Developmental processes are intricate networks of events by which a fertilized egg proliferates and differentiates into a fully functional multicellular organism (1 ). Mechanisms are used by dividing cells to create asymmetries in their information content that then allow the emergence of heterogeneous cell types through differential gene expression (2 ,3 ). Most developmental processes share the following characteristics:
| 1. | Consecutive steps of commitment and specialization are taken by dividing cells; |
| 2. | Time- and tissue-regulation of gene expression; and |
| 3. | Migration of specialized cells from their sites of origin to different locations. |
