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EIGHTEEN FIGURESInnumerable times the outgrowth of developing or regenerating nerve fibers has been watched, while taking place, both outside the organism in tissue culture (Harrison, '07, 'lo and others) and in the organism (Harrison, '08 ; Speidel, '32, '33). Yet to mere observation, the mechanism that operates in guiding the outgrowing nerve fibers into a definite pattern, peripheral and intracentral, has not revealed itself.In an attempt to elucidate the guiding principle, highly valuable contributions have been made both by experimental work and by speculation, and the question has been answered in several ways that sounded rather definitive. However, surveying the existing evidence, one realizes that none of the hypothetical solutions offered can be considered as wholly satisfactory, as long as their experimental foundation is so disproportionately small, and undoubtedly the disproportion between facts and speculation in this field is very great indeed.

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