At the end of the nineteenth century, controversy arose as to precisely when the first glial cells originate during development of the central nervous system, and to date, the issue has not been satisfactorily resolved. His (1889, 1890) noted that, even in the earliest developmental stages of the germinallayer, there appeared to be two distinct cell types. The cells which he called Spongioblasten were thought to be glial precursors from which all mature glial cells derive; Keimzellen, in contrast, were regarded as forming 1 neurons. His was working on the assumption that the very first...