This is evidently a species of appoggiatura, as also in the chorale Vater unser im Himmelreich (Clavierübung, Part III). But in this case, Bach uses a special notation:
In his Method for clavecin Ph. E. Bach, in speaking of a similar figure, thus explains it: "The first note of this figure must not be made too short, if the tempo be slow or moderate; for the second would then be held too long. It should be gently dwelt upon, not suddenly hammered."
"Play flautato," says W. Rust[195] upon the subject of such a fugue in an orchestral part; one should thus anticipate the beat with the flutist's stroke of the tongue, according to Quantz (Essai d'une méthode pour apprendre à jouer de la Flûte traversière. Berlin, 1752); that is to say, that the first of the two notes should be considered as written thus:
In a solfeggio lesson by J.G. Walther,[196] written in 1708, this indication, called punctus serpens, signifies that the notes are to be slurred; that is, bound together, two by two. This is, undoubtedly, the most correct interpretation, which fairly corresponds to what S. Scheidt calls "imitatio violistica."
Analogous notations of Frescobaldi[197] and Muffat[198] indicate a similar manner of execution.