Fig. 13.

Allometric growth and the development of the beak. Our interpretation of figures of the rostral aspect of the face and text in His (1875, pp. 204–206). (A, B) Fig. 139, 14 day (d) rabbit; (C, D) Fig. 139, 5 d chicken; (E, F) Fig. 140, 6 d chicken. According to His, the eye (yellow) of the chicken embryo grows enormously in size compared with that of the rabbit. As a consequence, the stomodeum (blue) and internasal region in the chicken are laterally compressed (arrows in D), causing the nasal pits (red) to converge (arrows in F). His argued that these changes contribute to the formation of a narrow, protruding beak. Key: nl, nasolachrymal duct.