68 



GENERAL PRINCIPLES OF ZOOLOGY 



Glandular Epithelium. Glandular epithelium is distinguished 

 physiologically from ordinary epithelium by the fact that it also produces 

 secretions or excretions; anatomically it is recognizable by the presence 

 of gland cells, which carry on the secretion and, to a greater or less extent, 

 reveal their character by their structure. Characteristic glandular cells 

 are, for example, the goblet cells; here the secretion, usually mucus, is 

 collected as a clear mass in the interior of the cell, the cytoplasm being 

 compressed into a thin external wall, reminding one of a goblet, contain- 

 ing the nucleus at its base (fig. 27,5, 29, d). Other gland cells are granular 

 cells, swollen bodies filled with secretory granules (fig. 27, A'o). Natu- 

 rally all grades between pavement and glandular epithelium occur. Com- 

 monly the latter name is only employed when the gland cells are especially 

 numerous, thereby giving to the area a pre-eminently secretory character. 

 This is especially the case with the structures which have the name of 

 glands, among which we distinguish unicellular and multicellular glands. 

 Unicellular Glands. Unicellular and multicellular glands increase 

 the secretory surface by invagination. Invagination of a single cell 



produces the unicellular glands which are 

 chiefly found among the invertebrate 

 animals (fig. 30) ; a gland cell here be- 

 comes so enormous that there is no room 

 for it in the epithelium, but it is pushed 

 into the deeper, the subepithelial layers, 

 the nucleated cell body, distended by 

 secretion, sending up a slender process, 

 the duct, to the epithelial surface. 



Multicellular Glands. In the forma- 

 tion of multicellular glands a considerable 

 area of glandular epithelium grows as a 

 tube or duct from the surface down into 

 the deeper tissues; this seldom remains 

 simple; it usually branches and forms 

 the compound glands, which may consist 

 of hundreds or thousands of glandular 



FIG. 30. Unicellular glands 

 from edge of the mantle of Helix 

 pomatia. e, epithelium; d, unicel- 

 lular glands; p, pigment cells. 



sacs, all emptying into a common duct. Among the multicellular glands 

 are to be distinguished tubular and acinmis (racemose) forms. In tubular 

 glands (fig. 31) the simple or branched glandular pouches preserve the 

 same tubular diameter from beginning to end; in the acinous glands 

 (fig. 32), on the contrary, the blind end of the glandular pouch widens 

 into a sac (acinus'), largely composed of secretory cells, and related to 

 the duct, as grapes to their stem. To the tubular glands belong the 



