44 



GENERAL BIOLOGY 



secretions of gland cells has arisen the necessity for 

 the production of large quantities of the secretion at 

 one time in a limited space. The natural extension 

 of the secreting area is ordinarily quickly limited 

 by the need for concentrating the secretion at a 

 certain point. In consequence the secretory surface 

 is increased by folding and by sinking below the 



FIG. 18. Diagram of the formation of glands by the sinking in of a 

 secretory area of the epithelium: /, simple tubular gland; 2 and '>. 

 branched tubular glands ; 4 and 5, simple alveolar glands ; 6', branched 

 alveolar gland. (Hertwig.) 



surface of the cell-layer (epithelium) of which it is a 

 part, so that a maximum amount of secreting surface 

 is produced in a minimum of space. In this way is 

 developed a gland (see fig. 18), the common channel 

 for the substance secreted being the duct. 



All organisms possess the power of reproducing 

 themselves, otherwise the species of which they are 

 members would become extinct. In the higher 

 plants and animals this, like the other properties 



