Fig. 8. Evo-Devo of ectodermal appendages.

A) X axis represents addition of new developmental mechanisms. Y axis represents emergence of new phenotypes. Glandular structures, invaginations, protrusion, branching, etc evolved from the flat epidermis. Complex feather forms have come a long way. Modified from Wu et al., 2004⁹⁰ B) Young chicks show downy feathers all over the body, while an adult pheasant shows diverse feather types that are sexually dimorphic. Female pheasant does not show these spectacular feathers (not shown). C) Mammalian ectodermal organs evolve with different emphasis under different physiological conditions as well as in evolutionary time. Top panel shows a fleece. Middle panel: scalp hairs and mammary gland. Bottom panel: long body hairs and tasks.