_Feet._ Bursæ under tendo achillis small but distinct. Present over both malleoli. Metatarso-phalangeal joints D 1, 2, 3, rudimentary D 4 and 5 absent. First phalangeal joints of D 1, 2, 3, 4, rudimentary absent over D 5. Second phalangeal joints absent on all digits. _Hapale Jacchus_ I. _Hand._ Lower end of ulna, which is very prominent, a bursa present, over end of radius, _which is much less prominent_, absent. Metacarpo-phalangeal joints, present in all. First phalangeal joints, which are prominent, present in all digits. Second phalangeal joints absent in all. _Foot._ Bursa under tendo achillis and over both malleoli. Metatarso-phalangeal joints absent on D 1; present on D 2, 3, 4, 5. First phalangeal joints, present in all. Second phalangeal joints, absent in all. _Cercopithecus Callitrichus_ J. _Hand._ Dorsal surface of the whole hand shows no localised bursæ, only a loose areolar tissue under the skin. Styloid processes of radius and ulna no bursæ. _Foot._ Dorsal surfaces over the whole foot similar to that of the hand. Bursæ present over both malleoli. Well-formed small bursæ under tendo achillis. _Cercopithecus Mona_ K. _Hand and Foot._ Dorsal surfaces similar to those of J and similar loose areolar tissue over styloid processes of ulna and radius. Bursæ over both malleoli. Well-formed bursa under tendo achillis. _Macacus Rhoesus_ L. This specimen showed more examples of bursæ than the two of Cercopithecus. Bursæ present over styloid processes of ulna and radius, also over metacarpo-phalangeal joints. Bursa well-marked over malleoli and under tendo achillis. Bursæ present over metacarpo-phalangeal and metatarso-phalangeal joints. No bursæ over phalangeal joints. Further Undesigned Experiments. The preceding facts as to the natural history of bursæ in man and some lower Primates, even if they stood alone, are enough to produce conviction as to the manner in which bursæ of all degrees of perfection are formed by function, and point to the origin of the initial stages of these structures. But they do not stand alone, for in man there have been carried out certain undesigned experiments in a similar direction, comparable to those described in the sections on direction of hair and arrangement of papillary ridges. These demonstrate the fact that frequent friction of skin over a hard surface has the power of producing adventitious bursæ in regions where they are not found in the normal state. These adventitious bursæ are the following:--