As has already been pointed out, the normal skin has the following appendages: hair follicles, both of the lanugo and deep hairs, with the adjoining sebaceous glands, and the sweat glands and ducts. From all of these organs both benign and malignant tumors may arise. In addition, there is unquestionably the possibility of there being congenitally displaced epithelial or glandular tissue, and it now seems fairly certain that at least one clinical and pathological entity, the syringocystadenoma, springs from congenitally misplaced coil ducts. Dermoids, of course, have their origin in congenitally dislocated tissue.
Owing to the degenerative changes, as well as the modifications in cell type, that naturally take place during the lifetime of a neoplasm, it is highly probable that a fair number of tumors originate from these appendages, and that they cannot be diagnosed as such under the microscope. In even a slightly advanced basal-celled carcinoma, it is impossible to determine whether the origin was in the basal layer of the epithelium or in the hair follicle. As Unna* well points out, the cells of the sebaceous glands so readily revert to simple-appearing epithelial cells that one might never suspect a carcinoma of having its origin in them. From these two illustrations it can be readily seen that it is often impossible to say in just what cells the malignant change first appeared.