Describe the physical change that has taken place in Jekyll?

From the text:

He put the glass to his lips and drank at one gulp. A cry followed; he reeled, staggered, clutched at the table and held on, staring with injected eyes, gasping with open mouth; and as I looked there came, I thought, a change—he seemed to swell—his face became suddenly black and the features seemed to melt and alter—and the next moment, I had sprung to my feet and leaped back against the wall, my arms raised to shield me from that prodigy, my mind submerged in terror.

“O God!” I screamed, and “O God!” again and again; for there before my eyes—pale and shaken, and half fainting, and groping before him with his hands, like a man restored from death—there stood Henry Jekyll!