Fig. 7. A hypothetical Evo-Devo Space.

X coordinate represents new developmental mechanism. Y coordinate represent new phenotypes. They all start from a flat layer of epidermis. Those in the right upper quadrant used more novel developmental mechanisms and are more complex. Arrows indicated possible topological transformation from one form to the other, not necessary indicate the evolutionary process. We are working to reveal the molecular basis of those arrows. The processes are in red, and the names of the forms are in blue. Yellow circle is the cross section of developing feather follicles with brown indicating the position of rachis. For comparison, the stage correspond to those used in Prum, 1999 is indicated in (green). Revised from Chuong et al., 2001.