CELLULAH DIFFERENTIATION 49 



physical structure of the protoplasm, in such a manner as to 

 cause great variation in their powers and activities. The 

 chemical differences are especially shown in excretion and se- 

 cretion, whereby various sorts of materials are deposited within 

 and between the cells of the different tissues. The material 

 deposited between the cells is known as intercellular substance. 

 The intercellular substance differs much in character and 

 amount. Both the cells and the intercellular substance are 

 important in enabling the tissue to perform its work. In gen- 

 eral if the tissue is active (as muscle) the cellular differentia- 

 tion is the important point; if, however, the function is a more 

 passive one, as support or protection, the nature of the inter- 

 cellular substance rather than the cells determines its character 

 (bone, connective tissue). 



66. Classification of Tissues. — From a physiological point 

 of view tissues may be classed in one of two groups: vegeta- 

 tive, and active. The vegetative tissues are those which per- 

 form the more passive functions, as nutrition, protection, sup- 

 port, etc. They resemble the plant tissues in their functions. 

 The two chief classes of vegetative tissues are: epithelial or 

 bounding tissues, and supportive or connective tissues. The 

 active tissues may be looked upon as the characteristic tissues 

 of animals. The muscular and nervous tissues belong to this 

 group. 



67. Epithelial Tissues. — This tissue is characterized by its 

 primitive form, i.e., by its relative lack of differentiation, by 

 the fact that it is the first to appear in individual development 

 (ectoderm and entoderm in the gastrula), and by the very small 

 amount of intercellular substance. It is a bounding tissue and 

 consists typically of a single layer of cells, although several 

 layers may occur. Epithelium bounds, by its own cells or 

 their products, the outside of the body, the lumen of the diges- 

 tive tract and its outgrowths, as well as the body cavity and 

 the structures contained in it. 



68. Kinds of Epithelial Tissue. — Located in a position 

 superficial to the other tissues, epithelium is subject to a wide 

 range of variation both as to form and function. Besides its 



