"Parts of a feather. I., Four barbs (B.) bearing anterior barbules (A.BB.) and posterior barbules (P.BB.);…
"Types of feathers. D., Down. 2, Developing feather in sheath (sh.). 3, Covert of heron showing aftershaft…
"A., Filoplume. B., very young feather within its sheath (sh.); c., the core of dermis; b., the barbs.…