As a vertebrate animal develops from a fertilized egg, progressive
differentiation of cell structure and function occurs,
resulting in fibroblasts, neurons, liver cells, muscle
cells, and many other types of cells. This diversity of differentiated
cells is believed to represent different regulatory
states of the same set of DNA sequences. How can different
states arise from a single cell? Viewing an early embryo
from any one of its cells allows us to distinguish two
sources of developmental information. One is internal, the
cells own inherited program. The second is external, including
the surrounding cells that provide the cell with
physical or chemical cues and create for it a unique microenvironment.
The general importance of the external
sources in vertebrate development is illustrated by two
kinds of experiments.