ORIGIN OF GLANDS 



53 



Multicellular glands that have not been invaginated are rare. Where 

 the unicellular mucous glands are collected closely on a surface, as in some 

 mollusks, and also where primary surfaces of the digestive tract are used 

 to produce some special fluid, we have examples of such surface glands. 



By far the greater number of multicellular glands are evaginated from 

 the primary surface. The simplest way in which this can occur is as a 

 mere single pocket which may be a simple tube-like depression called the 

 tubular, or a bag-like enlargement called the alveolar type of gland. 



TTTO 



FIG. 57. A-G. Diagram of the different sorts of glands. A, unicellular glands lying in the 

 epithelium and one of them projecting out of it; B, multicellular gland not invaginated; 

 C, simple saccular gland; D, simple saccular gland with neck elongated into duct; E, com- 

 pound tubular gland with individual ducts collecting into a single opening; F, compound 

 saccular gland; G, highly compound gland with one primary and many secondary invagina- 

 tions. Only the secondary epithelium secretes. 



These simple glands are the kind that usually produce the amplification 

 of a body surface (Fig. 57). Sometimes the lower part, or fundus, of 

 this simple gland does all the active secretion, and the upper part forms 

 the tube or duct that carries the secretion from the fundus to the surface. 

 When the fundus becomes a highly differentiated, saccular region on the 

 end of a duct, it is called an acinus. 



Many glands are compound in that they consist of two or more acini 

 on the ends of a branching duct, which acts as a common carrier for their 

 secretions. Compound tubular glands are to be seen in the digestive 

 gland of the lobster and the pepsin glands of the mammalian stomach. 



