Escher was fascinated by the concept of infinity bounded. In Circle Limit III, there is an infinite number of fish in a single circle. To create this image, Escher used arcs of circles to generate the axes of the fishes bodies: note the white curves. All fish whose body axes lies on the same white curve are colored the same color. As Escher lamented people's inability to see the "beauty of this infinite world-in-an-enclosed-plane."
Another example of an infinite world-in-an-enclosed-plane is Smaller and Smaller. Here the image is of reptiles in a repeated 4-leaf clover shape.
(You can see the four-leaf clover if you look at the black reptiles.)
The point of infinite in this image is the center rather than
the outer circle as it was in Circle Limit III.
All M. C. Escher works (c) Cordon Art B.V.-Baarn-the Netherlands. Used with permission.