Cells differ not only in their shape but also in their size. A few types
of cells are large enough to be seen by the unaided human eye. For
example, the nerve cells that extend from a giraffe's spinal cord to
its foot can be 2 n1 (about 6 1/2 ft) long. A human egg cell is about
the size of the period at the end of this sentence. Most cells, however,
are only 10 to 50 pm in diameter, or about 1/500 the size of the
period at the end of_ th~s ~entence.