SECRETOh'Y AX J) EI'ITIIELIAL CELLS 41 



region, then usually pass toward the surface of the cells \\here they 

 are discharged to become part of the active secretion. There is 

 considerable variation in the method by which secretion is carried 

 to completion in different types of glandular cells. The use of the 

 term glandular to indicate such secretory cells places the emphasis 

 u])on function, but the form of most of these cells will fit into the 

 classification of types already studied. 



Groups and organizations of actively secreting epithelial cells 

 are called glands, and two broad divisions are sei)arated on the basis 

 of whether the secretion formed is liberated into a hmien from the 

 free boundary of the cells or whether it passes in the opposite 

 direction to enter the vascular system. Those organizations of cells 

 secreting by the first method, that is by liberating their secretion 

 into ducts which carry it to the surface of the epithelial membrane 

 from which the gland developed, are called exocrine glands. The 

 endocrine glands are those glandular tissues lacking excretory duct 

 systems and whose secretions pass into the vascular system. 



Exocrine Glands. ^The secretory process is completed in se\eral 

 ways in different types of exocrine glands. In merocrine glands, 

 granules form in the cytoplasm of the cells and accumulate toward 

 the free boundary to be discharged as the active secretion; the 

 process is then repeated. Most glandular secretions, such as those 

 of the digestive glands, are derived in this manner. In a holocrine 

 gland, as exemplified by a sebaceous gland in the skin of a mammal, 

 the major portion of the active cell develops into a secretion mass 

 which, when dislodged to form the secretion, is accompanied by the 

 death and disintegration of the cell. A new cell then forms from 

 the underlying layers of the stratified epithelium forming such 

 glands and the process is repeated. Another type of secretion is 

 termed apocrine, and in this case a secretion mass forms in the apical 

 portion of the cell; this apical region becomes greatly enlarged and 

 is finally cut off from the basal nucleated portion. This t}'pe of 

 secretion occurs in the mammary gland. The cells whose apical 

 portions have been cut off do not disintegrate following the libera- 

 tion of the secretion but develop a new apical portion and repeat 

 the process. A convenient classification of the exocrine glands may 

 be built upon their organization regardless of the method of secretion. 



Unicellular Glands. ^In the skin of fishes and amphibians there 

 are scattered single cells that secrete a fluid substance. Also, 

 among the columnar cells lining the gut regions are the numerous 

 goblet cells, already mentioned under columnar epithelia. Mucin, 



