50 



ZOOLOGY. 



Sometimes the secretion is set free as quickly as it is formed ; 

 in other cases it accumulates in the secreting cell, until at 

 last it escapes as a large drop. A cell discharging a drop 

 in this way is termed a " goblet-cell " (fig. 12). We shall 

 return to this form of epithelium a little farther on ( 17). 



6. The Epidermis (outer portion of the skin) is a 

 many-layered epithelium, the deepest layer consisting of 

 columnar cells which, as they grow in size, are constantly 



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Fig. 18. MAMMALIAN SKIN. 

 Diagrammatic section. (After Wiedersheim.) 



undergoing division, cutting off smaller, natter cells on their 

 outer side, to form the superficial layers As these flatter 

 cells become pushed nearer and nearer the surface their 

 protoplasm undergoes change into a substance called keratin 

 or horn, and eventually becomes rubbed off the surface. 

 The innermost columnar layer and those above it in 

 which the cells are still protoplasmic, constitute the 

 Malpighian layer, or stratum mucosum : the outer ones 

 form the horny layer or stratum corneum (fig. 13). Hairs 

 and nails are specially developed portions of the horny 

 layers of the epidermis. 



