ORGANIZATION. 33 



pie, is moist: even the hairs, nails, and cuticle contain 

 water. The contents as well as the shape of the cells are 

 usually modified according to the tissue which they form : 

 thus, we find cells containing earthy matter, iron, fat, mu- 

 cus, etc. 



In plants, the cell always retains the characters of the 

 cell; but in animals (after the embryonic period) the cell 

 usually undergoes such modifications that the cellular form 

 disappears. The cells are connected together or enveloped 

 by an intercellular substance (blastema), which may be 

 watery, soft, and gelatinous, firmer and tenacious, still 

 more solid and hyaline, or hard and opaque. In the fluids 

 of the body, as the blood, the cells are separate ; i. e. 9 the 

 blastema is fluid. But in the solid tissues the cells coa- 

 lesce, being simply connected, as in the epidermis, or united 

 into fibres and tubes. 



In the lowest forms of life, and in all the higher ani- 

 mals in their embryonic state, the cells of which they are 

 composed are not transformed into differentiated tissues: 

 definite tissues make their first appearance in the Sponges, 

 and they differ from one another more and more widely 

 as we ascend the scale of being. In other words, the bod- 

 ies of the lower and the immature animals are more uni- 

 form in composition than the higher or adult forms. In 

 the Vertebrates only are all the following tissues found 

 represented : 



(1) Epithelial Tissue. This is the simplest form of cellu- 

 lar structure. It covers all the free surfaces of the body, 

 internal and external, so that an animal may be said to be 

 contained between the walls of a double bag. That which 

 is internal, lining the mouth, windpipe, lungs, blood-ves- 

 sels, gullet, stomach, intestines in fact, every cavity and 

 canal is called epithelium. It is a very delicate skin, 

 formed of flat or cylindrical cells, and in some parts (as in 

 the wind-pipe of air-breathing animals, and along the gills 



3 



