tance. A small number of signal transduction pathways are used in these interactions. There are approximately 10 kinds in early development and organogenesis. (Seven more are used by differentiated cell types.) The number of allelic variants that exist in these human genes remains to be studied. These pathways are conserved among animal phyla, as are many of the genetic regulatory circuits involved in the responses of cells to signals. In addition, many of the basic cell processes, such as proliferation, secretion, motility, and adhesion, are also highly conserved among animals. This extensive conservation gives strong justification to the use of model animals, including Drosophila, C. elegans, zebrafish, and mouse, to learn about basic aspects of mammalian, even human, development.

The understanding of development is far from complete. Although a number of main components have been identified for early processes, their interactions and use in combinations introduce substantial complexity to an inclusive understanding of development. Few of the many types of mammalian organogenesis have been analyzed, and only a few of the 300 types of cytodifferentiation have been studied. As more components and interactions are revealed, it will become important to establish readily accessible databases containing all the information about development.