GENERAL HISTOLOGY. 75 



shortened (fig. 25, a) and the separate cells have become changed 

 into thin plates. 



Flagellated and Ciliated Epithelia. Further differences which 

 obtain in the three kinds of epithelium mentioned above are 

 caused by the presence or absence of processes (cilia, or flagella) 

 on the peripheral end of the cells. Both are fine threads which 

 arise from the body of the cell, extend above the surface and here 

 maintain an extremely lively motion. In case of flagellated 

 epithelium (fig. 25, d) each cell has only one vibratile projection, 

 but this is strongly developed ; in the case of ciliated epithelium 

 (fig. 24, e), on the other hand, the surface of the cell is covered 

 with a thick forest of minute threads moving in unison. 



Cuticle. The majority of the one-layered epithelia are covered 

 by a cuticle, a membrane which is secreted by the epithelial cells 

 in general, and hence very frequently shows the impression of the 

 cells as polygonal markings. In many cases thin and inconspic- 

 uous, it may in other instances become thickened into a very con- 

 siderable layer, much thicker than the matrix layer of epithelium 

 which secretes this cuticle. The cuticle is plainly composed of 

 layers parallel with the surface, and forms a more effective protec- 

 tion for the surface of the body than does the epithelium; it 

 becomes a protective armor, as shown, among other examples, 

 by the calcareous shells of molluscs and the chitinous integument 

 of insects (fig. 25, /). 



Stratified Epithelia. The protection furnished by the cuticle 

 in the case of simple epithelium, may in the stratified be obtained 

 immediately through a chemical change of a part of the cells 

 themselves. In the stratified epithelia the cells of the various 

 layers always can be distinguished by their form. The deepest 

 layer consists of cylindrical cells; the superficial, on the other 

 hand, of more or less flattened elements; between lie several layers 

 of transitional forms, so that starting from the cylindrical cells we 

 gradually pass through the cubical cells to the flat cells of the sur- 

 face. As this arrangement shows, there exists a genetic con- 

 nexion between the cell -layers: the lower cylindrical cells are in a 

 state of active multiplication; their descendants, with gradual 

 changes of form, become the superficial layers, here to replace an 

 equal quantity of worn-out cells (fig. 26). 



In the course of this change of position, the protoplasmic 

 bodies may undergo an alteration; in the reptiles, birds, and 

 mammals (fig. 27) they became cornified, first the margins, then 

 the inner part of the cell, changing into horn. Of the living cell 



