1. E. Zwirner, Phonologische und phonometrische Probleme der Quantität,Proceedings of the Third International Congress of Phonetic Sciences, Ghent, 1939, p. 57–66.
2. L. L. Hammerich,Laryngeal before sonant, København, 1948 (Det Kgl. Danske Videnskabernes Selskab, Historisk-filologiske Meddelelser, Bind XXXI), p. 53 ff. “This must be due to a differentiation in Old High German. When the mutation ofẹreached the stage of closee, there arose a danger of confusion with the inheritedë. Now, the new ẹ could not escape, because it was still under the palatalizing influence of the following syllable (e.g. OHGgrẹbit“digs”,gẹsti“guests”,mẹnnisc“human”,bẹzziro“better”), but the inheritedëwhich was never preserved before ani/in the following syllable, could and did escape fusion, receding into a more open sound. In the great majority of cases High German—alone of all Teutonic languages—has worked out this distinction between the mutation-ẹ and the inherited è, that the former remains a close ẹ-sound, but the latter becomes an open e-sound, in dialects even ana.
3. A. Meillet,Caractères généraux des langues germaniques, 4th ed., Paris, 1930, p. 61: “Tout se passe donc à peu de chose près comme s'il y avait en germanique commun une voyelle unique, qui deviendraitiouesuivant les cas.”