92 



CHORDATE ANATOMY 



to distinguish different types of gland: (i) 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. 79, 80, u) or multicellular 

 (Fig. 80, 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 endo- 

 dermal epithelium and discharge at the surface of their native epithelium. 

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



Fig. 86. — Types of multicellular glands. A-D, tubular; E, F, alveolar or acinous. 

 A, simple; B, coiled; C-F, branched. The duct pierces the epithelium from which the 

 gland has been produced. (From Kingsley.) 



mesoderm gives rise to some secretory glands, especially in connexion 

 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, S6 A-D) , or alveolar 

 (acinous; Fig. S6E,F). Glands of either type, complicated by branch- 

 ing, are called compound (Fig. S6C-F). 



The larger multicellular glands, and 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 unstriated 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 epithelium of 

 all vertebrates to such massive compound multicellular glands as the 

 mammary glands and the liver. 



