I like this paper because it ties in molecular biology, classical embryology and evolution. I find the writing clear and use it as an example of the cross-disciplinary nature of developmental biology in my courses.

Here is the link; if you are subscribed to Science, you’ll be okay. If not, is it legal to propose that I can forward a PDF to readers?

http://www.sciencemag.org/cgi/content/full/299/5606/565

The Cellular and Molecular Origins of Beak Morphology

R. A. Schneider, J. A. Helms

Cellular and molecular mechanisms underlying differences in beak morphology likely involve interactions among multiple embryonic populations. We exchanged neural crest cells destined to participate in beak morphogenesis between two anatomically distinct species. Quail neural crest cells produced quail beaks in duck hosts and duck neural crest produced duck bills in quail hosts. These transformations involved morphological changes to non-neural crest host beak tissues. To achieve these changes, donor neural crest cells executed autonomous molecular programs and regulated gene expression in adjacent host tissues. Thus, neural crest cells are a source of molecular information that generates interspecific variation in beak morphology.