40 LABORATORY STUDIES IN ZOOLOdV 



A STUDY OF THE PRIMARY TISSUES 



Epithelium. — One of the most important functions of epi- 

 thelium is protection or the covering of other tissues. An exam- 

 ple of such an association of cells is the pavement-like layer of 

 flattened cells which constitute the outermost layer of the skin. 

 Examine a mounted preparation of such a layer which may be 

 prepared from the sloughed skin of the frog. This shows the 

 skin as it actually appears in surface view. Note that the cells 

 are all similar in appearance and so associated with one another 

 as to protract a sheet of cells which is the tissue. In studying 

 tissues it is always best to use a minimum of light. Tilt the 

 mirror of the microscope or reduce the opening in the diaphragm 

 in order to reduce the amount of light passing through the 

 preparation. 



Study a single cell and determine the parts which are common 

 to all cells, as follows: (a) cell membrane, the thin but dense mar- 

 gin of the cell; (b) Jiucleus, composed of dense protoplasm and 

 located near the center of the cell; (c) all the protoplasm excluding 

 the nucleus is termed the cytoplasm. Each of these parts and 

 many others not mentioned is an area of protoplasm differently 

 organized for particular services in the cell. 



Make an enlarged drawing of several cells showing their 

 relations and parts. Drawings of single cells should not be less 

 than three-quarters of an inch in diameter. 



Many adaptations to various functions are illustrated by epi- 

 thelial tissues and in each case the tissue takes a name descrip- 

 tive of its form, structure, or function. For instance, in some 

 regions, as in the lining of the mouth and oesophagus of the frog, 

 the exposed surface of the epithelial cells is provided with deli- 

 cate, vibratile extensions of protoplasm called cilia and it is 

 accordingly known as a ciliated epithelium. Examine a strip of 

 the lining from the roof of the frog's mouth stretched and mois- 

 tened with normal salt solution to demonstrate the transportation 

 of solid particles such as fine charcoal over the surface of the 

 epithelium by the effective stroke of the cilia. Examine a piece 

 of teased epithelium as a demonstration of ciliary action. 



Connective tissue performs varied functions and is corre- 

 spondingly modified in its nature. White fibrous connective 

 tissue, yellow elastic fibers, adipose tissue, cartilage, and bone 

 are here considered as forms of connective tissue. While in 

 some tissues the intercellular substance is relatively small in 



