In an effort to answer the question whether prosencephalic neural crest cells play an essential part in the differentiation of the facial skeleton, in twenty 2- to 6-somite stage chick embryos the anteriormost part of the neural crest was eliminated. Observations on 10 of these embryos, sacrificed 2 hours after the microlaser irradiation, showed that often also some damage is done to the prosencephalic neural tissue. Among the remaining 10 embryos, 6 survived until the age of 12 days. One of these embryos showed unilateral microphthalmia, two bilateral microphthalmia, two synophthalmia and one cyclopia. In the microphthalmic embryos, the facial skeleton proved to have differentiated normally; in the other embryos, several facial bones were entirely lacking, while others only had developed rudimentarily. All affected eyes lacked a cartilaginous sclera. On the basis of these results, the question posed could not be answered. On the other hand, however, the experiments revealed that (1) prosencephalic neural crest cells are indispensable for a normal differentiation of the scleral cartilage, (2) elimination of the prosencephalic neural crest and a part of the prosencephalon does not affect the development of the mandible and the hyoid, and (3) the mesodermal mesenchyme bordering the prosencephalon, has no developmental capacities of its own. Finally, the craniofacial anomalies observed in the synophthalmic and cyclopic chick embryos, proved to be highly similar to the corresponding anomalies in man.