42 THE EPITHELIAL TISSUES 



which is elaborated in the cytoplasm of these cells, collects toward 

 the free end, water is absorbed, and the volume of the secretion is 

 increased so that the apical end becomes much distended. The 

 distention progresses until there remains so thin a sheath of cyto- 

 plasm about the secretion that a rupture occurs and there is a 

 discharge of the mucus. Such goblet cells apparently may undergo 

 a number of repetitions of this apocrine type of secretion before 

 disintegrating. 



Secreting Areas.— As a further step beyond the unicellular condi- 

 tion, there are certain areas among cells forming an epithelial 

 membrane where a small aggregation of cells function as a special 

 secreting group, while the surrounding cells, although similar in 

 form, function chiefly in protection and absorption. Such areas 

 occur among the columnar cells lining the uterus, in the epithelial 

 membrane lining the trachea, and in the thin membrane of cuboidal 

 or low columnar cells forming the choroid plexes in the brain. 



Glandular Pockets.— Occuvrmg, at intervals in the epithelial mem- 

 branes lining the various ducts there are shallow pockets lined by 

 secretory cells. In the trachea, cloaca, and urethra, for example, 

 are small pockets usually lined with mucous secreting cells. Those 

 cells near the mouth of the pocket are cuboidal or short columnar, 

 while toward the bottom of the pocket are broader and longer 

 columnar cells. 



Simple Tubular Glands. — As the name implies, these glands result 

 from tubular invaginations of epithelial membranes during develop- 

 ment. The oviducts of the frog and other egg-laying forms are 

 usually lined with tubular glands whose secretions are added to the 

 descending eggs. The epithelial wall of the large intestine of mam- 

 mals possesses a multitude of simple tubular glands closely adjoining 

 each other, their walls composed of columnar cells, of which the 

 majority are of the goblet type. The cavity in these tubes is called 

 the lumen of the gland and may vary considerably in length. The 

 secretions of the cells pass into the lumen and out to the surface 

 of the epithelial membrane from which the gland forms. A 

 modification of these straight tubular glands can be seen in the type 

 represented by the sweat glands in mammalian integument. These 

 glands have an excretory superficial portion which is long and 

 spirals from the epidermis through the underlying connective tissue 

 to the deeper, much-coiled portion. The deeper portion is com- 

 posed of actively secreting cells and may be called the secreting 

 end-piece of the gland. The mucosa of the fundus of the stomach 



