What makes a rhinoceros look like a rhinoceros, or a petunia like a petunia? Understanding the basis of the generation of the forms, patterns, or shapes of complex organisms is a principal subject of biologist Gerald Edelman's stimulating book Topobiology, which explores the links among the development, evolution, form, and function of organisms.

In older scientific language, one could say that Edelman has brought a new dimension to morphology, the study of the form and shape of animate and plants, and to histology, the study of the minute structure of tissues. The major thrust of his book te what he calls a morphoregulatory hypothesis, which can be described as the outline of a molecular basis for the formation of pattern, involving three types of genes. In the morphoregulatory sequence, morphoregulatory genes control the production of molecules that cause the formation of clusters of cells. When the cell dusters begin to communicate, they activate another type (selector genes) that then select and switch on a third type of gene (historegulatory genes).

In the organisms they have studied, Edelman and his colleagues have found cell adhesion molecules (resulting from morphoregulatory gene action) that are attached to the surface of different cells and link them in clusters. These molecules can appear at different times in the development of a multicellular system, and at various specific sites on the cells. Furthermore, their chemical structure can vary. This tremendous variation allows a great flexibility in the kinds of linkages the molecules effect.

A second class of adhesion molecules plays a comparable role In attaching cells to the developing scaffolding of filaments that lie outside cells. Also of particular interest to researchers are what are called cell Junction molecules, composed of clusters of proteins providing small channels from the inside of one cell to the Inside of another. These three types of molecules facilitate a kind of conversation among eels that determines the course of development of organisms. It has been possible to demonstrate how Important these molecules are by treating tissues with agents that block their functions, thus effectively and drastically changing the organism's development.