44 



ZOOLOGY. 



Such a cell may become much enlarged and sink below the general 

 level of the epithelium, retaining in the meantime a narrow connection 

 with the exterior (Fig. 15, b). Multicellular glands represent areas of such 

 cells which have sunk below the surrounding surface, forming a tube- or 

 flask-shaped cavity, which may become very much branched. Glands with 

 such branched ducts are described as compound. They consist of numer- 

 ous final secretory sacs communicating by ductules with a common duct 

 or outlet to the surface. Transitional conditions between the simple 

 secretory epithelium and the compound gland may be seen in Fig. 16. 



68. Sensory Epithelium. In the lower animals there may be found 

 here and there over the surface of the body modified epithelial cells, which 



FIG. 1 6. 



FIG. 16. A series of diagrams showing progressive stages in the development of a 

 njulticellular gland from an area of glandular epithelial cells. C and D show two some- 

 what different types of gland, the cup-shaped and the tubular, e, bounding epithe- 

 lium; g, gland cells; d, duct; c, connective tissue. 



Questions on the figure. How do the compound glands seem to arise 

 from the simpler condition? What is the evidence that glands are lined 

 throughout with epithelium? What is gained in the sinking of the glands 

 below the surface? 



are specially capable of being stimulated by contact or other stimuli to 

 which the organism may be exposed. Likewise in higher forms we find 

 highly specialized areas of sensitive cells, which can be shown to belong 

 primarily to the epithelium. These are the end organs of special sense, 

 as touch, sight, and the like, and they get their special value from their 



