THE EPITHELIAL TISSUES. 



61 



cells surrounded by fibrous tissue, with no trace of a lumen among 

 the epithelial cells (Fig. 48). 



Glands develop from surfaces which are covered by epithelium. 



FIG. 48. 



Section of gland from human lip. (Nadler.) a, duct, cut in slightly oblique direction (lumen 

 oval), and probably near a branch, which would account for the apparent thickness of 

 its epithelial lining in the lower half; b, cross-section of acinus secreting mucus ; c, tan- 

 gential section of a similar acinus near its extremity and beyond the end of the lumen. 

 Cross-sections of the cells at the fundus occupy the centre, d, cross-section of an acinus 

 secreting a serous fluid, revealing a small lumen ; d', a similar acinus with a larger lumen, 

 probably cut near its junction with a duct ; e, acinus with cresceutic group of cells with 

 granular cytoplasm (e'), and other cells like those in 6. The granular cells of small size 

 are considered to be cells which have discharged their secretion and are accumulating 

 material for a fresh supply. /, nearly axial longitudinal section of a portion of a mucous 

 acinus ; g, tangential section of a serous acinus ; h, fibrous connective tissue between the 

 acini ; i, capillary bloodvessel in the fibrous tissue. 



The cells of this epithelium multiply and penetrate into the under- 

 lying tissues, forming little solid tongues or columns of cells (Fig. 181). 

 If the gland is destined to be of the simple tubular variety, this col- 

 umn of cells then becomes hollowed to form the lumen, the cells being 



