In some of the most famous experiments in embryology, Spemann and Mangold demonstrated that the potential for brain formation is not restricted to specific cell in the ectoderm of early amphibian embryos, but is induced by cell contact with this underlying chordamesoderm. The central concept which emerged from their work of cell interactions determining cell fate, remains a cornerstone of modern embryology. Nevertheless, over fifty years later, both the molecular signal that induces neural differentiation and the method of its communication remain to be discovered. We have begun to reinvestigate the problem of neural induction with modern microinjection techniques in embryonic ascidians, primitive marine chordates with a simple, archetypal development. We have chosen Ascidia interrupta for its relatively large and transparent eggs. These hermaphrodites produce fertile eggs and sperm throughout the year, and they self-fertilize with reasonable efficiency. Thus, control and experimental protocols can be carried out on a clutch of genetically identical embryos. Furthermore, the block to polyspermy does not reside in the extraembryonic membranes, so we have removed them enzymatically without disrupting development, which proceeds to the tadpole stage in less than 18 hours at room temperature. By injecting individual blastomeres with fluorescent dextrans at the eight cell stage, we have identified the cells that give rise to the embryonic nervous system. Injection of a smaller fluorescent indicator, Lucifer yellow, has also allowed us to visualize the pattern of cell communication through gap junctions during early development. We have discovered that when that communication is blocked briefly during gastrulation, the embryos fail to develop normally.
