SECRETORY AND EPITHELIAL CELLS 



45 



The liver in lower forms is a ('oiii])ouii(l tuhiilar j>;laii(l in form, hut 

 in the higher vertebrates this form may he lost during development, 

 as will be seen when studying this organ. 



Compound Alveolar Glands. Thv manner in wiiicli these glands 

 develop is similar to that in the case of tubular glands, but the 

 secreting end-pieces are expanded into alveoli. (Fig. 24.) The 

 lung develops as this type of gland but is not usually considered 

 as a gland, though it might be considered as secreting carbon 

 dioxide and absorbing oxygen. The mammary gland is another 

 example illustrating the structure of a compound alveolar gland. 

 The salivary glands and the pancreas, 

 considered as tubulo-alveolar, have the 

 outward appearance of compound 

 alveolar glands, but the lumens of the 

 secreting end-pieces are tubular in 

 form. (Fig. 28.) 



SeroNs and Mucous Glands. — On the 

 basis of secretory products there are 

 some glands whose secretions are of 

 the watery or serous type; others 

 secrete a thicker, viscous, mucilagin- 

 ous substance, called mucus. The cells 

 secreting these two ty^jes of material 

 have certain distinctive features. (Fig. 

 25.) The serous cells usually ha^•e a 

 clouded cytoplasm with a rounded 

 luicleus located toward the center of 

 the cell ; the mucous cells are generally 

 larger, haxe a clearer cytoplasm, aufl 

 have an oval, flattened nucleus located 



well tow^ard the base of the cell. The secreting end-pieces of the serous 

 glands have a small lumen when compared with the relatively wide 

 lumen in the end-pieces of mucous glands. The ])ancreas presents 

 an example of serous secretion. The salivary glands, which var\' 

 somewhat in this respect, may present either serous or mucous or a 

 combination of both, as will be observed in studying these glands 

 later. 



Endocrine Glands.^ These glandular organizations are very richly 

 supplied with blood vessels, which their secretions enter and are 

 circulated by means of the blood stream. The thyroid is an example 

 of this ty])e of gland; it begins its development as do other glands, 



Fig. 25. 



Diagram of mucous and 

 serous cells. 



