88 



THE TISSUES. 



5= ^ 3 k> Gland-cells. 



the form of rather coarse granules, which may be as large as i ]/ 2 p. 

 to 2 fJL. These granules are found in a hyaline substance, from 

 which they are probably formed, which substance is found in the 

 interspaces of a protoplasmic network with relatively wide meshes 

 (Langley). The granules as they develop and enlarge distend the 

 free portions of the cells. They are eventually extruded from the 

 cells, probably in the form of granules, as granules identical with 

 those found in the cells are found in the lumina of intestinal glands 

 in well-fixed material. After the extrusion of the secretion the 

 cell collapses, and may again assume a secretory function by the 

 elaboration of new granules. (See Fig. 50.) 



Multicellular glands originate by the metamorphosis of a num- 

 ber of adjacent cells into glandular cells. This is usually accom- 

 panied by a more or less 

 marked dipping down of 

 the epithelial layer into 

 the underlying connective 

 tissue. The glandular cells 

 are generally arranged in a 

 single layer, and rest on a 

 delicate membrane, known 

 as the basement membrane 

 (membrana propria); out- 

 side of this there is found 

 fibrous connective tissue, 

 containing the terminal 

 ramifications of capillaries 

 and lymph-spaces and of 

 nerve-fibers. The simplest 

 form of such an invagina- 

 tion is a cylindrical tube or 

 a small sac (known as an 

 alveolus) lined entirely by 

 glandular cells. A further 

 differentiation may take 



place in that all the invaginated cells do not assume a secretory func- 

 tion, those at the upper portion of the tube or sac forming the lining 

 membrane of an excretory duct. The originally uniform tube or sac 

 is thus differentiated into a duct and a secretory portion. Multi- 

 cellular glands may lie entirely within the epithelium, and are then 

 known as intra-epithelial glands, in contrast to the 'extra-epithelial 

 or ordinary type, the greater part of which lies imbedded in the 

 underlying connective tissue. Glands of the former type have been 

 studied in amphibian larvae, and, according to Sigmund Mayer, 

 occur also in the epididymis, conjunctiva, etc., of mammals. 



General Consideration of the Structure and Classification 

 of Glands. Since glandular tissue is composed almost wholly of 

 epithelial cells, it may not be out of place to consider at this time 



*_' T. propria. 



Muse, mucosse. 



Fig. 51. Simple tubular glands. Lieber- 

 ktihn's glands from the large intestine of man. 

 Sublimate fixation ; X 9- 



