The history of neuroscience is punctuated by oracular disclosures from in vitro systems. In 1907, a pivotal tissue culture study by Harrison proved that Ramón y Cajal’s theory on the developmental origin of nerve fibers was correct. Cajal had proposed in 1890, based on microscopic analysis of static histologic tissue sections, that the immature neuronal cell body sends out an axon that elongates freely, bearing a motile growth cone at its tip. Competing theories held that free growth of neurites did not occur, but that the neurites formed from the fusion of elements produced by other cells or from the stretching of a protoplasmic bridge between central and peripheral cell bodies of a multinucleated cell (1). These theories could not be tested by the histologic methods of the time, because axonal growth by a living neuron could not be directly observed. Harrison (2) pioneered a culture system for long-term microscopic observation of neuronal differentiation in living tadpole neural tube tissue. His observation that neurites grow out from cell bodies has been hailed as “one of the most revolutionary results in experimental biology” (3).