52 HISTOLOGY 



GLAND FORMATION 



Glands may be defined as portions of epithelial surface used for the 

 secretion or excretion of some particular substance or substances from 

 the body. This surface may be small, in fact it may consist of a single 

 cell, and often does. It usually consists, however, of quite a number 

 of cells, and there may be several different kinds of cells among them. 



This surface may be a part of some general body surface, but most 

 often it is removed from the surface to which it belongs by an invagina- 

 tion. It may be corrugated or evaginated, but the commonest form is 

 an invaginated gland. 



We shall study as an example of a unicellular gland the mucous cell 

 found in the digestive epithelium of the worm Cerebratulus (Fig. 56). 



FIG. 56. Portion of digestive epithelium of Cerebratulus lactatus, showing a deep 

 mucous cell, n, nucleus of mucous cell, x 1200. 



This epithelium shows, in section, a straight row of cells that are used, 

 evidently, for digestive purposes. At intervals among this layer of even 

 cells appears one which is so large that its body has grown down out of 

 the row and become many times its original bulk. It is forced to do this 

 by the kind of work that it must perform, the secretion of mucus. 

 This necessitates the large bulk of cell body, and as the surface would lose 

 its value as a digestive (and motor) surface were these great cells to spread 

 the others apart and wedge themselves in between, the large mass of 

 the mucous cell is kept below and only its distal end, with the average 

 diameter of the undifferentiated cells, remains in the row. The nu- 

 cleus is placed in the bottom of such a cell. See also the mucous cell 

 of Helix, pictured by Figure 338. 



An example of a unicellular gland that does not descend into the tissue 

 below the epithelium of which it is a part can be seen in the goblet cell 

 of the digestive epithelium in the intestine of any vertebrate animal (see 

 Fig. 263). 



