I40 



COMPARATIVE ANATOMY 



The great majority of secretory glands arise directly from the ectoder- 

 mal and endodermal epithelia and discharge at the surface of their native 

 epithelium. Such are the many kinds of integumentary glands and the 

 digestive glands. The mesoderm gives rise to some secretory glands, 

 especially in connection with the reproductive system— e.g., the albumen 

 glands and shell glands of oviducts and the mucous glands of the mam- 

 malian uterus. 



A multicellular secretory gland, other than endocrine, is an invagination 

 of an epithelium into adjacent tissue (Fig. 102). The secretory activity 

 is usually Hmited to cells of the deeper part of the gland, the more super- 

 ficial part of it serving merely as duct. 



Fig. 102. — 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.) 



Varying in the form which the invagination assumes, two main types 

 of gland are recognized. A tubular gland (Fig. 102, A-D) is one in which 

 the secretory portion and the duct are of approximately the same diameter. 

 An alveolar or acinous gland (Fig. io2£, F) has an enlarged and more or 

 less nearly globular secretory region. Glands of either type, complicated 

 by branching, are called compound. 



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 con- 

 nective tissue containing blood vessels and lymphatics. A thin layer of 

 unstriated muscle fibers may be present on the wall of a gland which dis- 

 charges its contents abruptly. The muscle would be accompanied 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 mam- 

 mary glands and the liver. 



NON-EPITHELIAL TISSUES 



The primarily essential parts of a metazoan animal are the epidermal 

 epithelium and the enteric epithehum. Certain of the organs which, in 



