Epithelium, being an epiblastic structure and capable of no other origin save from its kind, can only be supplied from those regions where it has preëxisted. Consequently, ulcers involving the external surface of the body demand a lively epithelial reproduction in order that they may have a normal covering. Epithelial activity sometimes becomes retarded, and is much slower toward the termination of the healing process than at the beginning. The epithelial covering of a healing ulcer is always marked by a delicate whitish or pinkish film, which proceeds from the periphery as well as from any little island of original epithelial structure left. It is well known that after a certain amount of this repair the process sometimes comes to a complete halt, and the various expedients for stimulating and promoting it, as sponge grafting and the different methods of skin grafting, have been devised solely to atone for such sluggishness or inability. Ulcers of small size, which are more or less exposed to the air in healthy individuals, while also exposed to possibility of infection, nevertheless seem to escape it, owing to the defensive power of the blood serum and the active cells. Such discharge as naturally comes from them, when not excessive, undergoes evaporation until a point is reached where a _dry crust_ or _scab_ is formed. Under this scab granulation proceeds to a point where the pressure of the scab itself, presumably on the level of the surrounding parts, checks its activity, while at the same time epithelial reproduction goes on until it has been completed. Then the scab, being no longer of use, drops off or is detached by slight friction.