GLANDS 219 



Non-nucleated portions of cells are probably not able to elaborate any true 

 secretion. When (see below) the entire cell enters the secretion as in the 

 mammary gland, the entire nucleus of course becomes a part of the secretion. 

 In any event, the onset of secretion is apt to be evidenced in the nucleus by en- 

 largement and irregularity, in some cases by the giving off of nuclear material 

 to the cytoplasm, in others by amitotic division. According to some observers, 

 both ergastoplasm and paranucleus arc of nuclear origin. 



In connection with the mucous cell (p. 217) it was noted that according to 

 some authorities the cell dies in secreting, while others believe that the cell 

 reconstructs itself and can again secrete. In certain glands, e.g., the mammary 

 and sebaceous, the cast off cells themselves form the secretion. More commonly 

 the cell merely gives off its secretion, the remainder of the cell recovering and 

 again going through the same process. 



Most glands are composed of more than one cell, usually of a large 

 number of cells, and these cells, instead of lying directly upon the sur- 

 face, line more or less extensive invaginations into vi^hich they pour 

 their secretions. 



In the simplest form of glandular invagination all the cells lining 

 the lumen are secreting cells. In more highly developed glands only 

 the deeper cells secrete, the remainder of the gland serving merely to 

 carry the secretion to the surface. This latter part is then know^n as 

 the excretory duct, in contradistinction to the deeper secreting portion. 

 In both the duct portion and secreting portion of a gland the epithe- 

 lium usually rests upon a more or less definite basement membrane or 

 mefnbrana propria (page 73). Beneath the basement membrane, 

 separating and supporting the glandular elements, is the connective 

 tissue of the gland. This varies greatly in structure and quantity in 

 different glands. 



When the secreting portion of the gland is a tubule, the lumen 

 of which is of fairly uniform diameter, the gland is known as a tubular 

 gland. When the lumen of the secreting portion is dilated in the 

 form of a sac or alveolus, the gland is known as a saccular or alveolar 

 gland. Intermediate forms have been described as tubulo-alveolar 

 glands. 



A gland may consist of a single tubule or saccule, or of a single 

 system of ducts leading to terminal tubules or saccules — simple gland. 

 A gland may consist of a number of more or less elaborate duct sys- 

 tems with their terminal tubules or saccules — compound gland. A 

 few glands, e.g., the thyreoid and thymus, have no ducts, and are 

 known as ductless glands. 



All compound glands are surrounded by connective tissue which 



