310 Basic Structure of Vertebrates 



the tracheal epithelium propel inhaled particles of dust upward and 

 outward. 



Cilia and mucous glands commonly occur in the same epi- 

 thelium. The simple external epithelium of an earthworm and the 

 stratified epidermis of a fish combine cuticular, glandular, and sensory 

 specializations. 



Glands 



"Glands" whose products are as different as are sweat, eggs, and 

 blood-cells hardly merit the same name. Accepting the name, it is 

 necessary to distinguish different types of gland: (1) secretory glands, 

 whose products are retained at least temporarily and serve some useful 

 purpose — e.g., mucous, salivary, and thyroid glands; (2) excretory 

 glands, which eliminate waste — e.g., kidneys; (3) cytogenic glands, 

 which produce living cells — e.g., reproductive glands producing eggs 

 or sperm, various lymph- and blood-glands in which white blood-cells 

 are produced. 



Secretory glands may be unicellular (Figs. 245; 246, u) or multi- 

 cellular (Fig. 246, g). Nearly all multicellular glands develop directly 

 from epithelia and retain their epithelial character. Some endocrine 

 glands are epithelioid. 



Most secretory glands develop from either the ectodermal or the 

 endodermal epithelium and discharge at the surface of their native epi- 

 thelium. Such are the many kinds of skin glands and digestive glands. 

 The mesoderm gives rise to some secretory glands, especially in con- 

 nection with the reproductive system — e.g., the albumen glands and 

 shell glands of oviducts and the mucous glands of the mammalian 

 uterus. 



Multicellular glands may be tubular (Fig. 252 A-D), or alveolar 

 (acinous : Fig. 252E, F). Glands of either type, complicated by branch- 

 ing, are called compound (Fig. 252C-F). 



The larger multicellular glands, especially those which are com- 

 pound, require certain accessory structures. A good blood-supply must 

 be provided. Therefore the gland may have an outer investment of 

 connective tissue containing blood-vessels and lymphatics. A thin layer 

 of nonstriated muscle fibers may be present on the wall of a gland 

 which discharges its contents abruptly. The muscle would be accom- 

 panied by nerve-fibers, and in some glands nerves may be traced to 

 the secretory cells. 



Secretory glands in vertebrates range from unicellular mucous 

 glands in the skin of fishes and amphibians and in the digestive epi- 

 thelium of all vertebrates, to such massive compound multicellular 

 glands as the mammary glands and liver. 



