Skull growth after partial prospective lower beak extirpation in chick embryos.

On one side of chick embryos in stage 28, part of the prospective lower beak or the entire prospective lower beak anterior to the buccal corner was removed surgically (CLB-A and CLB-B embryos, respectively). In another group the prospective lower beak was split in the midsagittal plane (CMS embryos). The controls were shame-operated embryos (CS). The skulls of stage 39 CLB-A and CLB-B embryos showed changes in the lower beak as well as in the skull base, the pterygoid, the quadrate, the palatine, and the upper beak. Stage 39 CMS embryos showed shortening of the dentary and Meckel's cartilage. At the tip of the CMS mandible, an excessive amount of osseous material was found. The results support Dullemeijer's (1974) views concerning the issue of dependency of functional components. The value of a combined developmental and functional approach to morphology, i.e., integrated investigation of development of structure and function of the beaks and related jaw musculature, is discussed.