octagon.
In the drawing W. P. 5b (see page 44 Fig. 1) the exterior only of the edifice is octagonal, the interior being formed by a circular colonnade; round chapels are placed against the four sides of the principal axes.
The elevation, drawn on the same sheet (see page 47 Fig. 3), shows the whole arrangement which is closely related with the one on Pl. LXXXVI No. 1, 2.
MS. B. 21a shows:
a) four sides with rectangular chapels crowned by pediments Pl. LXXXVII No. 3 (plan and elevation);
b) four sides with square chapels crowned by octagonal domes. Pl. LXXXVII No. 4; the plan underneath.
MS. B. 18a shows a variation obtained by replacing the round chapels in the principal axes of the sketch MS. B. l8a by square ones, with an apse. Leonardo repeated both ideas for