The development of the eye muscles of the chick

The anlage of the abducens musculature appears first at 24–25 somites as a condensation situated dorsal to the mandibular arch. The anlage of the superior oblique grows forward from a mesodermal condensation situated in the maxillomandibular region, termed for convenience the maxillomandibular condensation. This last consists of three parts: (1) the anlage of the superior oblique: (2) the anlage of the abducens musculature, and, (3) an intermediate region. The intermediate portion of the maxillomandibular mass forms a condensation with which the anlage of the abducens musculature fuses. Its fate is, therefore, similar to that of the so‐called ‘muscle E’ of elasmobranchs, which has been described as fusing with the lateral rectus. How much muscle is formed from the intermediate condensation in the chick has not been determined. The development of the pyramidalis and quadratus nictitans muscles, derivatives of the abducens complex, is described. The premandibular head cavities are replaced by solid mesodermal condensations, on the surface of which the anlagen of the oculomotor muscles appear. The premandibular mass expands laterally and anteriorly over the bulbus, carrying the oculomotor muscles to their respective positions on the bulbus. Portions of the premandibular and maxillomandibular condensations not involved in eye‐muscle formation take part in the formation of choroid and sclera. The growth shiftings of the eye muscles are analyzed. The order of their appearance is commented upon.