The mutual relations of structures in the growing bill of chick (Gallus domesticus L.) and duck (Anas platyrhynchos L.) Embryos. IV. The size, shape, and position of the upper and lower bills in unilaterally microphthalmic duck embryos.

Unilaterally microphthalmic duck (DUM) embryos studied in stage 39 were obtained by damaging the right eye in stage 17. The development between stages 19 and 39 was observed in samples. After prolonged incubation, one duck hatched and reached adulthood. In DUM embryos partial crossbeak develops. Both the upper and lower bills deviate lateralwards, the upper bill more than the lower. In stage 39 a small orbit is present on the treated side. In stained and cleared heads most of the elements of the skull (the bones surrounding the orbit, the brain-case, and the upper and lower bill elements) are translated in the direction of the rudimentary orbit on the treated side. The bills are twisted around each other at the tip. Measurements of external dimensions as well as those made in stained and cleared stage-39 DUM heads show elongation of the left part of the lower bill. Except for the crossed bills, the changes found in the embryos persisted in the adult. The findings support a hypothesis concerning the mutual influence of bills during morphogenesis. The coherence of the functional cranial components in the embryo is discussed.