he associated the furniture with his human life, in Part 3 Gregor accepts the changes, which suggests that he accepts his insect body as a permanent fact of life.
[H]e too was completely covered with dust; he dragged around with him on his back and along his sides fluff and hairs and scraps of food; his indifference to everything was much too deep for him to have gotten on his back and scrubbed himself clean on the rug (3.13)
In contrast to Part 1, where he could still feel a little thrill of well-being when he discovered some new aspect of his body, in Part 3 Gregor just seems defeated. He presents in this passage a particularly pathetic image.
"Look how these roomers are gorging themselves, and I'm dying!" (3.11)
Here we have another reference to Gregor as "dying." The passage draws a contrast between the human roomers and Gregor in order to ask the reader whether it's OK to maintain human life over an insect's, even if the insect is your erstwhile son/brother.