366 HISTOLOGY 



has a flat, round, and non-staining nucleus. Its strength comes from 

 a deposit of a substance called first kerato-hyalin, which then changes 

 into eleidin, and that into pareleidin. 



Just as the cuticle of the columnar epithelium is developed into a 

 variety of organs, so also is the outer protective layer of toughened dead 

 cells in the stratified epithelium developed into a vast number of struc- 

 tures that are used for a variety of purposes. In a very general way 

 it may be said that these structures represent and are made out of the 

 same kind of outer cells that cover the surface of the stratified epithelium. 

 They become so hardened and fitted together, however, that it is difficult 

 to distinguish them as cells. These structures are usually developed from 

 a portion of the epithelium that has been invaginated. 



One of the commonest and simplest forms of this structure is an 

 evagination, and is to be found on the tongue of most mammals, espe- 

 cially on those which eat living prey. The upper surface of the tongue 

 is evaginated into a series of close-set papillae covered with their strati- 

 fied epithelium. In the hollows this epithelium is thin and soft. On the 

 pointed tips it is immensely thick and strong, and is so hardened and 

 compacted that the animal can use its tongue as a rasp to scrape meat 

 off a bone. 



The same sort of development on flat areas separated by valleys 

 lined with a softer epidermis results in the "scale" of the reptiles. The 

 legs of birds show such structures well developed, and in the entire skin 

 of a fowl or pigeon early rudiments of this formation maybe seen. Such 

 scales are shed periodically and replaced by new ones which are devel- 

 oped beneath them. The method of fission between the old and new 

 layers is unexplained. It can be well seen in the snake. 



It is the analogous structures which are developed from invaginated 

 regions of an outer stratified epithelium that show the most perfect or- 

 ganization, however. Such are the hair and feathers of the mammals 

 and birds. Of these two, the feather seems to be the highest specializa- 

 tion. 



Hair is formed in general as follows. It begins in the embryo 

 mammal as a thickening of the epidermis, particularly of the basal layer. 

 This thickening soon develops into an invaginated pocket filled, of course, 

 with the basal cells. This pocket deepens into a long tube whose fundus 

 is widened and filled with the cells of a considerably thicker epithelium 

 than that which lines the sides. The bottom of this bulb is then evagi- 

 nated for a short distance by the growth of a mesodermal papilla, and the 

 epithelium on this papilla begins to grow much more rapidly than any 

 of the rest, forcing a pointed mass up through the lumen of the tube. 



This mass consists of a central core which is the hair shaft with an 

 outer layer called the inner sheath. This sheath travels with the hair 



