CELLULAR DIFFERENTIATION 



51 



in the life of all organisms. The position of the epithelium 

 also renders it specially liable to destruction. To compensate 

 for this its primitive or undifferentiated character makes it 

 particularly capable of regenerating portions of itself which 

 may have been lost. Epithelium is often especially active also 

 in the regeneration of other than simple epithelial structures, as, 

 for example, the regeneration of nervous cells in a cut earth- 

 worm. In close connection with this latter regenerative quality 



Fig. 18. 



3, A 



Fig. 18, A series of diagrams showing progressive stages in the development of a multicellular 

 gland from an area of glandular epithelial cells. C and D show two somewhat different types of 

 gland, — the cup-shaped and the tubular, e, bounding epithelium; g, gland cells; d, duet; f. connect- 

 ive tissue. 



Questions on the figures. — 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? 



is to be considered the fact that the reproductive or sexual cells, 

 by which new individuals are produced arise from an epithelium. 

 The foregoing enumeration of functions suggests the physiolog- 

 ical classification of epithelia: — bounding, glandular, sensory, 

 and reproductive epithelia. The same layer may fulfill several 

 of these functions at once. 



