The effects of experimental unilateral anotia on skull development in the chick embryo. II. Essentials of the development of the chondrocranium in normal embryos of 7-20 days of incubation.

In order to be able to describe adequately the developmental anomalies of the chondrocranium in unilateral anotia in the chick embryo, it seemed necessary to describe the essentials of the normal chondrocranial development. The following observations proved to be different from the available descriptions: 1. the occipital processes do not fuse with the ear capsules; 2. the tectum synoticum fuses only with the ear capsules; the occipital processes fuse later with the tectum synoticum by outgrowth to dorsal; 3. in the chick embryo there is no evidence for the existence of a tectum posterius; 4. the metotic fissure is in the cartilaginous stage of development not closed, but shifts laterally; 5. there is strong evidence that the pro-otic processes arise as independent structures which later fuse with the ear capsules. The results of the study of the development of the chondrocranium in the embryos in which experimentally unilateral anotia is produced, will be described in a following paper.