io8 



HISTORY OF THE HUMAN BODY 



complexity of development and in the nature of their secre- 

 tion. The principles underlying gland formation are very 

 simple, and may be briefly considered in this place before 

 taking up in detail their occurrence and distribution in verte- 



FIG. 29. Diagrams of various types of glands, shown as invaginations 

 from a layer of indifferent epithelium. 



(a) represents a region in which certain of the surface cells are differentiated 

 as unicellular glands. (b) is a simple tubular gland and (c) a simple acinous 

 gland, each formed from a complex of gland cells. Tubular glands may become coiled 

 (d), or branched (e). Acinous glands may consist of a single acinus, as in (c), 

 or of several, as in (f). A still greater complexity is seen in (g), where each 

 acinus possesses its own excurrent duct, all being collected into a common duct 

 which leads to a single outlet. 



brate integument. The protoplasm of all cells has the power 

 of storing up some form of secondary material, metaplasm, 

 extracted from the materials supplied to it, and a gland cell 

 differs from another mainly in the fact that its metaplasm is 

 of use to some other part of the organism and that its chief 

 value to the organism lies in the material which it produces. 



