DIFFERENTIATION OF THE ANIMAL BODY. 115 



body has over the single-celled animal with which we started. The 

 necessary functions are performed in a more complete fashion, 

 because of being given over to cells peculiarly fitted for their work. 

 In gaining this fitness the cells lose the power to do other kinds of 

 work. 



In the development and differentiation of tissues while the cell 

 itself is the original element it often conies about that the chief 

 work of the cells is to secrete among themselves a substance (inter- 

 cellular substance). After this is once produced, although it is dead 

 and inert, it may become the important part of the tissue, as in 

 bone or dentine. 



In general it may be said that the cellular structure is the impor- 

 tant thing in the active tissues. In passive tissue, however, the 

 intercellular secretion may be the more important. 



125. Classification of Tissues. We may classify tissues in several 

 ways. Perhaps the most meaningful classification for us is that 

 based on their function. We may classify tissues as passive and 

 active. The former are found in both plant and animals. The 

 latter are confined to the animals. The active tissues are the 

 muscular and nervous tissues. The passive tissues perform the 

 nutritive, the protective, and the supportive functions. There are 

 two main types of the passive tissues, the epithelial or bounding 

 tissues, and the connective or supportive tissues. 



126. Epithelial tissues (Fig. 22, e) always occur in layers, bounding 

 and finishing the surfaces of organs and cavities. Such are the 

 epidermis of the skin, the mucous membrane lining the mouth, the 

 digestive tract and its glands, and the lungs; the serous membrane, 

 which lines the body cavity and covers the outside of all the organs 

 in it. 



In addition to its finishing and bounding function, the epithelium 

 may secrete, as in the glands; may absorb fluids and gases as in the 

 intestines and lungs; may be sensitive as in the mucus membrane 

 of the nose and tongue; may produce new individuals (ova and 

 sperm), as in the reproductive glands. 



In the gastrula the ectoderm and entoderm are epithelial tissues. 

 At this stage the embryo is all epithelium. We may say, therefore, 

 that it is the most primitive kind of tissue. Ectoderm, entoderm, 

 and mesoderm all join in giving rise to the epithelial tissue of the 

 adult. The cells of this kind of tissue usually retain their power 

 of multiplication. This is the reason that new skin grows when 

 the old is injured. This is not so true of any other kind of tissue. 

 There is little or no intercellular substance in epithelial tissue. 



