Abstract
The form of multicellular animals and their organs is mainly defined by the curvature of cell layers. They are boundaries for solid tissues; and some organs and organisms consist mainly of distinct cell layers (Fig. I a). The form of adult organisms results from a complex interplay of tissue evagination, growth patterns, production of and interaction with extra-cellular material, and other effects; but the rudiments and basic features of the forms produced can often be traced back to processes of evagination or invagination of nearly flat cell sheets at defined locations in the course of embryogenesis.
