4 6 



HISTOLOGY 



They afterward divide one or more times by themselves, this time by 

 an amitotic division, and are then pushed farther outward by the next 

 divisions of the cells in the basal layer. The amitotic divisions do not 

 appear in such early stages. 



Thus, the basal layer remains where it started, continuing to divide 

 mitotically, and only such of its daughter cells as can keep in contact with 

 the basement membrane remaining basal cells. All others, having left 

 the basal layer, divide once or twice more by an amitotic division and are 

 pushed continually outward. When a certain distance from the basal 

 layer, they fail to get the proper amount of nourishment, and die. The 

 outer layer of dead cells is, as can be seen, constantly accumulating and 

 must be as constantly reduced in order to keep its volume at some nor- 

 mal point. This reduction is performed by the removal from the outer 



FiG. 50. Section of a single point of stratification on the umbilical cord of an embryo sheep. 

 Mitosis in the basal layer and in the underlying connective tissue. 



surface of as many of the dead cells as is necessary, by abrasion, 

 by the shedding of layers of these cells, by the solution and decay of 

 some of them on moist surfaces, and in certain cases by the pro- 

 cesses of oil formation that result in their degeneration and destruction 

 when the product is set free. They are also used by being built up 

 into various defensive and offensive structures, as hairs, horns, feathers, 

 etc. (see Chapter XX). 



An epithelium formed in the above manner from a simple or single 

 layered covering of cells is known as a stratified epithelium. Besides 

 being derived from the simple form of epithelium, it usually shows a ves- 

 tigial arrangement of its basal layer in a columnar or simple form. As 

 a secondary differentiation we may find the outer layer of cells elongated 

 and placed in a row so that they look like the columnar form. Such cells 

 are secreting cells, and a good example may be found in certain folds of 

 the conjunctiva of the young alligator, where these cells are engaged in 



