NEURULATION IN AMNIOTE VERTEBRATES - A NOVEL VIEW DEDUCED FROM THE USE OF QUAIL-CHICK CHIMERAS

Two apparently different mechanisms successively contribute to the for mation of the neural tube in the avian embryo: bending of the neural p late during the primary neurulation in the cephalo-cervico-thoracic re gion and cavitation of the medullary cord during the secondary neurula tion in the lumbo-sacral region. During both these processes, gastrula tion continues by the caudal regression of Hensen's node - also called cordoneural hinge in the secondary neurulation. Labeling of Hensen's node or cordoneural hinge by the quail chick marker system revealed th at this structure, which is the equivalent of the dorsal blastoporal l ip of the Amphibian embryo, i.e., of the Spemann's organizer, gives ri se to the midline cells of the three germ layers: the floor plate of t he neural tube, the notocord and the dorsal cells of the intestinal en doderm. Caudally to the organizer, both in primary and secondary neuru lation, the presumptive territory of the alar plates of the future neu ral tube overlies the precursors of the paraxial mesoderm. Regression of Hensen's node bisects the ectoderm in two bilateral neural plates l eaving in its wake the floor plate, the notocord and the dorsal endode rm.