CELLULAR DIFFERENTIATION. 4! 



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 with- 

 in and between the cells of the different tissues. The ma- 

 terial deposited between the cells is known as intercellular sub- 

 stance. 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 

 general if the tissue is active (as muscle) the cellular differ- 

 entiation is the important point; if, however, the function is 

 a more passive one, as support or protection, the nature of 

 the intercellular substance rather than the cells determines its 

 character (bone, connective tissue). 



63. 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. 



64. Epithelial Tissue. 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 absence 

 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 digestive tract and 

 its outgrowths, as well as the body cavity and the structures 

 contained in il 



65. Kinds of Epithelial Tissue. Located in a position 

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



