Chapter 13: Exercise 13-3, p. 210

There is a mistake in the comments of this exercise. The following

  // Draw the ellipse with size determined by Perlin noise
  fill(200);
  ellipse(__________,__________,__________,__________);
}

should read:

  // Draw the ellipse with location determined by Perlin noise
  fill(200);
  ellipse(__________,__________,__________,__________);
}

Incidentally, the answer to this exercise can be found here.