Chick embryos, during stages 14 to 25, undergo an arching of the hindbrain and cervical neural tube that is termed cervical flexure. We have found that if the truncus arteriosus is severed during stage 12–13, the embryos survive for more than 24 h and do not show cervical flexure. The embryos have a beating heart, the expected number of somites, and often have discernible wing and leg buds. Light and electron micrographs reveal no histological abnormalities. The percentage of cells that become labeled with tritiated thymidine is close to normal, indicating that most of the cells are healthy. These results suggest that cervical flexure is related to normal morphogenesis of the heart. At stage 10, the heart is almost straight, with the prospective ventricle cranial to the prospective sinus venosus. The heart tube loops between stage 10 and stage 23, first to the right and then caudad, so that the ventricle becomes caudal to the sinus venosus. The heart undergoes these morphogenetic movements autonomously. The truncus arteriosus does not increase in length during caudal movement of the ventricle, so the cervical region is pulled into an arch. Bending of the cervical region into an arch can be prevented in intact embryos by injecting agar into the foregut, so that the foregut cannot bend. However, after about 24 h of further growth, if the axis cannot bend, the truncus begins to leak blood and the embryo dies. We conclude that cervical flexure is a response of the embryonic axis to the morphogenesis of the hear
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