20 CLASSIFICATION OF TISSUES. 
the alimentary canal, to the neuroglia and nervous system. The 
mesoblast furnishes material for the genito-urinary organs and for 
voluntary and involuntary muscle. From the hypo-blast is developed 
the epithelium of: the respiratory, the genito-urinary, and digestive 
systems, and the various glands and passages in connection with them. 
From the parablast comes the material for the formation of the con- 
nective tissue, including cartilage, bone and fat, lymphatic vessels 
and tissues, bloodcells and bloodvessels, and the true endothelial cells. 
* EPITHELIAL TISSUES. 
What is the general structure of epithelial tissues ? 
It is a tissue composed entirely of cells, with a comparatively small 
amount of intercellular substance. It usually takes the form of a 
membrane covering free surfaces. j 
Give the most important surfaces covered with epithelial tissue. 
Surface of the skin. 
Surface of mucous membranes. 
Surface lining the ventricles of the brain and spinal canal. 
Free surfaces of serous membranes. 
In organs of special sense. 
To what cells is the name endothelium applied? 
To certain delicate or modified-epithelial cells lining serous cavities 
and bloodvessels. 
What is the classification of epithelial tissue ? 
Sinople, stratified, and transitional. 
What is the structure of simple epithelial tissue ? 
It is composed of a single layer of cells. 
Give its varieties. 
Pavement, columnar, glandular, and ciliated epithelial tissue. 
Describe pavement epithelium. 
Tkis is made up of a single layer of polygonal-shaped cells, placed 
together edge to edge, like stones in a mosaic, and united by a small 
amount of intercellular cement substance. 
Describe columnar epithelium. 
It is made up of cubical or cylindrical-shaped cells set on end, in 
profile looking like a close palisade. When seen from above they 
appear polygonal and resemble pavement epithelium. 
