142 



CELLS OF THE VERTEBRATE BODY 



thelium. In organs of secretion, columnar epithelium is modified 

 as glandular epithelium for the production of certain types of 

 secretions (Fig. 77 G). Epithehal cells, temporarily modified as 

 gland cells, occur as goblet cells (Fig. 77 H), so called because dis- 

 tended by a drop of mucus, in the mucous membrane of the diges- 

 tive tract. Sometimes glandular epithehal cells occur in groups, 

 and these cell groups may sink below the general surface (Fig. 78). 

 Such groups of cells may form simple tubes, constituting simple 

 tubular glands, like the gastric glands; or flask-shaped struc- 



FiG. 78. — Diagrams of glands. A. Unicellular glands. The one to the left 

 is shown extending below the surface epithelium. B. A group of gland cells 

 remaining in the surface epithehum. C. A simple alveolar gland. D. A sim- 

 ple tubular gland. E. A compound tubular gland. F. A compound alveolar 



gland. 



d, duct; gc, gland cell; se, cells of surface epithelium. 



tures, making simple alveolar glands, such as those of the frog's 

 skin. Secretion may occur in all cells of these glands; or it may 

 be confined to the basal cells, while those leading to the surface 

 form the duct or tube by way of which the secretion leaves the 

 gland. A compound tubular gland, like the liver, or a compound 

 alveolar gland, such as the pancreas, is formed by outpocketings 

 from an original simple type (Fig. 78). 



Sustentaf/ive Tissue. — The sustentative tissues are a very 

 heterogeneous group, classed together because they are all derived 

 during development from the same source — the stellate mesen- 



