Introduction and Biological Background ◾ 11
be compared to a colony of simple organisms and will show how systems
exhibiting complex collective behavior can be created within the colony.
A fundamental dierence between a multicellular organism and a col-
ony of single-cell organisms is the dierentiation of the cells into dier-
ent cell types, each of which has unique properties and well-dened roles
in the system. Dierentiation is mainly a unidirectional process, whereby
dierentiated cells cannot change into a dierent kind of cell and can sur-
vive only within the multicellular organism. For example, a neuron cannot
turn into a muscle cell. Only the genetic material existing in specialized
cells (the gametes or germ cells) is used