Unraveling the neuron jungle: The 1879-1886 publications by Wilhelm His on the embryological development of the human brain

The neuron doctrine, one of the central paradigms of the neurological sciences, states that neurons are individual units. Wilhelm His helped to lay the scientific foundation for the neuron doctrine. This Swiss researcher's experimental approach, which was based on the examination of embryological material, is of particular interest because few investigators at that time were using it. Only selected excerpts from His' seminal 1886 work have been translated into English. Therefore, details regarding his experimental materials and primary observations are unavailable to the American reader. Our objective was to translate additional sections of the seminal 1886 publication and, when relevant, portions of previously untranslated works dating back to 1879 and 1883 so that we could better understand this embryologist's experimental materials, his primary observations, and the basis for his conclusions. By 1886, His had compiled a collection of 12 human embryos of various measured lengths and estimated gestational ages ranging from 2.15 mm to 24 mm, and 2 weeks to 8.5 weeks. He had studied the embryological development of nerve fibers in successively older specimens. For example, he contrasted the young embryo N, whose "nerves ... stop only halfway and the distal part [of the extremity] is completely nerve-free," with the older embryo Zw, in which the nerves had already reached "the root of the fingers but none [were] in the territory of the end phalanges." In the 1870s and 1880s, using a discrete collection of 12 human embryos of consecutively older ages, His observed the development of nerve fibers and, from this pattern of growth, eventually suggested that nerve cells were individual units and that the transmission of impulses was possible without direct continuity between neurons. Although this age-based embryological approach seems relatively straightforward by today's standards, few other researchers were using it at the time.